Abstract

The massive sulfide deposit at Perkoa in Burkina Faso is the only one of its kind in the Paleoproterozoic Birimian Supergroup of West Africa. The estimated ore reserves are 5.7 Mt with 18.2 percent Zn, 10 percent BaSO4, 0.06 percent Pb, and 26 ppm Ag (50 ppm Cu). The deposit consists of a main orebody, which contains >90 percent of the reserves, and various satellite orebodies, hosted by tuffite and dioritic to granitic dikes, and structurally overlain by andesite, cumulate-rich basic rocks, and carbonaceous schist. The semitabular main orebody is 400 m long and 450 m wide, and has an average thickness of 10 m, whereas the satellite orebodies have an average thickness of 4 m. The tuffaceous rocks are intruded by a quartz diorite in the structural footwall (stratigraphic hanging wall), which has an U-Pb zircon age of 2175 ± 1 Ma. The main orebody is subparallel to the steeply dipping intrusive contact and is an average distance of 15 m from it. The satellite orebodies are discontinuous but also show a preferred orientation subparallel to the intrusive contact, on average 75 m stratigraphically below the intrusion.

Massive sulfide ore is mainly composed of Fe-rich sphalerite (30%), pyrite (25%), barite (10%), hexagonal pyrrhotite (5%), magnetite (5%), and white mica. Quartz is mainly present as host-rock relics. Significant deposition of hydrothermal quartz occurred only in the structural footwall of the deposit, where the main orebody is close to the quartz diorite (0.5 m away). Albite, (Ba, K) feldspar, Ba-rich biotite, chlorite, tourmaline, andradite-rich garnet, ilmenite, rutile, titanite, and galena are subordinate. Chalcopyrite, arsenopyrite, Ag-rich tetrahedrite, and molybdenite occur as trace minerals in the massive sulfide ore. An extremely altered quartz-microdiorite dike with disseminated zincian spinel + magnetite ± Fe-poor sphalerite occurs adjacent to the massive, Fe-rich sphalerite ore of one of the satellite orebodies.

The sulfur isotope compositions of sphalerite and pyrite from the main orebody have narrow ranges from –0.5 to 0.4 per mil and 0.4 to 1.0 per mil δ34S, respectively. These values are close to those of sulfur from mantle-derived rocks. Barite has δ34S values of 9.8 to 10.6 per mil. Pyrite and pyrrhotite in the sedimentary country rock of the deposit have a typical isotopic range of sulfides deposited in an oceanic environment with an input from sulfate-reducing bacteria (δ34S values of –7 to –22‰). A contribution of sedimentary sulfur can be ruled out for the massive sulfide ore.

The magmatic rocks in the Perkoa area have a juvenile character according to the Nd-Sm isotopic data, like most Birimian rocks, but the Perkoa basalts show the influence of a subduction component in various discrimination diagrams that incorporate Ta (or Nb) and Th. Birimian basalts from other areas have geochemical signatures of MORB or oceanic, within-plate basalts.

The massive sulfide ore has been metamorphosed during a regional thermal event under predominantly isotropic stress, probably 100 to 200 m.y. after the emplacement of the quartz diorite. Sphalerite and magnetite were remobilized, whereas pyrrhotite formed at the expense of pyrite. Peak metamorphic temperatures of >460°C at pressures of >1 kbar are indicated by almandine-rich garnet, the presence of andradite, and geothermometers involving arsenopyrite and pyrrhotite. Due to the metamorphic modifications of the ore and host rock, however, it is not possible to establish the age relationships between the main ore-forming event and granitoid intrusions. As a result, it is not known whether ore deposition took place close to a paleosea floor (i.e., an exhalative massive sulfide environment) or in a subvolcanic environment (i.e., a skarn).

You do not currently have access to this article.