Overview of the Lithophile Element-Bearing Magmatic-Hydrothermal System at Birch Creek, White Mountains, California
Mark D. Barton, 2000. "Overview of the Lithophile Element-Bearing Magmatic-Hydrothermal System at 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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A large lithophile element-bearing hydrothermal system is associated with the well-exposed Birch Creek bi-otite-muscovite granite and its metamorphic aureole in the White Mountains of eastern California. Elements enriched include F, Be, W, Zn, Pb, Ag, Cu, Au, Bi, and Sn although historic production (of Pb, Ag, Au, W) has been minor and likely resources are small. This system is one of several dozen Late Cretaceous intrusion-centered hydrothermal systems that are associated with two-mica granites along the Cordilleran miogeocline in the Great Basin. As a group, these occurrences resemble other Mesozoic W-Sn-F-Be bearing systems in circum-Pacific; however, these seemingly lack the economic deposits—an observation that begs the question: Why not? These granites are also of considerable interest because of their structural characteristics, notably intensely deformed and attenuated margins. They have figured prominently in literature on emplacement mechanisms for granites. This paper provides an overview of collaborative studies by several groups looking at the magmatic, structural, metamorphic, and hydrothermal development of the Birch Creek system.
Field, petrological, and geochemical studies demonstrate that the Birch Creek pluton (82 Ma, U-Pb, Ar-Ar) was episodically emplaced with alternating major pulses from at least two, probably three distinct magma sources (two crustal, one subcrustal). Compositions are peraluminous and range from biotite granodiorite to muscovite-biotite granite with episodic aplite formation. Hydrothermal features developed concurrently with each of these events. Field relations reveal a detailed history of fluid release from the evolving magma chamber. These fluids created high-temperature K feldspar-bearing quartz veins that are early and proximal to the magma chamber at any given time. With time, assemblages become K feldspar-destructive (albitization: albite-muscovite-fluorite ± quartz), and finally, only muscovite-stable (greisenization: muscovite-pyrite-fluorite ± quartz). This pattern is consistent with simple models of fluid evolution from the magma. Early veins, like the aplite dike swarms, have concentric and radial orientations consistent with formation in a localized, magma-chamber focused stress regime. Fractures hosting the later associations are consistently northeast oriented and controlled by far-field stress.
Features developed in Upper Proterozoic-Lower Cambrian carbonate and clastic host rocks can be linked to the intrusive history via map patterns and crosscutting relationships. Early grossular-rich garnet- and diop-sidic pyroxene-bearing skarnoids (in mixed siltstone-limestone units) and marbles (in massive limestone and dolomite) form and are then deformed (flattened) during early stages of pluton emplacement. Stable isotope data demonstrate at least some of this local metasomatic exchange was accompanied by magmatic fluid influx, whereas the marbles were largely impermeable and escaped metasomatism. “Anhydrous” calcic skarns consist of more iron-rich garnet plus salitic pyroxenes, idocrase, sodic plagioclase, quartz, and fluorite. In dolomite, equivalent vein skarns consist of humite-group minerals plus calcite and variable quantities of diopside, chlorite, spinel and grossular. Hydrous skarn assemblages formed next: an older group is characterized by combinations of clinozoisite-epidote, albitic plagioclase, fluorite, chlorite, and Mg-rich biotite; a younger group is characterized by muscovite, fluorite, pyrite, and fluorphlogopite. Scheelite, beryl, sphalerite, and other sulfides accompany these hydrous skarns. Structurally controlled, distal quartz-carbonate-sulfide veins and replacement bodies extend over 5 km from the intrusion. The skarns, replacement bodies and veins all formed from magmatic fluids as inferred from isotopic and fluid inclusion data.
An integrated time-space view of the hydrothermal, structural, and magmatic development is obtained from the field relationships and other physical and chemical constraints. Deformation in the form of locally intense subsolidus foliations, folded dikes and veins, and many local shears can be linked unequivocally to particular magmatic events during emplacement. These relationships demonstrate that much, perhaps all, of the intense deformation found around the western margin of the intrusion is syn-intrusive and driven by magma emplacement. Furthermore, these patterns help establish clear links between hydrothermal events in the intrusion and in the host. Skarns, both anhydrous and hydrous, formed in response to fluid release from each of the principal intrusive pulses. Overprinting events are found locally, yet their scarcity is consistent with a slowly evolving temperature field around the intrusion and predominant fluid flow upwards rather than outward.
Similarities and differences with other magmatic-hydrothermal systems, particularly those related to granitic composition rocks, are interesting to consider for this system. Level of exposure, early and continuous fluid production, and the lack of large-scale internal communication in the evolving magma chamber(s) all may have contributed to the lack of economically significant mineralization.
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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.