COMPOSITION OF SPHALERITES FROM BROADLANDS GEOTHERMAL FIELD AND THEIR SIGNIFICANCE TO SPHALERITE GEOTHERMOMETRY AND GEOBAROMETRY

Sphalerites from the Broadlands, New Zealand, geothermal field have been analyzed by electron microprobe and shown to have variable FeS and MnS contents. Samples analyzed come from drillcores at depths between 302 m and 1,841 m, where measured temperatures range from 140° C to 294° C and water pressures vary from 58.6 bars to 158.8 bars. The variable FeS contents of sphalerites coexisting with pyrite but not pyrrhotite are attributed to sulfur fugacity fluctuations. One core from drillhole Br 16 at 787 m depth (219°C) contains vugs in which pyrite, pyrrhotite and sphalerite (7.1 ± 0.6 mole % FeS) have crystallized together in equilibrium. The composition of the Broadlands sphalerites indicates that the slope of the sphalerite + pyrite + pyrrhotite phase boundary shifts sharply below 300° C toward the low FeS field, and means that Boorman's (1967) and Scott and Barnes' (1971) constant sphalerite composition value of 20.7 ± 0.6 mole % FeS between 300° C and 550° C cannot be extrapolated to lower temperatures. Similarly, the proposed sphalerite geobarometer (Scott and Barnes, 1971) cannot be applied to deposits in which sphalerite, pyrite and pyrrhotite crystallized together at temperatures below about 300°C. © 1973 Society of Economic Geologists, Inc.