Christopher J. Yeats

Chemical and boron isotopic compositions of tourmaline from the Archean Big Bell and Mount Gibson gold deposits, Murchison Province, Yilgarn Craton, Western Australia

Abstract Tourmaline is a commonly found gangue mineral in Archean greenstone belt lode-gold deposits worldwide. In this paper, we report major, trace, and rare-earth element (REE) and boron isotopic compositions of tourmaline from two major amphibolite-grade Archean gold deposits (Big Bell and Mount Gibson; Murchison Province, Yilgarn Craton, Western Australia), in an attempt to better understand the hydrothermal gold formation processes that were also responsible for tourmalinization. Tourmalines in these two major deposits all belong to the dravite–schorl series, and tend to be Mg-rich where closely associated with gold and sulfide ores. The ore-related tourmalines also contain high concentrations of ore metals such as Au, Ag, Pb, and Cu. In the Big Bell deposit, tourmalines occur along the foliation in lode schists, and show low Fe/(Fe+Mg) ratios of 0.11–0.30, TiO 2 contents of 0.15–0.73 wt.%, and negligible MnO ( δ 11 B values of −17.7‰ to −15.0‰. In the Mount Gibson deposit, the tourmalines closely associated with the sulfide ores are the most Mg-rich, with low Fe/(Fe+Mg) ratios of 0.19–0.22, while tourmalines from garnet-bearing biotite–muscovite schists that overly the lode-gold orebody are more Fe-rich (Fe/(Fe+Mg)=0.30–0.63). Tourmalines from an amphibolite schist are Mg-rich, with Fe/(Fe+Mg) ratios (0.27–0.34) that lie between the above two groups of tourmalines. The compositional trend from Fe-rich tourmalines in the sulfide-poor schists to Mg-rich in the sulfide ores at Mount Gibson is similar to that observed in massive sulfide ore deposits worldwide. Alternatively, the Mg-rich nature of the sulfide-related tourmalines may be due to metamorphic reactions that are accompanied by Fe incorporation into sulfides, leaving coexisting silicates more Mg-rich. The REE geochemistry of the tourmalines from the Mount Gibson ore zone schists displays strong HREE enrichments and positive Eu anomalies with Eu/Eu* ratios up to 2.4, which are similar to tourmalines associated with massive sulfide deposits. The ore-related tourmalines disseminated in the host schists show δ 11 B values of −18.6‰ to −17.0‰, whereas one coarse-grained tourmaline vein has a lower δ 11 B value of −21.9‰ to −21.4‰. These data support hypotheses which suggest that the schists that host the gold mineralization are the metamorphosed equivalents of a base metal-rich, hydrothermally altered, seafloor horizon that is typical of VMS-style mineralization, with the boron possibly derived from leaching of footwall sedimentary and volcanic rocks during hydrothermal circulation.

Contrasting evolution of hydrothermal fluids in the PACMANUS system, Manus Basin: The Sr and S isotope evidence

Ocean Drilling Program (ODP) Leg 193 investigated two sites of hydrothermal activity along the crest of the Pual Ridge in the eastern Manus Basin. A site of low-temperature diffuse venting, Snowcap (Site 1188), and a high-temperature black smoker site, Roman Ruins (Site 1189), were drilled to depths of 386 and 206 m below seafloor (mbsf), respectively. Although the two sites are <1000 m apart, the 87Sr/86Sr and ?34S signatures of anhydrite recovered at both sites are very different. The data suggest a complex interplay among hydrothermal fluid, magmatic fluid, and seawater during alteration and mineralization of the PACMANUS (Papua New Guinea–Australia–Canada–Manus) system. These new results significantly expand the subsurface data on seafloor hydrothermal systems and may begin to explain the earliest processes of multistage mineralization and alteration history that typify ancient massive sulfide systems.

New hydrothermal activity and alkalic volcanism in the backarc Coriolis Troughs, Vanuatu

The Vanuatu Australia Vents Expedition (VAVE) to the Coriolis Troughs in southern Vanuatu during September 2001 aboard the RV Franklin discovered a new hydrothermall vent field-herein informally named Nifonea-and recent alkallic volcanic activity. The Nifonea field in the central Vate Trough was located by coincident light transmission and CH4 anomalies in a hydrothermal plume of ∼60 km2 extent, best developed between 1600 and 1750 m depth at ∼150 m above the seafloor. Extensive hydrothermal fauna and yellow-brown crusts and mounds cover an area of ∼1 km2. Very fresh, glassy, variably vesicular, sparsely phyric and aphyric basalt, trachybasalt, and basaltic trachyandesite (with ∼5-6 wt% combined alkalies at ∼ 51%-53% SiO2 and enriched light rare earth elements, Nb, and Zr) samples were dredged from youthful curtain, tube, and sheet flows, plus iron oxyhydroxide deposits. The alkalic composition of lavas in this tectonic setting is unique and attributed to thin ocean crust being developed in an incipient rifting phase involving a relatively low percentage of source-mantle melting. The Coriolis Troughs are among Earths most youthful backarc basins and thus provide valuable insights to incipient rifting and hydrothermal processes.

Actively forming Kuroko-type VMS mineralization at Iheya North, Okinawa Trough, Japan: new geochemical, petrographic and δ34S isotope results

Actively forming Kuroko-type VMS mineralisation at Iheya North, Okinawa Trough, Japan: New geochemical, petrographic and δS isotope results Christopher Yeats, Steve Hollis, Crystal LaFlamme, Angela Halfpenny, Marco Fiorentini, JuanCarlos Corona, Gordon Southam, Richard Herrington and John Spratt Mineral Resources Flagship, CSIRO, Perth, Australia Geological Survey of New South Wales, Maitland, Australia iCRAG and School of Earth Sciences, University College Dublin, Dublin, Ireland (steve.hollis@icrag-centre.org) Centre for Exploration Targeting, University of Western Australia, Perth, Australia Microscopy & Microanalysis Facility, John de Laeter Centre, Curtin University, Perth, Australia Department of Geological Sciences & Environmental Studies, Binghamton University, USA School of Earth Sciences, University of Queensland, Australia Department of Earth Sciences, Natural History Museum, London, UK