Modeling hydrothermal alteration and dissolution in carbonate hosted ore deposits

There are a wide variety of ore deposit types in carbonate host rocks, from Carlin-type Au deposits, to polymetallic skarns, and Irish and Mississippi Valley type base metal deposits. The ubiquity and economic importance of these deposits mean that continued exploration to find new mineral resources is vital to supply the demands of the world economy. However, we have likely found most of the large deposits exposed at the Earth's surface, so discovery of new resources requires improved understanding of how to vector to these deposits. This thesis consists of five separate papers that examine different aspects of carbonate hosted deposits and their adjacent alteration to aid future exploration.

The first two papers are theoretical in nature with the first being an examination of the controls of isotopic alteration in carbonate hosted ore deposits. This paper centers around a sensitivity analysis of model in which hydrothermal flow through a carbonate host rock (based on interpreted the formation conditions of Carlin-type gold deposit) alters the ¹⁸O composition of a randomly generated domain. It concludes that the main control on alteration is the porosity distribution as this controls the water to rock ratio. The paper also examines the pulse of isotopically enriched fluid that passes through a system as it is being altered and compares the model results to zoned calcites associated with Upper Mississippi Valley alteration.

The second paper examines what parameters control the extent of carbonate mineral dissolution found in many deposits, specifically Carlin-type. The work finds there are two significant domains where carbonate dissolves: a lower temperature where the aqueous chemistry is dominated by HCO₃^- and a higher temperature one dominated by CO₂. These two fields act differently in response to changes in temperature and pressure, which has implications for temperature and depth interpretations in many Nevada Carlin-type gold deposits as well as other carbonate hosted deposits.

The third paper is an application of reactive transport modeling to the Paleoproterozoic sediment-hosted Mount Isa Cu deposit, Australia. Here we optimized an inlet fluid to match the observed geochemistry of the hydrothermally altered dolomitic rocks. The resulting model and sensitivity analysis provided insight into the mechanisms and controls on the alteration and extent of the footprint of the copper ore system at Mount Isa. By extrapolating the inlet fluid to flow through other formations, we gained insight to what similar alteration might look like in other host rocks.

The fourth paper examines the groundwater plume associated with weathering and oxidation of a Carlin-type deposit in the South Grass Valley of Nevada. This work contains a simple reactive transport model used to demonstrate that the system is dominated by vertical recharge with groundwater that has reacted with rocks containing sulfur and arsenic. We used the t-statistic to rank the importance of various parameters as tracers and found that gold was the best due to its low concentration and variation in background samples.

The final paper is a simple series of techniques used when optimizing geochemical models. Optimization of these models can be problematic due to the presence of local minima and flat areas. However, modelers can avoid these problems in some cases by understanding the geochemical causes of the minima.

The papers presented here collectively cover a range of modeling approaches, from theoretical to applied, for examining carbonate hosted ore deposits. The theoretical modeling of carbonate mineral dissolution and isotopic alteration provided context for the site-specific research at Mount Isa. While the outcomes of both the Mount Isa and the South Grass Valley studies provide guides to aid in vectoring towards specific deposit styles in specific geological environments, the theoretical modelling results do this as well, albeit in a more general sense vi through providing system constraints. Finally, the optimization paper provides general techniques for improving the optimization of models such as those conducted in the Mount Isa and South Grass Valley case studies. Taken as a whole, these papers demonstrate the value of modelling based insights for understanding the origins of, and aiding exploration for, carbonate hosted ore deposits.