Crustal controls on magmatic-hydrothermal systems: A geophysical comparison of White River, Washington, with Goldfield, Nevada

Abstract

The White River altered area, Washington, and the Goldfi eld mining district, Nevada, are nearly contemporaneous Tertiary (ca. 20 Ma) calc-alkaline igneous centers with large exposures of shallow (<1 km depth) magmatic-hydrothermal, acid-sulfate alteration. Goldfi eld is the largest known high-sulfi dation gold deposit in North America. At White River, silica is the only commodity exploited to date, but, based on its similarities with Goldfi eld, White River may have potential for concealed precious and/or base metal deposits at shallow depth. Both areas are products of the ancestral Cascade arc. Goldfi eld lies within the Great Basin physiographic province in an area of middle Miocene and younger Basin and Range and Walker Lane faulting, whereas White River is largely unaffected by young faults. However, west-northwest-striking magnetic anomalies at White River do correspond with mapped faults synchronous with magmatism, and other linear anomalies may refl ect contemporaneous concealed faults. The White River altered area lies immediately south of the west-northwest-striking White River fault zone and north of a postulated fault with similar orientation. Structural data from the White River altered area indicate that alteration developed synchronously with an anomalous stress fi eld conducive to left-lateral, strike-slip displacement on west-northwest-striking faults. Thus, the White River alteration may have developed in a transient transtensional region between the two strikeslip faults, analogous to models proposed for Goldfi eld and other mineral deposits in transverse deformational zones. Gravity and magnetic anomalies provide evidence for a pluton beneath the White River altered area that may have provided heat and fl uids to overlying volcanic rocks. East-to eastnortheast-striking extensional faults and/or fracture zones in the step-over region, also expressed in magnetic anomalies, may have tapped this intrusion and provided vertical and lateral transport of fl uids to now silicifi ed areas. By analogy to Goldfi eld, geophysical anomalies at the White River altered area may serve as proxies for geologic mapping in identifying faults, fractures, and intrusions relevant to hydrothermal alteration and ore formation in areas of poor exposure.

Episodic intrusion, internal differentiation, and hydrothermal alteration of the Miocene Tatoosh intrusive suite south of Mount Rainier, Washington

“…However, hydrothermal systems associated with the Tatoosh intrusive suite were generally weak and not known to have developed porphyry copper systems such as those related to several other Miocene plutons in the ancestral Cascades arc in Washington (Hollister, 1979;John et al, 2003;Blakely et al, 2007). Tatoosh intrusive suite rocks were variably affected by hydrothermal activity that accompanied emplacement and solidifi cation of Tatoosh intrusive suite magmas; fl uid inclusion characteristics, weak deuteric alteration, and widely spaced metal-poor veins indicate the former ubiquitous presence of magmatically derived hydrothermal fl uids.…”

“…The Stevens Ridge Formation is overlain by at least 700 m of Miocene Fifes Peak Formation, which is dominated by basalt, basaltic andesite, and andesite lava and associated volcaniclastic and pyroclastic deposits. Blakely et al (2007) report 40 Ar/ 39 Ar ages of between 20.6 and 21.1 Ma for rocks of the Fifes Peak Formation just northwest of Mount Rainier. Elsewhere in the region, rocks mapped as Fifes Peak Formation include silicic ash-fl ow tuffs low in the unit.…”

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“…In particular, porphyry copper deposits seem to be preferentially sited along compressional parts of magmatic arc segments in small extensional stepover zones between arc-parallel strike-slip faults. The White River deposit in southern Washington (Table 3) is located in just such a stepover zone (Blakely et al, 2007). Accordingly, the extensional regime that prevailed along the southern Washington-Oregon part of the ancestral arc during much of its evolution (Wells, 1990;Wells et al, 1998) appears to have fostered the diffuse, sporadic, unfocused mineralization characteristic of this arc domain.…”

Mapping and Interpretation of the Lithospheric Magnetic Field

“…By utilizing magnetic and gravity data in an integrated geological and geophysical study, Blakely et al (2007) establish that the White River area of Washington exhibits many similarities to the Goldfield mining district of Nevada, home to one of the largest epithermal gold deposits in North America. To date, White River has produced only silica commercially, but deep weathering, young surficial deposits, and dense vegetation have hindered the evaluation of its economic potential for base and precious metals in the near surface.…”