Tom E. McCandless

Rhenium behavior in molybdenite in hypogene and near-surface environments: Implications for Re-Os geochronometry

Abstract Rhenium is concentrated mainly in molybdenite (MoS2) and occurs as a major cation in only a few rare minerals. This affinity makes molybdenite an ideal mineral for Re-Os geochronometry, but research on the behavior of Re in molybdenite is limited. Infrared microscope, XRD, back-scattered electron (BSE), and microprobe techniques have been used in this study to document Re behavior in molybdenite affected by hypogene and near-surface processes. In the hypogene environment, both 3R and 2H molybdenite can experience Re loss during hydrothermal alteration, which also causes increased infrared transparency (IR). Alteration at temperatures as low as ~ 150°C can cause Re loss in the presence of advecting fluids, and will affect Re-Os dating if it occurs long after primary mineralization. Re loss in 3R and 2H molybdenite under supergene conditions does not increase IR transparency. Rhenium is not incorporated into supergene ferrimolybdite, but is enriched in K-Al-silicate intergrowths which may be illite(?). These minute intergrowths are present in several samples and would not be detected in a simple optical examination. In the supergene environment elemental Os is stable, whereas Re is not. Rhenium in molybdenite may be removed by supergene fluids after some has decayed to 187Os, causing erroneously old ages, or it may be adsorbed into the illite intergrowths, creating ages which are too young. In the weathering environment, Proterozoic molybdenites have altered to Re-enriched powellite, which can be detected using back-scattered electron imagery. Combined microprobe, XRD, BSE, and infrared microscopy can be successfully used to detect alteration in molybdenite prior to dating. These techniques are non-destructive and should be performed before any molybdenite is dated by the Re-Os system.

Diamonds and Their Mineral Inclusions, and What They Tell Us: A Detailed “Pull-Apart” of a Diamondiferous Eclogite

For the first time, three-dimensional, high-resolution X-ray computed tomography (HRXCT) of an eclogite xenolith from Yakutia has successfully imaged diamonds and their textural relationships with coexisting minerals. Thirty (30) macrodiamonds (≥1 mm), with a total weight of just over 3 carats, for an ore grade of some 27,000 ct/ton, were found in a small (4 × 5 × 6 cm) eclogite, U51/3, from Udachnaya. Based upon 3-D imaging, the diamonds appear to be associated with zones of secondary alteration of clinopyroxene (Cpx) in the xenolith. The presence of diamonds with secondary minerals strongly suggests that the diamonds formed after the eclogite, in conjunction with meta-somatic input(s) of carbon-rich fluids. Metasomatic processes are also indicated by the non-systematic variations in Cpx inclusion chemistry in the several diamonds. The inclusions in the diamonds vary considerably in major- and trace-element chemistry within and between diamonds, and do not correspond to the minerals of the host eclogite, whose compositions are extremely homogeneous. Some Cpx inclusions possess +Eu anomalies, probably inherited from their crustal source rocks. The only consistent feature for the Cpx crystals in the inclusions is that they have higher K2O than the Cpx grains in the host. The δ13C compositions are relatively constant at −5% both within and between diamonds, whereas δ15N values vary from −2.8% to −15.8%. Within a diamond, the total N varies considerably from 15 to 285 ppm in one diamond to 103 to 1250 ppm in another. Cathodoluminescent imaging reveals extremely contorted zonations and complex growth histories in the diamonds, indicating large variations in growth environments for each diamond. This study directly bears on the concept of diamond inclusions as time capsules for investigating the mantle of the Earth. If diamonds and their inclusions can vary so much within this one small xenolith, the significance of their compositions is a serious question that must be addressed in all diamond-inclusion endeavors.

Osmium isotopes and crustal sources for platinum-group mineralization in the Bushveld Complex, South Africa

The Bushveld Complex in South Africa is the largest layered mafic intrusion in the world and a major producer of platinum-group elements (PGEs). Economic mineralization is found in the Merensky Reef and the UG2 and UG1 chromitites and includes osmium-bearing laurite (Ru, Ir, Os)S 2 . The laurite-bearing samples have 187 Os/ 186 Os ratios of 1.28-1.60, which are more radiogenic than predicted for mantle-derived, 2.0 Ga Os (∼0.9) and indicate a system-wide assimilation of crustal Os. The UG1 chromitite is stratigraphically the lowest and the least radiogenic (1.28), whereas the Merensky Reef is the highest and most radiogenic (1.41-1.60; Hart and Kinloch, 1989). Radiogenic Os from crustal assimilation in mantle-derived magmas or from hydrothermal fluids can account for these high isotopic ratios. Assimilation models require that Os, and by inference other PGEs, are carried in Al-rich tholeiitic (A) magmas rather than high-Mg ultramafic (U) magmas. Pyritic black shales in contact with the intrusion may be high in radiogenic Os, and less than 1% shale assimilation in an A magma can explain the observed ratios. Late-stage hydrothermal fluids driven by the heat of the intrusion could also have carried Os and other PGEs from adjacent rocks into specific layers of the Bushveld. The Os-isotope data add to mounting evidence that PGEs in layered mafic intrusions may be derived from crustal sources and can no longer be considered as solely a product of mantle-derived magmatic processes.

Rhenium-Osmium Evidence for Regional Mineralization in Southwestern North America

More than 40 base metal porphyry ore deposits in southwestern North America are associated with the Laramide orogeny (about 90 million to 50 million years ago). Rhenium-osmium dates on molybdenite, a rhenium-enriched sulfide common in many of the deposits, reveal that in individual deposits mineralization occurs near the final stages of magmatic activity irrespective of the time of inception, magnitude, or duration of magmatism. Deposits that differ widely in location and in the extent and timing of magmatism have nearly identical ages for mineralization. Rhenium-osmium-ages suggest that mineralization occurred during two distinct intervals from about 74 million to 70 million years ago and from 60 million to 55 million years ago. Most deposits that formed in the oldest interval are within the older Precambrian basement of northwestern Arizona, whereas the younger deposits are restricted to the younger Precambrian basement in southern Arizona and northern Mexico. Synchronous, widespread mineralization indicates that similar crust-mantle interaction occurred on a regional scale for ore deposits once thought to be the product of localized processes.

Re-Os isotope and Pd/Ru variations in chromitites from the Critical Zone, Bushveld Complex, South Africa

Abstract Rhenium-osmium isotope measurements of selected chromitites from the Critical Zone of the Bushveld igneous complex reveal that a significant component of radiogenic Os was present in the magma when it formed 2.06 Ga ago. Measured 187 Os/ 188 Os ratios increase with stratigraphic height, and individual layers have similar values across the complex. Initial 187 Os/ 188 Os ratios for chromitites from the western Bushveld decrease from 0.140 at the LG to 0.129 at the MG2 chromitite, increasing to 0.153 at the UG2 chromitite. The inflection at MG2/MG3 is the boundary between the lower and upper Critical Zone, consistent with previous mineralogical and geochemical studies indicating a shift in magma composition at this point. Chromite separates have γOs values of ∼12–35 at 2.06 Ga and are too radiogenic for a purely mantle derivation. Assimilation of 5% mafic granulitic lower crust in a tholeiitic parent magma can explain the radiogenic values. Pd/Ru ratios for the intergranular and chromite separate components of each layer differ by orders of magnitude, but relative shifts from layer to layer are similar. Combined with the Re-Os data, the results suggest that PGE were concentrated into chromitites by magmatic or early deuteric processes that were specific to each layer. Post-layering, upward mobilization of PGE by hydrothermal processes probably did not play a significant role in PGE enrichment.

Re-Os isotope systematics of sulfides from felsic igneous rocks: Application to base metal porphyry mineralization in Chile

Dating sulfides and determining the source of ore-forming metals have been limiting factors in our understanding of processes that produce hydrothermal ore deposits. Here we analyze sulfides from base metal porphyry deposits from Chile to demonstrate the potential of the Re-Os system to determine both the age and the source of metals for hydrothermal mineralization. Cogenetic chalcopyrite, bornite, pyrite, and sphalerite from El Teniente (ca. 5 Ma) have 187 Re/ 188 Os ratios from 0.3 to 21.8 and initial 187 Os/ 188 Os ratios from 0.17 to 0.22. A paragenetically late pyrite that has an initial 187 Os/ 188 Os of 0.88 indicates that the ore deposit is not isotopically homogeneous in Os throughout the formation of the deposit. Pyrite samples from Andacollo (ca. 100 Ma) have 187 Re/ 188 Os ratios from 15 to 3600, and different isochrons yield ages between 87 and 103 Ma, consistent with different sericite K-Ar ages. Initial 187 Os/ 188 Os ratios of 0.2 to 1.1 are similar to those of El Teniente. The relatively homogeneous 187 Os/ 188 Os ratios (~0.19) for cogenetic sulfides from El Teniente suggest that most of the Os in the ore was from the causal intrusive. The latestage pyrite probably incorporated Os leached from the country rock by meteoric fluids, which mixed with magmatic fluids at the periphery of the hydrothermal system. We demonstrate that the Re-Os isotope system can be a powerful geochronological tool in hydrothermal ore deposits. The large range in Re/Os ratios of sulfides permits the age of mineralization to be well constrained, despite isotopic heterogeneities of the hydrothermal fluids.

New quantitative approach in trace elemental analysis of single fluid inclusions: applications of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS)

Laser ablation inductively coupled plasma mass spectrometry has been used for the in situ elemental analysis of individual fluid inclusions with artificially prepared fluid inclusions as external standards. Artificial fluid inclusions standards were prepared by drawing a standard solution of known composition into microcapillary tubes. An ICP-MS equipped with an Nd:YAG laser operating in the UV region (266 nm) was used for data acquisition. The laser was focused and fired on 25–30 µm spots on the microcapillary tubes until the fluid was reached. The content of the microcapillary tube then emptied and was transported with argon to the torch. The average volume of fluid released, per pulse of laser, was approximately 100 pl. Calibration curves were created by plotting signal intensity versus concentration. The long-term precision (RSD%) of measurements for Sr were 1.1–3.1% for a 1000 ppm solution, 2.7–4.9% for a 500 ppm solution and 3.1% for a 250 ppm solution. Similar measurements on the same solutions for Rb gave values of 1.9–2.3% for a 1000 ppm solution, 5.1–6.3% for a 500 ppm solution and 1.7% for a 250 ppm solution. The technique was tested on fluid inclusions in halite from the Palo Duro Basin, Texas, USA. The results show that concentrations for Sr range from 90 to 153 ppm, and for Rb from 98 to 100 ppm. The precision of analysis for individual natural fluid inclusions ranges from 5 to 31% for both Sr and Rb. The concentration of Sr calculated by this method is in agreement with earlier work.

Re-Os-isotope systematics of sulfides from base-metal porphyry and manto-type mineralization in Chile

Chile is a major world producer of copper, most of which occurs in base-metal porphyry and in manto deposits. A fundamental difference between these two types of deposits is the relative importance of intrusions spatially associated with the mineralization. The porphyry deposits are set within Mesozoic to Tertiary intrusive complexes. The manto-type deposits are restricted to volcanic and volcano-sedimentary sequences of Middle-Late Jurassic, Early Cretaceous, Late Cretaceous, and Early Tertiary times. Large intrusive centers are not spatially associated with these deposits, although minor intrusions are common. A central question in the metallogenesis of these deposits is the source of ore-forming components, in particular the ore metals. Initial 187Os/188Os isotopic data from sulfides from the El Teniente, La Disputada, and Andacollo base-metal porphyry deposits range from 0.19 to 1. These data indicate that the Os (and, by inference, Cu) is mostly crustally derived, since it is more radiogenic than tha...

Kimberlite xenocryst wear in high-energy fluvial systems: experimental studies

Abstract Xenocryst minerals from kimberlite have been mixed with gravels of varied grain size and artificially worn in a rotating drum. Results indicate that the minerals are transported with less wear when the finer-grained proportion of the gravel is increased. This is an important consideration when evaluating the natural dispersion of these minerals from their kimberlite host, and can aid in prioritizing stream-sediment anomalies in kimberlite exploration.