| Summary: | Located in Okanogan County, Washington, USA the Buckhorn mine is one of the largest gold skarns in North America (2.15 Mt at 14 ppm Au).
Buckhorn is hosted in Permian Anarchist Group metasedimentary rocks and Jurassic Elise Formation metavolcanic rocks. Monzodiorite comagmatic with the metavolcanic rocks is Jurassic in age (192.4 Ma) confirming the correlation. Two suites of granitoids intrude the local stratigraphy. The Middle Jurassic (170.4 Ma) post-accretionary Buckhorn Intrusive Suite is genetically related to skarn alteration and gold mineralization and comprised of a granodiorite stock, marginal diorite, and several generations of dikes. The Eocene Roosevelt Intrusive Suite (50.5 Ma) is comprised of a small (~0.03 km2) granodiorite stock that post-dates skarn alteration and gold mineralization.
Skarn alteration is zoned from dominantly magnetite-garnet in the proximal Magnetic Mine, to equal portions of magnetite-garnet-pyroxene in the Gold Bowl, and pyroxene dominated in the distal Southwest Ore-Zone. The Southwest Ore-Zone contains the majority of the gold mineralization and consists of massive calcic, Fe-rich, reduced skarn alteration along a low-angle shear zone at the contact between the carbonate metasedimentary rocks and the overlying metavolcanic rocks. Skarn alteration is divided into three categories based on the prograde mineralogy: pyroxene, garnet and magnetite skarn. Pyroxene skarn is further subdivided based on the retrograde mineralogy: amphibole-pyroxene, magnetite-pyroxene, and epidote-pyroxene skarn. Gold mineralization occurs in fractures in and intercrystalline space between skarn minerals and is intimately associated with bismuth. Re-Os geochronology of molybdenite bearing skarn confirms the Middle Jurassic age of skarn alteration and gold mineralization (162.8-165.5 Ma).
Based on the mineralogy, the hydrothermal fluids that caused prograde alteration were between 430-500° C, fO2=-25 to -20, fS2=-8 to -4.5, and near neutral pH. Fluids responsible for retrograde alteration were cooler (300-430° C), more reduced (fO2<-26), and had lower sulfur fugacity (fS2=-15 to -6). Gold mineralization occurred at the end of retrograde alteration. Gold was transported as bisulfide complexes in even cooler (241-300°C) and more reduced (fO2=-42 to -36) fluids than earlier retrograde alteration. Gold was scavenged by a bismuth melt and precipitated due to cooling and local retrograde oxidation reactions. === Science, Faculty of === Earth, Ocean and Atmospheric Sciences, Department of === Graduate
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