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

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