TRACE-ELEMENTS IN SULFIDE MINERALS FROM EASTERN AUSTRALIAN VOLCANIC-HOSTED MASSIVE SULFIDE DEPOSITS .1. PROTON MICROPROBE ANALYSES OF PYRITE, CHALCOPYRITE, AND SPHALERITE, AND .2. SELENIUM LEVELS IN PYRITE - COMPARISON WITH DELTA-S-34 VALUES AND IMPLICATIONS FOR THE SOURCE OF SULFUR IN VOLCANOGENIC HYDROTHERMAL SYSTEMS

Part I. Pyrite, chalcopyrite, and sphalerite from six volcanic-hosted massive sufide (Mount Chalmers, Rosebery, Waterloo, Agincourt, Dry River South, and Balcooma) deposits in eastern Australia were analyzed using a proton microprobe to determine trace element abundances. In pyrite, trace elements can be divided into three groups according to the most likely occurrence of the element: (1) elements that occur mainly as inclusions (Cu, Zn, Pb, Pa, Pi, Ag, and Sb), (2) elements that occur as nonstoichiometric substitutions in the lattice (As, Tl, Au, and possibly Mo), and (3) elements that occur as stoichiometric substitutions for Fe (Go and Ni) or S (Se and Te). Hydrothermal and metamorphic recrystallization cleans pyrite of group 1 and group 2 elements, but does not appear to affect the concentrations of group 3 elements. Colloform pyrite grains have the highest levels of As and Au (up to 200 ppm), suggesting that rapid precipitation is important in incorporating Au into auriferous pyrite. Elements that occur as inclusions in chalcopyrite include Pb, Pi, Zn (?), and Ba. The occurrence of As and Sb is unresolved, although consistently high values of As in some samples suggest that As may substitute into the lattice of chalcopyrite. Elements that substitute into the lattice include Ag (for Cu), In, Sn and Zn (?) (for Fe), and Se (for S). Lead, Ba Sb, possibly, and in some cases, Cu, occur commonly as inclusions in sphalerite. Lattice substitutions in sphalerite include Fe, Cd, Cu (to 4,500 ppm), Ni, In, Ag, Te, Ga and possibly Mo. In addition, consistently high (2,000-4,000 ppm) levels of As in the Rosebery barite zone may indicate As lattice substitution. Part II. The Se content of pyrite in volcanic-hosted massive sulfide deposits varies as follows: in Cu-poor, Zn-rich deposits, Se levels are low (mainly <5 ppm) throughout; in Cu-rich deposits, Se levels are highest ; (10-200 ppm) in stringer zones and the lower part of the massive sulfide lens, and decrease toward the top of the massive sulfide lens and into peripheral altered rocks. Metamorphic recrystallization does not affect these variations. Although delta(34)S values also vary systematically in individual deposits, no systematic differences were noted between Cu-rich and Zn-rich deposits. H2S and H2Se are the dominant aqueous S and Se species in volcanogenic fluids (1 m NaCl 0-2 units acid; Sigma H2S > Sigma SO42-) above 200 degrees C. Under these conditions, pyrite Se levels are governed by FeS2 + 2H(2)Se(aq) double left right arrow FeSe2 + 2H(2)S(aq), and H2Se/H2S approximates Sigma Se/Sigma S. Calculations using available thermodynamic data indicate that at constant H2Se/H2S, pyrite Se levels decrease with increasing temperature. Differences observed between Cu-rich and Cu-poor zones cannot be caused by temperature changes. The variations can be best accounted for by differences in Sigma Se/Sigma S of the hydrothermal fluids. Fluids that deposited pyrite in Cu-poor zones had Sigma Se/Sigma S ratios below 1 x 10(-6), which is typical of evolved seawater, with minimal magmatic input (either from magmatic volatiles or from leached volcanic rocks) of Se and S. Fluids that precipitated Cu-rich stringer ore had Sigma Se/Sigma S ratios of 0.05-4 x 10(-4), which is consistent with a significant (>10%) magmatic component. These interpretations are consistent with previous interpretations based on S isotopes (Ohmoto et al., 1983).