Structural controls on gold mineralization at the Ashanti Deposit, Obuasi, Ghana

Allibone, Andrew H., McCuaig, T. Campbell, Harris, David, Etheridge, Michael, Munroe, Stuart, Byrne, David, Amanor, J., and Gyapong, W. (2002) Structural controls on gold mineralization at the Ashanti Deposit, Obuasi, Ghana. In: Goldfarb, Richard J., and Nielsen, Richard L., (eds.) Integrated Methods for Discovery: global exploration in the twenty-first century. Special Publications of the Society of Economic Geologists, 9 . Society of Economic Geologists, pp. 65-93.

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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 evidence in the Birimian sedimentary rocks that host the deposit. The prominent northeast-striking structural grain and fold-thrust belt architecture that characterizes the Paleoproterozoic 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 south-east-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 assoicated with the F₄ folds, which implies that gold mineralization occured 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 are 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 north-east-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 exploitation tool in its own right, which allows the significance of scattered structural observations to be appreciated and incorporated into a robust targeting strategy.

Item ID: 27272
Item Type: Book Chapter (Research - B1)
ISBN: 978-1-887483-91-9
Date Deposited: 12 Jul 2017 03:30
FoR Codes: 04 EARTH SCIENCES > 0403 Geology > 040312 Structural Geology @ 100%
SEO Codes: 84 MINERAL RESOURCES (excl. Energy Resources) > 8401 Mineral Exploration > 840105 Precious (Noble) Metal Ore Exploration @ 100%
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