Diamond Potential of Wyoming

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Introduction

Wyoming may have the highest potential for the discovery of commercial diamond deposits in the United States. Currently, 20 diamondiferous kimberlites and one diamondiferous mafic breccia pipe have been identified in southern Wyoming. However, enormous portions of southern Wyoming, and essentially all of central and northern Wyoming remain unexplored for diamonds.

Geological, geochemical, and geophysical evidence gathered by the Wyoming State Geological Survey (WSGS) over the last two decades has provided direct evidence for dozens of undiscovered kimberlites in Wyoming – some of which could be diamondiferous. The State Agency also mapped the two largest kimberlite districts in the US, and one of the largest lamproite fields in the world. This is significant, as kimberlite and lamproite are the only two host rocks that are commercially mined for diamond.

From June 30^th, 1998 to July 1^st, 2000, the WSGS received a special appropriation from the Wyoming State legislature to accelerate the Survey’s diamond research project. Funding, which amounted to approximately $50,000/year, was used to pay field expenses and salaries for three contract geologists who assisted in data collection, sample preparation, geochemical and geophysical analyses, and aerial photography interpretation. During this project, dozens of stream sediment samples were collected for analysis, several geophysical surveys were completed, the Iron Mountain kimberlite district was mapped, and several samples were collected for diamond testing (see http://wsgsweb.uwyo.edu/metals/).

The results obtained during the two-year project were exceptional (although the project was designed to be a 6-year study). Several previously unknown kimberlites were discovered, numerous geophysical anomalies were detected, some kimberlitic indicator mineral anomalies were identified, and diamond-stability garnets were detected in some samples (most of the anomalies are interpreted as being related to undiscovered kimberlites: the diamond stability garnets are interpreted to be related to diamond deposits). Overall, the data supports that Wyoming is host to a major kimberlite and lamproite province.

Wyoming has the potential to produce both gem- and industrial-quality diamonds. In the past, approximately 50% of the diamonds produced from the Wyoming portion of the State Line district south of Laramie, have been high-quality gemstones.

Diamonds are valuable. Annual diamond production from individual mines is typically valued in tens of millions to hundreds of millions of dollars. Some diamonds are priceless. Rare diamonds, known as the Argyle pinks, are the most valuable gemstones on earth. Some have sold for as much as $1 million/carat (equivalent to the value of 2,700 ounces of gold). In 1996, world diamond sales by DeBeers amounted to more than $52 billion. In 2000, DeBeers sold $5.67 billion in uncut diamonds to the US.

Favorable Geologic Terranes

Commercial diamond deposits are only found in two rock types - kimberlite and lamproite. Secondary (alluvial) deposits known as placers derived from the erosion of these rocks, may also be important sources of diamonds.

Diamondiferous kimberlite and lamproite are restricted to cratonic environments (or very old continental cores). Cratons represent ancient, stable, continental cores that respond as rigid blocks to deformation and possess gentle, cool, geothermal gradients necessary for the generation of mantle-derived magmas from the diamond stability field more than 90 miles below the earth’s surface. Wyoming, which is underlain by very old rocks, is part of a larger craton known as the Wyoming craton, which extends from Wyoming under Montana, and small parts of Idaho, Utah, and Colorado.

Kimberlite and lamproite are two of the rarest rock types found on the earth. Thus the discovery of a new kimberlite or lamproite, is considered a significant event. Diamondiferous kimberlite and lamproite are even much rarer. Statistics show that only 1 in 10 kimberlites are diamond-bearing, and 1 in 100 contain commercial mineralization. Diamondiferous lamproites are even rarer.

Where found, diamondiferous kimberlite and lamproite are essentially restricted to cratons. Janse (1994) suggested that cratons should be separated into terranes of favorability to facilitate the exploration of diamonds ( Figure 1 ). Based on this concept, cratons are divided into stabilized Archean basement complexes (highly favorable); cratonized mid- to early-Proterozoic basement (moderately favorable); and stabilized late-Proterozoic basement (low favorability).

Of the three terranes, Archons are considered to have high potential for discovery of diamondiferous kimberlite. Protons are less favorable for diamondiferous kimberlite, but are considered to have the greatest potential for the discovery of diamondiferous lamproite. Tectons are considered to have very low potential for the discovery of diamond deposits associated with kimberlite or lamproite. Geological terranes younger than Tectons, are considered to have no potential for diamondiferous kimberlite and lamproite.

With the exception of southeastern Wyoming, the remainder of the state is underlain by an Archon, and is considered to have high potential for the discovery of commercial diamond deposits. Only the southeastern corner of the state is not underlain by an Archon. In southeastern Wyoming, a major suture zone, known as the Cheyenne Belt, separates older basement rocks of the Archon from rocks of a Proton. Based on geology, the Proton is considered to have moderate potential for the discovery of diamondiferous kimberlite and high potential for the discovery of diamondiferous lamproite.

Exploration Methods

Hausel and others (1979, 1981) reviewed the exploration methods for kimberlite. The classical exploration methods include stream sediment sampling, geological mapping, geophysical surveys, and remote sensing exploration.

Morphology

Kimberlite and lamproite exhibit different geomorphic characteristics. Kimberlite is essentially a serpentinized peridotite formed primarily of olivine. When kimberlite erupts at the earth’s surface, it is highly charged with CO₂ and H₂O gas that alters much of the olivine producing a carbonated serpentinite breccia. Through time, this breccia weathers to a distinct clay.

Serpentine, a relatively soft mineral with a hardness of only 2.5 to 5 on Mohs hardness scale (Mohs scale ranges from talc with a relative hardness of only 1 to diamond with a hardness of 10), is relatively susceptible to weathering and erosion than most minerals. This is particularly noticeable in the southern Laramie Range where serpentinized kimberlites intrude granite. The granite country rock is relatively resistant to erosion, whereas serpentinized kimberlites are susceptible to erosion.

Differential erosion results in the kimberlite either being buried by debris from the adjacent granites, or results in depressions over kimberlite. Most kimberlites are small, ranging from linear dikes to elliptical to circular pipes. Thus many anomalous circular to elliptical depressions are viewed as potential targets. This was particularly noted in the Northwest Territories of Canada. More than 300 kimberlites were found in northern Canada over the last decade (at least 6 are commercial), many were found under circular lakes.

Weathering of serpentinized kimberlite breccia produces ‘blue ground’ composed of montmorillonite clay with calcium carbonate. Many kimberlites are covered with a blanket of blue-ground, that ranges from a few to as much as 100 feet thick (Hausel and others, 1979).

The unique clay (blue ground) produced by the erosion of the kimberlite supports vegetation anomalies. Blue ground does not support tree growth, thus many kimberlites are devoid of trees. In forested areas, this may result in distinct circular to elliptical treeless parks.

Kimberlites and lamproites are emplaced along shear (fault) zones, which may hold considerable groundwater. The presence of the shear zones and water-rich clays associated with kimberlite, results in a distinct grassy vegetation anomaly that is particularly noticeable after several weeks of rain in late spring to early summer. The taller and thicker grass over weathered kimberlite is often used to map the extent of the kimberlites.

Lamproites are potassic mafic volcanic rocks with a unique mineral assemblage, which includes sanidine, leucite, phlogopite, volcanic glass, potassium richterite, and a host of rare accessory minerals. These minerals are relatively hard. As a result, many lamproites form positive features rather than depressions. Lamproites erupt as small volcanoes and produce restricted flows. However, diamond-rich lamproites contain considerable olivine, which serpentinizes during eruption similar to kimberlite. This results in the diamondiferous lamproites eroding relatively rapidly and disappearing under a thin soil cover. Such lamproites are difficult to find and often require geophysical surveys to aid in their discovery.

Stream sediment sampling methods

One of the more effective methods used to search for diamondiferous kimberlite, is stream sediment sampling for unique kimberlitic indicator minerals dispersed during erosion. These minerals include pyrope garnet, chromian diopside, ilmenite, and less commonly chromite, olivine, chromian enstatite, and diamond. Ideally, these mineral trains are traced upstream to the source kimberlite. Lamproites are much less susceptible to prospecting by this method, as they lack the classical kimberlitic indicator minerals.

Leighton and McCallum (1979), suggested that the maximum transportation distances before complete disaggregation for the classical indicator minerals was 1.5 miles (ilmenite), 1 mile (pyrope garnet), and 0.25 mile (chromian diopside). During mapping of the Iron Mountain district in 1998, the WSGS recovered samples of consolidated kimberlite cobbles 0.75 mile downstream from the source outcrop. This would suggest that the maximum transport distance for the three indicator minerals would be more on the order of 2 to 2.5 miles for ilmenite, nearly 2 miles for pyrope garnet, and 1 mile for chromian diopside. The presence of these minerals in stream sediment samples suggests a nearby source kimberlite intrusive(s).

Other indicator minerals include chromite, olivine, chromian enstatite, and diamond. Maximum transportation distances for chromite is similar to ilmenite, olivine similar to garnet, and chromian enstatite similar to chromian diopside. Diamond is not considered a good indicator unless found in a closed drainage basin. Diamond transportation distances in Africa are as much as 500 to 1000 miles.

Stream sediment sampling is labor intensive. It is accomplished by panning with traditional gold pans and sieves, then transporting the panned concentrates to a lab for microscopic examination to look for kimberlitic indicator minerals. Typically, it takes a field crew of two to recover a maximum of 3 to 4 samples per day. Ideally, the spacing of samples for optimum coverage is only about one mile on accessible drainages.

The sample concentrates are further processed in the laboratory, before microscopic examination, which is also labor intensive.

Once indicator minerals are identified in the concentrates, they are extracted and tested for geochemistry with an electron microprobe and compared to mineral inclusions in diamond. Diamond inclusion minerals provide a unique range of geochemistry, and minerals found in the stream sediments that have similar chemistry are interpreted to have originated from depths of 90 to 120 miles below the earth’s surface, where diamond is stable.

For example, many pyrope garnet mineral inclusions in diamond are sub-calcic, high-chrome, magnesian garnets (Gurney, 1984). These have been designated as “G10”. Similar calcic, high-chrome, magnesian garnets (G9) do not occur as diamond inclusions, but are found in many kimberlites and interpreted to have originated from shallower depths where graphite is stable, rather than diamond. A greater abundance of G10 garnets in a sample provides a general indication of higher ore grades. Potassic pyroxenes, magnesian chrome-rich chromites, and sodic pyrope-almandine garnets also provide information on diamond grades in kimberlite.

Another important indicator mineral found in kimberlite is ilmenite. Ilmenite is thought to provide an indication of oxidizing/reducing conditions of the kimberlite magma when it erupted, and thus an indication of diamond preservation. Diamonds being pure carbon, will burn producing CO₂ and graphite. Chrome depletion in ilmenite is thought to be a measure of the oxidizing conditions of the kimberlite, and a measure of diamond resorption and burning during emplacement.

McCallum and Waldman (1991) reported chrome depletion in ilmenites in the Iron Mountain kimberlites, Wyoming, and suggested that the district had no potential for diamonds. However, ilmenite from the Kelsey Lake diamond mine 45 miles to the south, is geochemically comparable to the Iron Mountain ilmenites. The Kelsey Lake kimberlite is a low-grade diamond deposit that has produced many exceptional high-quality octahedral gem diamonds that show little to no evidence of resorption. Some diamondiferous kimberlites in Africa also show similar ilmenite geochemistry to the Iron Mountain kimberlites. Thus the chrome/magnesian ratios of ilmenites appear to provide inconsistent information on diamond preservation (Coopersmith and Schultz, 1996).

When kimberlitic indicator minerals are identified in stream sediment sample concentrates, they are traced upstream to the source rock. When the kimberlite is found, a small sample (few hundred pounds) is collected and the indicator minerals are extracted for geochemical tests to provide an indication of diamond potential and ore grade. Kimberlitic indicator minerals are used to determine diamond potential, as these are many orders of magnitude more common than diamond (the richest diamond deposits typically contain less than 1 part per million diamond). If tests are favorable, further sampling is done to extract a sample of diamonds to appraise the quality and variety of stones. If these tests are favorable, a diamond exploration company may choose to bulk sample (10,000 tonnes) the kimberlite and extract enough diamonds to determine the average value of the ore.

Geological mapping

Detailed geological mapping is used to search for additional kimberlite or lamproite. Geological mapping is important for a number of reasons. Maps provide researchers and companies with an indication of the extent of the deposit and potential tonnage. Since kimberlites and lamproites are emplaced along deep fractures and typically occur in clusters, mapping often leads to the discovery of other kimberlites and/or lamproites.

Geophysical Exploration

Geophysical exploration (surface and/or airborne geophysical surveys) is employed to search for hidden kimberlite and lamproite. Geophysical exploration is often used in conjunction with geological studies to locate hidden kimberlite. In the Colorado-Wyoming State Line district, an airborne INPUT survey (combined conductivity and magnetics) successfully identified several diamondiferous kimberlites including kimberlite at Kelsey Lake, which was later developed into an open pit mine. Kelsey Lake has been a source of several spectacular gemstones.

The INPUT survey identified a group of magnetic anomalies within the State Line district that are interpreted as ‘blind’ diatremes (Paterson and MacFadyen, 1984). These magnetic anomalies remain untested even though they lie in the middle of a well-established diamond district. Similar geophysical anomalies identified in Riley County, Kansas, were recently drilled by the Kansas Geological Survey resulting in the discovery of three kimberlites lying under about 25 feet of soil (Berendsen and others, 2000).

Kimberlite and olivine lamproite often show geophysical signatures that contrast with the surrounding country rock. Both of these rock types are relatively dense compared to most other rocks. As a result, they tend to show up as gravity highs with high seismic velocities. However, the most effective geophysical tools used to prospect for diamond pipes are magnetics and conductivity.

One by-product of serpentinization is the production of disseminated magnetite. Serpentinized kimberlite and olivine lamproite may contain enough secondary magnetite to yield significant dipolar magnetic anomalies, especially if the intruded host rocks have highly contrasting magnetic susceptibility. Magnetic anomalies are typically higher where kimberlite outcrops at the surface. Where the kimberlite is intensely weathered and has a thick cover of ‘blue ground’, much of the near surface magnetite is replaced by non-magnetic hematite, yielding only weak magnetic signatures.

Weathered kimberlite produces blue-ground composed of appreciable montmorillonite clay. Clay being highly conductive, yields distinct conductivity highs and resistivity lows. Using geophysical instruments, such as a Geonics EM31, the conductivity highs associated with subsurface clays may be mapped. Such conductivity highs detected in the Laramie Mountains are considered of interest, as weathered granites and anorthosites, which form much of the outcrop in the range, generally do not produce much clay.

Most kimberlites investigated by the WSGS produced only subtle magnetic anomalies. Hausel and others (1981) identified very weak magnetic anomalies in weathered kimberlites that ranged from 20 to 150 gammas. These subtle anomalies are easily missed during exploration.

The Radichal kimberlite, which occurs as outcrop in the Middle Sybille Creek drainage, produced a 700 gamma high. Woodzick and others (1980) also detected a 1,000 gamma high over the Nix 2 kimberlite outcrop in Colorado. Where such distinct anomalies are detected, follow-up grid surveys over the anomaly may reveal a pipe-like (‘bull’s eye’), or dike-like (linear magnetic anomaly) feature.

Resistivity lows were detected in several weathered diamondiferous kimberlites in the State Line district (Hausel and others, 1979). A conductivity high (5 to 38 mmhos/meter) was also detected over a hidden kimberlite in the State Line district that was later verified by trenching. However, conductivity measurements over the Radichal kimberlite outcrop in Middle Sybille Creek were inconclusive due to the lack development of blue-ground over the outcrop (Hausel and others, 1981).

The Wyoming Craton

Wyoming has excellent exploration potential for the discovery of commercial diamond deposits, as the majority of the State is underlain by stabilized Archean basement (Archon) that is bordered on the southeast by cratonized mid- to early-Proterozoic basement (Proton). During the Laramide orogeny, portions of the craton were tectonically reactivated and subjected to a brittle, mountain-forming, deformation event. This event was essentially non-thermal and should not have affected the generation of diamondiferous magmas. Instead, the brittle style of deformation may have provided deep fractures (pathways) for Laramide and post-Laramide kimberlite and lamproite magmas. The potential for pre-Laramide diamondiferous kimberlites and lamproites is also considered good, as the Wyoming craton had already stabilized.

Nearly all of Wyoming and Montana is underlain by an Archean (>2.5 Ga) (Ma=million years old, Ga=billion years old) basement known as the Wyoming Province. This province forms the core of the Wyoming craton; however, only small slices of this ancient terrain are exposed in some of Wyoming’s mountain ranges. The remainder lies under thick layers of younger sediment in the Wyoming basins. According to Eggler and others (1988), the craton was established by 2.7 Ga, and later affected by a regional metamorphic event at 1.9 to 1.7 Ga.

The Wyoming Province is bounded along the south by cratonized Proterozoic volcanogenic gneiss and schist of the Green Mountain terrain, and along the east by Archean-Proterozoic basement of the Trans-Hudson orogen which stabilized during the Proterozoic (Goldich and others, 1966; Hausel and others, 1991). The Trans-Hudson orogen is interpreted as a Proterozoic fold belt that welded the Wyoming Province to the Superior Province (Williams and others, 1986; Eggler and others, 1988). Based on geology, the Green Mountain terrain and Trans-Hudsonian orogen are considered to have moderate favorability for hosting diamondiferous kimberlite and high favorability for diamondiferous lamproite.

Wyoming is highly anomalous. The Wyoming craton hosts the largest swarm of kimberlites and lamproites in the US (Hausel, 1998). This coupled with a few hundred kimberlitic indicator mineral anomalies identified in stream sediment samples collected over widespread regions (Hausel and others, 1988), provides evidence that the craton encloses a much more extensive kimberlite and lamproite province. Since kimberlite and lamproite are the only two host rocks mined for commercial diamond deposits, this could have significant implications on the future economics of Wyoming.

To date, the Wyoming craton has also been the most productive region in the US for diamonds. Even so, only a very small portion of Wyoming has been explored for diamonds. Vast regions of prospective ground remain unexplored.

Kimberlites, Lamproites & Kimberlitic Indicator Minerals

Presently, there are 40 diamondiferous kimberlites known in the Colorado-Wyoming State Line district, which have yielded more than 130,000 diamonds including gemstones as heavy as 28.3 carats. Twenty of these lie on the Wyoming side of the border (Hausel and others, 1981).

Geophysical evidence and kimberlitic indicator mineral anomalies indicate that there are still some undiscovered kimberlites in the Wyoming portion of the district (Hausel, 1998). Some of the more interesting anomalies were detected by an airborne geophysical survey by a Canadian diamond exploration group ( Figure 2 ). The data show a group of bull’s eye magnetic anomalies scattered across State and private land west of Highway 287.

These anomalies are interpreted as buried or ‘blind’ kimberlite pipes (Paterson and MacFadyen, 1984). To date, only the geophysical anomaly at the Kelsey Lake kimberlite (Schaffer 2) has been explored. This anomaly led to the discovery and developed of a commercial diamond mine! The remaining anomalies have not been drilled or explored due to access difficulty, even though several of the anomalies lie on State lands.

Kimberlites were also found in the Iron Mountain district southwest of Wheatland in 1971. Preliminary mapping by Smith (1977) indicated that the district had little potential to produce diamonds because the extent of the kimberlites based on the early mapping suggested very little tonnage was available for mining. Later geochemical data published by McCallum and Waldman (1991) suggested that these kimberlites were unfavorable targets for diamond.

Recent mapping by the WSGS ( Plate 1 ) increased the amount of kimberlite to more than twice what was originally known (with considerable potential for additional discoveries). In addition, the geochemistry of indicator minerals, indicates that many of the intrusives originated from the diamond-stability field. The type of kimberlite (hypabyssal facies) identified in the district indicates that much of the original kimberlite was removed during past erosion. This means there could be potential for secondary (placer) diamond deposits in the area.

North of the Iron Mountain district, kimberlitic indicator minerals recovered from stream sediment samples from the Elmers Rock greenstone belt, Middle Sybille Creek, Mule Creek, and Cooney Hills also originated from the diamond-stability field ( Figure 3 ). Other targets of interest include a reported diamondiferous kimberlite pipe east of Laramie Peak (Lee Peterson, personal communication, 2000), as well as magnetic and conductivity anomalies in the State Line diamond district, the Iron Mountain district, Elmers Rock greenstone belt, and Middle Sybille Creek. Kimberlitic indicator minerals and diamonds found in the Medicine Bow Mountains are also of interest.

A few hundred miles west of the Iron Mountain district, an extensive kimberlitic indicator mineral anomaly was discovered in the Greater Green River Basin of southwestern Wyoming more than two decades ago (McCandless and others, 1995). This anomaly, which covers a 500 mi² region, provides evidence for dozens to possibly hundreds of mantle-derived pipes, yet, only a small group of pipes have been found along the edge of Cedar Mountain north of the Utah border (Hausel and others, 2000). These pipes were discovered by Guardian Resources, which also reported finding three diamonds from one of the pipes. The geochemistry of the indicator minerals in this region; however, are not favorable, and are for the most part derived from the graphite stability field.

In the northern portion of the basin along the flank of the Rock Springs uplift, one of the largest lamproite fields in the world, known as the Leucite Hills, forms a group of geologically young volcanoes (1.1 to 3.1 Ma). Some lamproites in the northeastern portion of the field contain olivine, and a few have yielded diamond-stability chromites. Yet much of the district remains untested. This region also offers potential for hidden olivine lamproites, especially along the northeastern edge of the field in the vicinity of the Kilpecker dune field.

Many other areas with kimberlitic indicator mineral anomalies have been identified in Wyoming. Most of these have only been investigated in a cursory manner. These include placer diamonds reported from some locations in Wyoming (see Hausel, 1998), detrital G10 pyrope garnets reported in the southeastern Bighorn Basin, and G10 pyropes recovered from a paleoplacer in the Miracle Mile area near the Seminoe Mountains, as well as widespread anomalies in the Laramie, Medicine Bow, and Sierra Madre Mountains.

The Iron Mountain District

The Iron Mountain district was considered high-priority during this investigation, as it had only been partially examined by researchers and exploration companies in past years. Thus the WSGS began re-mapping the district and ended up identifying one of the two largest kimberlite districts in the US. Geophysical surveys provided further evidence of hidden kimberlite, and samples collected for geochemical analysis showed that most kimberlites originated from the diamond-stability field.

History and Location

The Iron Mountain district is located on the eastern flank of the Laramie Range 35 miles northwest of Cheyenne and 45 miles north-northeast of the State Line diamond district. The district was named for the nearby titaniferous magnetite deposits in the Laramie Range anorthosite (1.5 Ga).

At an average elevation of 7,000 feet above sea level, the district is characterized by rolling hills with sparse scattered sagebrush and local stands of coniferous trees. Rock exposures are good, although large grassy fields with soil and regolith are prevalent in many areas providing excellent cover for hidden kimberlite.

Kimberlite was first reported in the district by W.A. Braddock and D. Nordstrom in 1971 (Smith, 1977). Smith later mapped and sampled the district as a thesis project at Colorado State University and expanded the known extent of the district by finding several more kimberlites. Smith described a complex of small, discrete, discontinuous dikes and blows that were identified by 57 sample sites. Recent mapping by the WSGS greatly expanded the known extent of kimberlite in the district and provided evidence that the kimberlites mapped by Smith, were all connected. In addition, several other kimberlites were discovered. The appearance of the district is that of a deeply eroded root zone. We are essentially looking at the roots of a group of kimberlite pipes that have long been removed by erosion. As much as 3,000 to 5,000 feet of vertical column of rock is estimated to have been removed.

Age of Emplacement

Diamond provinces typically exhibit multiple intrusive episodes. To date, several kimberlitic and lamproitic intrusive events have been recognized in the Wyoming craton including Late Precambrian, Early Devonian, and a few Tertiary events (Hausel, 1996).

Kimberlites in the Wyoming craton have been dated as Late Precambrian, Early Devonian, and Middle Tertiary. Kimberlites in the State Line and Iron Mountain districts were initially dated as Early Devonian based on the presence of Paleozoic xenoliths, fission track dating of sphene (Larson and Amini, 1981) and zircon (Naesser and McCallum, 1977), and on Rb-Sr dating of phlogopite (Smith, 1977, 1983). More recently, two episodes of emplacement have been recognized in the State Line district: Late Proterozoic (600 to 700 Ma), and Devonian (A.P. Lester, personal communication, 1996). In eastern Montana, the Williams kimberlites were dated at middle Tertiary, and the age of the recently discovered diamondiferous Homestead kimberlite near Yellow Water Butte, is unknown, but assumed to be Tertiary. Lamproites and related lamprophyres in eastern Montana are Tertiary in age, and the Leucite Hills lamproites in western Wyoming are very young and dated at only 3.1 to 1.1 Ma.

In the Iron Mountain district, Smith (1977) found sedimentary xenoliths of probable Ordovician and Silurian age in kimberlite at the extreme southwestern end of the dike complex in the SW/4 section 17. The sedimentary xenoliths indicate the intrusive penetrated the former Silurian-Devonian surface and assimilated fragments of the sedimentary cover. According to Smith (1977) the xenoliths establish the oldest possible emplacement age for the primary episode of eruption (Late-Silurian or Early Devonian).

In addition to the sedimentary xenoliths, upper crustal xenoliths found in kimberlites at Iron Mountain include quartz monzonite, anorthosite, syenite, hypersthene syenite, amphibolite, biotite schist, and fragments of mafic igneous rock. Lower crustal xenoliths include pyroxenite and garnet granulite. Mantle xenoliths include garnet and spinel peridotite, websterite, eclogite, and diopside-ilmenite intergrowths. Metacrysts include clinopyroxene, orthopyroxene, garnet, ilmenite, and rare olivine (Smith, 1977).

Geology & Structure

The Iron Mountain district lies in a salient along the eastern flank of the Laramie Range, southwest of Chugwater. The salient was uplifted and thrust easterly during the Laramide orogeny. Iron Mountain is surrounded on the east, south, and west by Phanerozoic sedimentary rocks, which were distorted by crustal shortening during the Laramide orogeny. Several broad folds, anticlines, domes, synclines, low angle thrusts and associated tear faults are present in the Phanerozoic rocks. Within the Precambrian terrain at Iron Mountain, a northeast-trending zone of cataclasis was recognized in the northwestern part of the district that is unrelated to Laramide tectonism.

Kimberlites in the Iron Mountain district lie immediately south of a major suture zone known as the Cheyenne Belt, which marks the southern edge of the Wyoming Province (Archon), and the northern edge of cratonized Proterozoic basement (Proton). This structure is exposed in the Richeau Hills 6 miles north (Robert Houston, personal communication, 1997). The trend of the Cheyenne Belt approximately parallels the trend of the Iron Mountain kimberlites, and their emplacement was possibly influenced by this structure.

Kimberlites mapped at Iron Mountain by the WSGS ( Plate 1 ) form a deeply dissected feeder dike complex with periodic blows and sills that intrude Sherman Granite (1.4 Ga) exposed in the core of an eroded, south-plunging, anticlinorium. It is likely that the kimberlite complex continues to the east and west under the Phanerozoic cover. The possibility of additional kimberlites continuing to the north and further west is considered likely, and needs to be tested.

Mississippian and younger sedimentary rocks (dark blue on Plate 1) unconformably overlie the Sherman Granite and kimberlite, and dip off the core to the east, south and west. Immediately west of the district, sedimentary rocks lie in an asymmetrical southerly plunging syncline. Rocks of the Laramie anorthosite-syenite batholith and titaniferous magnetite deposits, crop out west of the syncline.

The oldest in situ sedimentary rocks consist of thin, calcareous, arkosic sandstone, underlying dolomitic limestone. These are thought to be a southern extension of the Mississippian Madison Formation, which sits on the Precambrian surface. The Madison is overlain by the Casper (Mississippian – Pennsylvanian), the Minnekahta and Opeche (Permian), and the Chugwater formations (Triassic).

A large acreage in the western portion of the district is covered by a Tertiary conglomerate (paleoplacer) lying on the Sherman surface. The conglomerate (shown in yellow on Plate 1 ) is unsorted, poorly consolidated, with rounded to angular boulders, pebbles and cobbles of Precambrian igneous and metamorphic rock interspersed with Paleozoic and Mesozoic fragments and occasional kimberlitic indicator minerals. The conglomerate is much younger than the kimberlite. Kimberlites were mapped to the edge of the conglomerate on both the east and west flanks, and presumably continues under the conglomerate.

The extent of hidden kimberlite beneath the conglomerate is unknown. Considerable clay in the conglomerate produced a conductive anomaly similar to kimberlite, possibly masking the electrical responses from hidden kimberlite. Therefore, it is recommended in the future to conduct a detailed grid magnetic survey in this region with the aim of identifying hidden intrusives. Although much more expensive, an airborne INPUT survey over this region could provide considerable information on the extent of kimberlite beneath the conglomerate cover.

Anorthosite (light-blue on the map) occurs in the northwestern portion of the district, as fragments in Tertiary conglomerate, xenoliths in some kimberlites, and in a poorly exposed north-trending string of xenoliths in the Sherman granite in the western portion of the area (Smith, 1977). The anorthosite was intruded by Sherman Granite (shown as orange on Plate 1) (Peterman and others, 1968).

Two varieties of granitic rock were distinguished by grain size. One is fine-grained granite in the northeast, northwest, west-central and southeast portions of the district. A coarse-grained porphyritic granitic rock (quartz monzonite) is found elsewhere. Discontinuous simple pegmatite and aplite dikes are scattered throughout the district.

North- to northwest-trending Precambrian mafic dikes (red on Plate 1 ) intrude Sherman Granite. These are 6 to 10 feet across, nearly vertical, and form sharp contacts with the granite. The dikes are cut by kimberlite at several places.

Kimberlite

The Iron Mountain kimberlites form an extensive, continuous, feeder-dike complex (shown in green on Plate 1 ). The dike complex contains several dike enlargements (blows) and a few possible pipes. For instance, in the SE section 3 (near the three geophysical lines 18, 20, and 21), a 'blow' or root zone of a diatreme was mapped. Further to the northeast, the root zone of a pipe was mapped in the NW section 2. From this point, the kimberlite complex continues to the east where it projects under the younger Paleozoic sedimentary rocks. The kimberlites also continue under sedimentary cover to the extreme southwest.

Kimberlite outcrops over a 12 mi² area. The complex has a general, northeasterly-trend and crops out over a 5-mile strike length with several branches and offshoots, suggestive of strong joint control. The dikes range from a few feet to 400 feet wide and dip vertically to steeply. A relatively narrow sill mapped in the eastern portion of the complex near the center of section 10, has a very shallow dip.

The dikes contain periodic outcrops of serpentinized porphyritic kimberlite that are connected by grus-covered kimberlitic soils (‘blue ground’). In places, the kimberlitic soils are distinct, particularly where the grus cover is thin. These soils contain intermixed rock fragments of kimberlite and granite in blue ground (weathered kimberlite) with variable amounts of indicator minerals (notably ilmenite, less commonly pyrope garnet, and rarely chromian diopside). The kimberlitic soils are often outlined by distinct vegetation anomalies.

Because kimberlite erodes faster than the surrounding granite, many kimberlites are outlined as slight topographic depressions, or flat areas. In uncommon circumstances, the adjacent wall rock and kimberlite will stand up in positive relief due to silicification.

Vegetation anomalies include dense, lush, grasses, devoid of trees. Kimberlitic soils, initially described as ‘blue ground’ in South Africa, consist of gray montmorillonite clay and carbonated soils produced from the weathering and decomposition of the kimberlite.

Grass anomalies are visually prominent at certain times of the growing season, such that the kimberlites can literally be mapped by the vegetation anomalies after a few weeks of rain in late May and early June ( Figure 4 ). The kimberlitic soil types are also favorable for burrowing animals and are an attractive food source for rattlesnakes (see Billman, 1998).

Mapping by the WSGS resulted in numerous discoveries in the district. In addition, a group of depressions were identified in the E/2 of section 8 continuing into the extreme NW of section 9 in the western portion of the district. These depressions contain distinct vegetation anomalies as well as some blue-ground. However, the origin of the depressions remains unsolved, as kimberlitic indicator minerals are rare to absent in the blue ground. XRD analysis of the clays showed a good match for montmorillonite, which also contain considerable carbonate similar to weathered kimberlite. EM surveys over all of the depressions identified good conductors as would be expected from weathered kimberlite. Future studies should focus on these depressions to determine if they are kimberlites, and if they are diamondiferous. If they are kimberlite pipes, they may represent some of the better targets in the district, based on size potential.

Further to the southwest, kimberlite continues under the Tertiary conglomerate in sections 8 and 17. This area undoubtedly conceals additional kimberlites. Material collected within the conglomerate included some picroilmenite megacrysts.

Petrography

Smith (1977) recognized three petrographic phases of kimberlite in the district:

porphyritic to massive kimberlite,
carbonatized kimberlite, and
carbonatized to silicified kimberlite breccia.

The porphyritic to massive hypabyssal kimberlite is highly serpentinized with rounded olivine megacrysts in a green, massive, serpentine groundmass. Very little primary olivine remains, and the megacrysts are pervasively replaced by magnetite and/or hematite. The kimberlite has only a minor xenocryst (foreign crystal) and xenolith (foreign rock) content. (Since kimberlites are highly explosive eruptions, the magmas are hybrids, and tend to collect a variety of rocks and minerals from the surface to depths as great as 120 miles). Uncommon mantle-derived xenocrysts include pyrope garnet, pyroxene, and ilmenite. The porphyritic kimberlite represents a magmatic dike-sill facies, and is the least differentiated kimberlite magma in the district.

Carbonatized kimberlite is a transitional kimberlite. It is dark grey to greyish-green with porphyritic to massive texture similar to the porphyritic kimberlite. It may contain up to 80% carbonate as replacements of olivine phenocrysts or xenocrysts in a serpentine groundmass, and locally may be diatreme facies. Carbonatized phenocryst ghosts are commonly visible in a massive carbonate rock. Groundmass minerals include hematite, magnetite, minor Cr-spinel and a variety of Fe-Ti oxides or hydroxides. Serpentine is usually present in small amounts dispersed through the matrix and intimately mixed with carbonate. Hematite mantles carbonated phenocrysts in some samples. Small amounts of secondary phlogopite are present.

The heterogeneous nature of this kimberlite is accented by a varied abundance of xenoliths. In general, a greater amount of mantle xenoliths and xenocrysts are found in this type of kimberlite compared to the other varieties at Iron Mountain. In the southwestern portion of the district, this kimberlite contains angular, elongate, granitic fragments that locally exhibit flow alignment.

The third variety of kimberlite is a carbonatized to silicified kimberlite breccia exposed in blows and in some feeder dikes. This type is gradational to the transitional variety. The breccias are serpentine deficient relative to normal intrusive kimberlite breccia. The rock is grey to black and may contain as much as 70% granitic fragments. The breccia is silicified to varying degrees, and exposures commonly express positive relief. The granitic wall rock adjacent to exposures is usually weakly silicified. This rock type is atypical of true kimberlite based on chemistry and mineralogy (Table 1).

Chemistry

Major oxide, minor and trace element concentrations of the Iron Mountain kimberlites are compatible with worldwide kimberlite averages. Many of the Iron Mountain kimberlites are similar to the State Line diamondiferous kimberlites.

Rare earth contents of six samples show chondrite normalized distribution patterns to be highly fractionated toward light rare earths. La/Yb ratios range from 75 to 173, and kimberlites from Iron Mountain (as well as State Line) display negative Eu anomalies. The REE patterns are similar to South African and Siberian diamondiferous kimberlites.

The kimberlites bare little resemblance to the initial mantle melt. This is due to extreme fractionation and/or differentiation, and incorporation of abundant xenolithic material.

Electron microprobe analyses of kimberlitic indicator minerals from several of the Iron Mountain kimberlites support that they originated from the diamond stability field. High-chrome magnesian pyrope garnets show both calcic and sub-calcic chemistry (Figure 5).

Diamondiferous kimberlites originate from great depth in the earth. Diamonds from kimberlite are actually derived from xenoliths brought up in the kimberlite. The principal diamondiferous xenoliths are pyrope harzburgite, eclogite and chromite harzburgite. The chemistry of the minerals that compose these xenoliths, are unique and similar to diamond inclusion minerals.

The data collected by the WSGS (Figures 5 & 6) show a significant sampling of the diamond stability field by the Iron Mountain kimberlites, as several garnets show similar chemistry to garnets derived from diamondiferous pyrope harzburgite. These garnets are sub-calcic, high chrome garnets seen on the left side of the inclined line in Figure 5. Plate 1 shows locations of some samples that yielded G10 (diamond-stability) garnets.

The geochemistry of garnets of eclogite paragenesis also show some derivation from the diamond stability field (Figure 6). Some of the titaniferous – sodic garnets are similar to diamond inclusion minerals. The microprobe analyses of kimberlitic indicator minerals supports that the Iron Mountain kimberlites should contain a low-grade resource of diamonds.

Geophysics

Several vegetation anomalies and depressions were identified during this project that suggest hidden kimberlite. Notably, grassy vegetation anomalies with sporadic carbonate-rich soil suggested weathered, soil-covered kimberlites at several localities. Detailed grid geophysical surveys, sampling, and trenching are recommended for these anomalies in order to determine if they have any economic significance.

During geological mapping, a group of northeasterly trending depressions, in sections 8 and 9 in the western portion of the district, were identified as possible hidden kimberlite pipes. Beginning with the northeastern-most depression in section 9, geophysical lines across the depression in an E-W and northeasterly direction identified weak conductors. Line A (E-W) yielded three weak conductors with maximum values of 12 to 14 mmhos/meter over widths of 45, 75 and 130 feet. The intersecting northeasterly line (line B) detected a 23 mmhos/meter conductor over a 220 foot width.

To the southwest, the next depression produced a conductor detected along both N-S and E-W survey lines. The conductor was a maximum of 19 mmhos/meter and occupied a large portion of the depression.

The next depression to the southwest yielded a strong conductor along the E-W line (line E). The conductivity was especially intense over 70 feet. The conductivity measured over much of this line averaged 18 mmhos/meter with the prominent EM spike of about 43 mmhos/meter, over a width of 70 feet. The data indicates an anomalous depression filled with clay. Whether or not the clay is derived from kimberlite is not known.

A very high conductor was detected in the next depression to the southwest. This depression also had a distinct vegetation anomaly. The survey over the feature showed a 315- to 360-foot wide conductor with a 75 mmhos/meter maximum.

The next depression to the southwest showed a similar, but elliptical-shaped conductor. A conductivity high of about 105 mmhos/meter was detected in the depression. This anomaly was persistent along much of the E-W line (I) over a width of 800 feet. The N-S line (8f) showed a conductor over a width of only 230 feet.

In the next depression to the southwest, EM profile 8d yielded a conductivity high of 100 mmhos/meter, indicating a clay-rich zone under the depression. The perpendicular line across the southern edge of the depression continuing to the west into the Tertiary paleoplacer, also yielded a distinct conductor along much of the line. The first few feet of the line began in the granitic terrain, which showed weak conductivity over the granite. However, conductivity highs, ranging up to 140 mmhos/meter were detected throughout the depression into the paleoplacer.

Line 8 was selected to test a small circular depression in section 8. A distinct conductivity anomaly with a maximum of 90 mmhos/meter was detected in this depression.

Two other lines were run in section 8. Line13-16 was run northeasterly along the northern edge of two depressions. The survey showed a distinct EM high ranging from 10 mmhos/meter to 91 mmhos/meter over a width of 750 feet. A second line (12) was run through the depressions, along the trend of a suspected dike. The EM yielded a conductivity high along the entire length of the line of about 2,000 feet.

EM lines were also set up to search for a continuation of the kimberlite trend under a Tertiary paleoplacer in section 8. In all probability, this paleoplacer covers one or more kimberlite. The paleoplacer hosts considerable clay as well as carbonate derived from the adjacent Paleozoic limestones, and tends to mimic kimberlitic blue ground, making it very difficult to prospect by electrical surveys.

EM line 5 (N-S) was ran in the paleoplacer near the projected trend of the belt of kimberlites. The results of the geophysical survey showed conductive material along its entire length (averaged about 55 mmhos/meter). However, a strong EM conductor was identified along the northern edge of the line, which peaked at 123 mmhos/meter. It is recommended that this anomaly be further evaluated.

To the east, blue-ground was exposed along the edge of the paleoplacer. EM line 9 (N-S) was set to test this area. A distinct conductivity anomaly was measured over a trend of 1,200 feet throughout the paleoplacer. The anomaly ranged from 22 to 100 mmhos/meter. The source of the high conductivity was due to clay in the paleoplacer, and possibly kimberlite. At any rate, we were unable to differentiate kimberlite from the paleoplacer.

Near the eastern edge of the district, kimberlite was traced to the easternmost extent of the Sherman granite. To the west, the kimberlite dike was traced under an intermittent stream valley near the western edge of Section 2. A geophysical line was run across the eastern edge of this valley. Line 17 in the valley adjacent to the Precambrian-Paleozoic contact yielded a EM anomaly ranging from 4 to 25.8 mmhos/meter over 450 feet. Although the anomaly is considered relatively low, the symmetry of the anomaly suggests that it deserves further exploration.

Line 2d, a N-S line east of the northeasterly-trending kimberlite dike detected a 60-ft conductor, suggestive of a buried dike (see Plate 1 ). A parallel profile (line 2c) detected a 90-ft conductor. This conductor is interpreted as a reflection of a dike beneath the heavily vegetated depression. Another line (2b) across the projected extent of the kimberlite yielded a 45-ft conductor over the projected kimberlite trend.

To the west, in the SE section 3, lines were run across the kimberlite dike. At this point, the dike is exposed as periodic outcrops of massive, serpentinized, hypabyssal facies kimberlite, with some blue ground containing calcium carbonate and periodic grains of picroilmenite, pyrope garnet, and rare chromian diopside in the soils. The dike also has a well-developed grassy vegetation anomaly. Adjacent to the kimberlite, outcrops of granite are evident. Since there are scattered outcrops of kimberlite and granite, one would not expect the soil profile to be very thick (i.e., a weak EM conductor).

Three lines (18, 20, and 21) were run across the dike (Figure 8) (see Plate 1 ). These produced distinct weak conductivity anomalies. The anomalies ranged from only 2 to 7.5 mmhos/meter, compared to the granite, which averaged about 1 mmho/meter. Based on geological mapping and the geophysical profiles, the dike is a maximum of 180 feet wide at this point.

About 0.75 mile west in the N/2 section 10, line 22 was set up along a dirt road crossing two kimberlite dikes (see Plate 1 ). The clay and soil profile over these two intrusives is assumed to be poorly developed and relatively thin. The first kimberlite yielded a 240-ft wide anomaly ranging from only 4 to 8 mmhos/meter. The second dike yielded a 4 to 19 mmhos/meter anomaly over a 330-ft width. Again, the low conductivity is interpreted to be due to a thin and poorly developed blue ground profile.

Further west, in NE section 9, the kimberlite dike complex separates into a maze of dikes. Some of these were identified based on the presence of distinct grassy vegetation anomalies along with carbonated soil. Line 9-3 cut across five of these dikes (see Plate 1 ). The line showed five prominent conductivity peaks associated with the mapped dikes.

An alluvial-filled depression with minor blue ground was found near the eastern edge of section 9. The depression continued into the western edge of section 10. Both EM and magnetics were run along a northwesterly trend over the depression (geophysical line 9-2, Plate 1 ). The magnetics was inconclusive. The EM showed a conductivity high ranging from 7.5 to 23.1 mmhos/meter over 300 feet. A line perpendicular to the first line showed background of about 1 mmho/meter over the line with a conductivity peak of about 4.5 mmhos/meter over a length of about 100 to 120 feet. The data suggests the presence of a hidden kimberlite blow in the depression.

In section 16, a depression yielded a 45-ft conductor with a high of 35 mmho/meter along geophysical line 16 ( Plate 1 ). The conductor may be a reflection of a hidden kimberlite.

Exploration of Other Anomalous Areas

Research was conducted in the Iron Mountain district, as well as several nearby regions to search for indications of diamonds. The results, combined with earlier data collected by the WSGS, show that Wyoming is highly anomalous and may be host to several undiscovered diamond deposits. It should be noted that not all of the samples collected by the WSGS have yet been processed, thus there is a real possibility that the areas of interest may be expanded. The samples collected included small bulk samples for diamond testing, smaller samples for geochemical testing, and stream sediment samples for kimberlitic indicator mineral extraction. These are being processed as time permits. As time permits, we are also examining hundreds of air photos in the search for airphoto anomalies suggestive of kimberlite.

Stream Sediment Sample Results

Worldwide, stream sediment sampling in the search for tracer minerals, commonly referred to as ‘kimberlitic indicator minerals’, has been one of the most successful means of exploration for diamond pipes. Tracer minerals eroded from kimberlite intrusives, or related rocks, are dispersed into the soils and adjacent stream drainages. Transportation distances, before complete disaggregation due to stream abrasion will vary from mineral to mineral. A general rule of thumb, is that kimberlitic indicator minerals found in stream sediment samples suggest that the source intrusive may lie 2 to 2.5 miles upstream of the anomaly. When abundant (>10) indicator minerals are found in a sample, the source kimberlite is assumed to lie within a mile of the sample.

Approximately 1,600 stream sediment samples were collected in southeastern Wyoming by the WSGS in the mid-1980ies, for the purpose of searching for kimberlite (Hausel and others, 1988). Of these, an incredible number (approximately 300) contained kimberlitic indicator minerals. Nearly 20% were anomalous which provides evidence that the Wyoming craton has been intruded by a significant kimberlite swarm. Other samples were collected during the past 2 years, the bulk of which are still being analyzed.

During the earlier (1980ies) project solid geochemical tests were not available, and research was limited to mineralogical studies. Geochemical testing of kimberlitic indicator minerals is a relatively new procedure that was developed in the late 1980ies to search for diamondiferous kimberlite in Canada, and is now used to identify potential diamond targets. Only a handful of Wyoming minerals have now been geochemically tested, but the results have been extraordinary, as several have yielded diamond-stability signatures!

Of the few samples that were recently processed for geochemistry, diamond-stability pyrope garnets were identified in samples DR9, DR29, DR30 from the Dodge Ranch in the Elmers Rock greenstone belt, SR1, SR10, and SR14 in the Middle Sybille Creek district, H890110-3 east of Baggs, NSW-2 from Mule Creek north of Iron Mountain, and IM80b, IM32, and IM35 from the Iron Mountain district.

Samples collected along the banks of the North Platte River north of the Seminoe Mountains in the Kortes paleoplacer several years ago yielded both gold and pyrope. Duplicate samples were collected during this present study from both west and east of the river, and microprobe analyses showed G10 (diamond-stability) signatures. Mapping in this region by Hausel (1994) in the past, suggested that the source region of the paleoplacer was in the vicinity of Bradley Peak on the western edge of the Seminoe Mountains. Presently, no kimberlites are known in this region, nor has this area been explored for kimberlite or diamond!

In the Iron Mountain district, two samples collected in the northwestern portion of the district were not associated with any known kimberlite. However, by the end of the 1999 field season, blue ground possibly related to kimberlite was discovered in this drainage basin, and may have contributed to the anomalies. An expansion of stream sediment sampling in the district will most likely lead to other discoveries.

Rock River 1:100,000 scale map

Several dozen samples were collected in the Laramie Range on the Rock River quadrangle (Plate 2). The Laramie Range in this region is formed of outcrops of Precambrian Sherman Granite intruded by anorthosite-syenite. Much of this terrain lies north of Eagle Rock within and south of Sybille Canyon. The Cheyenne Belt approximately parallels Highway 34 (Sybille Canyon) and is exposed in the Richeau Hills east of Sybille Canyon and north of Iron Mountain. This suture (1.9 to 1.7 Ga) is masked by the intrusion of the anorthosite batholith.

Iron Mountain anomaly

The Iron Mountain district is now recognized as one of the two largest kimberlite districts in the US due to the research by the WSGS. Samples collected along the western edge of the district during an earlier study by the WSGS provided direct evidence of kimberlite (Hausel and others, 1988). A group of steam sediment samples collected in Spring Creek in Hay Canyon were highly anomalous (see Plate 2). One sample collected in a tributary of Spring Creek draining from the district, on the west half of section 18, T19N, R70W, yielded more than 100 pyrope garnets, 10 chromian diopsides, and some picroilmenite. This sample was collected within 0.5 mile of the mapped kimberlites.

A short distance downstream, a second sample showed a dramatic decline in the number of kimberlitic indicator minerals. This sample yielded only 18 pyrope garnets, no chromian diopside, and some picroilmenite. The dramatic decrease in indicator minerals may have been due to a poor sample site. In all probability, the sample was disseminated due to the influx of sediment from Spring Creek.

The next sample collected in Spring Creek was located about 0.5 mile downstream from the previous sample. Concentrates from this sample yielded 23 pyrope garnets. Another sample located 0.8 mile from this site, yielded only 5 pyrope garnets. This was followed by another sample 0.5 mile downstream, which contained no indicator minerals.

These samples provide a general guide useful in the search of kimberlite in similar terrains in Wyoming. For example, they indicate:

That chromian diopside is greatly restricted in stream transport (maximum transportation distances of only 0.6 to 0.8 mile from a source kimberlite are assumed). Where chromian diopside is found, it suggests a proximal source.
Pyrope garnet may be transported up to distances of 2.25 miles from the source kimberlite.
When recovered in stream sediment samples, these indicator minerals suggest that a kimberlite source lies within 2.25 miles upstream.
Abundant indicator minerals (>10) suggests a proximal source.

Examination of Plate 2, shows several kimberlitic indicator mineral anomalies on the Rock River quadrangle that are suggestive of a widespread kimberlite swarm – possibly one of the largest in the world. Several areas stand out as being highly anomalous. These are Grant Creek, Middle Sybille Creek, and Dodge Ranch within the Elmers Rock Greenstone belt. Other notable anomalies include Plumbago Canyon, Rawhide Park, Mule Creek and a tributary of South Sybille Creek.

Grant Creek anomaly

Grant Creek lies north of the Iron Mountain district near the projected trend of the Cheyenne Belt. This is promising anomaly as it appears to be very localized. In the are of the sample sites, Grant Creek is relatively wide and flat with some unusual carbonates along the edge of the creek. A portion of the anomaly lies on State land.

Eleven samples were collected over distance of about one mile. Five of the samples yielded less than 6 to more than 100 kimberlitic indicator minerals with minor gold. Chromian diopside was recovered from four samples, and three of the samples contained 50 or more kimberlitic indicator minerals producing a similar distribution to the Iron Mountain anomaly described above. Samples collected a few hundred feet upstream from the anomaly were devoid of kimberlitic indicator minerals suggesting the presence of nearby, buried, kimberlite.

Two reconnaissance geophysical lines were run over the area in 1999 in an attempt to find the buried kimberlite. The Grant #2 EM line showed a 30-foot wide conductor. Other buried conductors were also detected along both lines, but none were conclusive.

It is recommended:

That an additional sample be collected and the kimberlitic indicator minerals be analyzed for geochemistry to determine the probability that the anomaly was derived from a diamondiferous kimberlite. If these results are favorable,
Conduct a detailed grid magnetic and electromagnetic geophysical survey over the entire area in order to isolate the source of the indicator minerals, and to determine the configuration of any conductors.
Once the anomaly is outlined, information about the discovery be published in mining journals in an attempt to attract interested diamond companies to Wyoming.

Middle Sybille Creek anomaly

Middle Sybille Creek, which lies along the Cheyenne belt, was initially investigated by the WSGS in 1980. A previously unreported kimberlite was first described by the WSGS along the northern bank of Middle Sybille Creek, and named in honor of Mr. and Mrs. Jack Radichal, the owners of the property at the time of the investigation (Hausel and others, 1981). A few years later, field crews conducted a district-wide stream sediment sample survey to search for additional kimberlites, which was funded by a University of Wyoming Mining and Mineral Resource Research Institute (Hausel and others, 1988).

The distribution of kimberlitic indicator mineral anomalies in this drainage supports the presence of additional, undiscovered, kimberlites. Kimberlitic indicator minerals were recovered from 42 separate sample sites over a distance of 10 miles along the entire length of the creek (Plate 2). Seven of the samples contained chromian diopside along with pyrope and picroilmenite, suggesting a nearby source(s). A few garnets were tested in 2000 for geochemistry and showed favorable G10 chemistry. The presence of G10 garnets makes this area of interest as the geochemistry supports some kimberlites could be diamond-bearing.

The Radichal kimberlite lies near the center of the indicator mineral anomaly. Many of the indicator minerals found downstream from the kimberlite, may have been derived from the intrusive. However, at least two sample sites probably had different sources, as these were taken in closed basins draining into Middle Sybille Creek. Additionally, more than a dozen anomalous sample sites lie upstream from the kimberlite. The data supports the presence of several undiscovered kimberlites in this drainage basin.

During the 2000 field season, geophysical profile lines were run in Middle Sybille Creek near the Radichal kimberlite. This drainage contains a hay meadow that could potentially hide several kimberlites. Lines were run in the meadow, and other lines were scheduled for several closed drainages, but funding for this project was terminated before the work was completed.

Geophysical Line 1 set up west of the Radichal kimberlite, ran across the intrusive, and continued east across the hay meadow in Middle Sybille Creek. No distinct EM response was detected over the kimberlite outcrop (kimberlite at the Radichal forms a massive outcrop with no weathered ‘blue ground’ soil profile, thus it was not expected to be conductive), but conductors were identified to the east in the hay meadow. The most distinct anomaly was east of Middle Sybille Creek, and peaked at 25.2 mmhos/meter. The data suggests the presence of a hidden, clay-rich zone under this line.

Lines 2 and 7 were run on an west-east line approximately parallel to line 1. Line 2 ended at Middle Sybille Creek, and line 7 continued on the east bank of Middle Sybille Creek. A 26.7 mmhos/meter maximum EM anomaly was detected over a width of 450 feet along line 7. Much of the line was located in wet ground and a portion of the line extended over dry ground. Line 2 produced an erratic anomaly with a maximum conductivity of about 20 mmhos/meter.

Line 3, south of line 2, ran east-west across the hay meadow. At a topographic low, a distinct conductor (maximum 16.2 mmhos/meter) was detected over a 90-foot width. Line 4, a west-east line south of Line 3, was designed to test grassy vegetation anomalies. The northern vegetation anomaly was associated with a spike in the EM (maximum of a 16.9 mmhos/meter over a 90-foot width). A second anomaly detected to the east, yielded an 18.9 mmhos/meter high over 360 feet.

Line 5-6 was run north-south crossing lines 1, 2, 3 and 4. Overall, the survey did not show any distinct anomalies. Line 8, a north-south line which crossed the Radichal kimberlite blow, continued to the south. The line produced two weak (8 mmhos/meter maximum) anomalies south of the kimberlite. One was located in a topographic low; the second was covered by sagebrush. Both anomalies were quite distinct compared to background. The low background was the result of exposed rock outcrop with thin soil cover. Line 9, a north-south line east of line 5, yielded a distinct EM anomaly with a maximum of 25 mmhos/meter over an 840-foot width.

The geophysical lines detected several buried conductors, some of which could be hidden kimberlite. With the presence of several kimberlitic indicator mineral anomalies (including G10 garnets) and one verified kimberlite in the district, the area should be given high priority for future research.

It is recommended that grid geophysical surveys be scheduled over some of the more distinct EM anomalies. Additional stream sediment sampling could assist in isolating the anomalies. Additional geophysical profiles should be conducted in some closed basins where kimberlitic indicator mineral anomalies have been identified. Some of the better conductivity anomalies should be augured to determine the source of the anomalies.

Dodge Ranch

Dodge Ranch lies within the Elmers Rock greenstone belt, an Archean age metasedimentary-metavolcanic synform sitting on an Archean basement complex of the Wyoming craton north of Sybille Canyon. This greenstone belt hosts one of the most impressive, widespread, kimberlitic indicator mineral anomalies in the US. The widespread anomaly suggests the presence of many, hidden, kimberlites. In addition, the presence of G10 diamond-stability garnets in this region, support that some intrusives originated from the diamond-stability field. Many of the anomalies are impressive, but only a few of the more prominent will be discussed.

A group of anomalous samples surround Bull Camp Peak in the western part of Dodge Ranch (Plate 2). Streams draining in nearly every direction from this area contain indicator minerals as well as some sapphires and visible gold. A few of the samples also contained grains of chromian diopside, and some contained more than 50 indicator minerals. One sample yielded 128 pyrope garnets. Several of the samples suggest a proximal source for the indicator minerals

It is recommended that additional stream sediment sample surveys be taken in the headwaters of all of the surrounding drainages of Bull Camp Peak, in an effort to tie down the source of the indicator minerals. This should be followed by geochemical analysis of the minerals to identify the better diamond targets, followed by detailed geological mapping and geophysical grids, where necessary.

Another prominent anomaly in the greenstone belt was identified near Wheatland tunnel on the Laramie River in section 36, T23N, R72W. A large group of samples in the area contained indicator minerals; however, samples within State section 36 show a dramatic increase in the number of indicator minerals. One sample yielded 73 indicator minerals (including some chromian diopside) indicating a proximal source. Three samples in this section also contained sapphire. It is recommended additional samples be taken, and a detailed geological map be completed to isolate the source of the indicator minerals and sapphires.

South Cooney Hills anomaly

Near the eastern edge of the Laramie Mountains, four samples were collected along the edge of South Cooney Hills. Two samples collected in the headwaters of restricted drainages, yielded pyropes with G10 chemistry. To the south in Mule Creek, another sample collected adjacent to Highway 34 yielded some pyrope garnets including some with G10 chemistry (Plate 2). Detailed sampling and mapping throughout the region is recommended to isolate the source of the anomalies.

Indian Guide District

The Indian Guide area is an enigma. Stream sediment samples in this area, show few anomalies (Plate 2). However, a group of geomorphic anomalies were discovered by the WSGS during airphoto reconnaissance of this area, west of the Iron Mountain kimberlite district. These structurally-controlled, circular to elliptical depressions occur in the Laramie anorthosite complex ( Figure 10 ). The depressions lie west of Hay Canyon and four miles west of the Iron Mountain district in sections 8, 9, 16, and 17, T19N, R71W. They also lie on-trend with the Iron Mountain kimberlites.

Most of the depressions support shallow intermittent lakes, and lie on distinct north-trending fractures. Northwesterly-trending fractures also are evident, and the majority of the depressions lie at the intersections of these with the north-trending fractures.

The circular depressions mimic several known kimberlite localities in the world. In particular, the recent discoveries of diamondiferous kimberlite in the Lac de Gras region of the Northwest Territories of Canada, are on similar water-filled depressions; however, the lack of nearby kimberlitic indicator mineral anomalies are of concern. Ground EM and magnetic surveys were initiated to determine if any of the depressions exhibited kimberlitic signatures.

Two lines were run over the southeastern-most depression. Line 1, a NW-SE line over the center of the depression, detected a 600-ft wide conductor. The conductor measured from a low of 9.6 to a high of 22.8 mmhos/meter. Line 2, a N-S line which crossed line 1 in the center of the depression, produced a 7.8 to 25.8 mmhos/meter conductor over 510 feet.

A short distance to the northwest, another relatively large depression was examined. An west-east line (Line 3) was run perpendicular to Line 4. A conductor ranging from 7.9 to 21.6 mmhos/meter over 420 ft was detected along line 3, and a similar conductor (9.6 to 21.9 mmhos/meter over a length of 480 ft) was detected along Line 4.

A short distance west, in the NE section 17, another depression yielded a similar anomaly. Line 5 (W-E) was run over the depression. The data showed a weak conductor (8.1 to 18.9 mmhos/meter) over 480 ft. Line 6 (NW-SE) crossed line 5 near the western edge of the depression. This line produced a conductor (7.6 to 23.4 mmhos/meter) over 540 ft.

Two lines were run across the next depression to the northwest. Line 7 (N-S) yielded a weak (7 to 14.4 mmhos/meter) conductor over 570 feet. Line 8, a W-E line along the southern edge of the water-filled depression, also yielded a weak conductor (8.1 to 16.8 mmhos/meter) over 390 feet. It should be noted that the water was frozen when the line was completed on March 7^th, 2000.

Line 9 and 10 detected a conductor over the next depression to the west. Line 9 was run N-S over the depression, and line 10 was run nearly E-W. A conductor (ranging from 3.2 to 18 mmhos/meter) was detected along the line over a distance of 700 feet. Line 10 (E-W) showed a weak conductor (9.6 to 18.3 mmhos/meter) over 300 feet.

Lines 11 and 12 were routed over the next depression to the west in section 8. A conductor was identified over 240 feet (>15 mmhos/meter maximum) along the E-W line (Line 11). The perpendicular line (S-N) showed a weak conductor (6 to 9.6 mmhos/meter) near the low point of the depression.

Line 13 (E-W) was located over another depression west of Line 11. No detectable conductor was identified along the line. Line 14 (E-W), west of line 13, ran over two depressions adjacent to one another. This line also yielded no significant conductors.

Another 0.5-mile further west, a large, 1,200 ft, circular, water-filled depression in the SE section 7 was investigated. Along the southern shore, a conductor (7.2 to 16.2 mmhos/meter) was identified before the signal was lost due to standing water. The northern shore produced a conductor ranging from 13.2 to 23.1 mmhos/meter. Line 15a was run S-N along the eastern shore of the pond to avoid standing water. This profile showed a conductor ranging from about 10 to 23.1 mmhos/meter over 1,100 feet. Line 16 and 16a were run E-W perpendicular to line 15 and 15a over the same depression. The data showed a similar conductor with an intensity of 7.8 to 21.6 mmhos/meter.

Still another depression was identified to the north. Line 17 (E-W) yielded a conductor (9.9 to 24.6 mmhos/meter) over about 600 feet. Line 18, a N-S line perpendicular to Line 17, yielded a 7.2 to 25.2 mmhos/meter conductor over about 930 feet. style='font-size:12.0pt;mso-bidi-font-size:10.0pt'>

We recommend drilling one or more of the depressions to determine the genesis of these cryptovolcanic structures. The possibly that these are impact features is highly unlikely, due to the fact that they are structurally controlled.

Laramie 1:100,000 scale map

Several dozen samples were collected on the Laramie quadrangle (Plate 3). The Laramie Range in this area includes vast outcrops of Sherman Granite. Along the northern edge, the granite intrudes anorthosite-syenite. Much of this terrain lies within the Proton of the Wyoming craton. The quadrangle encloses the State Line diamond district, which includes at least 20 known diamond-bearing kimberlites in Wyoming.

Eagle Rock anomalies

To the north of the State Line district, a highly anomalous area, known as Eagle Rock, shows an impressive dispersion of kimberlitic indicator mineral anomalies (Plate 3). A large concentration of anomalous samples were collected on North, Middle, and the South branches of Lodgepole Creek near Eagle rock to the north of Interstate 80. The anomalies were identified over an area of 50 to 60 mi². The widespread anomalies suggest the presence of many undiscovered kimberlites.

A distinct anomaly on North Lodgepole Creek east of Pilot Hill shows several samples, which yielded pyrope garnet, and a few scattered chromian diopsides and picroilmenite. These samples were collected in areas dammed by beavers, and are poor sample sites. Even so, several indicator mineral anomalies were identified. The area is heavily timbered, and difficult to prospect. It is recommended that additional samples be taken for geochemical tests to determine the probability of diamonds in this region.

This area also hosts a regional fluorite anomaly outlined by samples collected between North Lodgepole Creek and Interstate 80 over a surface area of about 150 mi². Most samples collected in the area, contained fluorite from unknown sources.

Boulder Ridge anomalies

Several samples collected in the Boulder Ridge area west of Tie Siding yielded kimberlitic indicator mineral anomalies (Plate 3). Most of the anomalies were produced by only one or two picroilmenite grains, with a few containing pyrope. This area is anomalous, but considered lower priority due to the limited number of indicator minerals.

Laramie Peak 1:100,000 scale map.

Several dozen stream sediment samples were panned and collected north of the Dodge Ranch on the Laramie Peak 1:100,000 quadrangle during this project. These were panned and examined microscopically by Woody Motten, contract geologist, and a few were reported to contain pyrope garnets. However, we have not yet completed examination and tests on the sample concentrates to verify the pyropes or their geochemistry.

Of additional interest, we were recently contacted by Lee Peterson from Douglas, Wyoming, who claims he leased a property to Cominco American in the early 1980s for diamonds. One or more diamondiferous kimberlites were apparently discovered according to Peterson, instyle='color:black'>section 31, T28N, R70W, east of Laramie Peak.

Saratoga 1:100,000 scale map.

Only a hand full of samples were collected in the Medicine Bow Mountains on the Saratoga quadrangle. These were collected in the vicinity of Centennial Ridge. Samples collected along the Middle Fork of the Little Laramie River and also on Libby Creek yielded pyrope garnets (Plate 4).

Two near-gem octahedral diamonds were recovered on Cortez Creek (section 35, T17N, R81W) on the west flank of the Medicine Bow Mountains by a prospector named Paul Boden in 1977. This area remains to be investigated.

Economic Geology

The potential of finding a commercial diamond deposit in Wyoming is considered high. Currently, 40 diamondiferous kimberlites have been identified in the Colorado-Wyoming State Line district within the cratonized Proterozoic basement. Grades of bulk samples from the district have ranged from 0.005 to 1.35 carats/tonne. About 50% of the stones are of gem quality. The diamonds range from microdiamonds to a 28.3 carat stone (larger stones are probable as the Kelsey Lake mill was incorrectly designed to recover diamonds <40 carats in weight). Ore grades for commercial diamond mines around the world range from 0.01 to 6.0 carats/tonne (Hausel, 1995a, b).

The Iron Mountain district is one of the more under-tested kimberlite districts in North America. During the early 1980s, Cominco American Incorporated collected samples for testing. Only two of their reclaimed trenches were found during this study. The trenches were small indicating that a very small test was made. Much of the kimberlite complex remains untested. The specifics on Cominco’s tests are unknown; however, one macrodiamond was reportedly recovered (Howard Coopersmith, pers. comm., 1999).

Recent sampling in the district by Kennecott Exploration Company resulted in the recovery of 6 diamonds, but three of the diamonds were synthetic. The diamonds were the result of contamination from a crusher normally used to crush core samples, which sometimes contain synthetic diamonds from drill bits. However, no explanation was provided for the 3 natural diamonds (Gordon Marlatt and Paul Graff, pers. comm., 1998).

Geochemical investigations by the WSGS indicate that several Iron Mountain kimberlites originated within the diamond-stability field. The geochemical data show several G10 (diamond-stability) garnets. The geochemical data are comparable to data from the Kelsey Lake diamond mine along the Colorado-Wyoming border.

Other samples were recently collected by backhoe by a Colorado Company searching for diamonds. The bulk of these samples were taken in the depressions discovered by the WSGS in section 8 of the Iron Mountain district. The samples consist almost entirely of montmorillonite clay, and indicator minerals are extremely rare. This is another enigma – as to why the blue ground is so poor in kimberlitic indicator minerals. A portion of the sample was provided to the WSGS in 2000, but diamond testing was not completed before funding terminated.

Diamond mining and extraction is one of the more inert types of mining. Mining and milling complexes require no toxic chemicals, as diamonds are recovered by gravity and x-ray extraction methods. Diamond mines disturb relatively small areas compared to most open pit operations. This is due to the inherit small size of the diamond pipes, which range from only a few feet to generally less that 2000 feet across.

Conclusions

Data collected by the WSGS indicates that the Wyoming craton encloses a major kimberlite-lamproite-lamprophyre province (Hausel, 1998). The presence of a large number of diamonds and diamond indicator minerals supports this craton to be a significant source of diamonds.

This study provides evidence of a major kimberlite district at Iron Mountain. The limits of the district have not yet been determined, as it is open to the north and west, and kimberlites probably continue under the sedimentary cover to the east of the district. Geophysical and topographic anomalies to the west in Indian Guide suggests a possible extension of the district.

Kimberlites also continue to the north for an unknown distance. Kimberlitic indicator mineral anomalies (including G10 pyrope garnets) surrounding the Radichal kimberlite 6 miles to the northwest, as well as indicator mineral anomalies in the Grant Creek and Mule Creek areas, suggest a sizable district may occur in this region.

Based on geochemistry, there is a high probability that additional diamonds will be found in the Iron Mountain district. However, the data collected to date are minimal, and ore grades can not be predicted until considerably more data is collected. The data produced during this study suggest some of the kimberlites may contain a low-grade diamond resource, but each kimberlite must be evaluated separately, as each diamondiferous kimberlite will vary in ore grade. It is recommended that bulk samples be collected from the kimberlites that have yielded G10 pyropes to search for diamond. Additional detailed geological mapping, stream sediment sampling, and the initiation of airborne INPUT and remote sensing surveys would provide much needed information on the extent of the district and province.

The results of a detailed survey could ultimately reap tremendous benefits to the economy of Wyoming, as some diamonds are the most valuable gemstones on earth, and some have been valued at 500,000 times an equivalent weight in gold! This project was financed by a special appropriation from the State Legislature.

References Cited

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Coopersmith, H.G., and Schultze, D.J., 1996, Development and geology of the Kelsey Lake diamond mine, Colorado in T.B. Thompson, ed., Diamonds to gold - I. State Line Kimberlite district, Colorado, II. Cresson mine, Cripple Creek district, Colorado: Society of Economic Geologists guidebook series, p. 5-19.

Eggler, D.H., Meen, J.K., Welt, F., Dudas, F.O., Furlong, K.P., McCallum, M.E., and Carlson, R.W., 1988, Tectonomagmatism of the Wyoming Province: Colorado School of Mines Quarterly, p. 25-40.

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Hausel, W.D., 1995a, Diamond, kimberlite, lamproite, and related rocks in the United States: Exploration and Mining Geology, v. 4, no. 3, p. 243-270.

Hausel, W.D., 1995b, Diamonds and their host rocks in the United States: Mining Engineering, v. 47, no. 8, p. 723-732.

Hausel, W.D., 1998, Diamonds and mantle source rocks in the Wyoming Craton, with a discussion of other US occurrences: Wyoming State Geological Survey Report of Investigations 53, 93 p.

Hausel, W.D., McCallum, M.E., and Woodzick, T.L., 1979, Exploration for diamond-bearing kimberlite in Colorado and Wyoming: an evaluation of exploration techniques: Geological Survey of Wyoming Report of Investigations 19, 29 p.

Hausel, W.D., Glahn, P.R., and Woodzick, T.L., 1981, Geological and geophysical investigations of kimberlites in the Laramie Range of southeastern Wyoming: Geological Survey of Wyoming Preliminary Report 18, 13 p., 2 plates (scale 1:24,000).

Hausel, W.D., Sutherland, W.M., and Gregory, E.B., 1988, Stream-sediment sample results in search of kimberlite intrusives in southeastern Wyoming: Geological Survey of Wyoming Open-File Report 88-11, 11 p. (5 plates) (revised 1993).

Hausel, W.D., Edwards, B.R., and Graff, P.J., 1991, Geology and mineralization of the Wyoming Province: SME Preprint 91-72, 12 p.

Hausel, W.D., Gregory, R.W., Motten, R.H., and Sutherland, W.M., 2000, Economic Geology of the Iron Mountain kimberlite district, Wyoming: Wyoming Geological Association Field Conference Guidebook, p. 151-164.

Hausel, W.D., Kucera, R.E., McCandless, T.E., and Gregory, R.W., 2000, Mantle-derived breccia pipes in the southern Green River Basin of Wyoming (USA): Proceedings of the 7th International Kimberlite Conference, University of Cape Town, South Africa, p. 348-352.

Janse, A.J.A., 1994, Is Clifford's rule still valid? Affirmative examples from around the world, in H.O.A. Meyer and O.H. Leonardos, eds., Diamonds: characterization, genesis, and exploration, v.2, Proceedings of the Fifth international kimberlite conference: Araxa, Brazil, 1991, Companhia de Pesquisa d