PLACER DIAMONDS

Wyoming attracts thousands of prospectors and rock hounds who search the hills, valleys, and streams for gold, jasper, gemstones, and other attractive rocks and minerals. On any given weekend during the summer, hundreds of people search for gold with metal detectors, gold pans, and dredges. Several clubs, groups and associations have been formed to take advantage of these treasures in the Cowboy State, but many of these treasure hunters may be overlooking some extraordinary treasure. For instance, in 1977, one gold prospector searching for gold in the Medicine Bow Mountains in Cortez Creek recovered two gem-quality diamonds along with gold. How many more diamonds occur in Cortez Creek and in other streams in the Medicine Bow Mountains?

Kimberlitic indicator minerals include a group of rare and unique minerals that often accompany diamond such as pyrope garnet (a rare purplish variety of garnet), chromian diopside (distinct emerald-green diopside), high-magnesian chromite, and picroilmenite (non-magnetic, magnesian ilmenite). Such minerals have been found in hundreds of streams, valleys and mountains in Wyoming indicating that the possibility of finding diamonds while searching for gold is high. A few years ago, a prospector from Arizona recovered considerable placer gold on Douglas Creek: along with the gold, he also recovered pyrope garnet. During a public field trip sponsored by the Wyoming State Geological Survey (WSGS) a few years ago, attendees were taught how to pan for gold along the Middle Fork of the Little Laramie River. Instead of panning out gold, several would be prospectors panned out pyrope garnet. Another prospector from Colorado asked for information on where to pan for diamonds – using only a gold pan, he shortly recovered a diamond in a stream south of Laramie. A research study by the WSGS during the 1980s resulted in the identification of approximately 300 kimberlitic indicator mineral anomalies suggesting the presence of a major diamond province in southeastern Wyoming (Hausel and others, 1988, 2003). It is also interesting to note that some samples collected during this project in the vicinity of Sybille Canyon, also had some sapphire and ruby.

In a later study by the WSGS, kimberlitic indicator minerals were found in anthills much farther north in the Big Horn basin, and numerous gold anomalies were also detected in along I-80 near Arlington, where no one had bothered to search for gold for a hundred years (Hausel and others, 1994). So how does one find diamonds or even recognize them?

DIAMOND RECOGNITION

In order to search for placer (stream-deposited) diamonds, a prospector needs to become familiar with natural diamond. Diamonds are isometric and have high symmetry. In their simplest form, they occur as a cube (Figure 1). One of the more common habits for diamond is an octahedron (8-sided diamond formed by two-pyramids attached at a common base). Many diamonds have crystal habits that are a modified octahedron and may include such varieties known to a mineralogist as hexoctohedral, rhombic dodecahedron, trisoctahedron, and others.

Figure 1. Some crystal habits of diamond include: (a) the cube - a relatively uncommon habit that is often reported in some placers in Brazil. Cubic diamonds often have several pyramidal depressions on the crystal faces. (b). Octahedral diamonds (8-sided) and (c) various modifications are more common. Octahedra often have distinct triangular-shaped growths or depressions on the crystal surfaces known as trigons- these are useful in the identification of diamond. Less common are (d) dodecahedral diamonds and (e) distorted dodecahedra. Numerous other crystal habits have been reported for diamond (f).

When identifying a raw diamond, it is important to look at the luster as well as the individual crystal faces under a microscope. Crystal faces are frequently rounded and may have distinct tiny triangles known as trigons. These are triangular depressions (or growth platelets) found on the octahedral crystal faces (Figure 1b). Cubic diamonds may show similar depressions with pyramidal morphology that appear as rotated squares or parallelograms. Twin diamond crystals often occur as a flatten triangular shaped diamond known as a macle.

Partial resorption the octahedron can result in a rounded dodecahedron (12-sided crystal) with rhombic faces (Figure 1d). Many dodecahedrons develop ridges on the rhombic faces producing a 24-sided crystal known as a trishexahedron. Four-sided tetrahedral diamonds are sometimes encountered that are distorted octahedrons. A tetrahedron by definition is a four-faced polyhedron, in which each face forms a triangle.

Diamonds have conchoidal fracture, are brittle, and will break from a mild strike with a hammer. Even so, they are the hardest of all natural minerals and are assigned a hardness of10 on Moh’s hardness scale. Diamond exhibits a slightly different hardness in different crystallographic directions, which allows for it to be polished with less difficulty in specific directions. For example, it is less difficult to grind the octahedral corners off the diamond, whereas grinding parallel to the octahedral face is nearly impossible.

Diamond has perfect cleavage in four directions parallel to the octahedral faces: thus an octahedron can be fashioned from an irregular diamond crystal by cleaving.

Diamond has greasy to adamantine luster – they often appear as if a thin film of Vaseline coats them. Diamonds are found in a variety of colors including white to colorless and less commonly shades of yellow, red, pink, orange, green, blue, brown and black. Those that are strongly colored are termed fancies, such as the distinctly blue Hope diamond.

The most common color is brown. Prior to the development of the Argyle mine in Australia in 1986, all brown diamonds were considered to be industrial. But due to Australian marketing strategies, these are now highly sought as gems. The lighter brown stones are labeled champagne diamonds and the darker brown referred to as cognac diamonds.

Yellow is the second most common color and these are referred to as “Cape” diamonds, after the Cape Province of South Africa. When the color is intense, the stone is referred to as “canary”. Pink, red and purple diamonds are rare and bring high values: according to Harlow (1998), orange is the rarest color in diamond. Even though there are many green diamonds, faceted green stones are rare, due to the fact that the color occurs as a thin layer of green on the surface of a white diamond.

Diamonds may also occur as grey, black and white crystals. Black diamonds are thought to be the result of numerous inclusions of graphite, which also make the diamond an electrical conductor. Such diamonds are difficult to polish due to abundant soft graphite. Black gem diamonds are uncommon.

The distinct fire seen in faceted diamonds is the result of the high coefficient of dispersion (0.044). Diamond also has a high index of refraction (IR=2.4195), due to its density. The high density diminishes the velocity of light passing through the mineral to only 77,000 miles per second, whereas the speed of light in a vacuum is 186,000 miles per second (Harlow, 1998).

Approximately 1/3rd of all gem diamonds will luminescent blue when placed under ultraviolet light. In most cases, luminescence will stop when the ultraviolet light is turned off, which is known as fluorescence. Many diamonds fluoresce in both long- and short-wavelength ultraviolet light. However, fluorescence is generally weak, and may not always be readily apparent to the naked eye. In some cases, the light emission from the diamond will still be visible for a brief second after the ultraviolet light is removed, which is known as phosphorescence.

Diamonds have tremendous thermal conductivity, such that they will feel cold to the lips when touched, since the gem conducts heat away from the lips. This is why diamonds are sometimes referred to as “ice” (Harlow, 1998). There are pockets size detectors (GEM tester) that are available for a minimal price that measure the surface thermal conductivity of diamond. These distinguish diamond from other gems and imitations.

Diamonds are hydrophobic (nonwettable) and repel water. Because they are hydrophobic, diamonds attract grease (grease will adhere to the surface of a diamond), providing an efficient method for extracting diamond from waste material. When heated in oxygen, diamond will burn to CO2. However, without the presence of oxygen, diamond will transform to graphite at 1900°C. Diamonds are also unaffected by acids.

PROSPECTING FOR PLACER DIAMONDS

Diamonds have moderate specific gravity (3.5) and tend to concentrate with black sands in creek and riverbeds. A prospector should be able to pan for diamonds as one would pan for gold, and Wyoming and Colorado provide excellent hunting grounds for placer diamonds.

When found in streams, diamonds may have been liberated from nearby kimberlite, lamproite, or related lamprophyric pipe or dike, or may have come from diamond pipes hundreds of miles away. Because of the extreme hardness of diamond, some diamonds are thought to be able to resist stream abrasion over great distances.

The greatest diamond placers in the world occur within the Orange River basin of southern Africa as well as in beach sands along the shoreline of the Atlantic Ocean of western Africa. The Orange River basin drains some of the richest diamond pipes in the world and includes a region with more than 3000 barren and diamondiferous kimberlite pipes. Erosion of these pipes over the past several million years resulted in the liberation of millions of diamonds into the Orange River and its tributaries that were carried downstream for hundreds of miles to the Atlantic Ocean. River sediments from Kimberley to the Atlantic Ocean, and beach sands extending from Port Nolloth, Namaqualand at the mouth of the Orange River northward to Luderitz, Nambia contain considerable placer diamonds eroded from the kimberlites.

Although not of the same scale as Africa, there is little doubt that thousands of diamonds occur in some streams in Colorado and Wyoming. In the Colorado-Wyoming State Line district south of Laramie, a minimum of 40 diamondiferous kimberlites has been eroded over a period of 300 to 600 million years. Hundreds of thousands (if not millions) of diamonds must have escaped these pipes during erosion. According to some early work by McCallum and Mabarak (1976), the state line diamond pipes may have lost 2,500 feet of vertical column of diamond-bearing rock (the Iron Mountain district to the north could have lost even more, possibly 4000 to 5000 feet) – yet where did all of these diamonds go? Many should still be found in nearby creek and riverbeds, waiting for someone to pick them up. A prospector has a much greater chance of getting rich by finding a valuable diamond panning in these streams than by winning the Colorado lottery.

For example, the largest diamond recovered from the Kelsey Lake mine along the Colorado-Wyoming border, weighed 28.3 carats, and a fragment from a broken diamond was projected to have come from an 80 carat stone (Howard Coopersmith, personal communication, 2002). So one should be able to find some large diamonds downstream.

Only a few placer diamonds have ever been reported in this region. During some of the early testing of the Kelsey Lake kimberlites, a 6.2-carat diamond was found in Fish Creek (Howard Coopersmith, personal communication (1998) (Figure 2). Earlier, some diamonds had been recovered on Rabbit Creek adjacent to the Sloan 1 and 2 kimberlites by a prospector searching for gold (Frank Yaussai, personal communication, 1977). Using only a gold pan, another prospector recently panned a diamond from the Poudre River (Vic Norris, personal communication, 2002).

Figure 2 Generalized location map of the Colorado-Wyoming kimberlite district showing locations of known kimberlites (named black areas), nearby streams (dashed lines) and roads (solid lines) (modified from Hausel, 1998).

Essentially, every kimberlite in the State Line district south of Laramie is diamondiferous. Some of the more significant kimberlites include the Kelsey Lake group, George Creek and the Sloan kimberlites. The George Creek kimberlites have yielded the greatest number of diamonds to date, and during testing, produced more than 89,000 diamonds from bulk sample tests in the 1980s. These diamond-rich dikes undoubtedly supplied tens of thousands of diamonds into George Creek and the adjacent tributaries during the geological past. Another good source for placer diamonds should be the Sloan kimberlites adjacent to Rabbit Creek. These yielded about 40,000 diamonds during bulk sampling tests in the early 1980s, including stones as large as 5.51 carats.

Historical placer diamond discoveries in Colorado and Wyoming have been rare (Figure 3). This may be due to the fact that most prospectors are not trained to recognize diamond, and that very historical little gold prospecting occurred in the vicinity of the State Line diamond district.

Figure 3 (a) Two gem-quality octahedral placer diamonds found in a placer gold deposit on Cortez Creek, Medicine Bow Mountains, Wyoming (WSGS photo). (b) Some diamonds mined from the Kelsey Lake mine, Colorado (photo courtesy of Howard Coopersmith).

Diamonds were never reported in any of the drainages downstream from the State Line district during the historical past, yet the district contains 40 diamondiferous kimberlites and some geophysical and mineral anomalies suggesting there are several more undiscovered diamondiferous kimberlites in this district. Thus the total diamond budget in the streams should be significant. So why didn’t gold prospectors of the past find gold and diamonds in this region? The answer is simple –the diamondiferous kimberlites occur in a region that is essentially barren of any significant gold veins and there are few indications of gold, so the old timers never spent much time looking in this region. However, if gold had been found in this district in the past, the prospectors would probably have found some diamonds – whether or not they would have recognized these rare minerals, is unknown.

If you think you have found a diamond, but are not sure, feel free to stop by our offices on the University of Wyoming campus in Laramie, or call Dan Hausel at 307-766-2286 extension 229 (email – dhausel@uwyo.edu). We will be happy to help you.

Metals and Precious Stones Section

PLACER DIAMONDS

by W. Dan Hausel

INTRODUCTION

DIAMOND RECOGNITION

PROSPECTING FOR PLACER DIAMONDS

RECOMMENDED READING