Fluid and source magma evolution of the Questa porphyry Mo deposit,New Mexico,USA

Combined fluid inclusion microthermometry and microanalysis by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) are used to constrain the hydrothermal processes forming a typical Climax-type porphyry Mo deposit. Molybdenum mineralisation at Questa occurred in two superimposed hydrothermal stages, a magmatic-hydrothermal breccia and later stockwork veining. In both stages, texturally earliest fluids were single-phase, of low salinity (~7 wt.% NaCl_equiv.) and intermediate-density. Upon decompression to ~300 bar, they boiled off a vapour phase, leaving behind a residual brine (up to 45 wt.% NaCl_equiv) at temperatures of ~420°C. The highest average Mo concentrations in this hot brine were ~500 μg/g, exceeding the Mo content of the intermediate-density input fluid by about an order of magnitude and reflecting pre-concentration of Mo by fluid phase separation prior to MoS₂ deposition from the brine. Molybdenum concentrations in brine inclusions, then, decrease down to 5 μg/g, recording Mo precipitation in response to cooling of the saline liquid to ~360°C. Molybdenite precipitation from a dense, residual and probably sulphide-depleted brine is proposed to explain the tabular shape of the ore body and the absence of Cu-Fe sulphides in contrast to the more common Cu-Mo deposits related to porphyry stocks. Cesium and Rb concentrations in the single-phase fluids of the breccia range from 2 to 8 and from 40 to 65 μg/g, respectively. In the stockwork veins, Cs and Rb concentrations are significantly higher (45–90 and 110–230 μg/g, respectively). Because Cs and Rb are incompatible and hydrothermally non-reactive elements, the systematic increase in their concentration requires two distinct pulses of fluid exsolution from a progressively more fractionated magma. By contrast, major element and ore metal concentrations of these two fluid pulses remain essentially constant. Mass balance calculations using fluid chemical data from LA-ICPMS suggest that at least 25 km³ of melt and 7 Gt of deep input fluid were necessary to provide the amount of Mo contained in the stockwork vein stage alone. While the absolute amounts of fluid and melt are uncertain, the well-constrained element ratios in the fluids together with empirical fluid/melt partition coefficients derived from the inclusion analyses suggest a high water content of the source melt of ~10%. In line with other circumstantial evidence, these results suggest that initial fluid exsolution may have occurred at a confining pressure exceeding 5 kbar. The source of the molybdenum-mineralising fluids probably was a particularly large magma chamber that crystallised and fractionated in the lower crust or at mid-crustal level, well below the shallow intrusions immediately underlying Questa and other porphyry molybdenum deposits. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Age and depositional environment of the Draa Sfar massive sulfide deposit, Morocco

The Draa Sfar mineralization consists of two main stratabound orebodies, Sidi M’Barek and Tazacourt, located north and south of the Tensift River (“Oued Tessift”), respectively. Each orebody is comprised by at least two massive sulfide lenses. The hosting rocks are predominantly black shales, although minor rhyolitic rocks are also present in the footwall to the southern orebody. Shales, rhyolitic volcanic rocks, and massive sulfides are all included into the Sarhlef Series, which is recognized as one of the main stratigraphic units of the Moroccan Variscan Meseta. Hydrothermal activity related with an anomalous thermal gradient, together with a high sedimentation rate in a tectonically driven pull-apart marine basin, favored the accumulation of organic-rich mud in the deepest parts of the basin and the sedimentary environment suitable for massive sulfide deposition and preservation. This took place by replacement of the hosting unlithified wet mud below the sediment–water interface. Geochemical data suggest a sedimentary environment characterized by oxic water column and anoxic sediment pile with the redox boundary below the sediment–water interface. The low oxygen availability within the sediment pile inhibited oxidation and pyritization of pyrrhotite. Biostratigraphic analysis, based on the palynological content of the hosting black shales, restricts the age of the sulfides to the Asbian substage (mid-Mississippian). This age is consistent with earlier geochronological constraints.     

Late Carboniferous porphyry copper mineralization at La Voluntad,Neuquén,Argentina: Constraints from Re–Os molybdenite dating

The La Voluntad porphyry Cu–Mo deposit in Neuquén, Argentina, is one of several poorly known porphyry-type deposits of Paleozoic to Early Jurassic age in the central and southern Andes. Mineralization at La Voluntad is related to a tonalite porphyry from the Chachil Plutonic Complex that intruded metasedimentary units of the Piedra Santa Complex. Five new Re–Os molybdenite ages from four samples representing three different vein types (i.e., quartz–molybdenite, quartz–sericite–molybdenite and quartz–sericite–molybdenite ± chalcopyrite–pyrite) are identical within error and were formed between ~312 to ~316 Ma. Rhenium and Os concentrations range between 34 to 183 ppm and 112 to 599 ppb, respectively. The new Re–Os ages indicate that the main mineralization event at La Voluntad, associated to sericitic alteration, was emplaced during a time span of 1.7 ± 3.2 Ma and that the deposit is Carboniferous in age, not Permian as previously thought. La Voluntad is the oldest porphyry copper deposit so far recognized in the Andes and indicates the presence of an active magmatic arc, with associated porphyry style mineralization, at the proto-Pacific margin of Gondwana during the Early Pennsylvanian.     

First seismic record for intra-arc strike-slip tectonics along the Liquiñe-Ofqui fault zone at the obliquely convergent plate margin of the southern Andes

A temporal seismic network recorded local seismicity along a 130 km long segment of the transpressional dextral strike-slip Liquiñe-Ofqui fault zone (LOFZ) in southern Chile. Seventy five shallow crustal events with magnitudes up to M_w 3.8 and depths shallower than 25 km were observed in an 11-month period mainly occurring in different clusters. Those clusters are spatially related to the LOFZ, to the volcanoes Chaitén, Michinmahuida and Corcovado, and to active faulting on secondary faults. Further activity along the LOFZ is indicated by individual events located in direct vicinity of the surface expression of the LOFZ. Focal mechanisms were calculated using deviatoric moment tensor inversion of body wave amplitude spectra which mostly yield strike-slip mechanisms indicating a NE–SW direction of the P-axis for the LOFZ at this latitude. The seismic activity reveals the present-day activity of the fault zone. The recent M_w 6.2 event near Puerto Aysén, Southern Chile at 45.4°S on April 21, 2007 shows that the LOFZ is also capable of producing large magnitude earthquakes and therefore imposing significant seismic hazard to this region.     

Age estimates of coastal terraces in the Andaman and Nicobar Islands and their tectonic implications

The great Indian Ocean earthquake of December 26, 2004 caused significant vertical changes in its rupture zone. About 800 km of the rupture is along the Andaman and Nicobar Islands, which forms the outer arc ridge of the subduction zone. Coseismic deformation along the exposed land could be observed as uplift/subsidence. Here we analyze the morphological features along the coast of the Andaman and Nicobar Islands, in an effort to reconstruct the past tectonics, taking cues from the coseismic effects. We obtained radiocarbon dates from coastal terraces of the island belt and used them to compute uplift rates, which vary from 1.33 mm yr^− 1 in the Little Andaman to 2.80 mm yr^− 1 in South Andaman and 2.45 mm yr^− 1 in the North Andaman. Our radiocarbon dates converge on  600 yr and  1000 yr old coastal uplifts, which we attribute to the level changes due to two major previous subduction earthquakes in the region.     

U–Pb SHRIMP zircon geochronology and T–t–d history of the Kampa Dome, southern Tibet

Structural, petrographic and geochronologic studies of the Kampa Dome provide insights into the tectonothermal evolution of orogenic crust exposed in the North Himalayan gneiss domes of southern Tibet. U–Pb ion microprobe dating of zircons from granite gneiss exposed at the deepest levels within the dome yields concordia ²⁰⁶Pb/²³⁸U age populations of 506 ± 3 Ma and 527 ± 6 Ma, with no evidence of new zircon growth during Himalayan orogenesis. However, the granite contains penetrative deformation fabrics that are also preserved in the overlying Paleozoic strata, implying that the Kampa granite is a Cambrian pluton that was strongly deformed and metamorphosed during Himalayan orogenesis. Zircons from deformed leucogranite sills that cross-cut Paleozoic metasedimentary rocks yield concordant Cambrian ages from oscillatory zoned cores and discordant ages ranging from ca. 491–32 Ma in metamict grains. Since these leucogranites clearly post-date the metasedimentary rocks they intrude, the zircons are interpreted as xenocrysts that are probably derived from the Kampa granite. The Kampa Dome formed via a series of progressive orogenic events including regional ~ N–S contraction and related crustal thickening (D₁), predominately top-to-N ductile shearing and crustal extension (D₂), top-to-N brittle–ductile faulting and related folding on the north limb of the dome, localized top-to-S faulting on the southern limb of the dome, and crustal doming (D₃), and continued N–S contraction, E–W extension and doming (D₄). Structural and geochronologic variability amongst adjacent North Himalayan gneiss domes may reflect changes in the magnitude of crustal exhumation along the North Himalayan antiform, possibly relating to differences in the mid-crustal geometry of the exhuming fault systems.     

Composition and formation conditions of pay zones in the lower Cretaceous Nizhnyaya Kheta and Yakovlevo formations within the Vankor oil-and-gas field (in the Northeast of West Siberia)

Sedimentologic analysis of cores from wells sunk in the Vankor petroleum field allowed refinement of the accumulation conditions producing the sandy strata of the Nizhnyaya Kheta River (Nizhnekhetsky) Formation accumulated in the coastal marine zone and of the Yakovlevo (Yakovlevsky) Formation accumulated under alluvial-deltaic conditions. Petrographic examination of the sandstones added information on the sources, transport, and accumulation conditions of the terrigenous material in the northeastern marginal part of the West Siberian sedimentation basin.     

Evolution of melt composition during intrusion of basalts into a silicic magma chamber

The article describes heat exchange between basaltic and rhyolite melts accompanied by fractional crystallization of phases in a basaltic melt. A numerical model has been developed for the homogenization mechanism of magma composition during intrusion of basaltic magma batches into felsic magma chambers. The results of numerical modeling demonstrate that the time needed for cooling the basalts and their fractionation to rhyolite melts is much shorter than the time required for chemical interaction based on diffusive mechanisms.     

The Finnmarkian Orogeny revisited: An isotopic investigation in eastern Finnmark, Arctic Norway

The Scandinavian Caledonides have been viewed as resulting from either a single Silurian (i.e. Scandian) event or from polycyclic orogenies involving several collisions on the margin of Baltica. Early studies of the Kalak Nappe Complex (KNC) in Finnmark, Arctic Norway, led to the hypothesis of an Early Cambrian-Early Ordovician (520-480 Ma) Finnmarkian Orogeny, though the nature of this tectonic event remains enigmatic. In this contribution we have employed in situ UV laser ablation Ar-Ar dating of fine-grained phyllite and schist from the eastern Caledonides of Arctic Norway to investigate the presence of pre-Scandian tectonometamorphic events. U-Th-Pb detrital zircon and whole rock Sm-Nd analyses have been used to test the regional stratigraphic correlations of these metasedimentary rocks. These results indicate that the Berlevåg Formation within the Tanafjord Nappe, previously assumed to be part of the KNC, was deposited after 1872 Ma and prior to a low temperature hydrothermal event at 555 ± 15 Ma. It has a likely provenance on the Baltica continent, lacks any Grenville-Sveconorwegian detrital zircons, and thus cannot be part of the KNC which contains abundant detritus in this age range. Instead the Berlevåg Formation is interpreted as part of the Laksefjord Nappe Complex, which structurally underlies the KNC. Laser-ablation argon-argon dating also shows that late Caledonian (i.e. Scandian) tectonometamorphism affected both the KNC and its immediate footwall at c. 425 ± 15 Ma. This is corroborated by a step-heating argon-argon muscovite age of 424 ± 3 Ma which is interpreted as dating cooling. However, within two samples from the KNC, an earlier (Middle-Late Cambrian) metamorphic event is also recorded. A biotite-grade schist yielded an Ar-Ar inverse isochron age of 506 ± 17 Ma from whole rock surfaces, in which the mineral domains are too fine-grained to date individually. An early generation of muscovite from a coarser-grained amphibolite-facies sample yielded an inverse isochron of 498 ± 13 Ma. Both isochron ages have atmospheric argon intercept values. Previous studies have documented similar Cambrian ages in the Caledonian nappes below the KNC. These results suggest correlative tectonometamorphic events in the eastern KNC and its footwall at c. 500 Ma. This Cambrian event may reflect the arrival of the Kalak Nappe Complex as a previously constructed exotic mobile belt onto the margin of Baltica. Combined with recent studies from the western Kalak Nappe Complex, the results do not support the traditional constraint on the Finnmarkian Orogeny sensu stricto. However they vindicate classic tectonic models involving a Cambrian accretion event.