Abstract

The Fire Tower zone at Mount Pleasant contains geologically complex deposits that are associated with two distinct phases of granite; an older fine-grained granite associated with tungsten-molybdenum orebodies and a younger granite porphyry to which tin and polymetallic zones appear to be related. Based on potassium-argon and rubidium-strontium isotope analyses, both phases of granite and their associated deposits were emplaced at approximately 340 to 330 m.y. ago. Geochemically, both phases are silica rich, aluminous, potassic, and fluorine rich.Porphyry tungsten-molybdenum orebodies occur mainly in Fire Tower breccia and, to a lesser extent, in quartz-feldspar porphyry country rock and the fine-grained granite which underlies the breccia. The orebodies consist of mineralized fractures, quartz veinlets, and disseminations in breccia matrix. Wolframite and molybdenite, the principal ore minerals, occur together with abundant fluorite and arsenopyrite and minor amounts of bismuth and bismuthinite. Associated greisen-type alteration consists of quartz + topaz + sericite + fluorite, which grades outward through an assemblage of quartz + biotite + chlorite + topaz into propylitic alteration, mainly chlorite + sericite. Crosscutting relationships between mineralized fractures and veinlets indicate that mineralization occurred in multiple stages, the earlier stages being more tungsten-rich and the later stages more molybdenum-rich.Tin-bearing polymetallic veins and replacement bodies, in many places spatially related to granite porphyry, are superimposed on the porphyry tungsten-molybdenum orebodies and surrounding rocks. They consist mainly of chlorite, abundant fluorite, and complex assemblages of disseminated to massive sulfide and oxide minerals. Some cassiterite-rich, relatively sulfide-poor zones occur also within greisen-altered granite porphyry. The relationship of these zones to the polymetallic veins and replacement bodies is not clear. Cassiterite-rich greisen occurs typically at deeper levels and may represent either the roots of the higher level polymetallic zones or an entirely separate stage of mineralization.The contrast in the styles of mineralization between porphyry tungsten-molybdenum ore-bodies on the one hand, and tin and polymetallic zones on the other, is striking. Formation of the porphyry deposits appears to be related to the development of high fluid pressures associated with crystallization of the underlying fine-grained granite magma and subsequent fracturing and breccia formation. The tin and polymetallic zones indicate not only a significant change in the composition of the ore-forming fluids but also formation at fluid pressures that were either too low or too localized to cause extensive fracturing and brecciation.

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