Volcanology, Geochemistry, Petrology [V]

 CC:Hall E  Tuesday  1400h

Innovative Applications of Stable Isotopes to Hydrothermal and High-Temperature Processes II Posters

Presiding:  K Graham, McGill University; J B Chapman, Geological Survey of Canada


A Multi-Technique Approach to Understanding Camp-Wide Mineralization Processes in Archean VMS Deposits

* Sharman, E R (libbysh@eps.mcgill.ca), Earth & Planetary Sciences, McGill University, 3450 University St., Montreal, QC H3A 2A7, Canada
Wing, B (boswell.wing@mcgill.ca), Earth & Planetary Sciences, McGill University, 3450 University St., Montreal, QC H3A 2A7, Canada
Taylor, B (Bruce.Taylor@NRCan-RNCan.gc.ca), Geological Survey of Canada, 601 Booth St., Ottawa, ON K1A 0E8, Canada
Jonasson, I (Ian.Jonasson@NRCan-RNCan.gc.ca), Geological Survey of Canada, 601 Booth St., Ottawa, ON K1A 0E8, Canada
Farquhar, J (jfarquha@essic.umd.edu), Department of Geology and Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD 20742, United States
Dubé, B (Benoit.Dube@RNCan-NRCan.gc.ca), Geological Survey of Canada, 490 rue de la Couronne, Quebec, QC G1K 9A9, Canada

Volcanogenic Massive Sulphide (VMS) deposits form on or below the seafloor, in association with submarine extrusive volcanism, and reflect the hydrothermal concentration of ore-forming components originating from various reservoirs within the submarine environment. A defining question about VMS deposits is the relative contributions of different sulfur sources to mineralization. Standard models for VMS formation include contributions from reduction of seawater sulfate, remobilization of sedimentary sulfur, and volcanic sources (e.g., direct magmatic degassing, hydrothermal dissolution of sulfides in volcanic wall rocks). We are using an array of geochemical techniques to assess a suite of sulphide mineral separates collected from numerous VMS deposits within the Archean Noranda camp of the Abitibi Belt, Superior Province, Canada. These techniques include ICP-MS analyses of dissolved sulphide separates, microprobe analysis, and multiple sulphur isotope analyses. Multiple sulphur isotope analysis provides a new and powerful tool for interpreting Archean ore deposits. In pre-2.45 Ga rocks, multiple sulphur isotope analyses (δ33S, δ34S, and δ36S) document mass-independent sulphur isotope fractionation (δ33S≠0.515×δ34S, δ36S≠1.9×δ34S), likely expressed because of the lack of an oxygenated atmosphere. Ore-forming processes in VMS deposits cannot create mass-independent fractionation; they can only dilute it away. Trace element geochemistry of sulphides has been used to identify where in a VMS system these minerals form, with contributions from sources such as sea-water, or from a plume having different geochemical 'footprints'. Coupled with multiple sulphur isotope measurements, trace element geochemistry can be used to help identify sulphur sources within Archean VMS deposits and can be used to interpret camp-wide ore-forming processes and controls on mineralization. This will in turn allow for a more comprehensive understanding of VMS mineralization processes.


Lithium Isotope Systematics in Azores Basalts

* Yu, H (huiminy@muohio.edu), Department of Geology, Miami University, Oxford, OH 45056, United States
Widom, E (widome@muohio.edu), Department of Geology, Miami University, Oxford, OH 45056, United States
Qiu, L (linqiu@geol.umd.edu), Department of Geology, University of Maryland, College Park, MD 20742, United States
Rudnick, R (rudnick@geol.umd.edu), Department of Geology, University of Maryland, College Park, MD 20742, United States
Gelinas, A (gelinaas@muohio.edu), Department of Geology, Miami University, Oxford, OH 45056, United States
Franca, Z (zfranca@uac.pt), Geoscience Department, University of the Azores, Ponta Delgada, Portugal

Basalts from the Azores archipelago and MORB from the nearby Azores Platform exhibit extreme chemical and isotopic variations attributed to the influence of a heterogeneous mantle plume, with compositions ranging from depleted mantle (DMM) to strong HIMU, EMI and EMII signatures. In order to assess the utility of Li isotopes as a mantle source tracer and to better constrain the origin of heterogeneous mantle beneath the Azores, we have analyzed Li isotopes in a suite of young, fresh, MgO-rich basalts from São Miguel and three Central Group islands including Pico, Faial and Terceira. Despite large variations in radiogenic isotope signatures (e.g. 206Pb/204Pb = 19.3 to 20.1), δ7Li varies only slightly (3.1-4.7‰), and is within the range for global and North Atlantic MORB [1, 2]. More extreme δ7Li values such as those reported previously for some EMII, EMI and HIMU ocean island basalts (-17‰ to +10‰; [3-5]) were not observed. Nevertheless, basalts from the Central Group islands with EMI-type signatures are, on average, slightly heavier in δ7Li than the São Miguel samples, and they exhibit positive correlations with 87Sr/86Sr and negative correlations with 206Pb/204Pb, Nd, and Hf isotopes. Li isotopes do not correlate with indices of fractionation such as MgO, suggesting that the δ7Li correlations with radiogenic isotopes may represent subtle variations in mantle source signatures. Positive and negative correlations of δ7Li with 87Sr/86Sr and 206Pb/204Pb, respectively, and relatively unradiogenic Os (187Os/188Os = 0.1244-0.1269), may reflect old, slab-fluid metasomatized mantle beneath the Central Group islands. In contrast, δ7Li signatures in the São Miguel basalts do not correlate with radiogenic isotopes. Rather, δ7Li is essentially constant despite extremely high 87Sr/86Sr and 206Pb/204Pb and low ΔεHf signatures that have been attributed to 3.5 Ga recycled E-MORB or evolved oceanic crust [6; 7]. This suggests either that the São Miguel source does not contain recycled oceanic crust or that recycled oceanic crust may not always result in anomalous Li isotope signatures in the mantle, as previously predicted [8-10]. [1] Tomascak et al. (2008), GCA 72, 1626-1637; [2] Simons et al. (2008), AGU abs; [3] Nishio et al. (2004), EPSL 217, 245-261; [4] Nishio et al. (2005), Geochem.J. 39, 91-103; [5] Ryan and Kyle (2004), Chem Geol. 212, 125-142; [6] Elliot et al. (2007), GCA 71, 219-240; [7] Beier et al. (2007), EPSL 259, 186-199, [8] Zack et al. (2003), EPSL 208, 279-290; [9] Marschall et al. (2007), EPSL 262, 563-580; [10] Chan et al. (2009), EPSL 277, 433-442.


Hydrothermal Fluid Evolution During Vein Formation in Arghash Gold Prospect, Northeast Of Iran

* Alirezaei, S (s-alirezaei@sbu.ac.ir), Saeed Alirezaei, Faculty of Earth Sciences, University of Shahid Beheshti, Tehran, 15875-4731, Iran (Islamic Republic of)
Ashrafpour, E (e-ashrafpour@sbu.ac.ir), Esmaeel Ashrafpour, Parskan East Company, Tehran, Iran (Islamic Republic of)
Ansdell, K M (k.ansdell@usask.ac.ca), Kevin M Ansdell, Department of Geological Sciences, University of Saskatchewan, Saskatoon, S7N 5E2, Canada

The Arghash gold prospect consisting of five gold-bearing vein systems is hosted by Eocene intermediate volcanic and pyroclastic rocks and Late Eocene-Oligocene granitic and dioritic rocks. The vein materials consist mostly of quartz, calcite and minor pyrite. Gold occurs as native particles in quartz, as well as submicroscopic particles in arsenian pyrite, as indicated by microprobe analyses. All auriferous vein systems show similar vein mineralogy and hydrothermal alteration assemblages, implying that they all belong to the same mineralization event in the region. The ä18O compositions of hydrothermal fluid(s) in equilibrium with vein quartz vary from +6.3 to +10.5 per mil. The äD values of the fluid in equilibrium with kaolinite from the immediate altered wall rocks vary from -53 to - 62 per mil. The oxygen isotope values fall in the range commonly accepted for magmatic waters; the calculated äD values overlap between meteoric and magmatic waters. Oxygen and hydrogen isotope compositions similar to those in Arghash can be produced from meteoric waters evolved through interaction with country rocks, boiling, and mixing with magmatic water. The effects of various water/rock ratios on the isotopic composition of the exchanged meteoric water could explain a shift of about +14 per mil in the ä18O values of the mineralizing fluids in the Arghash prospect. Water/rock interaction would also change the äD values of the hydrothermal fluids at low water/rock ratios. In the case of igneous country rocks, as in the Arghash, this process should lead to a shift in the äD of the presumed meteoric waters toward higher values. Boiling will increase the ä18O and äD values of hydrothermal fluids due to fractionation of 16O and H into the vapor phase and decreasing temperature. The magnitude of the enrichment depends on the boiling path and the mechanism of vapor separation. Boiling in Arghash is supported by mineralogical and textural evidences. Gold assays are highest where boiling occurred. The enrichment in 18O might thus be attributed, at least partly, to boiling processes. Mixing with a possible magmatic water could also explain the ä18O shift. However, any fluids of magmatic origin must have had a low salinity, consistent with the diluted fluids in Arghash prospect.