Molybdenite

Abstract: 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.% NaClequiv.) and intermediate-density. Upon decompression to ~300 bar, they boiled off a vapour phase, leaving behind a residual brine (up to 45 wt.% NaClequiv) 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.

Abstract: The magmatic-hydrothermal evolution of the El Teniente porphyry Cu-Mo deposit in the Central Andes in Chile is reconstructed based on field relationships, scanning electron microscopy cathodoluminescence, petrography, and fluid inclusion analysis by microthermometry and laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS). Three major stages of Cu-Mo mineralization are observed. Following the barren hydrothermal stage 1, the stage 2 mineralization is characterized by quartz-anhydrite stockwork veins and breccias with chalcopyrite, bornite, and molybdenite. Both stages 1 and 2 are associated with pervasive potassic alteration. Quartz-anhydrite veins with chalcopyrite, bornite, and molybdenite associated with phyllic alteration represent stage 3 mineralization. Stage 4 mineralization, linked to the formation of the large Braden diatreme, is characterized by breccias and rare veins containing a lower temperature assemblage with tourmaline, sericite, and lesser tennantite, bornite, and chalcopyrite with late gypsum in local vugs. Ten fluid types are distinguished in this study based on petrographic and microthermometric criteria, such as phase proportions, daughter minerals, homogenization behavior, and salinity. The hydrothermal evolution across the stages of Cu deposition is characterized by the contraction of a vapor phase originating by phase separation during stage 2. Overall cooling of the system at pressures fluctuating around the two-phase fluid surface led to a transition from a two-phase fluid state dominated by vapor at ~410°C and 300 bars in stage 2, to a single-phase low-salinity fluid derived from cooling and contraction of magmatic vapor to a liquid, which dominates during stage 3 mineralization at <350°C and 200 bars. Copper mineralization mainly formed from the vapor phase and its low-salinity liquid derivatives, representing a large volume of fluid with an initially high Cu content (1.2 ± 0.4 wt % Cu). Copper sulfides precipitated upon cooling between 410° and 320°C, indicated by a drop of Cu/(Na + K + Mn + Fe) ratios of over four orders of magnitude through the evolution of the deposit from stage 2 to stage 4. The highest Mo concentrations occur in residual brines resulting from extreme boiling, as indicated by concurrent halite saturation. Recent geochronology (Cannell, 2005; Maksaev et al., 2004) suggests a relatively long-lived magmatic-hydrothermal system at El Teniente. Our fluid chemical data show no evidence for major crystal fractionation in a large fluid-generating upper-crustal magma chamber, because Cs/(Na + K + Mn + Fe) is constant from pre-ore to all syn-ore fluids. However, the initiation of copper mineralization was associated with a 4- to 10-fold increase in the concentration of Cu, Mo, Li, and Fe in the inferred main ore-forming input fluid, compared with pre-ore fluids of intermediate salinity and otherwise very similar major and trace-element ratios. These data indicate that injection from depth of an exceptionally Cu, Mo, Li, and, probably, also S-rich volatile phase into an already actively evolving upper-crustal magmatic-hydrothermal system triggered the formation of this unusually large and rich copper deposit.