High-Temperature Hydrothermal Fluid Evolution, Archean Alkalic Intrusion-Related Mineralization, Karari-Whirling Dervish Gold Deposit, Western Australia

Genetic models that propose a magmatic-hydrothermal source for the gold ore fluid in Archean gold deposits hosted by, or spatially associated with, felsic to intermediate magmas are contentious. The Archean KarariWhirling Dervish gold deposit (>1 Moz at approximately 3 g/t Au), Kurnalpi terrane, Yilgarn craton, Western Australia, provides an opportunity to test such models because it is hosted by quartz-poor, volcaniclastic metasedimentary rocks that were intruded by Neoarchean oxidized syenitic magma. A carapace of hydrothermal alteration in felsic to intermediate volcaniclastic rocks that overlie the eastern intrusive complex evolved from pervasive potassic (biotite) alteration through structurally controlled tabular zones of sodic (albitic) alteration to localized potassic (K-feldspar) alteration within the albitized rocks; the last of these was directly associated with gold. Low-grade gold was deposited with hydrothermal K-feldspar in a pervasive fracture network as nanoparticles or crystallographic substitution in disseminated pyrite, probably as a result of redox reactions between an oxidized, potassic ore fluid and reduced, sodic host rocks. Subsequent fracturing of the volcaniclastic host rocks, under increasing fluid pressure, produced biotite-rutile-pyrite stringers and quartz-pyrite veins with K-feldspar-dolomite-pyrite alteration selvages. Pyrite associated with both of these events contains numerous inclusions of native gold and constitutes the high-grade component of mineralization (>3 g/t Au). Geothermometric data (biotite-apatite, Ti-in-biotite, Zr-in-rutile) indicate that these hydrothermal events took place at temperatures between 500 degrees and 650 degrees C; other mineral compositional (biotite, rutile) and S isotope data (delta 34S is -11 to 0 per mil) show that the hydrothermal fluids responsible for potassic (biotite) and potassic (K-feldspar) alteration were moderately to strongly oxidized. The triangle 33S signature of pyrite indicates a mantle source, and allows only minor or no crustal involvement, for the ore fluid. The fluid responsible for high-temperature, oxidized hydrothermal alteration and gold mineralization is identified as the magmatic-hydrothermal phase that exsolved from spatially associated syenitic magmas. On the basis of this interpretation, Karari-Whirling Dervish is classified as an alkalic intrusion-related gold deposit.