Tin-bearing greisens of Mount Bischoff, northwestern Tasmania, Australia

Abstract

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 beta -form quartz + K-feldspar dike material through muscovite + fluorite + or - 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 + or - 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 degrees to near 500 degrees C, with the greisen being produced between approximately 340 degrees to 420 degrees C 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, CO 2 , 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 1 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.