Tourmaline Veins Research Articles

Hydrothermal zircon and baddeleyite in Val-d'Or Archean mesothermal gold deposits: characteristics, compositions, and fluid-inclusion properties, with implications fortiming of primary gold mineralization

Zircon and baddeleyite occur within quartz–tourmaline veins at four gold deposits in the Val-d'Or district of the Archean Abitibi Southern Volcanic Zone. Host rocks have experienced intense metasomatic enrichment of Zr, Hf, Y, and rare earth elements. The zircons contain primary inclusions of quartz, tourmaline, pyrite, albite, K-mica, scheelite, and gold, and gold occurs in primary fluid inclusions in zircons. Magmatic zircons in host rocks do not have this suite of inclusions; consequently a wall-rock inheritance model for the vein zircons is implausible. Compositionally, the zircons feature pronounced interzone and intergrain variations of Hf, Y, Yb, Th, and U, and sporadic anomalous Ce contents of ~ 1100 ppm, distinct from magmatic counterparts. Two principal types of primary fluid inclusion occur in the vein zircons. Type 1 H2O–CO2 inclusions have low salinities, variable quantities of CO2 and homogenization temperatures of 260–380 °C, and type 2 CO2 rich inclusions contain minor H2O and CH4. The vein zircons coprecipitated at 260–380 °C and ~ 2 kbar (1 kbar = 100 MPa) with coexisting minerals of undisputed hydrothermal origin, such as vein quartz and gold. In the Superior Province, mesothermal gold deposits are related in space and time to translithospheric structures that mark the diachronous accretion of allochthonous subprovinces from north to south between ~ 2710 and 2680 Ma. Consequently, vein zircon ages of ~ 2680 Ma record the primary mineralizing event, whereas aberrantly young ages for rutile, titanite, scheelite, and micas in the same vein systems, that scatter over 2630–2579 Ma, reveal the age of secondary remobilization events.

Thermochronology of the Cornubian batholith in southwest England: Implications for pluton emplacement and protracted hydrothermal mineralization

The metalliferous ore deposits of southwest England are associated with biotite-muscovite granites that intruded upper Paleozoic sediments and volcanic rocks at the end of the Hercynian Orogeny. The hydrothermal mineralization can be subdivided into four stages: 1. (1) exoskarns 2. (2) high-temperature tin and tungsten oxide-bearing sheeted greisen bordered veins and Sn-bearing tourmaline veins and breccias 3. (3) polymetallic quartz-tourmaline-chlorite-sulfide-fluorite-bearing fissure veins, which represent the main episode of economic mineralization 4. (4) late-stage, low-temperature polymetallic fluorite veins. U-Pb dating of monazite and xenotime and 40Ar 39Ar dating of muscovite were used to determine emplacement ages and cooling times for individual plutons within the Cornubian batholith, as well as separate intrusive phases within the plutons. In addition, 40Ar 39Ar ages from hornblende and secondary muscovite and Sm-Nd isochron ages from fluorite were employed to determine the relationship between pluton emplacement and different stages of mineralization. The U-Pb ages indicate that granite magmatism was protracted from ~300 Ma down to ~275 Ma with no evidence of a major hiatus. There is no systematic relation between the age of a pluton and its location within the batholith. The U-Pb ages for separate granite phases within a single pluton are resolvable and indicate that magma emplacement within individual plutons occurred over periods of as much as 4.5 myrs. Felsic porphyry dike emplacement was coeval with plutonism, but continued to ~270 Ma. The geochronologic data suggest that the Cornubian batholith originated from repeated melting events over 30 myrs and was formed by a series of small coalescing granitic bodies. Cooling rates of the main plutons are unrelated to emplacement age, but decrease from the southwest to the northeast from ~210°C myr −1 to ~60°C myr −1 with a mean of 100°C myr −1. These slow cooling rates appear to reflect the addition of heat from multiple intrusive episodes. The mineralization history is distinct for each pluton and ranges from coeval with, to up to 40 myrs younger than the cooling age for the host pluton. Stage 2 mineralization is broadly synchronous with the emplacement of granite magmas, is dominated by fluids expelled during crystallization, and may be repeated by the emplacement of younger magmas within the same pluton. Sm-Nd isochrons for fluorite from stage 3 polymetallic mineralization give ages of 259 ± 7, 266 ± 3 and 267 ± 12 Ma, postdating stage 2 mineralization by up to 25 myrs within the same deposit. The similarity in age of the main polymetallic mineralization hosted by the oldest and youngest plutons, suggests that this stage of mineralization is unlikely to be related to hydrothermal circulation driven by the emplacement and cooling of the host granite. The mineralization is more likely the product of regional hydrothermal circulation driven by heat from the emplacement and crystallization of younger buried pulses of magma.

Allochemical retrograde metamorphism in shear zones: an example in metapelites, Virginia, USA

Abstract Metapelites in the Altavista area, southwest Virginia Piedmont, USA, underwent allochemical hydrothermal retrograde metamorphism in synmetamorphic shear zones. The metapelites of the Evington Group were metamorphosed in a prograde sequence of chlorite, staurolite, and sillimanite zones. Garnet–biotite geothermometry and phase relations support eastward increasing metamorphic grade, ranging from 570° C in the staurolite zone to 650° C in the sillimanite zone at c. 5.8 kbar. Sillimanite‐zone rocks later underwent progressive retrogression around shear zones which acted as fluid conduits. Retrograde assemblages are successively zoned around the shear zones with staurolite‐, chloritoid‐ and kyanite‐bearing assemblages. The shear zones commonly contain kyanite or tourmaline veins. Applicable phase equilibria indicate that retrogression occurred during isobaric cooling through c. 200–270° C. Rock compositional changes with retrogression occurred in steps: SiO2 was gained in the early stages of the retrogression but lost in the late stages; Al2O3, K2O, and H2O were increasingly gained through the sequence; CaO was increasingly lost. Addition of H2O and decreasing temperatures resulted in new ferromagnesian minerals (staurolite, chloritoid, chlorite) and changes in H2O, SiO2, Al2O3, K2O, and CaO contents produced muscovite and sodic plagioclase.Subsequent to prograde metamorphism, deeply derived fluids migrated upwards along shear zones, providing fluid and energy for the retrograde reactions. The sheared rocks underwent fluid infiltration with fluid fluxes of 1.8 × 107–4.3 × 107 cm3/cm2 corresponding to minimum estimated fluid‐to‐rock ratios of 7.5–21 as a function of position within the shear zone. Fluid flow was from high to low temperature early and low to high temperature later in the retrogression.

Chemical remobilization of volcanic-hosted sulphide deposits at Rosebery and Mt. Lyell, Tasmania

Devonian hydrothermal activity has resulted in localised chemical remobilization of the Cambrian massive sulphide deposits at Rosebery and Mt. Lyell in the Mt. Read mineral province of western Tasmania. At Rosebery, part of the massive pyrite-sphalerite-galena-chalcopyrite-quartz-phyllosilicate lens has been replaced, volume-for-volume, by a pyrite-pyrrhotite-chalcopyrite-magnetite-phlogopite-tourmaline-quartz assemblage, with complete removal of Cambrian sphalerite and galena. The replacement is later than folding of probable Devonian age. The likely temperature, the presence of tourmaline, and the relatively reduced nature of the secondary assemblage, suggest that the related fluids were derived from Devonian granitoids which, on independent evidence (tourmaline veins, pyrrhotite deposits, metamorphic biotite, and shallow granites nearby), appear to extend beneath the Rosebery mice. Lead and zinc dissolved from the Cambrian sulphide lens may have been redeposited, as veins and replacements in younger rocks. The resulting lead is likely to yield anomalous isotopic ages, and be associated with sulphur having the isotopic characteristics of Cambrian massive sulphide deposits. At Mt. Lyell, altered felsic volcanics host large disseminated pyrite-chalcopyrite-quartz-phyllosilicate lenses and minor stratabound exhalative(?) massive sulphide bodies, all of probable Cambrian age. In the North Lyell fault zone, late Cambrian/early Ordovician siliciclastic sediments have been replaced by cherty quartz-bornite-chalcopyrite-chalcocite-sericite-pyrophyllite ores of likely Devonian age. This assemblage probably formed from acid (pyrophyllite), oxidized (barite) solutions that may have derived high a Cu a Fe by dissolution of Cambrian chalcopyrite. Disseminated pyrite-chalcopyrite ores of Cambrian age have a well-developed Devonian cleavage in which hematite has crystallized, thereby indicating circulation of oxidized fluids during or after cleavage development. Precipitation mechanisms for the bornite ores may have involved fluid-sulphide and/or fluid-sediment reactions. The fluids are unlikely to have been derived directly from Devonian granitoids, but they may have been modified groundwaters circulating either during cleavage growth or as a result of heating by post-kinematic Devonian granitoid plutons.

Geological setting of Zambian emerald deposits

The Kafubu emerald deposits occur within rocks of the Muva Supergroup which form the youngest part of pre-Katangan basement immediately southwest of the well known Copperbelt of Zambia. Within the Muva Supergroup are persistent bands of talc—chlorite—amphibole (tremolite/actinolite) - magnetite schist, which are intruded by pegmatites. These pegmatites are believed to be related to neighbouring granitoids which originated or were rejuvenated during early stages of Pan-African orogeny. Pegmatites occur as feldspar—quartz—muscovite bodies or as minor quartz—tourmaline veins. Contact aureoles of the pegmatitic veins with the ultramafic schists are usually altered to phlogopite—biotite—tourmaline aggregates and the minor veins are often concordant to the foliation of the country rocks. The emeralds are found in the contact zones, and sometimes within the quartz—tourmaline veins. The emerald fields are considered comparable in quantity and quality of material to the well known Colombian fields, but their lithological setting differs considerably. Striking similarities were found between the Zambian occurrences and those of South Africa and Zimbabwe.

Societies and Academies

Societies and Academies