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Department of Earth Sciences, Laurentian U niversity, Volume MSc, p.78 (2013)


The Potter Cu-Zn volcanogenic massive sulfide (VM S) deposit is a bimodal-mafic deposit located in the eastern part o f the 2720-2710 Ma volcanic episode o f the A bitibi greenstone belt in Ontario. The deposit is hosted by geochemically unique Fe-, Mg-, Ni-, Cr-rich mafic volcaniclastic units, derived through submarine fire-fountain pyroclastic eruptions, which are enclosed entirely within a dominantly ultramafic flow succession; an unusual setting for Archean bimodal-mafic VMS deposits. The mineralization consists o f 11, stacked, tabular lenses o f semimassive to massive sulfide that are predominantly composed o f pyrrhotite, sphalerite and chalcopyrite.<br/>Hydrothermal alteration associated with the Potter mine, manifest primarily within the open-space matrix o f the volcaniclastic units and by alteration o f lapilli, is divisible into four m atrix-infill mineral assemblages that, with decreasing distance from massive sulphide, include: 1) actinolite-chlorite-albite, 2) chlorite-clinopyroxene, 3) albite-calcite, and 4) chlorite. The actinolite-chlorite-albite assemblage, which has a mineralogy that is similar to a background greenschist regional metamorphic assemblage, records a mass increase in K 20 , MnO, Cu, Nb, Rb, Sr, Zn and S, and<br/>depletion in Na20. The chlorite-clinopyroxene assemblage, which also contains albite, is associated with a mass increase in S, K.20 , MnO, Cu, Nb, Rb, Sr and Zn and depletion in CaO, MgO and Pb. Clinopyroxene in this assemblage results from the metamorphic recrystallization o f a Ca-enriched alteration zone. The albite-calcite assemblage is characterized by widespread replacement o f the matrix by variable amounts o f albite, calcite and minor chlorite, with a partial chloritization o f the fragments. Mass gains in S, K 20 , MnO, Na20, Cu, Nb, Rb, Sr and Zn, and losses in CaO andMgO characterize this assemblage when albite is the dominant matrix phase, whereas mass gains in S, CaO, K 20 , MnO, Co, Cu, Nb, Rb, Sr and Zn and losses in MgO, Na20 , Ti0 2, Ga and Pb characterize the assemblage when the matrix is rich in calcite. The chlorite assemblage is restricted to the immediate margins o f the massive sulfide lenses and is characterized by a mass change increase in S, FeO,, K 20 MgO, MnO, Co, Cu, Nb, Pb, Rb and Zn, and a decrease in CaO, Na20 , P2Os and Ni.<br/>The matrix mineral assemblages within VMS hosting volcaniclastic succession formed from fluids derived from 2 separate parts o f a VMS hydrothermal system. Only the chlorite assemblage shows a spatial association with massive sulfide mineralization, and is mineralogically and compositionally identical to chlorite (sericite) alteration associated with bimodal-mafic VMS deposits. The chlorite assemblage is interpreted to be direct product o f the interaction between glass-rich lapilli and a high temperature (250-350“C), hydrothermal fluid, presumably evolved seawater that moved laterally through the volcanic succession to produce the chlorite assemblage and to form the massive-sulfide lenses. A significant difference between the chlorite alteration at Potter and typical VMS deposits is its distribution. At Potter, the chlorite alteration is stratabound, which suggests that the deposit may be located at the margin or on the fringe o f a discordant alteration zone.<br/>The actinolite-chlorite-albite, chlorite-clinopyroxene and albite-calcite mineral assemblages differ from discordant alteration assemblages documented in other bimodal-mafic VMS deposits in that they are not assemblages that developed within the rock, but in an porous matrix, and they do not show spatial or compositional variations in whole rock modal mineralogy or in mass change that are typical o f chlorite and sericite alteration. These matrix mineral assemblages represent a semiconformable, stratabound background alteration that formed from progressively heated seawater which was drawn into and moved laterally within the porous and permeable volcaniclastic lithofacies within the lower temperature (150-200°C), upper part o f the Potter VMS hydrothermal system. Systematic, spatial variations in the mineral chemistry o f primary hydrothermal matrix minerals and subsequent metamorphic minerals, their modal mineralogy, and associated mass change indicate the influence o f a thermal gradient within this seawater dominated system that increased in temperature toward the Potter mine, and that these assemblages formed pre- and/or syn-mineralization.