--> --> Carlin-type gold deposit development as an analogue for unconventional hydrocarbon field development

AAPG Pacific Section and Rocky Mountain Section Joint Meeting

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Carlin-type gold deposit development as an analogue for unconventional hydrocarbon field development

Abstract

Carlin-type gold deposits are sediment-hosted, low-grade disseminated gold deposits with economic recovery cut-offs at or below 0.1 oz gold/ton. The gold in these deposits is not visible, but occurs as atoms to microscopic blebs in arsenian pyrite rims on pyrite. Though these deposits are low-grade, the Carlin-type gold deposits in the Carlin trend of northeastern Nevada contain the second largest concentration of gold in the world. Due to low gold concentrations, these deposits are generally developed as large surface mines. Initial development of many of these deposits began assuming fairly homogenous distribution of gold across the deposit. However, further analysis has shown that resource distribution is heterogeneous, with higher concentrations of gold in areas where the gold-depositing hydrothermal fluids were focused. Areas of focused fluid flow include along faults and folds, and within certain lithologic facies, including low-stand karsted shelf carbonates and deepwater carbonate turbidites. Thus, ore grade is a function of the structural and stratigraphic control of hydrothermal alteration. Therefore, mine development plans and economic models are critically linked to the deposit geology. Similarly, unconventional hydrocarbon resources were initially seen as mainly an engineering play; once a source rock target was identified, drill and produce the field under the assumption that the resource distribution is homogenous. However, as with Carlin-type deposits, the resource density within unconventional fields is heterogeneous and controlled by geologic factors, including (but not limited to) depositional setting, paleogeography, burial/maturation history, tectonic/structural history, mineralogy, diagenetic history, and geochemistry. Evaluation and exploitation of Carlin-type gold deposits and unconventional hydrocarbon fields have similar histories. Both were initially seen as having fairly homogenous resource density, thereby excluding geology as a factor once identified. However, the resource bases in Carlin-type deposits and unconventional fields are not homogenous and the heterogeneities are driven by geologic factors. Understanding those geologic factors is essential as an input to determine the most effective and efficient ways to develop these resources.