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Patterns of Lithology, Structure, Alteration, Trace Elements, Carbonate Mineralogy, and Stable Isotopes around High-Grade Carlin-type Gold Deposits: Turquoise Ridge Deposit, Getchell District, Nevada
AuthorCassinerio, Michael D.
AdvisorMuntean, John L.
Geological Sciences and Engineering
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Few data sets have been published that depict the extent of hydrothermal features surrounding high-grade ore bodies in Carlin-type gold deposits (CTGDs) in Nevada, making exploration, development, and production difficult, especially where visually unaltered, unmineralized rocks occur in close proximity to ore. This thesis, which is divided into 3 standalone chapters, investigates several hydrothermal features associated with the high-grade Turquoise Ridge CTGD in the Getchell District. Traditional exploration tools such as patterns of lithology, structure, visible hydrothermal alteration, trace elements, and clay mineralogy are presented in Chapter 1 to test for the presence of haloes that can potentially be used as vectoring tools in honing in on high-grade ore zones at the Turquoise Ridge CTGD. Carbonate staining was utilized to determine differences in carbonate mineralogy associated with more reactive and non-reactive carbonate host-rocks (Chapter 2). Stable isotopes (δ 18O and δ13C) are presented in Chapter 3 to test for the presence of potentially more pervasive cryptic alteration haloes relative to visible hydrothermal alteration and trace element anomalies. Closely-spaced diamond drill holes along two cross-sections across the High Grade Bullion (HGB) ore zone at the north end of Turquoise Ridge were logged in detail. The logging information, gold assays, multi-element analyses, and spectral reflectance analyses were plotted on a 1:600 scale cross-section, from which controls on gold (Au) grade and haloes to ore zones were interpreted. The Eocene-age Turquoise Ridge CTGD is hosted within a complexly deformed package of Cambrian-Ordovician sedimentary and volcanic rocks that is characterized by rapid facies changes and soft-sediment deformation. The lowermost unit comprises variably calcareous, carbonaceous mudstones and limestones that are overlain by a series of sedimentary debris flow breccias. Above the breccias are deformed tuffaceous mudstones and limestones, which inter-finger to the northeast with a thick sequence of pillow basalt. Both the sedimentary debris flow breccias and the basalt pinch out to the south, and are interpreted to be controlled by a Cambrian-Ordovician, west-northwest trending, northeast-facing, basin margin. Further up section the rocks are mainly mudstone, basalt, and chert. The rocks are intruded by Cretaceous dacite porphyry dikes. The Turquoise Ridge deposit occurs in the hanging wall of the east-dipping, north-northwest-striking Getchell fault zone, which is the main fault in the district. Much of the HGB ore zone occurs along complex intersections between steep, small-displacement faults, folds, and fractured calcareous lithologies, especially within the series of sedimentary breccias. The Au grades commonly show abrupt variations, locally changing from ≥17 ppm to <0.34 ppm within a few meters. Zones of ≥0.34 ppm Au extend up to 120 meters above and up to 150 meters below the HGB; occur within the Getchell fault; and occur along several steep, small-displacement, antithetic faults in the hanging wall of the Getchell fault. Visible alteration, consisting of decalcification, argillization, and silicification, strongly correlates with Au grades of >0.34 ppm. Correlation matrices and factor analysis of downhole multi-element data show Au is strongly associated with Hg, As, W, S, Tl, Sb, and Te, but has a strong negative correlation with Ca, Mg, and Sr that is consistent with decalcification being the main alteration type. Values of ≥100 ppm As form a halo up to 30 m beyond the 0.34 ppm Au contour and visible alteration, whereas the 5 ppm Sb contour forms a halo up to 12 m wide. With the possible exception of Hg, other features derived from the multi-element data, including Tl, Te, degree of sulfidation (DOS), Au factor scores (derived from factor analysis), and Ca (to monitor decalcification) do not form significant, coherent haloes to the 0.34 ppm Au contour or visible alteration. Kaolinite and illite commonly occur in zones of ≥0.34 ppm Au. Inspection of samples from ore zones using an SEM show they are intimately intergrown with arsenian pyrite. Kaolinite- and illite-bearing fractures surfaces form incoherent haloes extending up to 20 m beyond visible alteration in otherwise unaltered rocks. Limited analyses of these fractures using a Niton handheld XRF show the fractures can contain significantly higher As than the adjacent wall rock. Carbonate staining was utilized to determine the spatial distribution of carbonate mineralogy relative to high-grade gold ore at the Turquoise Ridge Carlin-type gold deposit. Specifically, dilute hydrochloric acid containing both alizarin red S and potassium ferricyanide was utilized to differentiate between calcite, ferroan calcite, ferroan dolomite and dolomite in the Cambrian-Ordovician carbonate host rocks at Turquoise Ridge. Detailed carbonate staining was conducted on 31 drill holes throughout the north end of Turquoise Ridge. Results reveal a distinct spatial relationship between gold mineralization, ferroan calcite, and the southern margin of a thick basalt within the Cambrian-Ordovician stratigraphic package that is referred to as the northern pillow basalt (NPB). High-grade gold ore (≥17 ppm Au) within the HGB ore zone occurs exclusively within ferroan calcite-bearing host rocks. The transition from ferroan calcite to calcite with depth occurs at the base of the HGB ore zone. Nonferroan calcite is distal to the NPB, both laterally and with depth. A close spatial association between the distribution of ferroan calcite and the NPB suggests iron was likely liberated from the basalt during seawater alteration of the basalts, and/or the emplacement of the Cretaceous intrusions and was mobilized into adjacent limestones, forming ideal host rocks for subsequent gold mineralization. Acidic, sulfur-rich, auriferous hydrothermal fluids dissolved ferroan carbonates, which liberated iron. Aqueous sulfide in the fluid reacted with the iron, which destabilized aqueous gold-sulfide complexes, resulting in the formation of auriferous pyrite and marcasite. The presence of interlayered basalts and ferroan carbonates could be important exploration criteria in the identification of future target areas in the Getchell District and throughout Nevada. Furthermore and most importantly, careful carbonate staining should be routinely employed, given that it provides inexpensive, real-time data. Analyses of carbonates from both visibly altered and unaltered rocks at Turquoise Ridge yielded δ18O values from 13.5‰ to 21.6‰ and δ13C values from -8.2‰ to 1.2‰. Most of the δ18O values associated with visibly unaltered recrystallized micrite are between 17.6‰ and 19.0‰, well below typical δ18O values associated with unaltered Paleozoic carbonates (22‰ to 28‰). The spatial distribution of δ18O values along an east-west cross-section through the northern portion of the Turquoise Ridge deposit reveal the lowest δ18O and δ13C values occur within visibly altered wall rocks associated with ore zones and defined fluid pathways. Stable isotopic alteration haloes are limited to visible alteration and do not extend beyond hydrothermal alteration into adjacent unaltered wallrocks. Similarly, elevated trace element concentrations (e.g., Au, As, Hg, Te, Sb,and Tl) are largely limited to visibly altered wall rocks, with elevated concentrations associated with bleached clay/calcite-lined fractures extending beyond continuous hydrothermal alteration in otherwise visibly unaltered wall rocks. The lack of wide, coherent haloes of various hydrothermal features surrounding ore at Turquoise Ridge is suggestive of a restrictive, fracture-controlled fluid flow network, rather than large-scale, pervasive, lithologically-controlled fluid flow. Ore fluids likely ascended along the pre-existing Getchell fault zone and into the steep, small-displacement, antithetic faults in the hanging wall and the main dacite porphyry dike before encountering highly fractured, reactive carbonate-bearing lithologies, especially the sedimentary breccias and deformed mudstones and limestones adjacent to the basalt. Increased surface area in these zones allowed more fluid:rock interaction, which resulted in more pervasive carbonate dissolution, sulfidation, and Au deposition in the HGB. A large zone <25% RQD values that surrounds ore supports exploitation of these pre-existing fracture networks and as well as fracturing caused by collapse during carbonate dissolution associated with ore formation. The results of this study show that few hydrothermal features extend beyond visible alteration (decalcification, argillization, and/or silcificiation) or low-grade gold assays. The features that do only form narrow, discontinuous haloes that are mostly restricted to fractures. None of the features investigated in this study, including visible alteration and low-grade gold assays, form pervasive haloes to ore on the scale of tens to hundreds of meters, suggesting widespread alteration haloes associated with CTGDs is a common misperception, especially high-grade CTGDs.