Search

In situ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb isotope dating of minerals that co-crystallised with disseminated sulphides in the exceptionally large aureole of metasomatic and contact metamorphic rocks around the Karaelakh mafic-ultramafic intrusion indicates that the sulphide mineralisation was coeval with emplacement of the intrusion. The dates agree with published isotope dilution-thermal ionisation mass spectrometry U-Pb zircon ages but are notably older than available Re-Os isochron ages of sulphides, which have long been interpreted as evidence for the prolonged duration of magmatic ore-forming processes at Norilsk., In situ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb isotope dating of minerals that co-crystallised with disseminated sulphides in the exceptionally large aureole of metasomatic and contact metamorphic rocks around the Karaelakh mafic-ultramafic intrusion indicates that the sulphide mineralisation was coeval with emplacement of the intrusion. The dates agree with published isotope dilution-thermal ionisation mass spectrometry U-Pb zircon ages but are notably older than available Re-Os isochron ages of sulphides, which have long been interpreted as evidence for the prolonged duration of magmatic ore-forming processes at Norilsk.

Mineral and melt inclusions in olivines from the most Mg-rich magma from the southern West Sulawesi Volcanic Province indicate that two distinct melts contributed to its petrogenesis. The contribution that dominates the whole-rock composition comes from a liquid with high CaO (up to 16 wt %) and low Al

Hypotheses for the formation of many types of hydrothermal ore deposits often involve the direct contribution of magma-related fluids (e.g., Cu–Mo–Au porphyries) or their superimposition on barren hydrothermal cells (e.g., volcanic-hosted massive sulfide deposits). However, the chemical and phase compositions of such fluids remain largely unknown. We report preliminary results of a comprehensive study of fluid bubbles trapped inside glassy melt inclusions in primitive olivine phenocrysts and pillow-rim glasses from basaltic magmas from different tectonic environments, including mid-ocean ridges (Macquarie Island, SW Pacific and Mid-Atlantic Ridge 43°N Fracture Zone), ocean islands (Hawaii) and a variety of modern and ancient backarc–island arc settings (eastern Manus Basin, Okinawa and Vanuatu Troughs, Troodos, New Caledonia and Hunter Ridge–Hunter Fracture Zone). Fluid bubbles from all localities, studied using electron microscopy with EDS and laser Raman spectroscopy, are composed of CO2-(±H2O±sulfur)-bearing vapor and contain significant amounts of amorphous (Na–K–Ca–Fe alumino-silicates and dissorded carbon) and crystalline phases. The crystals are represented mainly by carbonates (magnesite, calcite, ankerite, dolomite, siderite, nahcolite and rhodochrosite), sulfates (anhydrite, gypsum, barite and anglesite), and sulfides (pyrite, arsenopyrite, chalcopyrite and marcasite), though other minerals (brukite, apatite, halite, clinoenstatite, kalsilite, nepheline, amphibole and mica) may occur as well. We argue that chemical components (e.g., C, H, S, Cl, Si, Al, Na, K, Fe, Mn, Cr, Ca, Mg, Ba, Pb and Cu) that later formed precipitates in fluid bubbles were originally dissolved in the magmatic fluid, and were not supplied by host glasses or phenocrysts after entrapment. Magma-related fluid rich in dissolved metals and other non-volatile elements may be a potential precursor to ore-forming solutions.

Silicate-melt inclusions that coexist with extremely Cu-rich fluid inclusions in quartz veins from the Bajo de la Alumbrera deposit, Argentina, are used to gain insights into the extent of metal partitioning from magmas to hydrothermal fluids and to assess temporal changes in the bulk chemistry of exsolved magmatic fluids in their passage from the magma to their sites of accumulation in ore deposits. The coexisting melt and fluid inclusions preserve evidence that exsolution of volatiles from the melt was the link between the magmatic and hydrothermal systems. The metal distribution revealed by nuclear microprobe study confirms that Cu, Pb and Zn partitioned preferentially into the Cl-rich fluid phase, rather than into the silicate melt, and that an appreciable amount of Cu was partitioned into the accompanying vapour phase., Silicate-melt inclusions that coexist with extremely Cu-rich fluid inclusions in quartz veins from the Bajo de la Alumbrera deposit, Argentina, are used to gain insights into the extent of metal partitioning from magmas to hydrothermal fluids and to assess temporal changes in the bulk chemistry of exsolved magmatic fluids in their passage from the magma to their sites of accumulation in ore deposits. The coexisting melt and fluid inclusions preserve evidence that exsolution of volatiles from the melt was the link between the magmatic and hydrothermal systems. The metal distribution revealed by nuclear microprobe study confirms that Cu, Pb and Zn partitioned preferentially into the Cl-rich fluid phase, rather than into the silicate melt, and that an appreciable amount of Cu was partitioned into the accompanying vapour phase.

The epithermal deposit is hosted within a volcanic caldera of Neogene age, comprising a central granodiorite stock capped by comagmatic andesites and rhyolites. The intrusive felsic rocks have a close spatial and temporal relationship with the mineralisation and associated hydrothermal alteration. To investigate a possible genetic link, paragenetically constrained melt and fluid inclusions in magmatic quartz and vein minerals were studied using microthermometric techniques. The primary melt inclusions varied in composition from essentially silicate H2O- and Cl-rich melts with low salinity to high-density hypersaline brines. Salinities of secondary fluid inclusions decreased systematically, evolving along with temperature, pressure and density. This indicates that exsolved magmatic brines and aqueous chloride solutions were the primitive precursors to the hydrothermal ore-forming fluids, which produced epithermal mineralisation on mixing with meteoric waters., The epithermal deposit is hosted within a volcanic caldera of Neogene age, comprising a central granodiorite stock capped by comagmatic andesites and rhyolites. The intrusive felsic rocks have a close spatial and temporal relationship with the mineralisation and associated hydrothermal alteration. To investigate a possible genetic link, paragenetically constrained melt and fluid inclusions in magmatic quartz and vein minerals were studied using microthermometric techniques. The primary melt inclusions varied in composition from essentially silicate H2O- and Cl-rich melts with low salinity to high-density hypersaline brines. Salinities of secondary fluid inclusions decreased systematically, evolving along with temperature, pressure and density. This indicates that exsolved magmatic brines and aqueous chloride solutions were the primitive precursors to the hydrothermal ore-forming fluids, which produced epithermal mineralisation on mixing with meteoric waters.

The Olympic Dam iron oxide copper-gold (IOCG)-uranium-silver deposit is hosted in the large Olympic Dam breccia complex within the ca. 1.59 Ga Roxby Downs Granite. This breccia complex formed through multiple stages of hydrothermal activity and texturally destructive brecciation that affected the granite. The deepest diamond drill hole to date intersected weakly altered, in situ brecciated granite (370-2 329 m) and a quartz-phyric felsic unit (2 010-2 265 m). These two rock units host coarse-grained hydrothermal minerals, from ca. 2 150 m to the end of the drill hole (2 329 m). The main minerals in this assemblage are magnetite (± haematite), pyrite, fluorapatite, and quartz, with minor disseminated chalcopyrite, sericite, chlorite, rare earth element (REE)-fluorcarbonates, monazite, uraninite, thorite, galena, sphalerite, anhydrite, schorl, rutile, and pyrrhotite. The assemblage is cut by abundant multiphase veinlets and calcite (± fluorite ± barite) veins. A zircon U-Pb age for the felsic unit (1 591 ± 11 Ma) implies that this unit is broadly coeval with the granite, whereas U-Pb ages for hydrothermal uraninite (1 593.5 ± 5.1 Ma), fluorapatite (1 583.3 ± 6.5 Ma), and haematite (1 592 ± 15 Ma) indicate that deposition of the U-REE-rich hydrothermal magnetite-fluorapatite-pyrite-quartz assemblage and replacement of magnetite by haematite occurred soon after emplacement of the granitic host rocks. Sm-Nd dating of ubiquitous calcite veins suggests formation at ca. 1.54 Ga., The Olympic Dam iron oxide copper-gold (IOCG)-uranium-silver deposit is hosted in the large Olympic Dam breccia complex within the ca. 1.59 Ga Roxby Downs Granite. This breccia complex formed through multiple stages of hydrothermal activity and texturally destructive brecciation that affected the granite. The deepest diamond drill hole to date intersected weakly altered, in situ brecciated granite (370-2 329 m) and a quartz-phyric felsic unit (2 010-2 265 m). These two rock units host coarse-grained hydrothermal minerals, from ca. 2 150 m to the end of the drill hole (2 329 m). The main minerals in this assemblage are magnetite (± haematite), pyrite, fluorapatite, and quartz, with minor disseminated chalcopyrite, sericite, chlorite, rare earth element (REE)-fluorcarbonates, monazite, uraninite, thorite, galena, sphalerite, anhydrite, schorl, rutile, and pyrrhotite. The assemblage is cut by abundant multiphase veinlets and calcite (± fluorite ± barite) veins. A zircon U-Pb age for the felsic unit (1 591 ± 11 Ma) implies that this unit is broadly coeval with the granite, whereas U-Pb ages for hydrothermal uraninite (1 593.5 ± 5.1 Ma), fluorapatite (1 583.3 ± 6.5 Ma), and haematite (1 592 ± 15 Ma) indicate that deposition of the U-REE-rich hydrothermal magnetite-fluorapatite-pyrite-quartz assemblage and replacement of magnetite by haematite occurred soon after emplacement of the granitic host rocks. Sm-Nd dating of ubiquitous calcite veins suggests formation at ca. 1.54 Ga.