Geotimes
Highlights
Economic Geology
Dick Tosdal
Economic geology research links disciplines across the spectrum of geoscience
toward understanding how varied and seemingly disparate processes conspire to
form an anomalous concentration of elements in a specific spot in the crust. Research
focuses on the whole metal cycle from the genesis of deposits, through mining
and mineral extraction, and finally through site reclamation. Social, economic
and environmental issues of exploration and mine development, mine closure and
mitigation of any environmental legacy are becoming increasingly important in
the academic and business environment (Applied Geochemistry, v. 16, p.
1567).
Fluid flow
Metallic ore deposits result from large-scale flow of metal- and ligand-bearing
fluids and their concentrations at specific sites in Earth's crust. Computer
models help in evaluating potential processes. For example, models of lead-zinc
deposits hosted in sedimentary rock invoke mixing of two fluids derived from
different aquifers at the site of ore deposition. One basinal fluid carries
metal and the other fluid transports sulfur. Computer models of such large-scale
fluid flow and interaction, however, have shown that the two fluids might not
mix efficiently and therefore not deposit ore minerals. (Economic Geology,
vol. 95, p. 1605).
Whether the inefficiencies in the process shown in the computer model are real
or a function of the model parameters or other unrecognized factors is not clear.
Computer models also help visualize the end result of ore genesis processes.
In Australia, the Cooperative Research Centre for Predictive Mineral Discovery,
a collaborative university, government and industry research group, seeks to
create a computational environment to simulate 3-D and 4-D evolution of ore
genesis processes. Research integrates fluid flow paths, mineral reaction, deformation
and other ore genesis processes into predictive mineral deposit models applicable
to mineral exploration.
Within the exploration and mining community, technology imported from the energy
industry is increasingly used in exploration around active mines. Combining
geologic, geochemical and geophysical data provides an avenue to visualize ore
deposits in 3-D using computers or virtual reality theaters. The 3-D models
provide visual clues for exploration as well as how ore deposits form. Leaps
in understanding ore genesis will occur as the visualization technology and
tools to develop 3-D GIS analysis become more accessible.
The big and small views of metal
budgets
Advances
in ore genesis research are made on all scales of observation. Chemical analyses
of single fluid inclusions in Argentina's Bajo de Alumbrera porphyry copper
deposit (Economic Geology, v. 96, p. 1743) have shown that incredibly
efficient deposition of economic amounts of copper and gold occurred over a
very narrow temperature range. Furthermore, the study demonstrates that bulk
metal ratios in the deposit are identical to those found in the fluid exsolved
directly from the parental magma. The study further concludes that the chemical
evolution of the hydrothermal system will not affect metal ratios in a deposit,
but that the geochemical process will affect concentration processes and thus
the primary hypogene metal grades. The conclusion adds fuel to the longstanding
debate over the importance of site-specific geochemical processes vs. crustal-scale
controls over metal endowments.
Chemical
analyses of single fluid inclusions in Argentina's Bajo de Alumbrera porphyry
copper deposit have shown that incredibly efficient deposition of economic amounts
of copper and gold occurred over a very narrow temperature range.Courtesy of
Richard M. Tosdal
Taking a more regional view, research last year revealed that ore minerals and
their contained metals in epithermal deposits in Nevada reflect differences
in tectonic setting, which in turn strongly influenced the chemistry of the
magmas from which metal-bearing hydrothermal fluids are derived (Economic
Geology, v. 96, p. 1827).
Exploration techniques
Research drives development of new exploration techniques. Previously, airborne
geophysical instruments revolutionized the search for ore deposits, particularly
in vegetated and deeply weathered terranes. An emerging research frontier is
the process of converting potential field geophysical data collected at the
surface or from airborne platforms to images of geologic features of interest
in the subsurface in two or three dimensions. The imaging capability contributes
to the understanding of ore genesis and the relationship of the ore bodies to
geologic features, and also enhances exploration beneath cover terranes. Though
developed to examine metallic ore deposits, the techniques also have application
in the energy industry, environmental studies, and even in the search for unexploded
ordnance in areas used for military training or during times of armed conflict.
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Tosdal is the Director of the
Mineral Deposit Research Unit at the University of British Columbia. E-mail
