Structural Controls on Shallow Cenozoic Fluid Flow in the Otago Schist, New Zealand

The Otago Schist in the South Island of New Zealand represents an exhumed Mesozoic accretionary prism. Two coastal areas (Akatore Creek and Bruce Rocks) south of Dunedin preserve structural and geochemical evidence for the development of postmetamorphic hydrothermal systems that involved widespread fluid-rock reaction at shallow crustal depths. The Jurassic to Triassic pumpellyite-actinolite (Akatore Creek) to upper greenschist facies (Bruce Rocks) metamorphic fabrics were crosscut by sets of regionally extensive Cretaceous exhumation joints. Many of the joints were subsequently reactivated to form networks of small-displacement (<metres) strike-slip faults containing cemented fault breccias and veins composed of hydrothermal calcite, siderite, and ankerite. Paleostress analysis performed on infrequent fault slickenlines indicates an overall strike-slip paleostress regime and a paleo-σ1 orientation (azimuth 094°) similar to the contemporary σ1 orientation in Otago and Canterbury (azimuth c. 110°-120°). High δ18O values in vein calcite (δ18OVPDB=21 to 28‰), together with the predominance of Type I calcite twins, suggest that vein formation occurred at low temperatures (<200°C) in the shallow crust and was associated with strongly channelized fluid flow along the joint and fault networks. Mass-balance calculations performed on samples from carbonate alteration zones show that significant mobilisation of elements occurred during fluid flow and fluid-rock reaction. Whole-rock and in situ carbonate 87Sr/86Sr data indicate varying degrees of interaction between the hydrothermal fluids and the host rock schists. Fluids were likely derived from the breakdown of metamorphic Ca-rich mineral phases with low 87Rb in the host schists (e.g., epidote or calcite), as well as more radiogenic components such as mica. Overall, the field and geochemical data suggest that shallow fluid flow in the field areas was channelized along foliation surfaces, exhumation joints, and networks of brittle faults, and that these structures controlled the distribution of fluid-rock reactions and hydrothermal veins. The brittle fault networks and associated hydrothermal systems are interpreted to have formed after the onset of Early Miocene compression in the South Island and may represent the manifestation of fracturing and fluid flow associated with reverse reactivation of regional-scale faults such as the nearby Akatore Fault

Paragênese e evolução do depósito hidrotermal de óxido de ferrocobre-ouro Bacuri, Província Carajás (PA)

CNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOThe Bacuri copper deposit is located about 9 km east of the world-class Sossego iron oxide-copper-gold deposit in the Carajás Province. It is hosted by the ca. 2.84 Ga Serra Dourada granite, the Bacuri Porphyry, and subordinated gabbro. The Bacuri deposit is situated within the regional-scale, WNW-ESE-trending ductile Canaã shear zone, characterized by early albite, scapolite-magnetite, potassium feldspar and biotite-scapolite-magnetite hydrothermal alteration zones. Copper mineralization was controlled and largely simultaneous to the development of a NE-SW-trending transcurrent fault zone. Within this zone, intense chlorite alteration, silicification and quartz veining overlap previous hydrothermal stages. Copper ore ( chalcopyrite-pyrite- magnetite) is disseminated and related to veins and breccias. Chalcopyrite occurs along the mylonitic foliation in chlorite alteration zones, in brecciated quartz-(muscovite-fluorite) and undeformed milky quartz veins and in late potassium feldspar-epidote veinlets with open space textures. The relative temporal history of the Bacuri deposit reveals significant changes in physico-chemical parameters during the system evolution. Contribution of hot hypersaline metalliferous fluids was important during early sodic and potassic alteration stages and related to a regional hydrothermal system developed at relatively deep crustal levels. Influx of externally derived diluted fluids within the NE-SW-trending fault zone favoured the late shallow-emplaced chlorite alteration and silicification, and may have triggered the ore deposition due to decrease of temperaturend salinity, accompanied by fO2 increase and pH decrease. Such significant changes could indicate a hybrid system that evolved in multiple pulses during progressive exhumation.The Bacuri copper deposit is located about 9 km east of the world-class Sossego iron oxide–copper–gold deposit in the Carajás Province. It is hosted by the ca. 2.84 Ga Serra Dourada granite, the Bacuri Porphyry, and subordinated gabbro. The Bacuri deposit is situated within the regional-scale, WNW-ESE-trending ductile Canaã shear zone, characterized by early albite, scapolite-magnetite, potassium feldspar and biotite-scapolite-magnetite hydrothermal alteration zones. Copper mineralization was controlled and largely simultaneous to the development of a NE-SW-trending transcurrent fault zone. Within this zone, intense chlorite alteration, silicification and quartz veining overlap previous hydrothermal stages. Copper ore (chalcopyrite-pyrite-magnetite) is disseminated and related to veins and breccias. Chalcopyrite occurs along the mylonitic foliation in chlorite alteration zones, in brecciated quartz-(muscovite-fluorite) and undeformed milky quartz veins and in late potassium feldspar-epidote veinlets with open space textures. The relative temporal history of the Bacuri deposit reveals significant changes in physico-chemical parameters during the system evolution. Contribution of hot hypersaline metalliferous fluids was important during early sodic and potassic alteration stages and related to a regional hydrothermal system developed at relatively deep crustal levels. Influx of externally derived diluted fluids within the NE-SW-trending fault zone favoured the late shallow-emplaced chlorite alteration and silicification, and may have triggered the ore deposition due to decrease of temperature and salinity, accompanied by ƒO2 increase and pH decrease. Such significant changes could indicate a hybrid system that evolved in multiple pulses during progressive exhumation4417390CNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO472549/2009-0; 481969/2013-62003/11163-6; 2004/08126-4Augusto, R.A., Monteiro, L.V.S., Xavier, R.P., Souza Filho, C.R., Zonas de Alteração hidrotermal e paragêneses do minério cuprífero do Alvo Bacaba, Província Mineral de Carajás (2008) Revista Brasileira de Geociências, 38 (2), pp. 263-277Chiaradia, M., Banks, D., Cliff, R., Marschik, R., Haller, A., Origin of fluids in iron oxide-copper-gold deposits: Constraints from δ37Cl, 87Sr/86Sri and Cl/Br (2006) Mineralium Deposita, 41 (6), pp. 565-573. , DOI 10.1007/s00126-006-0082-6Craveiro, G.S., Villas, N.R., Silva, A.R.C., Depósito de Cu- Au Visconde, Carajás (PA): Geologia e alteração hidrotermal das rochas encaixantes (2012) Revista Brasileira de Geociências, 42, pp. 453-470Dardenne, M.A., Schobbenhaus, C.S., (2001) Metalogênese Do Brasil, p. 392. , Editora Universidade de Brasília/CNPq, BrasíliaRevisão litoestratigráfica da Província Mineral de Carajás - litoestratigrafia e principais depósitos minerais (1988) SBG, Congresso Brasileiro de Geologia, 35, pp. 11-54. , DOCEGEO, Anexo aos anaisFeio, G.R.L., Dall'Agnol, R., Dantas, E.L., Macambira, M.J.B., Santos, J.O.S., Althoff, F.J., Soares, J.E.B., Archean granitoid magmatism in the Canaã dos Carajás area: Implications for crustal evolution of the Carajás province (2013) Amazonian Craton, Brazil. Precambriam Research, (227), pp. 157-185Figueiredo, E., Silva, R.C., Hagemann, S., Lobato, L.M., Rosière, C.A., Banks, D.A., Davidson, G.J., Hergt, J., Hydrothermal fluid processes and evolution of the giant Serra Norte jaspilite-hosted iron ore deposits, Carajás mineral province (2013) Economic Geology, 108, pp. 739-779Fournier, R.O., Active hydrothermal systems as analogues of fossil systems (1983) The Role of Heat in the Development of Energy and Mineral Resources in the Northern Basin and Range Province, pp. 263-284. , Geothermal Resources Council Special Report No. 13,Eaton, G., (eds.)Frietsch, R., Tuisku, P., Martinsson, O., Perdahl, J.-A., Early Proterozoic Cu-(Au) and Fe ore deposits associated with regional Na-Cl metasomatism in northern Fennoscandia (1997) Ore Geology Reviews, 12, pp. 1-34Hirata, W.K., Rigon, J.C., Kadekaru, K., Cordeiro, A.A.C., Meireles, E.A., Geologia Regional da Província Mineral de Carajás (1982) SBG/NO, Anais I, Simpósio de Geologia da Amazônia, 1, pp. 100-110Hitzman, M.W., Oreskes, N., Einaudi, M.T., Geological characteristics and tectonic setting of Proterozoic iron oxide (Cu-U-Au-REE) deposits (1992) Precambrian Research, 58 (1-4), pp. 241-287Huhn, S.R.B., Souza, C.I.J., Albuquerque, M.C., Leal, E.D., Brustolin, V., Descoberta do depósito Cu-(Au) Cristalino: Geologia e mineralização associada região da Serra do Rabo-Carajás, PA (1999) Simpósio de Geologia da Amazônia, 6, pp. 140-143Hunt, J., Baker, T., Thorkelson, D., Regional-scale Proterozoic IOCG-mineralized breccia systems: Examples from the Wernecke Mountains, Yukon, Canada (2005) Mineralium Deposita, 40 (5), pp. 492-514. , DOI 10.1007/s00126-005-0019-5Lancaster Oliveira, J., Fanton, J., Almeida, A.J., Leveille, R.A., Vieira, S., Discovery and geology of the Sossego copper- gold deposit, Carajás district, Pará State, Brazil (2000) 31th International Geological Congress, , Rio de Janeiro, Anais [CD-ROM]Lindenmayer, Z.G., Depósito de Cu-Au do Salobo, Serra dos Carajás: Uma revisão (2003) Caracterização e Modelamento de Depósitos Minerais, pp. 69-98. , São Leopoldo, Unisinos,Ronchi L.H. & Althoff F.J. (eds.)Machado, N., Lindenmayer, D.H., Krough, T.E., Lindenmayer, Z.G., U-Pb geochronology of Archean magmatism and basement reactivation in the Carajás area, Amazon Shield, Brazil (1991) Precambrian Research, 49, pp. 1-26Melo, G.H.C., Monteiro, L.V.S., Xavier, R.P., Santiago, E.S.B., Santos, A.F.F., Torres, A., Aires, B., A new outlook on the giant Salobo IOCG deposit: A Mesoarchean basement-hosted deposit, Carajás province (2013) Simpósio de Geologia da Amazônia, , Belém, Anais [CD-ROM]Monteiro, L.V.S., Xavier, R.P., Hitzman, M.W., Carvalho, E.R., Johnson, C.A., Souza Filho, C.R., Torresi, I., Spatial and temporal zoning of hydrothermal alteration and mineralization in the Sossego iron oxide copper gold deposit, Carajás mineral province, Brazil: Paragenesis and stable isotope constraints (2008) Mineralium Deposita, 43, pp. 129-159Monteiro, L.V.S., Xavier, R.P., Hitzman, M.W., Juliani, C., Souza Filho, C.R., Carvalho, E.R., Mineral chemistry of ore and hydrothermal alteration at the Sossego iron oxide-copper- gold deposit, Carajás mineral province, Brazil (2008) Ore Geology Reviews, 34, pp. 317-336Monteiro, L.V.S., Xavier, R.P., Pestilho, A.L.S., Moreto, C.P.N., Juliani, C., Torresi, I., Souza Filho, C.R., O cinturão sul do cobre na província mineral de carajás: Reconstituição do paleossistema hidrotermal associado aos depósitos de óxido de ferro-cobre-ouro (2011) Contribuições À Metalogenia Do Brasil, 1, pp. 41-70. , Frantz J.C., Marques J.C., Jost H. (Org.)Porto Alegre, UFRGS/Instituto de GeociênciasMora, C.I., Valley, J.W., Halogen-rich scapolite and biotite: Implications for metamorphic fluid-rock interactions (1989) American Mineralogist, 74, pp. 721-737Moreto, C.P.N., (2013) U-Pb and Re-os Geochronology Applied to the Metallogenetic Evolution of the Southern Copper Belt of the Carajás Province, p. 216. , PhD Thesis, Universidade Estadual de Campinas, CampinasMoreto, C.P.N., Monteiro, L.V.S., Xavier, R.P., Amaral, W.S., Santos, T.J.S., Juliani, C., Souza Filho, C.R., Mesoarchean (3.0 and 2.86 Ga) host rocks of the iron oxide-Cu-Au Bacaba deposit, Carajás Mineral Province: U-Pb geochronology and metallogenetic implications (2011) Mineralium Deposita, 46, pp. 789-811Mougeot, R., Respaut, J.P., Briqueu, L., Ledru, P., Milesi, J.P., Macambira, M.J.B., Huhn, S.B., Geochronological constrains for the age of the Águas Claras Formation (Carajás province, Pará, Brazil) (1996) SBG, Congresso Brasileiro de Geologia, 6 (39), pp. 579-581. , Anais, SalvadorNiiranen, T., (2005) Iron Oxide-copper-gold Deposits in Finland: Case Studies from the Peräpohja Schist Belt and the Central Lapland Greenstone Belt, , PhD Thesis, University of HelsinkiNogueira, A.C.R., (1995) Análise Faciológica e Aspectos Estruturais da Formação Águas Claras, Região Central da Serra Dos Carajás, Pará, p. 167. , Dissertação de Mestrado UFPA, BelémOliver, N.H.S., Wall, V.J., Cartwright, I., Internal control of fluid compositions in amphibolite-facies scapolitic calcsilicates, Mary Kathleen, Australia (1992) Mineralium Deposita, 111, pp. 94-112Pestilho, A.L.S., (2011) Sistemática de Isótopos Estáveis Aplicada À Caracterização da Evolução Dos Paleo-sistemas Hidrotermais Associados Aos Depósitos Cupríferos Alvo Bacaba e Alvo Castanha, , Província Mineral de Carajás, PA. MS Dissertation, Universidade Estadual de Campinas, CampinasPidgeon, R.T., Macambira, M.J.B., Lafon, J.-M., Th-U-Pb isotopic systems and internal structures of complex zircons from an enderbite from the Pium Complex, Carajas Province, Brazil: Evidence for the ages of granulite facies metamorphism and the protolith of the enderbite (2000) Chemical Geology, 166 (1-2), pp. 159-171. , DOI 10.1016/S0009-2541(99)00190-4, PII S0009254199001904Pinheiro, R.V.L., (2013) Carajás, Brazil - A Short Tectonic Review. In: Simpósio de Geologia da Amazônia, , Belém, Anais [CD-ROM]Reed, M.H., Hydrothermal alteration and its relationship to ore fluid composition (1997) Geochemistry of Hydrothermal Ore Deposits, pp. 303-365. , In:Barnes H.L. (ed.) Wiley, New YorkRequia, K., Stein, H., Fontbote, L., Chiaradia, M., Re-Os and Pb-Pb geochronology of the Archean Salobo iron oxide copper-gold deposit, Carajaás mineral province, northern Brazil (2003) Mineralium Deposita, 38 (6), pp. 727-738. , DOI 10.1007/s00126-003-0364-1Ribeiro, A.A., (2008) Litogeoquímica e Geologia Isotópica Estável (C, S, O) Do Depósito Cupro-aurífero Do Alvo Cristalino Sul, Província Mineral de Carajás, Pará, p. 127. , MS Dissertation, Universidade Federal de Ouro Preto, Ouro PretoRigon, J.C., Munaro, P., Santos, L.A., Nascimento, J.A.S., Barreira, C.F., Alvo 118 copper-gold deposit: Geology and mineralization, Serra dos Carajás, Pará, Brazil (2000) International Geological Congress, 31. , Rio de Janeiro [CD-ROM]Rimstidt, J.D., Gangue mineral transport and deposition (1997) Geochemistry of Hydrothermal Ore Deposits, pp. 435-487. , In:Barnes H.L. (ed.) Wiley, 3rd ed., New YorkSantos, J.O.S., Hartmann, L.A., Gaudette, H.E., Groves, D.I., McNaughton, N.J., Fletcher, I.R., A new understanding of the Amazon Craton Provinces based on integration of field mapping and U-Pb and Sm-Nd geochronology (2000) Gondwana Research, 3, pp. 453-488Sardinha, A.S., Barros, C.E.M., Krymsky, M., Geology, geochemistry and U-Pb geochronology of the Archean (2.74 Ga) Serra do Rabo granite stocks, Carajás metalogenetic province, northern Brazil (2006) Journal of South America Earth Sciences, 20, pp. 327-339Seward, T.M., Barnes, H.L., Metal transportation by hydrothermal fluids (1997) Geochemistry of Hydrothermal Ore Deposits, pp. 435-486. , In:Barnes H.L. (ed.) Wiley, 3rd ed, New YorkSilva, M.G., Teixeira, J.B.G., Pimentel, M.M., Vasconcelos, P.M., Arielo, A., Rocha, W.J.S.F., Geologia e mineralizações de Fe-Cu-Au do Alvo GT46 (Igarapé Cinzento, Carajás) (2005) Caracterização de Depósitos Minerais em Distritos Mineiros da Amazônia., pp. 94-151. , In:Marini O.J., Queiroz E.T., Ramos B.W. (eds.) DNPM-CT-Mineral-FINEP- ADIMBSmith, M.P., Storey, C.D., Jeffries, T.E., Ryan, C., In situ U-Pb and trace element analysis of accessory minerals in the Kiruna district, Norrbotten, Sweden: New constraints on the timing and origin of mineralization (2009) Journal of Petrology, 50, pp. 2063-2094Sousa, F.D.S., (2007) Estudo da Alteração Hidrotermal, Com Ênfase No Metassomatismo Sódico, de Rochas Granitóides e Máficas da Região de Canaã de Carajás, Província Mineral de Carajás, , MS Dissertation, Universidade Federal do ParáSouza, S.R.B., Macambira, M.J.B., Sheller, T., Novos dados geocronológicos para os granitos deformados do Rio Itacaiúnas (Serra dos Carajás, PA): Implicações estratigráficas (1996) Simpósio de Geologia da Amazônia, 5, pp. 380-383. , Belém, ProceedingsStringham, B., Fields of formation of some common hydrothermal-alteration minerals (1952) Economic Geology, 47, pp. 661-664Tallarico, F.H.B., (2003) O Cinturão Cupro-aurífero de Carajás, Brasil, p. 229. , PhD Thesis, Universidade Estadual de CampinasTallarico, F.H.B., Figueiredo, B.R., Groves, D.I., Kositcin, N., McNaughton, N.J., Fletcher, I.R., Rego, J.L., Geology and SHRIMP U-Pb geochronology of the Igarapé Bahia deposit, Carajás copper-gold belt, Brazil: An Archean (2.57 Ga) example of Iron-Oxide Cu-Au-(U-REE) mineralization (2005) Economic Geology, 100 (1), pp. 7-28Tassinari, C.C.G., (1996) O Mapa Geocronológico Do Cráton Amazônico No Brasil: Revisão Dos Dados Isotópicos, p. 139. , Tese de Livre Docência, Universidade de São PauloTassinari, C.C.G., Macambira, M.J.B., Geochronological provinces of the Amazonian Craton (1999) Episodes, 22, pp. 174-182Teixeira, W., Tassinari, C.C.G., Cordani, U.G., Kawashita, K., A review of the geochronological of the Amazonian Craton: Tectonic implications (1989) Precambriam Research, 42, pp. 213-227Torresi, I., Bortholoto, D.F.A., Xavier, R.P., Monteiro, L.V.S., Hydrothermal alteration, fluid inclusions and stable isotope systematics of the Alvo 118 iron oxide-copper-gold deposit, Carajás mineral province (Brazil): Implications for ore genesis (2012) Mineralium Deposita, 47, pp. 299-323(2009) Report on the Bacuri Prospect, , http://saladeimprensa.vale.com/en/release/interna.asp?id=22000, Available from: Internal report (Unpublished). VALE 2012. Internal Report on the Bacuri. Vale obtains operation license for Salobo.,VALE,Cited 2013 February 4Vanko, D.A., Bishop, F.C., Occurrence and origin of marialitic scapolite in the Humboldtt Lopolith, N.W. Nevada (1982) Contributions to Mineral Petrology, 81, pp. 277-289Vasquez, L.V., Rosa-Costa, L.R., Silva, C.G., Ricci, P.F., Barbosa, J.O., Klein, E.L., Lopes, E.S., Silva, H.R., (2008) Geologia e Recursos Minerais Do Estado Do Pará: Sistema de Informações Geográficas - SIG: Texto Explicativo Dos Mapas Geológico e Tectônico e de Recursos Minerais Do Estado Do Pará, , 1:1.000.000. CPRM, BelémVillas, R.N., Santos, M.D., Gold deposits of the Carajás mineral province: Deposit types and metallogenesis (2001) Mineralium Deposita, 36, pp. 300-331Villas, R.N., Lima, L.F.O., Neves, M.P., Sousa, F.D.S., Lamarão, C.N., Fanton, J., Morais, R., Relações entre deformação, alteraç ão hidrotermal e mineralização no depósito Cu- u do Sossego, Província Mineral de Carajás (2005) Simpósio Brasileiro de Metalogenia, 1. , Gramado, RS. Short papers[CD-ROM]Williams, P.J., Barton, M.B., Johnson, D.A., Fontboté, L., Haller, A., Mark, G., Oliver, N.H., Marschik, R., Iron oxide copper- gold deposits: Geology, space-time distribution, and possible modes of origin (2005) Economic Geology, pp. 371-405. , 100th anniversary volumeWirth, K.R., Gibbs, A.K., Olszewski, Jr.W.J., U-Pb ages of zircons from the Grão Pará Group and Serra dos Carajás granite, Pará, Brasil (1986) Revista Brasileira de Geociências, 16, pp. 195-212+200Xavier, R.P., Monteiro, L.V.S., Moreto, C.P.N., Pestilho, A.L.S., Melo, G.H.C., Silva, M.A.D., Aires, B., Silva, F.H.F., The iron oxide copper-gold systems of the Carajás mineral province, Brazil (2012) Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard Sillitoe, , Special Publication, Society of Economic GeologistsXavier, R.P., Wiedenbeck, M., Trumbull, R.B., Dreher, A.M., Monteiro, L.V.S., Rhede, D., Araújo, C.E.G., Torresi, I., Tourmaline B-isotopes fingerprint marine evaporites as the source of high-salinity ore fluids in iron oxide-copper- gold deposits, Carajás mineral province (Brazil) (2008) Geology, 36 (9), pp. 743-746O depósito de cobre Bacuri está localizado a cerca de 9 km a leste da Mina Sossego, que representa um depósito de óxido de ferro-cobre-ouro de nível mundial da Província de Carajás. O depósito Bacuri é hospedado pelo Granito Serra Dourada de 2,84 Ga, pelo Pórfiro Bacuri, além de gabros, ao longo da Zona de Cisalhamento Canaã, regional, dúctil e de direção WNW-ESE. Essa zona regional caracteriza-se por alteração hidrotermal com albita, escapolita-magnetita e feldspato potássico, que evoluíram para zonas com biotita-escapolita-magnetita. A mineralização cuprífera foi controlada e simultânea ao desenvolvimento de uma falha transcorrente subsidiária de direção NE-SW, na qual alteração clorítica e silicificação se sobrepuseram aos estágios hidrotermais prévios. O minério cuprífero (calcopirita-pirita-magnetita) é disseminado e relacionado a veios e brechas. Calcopirita orienta-se ao longo da foliação milonítica na zona de alteração clorítica e preenche fraturas em veios brechados com quartzo-(muscovita-fluorita). Adicionalmente, é reconhecida em veios de quartzo tardios, indeformados, e em vênulas com feldspato potássico-epidoto, que apresentam texturas de preenchimento de espaços abertos. A evolução temporal do depósito Bacuri foi acompanhada por mudanças significativas nos parâmetros físico-químicos do sistema hidrotermal. Contribuição de fluidos metalíferos, hipersalinos e quentes foi importante nos estágios iniciais de alteração sódica e potássica desenvolvidos em níveis crustais mais profundos. Influxo de fluidos diluídos pode ter favorecido a deposição de minério devido à diminuição da temperatura e salinidade, que foi acompanhada por diminuição de pH e aumento de ƒO2. Tais mudanças podem indicar um sistema hidrotermal híbrido que evoluiu em múltiplos pulsos durante sua progressiva exumaçã

Evidence for a new shallow magma intrusion at La Soufrière of Guadeloupe (Lesser Antilles). Insights from long-term geochemical monitoring of halogen- rich hydrothermal fluids

International audienceMore than three decades of geochemical monitoring of hot springs and fumaroles of La Soufrière of Guadeloupe allows the construction of a working model of the shallow hydrothermal system. This system is delimited by the nested caldera structures inherited from the repeated flank collapse events and the present dome built during the last magmatic eruption (1530 AD) and which has been highly fractured by the subsequent phreatic or phreatomagmatic eruptions. Because it is confined into the low volume, highly compartmented and partially sealed upper edifice structure, the hydrothermal system is highly reactive to perturbations in the volcanic activity (input of deep magmatic fluids), the edifice structure (sealing and fracturing) and meteorology (wet tropical regime). The current unrest, which began with a mild reactivation of fumarolic activity in 1990, increased markedly in 1992 with seismic swarms and an increase of degassing from the summit of the dome. In 1997 seismic activity increased further and was accompanied by a sudden high-flux HCl-rich gas from summit fumaroles. We focus on the interpretation of the time-series of the chemistry and temperature of fumarolic gases and hot springs as well as the relative behaviours of halogens (F, Cl, Br and I). This extensive geochemical time-series shows that the deep magmatic fluids have undergone large changes in composition due to condensation and chemical interaction with shallow groundwater (scrubbing). It is possible to trace back these processes and the potential contribution of a deep magmatic source using a limited set of geochemical time series: T, CO2 and total S content in fumaroles, T and Cl- in hot springs and the relative fractionations between F, Cl, Br and I in both fluids. Coupling 35 years of geochemical data with meteorological rainfall data and models of ion transport in the hydrothermal aquifers has allowed us to identify a series of magmatic gas pulses into the hydrothermal system since the 1976-1977 crisis. The contrasting behaviours of S- and Cl- bearing species in fumarolic gas and in thermal springs suggests that the current activity is the result of a new magma intrusion which was progressively emplaced at shallow depth since ~1992. Although it might still be evolving, the characteristics of this new intrusion indicate that it hasalready reached a magnitude similar to the intrusion that was emplaced during the 1976-1977 eruptive crisis. The assessment of potential hazards associated with evolution of the current unrest must consider the implications of recurrent intrusion and further pressurization of the hydrothermal system on the likelihood of renewed phreatic explosive activity. Moreover, the role of hydrothermal pressurization on the basal friction along low-strength layers within the upper part of the edifice must be evaluated with regards to partial flank collapse. At this stage enhanced monitoring, research, and data analysis is required to quantify the uncertainties related to future scenarios of renewed eruptive activity and magmatic evolution

The Mercedes Au-Ag District, Sonora, Mexico: Geology, Geochemistry and Structure of a Sierra Madre Low-sulfidation Epithermal System

The Mercedes district contains a series of Oligocene-aged low-sulfidation quartz-carbonate veins with economic Au and Ag mineralization. The veins are hosted in dextral-normal faults contained within andesitic flows, flow breccias and lahar deposits within the Sierra Madre Occidental volcanic sequence. These are localized within two structural basins adjacent to a northwest-trending anticline that exposes pre-mineral tuffs and sedimentary units. The vein mineralogy is characterized by a unique ferruginous green quartz and Mn-oxide bearing calcite assemblage along with both primary and supergene Au-Ag mineralization. Three vein systems within the district were studied: the Mercedes, Klondike and Lupita/Diluvio systems. The Mercedes vein contains the highest grades within the district, is highly-brecciated and displays weak and anomalous geochemical zonation patterns attributed to a high degree of hydrothermal and tectonic brecciation and permeability controlled supergene remobilization. The vein possesses a 4-phase paragenesis. Phase I was the major mineralizing phase, introducing native Au as well as unidentified silver minerals (likely both electrum and silver sulfosalts) along with green quartz and pyrite. Phase II began during a hydrothermal/tectonic brecciation event and appears as a dark Mn-oxide bearing calcite and rhodochrosite cement between phase I breccias. Phase III was a later quartz and calcite veinlet event with local amounts of dark carbonate. Phase IV was the final event, and is a post-mineral oxidation event creating hematite after pyrite, limonite, zeolites and cerargyrite. Statistical evaluation of the geochemical data reveals that Au and Ag grades are poorly correlated with one another, due to post-mineral reworking of the silver mineralogy. Au is most strongly correlated with Pb, Cu, Hg, Zn and Se in that order. Ag is somewhat correlated with Se and Hg, with only weak (<0.5) correlations with other elements. Ca is positively correlated with only C and Mn, supporting the fact that the dark carbonate phase is a Mn-oxide/calcite intergrowth. This is interpretation is further supported by XRF analysis of the carbonates and the presence of boxworks of pyrolusite and other Mn-oxides in former carbonate sites leached by acidic groundwaters. The Klondike vein displays a similar mineralogy and paragenesis to the Mercedes vein, but retains a higher degree of structural order that results in a less-permeable system displaying a stronger and more apparent classical geochemical zonation. Geochemical correlations display a similar pattern to Mercedes with Pb, Cu and Hg being the most correlative elemental indicators of Au grade. Ag correlations with these elements are somewhat higher than in Mercedes. The Lupita/Diluvio system is hosted within a listric structure and consists of the fault-hosted Lupita vein and the overlying Diluvio stockwork, hosted within a gravitationally-displaced block of a quartz-lithic tuff that had been structurally prepared for mineralization during its displacement along the Lupita structure. The Lupita/Diluvio system displays a similar mineralogy and paragenesis to the Mercedes and Klondike vein systems. From these data, a number of inferences can be made. First is that the high degree of brecciation and transport within the Mercedes vein has destroyed the classical epithermal geochemical zonation that is somewhat present in the Klondike system and better defined in the Lupita/Diluvio system. This "washing-out" was accomplished through both physical transport and overprinting during multiple hydrothermal pulses. Second, supergene remobilization related to meteoric fluids has altered the grades and distribution of precious metals as well as further modified the geochemical zonation. The presence of cerargyrite and mineralogical evidence suggests that these processes may be related to the poor Ag recoveries (~30%) and erratic Au/Ag ratios within the vein systems. Third, it is apparent that the structural weaknesses that localized the mineralization on the property reflect the regional structural fabric of both dextral and extensional tectonics

Hydrothermal history of the Long Valley Caldera, California: Life after collapse

Thesis (Ph.D.) University of Alaska Fairbanks, 1995Drilling of the Long Valley Exploratory Well (LVEW) on the resurgent dome in the 760 ka Long Valley Caldera opened a window to view the geologic history of the central caldera. Stratigraphic relationships indicate piston/cylinder (Valles-type) collapse for this caldera, and a resurgent structure intimately linked to post-caldera-collapse rhyolitic intrusions. Samples recovered from this and other wells proximal to the resurgent dome were characterized through isotope microanalytical techniques, petrographic and microprobe study, and analysis of fluid inclusions within alteration minerals. This work revealed the complexity of primary magmatic and secondary hydrothermal activity involved in the formation of a resurgent dome. Measurements of the \rm\delta\sp{18}O composition of silicate components forming the intracaldera lithologies display disequilibrium within samples as a result of variable exchange with hydrothermal fluids. A maximum calculated temperature of 350\sp\circ\rm C at 1800 m depth in LVEW indicates paleohydrothermal temperatures exceeded the known present-day hydrothermal conditions by more than 100\sp\circ\rm C. Contouring of \rm\delta\sp{18}O values from wells on a line crossing the caldera define a pattern of convective flow with upwelling beneath the resurgent dome. Although surface volcanism at the LVEW site ended about 650 ka, laser probe \rm\sp{40}Ar/\sp{39}Ar microanalysis of samples from sill-like intrusions into the intracaldera ignimbrite reveals intrusive events at ${\sim}650$ ka, ${\sim}450$ ka, and ${\sim}350$ ka. Sanidine phenocrysts from the Bishop Tuff at 1772 and 1792 m depths and whole rock samples of the Mesozoic metavolcanic basement rocks at 1957 m depth record times of disturbance by hydrothermal pulses at ${\sim}530$ ka and ${\sim}350$ ka. Repeated emplacement of intrusions into the centrally located caldera ignimbrite was a primary process of resurgence. In turn, the feeders for the intrusions and the intrusions themselves supplied heat for resurgent-dome-centered hydrothermal flow. After approximately 300 ka, all activity shut off in the central caldera only to resume at ${\sim}40$ ka in response to renewed Holocene volcanic activity in the West Moat. Geophysical evidence of recent intrusive activity beneath the resurgent dome indicates this shallow magma emplacement mechanism is not totally extinct in the central caldera. Most likely a new cycle of volcanism and hydrothermal circulation is underway as the caldera matures

Recognising cryptic alteration surrounding the Mount Isa Copper Deposits: Implications for controls on fluid flow, and mineral exploration

This study investigated cryptic alteration haloes associated with copper mineralisation at Mount Isa, Northwest Queensland, Australia. New insights from hydrothermal alteration were used to constrain aspects of hydrothermal fluid flow and fluid-rock interactions that occurred as part of the mineralising system responsible for forming this world-class orebody. This study demonstrates the utility of several different approaches to identify and map cryptic alteration haloes associated with copper mineralisation at Mount Isa. Bulk geochemical techniques, such as carbon and oxygen stable isotope analysis and four-acid digest ICP-AES/MS analysis of assay pulps represent methods that can be readily applied during exploration to gain representative information about the mineralising system. Portable X-Ray Fluorescence (pXRF) represents a tool that can be used in a more targeted approach during exploration activities. With rigorous quality assurance and control procedures, pXRF can quickly and cost-effectively collect large datasets to identify broad trends across mineralising systems while minimising issues arising from sample heterogeneity at the scale of analysis. Integration of observations from previous studies and exploration data, with newly acquired petrographic and geochemical data collected across a range of scales, identified an extensive zoned alteration system that manifests as a series of interpreted reaction fronts, which extend at least 1500 m beyond mineralisation. Copper mineralisation is contained within a zone of visible mineral alteration, hydrothermal brecciation, and veining, locally known as the ‘silica-dolomite’. The silica-dolomite is characterised by silicification and brecciation of shales, recrystallisation of dolomite, and intense ¹⁸O-depletion, from δ¹⁸O ≈ 22‰ VSMOW in the least altered rocks to δ¹⁸O ≈ 10‰ in the most altered zone. These zones are spatially associated with structural dislocations on the contact between the Eastern Creek Volcanics and overlying metasediments of the Mount Isa Group that host copper mineralisation. These dislocations are interpreted to represent the fluid input zones to the mineralising system. A cryptic halo of K- and Ca-depletion extends from the inferred fluid input zones to include the region outboard of the visible mineral alteration envelope. This element depletion is interpreted to result from chloritisation and talc formation during silicification of white mica- and dolomite-bearing shale. Beyond the region of Ca-depletion and K-depletion, a large halo of cryptic potassic alteration is identified by whole-rock geochemical analysis. Potassium responsible for this alteration is interpreted to have been remobilised from zones of silicification and K-depletion at the core of the hydrothermal system. Although the ultimate sink of calcium mobilised from this core zone of element depletion remains unclear, a spatially extensive network of ore stage quartz-dolomite-calcite-pyrite veins has been documented. The change from dolomite- to calcite-dominated vein cement within individual veins is interpreted to be driven by increases in the relative activity of calcium during the evolution of the hydrothermal fluid as it moved away from the fluid input zone. Consequently, ore-stage veins with mixed dolomite-calcite vein cement potentially represent a distal expression of silicification and decalcification within the core of the mineralising system. The alteration haloes described above, both visible and cryptic, are all contained with the broad zone of ¹⁸O-depletion, representing the most spatially extensive alteration halo to copper mineralisation at Mount Isa. Copper mineralisation and associated hydrothermal alteration at Mount Isa developed due to fluid-rock interaction between ¹⁸O-depleted, low pH, silica-rich, cupriferous hydrothermal brines and variably carbonaceous and pyritic, carbonate-rich metasediments. Fluid flow responsible for developing the zoned hydrothermal alteration system was predominantly upward-directed and focused by structurally controlled permeability pathways. This fluid flow was sufficiently slow and warm enough to allow only moderate oxygen isotope disequilibrium between rock and the infiltrating hydrothermal fluids. In hydrothermal systems where fluids are undersaturated in metals, the size of the ore deposit will be limited by the time-integrated fluid flux. Not only can large hydrothermal systems with high time-integrated fluid fluxes form large ore deposits, but they will also be associated with large alteration haloes with greater distance between reaction fronts than smaller hydrothermal systems. Consequently, the zoned alteration system presented in this study assists in vectoring towards fluid input zones. At the same time, the spatial extent and spacing between reaction fronts provide an indicator of the prospectivity of the mineralising system at relatively early stages of exploration

Seventh Annual V. M. Goldschmidt Conference

Topics considered include: Subduction of the Aseismic Cocos Ridge Displaced Magma Sources Beneath the Cordillera de Talamanca, Costa Rica; Topography of Transition Zone Discontinuities: A Measure of 'Olivine' Content and Evidence for Deep Cratonic Roots; Uranium Enrichment in Lithospheric Mantle: Case Studies from French Massif Central; Rare-Earth-Element Anomalies in the Decollement Zone of the nankai Accretionary Prism, Japan: Evidence of Fluid Flow?; Rare Earth Elements in Japanese Mudrocks: The Influence of Provenance; The Evolution of Seawater Strontium Isotopes in the Last Hundred Million Years: Reinterpretation and Consequences for Erosion and Climate Models; From Pat to Tats: The Lead Isotope Legacy in the Studies of the Continental Crust-Upper Mantle System; Geochronology of the Jack Hills Detrital Zircons by Precise Uranium-Lead Isotope-Dilution Analysis of Crystal Fragments; Iridium in the Oceans; The Helium-Heat-Lead Paradox; Control of Distribution Patterns of Heavy Metals in Ganga Plain Around Kanpur Region, India, by Fluvial Geomorphic Domains; Geochemical and Isotopic Features of Ferrar Magmatic Provience (Victoria Land, Antarctica); Rare Earth Elements in Marine Fine-Grained Sediments from the Northwestern Portuguese Shelf (Atlantic); Aspects of Arc Fluxes; General Kinetic Model for Dolomite Precipitation Rate with Application to the Secular History of Seawater Composition; High-Precision Uranium-series Chronology from Speleothems; Trace-Element Modeling of Aqueous Fluid-Peridotite Interaction in the Mantle Wedge of Subduction Zones; Rainfall Variations in Southeastern Australia over the Last 500,000 Years from Speleothem Deposition; The Role of Water in High-Pressure Fluids; The Kinetic Conditions of Metamorphic Minearogenesis: Evidence from Minerals and Assemblages

Hydrothermally altered and fractured granite as an HDR reservoir in the EPS-1 borehole, Alsace,

As part of the European Hot Dry Rocks Project, a second exploration borehole, EPS-1, has been cored to a depth of 2227 m at Soultz-sous-Forets (France). The target was a granite beginning at 1417 m depth, overlain by post-Paleozoic sedimentary cover. Structural analysis and petrographic examination of the 800-m porphyritic granite core, have shown that this rock has undergone several periods of hydrothermal alteration and fracturing. More than 3000 natural structures were recorded, whose distribution pattern shows clusters where low-density fracture zones (less than 1 per meter) alternate with zones of high fracture density (more than 20 per meter). Vein alteration, ascribed to paleohydrothermal systems, developed within the hydrothermally altered and highly fractured zones, transforming primary biotite and plagioclase into clay minerals. One of these zones at 2.2 km depth produced a hot-water outflow during coring, indicating the existence of a hydrothermal reservoir. Its permeability is provided by the fracture network and by secondary porosity of the granitic matrix resulting from vein alteration. This dual porosity in the HDR granite reservoir must be taken into account in the design of the heat exchanger, both for modeling the water-rock interactions and for hydraulic testing