Economic Geology Vol. 94, 1999, pp. 123-128 ARSENOPYRITE COMPOSITIONAL VARIATION OVER VARIABLE TEMPERATURES OF MINERALIZATION, OTAGO SCHIST, NEW ZEALAND L. C. KF, nn,D. Cn^w? aND J. H. YOUNGSON Geology Department, University of Otago, PO. Box 56,Dunedin, New Zealand Abstract Compositions of arsenopyrite coexisting with pyrite from eight different mesothermal gold-beating systems in theOtago Schist fall mainly into a narrow band of 30 ñ i at.percent As. Intragrain and intergrain composi- tional variations, combined with analytical uncertainty, preclude resolution ofdifferences in arsenopyrite com- positions between samples ofless than i at. percent As. Only one ofthe vein systems has an arsenopyrite com- positional range that extends below 28 at.percent As. Arsenopyrite from a pyrrhotite-bearing assemblage has compositional range extending above 32 at.percent As. Thevein systems have formed over a wide range of temperatures between about 200 øand 400øC. It isnot possible to resolve the influence of temperature onar- senopyrite compositions for most samples. Sulfur and oxygen activity of the hydrothermal fluidmay be re- sponsible for thelowdegree of arsenopyrite compositional variation. Introduction Arsenopyrite isa common constituent ofhydrothermal gold deposits, and its presence and composition can place limits on physicochemical conditions offormation ofa deposit (Barton, 1969; Kretschmar andScott, 1976; Scott, 1983; Sharp et al., 1985). Arsenopyrite most commonly coexists withpyrite, and thestability field ofthis assemblage isgoverned by: FeS2 + As (s) = FeAsS + 1/2S2 and: Fef = FeS + 1/2Ss Equilibrium constants for these equations are dependent on temperature, sulfur activity (as,,. Igas/) and, toa lesser extent, pressure (Sharp et al.,1985). Thearsenopyrite-pyrite assem- blage has been proposed as a geothermometer on this basis (Kretschmar and Scott, 1976; Sharp et al.,1985), particularly when there is some independent control on as,,. (g•/ (Scott, 1983). Phase equilibria thatconstitute the arsenopyrite geother- toometer have been calibrated in terms of temperature and as2 lgas/by theory, experiment, and empirically with natural ex- amples, and a degree of consistency has been obtained (Bar- ton, 1969; Kretschmar andScott, 1976; Sharp et al., 1985). However, thegeothermometer has not been tested fora wide range of different temperatures in a single uniform host rock in which hydrothermal fluid compositions (including as2 (g•s/) might beexpected todisplay less variation than between com- pletely different terranes. In particular, thegeothermometer has had littleexamination below about 300øC, andSharp et al. (1985) suggest thatdisequilibrium limits itsusefulness at such low temperatures. Theobject of thepresent study was to compare arsenopyrite compositions froma selection of veins formed in different structural settings, and at different mineralization temperatures (<200 ø to >350øC) withinthe same host rock, theOtago Schist of NewZealand. Otago Schist The Otago Schist is a Mesozoic metamorphic bek formed during collision andamalgamation of metasedimentary ter- ranes of broadly similar composition. Mostof the exposed t Corresponding author: email: dave.craw@stonebow. otago.ac.nz portion of the beltwas extensively deformed andrecrystal- lized under pumpellyite-actinolite or greenschist facies con- ditions, and hasbeenthoroughly reconstituted. The host rocks were quartzofeldspathic (Torlesse terrane) or vol- canogenic (Caples terrane) graywackes withminor interca- lated mafic volcanics, especially in thewest (Aspiring litho- logic association; Fig. 1). The resultant metasedimentary schist is approximately uniform in composition over several thousand square kilometers, withonly minor compositional differences reflecting original grain size and provenance vari- ations, and some metamorphic segregation in greenschist fa- cies rocks. Minorhematite (lower grade), magnetite (higher grade), or pyrite occuras metamorphic minerals, with pyrrhotite in some rare graphitic layers. Arsenopyrite-bearing veins Arsenopyrite accompanies gold in quartz veins cutting the Otago Schist in a wide variety of structural settings. These veins formed fromfluids thatwereof metamorphic origin, witha possible meteoric component (Paterson, 1986; Craw and Norris, 1991). Theveins are found right across the schist belt(Fig.1), and their nature and origin have been summa- rizedby Crawand Norris(1991)and Craw(1992). Veins formed nearthe brittle-ductile transition under essentially greenschist facies conditions occur in shear zones which either lie subparallel tothe pervasive schistosity (Macraes de- posit; McKeag and Craw, 1989) or cutacross the schistosity (Invincible Vein;Hay and Craw,1993). These veins have been deformed partly in a ductile manner, and have green- schist facies alteration mineralogy very similar to that of the host rock, including albite in the Invincible Vein, as well as coexisting arsenopyrite and pyrite. Late Cenozoic uplift from middle crustal depths ofOtago Schist greenschist facies rocks has resulted in formation of arsenopyrite-bearing late-meta- morphic veins in crosscutting fractures withpyrrhotite, bi- otite, chlorite, andminor gold in the Callcry valley (Fig. 1 inset; Crawet al., 1987). Mineralization temperatures for these veins werein the range of typical greenschist facies processes (ca.300ø-400øC; Yardley, 1982). Fluidinclusion homogenization temperatures for the Callcryveinsare around 300øC, which is a minimum formation temperature 0361-0128/99/2046/123-656.00 123