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CALDERITE-RICH GARNET AND FRANKLINITE-RICH SPINEL IN AMPHIBOLITE-FACIES HYDROTHERMAL SEDIMENTS, GAMSBERG Zn–Pb DEPOSIT, NAMAQUA PROVINCE, SOUTH AFRICA

Marcel Stalder and Abraham Rozendaal

Department of Geology, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa

E-mail address: ar{at}sun.ac.za

Calderite-rich garnet and franklinite–jacobsite-rich spinel occur in amphibolite-facies hydrothermal metasedimentary rocks that stratigraphically overlie the Proterozoic Gamsberg Zn–Pb deposit, South Africa. The minerals are restricted in occurrence to an irregularly developed Mn–Fe-rich unit at the top of the ore-bearing horizon (the Gams Formation), which is interpreted to be genetically related to the underlying sulfide mineralization. The rocks are well laminated on a centimeter to decimeter scale and consist of alternating layers of garnet, pyroxenoid, Mn-rich clinopyroxene and Fe oxide (Zn–Mn-rich ferrite spinel, magnetite, hematite). The garnet contains up to 36% of the calderite component in solid solution with andradite and spessartine, whereas the spinel reaches a maximum franklinite component of 39% in solid solution with magnetite and jacobsite. Associated minor to accessory minerals include manganoan calcite, manganoan and zincian tremolite, Sr-rich epidote, hyalophane, pyrophanite, melanotekite and mimetite. The occurrence and spatial distribution of calderitic garnet, Zn–Mn-rich spinel and hematite constrain oxygen fugacities during metamorphism to conditions close to the HM buffer for the uppermost part of the Gams Formation, and indicate that considerable gradients in f(O₂) and f(S₂) existed within this specific horizon. The preserved assemblages are products of complex reduction and decarbonation reactions of protoliths composed of Mn–Ca-rich carbonate, Fe–Mn oxides and hydroxides, silica and aluminous clay. Under the observed conditions of P, T and f(O₂), the contrasting parageneses are a function of delicate variations in the bulk composition of the precursor sediments. The Mn–Fe-rich and Zn–S-poor nature of these metahydrothermal rocks indicates that they represent a dispersion halo of low-temperature and oxidized fluids that postdated base-metal mineralization. Collectively, the unusual mineralogical and chemical composition of these rocks provides an indication of the spectrum of rock types that may be associated with metamorphosed deposits of base-metal sulfides.

Keywords: calderite-rich garnet, franklinite-rich spinel, pyroxenoids, manganese silicate rocks, Gamsberg deposit, South Africa.

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