Raw petrographic microscopy photos and XRD data used in assessing the mineralogy of the Li-b enriched stratiform ore zone in the Cave Spring Formation, Rhyolite Ridge, NV, USA

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. The ioneer USA Corporation is developing this project and is the industry-sponsor for this research along with the USGS and CREG. A lacustrine section called Cave Spring Formation, measuring up to 1,500 feet thick, hosts the sediments of interest. The unit with economic potential is the middle unit, a 60-foot interval called the Searlesite layer that 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, hosted in smectite-rich interbeds.

This research characterizes the lithium and boron mineralization and examines whether diagenesis of tuffaceous sediments (including precipitation of zeolites and searlesite) enhances Li concentration in Li-bearing clays. Detailed mineralogical and geochemical analysis of 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. 17 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; zeolites such as analcime, heulandite and phillipsite; celadonite; and Li-bearing clays that form as disordered I/S mixed-layer clays composed of primarily montmorillonite and illite. The highly mineralized zone 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 distinguish the fine-grained sediments, and geochemistry assists with understanding the clay structural formulas of the I/S mixed-layer clays. These data enhance understanding of ioneer’s resource 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. The entire sample suite is represented here, and a model of clay structural formulas is informed by geochemical analysis and XRD interpretation of bulk material and clay separates.

Methods
Section 5.7 - Petrographic Microscopy

One hundred seventeen polished thin sections have been prepared using the Wagner Petrographic thin section service in Lindon, UT. Twenty-five of these thin sections were prepared from the 17 Crux Samples and are the focus of microscopy analysis. Fifteen of the non-Crux Sample thin sections were used to select samples for Ar-Ar geochronology, as described in section 5.9. Petrographic microscopy allowed for textural analysis of bedding, depositional features, position of zeolites relative to clays and glass, correlations between phases and sanidine freshness evaluation. It also refined target areas for Scanning Electron Microscopy (SEM) work. The ore zone is extremely fine-grained and optical microscopy is not powerful enough to characterize the relationships between minerals. Refining target areas by use of optical microscopy focused and expedited observations in preparation for SEM analysis.

Section 5.5 - X-Ray Diffraction (XRD)

X-ray diffraction (XRD) was used to identify mineralogical components of samples that are too small to be identified using standard techniques of optical microscopy. Using additional material saved during centrifugation (Section 5.3) of samples that were not submitted for geochemistry, clay-sized fractions of <0.5 μm and coarse size fractions of 2-595 μm were analyzed using the Bruker D-2 Phaser X-ray Diffraction instrument at the University of Nevada Reno. Typically, clay-sized material is <2 μm, however the fine-grained nature of the lacustrine sediments in Unit 2 resulted in clay separates with a significant percentage of impurities. Isolating the <0.5 μm size fraction is intended to concentrate the clay mineral phases and minimize non-clay phases.

Mineralogy was determined by XRD, using randomly oriented powder and oriented clay sample mounts. The random powder mounts – scanned from 4 to 70 degrees two-theta using Cu K-alpha radiation, with a step size of 0.02 degrees two theta, and a count time of one second per step – allow the crystals in the sample to be unoriented; this produces a pattern that is compared to the AMCSD 2019 database for mineral phase identification (Downs & Hall-Wallace, 2003) through the MDI Jade program (MDI, 2017). Oriented mounts – scanned from 4 to 45 degrees two-theta using Cu K-alpha radiation, and a count time of one second per step – are prepared by air-drying a wet sample onto a glass slide overnight while covered. With a properly prepared oriented slide, the clay crystals are oriented with the c-axis pointed up so the x-rays can interact with the 001 face and identify the d-spacing easily (Moore & Reynolds, 1997). Oriented mounts are run 4 times – first air dried, then glycolated, then heated to 400°C, and finally heated to 550°C – and peaks in the resultant pattern behave distinctly depending on the clay phases present. An air-dried swelling clay will have a d-spacing of between 12-14Å. Treatment with ethylene glycol will swell the d-spacing to approximately 17Å. Heating of the clay will then collapse the swollen clays to a d-spacing of approximately 10Å (Figure 30). A flow chart and the methods defined in Poppe, et al. (2001) are used to determine specific clay phases.