The search for precious metals, specifically gold (Au), has a storied history. Early cultures produced fashionable objects known for their beautiful designs incorporating gold. Throughout human history, technology has been at the forefront of the discovery, evaluation, and production of gold from the Earth’s crust. Most recently, the price of gold has approached $2100/oz. The exploration of gold throughout the world has been renewed to prove-up overlooked or second-rate deposits and discover new resources. So where can these resources be found?
Western United States
Carlin-type disseminated gold deposits in Western Conterminous United States remain one of the most highly productive targets in the world. The extraction of gold in Nevada over the past 60 years has pushed the United States to the forefront of leading global gold producers. It’s hoisted Nevada into second place behind South Africa as the host of the largest gold deposits on Earth. From a geological perspective, hypotheses on the genesis of Carlin-type deposits remain framed around the concepts of passive margin sedimentary rock deposition, structural controls, fluid flow, and mineralization. Since these deposits remain disseminated (read: microscopic gold incorporated in sulphides), they will not generate appreciable volumes of alluvial-hosted placer-type deposits. They are primarily associated with lode-type claims. For information on how to effectively stake claims, please review this post.
Geologists recognize several tectonic events that have shaped the Western United States into the modern geography we see today. These events set the stage for the Carlin-type deposition of gold. Roughly 400 million years ago (Ma), the proto-continent of North America (Laurentia) straddled the equator. At this point in earth history, a passive tectonic margin existed along the West coast of Laurentia near modern Utah that resulted in the shallow deposition of sedimentary rock such as limestone, shale, and sandstone. Ultimately, collisional tectonic events (orogeny) built out Western North America through the accretion of small continental landmasses over 100s of million of years. In addition to gaining landmass, these orogenies created conduits for fluid migration because of the thrusting and faulting associated with the tectonic events. Later, Basin and Range type extension of the continental crust uplifted blocks of these subterranean auriferous deposits.
Carbonate Rocks
Carlin-type deposits appear hosted in calcareous rich carbonate rocks such as limestone, silty limestone, and calcareous shales. These sedimentary units deposited in the shallow ocean of the passive margin during Ordovician, Silurian, and Devonian geological time periods (<400 Ma). Subsequently, these sedimentary units appear fractured, resulting in conduits for auriferous fluids. Tectonic events also contorted these rock units into structural traps such as domes and anticlines. Calcareous rocks are more permeable which allows fluids to dissolve preexisting rock, migrate throughout, and deposit minerals. In concert with the structural traps, fine-grained sedimentary units, such as shale, act as barriers to this migration, and “trap” fluids in the calcareous units which ultimately results in the deposition of minerals.
Ultimately, in the Eocene epoch (~40 Ma), passive migration of auriferous fluids from deep-seated magmatic and hydrothermal origins interacted with meteoric water from the surface. Geochemically, these fluids reacted with the calcareous sedimentary rocks and precipitated submicron gold in sulphides such as pyrite. This sequence of geological events resulted in the well-known gold-bearing zones. They range across Nevada, such as the Carlin Trend, the Eureka-Battle Mountain trend, and the Getchell trend. These auriferous rock units appear shallow in the surface in “windows”. Although most undiscovered Carlin-type deposits lay hidden beneath layers of younger, less favorable rock types. The team at Burgex Mining Consultants can help uncover these resources. We succeed through a combination of research, exploration, and precision sampling techniques to reveal immense value in this region! Even when buried deep beneath the surface, analytical techniques aid in the detection of valuable resources.
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References:
Cline, J.S., Hofstra, A.H., Muntean, J.L., Tosdal, R.M., Hickey, K.A., 2005, Carlin-type gold deposits in Nevada: critical geologic characteristics and viable models, Economic Geology, v. 100, p. 451-484.
Muntean, J.L., Cline, J.S., Simon, A.C., Longo, A.A., 2011, Magmatic-hydrothermal origin of Nevada’s Carlin-type gold deposits, Nature Geoscience, v. 4, p. 122-127.
