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
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
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.