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Structural Controls on Gold Mineralization at the Ashanti Deposit, Obuasi, Ghana

By
Andrew H. Allibone
Andrew H. Allibone
SRK Consulting, P.O. Box 943, West Perth, Western Australia, and School of Earth Science, James Cook University, Townsville, QLD, Australia
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T. Campbell McCuaig
T. Campbell McCuaig
SRK Consulting, P.O. Box 943, West Perth, Western Australia
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David Harris
David Harris
SRK Consulting, P.O. Box 943, West Perth, Western Australia
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Michael Etheridge
Michael Etheridge
SRK Consulting, P.O. Box 943, West Perth, Western Australia
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Stuart Munroe
Stuart Munroe
SRK Consulting, P.O. Box 943, West Perth, Western Australia
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David Byrne
David Byrne
SRK Consulting, P.O. Box 943, West Perth, Western Australia
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J. Amanor
J. Amanor
Ashanti Goldfields Company Limited, Obuasi, Ghana
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W. Gyapong
W. Gyapong
Ashanti Goldfields Company Limited, Obuasi, Ghana
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Published:
January 01, 2002

Abstract

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:

  1. Dilatant and subordinate compressional sites where mineralized shear zones step left and right, respectively, across F4 kink folds and reactivated D2 transfer faults;

  2. In pressure shadows associated with volcanic units, felsic and granitoid intrusions within the sedimentary sequence;

  3. 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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Contents

Special Publications of the Society of Economic Geologists

Integrated Methods for Discovery: Global Exploration in the Twenty-First Century

Richard J. Goldfarb
Richard J. Goldfarb
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Richard L. Nielsen
Richard L. Nielsen
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Society of Economic Geologists
Volume
9
ISBN electronic:
9781629490335
Publication date:
January 01, 2002

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