Stockwork

Abstract: The porphyry Cu-Mo deposit in Butte, Montana, formed where magmatic hydrothermal fluids, introduced with injections of porphyrytic dikes, fractured and permeated the Butte Quartz Monzonite. These fluids formed a stockwork of quartz and quartz-sulfide veinlets with a variety of styles of potassic and sericitic alteration envelopes. The distribution of vein and alteration types and the distribution of fluid inclusions in these veins record the progressive pressure, temperature, and compositional evolution of the hydrothermal fluids that formed this world-class deposit. Deep drilling and 1,300 m of offset along the Continental fault provide a vertical view of almost 3 km through the Butte deposit. Deep veins within and below the highest Mo grades are quartz dominated with thin K-feldspar or, less commonly, biotitic alteration rims. Fluid inclusions in deep veins trapped a single phase aqueous fluid containing 2 to 5 wt percent NaCl equiv and 2 to 8 mol percent CO2 at temperatures between 575° and 650°C and pressures between 200 and 250 MPa, corresponding to depths between 6 and 9 km. Although Cu grades are low in this region, abundant chalcopyrite daughter minerals in fluid inclusions indicate that the fluids were Cu rich. Fluids that formed these veins transported Cu from the magma below, upward into the region of Cu mineralization with only minor Cu precipitation. Over a kilometer above the bulk of deep quartz and quartz-molybdenite veins, the highest Cu grades are in and around chalcopyrite-bearing quartz-sulfide veins with biotitic alteration (early dark micaceous veins), and their upward, equivalent magnetite-chalcopyrite-pyrite-quartz veins with wide K-feldspar, green sericite, and chlorite alteration (pale-green sericitic veins). These veins contain more evidence for brine-vapor unmixing than any other vein type. The upward progression of early dark micaceous veins to pale-green sericitic veins formed where low salinity, CO2-bearing fluids, similar to those trapped in deep quartz veins, ascended, de-pressurized, sometimes unmixed, and cooled from ~650°C at 90 MPa to ~475°C at ~50 MPa. As low salinity, CO2-bearing, aqueous fluids, similar in composition to fluids trapped in deep quartz veins, cooled at shallow depths, they formed late pyrite-quartz veins with sericitic alteration. These veins formed from fluid cooling at temperatures between 370° and 450°C at transiently hydrostatic pressures between 40 and 70 MPa, corresponding to depths of 4 to 7 km. Most pyrite-quartz veins formed at pressures and temperatures above the H2O-NaCl-CO2 solvus, but evidence for brine-vapor unmixing is also present. Pyrite-quartz veins formed at progressively greater depths as the hydrothermal system cooled, overprinting much previous mineralization. Late Cu-Pb-Zn-Ag-As-rich Main stage veins formed from dilute fluids containing <3 wt percent NaCl equiv and <2 mol percent CO2. These fluids were trapped between 230° and 400°C under hydrostatic pressures between 20 and 60 MPa and depths of 2 to 6 km. No evidence of boiling is observed in Main stage veins. Fluid inclusion phase relationships indicate that the Butte porphyry Cu-Mo deposit formed at 5 to 9 km depth, greater than any other porphyry-type deposit. At Butte, the similarity in bulk composition of fluids trapped in early quartz-rich veins with potassic alteration and late pyrite-quartz veins with sericitic alteration implies that an underlying magma continually provided low salinity, CO2-bearing fluids of relatively constant composition during the entire life of the hydrothermal system. We hypothesize that rather than resulting from changes in fluid chemistry due to magma crystallization, the entire suite of vein and alteration types and the ore metal distribution reflect the path of cooling, depressurization, and wall-rock interaction of a parental magmatic-derived fluid of relatively constant initial composition. Fluid inclusions, vein and alteration relations, and ore metal distribution indicate that Cu and Mo were introduced into the hydrothermal system by the same fluids, but that the mechanisms of precipitation of these metals were decoupled. Early dark micaceous and, to a greater extent, pale-green sericitic, veins have wide alteration envelopes and contain more evidence for fluid unmixing than any other vein type, which suggests that chalcopyrite precipitation was driven by a combination of fluid unmixing, fluid-rock reaction, and fluid cooling between 650° and 475°C. Most molybdenite mineralization, however, is in quartz-dominated veins with little or no alteration that are dominated by low salinity inclusions. These veins formed in response to pressure decrease rather than cooling. After chalcopyrite and molybdenite precipitation, low salinity fluids cooled, usually at temperatures and pressures above the H2O-NaCl-CO2 solvus, to produce significant acid and voluminous sericitic alteration accompanied by pyrite-quartz vein formation that overprints much of the deposit and contains anomalous but noneconomic Cu.

Abstract: The Iberian Pyrite Belt, located in the SW Iberian Peninsula, contains many Paleozoic giant and supergiant massive sulphide deposits, including the largest individual massive sulphide bodies on Earth. Total ore reserves exceed 1500 Mt, distributed in eight supergiant deposits (>100 Mt) and a number of other smaller deposits, commonly with associated stockwork mineralizations and footwall alteration haloes. Massive sulphide bodies largely consist of pyrite, with subordi- nated sphalerite, galena and chalcopyrite and many other minor phases, although substantial diAerences occur between individual deposits, both in mineral abundance and spatial distribution. These deposits are considered to be volcanogenic, roughly similar to vol- canic-hosted massive sulphides (VHMS). However, our major conclusion is that the Iberian type of massive sulphides must be considered as a VHMS sub-type transitional to SHMS. This work is an assessment of the geological, geo- chemical and metallogenic data available up to date, including a number of new results. The following points are stressed; (a) ore deposits are located in three main geological sectors, with the southern one containing most of the giant and supergiant orebodies, whereas the northern one has mainly small to intermediate-sized de- posits; (b) ore deposits diAer one from another both in textures and mineral composition; (c) Co and Bi minerals are typical, especially in stockwork zones; (d) colloidal and other primary depositional textures are common in many localities; (e) a close relation has been found be- tween ore deposits and some characteristic sedimentary horizons, such as black shales. In contrast, relationships between massive sulphides and cherts or jaspers remains unclear; (f) footwall hydrothermal alterations show a rough zoning, the inner alteration haloes being charac- terized in places by a high Co/Ni ratio, as well as by mobility of Zr, Y and REE; (g) 18 O and D values indicate that fluids consist of modified seawater, whereas 34 S data strongly suggest the participation of bacterial-reduced sulphur, at least during some stages of the massive sul- phide genesis, and (h) lead isotopes suggest a single (or homogeneized) metal source, from both the volcanic piles and the underlying Devonian rocks (PQ Group). It is concluded that, although all these features can be compatible with classical VHMS interpretations, it is necessary to sketch a diAerent model to account for the IPB characteristics. A new proposal is presented, based on an alternative association between massive sulphide deposits and volcanism. We consider that most of the IPB massive orebodies, in particular the giant and su- pergiant ones, were formed during pauses in volcanic activity, when hydrothermal activity was triggered by the ascent and emplacement of late basic magmas. In these conditions, deposits formed which had magmatic activity as the heat source; however, the depositional environ- ment was not strictly volcanogenic, and many evolu- tionary stages could have occurred in conditions similar to those in sediment-hosted massive sulphides (SHMS). In addition, the greater thickness of the rock pile aAected by hydrothermal circulation would account for the enormous size of many of the deposits.

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.