Uranium-lead dating of felsic magmatic cycles in the southern Sierras Pampeanas, Argentina: Implications for the tectonic development of the proto-Andean Gondwana margin
Laurentia-Gondwana connections before Pangea, 1999
Uranium-lead dating of felsic magmatic cycles in the southern Sierras Pampeanas, Argentina: Implications for the tectonic development of the proto-Andean Gondwana margin
Laurentia-Gondwana connections before Pangea, 1999
Mineral exploration in areas of little bedrock exposure must increasingly rely on the predictive ... more Mineral exploration in areas of little bedrock exposure must increasingly rely on the predictive quality of geological, geochemical and geophysical data. In this contribution analysis of fluid inclusions is used to characterize fluid systems associated with alteration and mineralization in two locations in South Australia. In the Curnamona Province, fluids related to albitization and calc-silicate alteration with associated brecciation evolve from a saline sodic composition to a metalliferous, saline fluid also found associated with sulphide mineralization. The microthermometric distinction of inclusion types is evidence for pressure fluctuation as a cause for breccia formation. In the Central Gawler Craton gold province fluid inclusion studies have characterized the compositional and constrained the physical conditions of mineralizing fluids. Regionally identified low salinity CO 2-bearing fluids at temperatures b 350°C and pressures b 1.5kbar indicate a shallow-level orogenic [
Mineral exploration in areas of little bedrock exposure must increasingly rely on the predictive ... more Mineral exploration in areas of little bedrock exposure must increasingly rely on the predictive quality of geological, geochemical and geophysical data. In this contribution analysis of fluid inclusions is used to characterize fluid systems associated with alteration and mineralization in two locations in South Australia. In the Curnamona Province, fluids related to albitization and calc-silicate alteration with associated brecciation evolve from a saline sodic composition to a metalliferous, saline fluid also found associated with sulphide mineralization. The microthermometric distinction of inclusion types is evidence for pressure fluctuation as a cause for breccia formation. In the Central Gawler Craton gold province fluid inclusion studies have characterized the compositional and constrained the physical conditions of mineralizing fluids. Regionally identified low salinity CO 2-bearing fluids at temperatures b 350°C and pressures b 1.5kbar indicate a shallow-level orogenic [
The eastern Gawler craton hosts Australia's premier uranium-bearing iron oxide copper-gold (IOCG)... more The eastern Gawler craton hosts Australia's premier uranium-bearing iron oxide copper-gold (IOCG) belt, the >500-km-long Olympic Cu-Au-(U) province. In addition to the Olympic Dam Cu-U-Au and Prominent Hill Cu-Au deposits, numerous barren and weakly mineralized IOCG prospects are present in the province. New geochronological data for hydrothermal minerals combined with constraints from host-rock ages demonstrate that alteration and associated IOCG mineralization formed between ~1570 and ~1600 Ma in three districts of the province. This IOCG hydrothermal activity temporally overlaps with magmatism of the Hiltaba Suite and Gawler Range Volcanics. Titanites in paragenetically early magnetite-bearing alteration in the Olympic Dam and Prominent Hill districts yield U-Pb ion probe ages of 1576 ± 5 and 1567 ± 10 Ma, respectively. Molybdenite in veins crosscutting magnetite-biotite and albitic alteration in the Moonta-Wallaroo district have Re-Os ages of 1574 ± 6 and 1599 ± 6 Ma, respectively. These represent minimum ages of the IOCG alteration assemblages in this district. A muscovite 40 Ar/ 39 Ar age of 1575 ± 11 Ma provides a minimum age of paragenetically later hematitic alteration in the Olympic Dam district. Neodymium isotope compositions were determined for 44 whole-rock samples from barren and weakly mineralized Cu-Au prospects and host rocks in the Olympic Dam and Prominent Hill districts. The new geochronological framework enables comparison of the Nd isotope data across two IOCG districts at the time of formation of the Olympic Dam deposit (ca. 1590 Ma). Magnetite-rich weakly Cu-mineralized alteration from five prospects yields a relatively narrow range of εNd(1590) values of-5.8 to-4.1. Both hematite-and magnetite-rich alteration yield generally similar εNd(1590) values that match values from fresh and weakly altered Paleoproterozoic metasedimentary and metagranitic rocks (-6.6 to-3.5) as well as from most felsic Hiltaba Suite intrusions and Gawler Range Volcanics in the eastern Gawler craton (ca.-6 to-4). These data are consistent with crustal sources for REE and, by implication, for associated copper in the barren and weakly mineralized prospects. Mineralization and alteration in these minor IOCG systems can be geochemically discriminated from the giant Olympic Dam deposit, where greater inputs of mantle-derived REE and other ore components are evident.
Regionally extensive semiconformable zones of silicified, Fe-Mg metasomatized and epidotized, dom... more Regionally extensive semiconformable zones of silicified, Fe-Mg metasomatized and epidotized, dominantly mafic, volcaniclastic strata and lava flows are exposed 1 to 2 km stratigraphically beneath the Chisel Lake Zn-Cu massive sulfide deposit, Snow Lake district, Manitoba. The alteration zones occur within a Lower Proterozoic medium-grade regional metamorphic terrane of felsic and mafic volcanics and marine sediments, and lie between subvolcanic tonalite sills and the massive sulfide deposit. Semiconformable alteration occurs at two main stratigraphic positions. The lower zone of silicification in pillowed and massive mafic lavas is inferred to have occurred close to the sea floor and probably was not related directly to formation of the Chisel Lake massive sulfide deposit. The stratigraphically higher zone, which may be spatially and temporally associated with massive sulfide deposition, occurs in a-• 300-m-thick heterolithic mafic volcanic breccia and wacke unit. This volcaniclastic-hosted alteration is zoned laterally from dominantly silicification and epidotization to mainly Fe-Mg metasomatized, garnet-chlorite _+ biotite _+ staurolite rocks nearer the Chisel Lake sulfide deposit. A discordant footwall Fe-Mg alteration zone directly beneath the sulfide deposit extends toward, and may meet, the semiconformable Fe-Mg metasomatized zone. Silicification contributed to partial to complete replacement of volcanic clasts, beds in the volcaniclastic unit, rocks adjacent to some felsic dikes and pillow interiors by quartz and sodic plagioclase. Mass balance calculations for silicified rocks and equivalent least altered parts of volcaniclastic beds, dikes, and pillows indicate that SiO2 increased by up to 50 percent of its initial value, and Na20 by up to 30 percent. Up to 80 percent of the FeO, MgO, CaO, and Zn was removed during silicification. Elemental fluxes during Fe-Mg metasomatism are generally opposite those characterizing silicification and are of comparable magnitude. Epidotization resulted in depletion of Na, total Fe (but increased the Fe•+/Fe a+ ratio), Mg, Mn, K, Zn, and Ba, and enrichment in Ca and Sr relative to least altered rocks. Almost constant interelement ratios ofTi, Zr, and A1 in altered and less altered rocks indicate that these elements were essentially immobile during metasomatism and subsequent medium-grade regional metamorphisre. Limited data suggest that the heavy REE were also immobile during silicification. The subconcordant silicification in the mafic volcaniclastic unit is interpreted to have formed at subsea-floor depths of 1 to 2 km where a felsic dike swarm and subvolcanic tonalite sills heated Si-rich evolved seawater above the temperature of the silica solubility maximum ("• 340ø-450øC at pressures below 900 bars) causing silica deposition and metal leaching. A portion of the Fe, Mg, and possibly Zn that was leached from the subconcordant silicified zone may have been transported laterally away from this environment, thereby producing the semiconformable Fe-Mg metasomatized zones. Cross stratal structures similar to the hydrothermally altered synvolcanic faults that are known to cut the semiconformable alteration may represent fluid flow paths from a subconcordant metal reservoir in the volcaniclastic unit to the sea floor, where massive sulfides were deposited. Semiconformable pervasively silicified zones, particularly those associated with Fe-Mg metasomatized and/or epidotized rocks, are significantly larger exploration targets than areas of proximal alteration, and indicate large-scale hydrothermal mass transfer. Zonation of silicification to Fe-Mg metasomatism laterally within the alteration may provide a vector toward the discordant Fe-Mg-enriched alteration zones that commonly underlie volcanic-associated massive sulfide deposits.
Iron oxide-associated Au-Cu-Bi deposits of the Tennant Creek gold field range from magnetite-pyrr... more Iron oxide-associated Au-Cu-Bi deposits of the Tennant Creek gold field range from magnetite-pyrrhotitepyrite (reduced) Cu-rich deposits, through sulfide-poor, magnetite-rich high-grade Au-Bi (intermediate-f O 2) deposits, to hematite-rich high-grade Au (oxidized) deposits. Discordant magnetite-hematite-chlorite-quartz ironstone bodies that host the ore formed hydrothermally during the ~1840 to 1860 Ma Barramundi orogeny at temperatures up to ~350°to 400°C and ~2.5 to 5(?) kbars. Deposition of ironstone minerals was localized by dilatency that was generated during D 1 reverse shearing and folding. A separate Au-Cu-Bi hydrothermal stage overprinted selected ironstones during D 2 at or before ~1825 to 1830 Ma, where favorable D 1 structures and fluid pathways were reactivated. The extremes of the reduced-to oxidized-deposit spectrum represent divergent chemical conditions that cannot be reconciled by the action of a single fluid. We have identified both reducing and oxidizing fluids that helped to form the Au-Cu-Bi deposits. At the West Peko deposit, which represents the reduced end-member subtype, constraints from oxide-silicate-sulfide mineral stabilities, fluid inclusions, and chlorite thermochemistry indicate that Au-Cu-Bi ore was introduced into ironstone at 300°to 340°C by a reducing (pyrrhotite-stable), weakly acidic, sulfur-rich and N 2-CH 4-rich aqueous fluid containing 3 to 10 wt percent NaCl equiv. An ore fluid source outside the relatively oxidized metasedimentary host sequence (Warramunga Formation) is implied. We suggest similar ore fluids for other reduced Cu-and sulfide-rich deposits in the district. Chemical mass transfer modeling of reactions between reducing fluid and ironstone, incorporating estimated thermodynamic properties for aqueous bismuth complexes, indicates that highly efficient gold and native Bi deposition was predominantly in response to desulfidation and oxidation of the fluid. Chalcopyrite precipitated mainly in response to increased pH. Gold-bismuth ore in the Eldorado deposit, an oxidized and hematitic end member, is interpreted to have involved mixing of an oxidizing (hematite-stable) Ca-Na-Cl brine with an intermediate-f O 2 , low-to moderatesalinity ore fluid, as well as reaction of both fluids with ironstone. Furthermore, the mass transfer modeling suggests that mixing of reducing and oxidizing fluids in the absence of ironstones could have resulted in highgrade Au deposits lacking Fe oxides, a subtype that has received comparatively little exploration attention in the Tennant Creek district.
Metallogeny of the southern Sierras Pampeanas, Argentina: geological, 40Ar-39Ar dating and stable isotope evidence for Devonian Au, Ag-Pb-Zn and W ore …
Ore Geology …, 2000
... study areas in the provinces of San Luis, Córdoba and La Rioja are summarized in Table 1. The... more ... study areas in the provinces of San Luis, Córdoba and La Rioja are summarized in Table 1. The earliest mineralization recognized in these areas are deposits of NiCu sulfides hosted by Ordovician maficultramafic intrusions in the Sierra de San Luis (Skirrow and Sims, 2000). ...
Uploads
Papers by Roger Skirrow