ALBERT

Description: New age and whole-rock 87 Sr/ 86 Sr and 143 Nd/ 144 Nd isotopic data are used to assess petrogenetic and regional geodynamic processes associated with Late Cretaceous subvolcanic intrusions within the sparsely studied Timok Magmatic Complex (TMC) and Ridanj–Krepoljin Zone (RKZ) of eastern Serbia. The TMC and RKZ form part of the Apuseni–Banat–Timok–Srednogorie (ABTS) magmatic belt, a Cu–Au mineralized calc-alkaline magmatic arc related to closure of the Tethys Ocean that extends through Romania, Serbia, and Bulgaria in SE Europe. Zircon ages based on U–Pb laser ablation inductively coupled plasma mass spectrometry supplemented by existing isotope dilution thermal ionization mass spectrometry data respectively range from 89 to 79 Ma and from 76 to 71 Ma for the TMC and RKZ. This age pattern corresponds to cross-arc younging away from the European continent. Adakite-like trace element signatures (Y ≤18 ppm) are linked with samples that extend across the arc. These overlap in space and time with samples that conform to a normal arc differentiation trend. We performed energy-constrained assimilation–fractional crystallization (EC-AFC) modeling of Sr–La–Nd–Yb concentrations and Sr and Nd isotopic data. Results suggest that the two distinct fractionation trends may be explained in terms of a common mantle-derived parental magma but distinct fractionation and assimilation paths in the lower and upper crust. Petrogenesis of the adakite-like magmas is consistent with extensive high-pressure amphibole fractionation in the lower crust followed by ascent and plagioclase-dominant fractionation and assimilation in the upper crust. In contrast, normal arc signatures appear to have evolved exclusively via an upper-crustal differentiation process. Overall, our interpretation supports mantle wedge melting related to weak extension during progressive rollback of a subducting slab.

Description: The Serra Pelada Au-Pd-Pt deposit is located within the Carajás mineral province, which also hosts the world-class iron ore and iron oxide-hosted copper-gold deposits of the Amazon craton in Brazil. The unusual low-temperature hydrothermal mineralization at Serra Pelada is epigenetic, hosted by metasedimentary rocks of the Águas Claras Formation and structurally localized in the Serra Pelada overturned syncline. The orebodies are controlled by the intersection of subvertical NE-trending fault zones with metasiltstones, mainly at the syncline’s hinge, with minor ore occurrences at the upper and lower limb. Intense tropical weathering over the last 70 m.y. has completely overprinted the shallow ore in and near the flooded open pit, but primary hydrothermal features are preserved in deeper drill core delineating the remaining resource. Gold, platinum, and palladium mineralization is associated with intense argillic alteration, hematite breccias, and silicification, with the highest grade ore hosted by brecciated metasiltstones that are highly enriched in amorphous carbon. Distal alteration zones comprise a reducing and oxidizing alteration front. The hydrothermal mineral paragenesis comprises kaolinite, quartz, sericite, amesite (Mg-rich Al-silicate), amorphous carbon, hematite, monazite, rutile, pyrite, and a complex assemblage of Bi-, Ag-, Pb-, Cu-, Co-, Ni-, Pt-, Pd-, and Au-bearing, chalcogenide (S, Se), and arsenide (As, Sb) minerals. Major element changes during hydrothermal alteration include C and Mg addition, K depletion, localized silica loss, and silicification with notable introduction of trace elements including light rare earth elements (LREE), Bi, Pb, U, V, Cu, Co, Ni, and As. The hydrothermal alteration and element association of the Serra Pelada deposit show geochemical similarities with unconformity-related uranium deposits, which may also be enriched in Au, Pd, and Pt and were formed by mixing of fluids that interacted with a highly oxidized cover sequence and highly reduced rock packages in structures of brittle-ductile strain.

Description: Author(s): B. Heinrich, C. Burrowes, E. Montoya, B. Kardasz, E. Girt, Young-Yeal Song, Yiyan Sun, and Mingzhong Wu Spin injection across the ferrimagnetic insulator (YIG)/normal metal (Au) interface was studied by ferromagnetic resonance. The spin mixing conductance was determined by comparing the Gilbert damping in bare YIG films with those covered by a Au/Fe/Au structure. The Fe layer in Au/Fe/Au acted as a sp... [Phys. Rev. Lett. 107, 066604] Published Tue Aug 02, 2011

Description: Human brain networks have topological properties in common with many other complex systems, prompting the following question: what aspects of brain network organization are critical for distinctive functional properties of the brain, such as consciousness? To address this question, we used graph theoretical methods to explore brain network topology in...

Description: The formation of porphyry copper deposits requires a focused flux of magmatic fluid, expelled from a large reservoir of water-, metal-, and sulfur-rich magma. The dimensions of this usually hidden magma reservoir are difficult to determine but can be constrained by combining geophysical observations with thermal constraints and the mass balance imposed by the chemical enrichment of elements in the deposit. Here we show that an internally consistent scenario can be derived for the world-class Cu-Mo-Au deposit at Bingham Canyon (Utah, United States), which quantifies the essential characteristics, approximate dimension, and temporal evolution of a large pluton that generated the deposit. The mineralized district shows a distinct WSW-ENE–striking magnetic anomaly indicating a large intrusive body underlying the sedimentary host rocks of the Oquirrh Mountains. Modeling the deep body by geomagnetic methods is possible because of the high contrast in magnetic susceptibility between sedimentary host rocks and intrusive rocks and because a former volcanic edifice is largely eroded. Additional constraints from drilled geology and district-wide outcropping rocks, including partial demagnetization by hydrothermal alteration on the mine scale, restrict the range of possible solutions to a broadly laccolith-shaped intrusion with a volume of approximately 1,400 to 3,000 km 3 . From the roof of the laccolith, several smaller subvolcanic stocks and dikes protrude to the present surface, of which a major one is hosting the Bingham Canyon deposit. The roof of the laccolith probably lies between 2 and 3.5 km below the bottom of the present open-pit mine, and the average thickness of the laccolith is constrained between 2 and 3.5 km. Thermal modeling, using pluton dimensions derived from the geologic and geomagnetic modeling, indicates that a single laccolith with a magma volume of ~2,000 km 3 beneath Bingham would have solidified within about 230,000 years or less. Comparison of the thermal models with published high-precision geochronologic data and petrologic constraints suggests a scenario in which about 1,000 km 3 of magma was encapsulated by inward crystallization of the pluton after the preore equigranular monzonite stocks solidified and extrusive volcanism was probably terminated. This encapsulated reservoir was close to water saturation and contained approximately 150 billion metric tons (Gt) of magmatic water for subsequent closed-system fractionation and eventual fluid expulsion driving porphyry copper mineralization. Chemical mass balance shows that the known metal endowment and mapped mass of vein quartz within the deposit can be advected and precipitated by a fluid mass that is slightly smaller than the available 150 Gt of water. A conservative estimate indicates that 115 Gt of water is sufficient to precipitate all the quartz associated with successive Cu-Au-and Mo-stage veins as well as their barren precursors. According to our thermal model, approximately 250 km 3 of quartz monzonite magma with a temperature of about 690°C remained partially liquid some 215,000 years after initial intrusion of the laccolith. At that point, it expelled almost simultaneously the quartz monzonite porphyry and the main mass of accumulated fluid, generating most of the vein quartz in the quenched porphyry and the adjacent older rocks. Petrographic evidence indicates that the ore metals precipitated near the end of individual pulses of quartz veining that followed recurrent but waning pulses of porphyry intrusion. Considering published experimental solubility data as well as ore metal contents in fluid inclusions, a small fraction of the available fluid mass is sufficient to transport and precipitate all the ore metals after an initial fluid pulse precipitated most of the quartz. However, the total amount of sulfur present in the deposit, which includes Cu and Mo sulfides as well as a major addition of pyrite, would be facilitated by addition of a mafic magma input into the residual magma chamber that contained the evolved felsic magma. This magmatic injection probably triggered the emplacement of the mineralized porphyries, consistent with the more mafic composition of some of the latest porphyry dikes and the CO 2 -rich nature of ore-related fluid inclusions.

Description: The late Alpine evolution of the Rhodope Massif in southern Bulgaria and northern Greece involved postcollisional extension, which generated detachment faults, syndeformational sedimentary basins, and exhumation of a large metamorphic core complex composed of gneisses and marbles: the Central Rhodopian dome. Closely associated with this complex, subvolcanic rhyolite dikes and extrusive rocks were emplaced, shortly followed by major swarms of epithermal to mesothermal Pb-Zn veins and carbonate replacement orebodies. High-precision geochronology using complementary Ar-Ar, Rb-Sr, and U-Pb dating methods resolves how this process of tectonic denudation from deep crustal metamorphism to near-surface epithermal ore formation occurred within a period of about 12 m.y. After an early Alpine phase of accretion, eclogite-facies metamorphism, and orogenic nappe stacking, the late Alpine postcollisional evolution of the Central Rhodopian dome started with the intrusion of granitic bodies at about 42 to 41 Ma, probably marking the beginning of extension and core complex formation. The early stages of extension were characterized by normal faulting, rotation of fault blocks, and thinning that caused cooling of the hanging wall through ~300°C at about 40 to 38 Ma, as dated by Rb-Sr and Ar-Ar geochronology of metamorphic biotite. The main extensional phase occurred between 38 and 36 Ma and led to horizontal displacements of tens of kilometers in the hanging wall. In the footwall, high metamorphic temperatures and decompression persisted and resulted in partial melting and the formation of migmatites at 37 Ma and vuggy pegmatites at about 36 Ma. Cooling of the footwall below ~300°C occurred between 36 and 34 Ma, followed by emplacement of undeformed rhyolite porphyry dikes and the extrusion of volcanic products deposited onto the surface-exposed center of the dome at about 33 to 30 Ma. The hydrothermal ores were formed ca. 30.5 Ma in the south and ca. 29.3 Ma in the northern part of the dome during the last major event of focused heating to 270° to 330°C of near-surface rocks by hydrothermal fluid advection. Ore formation and localized, later fluid processes caused disturbance and younging of some Rb-Sr ages in the footwall of the dome. Field and geochronologic constraints indicate that the formation of the Pb-Zn deposits (~31–29 Ma) is up to 2 m.y. younger than the local rhyolitic magmatism, which is volumetrically minor in the mineralized core complex. This contrasts with ore formation related to calc-alkaline magmatism in the Eastern Rhodopes, where polymetallic Cu-Au-Ag-Pb-Zn mineralization was found to be coeval with the latest phases of igneous activity (~32 Ma). The chemically simpler but considerably larger metamorphic-hosted Pb-Zn deposits of the Central Rhodopian dome were generated by large-scale hydrothermal fluid circulation, driven by the high heat flow attending core complex formation, exhumation, and final fracturing of a rapidly thinned crust.

Description: Fluid mixing across unconformities between crystalline basement and overlying sedimentary basins is commonly invoked as an efficient chemical mechanism for ore deposition, but the origin of basement brines and the process of ore formation have rarely been linked by direct evidence. Using laser ablation–inductively coupled plasma–mass spectrometry microanalysis of individual fluid inclusions with an improved detection approach for anion components, we determined simultaneously the ore metal concentrations and the Cl/Br ratio in texturally well constrained inclusion assemblages from a basement-hosted quartz-fluorite-barite-Pb-Zn vein system. An inverse correlation between the Pb + Zn concentrations and the Cl/Br mass ratios in the fluid inclusions provides clear evidence for mixing of a basement-derived metal-rich brine and a metal-poor formation water that acquired its salinity from halite dissolution in Triassic evaporites of the sedimentary cover. This mixing of two distinct brines with comparable salinity is recorded during the growth of individual quartz crystals containing small galena inclusions, demonstrating the transient and episodic nature of fluid mixing during mineral deposition.

Description: The recently discovered Biely Vrch deposit in the Western Carpathian magmatic arc is the most extreme example of a porphyry gold deposit, being practically free of copper, molybdenum or any other sulfide minerals. Microanalytical data on fluid inclusions in quartz veinlets, including a characteristic type of banded veinlets, show that this deposit formed from nearly anhydrous Fe-K-Na-Cl salt melts containing ~10 ppm Au, coexisting with hydrous vapor of very low density. This exceptional fluid evolution required an Fe-rich dioritic source magma that was emplaced at shallow subvolcanic depth (〈3.5 km), directly exsolving a hypersaline liquid and magmatic vapor at high temperature (~850 °C). During ascent to the level of the porphyry intrusion (0.5–1 km), fluid expansion at high temperature but low pressure led to halite precipitation and further water loss to the vapor, generating an increasingly Fe-K-rich salt melt that transported high concentrations of Au but negligible Cu into the fractured porphyry stock. The low sulfur fugacity resulting from fluid expansion suppressed precipitation of sulfide, explaining the gold-only enrichment in this globally recurring but rare type of gold ore.

Description: The Altar porphyry Cu-(Mo-Au) deposit occurs in the Miocene to early Pliocene copper belt of the high Andes of western Argentina, close to the Chilean border. A cluster of late Miocene porphyries intruded a more extensive complex of early Miocene rhyolitic to andesitic volcanic and subvolcanic rocks. Potassic alteration in the Central Porphyry was partially overprinted by feldspar-destructive alteration and finally by argillic to advanced argillic alteration. Early stockwork veins have been cut successively by quartz-pyrite veins, tourmaline veins, and enargite veins. Based on correlation analysis of assay results and mapped abundances of these four vein types, we estimate that approximately 11 to 26% of the copper in the Altar orebody is associated with enargite veins, whereas the remainder is associated with the early stockwork veining and potassic alteration. Gold concentration is low compared with typical Cu-Au porphyries of the Andean back-arc region, but higher than in the giant Miocene deposits of Chile, at an average Au/Cu ratio of 0.14 x 10 –4 by weight across the Central Porphyry orebody at Altar. Gold is dominantly associated with chalcopyrite in the domain of stockwork veining and potassic alteration. Small-scale assays in combination with petrographic observations and laser ablation-inductively coupled plasma-mass spectrometry microanalyses show that a significant fraction of the gold in the stockwork veins is enclosed in pyrite as submicroscopic particles attached to inclusions of a Bi-Te-Pb-Ag–enriched Cu-Fe sulfide phase. Microscopic grains of native gold also occur along grain boundaries between pyrite and chalcopyrite, and rarely as larger particles in enargite veins.