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

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 (delta 18O and delta13C) 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.;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.;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.;The results of this study show that few hydrothermal features extend beyond visible alteration (decalcification, argillization, and/or silicification) 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. (Abstract shortened by UMI.)...