THE WHITE RIVER SILICA DEPOSITS, WASHINGTON: UPPER PARTS OF AN EARLY MIOCENE PORPHYRY CU-AU SYSTEM?

The White River hydrothermally altered area, located in the Cascade Range NW of Mt. Rainer, WA, occurs in Oligocene and Miocene andesite flows and silicic ash-flow tuffs of the Fifes Peak Formation. It was evaluated for alunite (aluminum) resources in the 1940s and is now being mined for industrial silica from two zones of massive silicification (Ash Grove and James Hardie quarries) within a 7 by 10 km, NE-elongated area of kaolinite-alunite-pyrophyllite alteration and hydrothermal breccia bodies. The two silica bodies are each continuous for 7 km and trend EW and NE, respectively. K-Ar and Ar-Ar dates of alunite range from 20.4 to 23.5 Ma.

Leaching of alkalis and alumina during alteration resulted in the formation of a vuggy residual rock with greater than 96% SiO₂. Locally, vuggy silica alteration is cut by hydrothermal breccias, and vugs and fractures are filled with chalcedonic silica. Enargite and pyrite are present in the unoxidized part of the silica ore bodies, mostly in narrow stockwork veins and breccia zones, and anomalous concentrations of As, Au, Bi, Cu, Ga, Hg, Mo, Sb, Se, Te, and F are present. The highest Au values, up to 1.7 ppm, occur in a series of N-striking high-angle normal fault zones containing hematite-rich, clay-supported breccias and clay-filled fractures in the Ash Grove quarry. Clay minerals are smectite ± dickite. Sedimentary structures in these open fractures, such as graded and cross bedding, indicate near-surface deposition.

The exposed part of the alteration zone lies in a saddle between 2 aeromagnetic lows mostly covered by glacial drift. The aeromagnetic lows originate from the shallow subsurface and may reflect subsurface alteration NW of the silica exposures. A gravity low west of the silica ore bodies probably reflects a deeper intrusion, possibly the source of hydrothermal fluids that formed the alteration zone. Northeast elongation of both the alteration zone and geophysical boundaries suggest that unmapped NE-striking faults controlled hydrothermal fluid flow and alteration.

The size, continuity, and intensity of alteration is comparable to near-surface alteration in other large acid-sulfate systems that are associated with quartz-alunite (high-sulfidation Au-Ag-Cu) deposits and porphyry Cu-Au mineralization. Deep drilling beneath the silica cap at White River has not been undertaken.