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R L Korotev - One of the best experts on this subject based on the ideXlab platform.
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40ar 39ar dating of apollo 12 regolith implications for the age of copernicus and the source of nonmare materials
Geochimica et Cosmochimica Acta, 2006Co-Authors: Fernando Barra, T D Swindle, R L Korotev, B L Jolliff, R A Zeigler, E OlsonAbstract:Abstract Twenty-one 2–4 mm rock samples from the Apollo 12 regolith were analyzed by the 40 Ar/ 39 Ar geochronological technique in order to further constrain the age and source of nonmare materials at the Apollo 12 site. Among the samples analyzed are: 2 felsites, 11 KREEP Breccias, 4 mare-basalt-bearing KREEP Breccias, 2 alkali anorthosites, 1 olivine-bearing impact-melt Breccia, and 1 high-Th mare basalt. Most samples show some degree of degassing at 700–800 Ma, with minimum formation ages that range from 1.0 to 3.1 Ga. We estimate that this degassing event occurred at 782 ± 21 Ma and may have been caused by the Copernicus impact event, either by providing degassed material or by causing heating at the Apollo 12 site. 40 Ar/ 39 Ar dating of two alkali anorthosite clasts yielded ages of 3.256 ± 0.022 Ga and 3.107 ± 0.058 Ga. We interpret these ages as the crystallization age of the rock and they represent the youngest age so far determined for a lunar anorthosite. The origin of these alkali anorthosite fragments is probably related to differentiation of shallow intrusives. Later impacts could have dispersed this material by lateral mixing or vertical mixing.
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feldspathic lunar meteorites pecora escarpment 02007 and dhofar 489 contamination of the surface of the lunar highlands by post basin impacts
Geochimica et Cosmochimica Acta, 2006Co-Authors: R L Korotev, R A Zeigler, Bradley L JolliffAbstract:Abstract PCA (Pecora Escarpment) 02007 and Dhofar 489 are both meteorites from the feldspathic highlands of the Moon. PCA 02007 is a feldspathic Breccia consisting of lithified regolith from the lunar surface. It has concentrations of both incompatible and siderophile elements that are at the high end of the ranges for feldspathic lunar meteorites. Dhofar 489 is a feldspathic Breccia composed mainly of impact-melted material from an unknown depth beneath the regolith. Concentrations of incompatible and siderophile elements are the lowest among Brecciated lunar meteorites. Among 19 known feldspathic lunar meteorites, all of which presumably originate from random locations in the highlands, concentrations of incompatible elements like Sm and Th tend to increase with those of siderophile elements like Ir. Feldspathic meteorites with high concentrations of both suites of elements are usually regolith Breccias. Iridium derives mainly from micrometeorites that accumulate in the regolith with duration of surface exposure. Micrometeorites have low concentrations of incompatible elements, however, so the correlation must reflect a three-component system. We postulate that the correlation between Sm and Ir occurs because the surface of the Feldspathic Highlands Terrane has become increasingly contaminated with time in Sm-rich material from the Procellarum KREEP Terrane that has been redistributed across the lunar surface by impacts of moderate-sized, post-basin impacts. The most Sm-rich regolith Breccias among feldspathic lunar meteorites are about 3× enriched compared to the most Sm-poor Breccias, but this level of enrichment requires only a few percent Sm-rich material typical of the Procellarum KREEP Terrane. The meteorite data suggest that nowhere in the feldspathic highlands are the concentrations of K, rare earths, and Th measured by the Lunar Prospector mission at the surface representative of the underlying “bedrock;” all surfaces covered by old regolith (as opposed to fresh ejecta) are at least slightly contaminated. Dhofar 489 is one of 15 paired lunar-meteorite stones from Oman (total mass of meteorite: ⩾1037 g). On the basis of its unusually high Mg/Fe ratio, the meteorite is likely to have originated from northern feldspathic highlands.
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compositional variation in apollo 16 impact melt Breccias and inferences for the geology and bombardment history of the central highlands of the moon
Geochimica et Cosmochimica Acta, 1994Co-Authors: R L KorotevAbstract:Abstract High-precision data for the concentrations of a number of lithophile and siderophile elements were obtained on multiple subsamples from 109 impact-melt rocks and Breccias (mostly crystalline) from the Apollo 16 site. Compositions of nearly all Apollo 16 melt rocks fall on one of two trends of increasing Sm concentration with increasing Sc concentration. The Eastern trend (lower Sm Sc , Mg Fe , and Sm Yb ratios) consists of compositional groups 3 and 4 of previous classification schemes. These melt rocks are feldspathic, poor in incompatible and siderophile elements, and appear to have provenance in the Descartes formation to the east of the site. The Western trend (higher Sm Sc , Mg Fe , and Sm Yb ratios) consists of compositional groups 1 and 2. These relatively mafic, KREEP-bearing Breccias are a major component (~35%) of the Cayley plains west of the site and are unusual, compared to otherwise similar melt Breccias from other sites, in having high concentrations of Fe-Ni metal (1–2%). The metal is the carrier of the low-Ir/Au (~0.3 × chondritic) siderophile-element signature that is characteristic of the Apollo 16 site. Four compositionally distinct groups (1M, 1F, 2DB, and 2NR) of Western-trend melt Breccias occur that are each represented by at least six samples. Compositional group 1 of previous classification schemes (the “poikilitic” or “LKFM” melt Breccias) can be subdivided into two groups. Group 1M (represented by six samples, including 60315) is characterized by lower Al2O3 concentrations, higher MgO and alkali concentrations, and higher Mg Fe and Cr Sc ratios than group 1F (represented by fifteen samples, including 65015). Group 1M also has siderophile-element concentrations averaging about twice those of group 1F and Ir Au and Ir Ni ratios that are even lower than those of other Western-trend melt rocks ( Ir Au = 0.24 ± 0.03 , CI-normalized). At the mafic extreme of group 2 (“VHA” melt Breccias), the melt lithology occurring as clasts in feldspathic fragmental Breccias from North Ray crater (group 2NR) is compositionally distinct from the melt lithology of dimict Breccias from the Cayley plains (group 2DB) in having higher concentrations of Sc, Cr, and heavy rare earth elements and lower concentrations of siderophile elements. The distinct siderophile-element signature (high absolute abundances, low Ir Au ratio) suggest that the four groups of mafic melt Breccia are all somehow related. Ratios of some lithophile elements also suggest that they are more closely related to each other than they are to melt Breccias from other Apollo sites. However, none of the Breccia compositions can be related to any of the others by any simple process of igneous fractionation or mixing involving common lunar materials. Thus, the origin of the four groups of mafic melt Breccia is enigmatic. If they were produced in only one or two impacts, then a mechanism exists for generating regimes of impact-melt Breccia in a single impact that are substantially different from each other in composition. For various reasons, including the problem of delivering large volumes of four different types of melt to the Apollo 16 site, it is unlikely that any of these Breccias were produced in basin-forming impacts. If they were produced in as many as four crater-forming impacts, then the unusual siderophile-element signature is difficult to explain. Possible explanations are 1. (1) the four groups of melt Breccia all contain metal from a single, earlier impact, 2. (2) they were each formed by related metal-rich meteoroids, or 3. (3) some common postimpact process has resulted in metal of similar composition in each of four melt pools. Within a compositional group, most intrasample and intersample variation in lithophile element concentrations is caused by differences among samples in the proportion of a component of normative anorthosite or noritic anorthosite. In most cases, this compositional variation probably reflects variation in clast abundance. For group 2DB (and probably 2NR), differences in abundance of a component of ferroan anorthosite (estimated Al2O3 ≈ 32%) accounts for the compositional variation. For groups 1M and 1F, the anorthositic component is more mafic (estimated Al2O3 ≈ 26%). Some group-2 samples may be related by a troctolitic component of varying abundance.
Karine Bernard - One of the best experts on this subject based on the ideXlab platform.
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epithermal clast coating inside the rock avalanche debris flow deposits from mount meager volcanic complex british columbia canada
Journal of Volcanology and Geothermal Research, 2020Co-Authors: Karine BernardAbstract:Abstract The observational and semi-quantitative sedimentological analyses of the lithofacies assemblage contribute to describe at different scales the Breccia matrix along the sheared and transformed contact between rock avalanche deposits and trailing debris flow deposits emplaced by a large landslide at Mt. Meager volcano in British Columbia in 2010. An inverted cataclastic gradient of crushed Breccias in a fluidized and mixed matrix implies a structurally controlled flow regime and rapid deposition. The coating matrix changed the initial polymodal distribution of the debris-flow lithofacies. Clast shape evolution helps to characterize the cataclastic sorting during transport and fluidized disaggregation. A plot of matrix percent against matrix/gravels helps to distinguish primary hot fracturing of about 61% from initial dilation and sheared fracturing involving ~22% matrix. Extensional disaggregation between 73 and 79% matrix is related to hydromagmatic fragmentation within an epithermal system. Clayey mineral assemblages identified by XRD patterns are related to colloidal aluminium gel, cataclastic shear bands, and quartz microstructures in epithermal Breccia zones (pH = 2–3, 200–350 °C, Microstructural analysis differentiates the inner rim of coated clasts from their border and the surrounding matrix in impact melt Breccias. Sequential coating stages are inferred during the propagation of the shock wave with an oscillatory relative speed during the inter-seismic period. We differentiate: 1) shock faulting which contributes to the impacted quartz (10–35 GPa) in a devitrified matrix and pseudotachylite coating related to frictional melting at the margin of a conduit; 2) shock response (85 GPa) in epithermal vein with calcic spheroids, CO2 dissociation, and basaltic melt (70–101 GPa, >1500 °C); and 3) the secondary fracturing with flash heating and pressure pulse during cavitation (ΔP ~ 10 GPa, >1000–1500 °C), which generates pockets of partial melting, quartz spheroids, and a roll-over effect for the inner rim of coated clasts. The formation of impact melt Breccias and the debris flow are related to the slowing elastic impact wave with an oscillatory relative speed during the inter-seismic period along a proximal strike-slip fault. This study helps identify how the proximal rock avalanche transformed into a highly mobile debris flow, larger examples of which pose a hazard to the town of Pemberton, at a distance of 65 km from Mt. Meager.
Roland Maas - One of the best experts on this subject based on the ideXlab platform.
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The Productora Cu-Au-Mo Deposit, Chile: A Mesozoic Magmatic-Hydrothermal Breccia Complex with Both Porphyry and Iron Oxide Cu-Au Affinities
Society of Economic Geologists, 2020Co-Authors: Angela Escolme, Roland Maas, David R Cooke, Julie Hunt, Ron F. Berry, Robert A. CreaserAbstract:Abstract The Productora Cu-Au-Mo deposit is hosted by a Cretaceous hydrothermal Breccia complex in the Coastal Cordillera of northern Chile. The current resource, which includes the neighboring Alice Cu-Mo porphyry deposit, is estimated at 236.6 Mt grading 0.48% Cu, 0.10 g/t Au, and 135 ppm Mo. Local wall rocks consist of a thick sequence of broadly coeval rhyolite to rhyodacite lapilli tuffs (128.7 ± 1.3 Ma; U-Pbzircon) and two major intrusions: the Cachiyuyito tonalite and Ruta Cinco granodiorite batholith (92.0 ± 1.0 Ma; U-Pbzircon). Previous studies at Productora concluded the deposit had strong affinities with the iron oxide copper-gold (IOCG) clan and likened the deposit to Candelaria. Based on new information, we document the deposit geology in detail and propose a new genetic model and alternative classification as a magmatic-hydrothermal Breccia complex with closer affinities to porphyry systems. Hydrothermal and tectonic Breccias, veins, and alteration assemblages at Productora define five paragenetic stages: stage 1 quartz-pyrite–cemented Breccias associated with muscovite alteration, stage 2 chaotic matrix-supported tectonic-hydrothermal Breccia with kaolinite-muscovite-pyrite alteration, stage 3 tourmaline-pyrite-chalcopyrite ± magnetite ± biotite-cemented Breccias and associated K-feldspar ± albite alteration, stage 4 chalcopyrite ± pyrite ± muscovite, illite, epidote, and chlorite veins, and stage 5 calcite veins. The Productora hydrothermal system crosscuts earlier-formed sodic-calcic alteration and magnetite-apatite mineralization associated with the Cachiyuyito stock. Main-stage mineralization at Productora was associated with formation of the stage 3 hydrothermal Breccia. Chalcopyrite is the dominant hypogene Cu mineral and occurs predominantly as Breccia cement and synBreccia veins with pyrite. The Alice Cu-Mo porphyry deposit is characterized by disseminated chalcopyrite and quartz-pyrite-chalcopyrite ± molybdenite vein stockworks hosted by a granodiorite porphyry stock. Alice is spatially associated with the Silica Ridge lithocap, which is characterized by massive, fine-grained, quartz-altered rock above domains of alunite, pyrophyllite, and dickite. Rhenium-Os dating of molybdenite indicates that main-stage mineralization at Productora occurred at 130.1 ± 0.6 Ma, and at 124.1 ± 0.6 Ma in the Alice porphyry. Chalcopyrite and pyrite from Productora have δ34Ssulfide values from –8.5 to +2.2‰, consistent with a magmatic sulfur source and fluids evolving under oxidizing conditions. No significant input from evaporite- or seawater-sourced fluids was detected. Stage 3 tourmalines have average initial Sr of 0.70397, consistent with an igneous-derived Sr source. The Productora magmatic-hydrothermal Breccia complex formed as a result of explosive volatile fluid release from a hydrous intrusive complex. Metal-bearing fluids were of magmatic affinity and evolved under oxidizing conditions. Despite sharing many similarities with the Andean IOCG clan (strong structural control, regional sodic-calcic alteration, locally anomalous U), fluid evolution at the Productora Cu-Au-Mo deposit is more consistent with that of a porphyry-related magmatic hydrothermal Breccia (sulfur-rich, acid alteration assemblages and relatively low magnetite contents, <5 vol %). The Productora camp is an excellent example of the close spatial association of Mesozoic magnetite-apatite, porphyry, and magmatic-hydrothermal Breccia mineralization styles, a relationship seen throughout the Coastal Cordillera of northern Chile.
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oxide sulphide and carbonate minerals in a mantle polymict Breccia metasomatism by proto kimberlite magmas and relationship to the kimberlite megacrystic suite
Chemical Geology, 2013Co-Authors: Andrea Giuliani, Vadim S Kamenetsky, Mark A Kendrick, David Phillips, B A Wyatt, Roland MaasAbstract:Abstract Polymict Breccias entrained by kimberlites are mantle xenoliths comprising coarse-grained mantle minerals (porphyroclasts) and rock clasts, cemented together by ilmenite, rutile, phlogopite, olivine and minor sulphides. These unusual xenoliths are generally considered to result from ascending primitive/precursor kimberlite magmas that crystallised in the magma conduit at lithospheric mantle depths. To enhance our understanding of these processes, we investigated the oxide, sulphide and carbonate minerals of a new polymict Breccia (DU-1) from the Bultfontein Dumps (Kimberley, South Africa). Xenolith DU-1 contains heterogeneous domains of ilmenite-rich Breccia surrounded by spinel harzburgite wall rock. The Breccia domains also host resorbed Cr-diopside porphyroclasts and occasional Fe–Ni–Co sulphides segregations. Ilmenite occurs as large (up to 5 cm), chemically zoned lenses, associated with minor rutile. The ilmenite has Hf isotope values (initial eHf = 2.1–3.0) in the range of South African Group I kimberlites and associated megacrysts. The ilmenite lenses host primary multiphase carbonate-rich, olivine-dominated, phlogopite-dominated and sulphide mineral inclusions. Carbonate-rich inclusions host abundant magnesite and dolomite, with subordinate kalsilite, phlogopite, alkali-carbonates, phosphates and chlorides. The occurrence of carbonate-rich inclusions suggests entrapment of a Ca–Mg-rich alkali-carbonate melt during ilmenite growth. However, geochemical modelling indicates that this melt was not parental to the ilmenite. Instead, it is suggested that the ilmenite (and other cementing phases) crystalised from ilmenite and the other cementing phases crystallised from a S-bearing Ti–Fe–K-rich ultramafic silicate melt, derived from an ascending proto-kimberlite melt, which was modified subsequently by wall rock assimilation and/or magma mixing, porphyroclast dissolution and crystal fractionation. The alkali-carbonate melt could be the residual liquid after these processes occurred. Alternatively, the ultramafic silicate and alkali-carbonate melts were produced by liquid immiscibility from a silicate-carbonate proto-kimberlite melt. The occurrence of ilmenite inclusions in sulphide segregations, and sulphide inclusions in ilmenite lenses is indicative of late-stage silicate-sulphide liquid immiscibility. The major and trace element concentrations, Hf isotope composition and mineral inclusion content of ilmenite are consistent with a genetic relationship between polymict Breccia formation and the kimberlite megacryst suite. We propose that polymict Breccias and megacrysts were produced at different depths by common silicate-carbonate magmas during the early stages of kimberlite magmatism.
S Kahraman - One of the best experts on this subject based on the ideXlab platform.
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the usability of the cerchar abrasivity index for the evaluation of the triaxial strength of misis fault Breccia
Bulletin of Engineering Geology and the Environment, 2015Co-Authors: S Kahraman, M Alber, Osman Gunaydin, Mustafa FenerAbstract:The development of some predictive models for the geomechanical properties of fault Breccias will be useful for geo-engineers, because the preparation of smooth specimens from the fault Breccias is usually tedious and expensive. In this study, simple and multiple regression analysis were applied to the data pertaining to Misis Fault Breccia to develop predictive models for the differential stress (Δσ) from non-destructive methods including the Cerchar abrasivity index (CAI). As a result of simple regression analysis, strong relations between Δσ and indirect properties were not found. The CAI was included in the best multiple regression model for the prediction of Δσ. The significance of derived models was statistically tested. It was concluded that the CAI is a useful property for the prediction of Δσ of Misis Fault Breccia.
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the usability of cerchar abrasivity index for the prediction of ucs and e of misis fault Breccia regression and artificial neural networks analysis
Expert Systems With Applications, 2010Co-Authors: S Kahraman, M Alber, Mustafa Fener, Osman GunaydinAbstract:The derivation of some predictive models for the geomechanical properties of fault Breccias will be useful due to the fact that the preparation of smooth specimens from the fault Breccias is usually difficult and expensive. To develop some predictive models for the uniaxial compressive strength (UCS) and elastic modulus (E) from the indirect methods including the Cerchar abrasivity index (CAI), regression and artificial neural networks (ANNs) analysis were applied on the data pertaining to Misis Fault Breccia. The CAI was included to the best regression model for the prediction of UCS. However, the CAI was not included to the best regression model for the prediction of E. The developed ANNs model was also compared with the regression model. It was concluded that the CAI is a useful property for the prediction of UCS of Misis Fault Breccia. Another conclusion is that ANNs model is more reliable than the regression models.
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evaluating the strength and deformability properties of misis fault Breccia using artificial neural networks
Expert Systems With Applications, 2009Co-Authors: S Kahraman, M Alber, Osman Gunaydin, Mustafa FenerAbstract:Since the preparation of smooth specimens from the fault Breccias are usually difficult and expensive, the development of some predictive models for the geomechanical properties of fault Breccias will be useful. In this study, artificial neural networks (ANNs) analysis was applied on the data pertaining to Misis fault Breccia to develop some predictive models for the uniaxial compressive strength (UCS) and elastic modulus (E) from the indirect methods. The developed ANNs models were also compared with the regression models. As a result of ANNs analysis, very good models were derived for both UCS and E estimation. It was shown that ANNs models were more reliable than the regression models. Concluding remark is that UCS and E values of Misis fault Breccia can reliably be estimated from the indirect methods using ANNs analysis.
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predicting the uniaxial compressive strength and elastic modulus of a fault Breccia from texture coefficient
Rock Mechanics and Rock Engineering, 2009Co-Authors: M Alber, S KahramanAbstract:Fault rocks can be divided into six types: fault cataclastic rocks, fault block rocks, unindurated fault Breccias, fault gouge, fault Breccias and mylonites. These fault rocks always have very poor engineering properties and fall within the category of weak or soft rocks. Weak rocks usually cause problems, not only in construction works, but also in slopes, or in underground works. For this reason, knowing the mechanical and elastic properties of fault Breccia is very important in rock engineering. However, no published material on the geomechanical properties of fault Breccias was encountered. Several researchers (Chester and Logan, 1986; Lindquist and Goodman, 1994; Medley, 1994, 2001, 2002; Medley and Goodman, 1994; Ehrbar and Pfenniger, 1999; Goodman and Ahlgren, 2000; Burgi et al., 2001; Habimana et al., 2002; Laws et al., 2003; Sonmez et al., 2004, 2006) have investigated the properties of complex geo-materials such as melanges, sheared serpentinites, coarse pyroclastic rocks and fault rocks. However, none of them has investigated the properties of fault
David R Cooke - One of the best experts on this subject based on the ideXlab platform.
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The Productora Cu-Au-Mo Deposit, Chile: A Mesozoic Magmatic-Hydrothermal Breccia Complex with Both Porphyry and Iron Oxide Cu-Au Affinities
Society of Economic Geologists, 2020Co-Authors: Angela Escolme, Roland Maas, David R Cooke, Julie Hunt, Ron F. Berry, Robert A. CreaserAbstract:Abstract The Productora Cu-Au-Mo deposit is hosted by a Cretaceous hydrothermal Breccia complex in the Coastal Cordillera of northern Chile. The current resource, which includes the neighboring Alice Cu-Mo porphyry deposit, is estimated at 236.6 Mt grading 0.48% Cu, 0.10 g/t Au, and 135 ppm Mo. Local wall rocks consist of a thick sequence of broadly coeval rhyolite to rhyodacite lapilli tuffs (128.7 ± 1.3 Ma; U-Pbzircon) and two major intrusions: the Cachiyuyito tonalite and Ruta Cinco granodiorite batholith (92.0 ± 1.0 Ma; U-Pbzircon). Previous studies at Productora concluded the deposit had strong affinities with the iron oxide copper-gold (IOCG) clan and likened the deposit to Candelaria. Based on new information, we document the deposit geology in detail and propose a new genetic model and alternative classification as a magmatic-hydrothermal Breccia complex with closer affinities to porphyry systems. Hydrothermal and tectonic Breccias, veins, and alteration assemblages at Productora define five paragenetic stages: stage 1 quartz-pyrite–cemented Breccias associated with muscovite alteration, stage 2 chaotic matrix-supported tectonic-hydrothermal Breccia with kaolinite-muscovite-pyrite alteration, stage 3 tourmaline-pyrite-chalcopyrite ± magnetite ± biotite-cemented Breccias and associated K-feldspar ± albite alteration, stage 4 chalcopyrite ± pyrite ± muscovite, illite, epidote, and chlorite veins, and stage 5 calcite veins. The Productora hydrothermal system crosscuts earlier-formed sodic-calcic alteration and magnetite-apatite mineralization associated with the Cachiyuyito stock. Main-stage mineralization at Productora was associated with formation of the stage 3 hydrothermal Breccia. Chalcopyrite is the dominant hypogene Cu mineral and occurs predominantly as Breccia cement and synBreccia veins with pyrite. The Alice Cu-Mo porphyry deposit is characterized by disseminated chalcopyrite and quartz-pyrite-chalcopyrite ± molybdenite vein stockworks hosted by a granodiorite porphyry stock. Alice is spatially associated with the Silica Ridge lithocap, which is characterized by massive, fine-grained, quartz-altered rock above domains of alunite, pyrophyllite, and dickite. Rhenium-Os dating of molybdenite indicates that main-stage mineralization at Productora occurred at 130.1 ± 0.6 Ma, and at 124.1 ± 0.6 Ma in the Alice porphyry. Chalcopyrite and pyrite from Productora have δ34Ssulfide values from –8.5 to +2.2‰, consistent with a magmatic sulfur source and fluids evolving under oxidizing conditions. No significant input from evaporite- or seawater-sourced fluids was detected. Stage 3 tourmalines have average initial Sr of 0.70397, consistent with an igneous-derived Sr source. The Productora magmatic-hydrothermal Breccia complex formed as a result of explosive volatile fluid release from a hydrous intrusive complex. Metal-bearing fluids were of magmatic affinity and evolved under oxidizing conditions. Despite sharing many similarities with the Andean IOCG clan (strong structural control, regional sodic-calcic alteration, locally anomalous U), fluid evolution at the Productora Cu-Au-Mo deposit is more consistent with that of a porphyry-related magmatic hydrothermal Breccia (sulfur-rich, acid alteration assemblages and relatively low magnetite contents, <5 vol %). The Productora camp is an excellent example of the close spatial association of Mesozoic magnetite-apatite, porphyry, and magmatic-hydrothermal Breccia mineralization styles, a relationship seen throughout the Coastal Cordillera of northern Chile.
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bismoclite biocl in the san francisco de los andes bi cu au deposit argentina first occurrence of a bismuth oxychloride in a magmatic hydrothermal Breccia pipe and its usefulness as an indicator phase in mineral exploration
Minerals, 2016Co-Authors: F J Testa, David R Cooke, L Zhang, Graciela R MasAbstract:The rare bismuth oxychloride, bismoclite (BiOCl), has been identified in the weathered tourmaline–cemented, magmatic–hydrothermal Breccia complex at the San Francisco de los Andes Bi–Cu–Au deposit, Argentina. A wide variety of supergene minerals were detected in the oxidized zone, but only preisingerite (Bi3(AsO4)2O(OH)) is intimately associated with bismoclite. Bismuth arsenate is present either as minor accessory phases or as traces in bismoclite-rich samples. This is the first documented occurrence of bismoclite in a porphyry-related, and magmatic–hydrothermal Breccia pipe deposit. Bismoclite is interpreted to have formed by weathering of hypogene bismuthinite (Bi2S3), which originally occurred with arsenopyrite to cement the Breccias. These appear to have reacted with O2- and HCl-bearing meteoric waters to produce pockets of supergene bismoclite–preisingerite assemblages. Bismoclite samples have been characterized by means of X-ray diffractometry (XRD), geochemistry, petrography, scanning electron microscopy (SEM), differential thermal analysis–thermogravimetry analysis (DTA–TGA) and infrared analysis (IR) providing useful insights and updated information regarding this rare bismuth oxychloride and associated arsenate mineral. The San Francisco de los Andes Breccia complex shows similar geometry, morphology and internal organization as those found in traditional magmatic–hydrothermal Breccias associated with Cu–Mo porphyry deposits. Bismoclite and preisingerite form due to the presence of hypogene Bi-bearing minerals followed by appropriate supergene conditions. These hypogene minerals commonly occur only as trace phases, or are entirely absent, in porphyry and related magmatic–hydrothermal Breccia deposits. The scarcity of hypogene Bi–mineral phases in porphyry and related magmatic–hydrothermal Breccia deposits is the main reason why bismoclite has not previously been reported in these types of deposits. The detection of bismoclite as a mineral phase in the oxidized zone of weathered deposits highlights hypogene Bi mineralization at depth, and associated metals. Bismoclite is an insoluble mineral of particular interest in those supergene profiles which have been completely leached out of distinctive, water-soluble phases, such as Cu sulfate minerals, which are diagnostic of Cu mineralization at depth. Consequently, bismoclite could potentially be the only indicator of hypogene and supergene mineralization in lower portions of a bismuth bearing ore deposit.
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hydrothermal Breccias and veins at the kelian gold mine kalimantan indonesia genesis of a large epithermal gold deposit
Economic Geology, 2008Co-Authors: Andrew G S Davies, David R Cooke, Bruce J Gemmell, Theo Van Leeuwen, Pat Cesare, Greg HartshornAbstract:Mineralized hydrothermal Breccias and veins formed during and after the waning stages of maar-diatreme-related volcanic activity at Kelian, Kalimantan, Indonesia. Subsurface phreatic explosions occurred around the margins of the diatreme Breccia complex, focusing high-temperature fluid flow and generating several large, mineralized hydrothermal Breccia bodies. Tectonic, phreatomagmatic, and hydraulic processes also contributed to hydrothermal Brecciation. Explosive phreatic Brecciation was followed by in situ hydraulic Brecciation, and then by minor veining as the system returned to steady-state geothermal conditions. Brecciation and mineralization mostly occurred 400 to 700 m below the paleowater table. The Kelian deposit contained more than 240 t Au prior to mining. Precious metals occur with sulfide minerals as disseminations, in sheeted and conjugate veins and as Breccia cement. There is a progression from pyrite-dominated (stage 1) to base metal sulfide-dominated (stages 2 and 3) to sulfosalt-dominated hydrothermal Breccias and veins (stage 4). In terms of gangue minerals, the system evolved from illite-quartz to adularia- and/or quartz- and/or illite-dominated and then carbonate-dominated. Boiling produced abundant bladed carbonate during stages 3 and 4. Overall, carbonate and base metal sulfide minerals are abundant, whereas quartz is comparatively minor. Free gold is most abundant in stages 3 and 4 but also occurs in stages 1 and 2. Native gold occurs principally as inclusions in pyrite, sphalerite, galena, arsenopyrite, quartz, bladed carbonate, and sulfosalts. Hydrothermal alteration assemblages are zoned about contacts, faults, Breccias and veins, and consist of secondary quartz, illite, pyrite, chlorite, and various carbonate minerals. Kelian preserves evidence of a magmatic component to the mineralizing fluids. Fluid inclusion analyses revealed the presence of saline fluid inclusions, in particular from stage 3. Isothermal mixing of low-salinity (∼0 to 2 wt % NaCl equiv) with moderate-salinity fluids (10 to 25 wt % NaCl equiv), rather than boiling, resulted in the range of salinity and homogenization temperatures of fluid inclusions in sphalerite, carbonate and quartz deposited during stage 3. Salinities of ∼4 to 6 wt percent NaCl equiv for inclusions in adularia and quartz from stages 2 and 4 and in rhodochrosite and proustite-pyrargyrite from stage 4 may also reflect a component of mixing of low-salinity water with a moderately saline fluid. Sulfur isotope values from Kelian sulfides (δ 34S = -1.4 to 5.5 per mil) are consistent with a magmatic sulfur source. Both boiling and fluid mixing contributed to high-grade Au mineralization at Kelian. Rapid formation (hundreds of years or less) is inferred for the individual hydrothermal Breccia bodies. © 2008 Society of Economic Geologists, Inc.