ALBERT

Description: Apatite is a common resistate mineral occurring in a range of host rocks and ore-related hydrothermal alteration assemblages. Apatite in several porphyry copper deposits in British Columbia has a unique set of physical and compositional characteristics that can be used to evaluate the chemical conditions of magmas that formed the causative intrusions or associated hydrothermal alteration. Apatite under visible light and SEM shows no notable variations between unaltered and altered varieties but cathodoluminescence reveals significant differences. Apatite in unaltered rocks displays yellow, yellow-brown, and brown luminescence, whereas in K silicate-altered rocks apatite displays a characteristic green luminescence. The green-luminescent apatite replaces yellow- or brown-luminescent apatite and locally overgrows it. Apatite occurring with muscovite (i.e., phyllic)-altered rocks displays characteristic gray luminescence. The chemistry of apatite, as determined by electron microprobe and laser ICP-MS analyses, directly reflects its alteration and luminescence. The unaltered yellow-luminescent apatite has high concentrations of Mn (0.3–0.5 wt % MnO) and a high Mn/Fe ratio (〉1), whereas the brown-luminescent apatite has low Mn, but higher concentrations of S and REE + Y. The green K silicate alteration-related luminescence is caused by lower Mn/Fe ratios (ca. 1) along with depletions of other trace elements such as Cl, S, and Na. Gray-luminescent apatite occurring with muscovite-altered rocks results from significant Mn loss (〈0.15% MnO) contemporaneous with depletion in Na, S, Cl, and REE during low pH phyllic alteration in calc-alkalic porphyry deposits. The correlation between apatite texture, luminescence, and chemical composition with the type and intensity of porphyry alteration offers a potentially fast and effective method to utilize it as an indicator for porphyry mineralization in a range of exploration materials including soils, regoliths, and heavy mineral concentrates from glacial and fluvial materials.

Description: The stable isotope ratios of various elements (e.g., H, C, O, S) have numerous uses to improve the understanding of the genesis and formation of hydrothermal and magmatic ore deposits, as well as having various applications to mineral exploration. However, stable isotope data has not been routinely collected during mineral exploration for various reasons related to cost per sample, the speed at which analytical data can be collected, and uncertainty regarding the benefits of stable isotope measurements to mineral exploration. Recent advances in analytical technologies which utilize infrared absorption spectroscopy (e.g., off-axis integrated cavity output spectroscopy [OA-ICOS]) mean that stable isotope data can now be collected in far greater quantities than has been previously possible. This advance in analytical technology, which allows for significantly more rapid and less expensive stable isotope analyses, has significant implications for the way in which stable isotope data can be collected and utilized during mineral exploration. Potential applications of stable isotope ratios to mineral exploration include delineating property- to district-scale stable isotope alteration halos and identifying "blind deposits" at depth, as well as vectoring toward new deposits within endowed districts. Stable carbon and oxygen isotope data collected using OA-ICOS from carbonate rocks surrounding the Screamer Carlin-type gold deposit in Nevada demonstrate that stable isotope alteration can be detected at distances of up to (and potentially more than) 3 km laterally around mineralization.

Description: Oxygen isotope ratios measured from microdrilled calcite in limestone wall rocks at the Banshee Carlin-type Au deposit show marked depletion proximal to Au mineralization, indicating isotopic exchange between wall-rock calcite and hydrothermal fluids. Isotopic alteration is spatially coincident with mineralization and puts minimum constraints on hydrothermal fluid infiltration outside of visible indicators (i.e., carbonate dissolution and silicification). Additionally, 18 O depletion is nearly homogeneous at hand-specimen scale indicating near-complete alteration of limestone protolith. The primary mechanism of isotopic exchange is coupled dissolution- precipitation leading to pseudomorphic replacement of calcite during hydrothermal fluid infiltration. Surface reactions between calcite and the hydrothermal fluids are evidenced by textural and chemical variations between altered and unaltered calcite in wall-rock limestone and limestone breccia. Cathodoluminescence (CL) reveals distinct changes in the luminescence of altered calcite relative to unaltered equivalent calcite, and laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) data from altered and unaltered calcite show that observed CL responses are due to changes in calcite mineral chemistry. Altered samples exhibit bright CL responses in calcite with increases in Mn and Fe. In addition to wall-rock calcite alteration, calcite veins without clear paragenetic relationships can be related to hydrothermal alteration owing to 18 O depletion, CL response, and positive Eu* anomalies. Together, isotopic alteration of wall-rock calcite and evidence of hydrothermal calcite veins define the distal expression of low-temperature hydrothermal alteration in calcite-bearing rocks at the Banshee deposit.

Description: Many Carlin-type gold deposits in northern Nevada are found adjacent to steeply dipping faults that channeled hydrothermal fluids upward into mineralized regions. However, some Carlin deposits comprise gently dipping tabular ore zones lacking any obvious large, steeply dipping, feeder structure beneath them. The goal of this paper is to test whether fluid flow into such ore zones also involved largely vertical upwelling, or whether lateral fluid flow, particularly along low-angle fault structures, may have played a significant role in their formation. In this study we use carbon and oxygen isotopes in conjunction with trace element geochemistry to investigate pathways taken by hydrothermal ore-forming fluids into the tabular subhorizontal ore zones of the Pipeline deposit, a giant carbonate rock-hosted Carlin-type deposit in Nevada. We sampled deep drill holes within, below, and to the side of the main ore zone to assess the extent of lateral versus vertical fluid flow up to 600 m below mineralization. The main ore zone at the Pipeline deposit is dominated by carbonate host rocks having 18 O and 13 C values that are depleted in the heavier 18 O and 13 C isotopes compared to global background values for rocks of the same age. Depletion results from the progressive buffering of the rock by an infiltrating auriferous hydrothermal fluid. There is significant heterogeneity in the spatial pattern and relative degree of 18 O and 13 C depletion through the ore zone, most likely reflecting multiple flow paths with different total integrated fluid fluxes, path lengths, rates of fluid cooling, and possibly fluid mixing. A lack of a pervasive reduction in 18 O and 13 C values, and low concentrations of trace elements in the rocks immediately beneath the main ore zone indicate that the deposit was not a product of large-scale vertical upwelling of auriferous hydrothermal fluid directly into the mineralized region. Rather, flow was focused along preexisting low-angle thrusts, particularly the Abyss fault, with fluids flowing laterally underneath and then up into the area of the main ore zone. Enhanced permeability along these low-angle structures most likely derived from fault reactivation and the generation of fracture networks in damage zones peripheral to a relatively impermeable fault core. Such reactivation would have required suprahydrostatic fluid pressures, relatively shallow crustal depths, and a lack of more optimally oriented, high angle faults (〉50°) that would otherwise have preferentially failed and focused fluid flow steeply upward.

Description: The duration of hydrothermal activity required to form ore deposits is poorly constrained. We demonstrate that thermochronology data, coupled with thermal modeling, can be used to constrain the duration of hydrothermal fluid flow. Apatite fission-track (AFT) thermochronology data define a conductive halo around an Eocene hydrothermal system that formed the Betze-Post gold deposit on the northern Carlin trend in Nevada. The premineralization Goldstrike stock acted as an essentially impermeable side to the auriferous Carlin hydrothermal system. The hydrothermal fluid conductively heated the intrusion over the time that it flowed past it. To derive first-order estimates for the maximum duration of this flow we numerically modeled one-dimensional conductive heat flow into the intrusion and used the results to forward model ensuing AFT annealing. Modeled levels of annealing were compared to AFT dates and track length data measured across the intrusion. Our results indicate that the episode of main ore-stage hydrothermal fluid flow (mean temperature of 200°C) that formed the ~1,150 metric ton (t) Betze-Post gold deposit had a maximum duration of 〈15 to 45 ka. The average gold flux over this period was ~80 to 30 kg yr –1 , comparable to that measured in the deep reservoirs of several modern geothermal fields. Conservative estimates of gold concentration in the main ore-stage fluids imply that fluid upflow rates and total advective heat flow were also comparable to modern geothermal systems. This suggests that the most important factors for generating the large gold deposits of the northern Carlin trend were a large and/or continuous source of gold, and a very efficient means of removing it from the fluid, rather than the hydrologic system itself. The short duration of main ore-stage fluid flow is unlikely to represent a steady-state convective system. Instead, it most likely reflects a transient period of flow following slip and permeability generation along the steeply dipping Post-Genesis fault system that hosts many of the deposits along the northern Carlin trend. A sudden increase in the permeability of a fault may have led to a transitory period of peak fluid temperature as the fault initially tapped meteoric fluid that had resided at depth and had thermally equilibrated with the host rocks. With continued convection the flow drew cooler, less rock-buffered meteoric water down from higher in the system.

Description: Quartz at the Betze-Post deposit, the largest Carlin-type gold system in the world, was examined to better understand ore fluid properties and sources that generated Carlin-type gold deposits. Detailed petrography and paragenesis investigations distinguished textural relationships, luminescence, various generations of quartz, and fluid inclusion populations; microthermometry, chemical analyses of quartz, and in situ and conventional oxygen isotope analyses of quartz provided information about hydrothermal fluid conditions as well as fluid sources and ore formation processes. Pre-, syn-, late-, and post-ore stages of quartz were distinguished during petrographic studies and confirmed by cathodoluminescence analyses. Ore-stage jasperoid and late-ore drusy quartz are spatially associated with Au-bearing pyrite; both lack luminescence. Two generations of post-ore drusy quartz overgrew ore and late-ore quartz and exhibit bright and multiply zoned luminescence. Electron probe microanalyses determined that nonluminescing ore and late-ore quartz have elevated concentrations of Al, in contrast to post-ore luminescent quartz that contains low trace element concentrations. Fluid inclusion microthermometry determined that ore-stage fluids had low salinities, generally 〈5 wt % NaCl equiv, and were trapped at 180° to 240°C. The 18 O values, calculated for inclusion fluids and determined for quartz using in situ ion probe and conventional analyses, indicate that the isotopic composition of hydrothermal fluids became 18 O depleted with time, from the ore stage through the late-ore stage to the post-ore stage. The 18 O composition of each stage of quartz also varies spatially across the Betze-Post deposit, and 18 O VSMOW values of ore-, late-ore, and post-ore stage quartz became more depleted with increasing distance from the master Post fault. These patterns are interpreted to result from increasing dilution of the 18 O-enriched ore fluid by unevolved meteoric fluids over time and with increased ore fluid transport distance to the west, away from the Post fault, the master fault that transmitted hydrothermal fluids to the deposit. Oxygen isotope results are consistent with a low fluid/rock ratio during upward transport of the ore fluid via the Post fault. Below the deposit, some ore fluids were diverted into the JB Series faults, associated with the Betze ore zones, and the Shalosky fault, associated with the Screamer ore zone in the western part of the deposit. Ore fluids were increasingly diluted by meteoric fluids with distance as ore fluids traveled away from the Post fault and over time as different stages of quartz precipitated. Acidic, 180° to 240°C ore fluids accessed the level of the deposits where they reacted with the Devonian Popovich Formation and precipitated nonluminescing, Al-enriched ore-stage jasperoid and late-ore drusy quartz, along with Au and trace element-rich pyrite. Fluid-rock reaction, dissolution, and replacement of silty carbonate host rocks may have produced Al that substituted for Si in non-luminescing jasperoid and late-ore stage quartz. As the hydrothermal ore system began to cool and collapse, late-ore drusy quartz and realgar precipitated in open space as ore-stage pyrite precipitation declined. The hydrothermal system collapsed with meteoric water flooding; post-ore minerals, including luminescent, Al-poor post-ore drusy quartz and later calcite and barite, precipitated either from latest Carlin-related fluids, neutralized by infiltration of meteoric waters, or as part of a later, separate hydrothermal event.

Description: Hydrothermally altered country rocks surrounding hydrothermal ore deposits are commonly characterized by altered 18 O and 13 C values relative to distal, unaltered rocks. Stable isotopes have been recognized as a powerful tool to complement traditional geochemical and geophysical approaches during mineral exploration, particularly in carbonate replacement and Carlin-type deposits. However, wall-rock alteration and carbonate veining during ore formation is often accompanied by the precipitation of sulfide minerals, complicating stable isotope analysis. The classic phosphoric acid extraction technique for carbonate minerals results in the release of additional H 2 S gas, interfering with isotope ratio mass spectrometry (IRMS), and laser absorption spectrometry (LAS) analysis of CO 2 . Here we present the first results from a novel off-axis integrated cavity output spectrometer (OA-ICOS) instrument, capable of determining the stable isotope composition ( 13 C, 18 O) of mixed CO 2 -H 2 S gas evolved from carbonate-sulfide mixtures. Instrument calibration was performed by analyzing in-house and commercially available CaCO 3 stable isotope reference materials. Analyses of mixed CO 2 -H 2 S tank gas and sulfide-spiked stable isotope reference materials containing up to 40 wt % chalcopyrite and sphalerite demonstrate that the method outlined here is capable of effectively analyzing previously problematic high sulfide samples. The analytical capabilities, together with the robust nature of the OA-ICOS instrument, may be of direct practical use for mineral exploration in carbonate-hosted hydrothermal systems including ore deposits.