The Long Canyon Deposit: Anatomy of a New Off-Trend Sedimentary Rock-Hosted Gold Discovery in Northeastern Nevada

Abstract

Abstract The Long Canyon deposit is a new sedimentary rock-hosted gold discovery located in northeastern Nevada, over 150 km east of the well-explored Carlin trend. Unlike most examples of this type, the deposit is hosted in a sequence of Cambro-Ordovician carbonate rocks deposited in a periodically emergent, platform to shelf-edge environment. The strata were strongly deformed and weakly metamorphosed during a mid-Mesozoic orogenic event that segmented and extended a brittle, 80-m-thick dolomite layer into a series of elongate, N- to NE-trending boudin blocks, while the enclosing limestone underwent ductile deformation. This unusual structural architecture focused subsequent contractional deformation, as well as Tertiary extensional brittle deformation, forming pathways for meteoric and hydrothermal fluids, which formed and enlarged dissolution cavities and dissolution breccias along boudin margins, boudin necks, and normal faults. Gold occurs primarily in zones of polyphase dissolution breccias that are localized in minor faults and fold hinges in the structurally complex areas along and adjacent to dolomite block margins. Two general phases of brecciation are present, including calcareous breccias and later, Fe ± As oxide-rich breccias, which carry the highest gold grades. Breccias are accompanied by variable pervasive silicification. Alteration assemblages include zoned dickite + white mica + hematite in the cores of mineralized zones, with peripheral kaolinite + sericite and goethite + hematite and, ultimately, more distal illite + smectite. Local preservation of fine-grained sulfides encapsulated in calcite and silica suggests that the iron oxide assemblages may represent oxidation of fine-grained pyrite + marcasite + arsenopyrite assemblages. As currently defined, the Long Canyon deposit extends for 3 km in a northeasterly direction and consists of several subparallel zones of mineralization focused on boudin block margins, boudin necks, and incipient boudins. It contains a combined (measured, indicated, and inferred) estimated resource comprising 2.2 million ounces of gold, at an average grade of ~2.3 g/t gold. Metallurgical work indicates that the deposit is highly amenable to cyanidation by heap leaching or milling. The Long Canyon deposit is open to further exploration and resource additions, and is shallowly buried, relatively high grade, and thoroughly oxidized. The stratigraphic position of the Long Canyon deposit on the Paleozoic shelf and its location well east of the majority of identified major trends indicate significant potential for new discoveries in this underexplored area of eastern Nevada.