The use of trace elements in pyrite and carbonate minerals as a petrogenetic and exploration tool for carbonate-bearing Archean VMS: example from B26, Abitibi, Canada
摘要
Volcanogenic massive sulfide (VMS) deposits are important sources of Zn and Cu and their kilometric-scale alteration halo, composed of chlorite-sericite, is commonly used in exploration. In some cases, VMS are also associated with widespread, semi-concordant carbonate alteration (Mattabi-type deposits), but the origin of the carbonate minerals is poorly constrained, and vectoring tools using carbonate alteration is under-developed. The Archean B26 VMS prospect (Selbaie district, Abitibi greenstone belt, Canada) comprises Cu-Zn-Ag-Au mineralization which is spatially associated with a multi-km, stratabound carbonate alteration within the hanging wall rhyolitic tuffs. Zonation of the carbonate alteration is typical of Mattabi-type VMS, with siderite proximal to the mineralization, surrounded by a large ankerite halo, and distal calcite. We propose that the carbonate alteration, proximal to the mineralization, is volcanogenic in origin - formed by degassing of CO2-rich fluids from the synvolcanic intrusion that mixed with the sub-seafloor hydrothermal convection cells in a shallow water environment (< 500 m). Carbonate minerals hosted in the mineralization have negative Eu anomalies (type 1), indicative of magmatic derived fluids, whereas carbonate minerals from the proximal alteration have positive Eu anomalies (type 2), more typical of VMS fluids (i.e. modified seawater). The trace element pattern of pyrite from the carbonate alteration reflects that of the mineralization suggesting that they are part of the same mineralizing system. The semi-volatile (Ag-Se-Bi-Tl) and Co contents of pyrite show a progressive depletion away from B26 as temperature decreases and dilution with seawater increases. The most distal carbonate alteration precipitated from seawater trapped in the porous rhyolitic tuffs. These carbonate minerals (type 3) have a REE + Y pattern typical of diagenetic carbonate, and pyrite is enriched in Mn and V also suggestive of sedimentary origin. Thus, the in-situ trace element content of carbonate minerals and pyrite are effective petrogenetic and exploration tools for carbonate-bearing VMS.