Oxygen and hydrogen isotope characteristics of hydrothermal alteration at the Ann-Mason porphyry copper deposit, Yerington, Nevada

Abstract

Late Cenozoic normal faulting has tilted a 3- by 5-km block containing the mid-Jurassic Ann-Mason porphyry copper deposit ≈ 90o W and exposed the hydrothermal alteration pattern in cross section from 1- to 6-km palcodepth. Oxygen and hydrogen isotopes have been used to deduce sources of hydrothermal fluids. Fresh samples of the host Yerington batholith pluton have typical calc-alkaline magmatic isotopic compositions (δ^(18)O_(whole rock) = 6.7-7.0‰; δD_(biotite) = -85 to -88‰). The various types of hydrothermally altered rocks have similar δD and δ^(18)O values, only slightly isotopically shifted from original magmatic values, except along an axial zone within and above the orebody. In this zone, altered rocks are enriched in ^(18)O by 1 to 3.5 per mil, and δ^(18)O values increase with decreasing palcodepth. The δ^(18)O values of the whole rock and/or feldspar and the δD values of biotite, chlorite, and/or actinolite in the various alteration zones are as follows: propylitic alteration is widespread and is isotopically similar to fresh rock: δ(18)O = 6.2 to 7.2 per mil, δD = -82 to -92 per mil. Potassic alteration, characterized by 100 percent alteration of hornblende to biotite and Cu mineralization, occurs at ≈ 3.5-km palcodepth along the axis of the mineralizing granite porphyry dike swarm: δ^(18)O = 6.5 to 8.4 per mil, δD = -68 to -96 per mil. Sodic-calcic alteration, characterized by hydrothermal oligoclase-actinolite-sphene, occurs laterally alongside and below the ore zone at 3- to 6-km palcodepth: δ(18)O = 5.7 to 8.4 per mil, δD = -67 to -90 per mil. Late-stage sodic (albite-chlorite) alteration occurs at 1- to 4-km palcodepth in the vicinity of the axial zone: δ(18)O = 6.7 to 9.6 per mil, δD = -79 to -85 per mil. The youngest, late-stage sericitic alteration occurs at the shallowest levels, primarily at <1- to 2-km palcodepth, and displays a distinct ^(18)O and D enrichment: δ^(18)O = 9.8 to 10.4 per mil, δD = -61 per mil. Integration of stable isotope data, geologic field relations, and phase equilibria constraints indicates that the high-salinity fluids responsible for potassic alteration and Cu mineralization were of internal (magmatic) origin, but that synchronous, deep, convecting, saline, sodic-calcic fluids were largely derived from external (nonmagmatic) sources. A third type of fluid, also external but lower in temperature and much more dilute, was prevalent at shallow levels and caused latestage sericitic alteration. This was either an isotopically heavy, coastal meteoric water or a seawater.