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Zn-Pb-Ag ores at Nanisivik, northwest Baffin Island, are hosted by upper Proterozoic laminated dolostone of the Society Cliffs Formation. Sulfide orebodies containing pyrite, sphalerite, galena, sparry dolomite, and pyrite pseudomorphous after marcasite are characterized by well-banded ore textures considered to have resulted from the progressive replacement of carbonate wall rock. Textural and mineralogical variations enable the Main orebody to be divided into three vertical and four horizontal ore zones. The upper lens of the Main orebody may be further subdivided into six texturally distinct, laterally extensive mine units.In the eastern upper lens, where textural variations are best developed, fluid inclusions indicate that most ore mineralization took place over the temperature range of 165 degrees to 210 degrees C and involved a brine containing 20 to 38 wt percent NaCl equiv. The sulfur isotope compositions of main- and late-stage pyrite crystals range from (delta 34 S = 27.4 to 28.0 per mil. Iron contents of sphalerite vary from 14 to 0 mole percent FeS, corresponding to well-developed color zoning, and constrain the oxygen activity of the ore fluid to 10 (super -46) to 10 (super -41) at 200 degrees C during sphalerite precipitation. Interbanded primary marcasite and sparry dolomite gangue indicates that the ore fluid fluctuated around pH = 5.Ore formation is modeled based on processes involving in situ reduction of sulfate by hydrocarbons and minor H 2 S encountered at the site of ore deposition. The location and configuration of the orebodies was probably controlled in part by the former position of a horizontal fluid interface. Banding is likely the result of repetitive sulfate reduction, metal precipitation, wall-rock dissolution, and dolomite recrystallization in response to the pulsatory influx of ore fluid. Textural and mineralogic variations probably resulted from slight variations in the availability of reduced sulfur.

A mineral assemblage consisting of calcite–tremolite–quartz–clinozoisite–diopside–muscovite has been found associated with a small, stratiform pod of galena–sphalerite–chalcopyrite–pyrite–pyrrhotite. This alteration assemblage resembles that of a skarn, although lacking, significantly, some typical minerals including garnet, wollastonite, and vesuvianite. Limits on possible condition of T, Pfluid, and [Formula: see text] can be estimated for the formation of the assemblage. The assemblage must have formed in a water-rich fluid, as [Formula: see text] is sharply restricted by the reaction 2 clinozoisite + CO2 = calcite + 3 anorthite + H2O. In Pfluid–T space, the assemblage is stable between the univariant curves for the assemblages tremolite + calcite + quartz + diopside at fixed [Formula: see text] and clinozoisite + anorthite + grossular + quartz + calcite. These curves bracket the kyanite–sillimanite boundary. As kyanite and sillimanite occur in the wall rocks, the minimum Pfluid–T are those of the aluminosilicate triple point. Measurement of sphalerite–sphalerite/galena dihedral angles in the thermally annealed area yielded a calculated temperature of 390 °C. Metamorphic conditions suggest that a sequence of progressive contact metamorphic reactions of an original talc and (or) chlorite–quartz–calcite assemblage produced the presently observed assemblage. Proximity to a later granitic batholith is believed to have brought about these changes by reactions in the original alteration mineral assemblage.

A hydrothermal recrystallization method was employed to determine the phase relations in the Fe-S system involving pyrrhotite between 350 degrees and 115 degrees C. The superstructures of pyrrhotite, as designated by Nakazawa and Morimoto (1970, 1971), are considered to be thermodynamic phases. The solvus between pyrite and hexagonal (1C) 1 pyrrhotite continues below 325 degrees C along the slope established by Toulmin and Barton (1964) to 308 degrees C, where a discontinuity marks the upper stability of the MC superstructure. The solvus then continues along a similar slope to 262 degrees C where, with the onset of the NA superstructure, there is a discontinuity and the curve reverses slope sharply. At 254 degrees C, monoclinic (4C) pyrrhotite becomes stable in the peritectoid reaction hexagonal (NA) pyrrhotite (47.30 at. % Fe) + pyrite = monoclinic (4C) pyrrhotite (47.25 at. % Fe). Monoclinic pyrrhotite exists over a range of compositions, the maximum being 46.4 to 46.9 at. percent Fe at 115 degrees C. Hexagonal pyrrhotite is separated from monoclinic pyrrhotite by a narrow miscibility gap down to the upper stability limit of the NC superstructure at 209 degrees C. Below 209 degrees C, the miscibility gap widens, with hexagonal (NC) pyrrhotite of 47.3 at. percent Fe coexisting with monoclinic pyrrhotite of 46.9 at. percent Fe. At the sulfur-rich limit of the stability field, monoclinic pyrrhotite coexists with pyrite. The phases greigite, smythite, and marcasite were not encountered.Our results are in reasonable agreement with those obtained previously by hydrothermal synthesis methods, but they are in disagreement with some results obtained in "dry" experiments which have produced metastable phase assemblages. The phase diagram is compatible with natural pyrrhotite occurrences and in particular with those from geothermal boreholes at Broadlands, New Zealand, from which samples were recovered at known quench temperatures. On the other hand, our studies cast considerable doubt on the validity of the "pyrrhotite geothermometer" as applied to ores.

The Proterozoic Willyama Complex hosts metamorphosed massive lead-zinc-silver sulfide deposits (including the main Broken Hill lode) distributed across three prograde regional metamorphic zones ranging from middle amphibolite to granulite facies. FeS contents of sphalerites coexisting with pyrrhotite are consistent within a single sample but range widely within a deposit and across the complex showing no correlation with metamorphic grade. Primary (metamorphic) pyrrhotite was iron rich and now consists of hexagonal pyrrhotite with exsolved troilite. Although most primary pyrrhotites were altered to monoclinic pyrrhotite, pyrite, marcasite, or mixtures of these minerals during retrograde or postmetamorphic events, sphalerites were not affected significantly. Closely spaced samples from No. 3 Lens at North mine in the main Broken Hill lode indicate that the domains of sulfide equilibria are less than 30 cm and may not be much larger than the dimension of a polished section.The partitioning of FeS between sphalerite and hexagonal pyrrhotite is a function of P, T, and a FeS . The determination of a FeS for the bulk compositions of pyrrhotites together with analyses of coexisting sphalerites permits an estimation of pressure during regional metamorphism provided that the pyrrhotites have not been altered and the temperature of sulfide equilibration is known. Four samples from the main Broken Hill lode in granulite rocks give mean pressures of 8.1 + or - 0.6 kb for an assumed temperature of final sphalerite equilibration of 650 degrees C, as suggested from sulfur isotopic data, or 9.9 + or - 0.7 kb at 800delta C which is the estimated peak metamorphic temperature.

Phase relationships involving sphalerite in the Zn-Fe-S system have been estimated below 300 degrees C by combining new experimental data with microprobe analyses of sphalerites from naturally occurring, low-temperature assemblages. Sphalerite compositions from the various sources are in agreement and the univariant, three-phase boundaries derived from them are compatible with the arrangement of univariant assemblages in the Fe-S system as determined by Kissin and Scott (1972). These results show that sphalerite composition varies in response to a FeS in similar manner to its behavior at higher temperatures producing lower FeS contents in sphalerites in the more sulfur-rich iron sulfide assemblages.