Field Trip Day Three: Skarns of the Yerington District, Nevada: A Triplog and Commentary
Marco T. Einaudi, 2000. "Field Trip Day Three: Skarns of the Yerington District, Nevada: A Triplog and Commentary", 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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THE CONTACT between the Yerington batholith and metased-imentary and metavolcanic rocks east of Ludwig, Nevada, is exposed over 3.5 km of paleodepth (Fig. 1) due to 90° of westward rotation during Basin-and-Range faulting (Proffett, 1977; Geissman et al., 1982). Here we have the opportunity to study the effects of depth, distance, and time in the generation of metamorphic rocks and skarns. As we walk over the terrain, keep reminding yourself that “original up” is to the west. The main emphasis of the trip will be to examine the structural and lithologic controls on the formation of calc-sil-icate hornfels, skarn, and related ores, the mineralogy of these rocks, and their temporal relation to quartz monzodior-ite and granite porphyry intrusions of the Yerington batholith. This examination will yield a conceptual framework for understanding skarn-forming processes and will generate ideas useful in mineral exploration. Some background material on terminology and phase equilibria is given in the first section of the chapter, preceding the description of individual stops. The actual trip log gives descriptions of outcrops for each stop, followed by background material, interpretation, and application where appropriate. An overall summary and conclusions is beyond the scope of this chapter.
Many of the conclusions presented below rely on the parallel studies of igneous and hyrothermal events associated with emplacement of the Yerington batholith (Proffett, 1977; Proffett and Dilles, 1984; Dilles, 1987; Dilles and Einaudi, 1992; Dilles et al., 1992; Dilles and Proffett, 1995), summaries in this Fieldtrip Guidebook (Dilles et al., 2000, etc.), as well as studies by others on skarn deposits around the world (cited individually in text).
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.