SILICIFICATION AND METAL LEACHING IN SEMICONFORMABLE ALTERATION BENEATH THE CHISEL LAKE MASSIVE SULFIDE DEPOSIT, SNOW LAKE, MANITOBA

Regionally extensive semiconformable zones of silicified, Fe-Mg metaso matized and epidotized, dominantly mafic, volcaniclastic strata and la va flows are exposed 1 to 2 km stratigraphically beneath the Chisel La ke Zn-Cu massive sulfide deposit, Snow Lake district, Manitoba. The al teration zones occur within a Lower Proterozoic medium-grade regional metamorphic terrane of felsic and mafic volcanics and marine sediments , and lie between subvolcanic tonalite sills and the massive sulfide d eposit. Semiconformable alteration occurs at two main stratigraphic po sitions. The lower zone of silicification in pillowed and massive mafi c lavas is inferred to have occurred close to the sea floor and probab ly was not related directly to formation of the Chisel Lake massive su lfide deposit. The stratigraphically higher zone, which may be spatial ly and temporally associated with massive sulfide deposition, occurs i n a approximately 300-m-thick heterolithic mafic volcanic breccia and wacke unit. This volcaniclastic-hosted alteration is zoned laterally f rom dominantly silicification and epidotization to mainly Fe-Mg metaso matized, garnet-chlorite +/- biotite +/- staurolite rocks nearer the C hisel Lake sulfide deposit. A discordant footwall Fe-Mg alteration zon e directly beneath the sulfide deposit extends toward, and may meet, t he semiconformable Fe-Mg metasomatized zone. Silicification contribute d to partial to complete replacement of volcanic clasts, beds in the v olcaniclastic unit, rocks adjacent to some felsic dikes and pillow int eriors by quartz and sodic plagioclase. Mass balance calculations for silicified rocks and equivalent least altered parts of volcaniclastic beds, dikes, and pillows indicate that SiO2 increased by up to 50 perc ent of its initial value, and Na2O by up to 30 percent. Up to 80 perce nt of the FeO, MgO, CaO, and Zn was removed during silicification. Ele mental fluxes during Fe-Mg metasomatism are generally opposite those c haracterizing silicification and are of comparable magnitude. Epidotiz ation resulted in depletion of Na, total Fe (but increased the Fe2+/Fe 3+ ratio), Mg, Mn, K, Zn, and Ba, and enrichment in Ca and Sr relative to least altered rocks. Almost constant interelement ratios of Ti, Zr , and Al in altered and less altered rocks indicate that these element s were essentially immobile during metasomatism and subsequent medium- grade regional metamorphism. Limited data suggest that the heavy REE w ere also immobile during silicification. The subconcordant silicificat ion in the mafic volcaniclastic unit is interpreted to have formed at subsea-floor depths of 1 to 2 km where a felsic dike swarm and subvolc anic tonalite sills heated Si-rich evolved seawater above the temperat ure of the silica solubility maximum (approximately 340-degrees-450-de grees-C at pressures below 900 bars) causing silica deposition and met al leaching. A portion of the Fe, Mg, and possibly Zn that was leached from the subconcordant silicified zone may have been transported late rally away from this environment, thereby producing the semiconformabl e Fe-Mg metasomatized zones. Cross stratal structures similar to the h ydrothermally altered synvolcanic faults that are known to cut the sem iconformable alteration may represent fluid flow paths from a subconco rdant metal reservoir in the volcaniclastic unit to the sea floor, whe re massive sulfides were deposited. Semiconformable pervasively silici fied zones, particularly those associated with Fe-Mg metasomatized and /or epidotized rocks, are significantly larger exploration targets tha n areas of proximal alteration, and indicate large-scale hydrothermal mass transfer. Zonation of silicification to Fe-Mg metasomatism latera lly within the alteration may provide a vector toward the discordant F e-Mg-enriched alteration zones that commonly underlie volcanic-associa ted massive sulfide deposits.