ALBERT

Description: 〈span〉〈div〉Abstract〈/div〉The transport and deposition of gold from colloidal suspensions in hydrothermal fluids has been a persistent theme in ore deposits research. Studies of active geothermal systems show that a complete model of gold transport must include both dissolved and particulate forms. However, samples of the hydrothermal fluids are commonly spiked with aqua regia after collection in order to put any solids back into solution, thus preventing a quantitative assessment of the particle load. Although attempts have been made to filter the solids, gold nanoparticles (Au NPs) will mostly pass the 0.2-〈span〉μ〈/span〉m filters that are in common use, and a simple technique for analyzing suspended particles in the liquids has been lacking. In this study, we demonstrate how time-resolved acquisition of mass 197 in a conventional inductively coupled plasma-mass spectrometer (ICP-MS) can be used to detect and measure Au NPs in the filtered liquids, with an example of well-characterized fluids from the Reykjanes geothermal field on Iceland. The technique allows for precise monitoring of the solution as it is introduced into the plasma with the capability of identifying individual particles carried in suspension. Results show that Au particles passing the 0.2-〈span〉μ〈/span〉m filters are abundant in the studied samples, and measurements of the individual particles can be used to determine their size. The experiment highlights the potential of emerging ICP-MS techniques, including very fast data acquisition and multielement analysis of single particles in time-of-flight mode, for characterization of NPs in hydrothermal fluids.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Four porphyry Cu-Mo systems were investigated by Re-Os molybdenite geochronology to constrain their timing with respect to the geodynamic and magmatic evolution of the eastern Pontides, Turkey. Molybdenite from the Ispir-Ulutaş deposit yielded an Re-Os age of 131.0 ± 0.7 Ma, which is consistent with Early Cretaceous U-Pb laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon ages of local calc-alkaline intrusions. It demonstrates that porphyry deposits were already formed during Early Cretaceous subduction of the Neotethys along the eastern Pontides, and that they can be correlated with porphyry Cu events in the adjacent Lesser Caucasus. Molybdenite Re-Os ages of 76.0 ± 0.4 and 75.7 ± 0.4 Ma at the Elbeyli prospect and 77.2 ± 1.0 Ma at the Emeksen prospect overlap with U-Pb LA-ICP-MS zircon ages of shoshonitic to high-K calc-alkaline intrusions in the region, which were emplaced during Late Cretaceous Neotethys subduction. A 50.7 ± 0.3 Ma molybdenite Re-Os age at the Güzelyayla deposit confirms porphyry Cu-Mo emplacement coeval with Eocene postcollisional, calc-alkaline adakitic magmatism of the eastern Pontides.An electron microprobe study of molybdenite samples, supplemented by data obtained during Re-Os dating, shows that the Eocene Güzelyayla deposit and the Late Cretaceous Emeksen prospect have the highest Re enrichment. Postcollisional melting of a thickened mafic lower continental crust and melting of a metasomatized lithospheric mantle with little to no interaction with upper crustal rocks may explain the Re enrichment at Güzelyayla and Emeksen, respectively.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Haigou lode gold deposit (〉40 tons [t] at 3.4 g/t), which is located near the eastern boundary of the Central Asian orogenic belt and the North China craton, is one of the largest gold deposits in northeastern China. Native gold is intergrown with molybdenite and pyrite in auriferous quartz veins hosted by a monzogranite-monzonite stock and locally by Proterozoic gneiss, thereby offering an excellent opportunity to directly date the mineralizing event. Uranium-Pb age determinations for zircon yielded ages for the monzogranite and monzonite of 327.1 ± 1.1 and 329.5 ± 1.0 Ma, respectively. Numerous mafic to felsic dikes, which are crosscut by ore veins (pre-ore), parallel to these veins (possibly synore), or crosscut by them (post-ore), were carefully examined and dated. Their zircon 〈sup〉206〈/sup〉Pb/〈sup〉238〈/sup〉U ages are 318.3 ± 1.0, 310.9 ± 1.1, and 134.9 ± 0.4 Ma, respectively, thereby placing the timing of gold mineralization within the relatively large interval of 318.3 ± 1.0 to 134.9 ± 0.4 Ma. The age of mineralization was determined directly using the Re-Os method applied to molybdenite. A total of 19 molybdenite samples separated from auriferous quartz veins yielded widely differing Re-Os model ages of 467 to 155 Ma, and replicate analyses of individual samples also yielded widely differing ages. Significantly, the wide range is attributable entirely to the results obtained for some coarse-grained molybdenite samples and is interpreted to be due to Re and Os isotope decoupling, the considerable spatial Re heterogeneity, the analytical procedure (e.g., use of small sample aliquots), and the post-ore deformation. Nine of the samples, which are all fine grained, yielded a robust weighted mean model age of 310 ± 3 Ma and an isochron age of 309 ± 8 Ma. Thus, the molybdenite Re-Os ages are identical, within uncertainty, to those of the dikes that are parallel to the ore veins, indicating that these dikes were emplaced contemporaneously with the ore and that they and the Haigou gold mineralization are of late Paleozoic age (ca. 310 Ma). Finally, a sericite sample obtained from an auriferous vein returned a 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar plateau age of 165.3 ± 1.2 Ma, which is much younger than the age of the mineralization constrained by Re-Os age determinations of molybdenite. This indicates that the 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar isotope system was reset by post-ore thermal events.Our new geochronological data provide evidence for late Paleozoic gold mineralization in Haigou, which makes it the oldest known lode gold deposit in the easternmost Central Asian orogenic belt, a finding that has important implications for precious metal mineral exploration in the eastern part of the Solonker-Xar Moron-Changchun-Yanji suture zone between the Central Asian orogenic belt and the North China craton. This study also indicates that accurate and reproducible molybdenite Re-Os ages representing the true timing of ore deposition need an integrated combination of careful petrography, proper sampling procedures, sufficiently large analyzed aliquots, multiple analyses of individual samples, and multiple dating methods.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉With longshore processes enabling sediment littoral migration along coastal shelves for tens to hundreds of kilometers, beach placers form important locations of heavy mineral accumulation and mining. We report a method for rapid composition and morphology characterization of placer sediment mineralogy using scanning electron microscope element mapping and image analysis, and we apply this technique to investigate garnet from 13 strandline samples from beaches along 427 km of the Westland coast of New Zealand’s South Island with the purpose of establishing patterns of mineral composition, morphology, and distribution in modern and associated raised beaches. Mineral modes show garnet to be least abundant on the South Westland beaches, intermediate in abundance along the Paparoa coast, and most abundant in Central Westland and North Westland. The garnets are typically almandine rich and are mostly derived from the Alpine Schist, even when this unit is distal such as along the Paparoa coastline. Garnet average grain size, which is an important parameter for a garnet resource, is coarsest in Central and North Westland (〉200 〈span〉μ〈/span〉m) and finest along the Paparoa coast. Once it is abraded to ≤40 〈span〉μ〈/span〉m, it is largely removed from the sediment load. Inclusions in garnet, which reduce the viability of a resource, decrease northward. As extensive historical prospecting has been undertaken on ilmenite on narrow but elongate stranded beach sand terraces formed during higher sea levels in Westland, the calculated garnet-ilmenite area ratio for the beach placer samples coupled with the historical ilmenite bulk values indicate that several raised beach terraces likely have 〉5 million tonnes (Mt) of detrital garnet. Our study therefore shows that (1) garnet and other heavy minerals (including ilmenite and gold) are moved northward along the Westland coastline except where submarine canyons intercept the transport direction, (2) garnet composition can be used to fingerprint the heavy mineral sources, and there are systematic morphology variations with distance from source, (3) many beach placers and stranded terraces that formed during higher sea levels in Westland are extremely rich in garnet, and (4) the Central Westland beaches contain garnet of suitable composition, morphology, and lack of inclusions to be worthy of further mineral exploration. The scanning electron microscope and image analysis techniques employed could be readily applied to other sedimentary mineral deposits.〈/span〉