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 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 . INIRODUCI10NOxygen and hydrogen isotopic ratios are the only intrinsic properties of H20 that are known to be both variable and easily measured; hence they provide the only unequivocal data on the origin of natural aqueous fluids [Craig et a/., 1956]. Application of these principles proves that fluids derived from meteoric waters played a critical role in the formation of numerous ore deposits of the western United States. Prominent examples are the epithermal Au and Ag deposits of Nevada, such as Tonopah, Goldfield, and the Comstock Lode [Taylor, 1973[Taylor, , 1974aO'Neil and Silberman, 1974], and several copper porphyry and vein deposits [Sheppard et al., 1969[Sheppard et al., , 1971. A particularly important example is the Main Stage mineralization of the zoned Cu-Zn-Pb-Mn deposit at Butte, Montana [Garlick and Epstein, 1966;Sheppard and Ta...

Brief duration of hydrothermal activity at Round Mountain, Nevada, determined from Ar 40 /Ar 39 geochronology

“…Our conclusion of brief activity at Round Mountain contrasts with a common interpretation that hydrothermal systems, even shallow epithermal systems, are long-lived, with life times of up to several million years (Silberman et al, 1979;Whalen et al, 1982;Silberman, 1985;Criss et al, 1991;Conrad et al, 1993). This is less of a contradiction than it first appears.…”

Life and reproduction of titanosaurians: Isotopic hallmark of mid-palaeolatitude eggshells and its significance for body temperature, diet, and nesting

“…While this value is relatively high for purely surface-derived meteoric water, it is not uncommon in arid continental environments. In addition, the enrichment in 18 O may be related to hydrothermal fluid-rock interaction with the underlying basement, if the meteoric waters were heated to close to 100 • C (McKibbin et al, 1986;Criss et al, 1991;Rollinson, 1993). The validity of this result can be cross-checked using the quartz-water oxygen isotope fractionation.…”

Variations of Stable Isotope Ratios in Nature

“…Beaty and Taylor 1982;Green et al 1983). Criss et al (1985Criss et al ( , 1991 found excellent spatial correlations between low δ 18 O-values and economic mineralization in siliceous rocks. Similar zonation around ore deposits in carbonate rocks have also been observed (e.g.…”

“…They exhibit great variations in size from relatively small intrusions (<100 km 2 ) to large plutonic complexes (>1000 km 2 ). Amongst the best documented examples are the Skaergaard intrusion in Greenland, the Tertiary intrusions of the Scottish Hebrides, and the Tertiary epizonal intrusions of the northwestern United States and southern British Columbia, where 5 % of the land surface has been altered by meteoric hydrothermal water (Criss et al 1991).…”