Arkose-hosted, aquifer-controlled, epithermal Au-Ag mineralization, Wenatchee, Washington

Abstract

The Wenatchee district, site of the Cannon mine, is located on the east flank of the Cascade Range in central Washington. Epithermal gold mineralization of Eocene age (44 Ma) is hosted by the Eocene Chumstick Formation, a sequence of interbedded arkosic sandstone, conglomerate, and mudstone. This study focuses on the structure and hydrothermal alteration and mineralization south of the Cannon mine.Hydrothermal alteration and mineralization is hosted by Chumstick arkose, minor Eocene dacite, and andesite dikes, and locally, Eocene felsic volcanic rocks (flows, ash flows, eruption breccia) and extends for at least 4 km southeast of the Cannon mine. The district-scale structure is dominated by a postmineralization, northeast-verging fold and thrust belt which consists of three major southwest-dipping reverse faults and associated folds produced by the propagation of the faults.Arkose-hosted alteration consists of zones of silicification which grade symmetrically outward to argillization and a widely dispersed, district-scale propylitic alteration. Alteration is strata bound within units of arkosic sandstone and conglomerate bound or capped by mudstone-rich sections. Features of the propylitic zone include the growth of pyrite within detrital biotite and the replacement of plagioclase by carbonate and epidote. The argillic assemblage includes hydrothermal kaolinite and sericite. Silicification consists of silicic hydrothermal breccias and pervasive quartz flooding closest to the argillic zone; hydrothermal K feldspar is common. Typically, a transitional subzone containing abundant quartz veins (quartz stockwork) occurs between the argillic and silicic zones.In decreasing order of abundance, metallic phases are pyrite, arsenopyrite, marcasite, stibnite (latest in paragenesis), chalcopyrite, hessite, electrum, native silver, sphalerite, galena, pyrargyrite, and boulangerite. Gold contents are highest in silicic alteration and the quartz-stock-work subzone. Where arsenic contents are high, gold is concentrated in the argillic zone. In the latter case, strong correlations between gold and arsenic suggest that gold may have been transported as a gold thioarsenide complex; precipitation of the gold and arsenic may have been promoted by the progressive sulfidation of biotite.Two laterally extensive strata-bound alteration zones (> or =2 km along strike) have been delineated, one largely bound by mudstones in the hanging wall of a postmineralization reverse fault, and one in the footwall of the fault and bound above by a mudstone and ash-flow tuff. The absence of alteration stratigraphically above the footwall zone indicates that it is the stratigraphically highest mineralized aquifer in the district. Hydrothermal fluids migrated laterally along at least two laterally extensive aquifers which were stratigraphically stacked. The shallower footwall zone is enriched in As, Sb, and Au relative to the deeper hanging wall zone, which is richer in Ag, Cu, Te, and Se. This variation in depth is similar to patterns observed in active geothermal systems and epithermal precious metal deposits and implies some degree of exchange of hydrothermal fluids between the aquifers, possibly via leaks in aquitards.