Tertiary meteoric hydrothermal systems and their relation to ore deposition, northwestern United States and southern British Columbia

Abstract

Tertiary meteoric hydrothermal systems have altered the rocks exposed over more than 5 % of the land surface of the northwestern United States and southern British Columbia, including at least 25,000 km2 in Idaho. The systems typically involved convective circulation of fluid derived from ordinary meteoric groundwaters around crystallizing, calc‐alkaline, epizonal plutons emplaced into coeval volcanic cover rocks. These individual systems had widely ranging “lifetimes” of 103 to 106 years and operated locally throughout the Cenozoic, although the most profound development of such activity occurred during Eocene time. Individual systems varied in size from a few tens of square kilometers (Yankee Fork, Idaho) or less to several thousand square kilometers (Sawtooth and Castro ring zones, Idaho) Typically, regional propylitization aacompanied the fluid circulation, although the higher‐temperature alteration assemblages were developed locally, as were intense alteration effects (e.g., silicification, sericitization, etc.) near some veins and in mining districts. A significant amount, probably 25–50%, of the mineral production and potential in the region is closely related to Tertiary meteoric hydrothermal systems. Oxygen and hydrogen isotopic data clearly demonstrate the close geologic association of meteoric hydrothermal systems and mineralization in (1) the Paleocene, Cu‐Zn‐Pb‐Mn Main Stage mineralization at Butte, Montana; (2) numerous Eocene epithermal deposits principally valued for Au and Ag but also including significant deposits of Cu, Pb, Zn, F, Sb, etc., as at Republic, Washington, and in several mining districts in the Idaho batholith and the Challis volcanic field; (3) several Eocene skarn deposits valued for W (Ima, Idaho) and Cu (Mackay, Idaho); (4) important lead‐silver vein and replacement deposits of Tertiary (Bluebell, British Columbia) and of probable Cretaceous and early Tertiary age (Wood River, Idaho); (5) several potentially economic Mo‐, Be‐, and U‐bearing Eocene “porphyry” plutons; and (6) Miocene epithermal deposits, most prominently the Au and Ag bearing veins at Silver City and DeLamar, Idaho, the Hg deposits at the McDermitt caldera, Nevada and Oregon, and at Weiser, Idaho, and Au deposits in the Western Cascade Range and Lake County, Oregon. A close spatial association has been demonstrated between ore deposits and rocks having anomalous δ18O values and low δD values. The most important deposits are associated with relatively small (generally 5–300 km2) zones of low δ18O values, and they are particularly closely linked with zones of very steep 18O/16O gradients in the altered rocks. These associations hold much promise for the use of δ18O and δD contour maps in future exploration efforts.