Mineralogy and Geochemistry of a Lithium and Boron Enriched Stratiform Ore Zone in the Cave Spring Formation, Rhyolite Ridge, NV, USA

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Chafetz, Daniel Addison

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2023

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Boron , Diagenesis , Lithium , Microscopy , Searlesite , X-ray Diffraction

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The Rhyolite Ridge Project in the Silver Peak Range contains lithium and boron mineralization hosted by late Miocene-early Pliocene strata-bound sediments in an intermontane valley. ioneer USA Corporation is developing this project and is the industry-sponsor for this research along with the USGS and CREG. A lacustrine section, the Cave Spring Formation, measures up to 1,500 feet thick and hosts the sediments of interest. The unit with economic potential is the middle unit: a 60-foot interval called the Searlesite layer (B5). It is mineralized with both lithium clays and boron and is the targeted ore zone and the focus of this research. The younger, marl-rich member of this unit, the Li-Marl layer, shows no searlesite but has higher concentrations of lithium, which is hosted in smectite-rich interbeds.This research tests the hypothesis that precipitation of the Searlesite and Li-Marl layers of the Cave Spring Formation fits within the framework of the Closed Hydrologic System Diagenesis (CHSD) Model: these pelitic sediments required a saline alkaline lake with inputs of tuffaceous material to create the favorable chemistry for diagenesis of volcanic glass and subsequent precipitation of clays, zeolites, searlesite, and K-feldspar. This diagenetic alteration sequence shows a progression towards altered mineral assemblages observed in the Searlesite layer, where Li concentration in clay positively correlates with the degree of smectite-illite transformation and searlesite precipitation.Detailed mineralogical and geochemical analysis of 17 samples across the Li-Marl and Searlesite layers distinguishes the two layers and provides insight into the depositional environment of the ore zone and the evolution of the pore water during deposition of the younger material. The abrupt change in geochemistry between these layers is striking, and the genetic relationship between the mineralogy and the economic value of the sediments is directly related to the paleo lake conditions. The CHSD model states that rocks in deeper alkaline lakes with density-driven circulation will show more illitization of smectite and more authigenic K-feldspar than those in shallower waters or near basin margins; dense, diagenetic fluids will concentrate in basin-lows, driving precipitation these alteration products. Illite has been demonstrated to be more favorable in processing for lithium due to its lack of swelling characteristics while also being observed to concentrate lithium more effectively. The Searlesite layer exhibits characteristics of a deeper lake with a mineral assemblage reflecting the more highly progressed alteration sequence described in the CHSD model, including mixed-layer illite-smectite clays. The overlying Li-Marl layer is primarily characterized by a minimally altered mineral assemblage reflecting a shallower lake, including a dominant proportion of neoformed Mg-smectite clays.Seventeen samples from one core hole �" SBH-086 �" characterize the lacustrine sediments of the Li-Marl and Searlesite layers of the Cave Spring Formation. The minerals identified include carbonates such as calcite, dolomite and strontianite; searlesite; celadonite mica;zeolites such as analcime, heulandite and phillipsite; and Li-bearing clays that form as Mg-smectite or disordered I/S mixed-layer clays �" interpreted to be composed of Mg-smectite resembling hectorite and Al-illite. The highly mineralized Searlesite layer represents the most diagenetically altered material and the mineral suite in the Searlesite layer is the result of this diagenesis in the presence of boron. X-ray diffraction techniques are employed to interpret mineralogy in the fine-grained sediments, and geochemical analysis and modeling determined the clay structural formulas of the clays. These data enhance understanding of the Rhyolite Ridge deposit and the conceptual deposit model, which could prove useful for additional discovery of analogous deposits in other locations. Additionally, recovery and processing of the ore may be enhanced by quantifying variation in grade as a function of clay and borate mineralogy, as well as cation ratios of important elements.

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Creative Commons Attribution 4.0 United States

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