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 Riv er South, and Balcooma) deposits in eastern Australia were analyzed us ing a proton microprobe to determine trace element abundances. In pyri te, trace elements can be divided into three groups according to the m ost likely occurrence of the element: (1) elements that occur mainly a s inclusions (Cu, Zn, Pb, Pa, Pi, Ag, and Sb), (2) elements that occur as nonstoichiometric substitutions in the lattice (As, Tl, Au, and po ssibly Mo), and (3) elements that occur as stoichiometric substitution s for Fe (Go and Ni) or S (Se and Te). Hydrothermal and metamorphic re crystallization cleans pyrite of group 1 and group 2 elements, but doe s not appear to affect the concentrations of group 3 elements. Collofo rm 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 chalcopyr ite include Pb, Pi, Zn (?), and Ba. The occurrence of As and Sb is unr esolved, although consistently high values of As in some samples sugge st that As may substitute into the lattice of chalcopyrite. Elements t hat 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, C u, occur commonly as inclusions in sphalerite. Lattice substitutions i n 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 a re 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 mas sive 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 domi nant aqueous S and Se species in volcanogenic fluids (1 m NaCl 0-2 uni ts acid; Sigma H2S > Sigma SO42-) above 200 degrees C. Under these con ditions, pyrite Se levels are governed by FeS2 + 2H(2)Se(aq) double le ft 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 tempe rature. 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. Fl uids that deposited pyrite in Cu-poor zones had Sigma Se/Sigma S ratio s below 1 x 10(-6), which is typical of evolved seawater, with minimal magmatic input (either from magmatic volatiles or from leached volcan ic rocks) of Se and S. Fluids that precipitated Cu-rich stringer ore h ad Sigma Se/Sigma S ratios of 0.05-4 x 10(-4), which is consistent wit h a significant (>10%) magmatic component. These interpretations are c onsistent with previous interpretations based on S isotopes (Ohmoto et al., 1983).