The Character of Sn (-W-Mo) Mineralizing Fluids, North Zone, Mount Pleasant, New Brunswick
Several major Sn deposits and minor W-Mo mineralization occur within hydrothermally altered breccias in the North Zone (NZ), Mount Pleasant, New Brunswick. The principal hydrothermal minerals, which either replace the host granites or have been precipitated in cavities and veins, are quartz, fluorite, topaz and chlorite. Previous studies carried out on the Endogranitic Tin Zone (ETZ), one of the mineralized zones in the NZ, have reported that cassiterite is associated with all stages of alteration, but mostly occurred during intense chloritization-topazification. Our studies indicate, however, that cassiterite principally occurs in association with fluorite as an open space filling, and that chlorite alteration was a later event. Wolframite is also associated with fluorite, and with topaz and arsenopyrite. Molybdenite occurs either in quartz veins or in veins associated with quartz-topaz greisen. Previous work on the Endogranitic Tin Zone (ETZ) indicated that primary fluid inclusions in quartz, topaz, and fluorite contain liquid + vapour ± halite, with trapping temperatures of less than ∼ 225°C and salinities of < 30 equiv wt % NaCl. In the Fire Tower Zone (FTZ), fluid inclusions associated with W-Mo mineralization are reported to be liquid + vapour ± solid inclusions, with the latter containing up to 6 daughter crystals, and which homogenize by halite disappearance at temperatures from 146 to 492°C, with salinities ranging from 30 to 60 equiv wt % NaCl. Our studies show that primary fluid inclusions in fluorite that is intimately associated with cassiterite have very complex solid phase assemblages at room temperature. These solids occupy up to ∼ 75 volume % of these inclusions, the remainder being a vapour bubble and liquid. At least 7 of the solids melt between 89 and 385°C, indicating that they are daughter minerals, and the inclusions homogenize to liquid by vapour disappearance at between 469 and 493°C. These complex fluid inclusions indicate that the fluids responsible for Sn mineralization in the NZ were more saline and higher temperature than previously thought. Even more complex inclusions occur in fluorite associated with wolframite, with solids that fill up to ∼ 80% of the volume of an individual fluid inclusion. Homogenization temperatures (to liquid) for these inclusions are > 600°C, and indicate that conditions for W mineralization in the NZ were somewhat different from those in the FTZ. In summary, these data suggest that cassiterite and wolframite in the NZ were deposited from high-temperature, hypersaline fluids of magmatic origin.
The Riviera Deposit: Endo-skarn and Vein-hosted W-MO-REE Mineralization in I-type Granites of the Cape Granite Suite, South Africa
The blind Riviera deposit is located in the western Cape Province and was discovered by stream sediment sampling in the mid 1970's. Resources total 46 million metric tons assaying 0,216 per cent tungsten and 200 parts per million molybdenum, a marginal grade that has prohibited development into an open cast mine. Mineralization is mainly hosted by granitoids of the Riviera Pluton which intruded the regionally metamorphosed volcano-sedimentary Malmesbury Group. These granitoids form part of the Cape Granite Suite, a series of batholiths and plutons with S-, I- and A-type characteristics. The composite Riviera Pluton comprises a suite of metaluminous to slightly peraluminous granitoids. The rocks least affected by hydrothermal alteration are granodioritic to adamelitic in composition whereas the more altered host rocks include quartz-monzonite, granite and quartz syenite. As a whole the suite is subalkaline to K-calcalkaline and conforms to the characteristics of I-type granites. The pluton was emplaced into a dome-shaped interference structure, late in the Neoproterozoic Saldanian orogenic cycle. Alteration, particularly prevalent in the roof or cupola of the pluton, occurs as zones of pervasive sericitization, argillization, silicification and potassic alteration. Their spatial and temporal relationship is complex and indicates several superimposed alteration events. Wall rocks display limited alteration and have acted as an impermeable cap. The cross-cutting granitoid intrusions produced wall rock xenoliths of various dimensions consisting mainly of meta-carbonates displaying various stages of digestion. Economic concentrations of scheelite are spatially linked to these assimilations, particularly proximal to the wall rock contact. The occurrence of diagnostic minerals such as vesuvianite, hornblende, hedenbergite, grandite garnets define a typical endo-skarn association. Accessory minerals include pyrite, pyrrhotite, chalcopyrite, sphalerite and the LREE enriched mineral allanite. Scheelite and molybdenite occurs as fine disseminations, but also as coarse grains within cross-cutting, late stage quartz and calcite veins in the granite and the wall rocks. No spatial or paragenetic correlation exists between the concentration of tungsten and molybdenum, suggesting that their distribution is related to sequential introduction of multiple magmatic phases and associated mineralizing fluids.
Solubility of Pt and Rh in Basalt-Rhyolite Mixtures: Implications for PGE Saturation During Crustal Contamination
Past work on platinum-group element solubility in mafic magmas has largely focussed on the role of oxygen fugacity, with little attention to melt composition effects. However, the latter may be important to PGE saturation behaviour during the interaction of mafic magmas with felsic crustal contaminants. To better assess this behaviour, we have measured the solubility of Pt and Rh in basalt-rhyolite mixtures at 1400° C, 1 bar, and FMQ + 3.8. Mixtures of calcined natural basalt and rhyolite were equilibrated for 5-50 hours on loops of Pt10%Rh then quenched in water. The Pt and Rh content of run-product glasses was measured with LA- ICPMS. Glasses were found to be homogeneous, and solubilities nearly constant over the duration investigated. The concentration of Pt and Rh in run-product glasses are similar, and decrease from 0.8 ppm in pure basalt to 0.1 ppm in mixtures containing 75% rhyolite. The change in solubility with rhyolite content decreases in an exponential fashion, such that the concentration of Pt or Rh in a mixture of saturated basalt with PGE-free rhyolite will exceed the solubility. Simple models of diffusive exchange between basalt and rhyolite have shown that cation concentration gradients parallel that for silica. If this is true for Pt and Rh, then our results predict that PGE oversaturation can occur within the zone of diffusive exchange, even if the basalt is initially up to 40% below the solubility limit. If the degree of oversaturation is sufficient to overcome the surface energy term for metal nucleation, or if pre-existing nucleii are available, then metal grains will form in the zone of contamination. This process may be significant in systems for which chromite precipitation is triggered by contamination, providing a mechansim for the simultaneous growth of chromite and PGE alloys, and possibly accounting for the PGE-chromite association.
Apatite Chemistry in a Felsic Magmatic System From the El Teniente District (Chile) as Monitor of an Early, Single-phase, Cl and S-rich Magmatic Volatile Phase Evolution.
Apatite (Ap) is a ubiquitous accessory mineral phase in igneous rocks, that can incorporate several geochemically important elements among which are volatiles as Cl, H2O, S, As, F. Furthermore, as Ap starts to crystallize early in felsic magmas, and continue through a wide temperature range, it can potentially be used to monitor the evolution of magmatic volatiles in porphyry copper systems. In this work, we have studied magmatic Ap from Late Miocene dacitic porphyries spatially and temporally associated to the Cu-(Mo) La Huifa- La Negra prospect (4 km NE from the giant El Teniente porphyry copper deposit, Chile). These felsic rocks formed from hydrous magmas as indicated by early crystallized Hb (before Bt). Al-Hb geobarometer indicates that phenocrysts formed at ∼2 Kb, while fine grained groundmass suggests a depressurization. Ap is present as small (∼10-50 microns) subhedral to euhedral prisms included in Fe-Ti oxides, plagioclase, amphibole and biotite phenocrysts (IAp), and as bigger (up to ∼300 microns) isolated microphenocrysts (MAp), indicating crystallization throughout magmatic evolution. About 300 EPM analyses of Cl, F and SO3 have been performed on Ap in different textural positions. Only Ap included in anhydrous phases (mostly Pl and oxides) and unaltered grains from the groundmass were used to evaluate volatile evolution. Calculated apatite saturation temperature following Piccoli and Candela (1994) indicates that they started to crystallize ∼900°C. The most outstanding chemical feature of the studied Ap is their high Cl (up to 4.52 wt%) and SO3 (up to 0.98 wt%) contents, being highest in IAp. Cl/F and Cl/OH strongly decrease from ApI to MAp within all studied samples varying in SiO2 content from 66.3 to 69.7 wt%. They display a continuous and well defined trend. This variation is controlled mainly by Cl decrease and F increase, meanwhile OH remains roughly constant. SO3 in Ap varies from 0.98wt% in IAp to below detection limit (0.02 wt% SO3) in MAp. S and Cl in Ap show a general positive correlation being their contents in IAp higher than those in MAp. The results are consistent with Ap starting to crystallize at ∼2Kb, in equilibrium with an early formed, high temperature, highly saline S-rich magmatic volatile phase that evolved toward less saline less S-rich compositions. The strong decrease of Cl/OH as F increases indicates that the exsolved volatile phase is a single phase (supercritical), in agreement with its P and T of formation. High sulfur contents in IAp suggest the host magma was sulfate-rich, and thus oxidized as it's also suggested by high Mg# (0.66-0.75) in primary ferromagnesian minerals. Sulfur presence, along with Cl, would enhance metal (Cu) partitioning from melt into magmatic volatile phase (Simon et al, 2006), while low crystal charge prevailing during early volatile exsolution would favour coalescence and upward migration processes, to finally accumulation in upper parts of the system. Thus, the aqueous Cl and S-rich fluids exsolved, at high pressure (∼2 Kb), upon magma differentiation at La Huifa-La Negra prospect, could have efficiently extracted metals from the magma and hence, it would have the potential to create a hydrothermal ore deposit provided that the magma was not erupted and that the necessary conditions for subsequent ore deposition prevailed. This is a contribution to DIUC 203-320-013-1 Piccoli, P. and Candela, P. 1994. Am. J. of Sc., 294, 92-135. Simon, A.C., Pettke, T., Candela, P., Piccoli, P. y Heinrich, C.A., 2006. GCA, 70, 5583-5600.
The Geochemistry of Aplite Dikes at Elliot Lake, Ontario
We present an analysis of the bulk geochemistry of heretofore unstudied aplites associated with the Matinenda Formation at Elliot Lake, Ontario, Canada. We present major and trace element data from six aplites and their quartz pebble conglomerate hosts. Our preliminary data suggests that the aplites are sodic (10-11%) and are only moderately enriched in SiO2 (up to 72%). In contrast the host rock is potassic (3.8%) or sodic (4-6%) but universally enriched in SiO2 (over 85%) in comparison to the aplites. The aplites are characterized by light pink-grey to medium pink coloration, sugary texture, and fairly uniform sub-millimeter quartz and feldspar crystals. Aplites exhibiting darker or pinker coloration contain higher concentrations of sub-millimeter garnet, biotite, muscovite, and pyrite. Pyrite is present in all aplite and quartz pebble conglomerate samples, accumulating preferentially on and around fractures. Quartz pebble conglomerate samples range from light tan-grey to dark pink-grey in color and contain varying amounts of rounded clear to white-yellow quartz grains up to 3 mm in diameter. The quartz pebble conglomerate of the Matinenda Formation, through which the aplite dikes have passed, is known to contain economic concentrations of paleoplacer uraninite. Uraninite is concentrated near the axis of the regional Quirke Lake Syncline structure, possibly as a result of past activity of an underlying metavolcanic-metasedimentary belt (Robertson 1977, OGS). Some aplite and quartz pebble conglomerate drill cores exhibit radioactivity, however we have selected only non-radioactive samples for analysis. This study will examine the petrogenesis of the aplite dikes, and determine if the intrusion of the aplites triggered a hydrothermal circulation system that remobilized the paleoplacer uraninite in the host rock and transported uranyl ions into the aplite magma.
The Genesis of Precious and Base Metal Mineralization at the Miguel Auza Deposit, Zacatecas, Mexico
The Miguel Auza mine located in Zacatecas State, Mexico, is a vein-type polymetallic epithermal deposit hosted in deformed argillite, siltstone and, greywacke of the Cretaceous Caracol Formation. Silver-rich base metal veins (0.2 m to >1.5 m wide) are spatially associated with the NE-striking, steeply SE- dipping (70-80°) Miguel Auza fault over a strike length of 1.6 km and a depth of 460 m. A 2 km2 monzonitic stock located in the proximity of the mineralized zones, has previously been interpreted as the source of the mineralizing fluids. Four distinct structural stages are correlated with hydrothermal mineral deposition: (I) The Pre-ore stage is characterized by normal faulting, fracturing of host rock, and rotation of bedding planes. This stage consists of quartz, illite, chlorite, +/- pyrite alteration of sedimentary wall rocks. (II) The Pyrite-vein stage is associated with reverse-sense reactivation of early normal faults, dilation of bedding planes/fractures, and deposition of generally barren calcite + pyrite veinlets. (III) The Main-ore stage is related to the development of reverse-fault- hosted massive sulphide veins. During this stage three phases of mineral deposition are recorded: early pyrite and arsenopyrite, intermediate chalcopyrite, pyrite, arsenopyrite, and base metals, and late base metals and Ag-bearing minerals. Associated gangue minerals during the main ore stage are quartz, muscovite, calcite and chlorite. (IV) The Post-ore stage involves late NW-SE striking block faulting, brecciation and calcite veining. Later supergene oxidation of veins led to deposition of Fe-oxides and hydroxides, commonly filling fractures or replacing early-formed sulphide assemblages. The various vein types display classic epithermal textures including open space filling, banding, comb quartz and brecciation. The Ag-bearing minerals comprise pyrargyrite [Ag3(Sb,As)S3], argentotennantite [(Cu,Ag)10(Zn,Fe)2(Sn,As)4S13], polybasite-pearceite [(Ag,Cu)16(Sb,As)2S11], and acanthite [AgS2]; associated sulphides include galena, sphalerite, chalcopyrite, arsenopyrite and pyrite. In the main ore zone, base metal sulphides are commonly intergrown with the Ag-bearing sulfosalts. Analyses of galena show no significant silver values indicating that silver grades are exclusively associated with the Ag-bearing sulfosalts and sulphides. The distribution of the Sb/(Sb + As) ratios in the silver sulfosalts indicate that the ore forming fluid(s) was consistently antimony-rich during the Ag-rich ore deposition with no significant variation laterally, vertically, or along strike of the vein systems. However, Ag/(Ag + Cu) values in argentotennantite decrease along-strike from NE to SW and with depth. Compositions of argentotennantite + pyrargyrite + sphalerite indicate a primary depositional temperature around 325-350° C for the late phase of the Main-ore stage. Compositions of sphalerite also show an increasing trend in FeS (mol %) along strike of the deposit from NE to SW. The geometric relationship between the various structures, vein types, and the regional Miguel Auza fault zone suggest episodic reverse-sense reactivation of normal faults. It is argued that the structural evolution of the area, and, in particular, the Main-ore stage, provided transport pathways for metal-rich fluids and controlled the orientations of ore-bearing veins. Variations in mineral chemistry suggest that the rocks in the NE sector interacted with hotter fluids than in the SW part of the deposit.