Historic Mining Districts: A Gateway to Future Critical Mineral Potential
by James McVey and Stephanie E. Mills
A vein in the Clifton vein swarm. Minerals include quartz, calcite, and oxide ores containing copper, lead, gold and molybdenum.
The Utah Geological Survey (UGS) has been a leader in critical minerals research, long before the U.S. Geological Survey (USGS) formalized the definition and list of critical minerals in 2018. One of the UGS’s recent critical mineral projects, funded by the USGS Earth Mapping Resources Initiative (EMRI), began in 2020 and focused on geological mapping and tungsten evaluation in the Gold Hill mining district in western Tooele County. During this study, UGS geologists recognized a unique mineralization feature in the district, known as the Clifton vein swarm, that had not previously been researched. The veins are an array of sheet-like minerals consisting of quartz, calcite, and oxide ores containing gold, copper, molybdenum, and lead. The vein swarm is like those associated with giant copper porphyry deposits in Utah and worldwide, suggesting the district may have unrecognized mineralization potential.
Importance of Copper Porphyry Deposits
Copper is one of the most foundational metals in modern economies and is essential to many aspects of modern life. It is critical to electric/hybrid vehicles, power transmission lines, plumbing components, solar panels, electronics, and wind turbines, among many other applications. As more emphasis is placed on domestic mineral resources, attention has returned to many historical mining districts and the opportunity for unrecognized mineralization. In Utah, this attention is largely directed at the potential for copper (+/- molybdenum, gold, and silver) porphyry deposits—the same kind of deposit that is currently being mined at Kennecott’s Bingham Canyon.
The extent and scale of veins at Gold Hill, based on historical mapping by Dumont Exploration.
The focus on copper porphyries is due to their economy of scale, characterized by large tonnage and consistent grade, and their common association with other deposit types and potential byproducts. Porphyry copper systems are the world’s primary source of mined copper, accounting for ~60% of production globally and over 90% of production in the United States. Additionally, because such a large tonnage of rock is processed to extract copper from porphyry deposits, many otherwise sub-economic minerals and commodities become viable to produce. These minerals include several critical minerals, such as tellurium, which is a byproduct of production at Bingham Canyon.
Despite their importance, copper porphyry deposits are becoming more difficult to discover, and there is increasing focus on methods to discover “blind” deposits, i.e., those that do not outcrop at the surface. The modern prospecting and exploration approach for porphyries focuses on identifying geologic features, such as vein swarms that are commonly associated with, but distant from, the porphyry itself, and using those features to point towards porphyry mineralization. Porphyry copper deposits are created in areas of volcanism and from magmatic processes that generate water and metal-enriched magmas. The ore zones (copper sulfide minerals such as chalcopyrite, chalcocite, bornite, and covellite) are often found in a “cupola,” a distinctive upside-down bowl shape that sits as a cap over shallow subsurface magmatic bodies, which often have a porphyritic texture (large crystals in a fine-grained matrix— hence the term copper porphyry deposit). Utah contains several ancient volcanic/magmatic terrains which are prospective for porphyry mineralization, such as Gold Hill.
Importance of Copper Porphyry Deposits
The Gold Hill mining district is one of the oldest mining districts in Utah and is the leading historical producer of tungsten and arsenic in the state. Much focus has been paid to skarn, replacement, epithermal, and sediment-hosted gold potential in the district, with relatively little interest in the dense vein swarm that forms a ~1.5 by 2 km footprint along a distinct northeast-southwest trend. Veins such as these can sometimes form in connection to porphyry deposits, particularly when occurring in such density. As an example, a similar size vein swarm occurs at the Bingham Canyon porphyry deposit. Since identifying the porphyry prospectivity associated with the veins at Gold Hill, UGS geologists have been mapping, characterizing, sampling, and dating the veins to understand their formation and potential indications of associated mineralization. This research and similar studies will provide new interpretations for the formation of mineralized geological features and offer the potential to better predict the location of unrecognized buried porphyry deposits, with the objective of expanding Utah’s critical mineral portfolio to stimulate exploration and development of these resources in Utah.
View from the Southern Mine with the remains of the mine’s winch and “hot bulb” semi-diesel engine, which was used to hoist ore from one of the veins in the Clifton Shears vein swarm.
