The large Mount Bischoff tin deposit occurs within an inlier of Precambrian sedimentary rocks which are surrounded by lower Paleozoic and Tertiary sedimentary and volcanic rocks. Tin is contained within a network of altered Devonian porphyry dikes (endogreisens), replaced dolomite (exogreisen), mineralized hydrobreccias, thin veins, and alluvials.Greisen-style alteration of porphyry dikes is zoned inward and downward from nearly fresh /%form quartz + K-feldspar dike material through muscovite + fluorite _ tourmaline greisen and topaz + quartz greisen to quartz greisen. Tin grades of up to 2 wt percent occur within endogreisen, with higher grades occurring near the topaz + quartz-quartz greisen contact.Replacement of dolomite by greisen assemblages and greisen-style alteration of porphyry dikes were cogenetic. The mineral assemblages and the spatial distribution of greisen minerals, such as mica, topaz, and tourmaline, in endo-and exogreisens are similar. Topaz, quartz, fluorides, cassiterite, and tourmaline occur in endo-and exogreisen, although the proportion of fluoride minerals (sellaite and fluorite) is high where dolomite is replaced. Within exogreisens the highest grades of Sn occur at the contact between topaz-bearing endo-and exogreisens, grades obtained from drill core range up to 22 wt percent (1. 3-m intersection).The greisens were overprinted by assemblages containing phyllosilicates (chlorite, talc, phlogopite, serpentine, phengite), carbonates (magnesite, siderite, dolomite), abundant sulfides (pyrrhotite _ pyrite, sphalerite), late fluorite, and later argillic alteration. Only limited quantities of dolomite were replaced by these later postgreisen assemblages.Fluid inclusion temperatures range from 90 ø to near 500øC, with the greisen being produced between approximately 340 ø to 420øC or higher and the later overprints at progressively lower temperatures. Salinities at the greisen stage of mineralization were high, with measurements commonly between 30 to 40 percent total dissolved salts. Ca-Na-K-Fe-Mn chlorides, possible fluorides, CO2, and hydrocarbons(?) were deduced as components of the fluid phase.Lesser amounts of dissolved Si, Al, and B and rare Ti, Ca, La, P, Sn, Cu, and S were inferred from daughter mineral identifications.The deposit was formed from acidic and relatively reduced hydrothermal solutions in subvolcanic conditions, at a depth of approximately i km, above a leucogranite pluton (granite inclusions exist in the dikes). High-level brecciation was cogenetic with porphyry dike intrusion and both occurred in several separate intrusion-mineralization cycles.
Endogreisens which replace K-feldspar-quartz dykes in a Devonian (360 Ma) tin deposit at Mt Bischoff, north-west Tasmania, formed from the interaction of unusual solutions, probably derived from an underlying leucogranite pluton, porphyry dykes and limited quantities of local dolomitic country rock components. The intensity of greisenization and pH of the solutions increase inward to the greisenized dykes' cores and downward. The following types of greisen assemblages indicate increasing degrees of greisenization: 'sericite' muscovite + quartz * tourmaline * fluorite, topaz + quartz 2 tourmaline ? fluorite, weberite, prosopite, ralstonite, Ca-ralstonite; and quartz ? topaz ? fluorite. Where the solutions interacted with dolomite, exogreisens consisting of topaz-or tourmaline-bearing assemblages were formed. The greisens were subsequently overprinted to varying degrees by siderite, sulphides and hydrous silicates (talc, serpentine, chlorite, micas).The temperature during greisenization ranged from 180 to 414"C, based on fluid inclusions in topaz, quartz, fluorite, sellaite and cassiterite. The main greisen-forming event occurred at temperatures of 360*20"C. The fluids boiled intermittently. Their salinities ranged from 31.5 to 38.9 wt% total dissolved salts, consisting of Ca-K-Na-Fe-CI? hydrocarbon species. Fluid inclusion data indicate that only 0.5-1.5 km of cover were present above this deposit at the time of formation.The greisenized dykes were intruded by and intrude different stages of breccias. The breccias consist mainly of country rock and greisenized dyke fragments, with rock-flour and later tourmaline alteration. The Mt Bischoff greisen system is possibly part of a 'porphyry tin' style deposit formed at near-surface conditions (0.5-1.0 km).
Abstract:The Caribou Zn-Pb-Cu-Ag volcanogenic massive sulphide deposit located in northeast New Brunswick represents a significant base-metal resource in the Bathurst Mining Camp. Zinc, Pb and Cu are the primary resources that are being extracted from this deposit; however, Au and Ag are important by-products that could help offset costs. This study used mineral liberation analysis supported further by in situ laser ablation inductively-coupled plasma-mass spectrometry methods to document variations in Au and Ag distribution between and within sulphide minerals. The variations in Ag and Au distribution provide critical inputs to the optimization of mineral processing design. The greatest influence on Au recovery at Caribou is the proportion of Au hosted in arsenopyrite and pyrite; consequently, considerable Au will report to the tailings. Silver recovery at Caribou is highly affected by the proportion of Ag hosted in galena and tetrahedrite-tennantite. Proximal to the vent complex, Ag values are primarily hosted in galena, whereas further from the vent complex, Ag values are likely primarily hosted in tetrahedrite-tennantite. Galena Ag values will report mostly to the Pb concentrate, while tetrahedrite-tennantite Ag values will report to the Cu concentrate.
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