Structural Controls on Gold Mineralization at the Ashanti Deposit, Obuasi, Ghana
Andrew H. Allibone, T. Campbell McCuaig, David Harris, Michael Etheridge, Stuart Munroe, David Byrne, J. Amanor, W. Gyapong, 2002. "Structural Controls on Gold Mineralization at the Ashanti Deposit, Obuasi, Ghana", Integrated Methods for Discovery: Global Exploration in the Twenty-First Century, Richard J. Goldfarb, Richard L. Nielsen
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Fault zones that cut Paleoproterozoic Birimian Supergroup sedimentary and mafic volcanic rocks in southwestern Ghana, west Africa, host numerous gold deposits that form one of the richest mesothermal lode gold provinces in the world. The Ashanti gold deposit is the largest discovered to date in west Africa, with past production and current reserves exceeding ~1,200 tonnes (t) of gold. A complex multiphase deformation history is evident in the Birimian sedimentary rocks that host the deposit. The prominent northeast-striking structural grain and fold-thrust belt architecture that characterizes the Paleoprotero zoic rocks of southwestern Ghana was established during regional-scale southeast-directed shortening (D2) after development of a widespread bedding-parallel cleavage (S1). A further minor episode of southeast-directed shortening (D3) overprints D2. Structures associated with D1-D3 are folded around 300- to 500-m- scale upright folds (F4) that plunge to the northeast and have axial planes that strike ~east-west and dip 50° to 80° N. Upright folding was followed by development of north-striking, small-displacement, sinistral strike-slip faults (D5) and local sinistral reactivation of some older D2 thrust faults.
Disseminated auriferous arsenopyrite grains in rocks adjacent to the mineralized faults are either localized on or cut the crenulation cleavage associated with the F4 folds, which implies that gold mineralization occurred towards the end of, or after, F4. Mineralization along the faults themselves is hosted in quartz vein arrays that commonly have sinistral asymmetries at scales ranging from a few centimeters to several hundred meters, implying that the main gold event occurred during D5. Mineralized faults locally cut across F4 folds without deflection, again implying that ore deposition occurred after F4 folding.
Ore shoots within the Ashanti deposit and adjacent satellite deposits are predominantly structurally controlled and are located in the following:
Dilatant and subordinate compressional sites where mineralized shear zones step left and right, respectively, across F4 kink folds and reactivated D2 transfer faults;
In pressure shadows associated with volcanic units, felsic and granitoid intrusions within the sedimentary sequence;
At the intersections of major structures that were active during mineralization.
The Ashanti deposit as a whole occupies an ~8-km-long segment of an otherwise unmineralized northeast-striking D2 thrust fault known as the Obuasi/Main Reef fissure. Sinistral reactivation of this specific fault segment during the D5 mineralization event occurred in response to movement on the younger north-striking Ashanti fissure, which merges with the Obuasi/Main Reef fissure at the northern end of the Ashanti deposit. The southern end of the mine is marked by a sharp right-hand flexure in the Obuasi fissure where it steps across a D2 transfer zone.
Recognition of these structural controls on mineralization allowed extensions to ore shoots within the Ashanti deposit to be targeted with a greater degree of confidence and has led to delineation of significant additional resources. Similar structural sites were targeted during exploration of the surrounding area using “integrated” geologic maps that combined the results of geologic mapping, airborne geophysical surveys, soil geochemical data, aerial and satellite photography, and local costeaning. Detection of mineralized faults was best achieved with a combination of geologic mapping, soil geochemical surveys, and costeaning. Routine recognition of structural sites similar to those noted above is probably only possible with geologic mapping at scales larger than 1:50,000. Attempting to remotely detect 200- to 400-m-long bends in poorly exposed faults was the most difficult aspect of this program. However, the detailed understanding of the timing and structural controls on mineralization gained in the mine area is a powerful exploration tool in its own right, which allows the significance of scattered structural observations to be appreciated and incorporated into a robust targeting strategy.
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Integrated Methods for Discovery: Global Exploration in the Twenty-First Century
The dominant forces that affect the mining industry in today's economy are globalization, entrepreneurship, and unprecedented minerals availability. The current malaise and diminished economic importance of the base and precious metal mining industry in the world economy are the result of ongoing, longterm trends. Shifting market forces within a changed geopolitical landscape have resulted in excess supplies and reduced demands for high-unit-value mineral products and for geologists and engineers in the exploration and mining industries. Although these changes are still evolving, professionals and the mining companies that employ them must realize that these changes are irreversible.
Mining has five dominant characteristics: (1) it is essential to society, (2) it is effective in keeping society supplied with abundant, low-cost mineral products, (3) historically, it has been wealth destructive for investors, (4) it has been subsidized, and (5) it is shaped by social and political forces. What is different now is that instead of being subsidized, the industry is being handicapped by governments and abandoned by investors. The industry has reacted to a marketing problem with an inappropriate strategy of cost reduction and increased production. The increased supply has resulted in decreased prices. Mining companies are earning low or negative rates of return.
Remedies for the larger mining companies and the high-unit-value metals businesses include: Exemplary behavior to regain the trust of the public, governments and investors; Continued consolidation to become sector leaders; Profit through value-added vertical integration and direct marketing to consumers Development of innovative consumer-based financing mechanisms, especially for exploration ventures Individuals and small groups that have been displaced from the mainstream must become entrepreneurs. They must engineer their own survival by shifting careers or by finding, developing, and capitalizing on exploration and production opportunities that are unrecognized by, or are too small for, the major companies.
The mining industry is here to stay. It is too necessary to society to be abandoned; however, the future size and shape of the industry will probably be unrecognizable to most of us. It will be leaner and more opportunistic. It will be characterized by entrepreneurial corporations of all sizes that dominate niches and sectors and that anticipate and profit from the changing needs of society