Application of chlorite-sulfide-oxide equilibria to metamorphosed massive sulfide ores, Snow Lake area, Manitoba

Abstract

The Stall Lake and Anderson Lake massive Cu-Zn sulfide deposits, situated in the Snow Lake area of northern Manitoba, occur in hydrothermally altered felsic volcanic rocks which have been subjected to amphibolite-grade metamorphism. Mineral assemblages from the two deposits contain in addition to chlorite, the f (sub O 2 ) -f (sub S 2 ) buffer assemblage of pyrite, pyrrhotite, and magnetite as well as quartz, aluminosilicate, and sphalerite.In P-T space, the mole fraction of daphnite in a chlorite solid solution may be represented by a series of isopleths of Fe/Sigma R (super +2) calculated at P (sub H 2 O) = P total for f (sub O 2 ) and f (sub S 2 ) buffered by pyrite, pyrrhotite, and magnetite. Calculations were based on an ideal ionic solution model for the daphnite component in chlorite (i.e., a daphnite = Fe (super +2) /Sigma R (super +2) = X daphnite ) and the free energy data for daphnite from Bryndzia and Scott (1987). Chlorite isopleths are very sensitive to changes in ambient oxygen and sulfur fugacity conditions. The very small deviations from average values of daphnite mole fractions in chlorite from the pyrite-pyrrhotite-magnetite-bearing assemblages (X = 0.41 + or - 0.03, n = 3, at Anderson Lake and 0.35 + or - 0.02, n = 8, at Stall Lake) demonstrates that fugacities of both oxygen and sulfur were buffered by pyrite, pyrrhotite, and magnetite.Intersection of chlorite and sphalerite isopleths gives peak metamorphic conditions of 6.5 to 7.0 kb and 580 degrees to 600 degrees C. Such high pressures are incompatible with metamorphic assemblages. The intersection of chlorite isopleths with Holdaway9s (1971) sillimanite-kyanite phase boundary indicates peak metamorphic pressure and temperature to be 5.6 kb and 580 degrees C, in excellent agreement with a previous estimate of 5.5 kb and 600 degrees C by Froese and Gasparrini (1975).Chlorite isopleths in P-T space are very steep and, therefore, sensitive to variations in temperature. P-T-X daphnite diagrams have potential for geothermometry, particularly when combined with other experimentally reversed equilibria defining reactions on a petrogenetic grid.