The geology of the Antakori deposit consists of Cretaceous basement rocks with a lower sequence of quartz sandstones/siltstones with intercalated arkose sandstones/siltstones and minor calcareous siltstones of the Farrat Formation. The Farrat Fm. is overlain by the Inca Formation which consists of fine-grained calcareous siltstones with minor intercalated quartz-rich siltstones. The Inca Fm. is overlain by the Chulec Formation which consists predominantly of limestones with minor intercalations of fine-grained calcareous siltstones.
The Cretaceous sequence is intruded by numerous feldspar – biotite ± quartz porphyry dykes and stocks with spatially associated breccia bodies. These intrusions have generated contact metamorphic haloes and metasomatic skarn alteration in the surrounding sedimentary rocks. Common skarn minerals include prograde-facies red and green garnets, pyroxene, magnetite, and vesuvianite, as well as very strongly developed retrograde-facies epidote, chlorite, magnetite and calcite. Skarn development is controlled both by stratigraphic, bedding-parallel permeability and favourable-bed reactivity, as well as by crosscutting structural fractures and fault zones.
The Cretaceous sedimentary rocks are unconformably overlain by Middle Miocene (12.7-13.2 Ma) pyroclastic volcanic rocks of the Calipuy Formation. The unconformity is characterized by a previously unrecognized paleo-regolith which has affected the underlying skarn mineralization. The Calipuy Fm. volcanic package is intruded by a series of co-magmatic hypabyssal intrusive complexes. High-sulphidation style Cu-Au-Ag mineralization is associated with this volcanic-magmatic stage in the form of disseminations, veinlets and larger structures of enargite-tennantite-pyrite, associated with a strong advanced argillic alteration with pyrophyllite-alunite-kaolinite. The entire sequence is cut by an array of high-level phreatic and phreatomagmatic mill-matrixed breccias.
The penultimate event consists of hypabyssal rhyolitic dykes, stocks and locally developed flow-dome complexes that intrude both the Cretaceous sedimentary rocks and the Calipuy Fm. volcanic rocks. These intrusions and flow-dome complexes have associated intermediate-sulphidation Au-Ag-Zn-Pb-Cu, “Base Metal Carbonate” mineralization in the form of veinlets and disseminations, associated with variably developed intermediate argillic alteration. A final stage of magmatic activity consists of post-mineral felsic dykes that cut all previously described rocks.
AntaKori is characterized by the superimposition of several different mineralizing hydrothermal systems:
- a Cu-Au-Ag calcic skarn developed in Cretaceous sedimentary rocks associated with massive replacement sulphide bodies;
- a weakly mineralized porphyry Cu-Au-Ag-(Mo) system associated with several feldspar-biotite porphyry dikes and breccias;
- a second porphyry system associated with sericite-chlorite-anhydrite alteration (SCC) with significant mineralization of Cu-Au-Ag-(Mo);
- a high-sulphidation epithermal system with Cu-Au-Ag-As-Sb developed in Miocene volcanic rocks and subvolcanic intrusions with enargite-pyrite structures, and finally;
- an epithermal intermediate sulphidation, “Base Metal Carbonate” system with Au-Ag-Pb-Zn-Cu mineralization associated with late stage rhyolite stocks and flow domes of Upper Miocene age (8.7-8.5 Ma).
An intense alteration of the quartz – sericite – pyrite ± tourmaline is developed in the dykes and the surrounding wall rocks, leaving only relicts of propylitic and potassium alteration. The dykes also have weak Cu-Au-Ag-(Mo) mineralization in the form of pyrite – chalcopyrite ± bornite disseminations and type “B” and “D” veinlets. The intrusive rocks and associated breccias are affected by an intense sericite-chlorite-anhydrite alteration (SCC from Sillitoe, 2010) with significant Cu-Au-Ag-(Mo) mineralization in the form of disseminations, veinlets and breccia matrix replacement cement.