Ryan Mathur

Relationship between copper tonnage of Chilean base-metal porphyry deposits and Os isotope ratios

Re-Os isotopes in pyrite and chalcopyrite from early high-temperature hypogene alteration assemblages in Chilean porphyry copper deposits identify the source of Os and, by inference, Cu in these ore systems. Typical concentrations for Os in both pyrite and chalcopyrite are between 7 and 30 ppt (10 ‐12 g/g), and for Re between 0.200 and 10 ppb (10 ‐9 g/g). Re-Os isochrons yield ages and initial 187 Os/ 188 Os ratios for the sulfides. The isotopic data reflect the relative contributions of copper from the mantle and crust in Chilean porphyry copper deposits. Seven ore deposits that reside in different tectonic terranes and represent distinct epochs of mineralization in Chile were studied. The initial osmium ratios of the first stage of mineralization at each of the deposits range from 0.15 to 5. These values are more radiogenic than the present chondritic mantle (~0.13), and indicate significant crustal contributions of Os to the magmatic and/or hydrothermal systems. There is a strong correspondence between the total copper content and initial Os isotopic ratios in base-metal porphyry deposits. The larger deposits have lower initial Os ratios than the smaller, less significant deposits. This relationship implies that larger deposits acquire a greater proportion of Os from the mantle. The initial Os ratio of samples in the central segment of porphyry copper deposits of northern Chile also decreases with decreasing age of the deposit. A plausible interpretation of the Re-Os data is that the later and larger deposits use regional tectonic and structural features that allow sampling of deeper more primitive magmatic sources.

Different crustal sources for Au-rich and Au-poor ores of the Grasberg Cu–Au porphyry deposit

Abstract The Grasberg is a porphyry copper deposit that is crosscut by a second stage mineralization which is greatly enriched in gold. Sulfides from the porphyry-type event yield a 2.9±0.3 Ma Re–Os isochron that agrees with published geochronology. The initial 187Os/188Os ratio of the isochron is 0.56±0.02, and implies a significant crustal component for the source of Os and by inference the other base metals. The samples from the crosscutting secondary event do not form an isochron, but form a mixing line with the older porphyry style mineralization as one of the end members and a very radiogenic end member possibly shale. The initial 187Os/188Os ratios from samples of the second event range from 0.81 to 1.26 and correlate with gold content. The most radiogenic samples have the highest gold. The Re–Os isotope data indicate different crustal sources for the ore-forming elements at the Grasberg Cu–Au deposit and support a model in which gold is derived from sedimentary protoliths, that may have been pre-enriched by sedimentary processes. The gold derived from these sources is concentrated by magmatic/hydrothermal systems. The requirement of a sedimentary source for gold to produce hydrothermal gold deposits has been debated for decades and this study provides strong support for the model.

Isotope fractionation during microbial metal uptake measured by MC-ICP-MS

High-precision isotopic analyses by MC-ICP-MS were used to investigate the mass-dependent fractionation of Mo and Fe isotopes during bacterial metal assimilation in experiments with Azotobacter vinelandii. A. vinelandii is a diazotroph with high demand for both Mo and Fe during nitrogen fixation. Our results demonstrate that the growth medium became progressively enriched in heavier isotopes of Mo during bacterial growth, indicating preferential assimilation of lighter isotopes. In contrast, for Fe, the medium become isotopically lighter as Fe was removed from solution. The experimental data can be interpreted in terms of Rayleigh fractionation, yielding fractionation factors of 0.9997 and 1.0011 for Mo and Fe, respectively. Hence, we infer Δ97/95Mocells-medium = –0.3‰ and Δ56/54Fecells-medium = 1.1‰. Fractionation of Mo isotopes could result from simple kinetic effects during assimilation, but may also be affected by complexation with high-affinity metal binding ligands. Kinetic effects cannot easily account for the sense of Fe isotope fractionation, and so equilibrium effects, possibly between different Fe complexes, are implied. Adsorption of Mo and Fe onto cell surfaces may also play a role and requires further examination. Isotope fractionation studies using MC-ICP-MS may provide new constraints on the processes by which microbes extract metals from their surroundings, ultimately yielding insights into the mechanisms of metal assimilation into the metallome.

Extension of Laramide magmatism in southwestern North America into Trans-Pecos Texas

The Red Hills intrusion hosts the easternmost porphyry copper-molybdenum system in southwestern North America and consists of quartz-sulfide stockwork veins in sericitized porphyritic quartz monzonite. Zircon U-Pb and molybdenite Re-Os analyses yield ages of 64.2 ′ 0.2 Ma and 60.2 ′ 0.3 Ma, respectively, indicating that the Red Hills intrusion and mineralization are distinctly older than all other Tertiary magmatism (48-17 Ma) in the Trans-Pecos region of Texas, including the nearby 32 Ma Chinati Mountains caldera. The Red Hills intrusive system is contemporaneous with and genetically related to other Laramide magmatic systems (75-54 Ma) that host porphyry copper deposits in Arizona, southwestern New Mexico, and northern Mexico. These results significantly extend the Laramide magmatic province eastward and suggest that Laramide subduction-related magmatism and deformation are coextensive over a broad area of southwestern North America.

A Flow-through Reaction Cell that Couples Time-resolved X-ray Diffraction with Stable Isotope Analysis

A non-metallic flow-through reaction cell is described, designed for in situ time-resolved X-ray diffraction coupled with stable isotope analysis. The experimental setup allows the correlation of Cu isotope fractionation with changes in crystal structure during copper sulfide dissolution. This flow-through cell can be applied to many classes of fluid-mineral reactions that involve dissolution or ion exchange.

New isotopic evidence bearing on bonanza (Au-Ag) epithermal ore-forming processes

New Cu, S, and Pb isotope data provide evidence for a magmatic source of metal(loid)s and sulfur in epithermal Au-Ag deposits even though their ore-forming solutions are composed primarily of heated meteoric (ground) waters. The apparent isotopic discrepancy between ore metals and ore-forming solutions, and even between the ore and associated gangue minerals, indicates two different sources of epithermal ore-forming constituents: (1) a shallow geothermal system that not only provides the bulk of water for the ore-forming solutions but also major chemical constituents leached from host rocks (silica, aluminum, potassium, sodium, calcium) to make gangue minerals and (2) metals and metalloids (As, Te, Sb, etc.) and sulfur (±Se) derived from deeper magma bodies. Isotopic data are consistent with either vapor-phase transport of metal(loids) and sulfur and their subsequent absorption by shallow geothermal waters or formation of metallic (Au, Ag, Cu phases) nanoparticles at depth from magmatic fluids prior to encountering the geothermal system. The latter is most consistent with ore textures that indicate physical transport and aggregation of nanoparticles were significant ore-forming processes. The recognition that epithermal Au-Ag ores form in tectonic settings that produce magmas capable of releasing metal-rich fluids necessary to form these deposits can refine exploration strategies that previously often have focused on locating fossil geothermal systems.

Methods for the determination of stable Te isotopes of minerals in the system Au–Ag–Te by MC-ICP-MS

The measurement of stable isotopes in ore- and ore-related minerals can provide insight into the geochemistry and formation of metal-bearing ore systems. Currently, there are few high-precision studies of the natural variability of stable tellurium isotopes, most of which are focused on meteorites and sulfides and are related to cosmogenesis; there are no modern studies on the variability of tellurium isotopes within native tellurium and tellurides from ore-forming systems. Tellurium is an element of interest due to its common association with gold in geologic systems, as well as its rarity in the Earths crust and increasing industrial demand for applications such as photovoltaics. This study presents a method by which tellurium can be sampled from Au–Ag tellurides and native tellurium, isolated by ion exchange chromatography, and analyzed for isotopic composition by multi collector-inductively coupled plasma-mass spectrometry (MC-ICP-MS). Using a micromill, a sufficient mass of telluride or native Te sample can be extracted from coexisting ore and gangue minerals from ∼100 μm wide by ∼50 μm deep drilled holes. Acid digestion of micromilled samples and subsequent ion exchange chromatography isolated Te from matrix metals. The chromatography procedure has Te yields of 96% and produces no net fractionation of Te isotopes. MC-ICP-MS analyses were performed using two techniques, both of which employed the doping of samples using Cd to correct for instrumental mass bias. The first method was the introduction of 100 ppb Te-bearing solutions (with 100 ppb Cd) using a desolvating nebulizer (Aridus II). The second method involved solution nebulization of 2 ppm solutions of Te (with 1 ppm Cd) into the plasma of the MC-ICP-MS. Although the wet and dry methods produce statistically identical delta values, precision is increased using the wet method. The average uncertainty (two standard deviations of the mean) using the Aridus II is ±0.20‰ for 130/125Te (dry method), whereas that for the wet method is ±0.08‰. Repeated analyses of the Te standard over a period of ∼15 months by solution nebulization yielded an external precision of ±0.10‰ for 130/125Te. Natural, hypogene tellurides (calaverite, hessite, krennerite, and sylvanite) and native tellurium samples (n = 32) have a range of 1.64‰ in the isotope composition 130/125Te, demonstrating resolvable, disparate isotope ratios between samples from different areas and between samples from the same locality (e.g., Cripple Creek, Colorado). Fractionation of Te isotopes was caused by mass-dependent geological processes.

Erratum to: Re–Os isotope evidence for mixed source components in carbonate-replacement Pb–Zn–Ag deposits in the Lavrion district, Attica, Greece

The Lavrion ore district contains carbonate-replacement and vein-type Pb–Zn–Ag deposits as well as low-grade porphyry Mo, Cu–Fe skarn, and minor breccia-hosted Pb–Zn–Cu sulfide mineralization. These ore types are spatially related to a Late Miocene granodiorite intrusion (7 to 10 Ma), and various sills and dikes of mafic to felsic composition. Samples of sphalerite and pyrite from the Ilarion carbonate replacement deposit, and galena from Vein 80 (vein-type mineralization) in the Adami deposit show heterogeneous Re–Os values. These values were partially disturbed by hydrothermal activity associated with the formation of hydrothermal veins (e.g., Vein 80). A plot of initial 187Os/188Os versus 1/Oscommon ratios for pyrite and sphalerite from the Ilarion deposit form a mixing line (r2 = 0.78) between high concentration crustal-like and low concentration mantle-like end-members, or two crustal end-members one of which was more radiogenic than the other. Based on the Re–Os systematics and previously published geological and geochemical evidence, the most plausible explanation for the Re–Os isotope data is that ore-forming components were derived from mixed sources, one of which was a radiogenic crustal source from schists and carbonates probably near intrusion centers and the other, intrusive rocks in the district that are less radiogenic. Although the Re and Os concentrations of galena from Vein 80 are above background values they cannot be used as a chronometer. However, the results of the current study suggest that although pyrite, sphalerite, and galena are poor geochronometers in this ore deposit, due to partial open-system behavior, they still yield valuable information on the origin of the source rocks in the formation of bedded replacement and vein mineralization in the Lavrion district.

Challenges of Using Copper Isotope Ratios to Trace the Origin of Native Copper Artifacts: An Example from the Keweenaw Peninsula

ABSTRACT In an effort to understand how and if Cu isotopes can be used to trace native copper artifacts to their mineral deposits of origin, this study presents Cu isotope measurements from weathered native Cu artifacts and ores known to be derived from the Precambrian native copper deposits of Michigan. The five weathered artifacts have Cu isotope compositions ranging from &dgr;65Cu= +0.54 to -1.15%o. Weathered glacial till native copper nuggets range from &dgr;65Cu= -0.12 to +0.54%o, non-weathered ores have &dgr;65Cu= +0.33 ± 0.2%o (n= 42 from the literature and this study), a completely oxidized copper rind derived of large glacial boulder of native copper has -0.04%o. The oxidized rinds along with the weathered artifacts possess isotopically lighter signatures in comparison to the non-weathered ores and interiors of weathered copper nuggets. The copper isotope data indicate the interiors of oxidized nuggets correlate with the non-weathered ores. Copper from the artifacts was sampled as micro drill bits (0.001–0.0009g) and larger cut pieces of the artifacts (>0.5g). Only the larger sample artifacts have the same copper isotope composition as the non-weathered ores, and not the oxidized rinds, and non-weathered interiors of copper nuggets. Therefore, when considering the unreacted interior of the native copper artifacts, the copper isotopic composition matches that of the known copper ore source. In contrast, weathering clearly depletes 65Cu on the surfaces of artifacts and micro-sampling of the outer rims does not yield similar isotope results between sources and artifacts.

The Aguablanca Ni-Cu Deposit: A Re-Os Isotope Study

The Aguablanca Ni-Cu ore deposit is hosted by subvertical magmatic breccia bodies in which fragments of pyroxenite are cemented by sulfides. It is located within the Aguablanca Stock, which includes diorite, gabbro, norite and gabbronorite. Re-Os isotope data of magmatic pyrrhotite, chalcopyrite, and pentlandite and hydrothermal pyrite show that the Os has a mixed origin, derived from a juvenile mantle source and a crustal source. The more likely contaminant of the primitive melt is the late Neoproterozoic to early Cambrian sequence that extensively crops out in the area. Data from the disseminated ore are aligned along a reference line yielding 383 ± 59 Ma (MSWD = 3.0; 187Os/ 188Osi = 0.17 ± 0.01). This age is similar to previous 40Ar/39Ar and U-Pb data of the host rocks (ca. 340 Ma) and confirms that both the host rocks and the mineralization are Variscan in age. These results also show that both the magmatic and hydrothermal processes took place in a short time span, with the hydrothermal pyrite being isotopically similar to the magmatic sulfides. However, some Re-Os results from massive sulfides are discordant to the observed trend and are attributed to a late alteration of the Re-Os system during late- to post-Variscan hydrothermal and supergene alteration. This alteration produced a variable and unsystematic variation of both Re and Os in the sulfides.