NORTHERN NEVADA GOLD OVERVIEW – THE OPPORTUNITY
Nevada boasts 75% of US gold production and 5% of the world’s output. Of the 27 gold mines currently active in the state, 19 are located in Lander, Eureka, Humboldt, and Elko counties. First discovered in the 1930s, prospectors referred to “invisible” gold since it could only be determined through assay. Workers discovered this was due to lattice substitution of gold in normally gangue minerals pyrite and arsenopyrite. When the Carlin deposit was discovered in 1962, it put these low-grade deposits on the map and came to define a type of mineralization now known as a Carlin-Type Deposit. The Carlin discovery is considered in Nevada mining lore to be second in economic and historical importance only to the Comstock silver deposits discovered on the eastern slopes of the Sierra Nevadas in 1859. Today, Carlin-type and regionally associated gold mines account for 89% of Nevada’s total gold production.
Active gold mines, Reno and Las Vegas included for reference
Nevada gold plays and trends
Prior to Carlin, the region was tapped for base metal porphyry deposits of copper, molybdenum, tungsten, mercury, lead and zinc, with local vanadium and uranium plays as well. Yet the Carlin discovery, coupled with relaxation of federal pricing controls, shifted interest to gold, and graphics tell the rest of the story:
The western margin of the North American continent was underwater for nearly all the Paleozoic and above sea level since, subject thereafter to terrestrial sedimentation and extrusive igneous activity in places. The depositional environments have evolved beneath tectonic constraints but broadly include siliceous and carbonate sedimentary sequences stacked like pancakes, older rocks on younger rocks and the other way around.
The northern Nevada gold province is mostly contained within and/or bounded by the Roberts Mountain Allochthon, an early Paleozoic siliceous eugeoclinal sedimentary plate thrust to the east over an Ordovician-lower Mississippian dirty carbonate miogeoclinal assemblage during the Devono-Mississipian Antler orogeny. The imbricate thrusting of the region’s first orogeny capitalized on zones of structural weakness dating from Pre-Cambrian to Silurian normal faulting along the stable North American continental margin. The resulting structural porosity was then exacerbated by continuing compressive tectonism during the Pennsylvanian (Humboldt orogeny) the Triassic (Sonoma orogeny) and the early Cretaceous (Sevier orogeny). Coeval and subsequent mineralizing fluids are sure to have had plenty of pathways in the structurally complex and evolving terrain.
Intrusive magmatism is regionally documented from the Triassic, Jurassic Cretaceous, Eocene, Oligocene and Pliocene periods. The predominant lithologies are granodiorite and monzogranite. Plutons occur as dike swarms along zones of structural weakness, like the BME. Despite a profusion of mineral deposits, 3000 registered occurrences by one count, no empirical association exists between igneous lithology and proximal metal concentrations.
It would appear that igneous intrusion is surely a prime mover of mineralization but that it is not in itself much of a diagnostic prospecting tool.
While a genetic linkage between regional deposits is implicit, the province as a whole remains unusually heterogeneous and defies easy metallogenic prescription. Oxygen and hydrogen isotopes show that both magmatic and meteoric water were locally important, often in combination. Likewise, sulfur isotopes point to both magmatic and sedimentary sources of sulfide. With these key drivers of mineralization coming from such different sources, it appears likely that a source of metals and heat was present through at least the Mesozoic and Cenozoic eras and that geothermal waters of the time circulated into mineralizing traps regionally as a coincidence of time and local circumstance.
Low mineralogical diversity of ore assemblages and the “invisible” quality of the Carlin-type gold has complicated classic approaches to interpretation. The possible “re-setting” of temperatures in key age-dating isotopic pairs by Oligocene intrusions has also been raised as a complication in confidence of other age-dating conclusions. Plus it’s just complicated. Whatever the excuses, a definitive metallogenetic model for the region as a whole remains elusive.
A purely Carlin-type mineralization model has been advanced, however.
In broad strokes, regional mineralization began with porphyry base metal deposits in the Cretaceous that favored the BME zone of crustal weakness. Though gold was present in the Cretaceous mineralizing fluid, the Carlin gold front came 10 million years later in the Eocene and is certainly the region’s greatest gold mineralization event. Yet Oligocene deposits and even Tertiary tuffs show ongoing vulcanism and probably continuing mineralization phases that complicate interpretation, particularly where two or more mineralization styles overlap, as they do in the BME Trend.
MINING AND REFINING
Open pit is the predominant mining paradigm for Carlin-type gold, with some underground operations. Heap cyanide leaching of milled ore is the extractive technology. Due to the low grade of these deposits, most were economically marginal until the Fed relaxed its pricing controls and gold began its spot climb in the 1970s. The most notable exception is the Carlin Mine itself, which has remained in operation since its 1965 launch, due largely to its large orebody volume and partly due to smart extraction technology improvements amid the vagaries of spot pricing through the years.
So what is it about this gold province that makes it both so productive and so heterogeneous and unique?
- Mineralization Pathways. The terrain has experienced structural stresses from basin subsidence, sea-floor spreading, compressive orogeny, igneous intrusion and crustal extension. It has fractures on top of fractures, cleaved into fault planes lubricated by tectonic juice. The complex network of structural porosity provides channels to ongoing mineralizing fluids rising from or powered by proximal intrusions.
- Structural Serendipity. The Antler Orogeny overthrust early Paleozoic siliceous sediments across Devonian-Mississippian carbonates to comprise what we call the Roberts Mountains allochthon. The hydrothermal effect of this event was to cap a chemically active carbonate rock sequence with a siliceous aquitard and create an ideal trapping structure for metals . . . should they ever migrate in that direction.
- Mineralization. Sure enough, regional base-metal mineralization kicked off in the Cretaceous, a mineralizing front expelling its outermost and less intransigent minerals to take solid form as minerals all across the province, mostly as porphyry Cu-Mo deposits. But Carlin gold arrived ten million years later, overprinting Cretaceous base metal concentrations during the Eocene with a massive auric overprint.
- Uniqueness. The Carlin-type gold model is today known to occur elsewhere (China, Iran, Macedonia) and to be a major style of gold concentration. Yet the evolving model from afield remains inadequate to substantiate a comprehensive Roberts Mountain Gold Province model. With words like “poorly understood,” “disputed,” and “unsubstantiated” littering the trade literature, Nevada’s BME mineralization remains, at least for the moment, beyond the full grasp of geochemical ratiocination.
- Temporal Mélange. The north-central Nevada gold province has existed within a highly-complex tectonic setting active semi-continuously for the past 400 million years. Processes in play across its life include continent-margin sedimentation, plutonic intrusion, metamorphism, orogeny, crustal extension and mineralization. The tectonic constancy and variety have left a shifting veneer on its ore deposits, which do not lend themselves to a simple metallogeny.
- Commonality: The province does have a common dominator however: gold.
ALL THIS AND TWO BITS . . .
While all of that is nice, perhaps the most optimistic inference lies in the gold-endowment comparison of the Carlin and BME districts. This analysis compares known and exploited deposits in a given district with logarithmic production on the X-axis with their statistical relevance reported as numeric quantile percentage on the Y-axis. The result is a deposit-size probability model specific to the district, allowing districts to be compared.
By its own internal metrics, the BME curve approaches the Carlin curve for large accumulations (lower right portion of the model). Indeed the statistical curve softening of the BME model appears to under-represent this effect. This graph implies based on historical production evidence alone that undiscovered BME deposits exist that are within spitting distance of the largest Carlin deposit found to date.
It’s enough to keep a geologist awake at night.