Abstract Obuasi, with a total mineral resource plus past production of 70 Moz, is the largest gold deposit in West Africa, and one of the largest in the world. It is hosted by ~2135 Ma siliciclastic rocks of the Eburnean Kumasi Basin, which were obliquely shortened along an inverted boundary with the older Eoeburnean Ashanti belt to the east. Greenschist facies metamorphism was coeval with mineralization and related alteration at ~2095 Ma. The steeply dipping, ENE-plunging lodes extend over an 8-km strike length and to depths of >2.5 km. They include paragenetically complex gold-rich quartz veins surrounded by refractory auriferous arsenopyrite and closely associated carbonate-muscovite alteration halos in deformed carbonaceous phyllites and subordinate metaigneous host rocks. Gold and arsenic were initially precipitated during deformation-assisted interaction with reduced host rocks at ~350°C and 100 to 200 MPa. The mineralizing fluids were derived primarily from deeper, As-rich metasedimentary sources by basinal fluid expulsion and metamorphic devolatilization triggered by inversion and shortening, followed by transpression. Continued fluid injection during and after the metamorphic peak produced changes in gold fineness, sulfide assemblages, repeated dissolution (stylolites) and reprecipitation of mineralized veins, and a change from early deformed shear-related, sulfide-rich lodes to later quartz-rich lodes that plunge down or across the axes of younger transpressional folds. Channelized fluid flow due to reactivation of basin-edge transfer structures, and/or irregularly distributed gold source rocks, may explain the variation in gold endowment along the former basin boundary.

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 (D 2 ) after development of a widespread bedding-parallel cleavage (S 1 ). A further minor episode of southeast-directed shortening (D 3 ) overprints D 2 . Structures associated with D 1 -D 3 are folded around 300- to 500-m- scale upright folds (F 4 ) 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 (D 5 ) and local sinistral reactivation of some older D 2 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 F 4 folds, which implies that gold mineralization occurred towards the end of, or after, F 4 . 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 D 5 . Mineralized faults locally cut across F 4 folds without deflection, again implying that ore deposition occurred after F 4 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 F 4 kink folds and reactivated D 2 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 D 2 thrust fault known as the Obuasi/Main Reef fissure. Sinistral reactivation of this specific fault segment during the D 5 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 D 2 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.

Student Field Trip 17 visited a number of paleoplacer and orogenic gold deposits throughout southwest Ghana over a week. The itinerary and route map include visits to active open-pit and underground operations, including Damang, Iduapriem, Nkran/Esaase, Obuasi, Prestea-Bogosu, and Wassa, as well as exploration projects at Homase-Akrokerri, all hosted within the Birimian Supergroup. The course material covers the geology and mineralization of these deposits in the context of regional and local geological settings, focusing on their similarities and differences, ore resources, deposit models, and exploration methods.

Student Field Trip 17 visited a number of paleoplacer and orogenic gold deposits throughout southwest Ghana over a week. The itinerary and route map include visits to active open-pit and underground operations, including Damang, Iduapriem, Nkran/Esaase, Obuasi, Prestea-Bogosu, and Wassa, as well as exploration projects at Homase-Akrokerri, all hosted within the Birimian Supergroup. The course material covers the geology and mineralization of these deposits in the context of regional and local geological settings, focusing on their similarities and differences, ore resources, deposit models, and exploration methods.