Geology Reports⌕ Search
Home/Archive/USGS

USGS · 70021396

Age constraints on Jerritt Canyon and other Carlin-type gold deposits in the western United States-relationship to mid-Tertiary extension and magmatism

A. H. Hofstra·L.W. Snee·R. O. Rye·H. W. Folger·J.D. Phinisey·R.J. Loranger·A.R. Dahl·C. W. Naeser·H. J. Stein·M. Lewchuk
View original source ↗Export metadata ↓

Abstract

Carlin-type gold deposits are difficult to date and a wide range of ages has been reported for individual deposits. Therefore, several methods were employed to constrain the age of the gold deposits in the Jerritt Canyon district. Dated igneous rocks with well-documented crosscutting relationships to ore provided the most reliable constraints. K/Ar and 40 Ar/ 39 Ar dates on igneous rocks are as follows: andesite dikes 324 Ma, sericitic alteration in andesite dikes 118 Ma, basalt dikes 40.8 Ma, quartz monzonite dikes 39.2 Ma, and calc-alkaline ignimbrites 43.1 to 40.1 Ma. Of these, only the andesite and basalt dikes are clearly altered and mineralized. The gold deposits are, therefore, younger than the 40.8 Ma basalt dikes. The sericitic alteration in the andesite dikes is unrelated to the gold deposits. A number of dating techniques did not work. K/Ar and 40 Ar/ 39 Ar dates on mica from mineralized Ordovician to Devonian sedimentary rocks gave misleading results. The youngest date of 149 Ma from the smallest <0.1-mu m-size fraction shows that the temperature (120 degrees -260 degrees C) and duration (?) of hydrothermal activity was insufficient to reset preexisting fine-grained micas in the host rocks. The temperature and duration was also insufficient to anneal fission tracks in zircon from Ordovician quartzites as they yield Middle Proterozoic dates in both mineralized and barren samples. Apatites were too small for fission track dating. Hydrothermal sulfides have pronounced crustal osmium isotope signatures ( 187 Os/ 188 Os initial = 0.9-3.6) but did not yield a meaningful isochron due to very low Re and Os concentrations and large analytical uncertainties. Paleomagnetic dating techniques failed because the hydrothermal fluids sulfidized nearly all of the iron in the host rocks leaving no remnant magnetism.When published isotopic dates from other Carlin-type deposits in Nevada and Utah are subject to the rigorous evaluation developed for the Jerritt Canyon study, most deposits can be shown to have formed between 42 and 30 Ma. K/Ar and 40 Ar/ 39 Ar dates on the youngest preore igneous rocks range from 41 to 32 Ma, whereas the oldest postore igneous rocks range from 35 to 33 Ma. Hydrothermal adularia from the Twin Creeks deposit yields similar 40 Ar/ 39 Ar dates of 42 Ma. K/Ar dates on supergene alunite range from 4 to 30 Ma. K/Ar and 40 Ar/ 39 Ar dates on micas separated from sedimentary (395-43 Ma) and igneous (145-38 Ma) rocks are usually much older than the gold deposits and most are suspect because they are from incompletely reset preore micas or from mixtures of preore and ore-stage mica. Fission track dates on zircons are also generally older than the deposits (169-35 Ma) and are not completely reset by mineralization. Apatites are likley to be reset by the hydrothermal systems (and by younger thermal events) and yield dates (83-22 Ma) that are younger than those from zircon. Independent support of a mid-Tertiary age is provided by the low delta D (sub H 2 O) values of hydrothermal fluids from 10 widely separated Carlin-type deposits (-134 + or - 23ppm). The low delta D (sub H 2 O) values are consistent with a mid-Tertiary age when the climate was cool but are inconsistent with the Cretaceous and Jurassic ages favored by some workers when the climate was warm. The age and distribution of Carlin-type deposits coincides with areas that underwent extension and calc-alkaline magmatism over the same time period. Despite this relationship, the deposits lack clear spatial or genetic relationships to mid-Tertiary epizonal plutons. Rather, many deposits are localized along preexisting crustal fault zones. In the Getchell trend, structures tapped deep-sourced metamorphic (or magmatic) fluids and subsequently variably exchanged meteoric water. In the other trends and districts only variably exchanged meteoric water has been detected. These relationships suggest either that all of the deposits formed from deep-sourced fluids and were subsequently flooded by meteoric water, or, that fluids from different sources evolved to produce deposits with similar charateristics.

Explore related subjects

Economic GeologyArticle

Keep this discovery

Explore connections, maps & timelines

BibTeXRIS

A. H. Hofstra, L.W. Snee, R. O. Rye, H. W. Folger, J.D. Phinisey, R.J. Loranger, A.R. Dahl, C. W. Naeser, H. J. Stein, M. Lewchuk. 1999-09-01. Age constraints on Jerritt Canyon and other Carlin-type gold deposits in the western United States-relationship to mid-Tertiary extension and magmatism. https://doi.org/10.2113/gsecongeo.94.6.769

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related USGS reports

USGS2026

Origin of the Pd/Pt ratio of the J-M Reef, Stillwater Complex, Montana, USA

The J-M reef of the Stillwater Complex is characterized by a high Pd/Pt ratio (mean ~3.8 with a standard error of 0.03) with a homogeneous geospatial distribution at the deposit scale. In this contribution, we demonstrate that the Pd/Pt ratio of the reef is the product of equilibration of an immiscible sulfide liquid with a silicate melt rich in Pd relative to Pt. Despite the high tenors of the J-M reef sulfides (avg 2,700 ppm Pt and 770 ppm Pt), numerical modeling shows that the parental melts did not have extraordinary Pd and Pt concentrations. Instead, the initial composition of a plausible parental silicate melt can have Pd and Pt contents well within the expected range of a normal, mantle-derived partial melt (i.e., ~10–20 ppb for both Pd and Pt with Pd/Pt of ~1). The relative differences in the partitioning behavior of Pt and Pd between sulfide liquid and silicate melt are unlikely to produce a consistent Pd/Pt ratio across a wide range of silicate melt to sulfide liquid mass ratios (i.e., R factors). Instead, the pre-emplacement fractionation of Pt alloy from S-undersaturated silicate magma accounts for the homogeneous and high Pd/Pt ratio of the J-M reef. We show that batch equilibration of sulfide liquid with silicate melt can produce the high Pd/Pt ratios of the reef if the partition coefficients between sulfide liquid and silicate melt for Pd and Pt are extremely high (>10 6 ). In an alternative model, Pd enrichment could be achieved by sulfide upgrading in resident footwall mush even if the partition coefficients between sulfide liquid and silicate melt are relatively small (between 10 4 and 10 6 ) because the instantaneous mass ratio of silicate melt to sulfide liquid is small (R ≈ 100–700), so the partitioning behavior of Pt and Pd has little impact on the composition of sulfide liquid.

Montana↗
USGS2025

Depth of magma crystallization and fluid exsolution beneath the porphyry-skarn Cu deposits at Santa Rita and Hanover-Fierro, New Mexico, USA

The depth level at which porphyry Cu–forming magmas fractionated and exsolved mineralizing fluids is actively debated. In the classic model, extensive magma fractionation occurs in large, upper crustal magma chambers, and concomitant fluid exsolution leads to forceful expulsion of residual magmas in the form of porphyry dikes, stocks, and breccia pipes, which subsequently serve as pathways for the mineralizing fluids. In contrast, some recent studies highlighting the role of deep crustal magma fractionation in the production of fertile magmas essentially deny the existence of upper crustal magma chambers at the time of mineralization. To address this, we conducted a detailed thermobarometric investigation of 13 intermediate to felsic, porphyritic intrusive rocks related to porphyry-skarn Cu mineralization at Santa Rita and Hanover-Fierro, New Mexico, United States, representing two premineralization magmas (61–60 Ma), seven synmineralization magmas (60–58 Ma), and four late- to postmineralization magmas (58–57 Ma). For each sample, the pressure of last magma crystallization before final magma ascent to the current exposure level was reconstructed based on Al-in-hornblende barometry of small hornblende inclusions trapped within quartz phenocrysts and through titanium-in-quartz (TitaniQ) thermobarometry of the host quartz phenocrysts themselves. Since quartz is one of the last crystallizing magmatic minerals, and no significant phenocryst growth could have occurred in small dikes and stocks after final magma emplacement, quartz phenocrysts and their contained hornblende inclusions record the depth of last magma crystallization before final magma ascent. When present, hornblende phenocrysts and hornblende inclusions within other major phenocrysts were also analyzed. Both quartz and hornblende barometers return consistent average pressures of 3.2 ± 0.4 kbar for the entire suite of pre- to postmineralization magmas, corresponding to depths of 11 to 14 km. The synmineralization magmas return even more consistent average pressures of 3.1 ± 0.2 kbar, corresponding to a depth of 12 ± 1 km. The volume of the mineralizing porphyry dikes and stocks at the emplacement level is far too small to have provided all the fluids and metals required to form the observed ore deposits. Therefore, the majority of the ore-forming fluids must have originated from the magmas that crystallized at 12 ± 1 km depth. The ore deposits, conversely, formed at ~5-km paleodepth. This implies that most of the mineralizing fluids traveled an average vertical distance of ~7 km from their magmatic source to the eventual site of ore precipitation. The relatively unaltered nature and low veining degree of deeper parts of mineralized porphyry dikes and stocks suggest that the fluid transport through these intrusive bodies occurred mostly at near-solidus conditions by means of fluid percolation along grain boundaries. In summary, our results suggest that (1) a large, upper crustal pluton exists ~7 km beneath the Santa Rita and Hanover-Fierro deposits; (2) abundant phenocryst crystallization occurred at this depth level; and (3) this pluton was the main source for the exsolution of ore-forming fluids. However, the investigated rocks have elevated whole-rock Sr/Y ratios, indicating magma fractionation at deep crustal levels. As a result, our preferred model is a combination of the two end-member models introduced above, with most magma fractionation having occurred in the deep crust and with residual, intermediate to felsic melts having ascended and accumulated at 11 to 14 km paleodepth, where they continued to crystallize with comparatively little crystal-liquid separation, before some of these magmas ascended further to shallow levels and quenched to porphyries.

New Mexico↗
USGS2025

Assimilation of reduced carbon triggers platinum alloy saturation in mafic and ultramafic magmas

It is generally observed that magmatic sulfide ores have higher ratios of Pd/Pt than the mantle-like values of their parental magmas. This discrepancy has defied simple explanation because the partitioning behavior of both elements between sulfide and silicate liquids is very similar. Assimilation of sulfur- and carbon-rich country rocks by mafic and ultramafic magmas is considered a critical, if not essential, step in the formation of magmatic base metal sulfide deposits. Although there is general consensus that the assimilation of external sulfur and carbon promotes sulfide saturation, the effect of carbon assimilation on the solubilities of platinum-group elements in natural S-bearing silicate melt has been overlooked. In this study, we investigate the variations of platinum and palladium solubilities during assimilation of graphite and methane through thermodynamic modeling, in comparison with data from an array of highly distinctive magmatic sulfide ore systems representing ages from Archean to Paleozoic, melt compositions from komatiite to basalt, and magmatic settings including lavas, hypabyssal intrusions, plutonic continental arc roots, and plutonic layered intrusions, namely: Raglan, Norilsk-Talnakh, Lac des Iles, and the J-M Reef of the Stillwater Complex. We model assimilation-fractional crystallization processes to estimate the reduction of oxygen fugacity ( ⁠fO2⁠ ) of the melt due to incorporation of graphite and methane. The simulations show that although Pd remains highly soluble during the progressive assimilation of reduced carbon, Pt solubility decreases significantly as the silicate melt becomes increasingly reduced. With less than 8 % of sediment assimilation, Pt alloy may saturate and then deviate from sulfide-undersaturated silicate melts, concomitantly increasing the Pd/Pt value of the remaining melts of the Raglan and Norilsk-Talnakh systems. For the Lac des Iles and Stillwater systems, a higher extent of assimilation is needed to reach Pt saturation because of the relatively carbon-poor nature of the lower crustal rocks. The assimilation of methane volatiles is shown to be more effective than graphite assimilation, and it provides a pathway to Pt alloy fractionation in the absence of detectable amounts of bulk host-rock assimilation. High Pd/Pt values have been documented in many world-class magmatic sulfide deposits whose parental magmas have demonstrably experienced crustal contamination. Our model suggests that although anomalous Pd/Pt values may be explained by other mechanisms such as incongruent melting of preexisting sulfide or differences in the diffusivities of the metals within achieving equilibration, the assimilation of graphite or methane may play an important role in the global occurrence of magmatic sulfide ores with elevated Pd/Pt values.

Economic Geology↗