Abstract

Tin and tin-bearing minerals are prominent in the metamorphosed (amphibolite-granulite facies) Geco Cu-Zn volcanogenic massive sulfide deposit as well as in some of the smaller massive sulfides in the Manitouwadge district. Cassiterite and minor Sn-bearing andradite, epidote, rutile, and phlogopite occur within the massive sulfide ores. Nigerite-24R of composition (Sn, Ti) x (Zn, Mg, Fe, Mn) (sub 8-2x) (Al, Fe) 16 O 32 , with x [asymp] 2, and mixed polytypes of hoegbomite with the general formula (Ti, Sn) x (Fe, Mg, Zn, Mn) (sub 8-2x) (Al, Fe) 16 O 32 , with x [asymp] 1, occur in the footwall rocks which comprise the feeder zone. Hoegbomite is the primary tin-bearing mineral in the districtwide cordierite-gedrite gneisses which represent footwall conformable alteration zones.A relatively high f (sub O 2 ) during metamorphism and also during the formation of the ores accounts for the presence of cassiterite rather than stannite or the simple tin sulfides in the ores. Nigerite and hoegbomite occur only in Ca-Na-deficient Mg-Fe-rich aluminous rocks and are associated with Zn staurolite, corundum, cordierite, sillimanite, (Zn, Fe) spinels, and Fe-Ti oxides.Bulk-rock tin assays systematically increase as the Geco orebody is approached: districtwide cordierite-gedrite gneisses (10-30 ppm), feeder zone (50-330 ppm), and orebody (100-3,000 ppm). Hanging-wall felsic gneisses and nearby mafic gneisses and granites contain less than 3 ppm tin. Small nearby pegmatites contain 2 to 10 ppm tin.From textural evidence, phase equilibria calculations, and the lithologic distribution of tin, a model for the Geco deposit is developed in which the districtwide cordierite-gedrite gneisses represent one segment of the fossil hydrothermal system and in which Cu-Zn and Sn mineralizations are closely associated. Nigerite, hoegbomite, and spinel from Geco occupy distinct and characteristic (Zn, Fe, Mg, Mn) solid solution fields. Hence, these species, which are known to occur as detrital heavy minerals may be useful pathfinders in geochemical exploration for eroded volcanogenic massive sulfide deposits.

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