Abstract

Orebody 2 is 21 million metric tons grading 10.5 percent Zn, 0.17 percent Cu, and 0.014 oz Au plus 1.15 oz Ag per short ton. Quartz in the orebody has a delta 18 O of 10 to 11.9 per mil or approximately 2 per mil heavier than footwall quartz, signifying precipitation temperatures 250 degrees to 280 degrees C; this is corroborated by filling temperatures of primary fluid inclusions within gangue quartz (T f = 240 degrees -270 degrees C). Both isotopic and fluid inclusion temperatures signify a broad thermal zonation from approximately 270 degrees C at the base to approximately 240 degrees C at the top of the orebody. Minimum water depths of 300 m are indicated from nonboiling hydrothermal fluids of 3 to 5 weight percent NaCl equivalent at approximately 250 degrees C. A approximately 50 degrees C cooling of fluids during passage from the footwall to a suggested sea-floor brine pool above the ore zone may have been induced by turbulent mixing of approximately 20 percent pristine Archean marine bottom water at approximately 70 degrees C downward into the brine pool. Chert in the overlying Key Tuffite has delta 18 O of 10 to 12 per mil, probably representing equilibrium precipitation from brine pool fluids at 200 degrees to 250 degrees C spreading laterally across the sea floor in a gravitationally stable boundary layer.--Modified journal abstract.

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