Chemical evolution and zonation of massive sulfide deposits in volcanic terrain

Base metal massive sulfide deposits within felsic volcanic piles are typically zoned from a copper-rich base to a zinc [plus-minus sign] lead-rich top. In the Noranda and Matagami mining camps in the Archean Superior Province of eastern Canada chalcopyrite is concentrated with pyrrhotite-pyrite [plus-minus sign] magnetite in the base of the massive sulfide ore lenses or xvithin a stringer zone below the lenses. Passing stratigraphically upward the pyrite-pyrrhotite ratio generally increases, magnetite content decreases, and the tops of the orebodies are characterized by banded pyrite-sphalerite mineralization. The Cu-Zn-Pb-bearing massive sulfides in the kuroko district of Japan exhibit similar mineralogical zonation except for the general lack of magnetite and pyrrhotite and the abundance of sulfate gangue minerals. Massive pyritic ores within ophiolitic environments (the Cyprus type) are copper rich with nilnor zinc and negligible lead. They lack appreciable pyrrhotite and magnetite and are poorly zoned. All three ore types (Archean, kuroko, Cyprus) overlie pipes of hydrothermal alteration and are considered to have been deposited at or just below the sea-water-rock interface. They occur toward the center of the volcanic pile and are termed proximal ores. Banded sulfide ore lenses and metalliferous sediments that are not underlain by strong footwall alteration were probably deposited within restricted basins directly onto the sea floor from metal-bearing brines which have moved some distance from their fumarolic outlet. These ores are commonly Pb-Zn-rich types, well banded, occurring within a mixed volcanic-sedimentary environment, and are termed distal ores. Experimental data on the solubility of chalcopyrite, sphalerite, and galena indicate that there are two distinct environments in which Cu, Pb, and Zn may be transported to form massive sulfides: I. Reduced, high temperature (>275 degrees), acid to neutral pH environment. II. Oxidized, any temperature, any pH environment. Consideration of mineral-solution equilibria in the Fe-S-O system suggests that proximal Archean-type massive sulfides are deposited from high temperature, mildly acid, and reduced chloride-rich solutions (within environment I) which mix with sea water at the top of the volcanic pile. Mixing results in an increase in solution pH, fo2, and [Sigma]S accompanied by a drop in temperature and leads to the precipitation of chalcopyrite-pyrrhotite ([plus-minus sign] pyrite [plus-minus sign] magnetite) assemblages in the stringer and lower massive ore zones below the sea-water-rock interface, whereas pyrite-sphalerite [plus-minus sign] galena assemblages are deposited around the hydrothermal vent directly onto the sea floor. Solutions exhaled from the submarine vent move along the sea floor and deposit silica and pyrite to form the laminated cherty tuffs which are commonly associated with the ores. The presence of pyrite rather than hematite or magnetite in these sediment suggests that the exhaled solution probably contained an excess of reduced sulfur (Sr) over oxidized sulfur (So). This may have been a consequence of the reduced nature of the Archean ocean. Kuroko-type ores were probably deposited in a similar geological environment from solutions more rich in lead at a higher oxygen fugacity level. Whereas proximal massive sulfides develop under a trend of increasing So/Sr as the solution passes from environment I to II, distal ores on the other hand develop under a trend of decreasing So/Sr.