Prospecting For Diamonds

To search for placer diamonds, the prospector first needs to become familiar with the commodity. Diamonds are extraordinary minerals with extreme hardness, making them useful in industrial applications. They also have inherent beauty that is sought for personal adornment. Genesis of this unique mineral requires extreme temperature and pressure generated at great depth within the
earth, making diamonds rare. In fact, the environment for diamonds is so rare that diamond is one of the more valuable commodities on earth, and arguably, the most valuable based on price and size. Few commodities can match the value of a high-quality faceted diamond gemstone when based on comparable weight.

Diamond habit

Diamonds are isometric and have high symmetry. In their simplest form, they occur as a cube, which is a 6-sided solid. 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 modified octahedrons and may include such varieties known to the mineralogist as hexoctohedron, rhombic dodecahedron, trisoctahedron, and others.

Crystal faces are frequently rounded and may have distinct tiny triangles known as trigons which you may need a 10x hand lens to see. Trigons are triangular depressions (or growth plates) found on the octahedral crystal faces. Cubic diamonds may show similar depressions with pyramidal morphology that appear as rotated squares or parallelograms. Twinned diamond crystals often occur as a flattened triangular-shaped diamond known as a macle.

Partial resorption of the octahedron can result in a rounded dodecahedron (12-sided crystal) with rhombic faces along with some distorted variations. 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 can also occur in a variety of crystal habits.

Fracture, hardness, and cleavage

Diamonds have conchoidal fracture, are brittle, and break from a mild strike with a hammer. Even so, they are considered the hardest of all natural minerals and are assigned a hardness of 10on Mohs’ hardness scale. Diamond exhibits a slightly different hardness in different crystallographic directions, which allows it to be polished with less difficulty in specific directions. For example, it is easier to grind the octahedral corners off a diamond than to grind parallel to the octahedral face, which 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.

Appearance

The luster of diamonds is greasy to adamantine, appearing as if they are covered with a thin coat of grease. Diamonds are found in a variety of colors including the more common white to colorless and less commonly, shades of yellow, red, pink, orange, green, blue, brown, gray, and black. Those that are strongly colored are termed fancies; a 0.95-carat fancy purplish-red Argyle diamond recently
attracted attention by selling for nearly $1 million (U.S.). A yellow-brown fancy diamond (the 545.7 carat Golden Jubilee) is known as the world’s largest faceted gemstone. Possibly the most famous fancy diamond in the world is the 45-carat blue Hope Diamond.

The most common colored diamond is brown. Prior to the 1986 development of the Argyle mine in Australia, all brown diamonds were considered industrial stones, 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 colored diamond; these are referred to as Cape diamonds, after the Cape Province of South Africa. When the yellow color is intense, the stone is referred to as canary.

Pink, red, and purple diamonds are rare and bring high values. Orange is the rarest color in diamond. There are many green diamonds because the color occurs as a thin layer of green on the surface of a white diamond. Faceted green stones, however, are rare. Black diamonds are thought to be the result of numerous inclusions of graphite, which also make diamond an electrical conductor. Such diamonds are difficult to polish due to the presence of abundant soft graphite, so black gem diamonds are uncommon.

Other traits

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, and is responsible for the distinctive adamantine luster. 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.

Approximately one-third of all gem diamonds will luminesce blue when placed under ultraviolet light. In most cases, luminescence will stop when the ultraviolet light is turned off, a property known as fluorescence. Many diamonds fluoresce in both long and shortwave length ultraviolet light. However, fluorescence is generally weak, and may not be readily apparent to the naked eye. In some cases, the light emission from the diamond will still be visible for a brief moment after the ultraviolet light is removed, which is known as phosphorescence.

Diamonds have tremendous thermal conductivity, i.e., diamond will feel cold to the lips when touched, since the gemstone conducts heat away from the lips. This is why diamonds are sometimes referred to as “ice”. There are pocket-sized detectors (e.g., GEM® testers) that are available for a minimal price that are designed to measure the surface thermal conductivity of diamond. These will distinguish diamond from other gems and diamond imitations.

Diamonds are hydrophobic (non-wettable) and repel water. Because they are hydrophobic, diamonds attract grease (which will adhere to the surface of a diamond), providing an efficient method for extracting diamonds from waste material by the use of grease tables.

When heated in the presence of oxygen, diamond will burn to produce CO2. However, when heated in the absence of oxygen, diamond will transform to graphite at 1900°C. Diamonds are also unaffected by acids.

Prospecting for placer diamonds

The greatest diamond placers in the world occur within the Orange River basin of southern Africa and in beach sands along the Atlantic Ocean shoreline of western Africa. The Orange River basin drains some of the richest diamond pipes in the world and includes a region with more than 3000barren and diamondiferous kimberlite pipes. Erosion of these pipes over the past several million years resulted in the liberation of millions of diamonds. The Orange River and its tributaries captured the diamonds and carried them hundreds of miles downstream 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 also occur in some streams and stream sediments in Colorado and Wyoming.

Diamonds in Canada

Diamonds have been known from glacial tills and gravels in scattered locations throughout Canada (and the northern U.S.A) for decades, but it was not until the late 1980s that exploration for kimberlites as a primary diamond source became significant. DiaMet Minerals Ltd. led explorers in the Northwest Territories, and in 1991 discovered the first pipe of what was to become BHP’s Ekati Diamond Mine. This discovery touched off an unprecedented staking rush and exploration of the Northwest Territories and Nunavut. More than 300 kimberlites have since been discovered in the Northwest Territories and Nunavut. About half of these have at least trace amounts of diamonds. In addition to Ekati, at least three other projects in the Slave Province are in advanced exploration or feasibility stages. It is projected that Canada will account for up to 10% of world wide diamond production in the future.

Alberta

DeBeers Canada (formerly Monopros) discovered the Mountain Lake alkaline ultrabasic volcanic in the early 1990s. It was originally thought to be possibly a kimberlite. It’s discovery fostered increased exploration in Alberta, which led Ashton Mining of Canada Ltd. and partners to the discovery of several kimberlites in the Buffalo Head craton in north central Alberta. To date, 41 kimberlites have been discovered in Alberta.

Other Provinces

Exploration for diamonds is ongoing in the Hudson Bay lowlands of Manitoba and Ontario, on Palaeozoic flat lying sediments underlain by the Superior Province of the Canadian Shield. This area hosts De Beers Victor property, where 16 of the 18 pipes discovered to date are diamondiferous. In Ontario diamonds have also been recovered from kimberlite diatremes near Kirkland Lake, and dykes near Wawa. In northern Quebec, kimberlitic dykes are being explored for diamond potential.

British Columbia

Generally, British Columbia has not been considered favourable for diamonds because the bulk of the province is underlain by orogenic “mobile” belts which formed in sedimentary rocks at the margin of the North American craton and in exotic accreted terranes. Abundant magmatism accompanied the orogeny. The tectonic disturbance, magmatism and relatively higher geothermal gradients in the orogenic belts were unfavourable for the intrusion of kimberlites or diamond sampling and preservation. It should be noted, however, that LITHOPROBE seismic data suggests that the crystalline basement rocks are essentially undisturbed under the Phanerozoic cover at least as far west as the Rocky Mountain Trench. This may explain the occurrence of the Crossing Creek kimberlite. Walker (1994) has presented arguments that diamonds could occur in the mobile belt of the Canadian Cordillera, and outlines models that could account for the rare known occurrences and the paucity of accompanying indicator minerals.

The northeastern part of B.C. is within the Interior Platform, comprising relatively flat lying Phanerozoic sediments lying on crystalline basement rocks, beyond the eastern limit of deformation of the Cordilleran orogeny. The Interior Platform of northeastern B.C. is potentially a favourable environment for diamonds because like Alberta, it is underlain by stable continental cratonal basement rocks.

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