Oxygen isotopic evidence for the origin and evolution of a scheelite ore-forming fluid, Glenorchy, New Zealand

Abstract

The Glenorchy lode field in the Otago Au-W-Sb metallogenic province of New Zealand is hosted by metasediments of pumpellyite-actinolite facies to lowest greenschist facies. The lodes cut schistosity at a high angle and consist of anastomosing veins of quartz-scheelite-calcite-pyrite-arsenopyrite within zones of crushed and deformed schist up to 6 m wide.Lode quartz delta 18 O values range from 16.0 to 17.4 per mil, background schist contains quartz with delta 18 O values between 13.0 and 14.8 per mil, and the alteration zone quartz delta 18 O values are gradational between the two extremes. Albite and muscovite also exhibit increased delta 18 O values in altered schists relative to background schists. The width of the oxygen isotopic alteration aureole is dependent on schist permeability during mineralization and extends up to 10 m from the lodes. Oxygen isotopic equilibrium conditions were approached in the schists during metamorphism and in the lodes and immediate wall rocks during mineralization. The remainder of the alteration zone is characterized by isotopic disequilibrium. The quartz delta 18 O values in the alteration zone schists probably represent mixtures of hydrothermal quartz (delta 18 O = 17ppm) and metamorphic quartz (delta 18 O = 13-14ppm).The temperature of metamorphism was estimated by oxygen isotope geothermometry to be between 370 degrees and 445 degrees C, although pumpellyite stability experiments by Schiffman and Liou (1977) suggest that 400 degrees C would be the upper limit. Oxygen isotope geothermometry gave a temperature range for mineralization of 280 degrees to 350 degrees C. Relative temperatures can be more reliably stated--mineralization occurred after metamorphism at a temperature at least 80 degrees C less than that of metamorphism.The ore-forming fluid delta 18 O was in the range 7 to 10 per mil, and on this evidence could have been either metamorphic or magmatic in origin, but because there is no spatial association between Au-W-Sb mineralization and igneous activity in the Otago area, a metamorphic origin is favored. A model for the origin and isotopic evolution of the ore-forming fluid is proposed whereby the fluid was derived from metamorphic dehydration reactions in rocks of at least upper greenschist facies. The fluid was trapped in the rocks until uplift of the schist pile led to hydraulic fracturing and release of the fluid to existing fault zones. The fluid migrated rapidly upward, retaining a constant delta 18 O value, and deposited hydrothermal minerals which were 18 O enriched relative to those in the country rock. Ore-forming elements (W, Au) were derived deep in the schist pile, probably also from metamorphic reactions.The oxygen isotope alteration aureole is probably too narrow to be of any practical use in exploration for similar lode deposits, at least in the Glenorchy area.