Tertiary Meteoric Hydrothermal Systems and their Relation to Ore Deposition, Northwestern United States and Southern British Columbia

Tertiary meteoric hydrothermal systems have altered the rocks exposed over more than S% of the land surface of the northwestern United States and southern British Columbia, including at least 25,000 1m? in Idaho. The systems typically involved convective circulation of fluid derived from ordinary meteoric groundwater& around crystallizing, calc-alksline, epizonal plutons eiiiP.laced into coeval volcanic cover rocks. These individual systems had widely ranging "lifetimes" of 10' to 1Cf >:~and operated ~ocally throu~out the Cenozoic, although the most profound devolopment of such activity occurred dunng Eocene time. Individual systems varied in size from a few tens of S'JUBre kilometers (Y ankec: Fork, I~o) or less ~ ~v~ral thousand square kilometers (Sawtooth and Castro nng zones, Idaho) Typically, reg~onsl propyhtization aacompanied the fluid circulation, although th~ .lJ!ghe~-tempe':B~re ~teration assemblages. were ~evel~ed locally as were intense alteration effects (e.g., silicification, senc1tization, etc.) near some ve1ns and m Jrurung districu. A significant amount, PC?bably 25-SO%, of the mineral production and pote!ltial i~ the region is closely related to Tertiary meteonc hydrothermal systems. Oxygen and hydrogen 1sotop•c data clearly demonstrate the close geologic association of meteoric hydrothermal systems amd 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 Challi~ volcanic field; (3) several Eocene skarn deposits valued for W (lma, Idaho) and Cu (Mackay, Idaho); (4) Important leadsilver vein and replacement deposits of Tertiary (Bluebell, British Columbia) and of probable Cre~ceous and early Tertiary age (Wood River, Idaho); (S) several potentially economic Mo-, Be-, and U-beanng 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, Ore-eon. A close spat1al association has been demonstrated between ore deposits and rocks having anomalous 8' 0 values and low fJD values. The most important deposits are associated with relatively small (generaii.I S-300 len() zones of low 80 values, and they are particularly closely linked with zone 1 s of very steep 0/"'0 Jn!dients in the alte~d rocks. These associations hold much promise for the use of fJ "o and I)D contour maps m future exploration efforts.