Graduate Thesis Or Dissertation
 

What lies Beneath? Origin and Transition from the Shallow Epithermal Au-Ag Lithocap to Deeper Porphyry Cu Environment: Insights from the Magmatic Hydrothermal Deposits at Yerington, Nevada, and Summitville, Colorado

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/1v53k503g

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  • Near-surface advanced argillic hydrothermal alteration zones, also referred to as lithocaps, are characterized by quartz, alunite, clays (pyrophyllite and kaolinite group minerals), and minor F-bearing aluminosilicates (i.e., topaz, zunyite, diaspore, and APS) that form where water-, SO2-, and HF-rich gas condenses into shallow groundwater, acidifies, and reacts with rocks. The common environment of formation is subvolcanic and ranges from surface to 1.5-2.0 km depth. These lithocaps globally host a number of economic high-sulfidation epithermal Au-Ag ± Cu deposits or overlie deeper porphyry Cu-Mo-Au deposits. Nonetheless, most lithocaps are not associated with known epithermal or porphyry ore deposits. This study documents the conditions favorable to ore deposit formation in the lithocap environment, using field and mineralogical studies of two lithocaps: the Buckskin Range (Alunite Hill) zone that overlies porphyry Cu-Mo deposits of the Yerington district, Nevada; and the open-pit, surface, and drill core samples of the epithermal Au-Cu-Ag ore bodies at Summitville, Colorado. At Yerington district, the lithocap exposures formed at ~1,000 m depth and are characterized by voluminous (>1 km3) zones of quartz-alunite-topaz and quartz-pyrophyllite-topaz that are partially replaced by later muscovite ± zunyite. The replacement of pyrophyllite by muscovite and of topaz and pyrophyllite by zunyite requires alkali (K+) and Cl- addition at chemical conditions of higher K+/H+, pH, and HCl/HF than the conditions that produced the earlier lithocap zones. Common ‘pathfinder trace elements’ (e.g., As, Li, Sb, Tl, Rb, and Cs) associated with epithermal Au-Ag mineralization worldwide are enriched in whole rock samples muscovite ± zunyite replacement zones, which is consistent with their transport as chloride complexes. Both the mineral replacement reactions and the trace elements are consistent with formation by a relatively late-stage Cl-rich magmatic hydrothermal fluid. At Summitville, an early stage lithocap zone that contains alunite, kaolinite group minerals, local pyrophyllite, and topaz is partly replaced by late muscovite ± zunyite in a zone that extends from the base of the ore bodies at ~500 m original depth to >1.5 km greater depth. In the deep samples, the magmatic-hydrothermal alteration is characterized by quartz-sericite-pyrite alteration zones typical of the upper parts of porphyry Cu deposits. These zones are enriched in trace elements (i.e., Te, Se, Sn, W, and Mo) as in porphyry Cu deposits, and at their upper extent spatially overlap with the lithocap that hosts epithermal Au-Cu-Ag ores and associate trace metal enrichments (Tl, Sb, As, and Bi). These observations further support the hypothesis that a late and high temperature magmatic-hydrothermal fluid caused partial replacement of an earlier formed lithocap alteration. Short wave infrared spectroscopy (SWIR) of numerous rock samples from both Yerington and Summitville was used to identify pyrophyllite, kaolinite group minerals, alunite, muscovite, topaz and zunyite. The spectral identifications were calibrated using X-ray diffraction and a set of K-Na alunite samples synthesized experimentally. The SWIR data allow construction of a three-dimensional distribution map of the zones in which muscovite and zunyite have replaced pyrophyllite and topaz provides a proxy for mapping the effects of distribution of late magmatic hydrothermal fluids that introduced ore metals and trace metals. Alunite compositions determined by electron microprobe analysis (EMPA) at both Yerington and Summitville are strongly correlated with the original pre-alteration composition of both the host whole rock and its feldspars. The data demonstrate that host rock exerts a fundamental control on the alkali and alkali earth content (K, Na, Ca) of alunite. Samples from the Yerington district exhibit a wide range of compositions from near-end-member alunite (KAl3(SO4)2(OH)6) to natroalunite (NaAl3(SO4)2(OH)6) due to the chemically heterogeneous nature of the volcaniclastic and andesite-dacite volcanic host rocks whereas at Summitville the narrow compositional range (66.6-69.5 wt.% SiO2 and 7.6-8.4 wt.% K2O + Na2O) of alunite reflects the relatively chemically homogeneous nature of host high-silica dacite. These observations suggest that SO2-rich magmatic vapors that condensed into shallow groundwater and formed alunite did not transport or add alkalis (or Ca) into the lithocaps at Yerington or Summitville. The integrated exposures from Yerington and Summitville capture the nearly full vertical extent of the advanced argillic lithocap alteration zones containing epithermal Au-Cu-Ag ores and trace metal enrichments from the near-surface to 1.5-2.0 km depth and are transitional into deeper zones of porphyry Cu-Mo ore and metal enrichments. In both examples, early-stage advanced argillic alteration produced by magmatic gas condensation is replaced and overprinted by a late stage of magmatic hydrothermal fluid that was less acidic, contained chloride, and was capable of alkali and metal transport. This late magmatic hydrothermal fluid was likely an intermediate density and single-phase fluid, consistent with fluid phase properties and with fluid inclusion studies at Summitville (Bethke). The late fluid altered early formed pyrophyllite-topaz to muscovite-zunyite and likely both transported alkali and trace metals into the lithocap and produced the Au-Cu-Ag ores at Summitville.
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  • Pending Publication
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  • 2022-03-18 to 2024-04-18

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