Geometry and genesis of the giant Obuasi gold deposit, Ghana

The supergiant Obuasi gold deposit is the largest deposit hosted in the Paleoproterozoic Birimian terranes of West Africa (62 Moz, cumulative past production and resources). The deposit is hosted in Kumasi Group sedimentary rocks composed of carbonaceous phyllites, slates, psammites, and volcaniclastic rocks intruded by different generations of felsic dykes and granites.

In this study, the deformation history of the Obuasi district was re-evaluated and a three stage sequence defined based on observations from the regional to microscopic scale. The D1_Ob stage is weakly recorded in the sedimentary rocks as a layer-parallel fabric. The D2_Ob event is the main deformation stage and corresponds to a NW-SE shortening, involving tight to isoclinal folding, a pervasive subvertical S2_Ob cleavage striking NE, as well as intense sub-horizontal stretching. Finally, a N-S shortening event (D3_Ob) formed an ENE-striking, variably dipping S3_Ob crenulation cleavage.

Three ore bodies characteristic of the three main parallel mineralised trends were studied in details: the Anyankyerem in the Binsere trend; the Sibi deposit in the Gyabunsu trend, and the Obuasi deposit in the main trend. In the Obuasi deposit, two distinct styles of gold mineralisation occur; (1) gold-bearing sulphides, dominantly arsenopyrite, disseminated in metasedimentary rocks and (2) native gold hosted in quartz veins up to 25 m wide. Both mineralisation styles are contained in high grade ore shoots classified in three groups on the basis of 3D modelling of deposit geometry from drill hole and underground development data; (1) volcanic rock controlled shoots, (2) fault intersection and bifurcation, and (3) F3_Ob fold hinge controlled shoots. Strain shadows surrounding gold-bearing arsenopyrite parallel with S2_Ob but folded by S3_Ob, indicate that the sulphides were formed during D2_Ob. In contrast, although the mineralised quartz veins formed during D2_Ob, field, SEM and microtomographic observations demonstrate that the gold is hosted in microcrack networks in the veins, located in hinges of F3_Ob folds. These observations provide the first evidence for multiple stages of gold deposition at the Obuasi deposit.

The crystal plasticity and element mobility behaviour of the gold-bearing arsenopyrite during metamorphism (340° - 460° and 2 kbars) was investigated using quantitative electron backscatter diffraction analysis, ion microprobe imaging and synchrotron XFM mapping. The results show that the arsenopyrites remained remarkably robust during the high strain deformation (D2_Ob) and preserved their gold content (300 to 3000 ppm Au). However, small amounts of crystal plasticity or intragranular microcracking, during D3_Ob, enabled low volume fluid infiltration into the arsenopyrites and activation of a dissolution-replacement reaction, involving replacement by Au-poor (bellow detection limit of all techniques), Ni-rich (up to 3000 ppm Ni) arsenopyrite coeval with Ni-rich pyrite and native gold precipitation in the microcracks. The newly formed arsenopyrite is also S- and Fe-depleted and As-enriched in comparison to the primary arsenopyrite. Thin-section scale mass balance calculations indicate gold was mobilised over distances greater than the centimetre scale; likely contributing to native gold along wall rock cleavage and at high concentrations in the fracture networks of the quartz veins. The gold remobilisation mechanism was likely controlled by strong chemical gradients at crystal-fluid interfaces induced by the salinity of the infiltrating fluid and S-release during the replacement reaction. It is suggested that the high grade native gold hosted in quartz veins was sourced from the arsenopyrite mineralisation.