The Crown Jewel gold skarn deposit.

摘要

The Crown Jewel deposit, located in north-central Washington, is one of the largest Au-skarns in North America (7.7 Mt 6.2g/t). The deposit is hosted by Permian Anarchist Group metasedimentary rocks and Jurassic Elise Formation metavolcanic rocks. Metasedimentary rocks deposited in a tectonically active back-arc basin. Metavolcanic rocks are calcalkaline basalts, emplaced in a continental-arc setting. Middle Jurassic granodiorites (168 Ma; U/Pb) and associated dikes represent magmatic activity associated with accretion of the Quesnel terrane. 53 Ma granodiorites relate to Eocene extension. Both Jurassic and Eocene intrusions are reduced, metaluminous, calcalkaline, LREE- and LILE-enriched, and HFSE-depleted, characteristics of subduction-related intrusions. They could both be associated with reduced Au-skarns but their relationship with Crown Jewel is still poorly constrained.; The deposit includes multiple ore zones with complex stratigraphic and structural controls. The Southwest zone (>70% of the ore reserves) consists of a massive, calcic, Fe-rich, and reduced skarn locally controlled by faults and shear zones. The early stages of alteration are characterized by extensive biotite hornfels, coeval with numerous shear zones. Prograde skarn consists of pyroxene (Hd36-93) +/- grandite (Adr30-99) +/- magnetite formed at relatively high fO2 and fS2 and around 450-500°C. Retrograde alteration is dominated by Fe-rich minerals (amphibole + epidote +/- phyllosilicates) formed at lower fO2 and fS 2, and down to 300°C. Opaque minerals include pyrrhotite and minor chalcopyrite, pyrite, marcasite, arsenopyrite/cobaltite, bismuth minerals. Au precipitates in the last stages of retrograde alteration associated with Bi-minerals.; LA-ICP-MS analyses of garnets reveal a positive correlation of SigmaREE 3+ with Al content. Al-rich garnets are anisotropic, HREE-enriched and LREE-depleted. Isotropic Fe-rich garnets have low SigmaREE, a positive Eu anomaly, and are LREE-enriched and HREE-depleted. Thermodynamic data suggests preferential incorporation of HREE in grossular and LREE in andradite, which follows a coupled substitution mechanism ( X2+VII I-1REE3+ VIII+1Si4+ IV-1Z3+ IV+1 ). Variations in the Crown Jewel garnets are largely controlled by external factors. Al-rich garnets formed by diffusive metasomatism, under low W/R ratios, high fluid residence times, and in equilibrium with fluids compositionally buffered by the host rocks. Fe-rich garnets growth rapidly by advective metasomatism, under relatively high W/R ratios, and in equilibrium with a magmatic-derived fluid.