Samuel Olusegun Akande

Promoted to and hence becoming the first
full Professor of Geology
installed at the Dept. of Geology and Mineral Sciences
University of Ilorin
P.M.B. 1515 Ilorin, Nigeria

The Gays River lead-zinc deposit consists of stratiform bodies and discordant fault controlled vein systems within a Mississippian dolomitic reef that overlies unconformably a Cambro-Ordovician metasedimentary basement and is overlain by Mississippian evaporites.

Detailed underground mapping and laboratory studies suggest 3 main states of evolution: 1) pre-ore evaporite deposition, pervasive reef dolomitization and growth of marcasite, 2) precipitation of ore sphalerite, galena, chalcopyrite and calcite, and 3) post-ore deposition of calcite, fluorite, barite, marcasite, pyrite and selenite. Fluid inclusions homogenize at the following temperatures: sphalerite, 215^oC; ore-stage calcite, 172^oC; post-ore calcite, 142^oC; fluorite, 143^oC, and barite, 137^oC. Salinity of the hydrothermal fluids was approximately 20.4 equivalent weight percent NaCl during deposition of post-ore calcite and fluorite.

Gypsum and anhydrite in the overlying evaporites and barite within the ore, are enriched in heavy sulfur ( 34S +13.1 to +16.5o/oo); ore-stage sphalerite, galena and chalcopyrite show a limited spread of 34S (+8.0 to +13.65o/oo); whereas post-ore marcasite and pyrite give widely scattered light

34S values (-9.7 to -4.6o/oo). The similarities of the 34S values of gypsum, anhydrite and barite with that of Mississippian seawater confirm this as the dominant sulfur source for the sulfates. The heavy sulfur of the ore-stage sulfide minerals is either directly contributed from seawater or from a solution of equilibrium with evaporites. A biogenic source is preferred for the sulfur in post-ore marcasite and pyrite. Mineralized carbonates are significantly depleted in the heavier isotopes of carbon and oxygen with respect to the unmineralized equivalents. Preliminary lead isotope data for galena suggest that the source of the lead is in the Cambro-Ordovician metasedimentary basement.

The present evidence eliminates the possibility that the lead-zinc ores are an early low-temperature diagenetic cement of the dolomitic reef as previously envisaged. Contact relationships and remarkable similarities between the stratiform and vein ores suggest that the discordant veins acted as feeders for the epigenetic conformable mineralization. A model involving deeply circulating brines released during gypsum dehydration best explains the metal leaching, transportation and subsequent deposition of lead and zinc ores as replacements and open-space fillings within the dolomitic reef.

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Pages: 366
Supervisor:  Marcos Zentilli