1. Age Constraints on Jerritt Canyon and Other Carlin-Type Gold Deposits in the Western United States -
   Relationship to Mid-Tertiary Extension and Magmatism

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 ⁴⁰Ar/³⁹Ar 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 ⁴⁰Ar/³⁹Ar dates on mica from mineralized Ordovician to Devonian sedimentary rocks gave misleading results. The youngest date of 149 Ma from the smallest <0.1-mm-size fraction shows that the temperature (120°-260°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 (¹⁸⁷Os/¹⁸⁸Os_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 ⁴⁰Ar/³⁹Ar 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 ⁴⁰Ar/³⁹Ar dates of 42 Ma. K/Ar dates on supergene alunite range from 4 to 30 Ma. K/Ar and ⁴⁰Ar/³⁹Ar 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 likely 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 dDH₂O values of hydrothermal fluids from 10 widely separated Carlin-type deposits (-134 ± 23‰). The low dDH₂O 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.
 
 

Origin of the Eskay Creek Precious Metal-Rich Volcanogenic Massive Sulfide Deposit: Fluid Inclusion and Stable Isotope Evidence

The Eskay Creek deposit is an unusual, polymetallic, Au-Ag-rich volcanogenic sulfide-sulfosalt deposit located in the Iskut River area of northwestern British Columbia, Canada. Economic concentrations of precious and base metals are contained in the 21 zone, which is divided into a number of subzones. As of December 31, 1998, total production and proven-probable reserves are 1.9 million tons (Mt) at 60.2 g/t Au, 2,652 g/t Ag, 3.2 percent Pb, 5.2 percent Zn, and 0.7 percent Cu. The 21B zone, which contains the bulk of the reserves, began production in 1995. The mineralization occurs mainly as well-preserved stratiform clastic beds of sulfide-sulfosalt debris and also as discordant footwall quartz sulfide veins.

The hydrothermal system that formed the Eskay deposit was low temperature (<200°C) with a relatively high gas content. Fluid inclusion petrography and measured gas ratios are consistent with liquid-vapor phase separation occurring in the hydrothermal system. The calculated fluid pressures, from fluid inclusion data, are variable as a result of contributions from vapor-rich fluid inclusions. Three samples gave calculated fluid pressures of ~150 bars that equate to a 1,500-m water depth. These analyses are considered the best indicators of the boiling depth since they most likely had the minimum contribution from vapor-rich fluid inclusions. Oxygen isotope ratios of quartz separates and whole-rock data suggest that the dominant hydrothermal fluid was normal seawater at temperatures of around 200°C. Fluid inclusion leachates suggest mixing between a seawater-derived fluid and a lower temperature (~100°C), more saline fluid which has high K/Na and Cl/Br ratios compared to normal seawater. The high-salinity fluid has halogen and cation ratios that are consistent with a magmatic-derived fluid. The relationship of this fluid to mineralization is uncertain. Sulfur isotope data suggest that the sulfide sulfur may have been derived from either an igneous source or by reduction of seawater sulfate. The dominant origin of the sulfate sulfur was seawater, although one barite sample may contain oxidized igneous sulfur. Mineralization at Eskay Creek is inferred to have formed at, or near, the sea floor in a relatively shallow-water setting, by fluid boiling which is an effective precipitation mechanism for gold and silver. The low solubility and ineffective precipitation mechanisms for base metals at these low temperatures resulted in mineralization with a high precious to base metal ratio. The best modern-day analogue of Eskay Creek mineralization is the JADE hydrothermal field in the Okinawa trough.
 

The Highway-Reward massive sulfide deposit is hosted by a submarine (below storm-wave base), silicic, synsedimentary intrusion-dominated volcanic succession. The succession includes at least 13 porphyritic units in a volume of 1 X 1 X 0.5 km. Peperitic upper margins suggest that most of the rhyolites, rhyodacites, and dacites were emplaced as small (<350 m diam) synsedimentary sills and cryptodomes. The intrusions are separated by thin (0.2ñ30 m) disrupted intervals of siltstone, sandstone, nonwelded pumice breccia, and polymictic lithic breccia. Evidence for eruption of magma onto the sea floor is limited to a single, partly extrusive cryptodome.

The Highway and Reward pyrite-chalcopyrite pipes occur within, but close to, the margins of the intrusions. The pipes are discordant to local bedding and contain relic patches of rhyolite, rhyodacite, and peperite. Pyrite ± quartz stringer veins extend beneath the massive sulfide pipes, and in some sections also occur in strongly altered strata above the pipes. The pyrite-chalcopyrite pipes are enveloped by a halo of pyrite-sphalerite ± chalcopyrite ± galena ± barite ore, which includes a small strata-bound lens. Near-surface pyritic ores have oxidized to form gossanous zones.

Massive sulfide ores are enclosed within a discordant hydrothermal alteration envelope that extends at least 150 m below the orebodies to over 60 m above the Highway pipe. The envelope exhibits a mineralogical zonation, with central quartz-sericite ± pyrite zones surrounded by zones of chlorite ± anhydrite ± gypsum, chlorite-sericite-quartz, and lastly, chlorite-sericite at the margins. Outside the hydrothermal alteration envelope, felsic volcanic rocks have altered to various assemblages of feldspar, sericite, chlorite, epidote, calcite, quartz, and hematite.

Overprinting relationships and isotopic values are consistent with syngenetic accumulation of the massive sulfides. Most of the ores formed by subsea-floor replacement of rhyolite, rhyodacite, and volcaniclastic units because: (1) massive sulfide ores are enclosed within intrusive or mass-flow emplaced units; (2) discordant and strata-bound ores contain relics of coherent facies or precursor volcanic particles; (3) peperite and massive sulfides are not mixed, implying ore deposition postdated emplacement of the enclosing succession; (4) pyrite pipes are discordant to local bedding; (5) there are replacement fronts passing from discordant pyrite pipes into a strata-bound sphalerite-rich lens; and (6) zones of strong hydrothermal alteration and veining extend into the hanging wall without any abrupt breaks in intensity.

At the Highway-Reward deposit, deformation, disruption of bedding, resedimentation, and induration of the host succession accompanied emplacement of sills and cryptodomes. The resultant variations in secondary permeability and porosity are interpreted to have focused ascending hydrothermal fluids within the fractured glassy margins of synsedimentary intrusions. Pyrite-chalcopyrite pipes formed from relatively oxidized, mildly acidic (pH 4.5-5.0), high-temperature (>300°C), H₂S-dominant fluids by replacement of rhyolite, rhyodacite, and peperite. Lower temperature (<300°C) fluids that diffused from the margins of the pipes deposited a halo of sphalerite-rich ore.

The genesis of komatiites, basalts, and associated Ni mineralization in the Dundonald township area, Ontario, is critical to understanding the metallogenic evolution of the Kidd-Munro assemblage, one of the most primitive volcanic assemblages in the world. The 2.5-km-thick stratigraphic succession has a basal calc-alkalic basalt-dacite-rhyolite sequence (2716.8 ± 2.1 Ma, U-Pb zircon) cut by the Dundonald peridotite-gabbro sill (2707^-3₊₅ Ma, U-Pb zircon), overlain by komatiitic and basaltic flows containing magmatic Ni sulfide mineralization, which are, in turn, overlain by chemically distinctive low Ti basalt flows. The Dundonald komatiites correlate with komatiites in the footwall of the giant Kidd Creek volcanic-associated massive sulfide deposit 40 km to the west, but are slightly older than the komatiites of Munro township which are 40 km to the east.

The Dundonald komatiites comprise a Munro-type, Al-undepleted komatiite suite, with liquid compositions having Al₂O₃/TiO₂ = 18 to 22 and MgO up to 30.3 wt percent (anhydrous, n = 24). Most samples in the komatiite suite have slightly depleted light REE contents, with La_N/Sm_N = 0.6 to 1.1 and La_N/Yb_N = 0.4 to 0.9 (n = 21), and negligible Ti/Ti* and Zr/Zr* anomalies. Their major and trace element geochemistry is consistent with a derivation by significant partial melting of a chemically primitive mantle with little or no influence of majorite garnet. Flows that are host to the Dundeal and Alexo Ni deposits have distinctively high Th_PM/Nb_PM ratios (>1.7, n = 3), suggesting crustal contamination, and peperite textures that are consistent with contamination from nearby graphitic argillite. The chemically distinctive low Ti basalts have 49 to 53 percent SiO₂, 4.5 to 9.5 percent MgO, 18.8 to 21.5 percent Al₂O₃, 0.30 to 0.37 percent TiO₂, moderately depleted light REE contents, with La_N/Sm_N = 0.9 to 1.2 and La_N/Yb_N = 0.4 to 0.6, high Ni, Cr, and Ba contents (570-6,800, 390-630, 90-320 ppm, respectively; n = 6). Their high Al₂O₃ contents place them in the calc-alkalic field on a Jensen cation plot, although their trace element ratios (Zr/Y, La/Yb) are more consistent with a tholeiitic affinity. They are compositionally identical to, and they correlate with, low Ti basalts in the Kidd Creek Volcanic Complex. Their geochemistry is consistent with a derivation by partial melting of a refractory harzburgitic mantle that has undergone a previous melt extraction, possibly influenced by a large ion lithophile-enriched hydrous phase.

The Empire flow is the thickest and most primitive komatiite flow and is host to the Dundeal Ni deposit (resource of ~0.4 Mt at 2.0% Ni). The Empire flow contains an elongate dunite-peridotite basal unit interpreted as the core of a flow that thermally eroded into footwall heterolithic breccias and pillowed andesites. The main Dundeal Ni horizon overlies part of the basal dunite-peridotite unit within the flow channel and extends over the cutbank wall. The Dundeal deposit and the nearby Dundonald South Ni deposit have low total sulfide contents and high Ni/S ratios (0.9 and 1.0, respectively) in comparison to most komatiite-hosted magmatic sulfide deposits. The high Ni tenor in sulfide is believed to be due to assimilation of reducing carbonaceous sediment which causes greater Ni partitioning into the magmatic sulfide phase.
 

The Iberian pyrite belt volcanogenic sulfides are hosted in volcano-sedimentary successions of Upper Devonian to Lower Carboniferous age and represent the greatest concentration of large massive sulfide deposits on Earth. Most of the ore deposits are exposed at the surface, and little investment in geological research has been needed to mine them. As a consequence, fundamental aspects of the ore geology, such as the depositional environment, the eruptive style of volcanism, the chronostratigraphic relations between facies types, and the final mode of emplacement of volcanic rocks have not been studied previously in detail.

A facies analysis has been systematically carried out on several volcano-sedimentary successions of the ore-host unit. Seventeen volcanic and sedimentary facies are defined. The main facies are silicic volcanic facies, mafic volcanic facies, reworked volcaniclastic facies, siliciclastic facies, slope instability facies, and hydrothermal facies. Facies analysis suggests that the Iberian pyrite belt volcanism took place in a submarine, below-wave-base, depositional environment and that water depth increased to the east and north in the ore-hosting basin. Fragmentation mechanisms suggest that the eruptive style of volcanism was mainly nonexplosive, though explosive volcanism may have occurred outside the basin. Transport and depositional processes suggest that most of the volcaniclastic rocks were emplaced from syneruptive, nonpyroclastic mass flows. Contact relationships of volcanic and sedimentary facies reveal that peperitic facies are very widespread, silicic and mafic magmas intruded simultaneously at shallow levels in the volcano-sedimentary pile, and low-temperature hydrothermal processes may have occurred prior to volcanism at many sites.

Fine-grained pyrite is the earliest generation of pyrite and the most abundant sulfide within the Urquhart Shale at Mount Isa, northwest Queensland. The pyrite is intimately interbanded with ore-grade Pb-Zn mineralization at the Mount Isa mine but is also abundant north and south of the mine at several stratigraphic horizons within the Urquhart Shale. Detailed sedimentologic, petrographic, and sulfur isotope studies of the Urquhart Shale, mostly north of the mine, reveal that the fine-grained pyrite (d³⁴S = -3.3 to +26.3‰) formed by thermochemical sulfate reduction during diagenesis. The sulfate source was local sulfate evaporites, pseudomorphs of which are present throughout the Urquhart Shale (i.e., gypsum, anhydrite, and barite). Deep-burial diagenetic replacement of these evaporites resulted in sulfate-bearing ground waters which migrated parallel to bedding. Fine-grained pyrite formed where these fluids infiltrated and then interacted with carbon-rich laminated siltstones.

Comparison of the sulfur isotope systematics of fine-grained pyrite and spatially associated base metal sulfides from the Mount Isa Pb-Zn and Cu orebodies indicates a common sulfur source of ultimately marine origin for all sulfide types. Different sulfur isotope ratio distributions for the various sulfides are the result of contrasting formation mechanisms and/or depositional conditions rather than differing sulfur sources. The sulfur isotope systematics of the base metal and associated iron sulfide generations are consistent with mineralization by reduced hydrothermal fluids, perhaps generated by bulk reduction of evaporite-sourced sulfate-bearing waters generated deeper in the Mount Isa Group, the sedimentary sequence which contains the Urquhart Shale. The available sulfur isotope data from the Mount Isa orebodies are consistent with either a chemically and thermally zoned, evolving Cu-Pb-Zn system, or discrete Cu and Pb-Zn mineralizing events linked by a common sulfur source.
 

A growing body of evidence indicates a relationship in the genesis of the Pb-Zn-Cu ores of the Southeast Missouri district to sedimentary brines mobilized during Late Paleozoic tectonism along the Appalachian-Ouachita orogenic belt. An effective mechanism for this brine mobilization would have been the steep topographic gradients created in the Ozark region during the Ouachita orogeny. The present study is an effort to reconstruct the paleohydrology of the flow system and explore its implications for ore formation.

Transient finite element simulations of fluid flow, heat transport, and solute transport were used to describe the evolution of the fluid velocity, temperature, and salinity fields as a result of uplift and erosion of Ozark topography. A vigorous south to north flow regime was predicted, characterized by strong recharge near the southern boundary of the Arkoma basin and strong discharge over the crest of the Ozark dome. Ground-water temperature and velocity were found to increase with time during the early stages of uplift, before reaching a maximum and thereafter declining to somewhat lower values that remained steady over time. Continuous meteoric recharge led to the development of a freshwater plume that gradually migrated through the flow system, displacing the more saline pore fluids that were present initially. The calculations showed further that the onset of fluid velocities and temperatures optimal for forming the ores to coincide very closely with the onset of low salinities unsuitable for forming the ores. Unless salinity was somehow replenished, a relatively short period of time, on the order of hundreds of thousands of years or perhaps less, was available for mineralization in Southeast Missouri. Temperature variation in the Cambrian sediments was found to be minimal south of the Viburnum Trend after the earliest stages of uplift. Within the Viburnum Trend, however, a temperature gradient of about 25°C over the length of the trend was predicted. The magnitude of the temperatures generated in the modeling also agreed well with the fluid inclusion data, matching most of the range of measured values.

Once uplift had ceased, erosion would have gradually diminished topographic relief and hence, the driving mechanism for fluid flow and much of the heat transfer. The modeling results showed that ore-forming temperatures in Southeast Missouri would have been maintained for no more than a few million years once uplift had ceased, whereas ore-forming fluid velocities would have been maintained much longer, on the order of tens of millions of years.
 
 

Chuquicamata is the world's largest porphyry copper deposit, notwithstanding the fact that a portion of the orebody has been faulted off by postmineralization movement along the West fault. In order to locate the missing portion of the orebody in the vertical dimension, a study was designed to estimate the sense and amount of vertical displacement along this major structure by measuring the (U-Th)/He and fission-track ages of vertically distributed apatite samples from each of the crustal blocks (Fortuna and Chuquicamata Intrusive Complexes) bordering the fault. Apatite (U-Th)/He ages range from 32 to 16 Ma, whereas apatite fission-track ages range from 33 to 28 Ma, reflecting the lower closure temperature of the (U-Th)/He thermochronology method (~75° vs. ~125°C for cooling rates of ~10°C/m.y.). The (U-Th)/He ages decrease systematically with depth in both blocks, however, the age-elevation curve for the western Fortuna block is shifted vertically with respect to the eastern Chuquicamata block, indicating that the postmineralization denudation was significantly greater to the west. The minimum vertical displacement along the West fault is estimated to be 600 ± 100 m, implying that the missing portion of the Chuquicamata deposit should be located at a present-day elevation of at least 3,600 m. The new apatite ages, combined with previous thermochronometric data (Rb-Sr, U-Pb, Ar-Ar), reveal rapid cooling rates (~100°C/m.y.) for the Chuquicamata deposit following emplacement at about 35 Ma, thereby indicating that the Cu mineralization took place at a depth of less than 4 km.
 

The Banska Stiavnica Au-Ag base metals epithermal deposit is hosted within a Neogene-age volcanic caldera in central Slovakia. The caldera comprises a central granodiorite stock that has been capped by comagmatic andesite and rhyolite extrusions. The intrusive felsic rocks possess a close spatial and temporal relationship with the mineralization and associated hydrothermal alteration. To investigate the possible genetic link between magmatic and hydrothermal activity, paragenetically constrained melt and fluid inclusions in magmatic quartz and vein minerals were studied, using microthermometric techniques. Primary melt inclusions in magmatic quartz from the granodiorite vary in composition from essentially silicate H₂O- and Cl-rich melt with low-salinity fluid (8.3-9.6 wt % NaCl equiv) to high-density hypersaline brines (~80 wt % NaCl equiv). Salinities of secondary fluid inclusions in magmatic quartz systematically decrease along the NaCl saturation curve toward lower temperatures and salinities equivalent to those determined for primary fluid inclusions in sphalerite and vein minerals (quartz, barite, fluorite) within the deposit (<400°C, <12 wt % NaCl equiv). This systematic evolution in measured and calculated characteristics (temperature, pressure, salinity, and density) of the studied fluid inclusions indicates that exsolved magmatic brines and aqueous chloride solutions were the primitive precursors to the hydrothermal ore-forming fluids that produced epithermal mineralization upon mixing with meteoric waters in the near-surface environment.