Goldschmidt Conference Abstracts 609 Low-temperature Pt–Pd Iodine fingerprints biogenic fixation mineralisation: Examples from Brazil of platinum and palladium A.R. CABRAL1*, B. LEHMANN1 AND M. BRAUNS2 A.R. CABRAL1*, M. RADTKE2, F. MUNNIK3, 1 2 2 1 B. LEHMANN , U. REINHOLZ , H. RIESEMEIER , Mineral Deposits, TU Clausthal, 38678 Clausthal-Zellerfeld, M. TUPINAMBÁ4 AND R. KWITKO-RIBEIRO5 Germany (*correspondence: [email protected]) 1Mineral Deposits, TU Clausthal, 38678 Clausthal-Zellerfeld, 2Curt-Engelhorn-Zentrum Archäometrie, 68159 Mannheim, Germany Germany (*correspondence: [email protected]) 2BAM Federal Institute for Materials Research and Testing, Hematite-bearing Au–Pd mineralisation in Brazil 12489 Berlin, Germany commonly has a platiniferous component [1, 2]. Examples are 3Institute of Ion Beam Physics and Materials Research, Hg-bearing hongshiite, PtCu, and Pt2HgSe3, both from Itabira, HZDR, P.O. Box 510119, 01314 Dresden, Germany Minas Gerais. These minerals occur in specular hematite-rich 4Faculdade de Geologia, UERJ, 20550-050 Rio Janeiro-RJ, veins that cross-cut the ~0.6-Ga Brasiliano tectonic foliation Brazil of the host rock (itabirite). This vein mineralisation is called 4Centro de Desenvolvimento Mineral, VALE, Caixa Postal 09, ‘jacutinga’. The presence of barite in hongshiite and Na/K– 33030-970 Santa Luzia-MG, Brazil Na/Li fluid–mineral geothermometers indicate that oxidising brines of evaporitic origin were instrumental to the Au–Pd–Pt Botryoidal aggregates of platinum (Pt) and palladium (Pd) mineralisation at a maximum temperature of about 350°C [3]. from an alluvial deposit (Córrego Bom Sucesso) in Serro, Platiniferous alluvia are found in the quartzitic domains of Minas Gerais, Brazil, were likely the sample material from the Palaeo-Mesoproterozoic southern Serra do Espinhaço, which Wollaston [1] isolated and identified Pd for the first Minas Gerais, north of Itabira. Alluvial palladiferous gold and time [2]. We recovered millimetre-sized botryoidal and rod- specular hematite point to ‘jacutinga’-like veins in the shaped grains of Pt and Pd from the alluvial deposit. Their quartzite country rock in the Córrego Bom Sucesso area. We arborescent morphologies indicate that the Pt–Pd aggregates found high Pd/Ag ratios (~4–3700) in the alluvial formed in situ within the alluvium [2]. We carried out palladiferous gold. Such ratios are thermodynamically synchrotron radiation-induced X-ray fluorescence (SR-XRF) restricted to very oxidising brines [4]. In addition, Córrego spectrometry on the Pt–Pd aggregates to determine iodine. We Bom Sucesso is famous for its botryoidal Pt–Pd aggregates, found high concentrations of iodine, in the range from 10 to reaching several millimetres across, which formed within the ~120 !g/g [3]. Iodine is a strongly biophile element [4], which alluvium [1, 2, 5]. is enriched in peatlands by microbial activity [5]. Its high In northern Brazil, Au–Pd–Pt bonanza mineralisation concentration in the Pt–Pd nuggets suggests that microbial triggered the gold rush that made Serra Pelada known activity took place during precious-metal fixation in the worldwide. The near-surface mineralisation is hosted by aqueous alluvial milieu. The Pt–Pd nuggets have an average weakly metamorphosed metasedimentary rocks of supposedly thallium/selenium ratio of about 0.08, a value close to that for Neoarchaean age, but the bonanza ore is coeval with a Mn–Ba fluvial waters, suggesting that Pt and Pd were fixed from oxide, which has a Late Cretaceous 40Ar/39Ar age [6]. Fluid- highly dilute solutions within the alluvium [6]. Inorganic and inclusion microthermometric data from quartz and the mineral biogenic processes, i.e. electrochemical metal accretion [7] assemblage of Mn–Ba oxide and fine-grained specular and bioreduction, are thought to have contributed to the hematite give evidence for very oxidising brines at growth of biogenic Pt–Pd nanoparticles that formed on temperatures between ~100 and 170°C. organic templates such as humified plant remains. [1] Hussak (1904) Sitz.-Ber. math.-naturwiss. Kl. Kais. Akad. [1] Wollaston (1809) Phil. Trans. 99, 189-194. [2] Hussak Wiss. 113, 379-466. [2] Cabral et al. (2009) Econ. Geol. 104, (1906) Z. prakt. Geol. 14, 284-93. [3] Cabral et al. (2011) 1265-1276. [3] Lüders et al. (2005) Miner. Deposita 40, 289- Chem. Geol. 281, 125-132. [4] Goldschmidt (1958) 306. [4] Gammons et al. (1993) Geochim. Cosmochim. Acta Geochemistry. Oxford University Press. [5] Keppler et al. 57, 2469-2479. [5] Cabral et al. (2011) Chem. Geol. 281, 125- (2004) Environ. Chem. Lett. 1, 219-223. [6] Cabral et al. 132. [6] Cabral et al. (2011) Econ. Geol. 106, 119-125. (2009) Econ. Geol. 104, 1265-1276. [7] Cabral et al. (2009) Eur. J. Mineral. 21, 811-816. Mineralogical Magazine www.minersoc.org Downloaded from http://pubs.geoscienceworld.org/minmag/article-pdf/75/3/609/2918879/gsminmag.75.3.03-C.pdf by guest on 30 September 2021 610 Goldschmidt Conference Abstracts Volatile and trace element Mineralogy of stream sediments and abundances in HIMU melt inclusions soils of Santiago Island, Cape Verde R.A. CABRAL1*, M.G. JACKSON1, E.F. ROSE-KOGA2, M.M.S. CABRAL PINTO1,2*, M.M.V. SILVA2, J.M.D. DAY3AND N. SHIMIZU4 R. HERNANDEZ1 AND E.A. FERREIRA DA SILVA1 1Department of Earth Sciences, Boston University, Boston, 1Geobiotec Center, University of Aveiro, Portugal MA 02215, USA (*correspondence: [email protected]) 2Geociences Center, University of Coimbra, Portugal 2Laboratoire Magmas et Volcans, Universite Blaise Pascal, (*[email protected]) CNRS, UMR 6524, IRD, R 163, Clermont-Ferrand, France Santiago Island covers an area of 991 km2 and is 3Scripps Institution of Oceanography, La Jolla, CA 92037, characterized by a rough relief. (up to 1392 m) and valleys USA with almost vertical slopes and large flat areas in the coastal 4Woods Hole Institute of Oceanography, Woods Hole, MA zones. The climate is semi-arid, with torrential rains. The 02543, USA lavas occupy most of the island, the pyroclasts are subordinated and the quaternary sedimentary cover occurs in Water distribution within the mantle influences magma small areas. The mineralogical composition of the 70 stream chemistry and evolution, location and extent of melting, and sediments and 70 soil samples, collected from all geological volcanism over geological time. During subduction, oceanic formations of the island, was studied in the < 2mm fraction. crust has been suggested to transport significant quantities of The samples are dominated by primary silicate minerals, such water and other volatiles to post-arc depths [1]. Despite the as feldspar (15.0 to 35.4 %), pyroxene (7.8 to 37.4 %) and important control water has over mantle characteristics and olivine (0.0 to 9.0 %), reflecting the mineralogical signature of behavior, the amount of water that is retained within the descending slab is poorly constrained [2, 3]. the igneous rocks that support the island. Quartz, One possible method for constraining the amount of phyllosilicates (smectite, kaolinite, mica/illite), calcite, surviving volatiles is to examine oceanic hotspot lavas that are hematite, leucite, apatite, nepheline, magnetite, thought to sample melts of subducted oceanic crust. It is titanomagnetite, ilmenite, chromite, garnet, zeolites, siderite, hypothesized that subducted material can be returned to the opal, barite, titanite, zircon, halite, aragonite, dolomite, brucite shallow mantle in areas of mantle upwelling, where it is larnite and chlorite were also identified. Higher proportion of partially melted and erupted during hotspot volcanism. Lavas feldspar and pyroxene were detected on stream sediments from Mangaia represent the HIMU (high-µ, or high (27.3 % and 25.6 %, respectively), than the soils (24.1 % and 238U/204Pb) mantle end member, which contains geochemical 17.0 %, respectively). The soils have higher relative signatures associated with recycled oceanic crust. proportion of quartz (24.5 %), phyllosilicates (16.1 %), calcite Earlier work on olivine-hosted melt inclusions from (2.8 %) and hematite (9.6 %) than stream sediments (12.2 %, Mangaia found the inclusions to host volatile-rich phases (e.g., 14.5 %, 0.8 % and 9.0 %, respectively). These differences are amphibole, phlogopite, apatite, and carbonatite [4]), an due to pedogenetic processes and wind-transported materials observation that is consistent with Mangaian melt inclusions that affect the soils. Soil and stream sediments that cover being volatile-rich. High volatile abundances, coupled with formations affected by intense weathering are enriched in major- and trace-element compositions in melt inclusions that phyllosilicates and hematite and impoverished in pyroxene are consistent with HIMU source derivation, may indicate that subducted oceanic crust has retained a significant amount of and olivine. water [1]. This would imply that dehydration during subduction of oceanic crust is not an efficient process. Here, we will place new constraints on deep cycling of volatiles into the mantle through examination of olivine- hosted melt inclusions, and we will present the first ever volatiles data on HIMU end member glasses. [1] Hacker (2008) G3 9, doi:10.1029/2007GC001707. [2] Hilton et al. (2002) Rev.Miner 47, 319-370. [3] Wallace (2005) Volcan.Geotherm.Res. 140, 217-240. [4] Saal et. al. (1998) Science 282, 1481-1484. Mineralogical Magazine www.minersoc.org Downloaded from http://pubs.geoscienceworld.org/minmag/article-pdf/75/3/609/2918879/gsminmag.75.3.03-C.pdf by guest on 30 September 2021 Goldschmidt Conference Abstracts 611 Contrasting mechanisms for two The characteristics of organic matter pulses of garnet growth at Stillup Tal, adsorbed on clay minerals and its Tauern Window, Austria significance in carbon cycling MARK J. CADDICK1 , ETHAN F. BAXTER1,2 AND CAI JINGGONG1*, JI JUNFENG2, LU LONGFEI1, DING FEI1 ANTHONY D. POLLINGTON2,3 AND CAI YUANFENG2 1ETH Zürich, 8092 Zürich, Switzerland 1State key laboratory of marine geology, Shanghai, 200092, 2Boston University, Boston MA, 02215, USA China (*correspondence: [email protected]) 3University of Wisconsin, Madison, WI, 53706, USA 2State Key Laboratory of Mineral Deposits Research, Nanjing; 210093,china Growth of ca.