Field Trip Day One: Birch Creek, White Mountains, California
Mark D. Barton, 2000. "Field Trip Day One: Birch Creek, White Mountains, California", Part I. Contrasting Styles of Intrusion-Associated Hydrothermal Systems: Part II. Geology & Gold Deposits of the Getchell Region, John H. Dilles, Mark D. Barton, David A. Johnson, John M. Proffett, Marco T. Einaudi, Elizabeth Jones Crafford
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This Paper provides the log for the first full day of the field trip. Day 1 will be spent in the Birch Creek area, east of Bishop in the southeastern White Mountains. There are several goals to the Birch Creek day:
To see one of the best-exposed examples of a Cretaceous two-mica granite pluton in the Great Basin with its distinctive style of emplacement and hydrothermal alteration;
To examine the relationships among magmatism, structural development, and emplacement;
To examine the links between magmatism and hydro-thermal activity;
To ask the questions: Why are these Late Cretaceous systems in the Great Basin not well mineralized? What differs between these systems and better-mineralized Late Mesozoic examples elsewhere in the circum-Pacific?
To consider how the characteristics of and processes in these granite-related systems compare with other intrusion-related hydrothermal systems, specifically in the Yerington district and in the Humboldt mafic complex.
See the accompanying paper (Barton, 2000) for an introduction to the Birch Creek system as a whole and its broader context. Some local terminology used in this field guide is defined there.
It is important to get an early start as this is the most challenging day of the field trip because of logistics and length and because it is second only to the skarn traverse at Yering-ton in the physical exertion required. If the upper Mollie Gibson road is impassable due to snow, ice, or obstacles, hiking in and/or alternative stops can be considered.
Figures & Tables
Part I. Contrasting Styles of Intrusion-Associated Hydrothermal Systems: Part II. Geology & Gold Deposits of the Getchell Region
Intrusion-related hydrothermal systems represent a large variety of geologic environments that in some cases form large metallic mineral deposits. The deposits examined in this trip represent the spectrum from systems dominated by magmatic fluid (Birch Creek, California and Yerington, Nevada) to those systems in which intrusions serve as heat engines to drive convectively circulating brines derived from sedimentary rocks (Hum-boldt, Nevada). In these examples, nonmagmatic fluids are largely excluded from more deeply emplaced intrusions in a compressive environment, and the hydrothermal composition and ores (e.g., granite W-F, Cu porphyry and skarn) are dictated by the composition of the magma and its mechanism of crystallization and aqueous fluid generation. Magmatic fluids are less important in the shallow crustal ore environment, but apparently contribute to acidic alteration zones located vertically above source intrusions. Using Humboldt as an example, we propose that the Fe oxide Cu-Au ores in the shallow environment require an abundant source of sedimentary brines (typical of evaporitic environments), high fracture permeability (promoted by an exten-sional setting) to allow aqueous fluid flow and dike emplacement, and shallowly emplaced intrusions to serve as heat sources.
IGNEOUS-RELATED hydrothermal systems constitute the most varied type of geologic environment, ranging in tectonic setting from spreading centers to collisional belts, in depth from the surface to the deep crust, and in sources of materials from purely magmatic to largely external. They comprise perhaps the single most important ore-forming environment, yet most igneous systems lack economically significant mineralization. This variety is attributable to igneous factors such as volatile content and its evolution from the intrusion, and to external factors that include depth of emplacement, host rocks, tectonic environment, and structural setting, which control permeability and access of external fluids to the crystallized intrusion and its contact aureole.
This field trip examines three large but markedly different intrusion-centered hydrothermal systems in the western Great Basin of California and Nevada (Fig. 1, Table 1). Each example represents a major group of these systems worldwide. The field emphasis will be on examining mass transfer features—such as mechanisms for igneous emplacement, degassing of magmatic-aqueous fluids, and fracturing and ductile deformation—that allow variation from near-lithostatic to hydrostatic conditions, incursion of nonmagmatic fluids into the high-temperature environment, and hydrothermal alteration, vein deposition, and wall-rock replacement via aqueous fluids. The broader questions of metallogenic provinces and processes will be raised as a context for the specific sites examined.
The overall emphasis of this trip will be on documenting and understanding the dynamics of igneous-related hydrothermal systems.