QUARTZ-ALUNITE HYDROTHERMAL ALTERATION SYSTEMS IN THE CASCADES ARC, NORTHWESTERN UNITED STATES

Hydrothermal systems containing quartz-alunite alteration are present in the ancestral Western Cascades arc and in the present-day High Cascades arc. Hydrothermal alteration and geochemistry of these systems are similar to those in high-sulfidation gold deposits. However, none of these high-sulfidation Cascade hydrothermal systems are known to contain a resource of precious or base metals. Residual silica characteristic of acid-sulfate alteration is present in most systems and late stage quartz-chalcedony veins and replacements of acid-leached rocks locally are associated with introduction of Au-Ag-Sn-W±Cu±Bi±Mo. In many of the deposits the uppermost part of the hydrothermal system is exposed as evidenced by fine grained, bedded sediment present in open spaces within brecciated vuggy silica. Only two of the systems are spatially associated with known porphyry mineralization, but the quartz-alunite systems near the Margaret porphyry Cu deposit, WA, are older and thus not genetically related to it. Two of the quartz-alunite systems, White River, WA, and Quartz Mountain, OR,have been mined for silica and constitute a significant silica resource. Silica from Quartz Mountain has been used primarily for smelter flux and silica from White River has been used for cement and as filler for wallboard. Acid rock drainage (ARD) and deposits of ferricrete are associated with most of the quartz-alunite systems in the Cascades Arc. The waters are characterized by high concentrations of sulfate, Al, and Fe, but relatively low concentrations of As, Sb, and base metals in comparison to ARD from quartz-alunite alteration associated with economically productive high-sulfidation Au-Ag deposits. Ferricrete is enriched in As, Se, Hg, ±Cu, ±Ni, and ±Zn. Removal of these elements from ARD by FeOOH sorption results in metal enrichment in the ferricrete and low levels of these metals in ARD. The Hg concentration in ARD is typically very low (generally ≤5 ng/L) and commonly below background levels outside the zone of quartz-alunite alteration. Both ARD and rock geochemistry of quartz-alunite hydrothermal systems in the Cascades arc indicate that these systems have low mineral resource potential for precious metals, but in some cases may be the near-surface expression of genetically related porphyry mineralization.