The endogranitic tin zone,Mount Pleasant,New Brunswick,Canada and its metallogenesis

Abstract

The Mount Pleasant Sn–W–Mo deposits occur within the margin of an eroded caldera underlain by a Devono–Mississippian intrusive-subvolcanic-eruptive complex of granitic, porphyritic and felsic pyroclastic rocks. The separate North and Fire Tower mineralised zones are centred in volcanic necks defined by hydrothermal breccias within this complex. The oldest of three successive granites in both Zones hosts W–Mo mineralisation, whereas the two younger granites in the North Zone contain various Sn deposits. One of these, the endogranitic tin zone, is an irregular but generally tabular body hosted within a distinctive, highly evolved phase of the third, and youngest granite.

The endogranitic tin zone was formed by successive stages of sericitisation, chloritisation/topazification-silicification, brecciation, dense chloritisation-topazification and late replacement. It contains quartz, chlorite, topaz, fluorite, cassiterite, arsenopyrite, base metal sulphides, pyrite, stannite, wolframite, native bismuth, molybdenite, kaolinite and carbonates. The ore minerals occur about equally in veinlets and disseminations. Cassiterite was deposited in all stages, but more abundantly during brecciation and particularly during dense chloritisation-topazification where it accompanies a distinctive, avocado-green, Fe-rich chlorite. The host, endogranitic tin zone granite, has undergone significant depletions and enrichments in major and minor elements.

Fluid inclusions indicate that the chloritised/topazified-silicified rock formed from early, low salinity dominantly meteoric fluids at ∼200–225°C. Later, saline, magmatic-hydrothermal fluids, probably differentiated in the upper part of the endogranitic tin zone granite, mineralised the brecciated and densely chloritised-topazified rocks at similar temperatures. Latest low salinity meteoric fluids formed the late replacement bodies at 175–185°C. Sn was deposited under acidic conditions at oxygen fugacity of 10⁻³⁹–10⁻³⁴ atm from dominant SnCl₂⁰ and SnF⁺ complexes due to fluid oxidation and neutralisation, and cooling.

Endogranitic tin zone cassiterite has a trace element signature that is intermediate between that of cassiterite from deep-seated greisens and pegmatites, and shallower cassiterite–sulphide and Bolivian porphyry–tin ores. The late replacement cassiterite, however, is high in Nb, Ta, Ti and Fe, and thus has a pegmatitic signature. This, together with their size and distribution within the endogranitic tin zone and elsewhere in the complex, suggests that the late replacement bodies are aborted pegmatites formed when the magmatic-hydrothermal fluids became largely diluted by the late meteoric fluids. Wood tin and colloform cassiterite contain abundant Fe, resembling wood tin in the high-level topaz rhyolites and Russian subvolcanic tin deposits. Sphalerite contains abundant In, as do other high-level, stanniferous polymetallic ores. The endogranitic tin zone at Mount Pleasant appears to be unique, differing from both greisen– and porphyry–tin deposits and no close analogues are known elsewhere.