sphalerite geobarometry in the Balmat-Edwards district , New yorkl
Sphaleri te-pyri te-pyrrhoti te barometry was undertaken in the Balmat-Edwards distr ict in the NW Adirondacks to determine the pressures in the metamorphosed ore body. Metamorphic temperatures were estimated at 625 t 25"C using calcite-dolomite thermometry. Sphaleri tes with extensive exsolut ion of chalcopyrite are very inhomogeneous, even within ten microns of the po + py + sp tr iple point, and have iron contents of 16-12 mole percent FeS, corresponding to 3-7 kbar (Scott, 1973). This local variat ion in pressure is unreasonable; either the system did not equi l ibrate during metamorphism, errat ical ly reequil ibrated afterwards, or exsolved chalcopyrite has variably affected the barometer. A few sphaleri tes without microscopic chalcopyrite blebs yielded consistent FeS contents of 13-12 mole percent where touching pyrrhoti te-pyri te and correspond to pressures of 6.5-7.5 kbar. These data and the presence of si l l imanite, indicative of pressures less than 6-6.5 kbar at 600-650.C, in the adjacent paragneisses uggest hat the Balmat distr ict was subject to a regional metamorphism of 6.5 a 0.5 kbar. Incautious application of the sphaleri te barometer based on a few samples or on heterogeneous materials can lead to large errors in est imated pressures.
A recalibration of the garnet‐olivine geothermometer and a new geobarometer for garnet peridotites and garnet‐olivine‐plagioclase‐bearing granulites
The garnet‐olivine Fe‐Mg exchange geothermometer and the garnet‐olivine‐plagioclase geobarometer have been simultaneously calibrated using reversed experimental data based on the model reactions and between 900 and 1500 °C at 9.1–95.0 kbar and between 4.7 and 7.0 kbar at 750–1050 °C, respectively. The resulting garnet‐olivine thermometer reproduces experimental temperatures mostly within ±75 °C and the garnet‐olivine‐plagioclase barometer reproduces experimental pressures well within ±0.19 kbar. These new thermobarometers use the same garnet and olivine activity models and are thermodynamically consistent. Application of these thermobarometers to garnet peridotites from mantle xenoliths, orogenic garnet peridotites over the world and the Adirondack olivine‐bearing metagabbros yielded reasonable P–T results. The present garnet‐olivine thermometer can be used to measure medium‐high‐grade to ultrahigh‐grade, low‐pressure to ultrahigh–high‐pressure garnet peridotites and metagabbros, whereas the garnet‐olivine‐plagioclase barometer has limited application to garnet‐olivine‐plagioclase‐bearing granulites.
Eine Sulfidparagenese mit kupferhaltigem Zonarpyrit von Nukundamu/Fiji
ZusammenfassungVon der bisher kaum bekannten Cu-Zn-Lagerstätte Nukundamu auf Vanua Levu/Fiji wird eine Sulfidparagenese mit Pyrit, Markasit, Idait, Covellin, Wurtzit, Zinkblende, Luzonit und Bleiglanz mit dem Erzmikroskop und der Elektronen-Mikrosonde untersucht. Anlaß der Untersuchung waren die bemerkenswert grobtafeligen Idait- und Zinkblende-Wurtzit-Aggregate und die Bravoit-ähnlichen Zonarpyrite, die statt Ni oder Co bis maximal 10 Gew.-% Cu in offenbar fester Lösung zeigen. Eine vorlāufige Zuordnung des Vorkommens zur meso- bis epithermalen Kupfer-Arsenformation der subvulkanischen Abfolge bei deutlicher Tendenz zu verkieselten Subvulkangesteinen mit Pyrit-Imprägnationen ist angezeigt.AbstractFrom the little known copper-zinc-deposit Nukundamu in Vanua Levu/Fiji a paragenesis of pyrite, marcasite, idaite, covellite, wurtzite, sphalerite, luzonite and galena is described by means of oremicroscope and electron probe microanalysis. The examination of this material was undertaken because of the remarkably coarse and clear aggregates of idaite and sphalerite-wurtzite, as well as the zoned pyrite which revealed a copper content of up to 10 weight percent, apparently in solid solution. A preliminary classification of the prospect as a meso-, respectively epithermal arsenic copper-sulfide deposit of subvolcanic origin with a distinct transition to silicified effusive rocks showing disseminated pyrite is indicated.