Program Speaker: William Mansker, Prospecting for Diamonds

by Nancy L. Attaway

Professional geologist William Mansker, Ph.D. spoke to the New Mexico Faceters Guild about his experiences in searching for kimberlite deposits in the United States. Dr. William Mansker received his B.S. and M.A. from the University of Missouri. He received his Ph.D. from the University of New Mexico with a dissertation on “The Petrology of Nephelinites and Melilite Nephelinites of Oahu, Hawaii”. Dr. William Mansker has worked as a project geologist for diamond exploration in the United States for the last two decades, and he has served as an expert witness on environmental compliance. He is involved in the research and development of innovative geologic and environmental tools and is currently preparing a manuscript titled “A Prospecting Guide for Diamonds”.

Dr. Mansker began by stating that kimberlites and lamproites were the primary sources for diamonds. Placer deposits were the secondary sources. Diamonds have also been found in meteorites. He defined a kimberlite as a rare type of volcanic rock, named after the South African town of Kimberly, that is the major and primary source of naturally occurring diamonds. He mentioned that some kimberlites were barren, as not all kimberlites contain diamonds. Volcanic in origin, kimberlites rise upward from the depths of 100 to 150 miles within the earth. At such depths, diamond and other minerals, such as pyrope garnets and magnesium ilmenite, are stable. He explained that the structure of the kimberlite is usually called a pipe, which in a cross section appears to be funnel shaped. The pipe is broader near the surface and narrows as it travels downward. This typical funnel shape arises from a violent, explosive eruption of the kimberlite from the depth to the surface, as extremely high internal pressures are released.

Dr. Mansker said that since kimberlites are the very old volcanic rocks found at great depths, lamproites are then the shallow, more magmatically evolved rocks with an unusual chemistry. Lamproites have only recently been found to carry diamonds. Diamonds from the Argyle mine in the Kimberley region of western Australia, famous for pink diamonds, were traced to the weathered remains of a lamproite volcano. Kimberlites are thick, stable craton areas, and lamproites are the thinner cratons.

Dr. Mansker explained that several major minerals are found in kimberlite. These minerals serve as locator minerals for diamonds. The lists includes pyrope garnets, magnesium ilmenite, olivine, chrome diopside, and serpentine. Other accessory minerals include phlogopite, a magnesium-rich mica; apatite, a calcium phosphate; calcite, a calcium carbonate; and spinels, various iron, magnesium, aluminum, and chrome oxides. He said that color change garnets and garnets with high chrome and low calcium content reveal the presence of diamonds. An orange peel texture on a garnet also indicates the presence of diamonds.

Dr. Mansker described diamonds as pre-cambrian and composed of carbon in high pressure form. Diamonds occur as xenocrysts and contain cognate mineral inclusions. Diamonds are transparent to translucent and usually occur as octahedral grains or as other variations of cubic form. Graphite is a common low pressure form of carbon.

Diamonds do not crystallize in kimberlite or lamproite as a primary mineral. Instead, they are transported through kimberlite and lamproite pipes from deeper areas. The parent rocks where diamonds are crystallized lie deep in basement rock beneath the pipes. Small crystals of diamond will sometimes be found still encased in fragments of their original parent rocks. These fragments are called xenoliths.

Other materials in kimberlites include exotic fragments of rock material that was plucked from the earth’s crust and mantle during the kimberlite’s rapid ascent to the surface of the earth. These exotic materials include: eclogite, a rock saturated with garnet and chrome diopside from deep within the earth’s mantle, where diamonds can crystallize; granite, crustal fragments from immediately below the surface to depths of about 30 miles; sedimentary rocks, shale, limestone, and sandstone from very near surface rocks from zero to two miles down, varying from place to place.

Dr. Mansker said that kimberlites have been found in the United States in North Carolina, South Carolina, Georgia, West Virginia, Michigan, Kansas, Arkansas, and Louisiana, as well as in Texas, Colorado, Wyoming, California, Oregon, Utah, New Mexico (in the Pedernal Mountains and Zuni Mountains), and Alaska. Diamonds have been found in commercial quantities in Canada’s Northwest Territories. Dr. Mansker stated that ten to eleven carats per metric ton is required for commercial diamond potential.
In searching for diamonds, Dr. Mansker stated that a large-scale geologic area is first targeted. Landsat imagery from satellites enable geologists to locate the long linaments that outline the cratons. Low altitude photography is also used. Diamond-bearing kimberlites of economic significance are found in archons, those portions of cratons that are older than 2.5 billion years. Cratons are magnetic and can be traced. Kimberlites also occur in clusters.

Dr. Mansker said that a common method for kimberlite exploration and prospecting include taking samples from stream sediment to look for the indicator minerals, like pyrope garnet, ilmenite, chrome diopside, and olivine. A second method involves magnetic investigations of probable kimberlite areas. Kimberlites are usually magnetic due to their composition of magnetic minerals, such as ilmenite, olivine, diopside, and magnetite. A third method utilizes the geochemical investigations of probable kimberlite areas and stream sediments. Kimberlites typically contain higher amounts of nickel, chrome, and niobium, and they contain more of these elements than other rock types.

Dr. Mansker stated that once a kimberlite has been located by exploration geologists, it must be evaluated for economic potential by checking the diamond content. The usual steps in evaluation include the determination of the kimberlite type and its extent and also bulk sampling. The kimberlite must be drilled to have samples taken, the rock must be mapped geologically, and geochemical analysis of the kimberlite and its minerals must be performed. Through bulk sampling, the kimberlite is determined either to be diamondiferous or barren. If it is discovered to be diamondiferous, then the grade of diamonds must be ascertained.

After a kimberlite has been evaluated, assuming all results have been positive, then mining and diamond recovery may commence. Dr. Mansker described the several operations involved that include: 1) mining of the kimberlite and shipping, 3) milling and crushing of the kimberlite, 3) separation of heavy minerals, such as garnets, ilmenites, diamonds (now called concentrate), and 4) separation of diamonds from the heavy mineral concentrate.

The total time frame from initial prospecting and exploration to completion of actual mining varies, according to Dr. Mansker. Prospecting usually entails several months of rigorous field work by geologists. Once a kimberlite has been located, evaluation may take one to three years. Following the evaluation of the kimberlite, another three to four years are required to bring a mine into full production. The producing lifetime of a mine is also variable, depending upon the size and grade of the kimberlite and other factors. From an economic standpoint, Dr. Mansker said that a mine should have an active lifetime of at least seven to ten years once mining has begun in earnest.

Dr. Mansker brought various kimberlite mineral samples to show. He made gifts to Guild members of small vials of concentrate from a Kansas kimberlite. The New Mexico Faceters Guild thanks Dr. Mansker very much for a fine presentation on the search for diamonds.