Chlorite Zone Research Articles

Multispectral and Radar Data for the Setting of Gold Mineralization in the South Eastern Desert, Egypt

Satellite-based multi-sensor data coupled with field and microscopic investigations are used to unravel the setting and controls of gold mineralization in the Wadi Beitan–Wadi Rahaba area in the South Eastern Desert of Egypt. The satellite-based multispectral and Synthetic Aperture Radar (SAR) data promoted a vibrant litho-tectonic understanding and abetted in assessing the regional structural control of the scattered gold occurrences in the study area. The herein detailed approach includes band rationing, principal component and independent component analyses, directional filtering, and automated and semi-automated lineament extraction techniques to Landsat 8- Operational Land Imager (OLI), Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), Phased Array L-band Synthetic Aperture Radar (PALSAR), and Sentinel-1B data. Results of optical and SAR data processed as grayscale raster images of band ratios, Relative Absorption Band Depth (RBD), and (mafic–carbonate–hydrous) mineralogical indices are used to extract the representative pixels (regions of interest). The extracted pixels are then converted to vector shape files and are finally imported into the ArcMap environment. Similarly, manually and automatically extracted lineaments are merged with the band ratios and mineralogical indices vector layers. The data fusion approach used herein reveals no particular spatial association between gold occurrences and certain lithological units, but shows a preferential distribution of gold–quartz veins in zones of chlorite–epidote alteration overlapping with high-density intersections of lineaments. Structural features including en-echelon arrays of quartz veins and intense recrystallization and sub-grain development textures are consistent with vein formation and gold deposition syn-kinematic with the host shear zones. The mineralized, central-shear quartz veins, and the associated strong stretching lineation affirm vein formation amid stress build-up and stress relaxation of an enduring oblique convergence (assigned as Najd-related sinistral transpression; ~640–610 Ma). As the main outcome of this research, we present a priority map with zones defined as high potential targets for undiscovered gold resources.

Timescale of material circulation in subduction zone: U–Pb zircon and K–Ar phengite double‐dating of the Sanbagawa metamorphic complex in the Ikeda district, central Shikoku, southwest Japan

AbstractWe have estimated the timescale of material circulation in the Sanbagawa subduction zone based on U–Pb zircon and K–Ar phengite dating in the Ikeda district, central Shikoku. The Minawa and Koboke units are major constituents of the high‐P Sanbagawa metamorphic complex in Shikoku, southwest Japan. For the Minawa unit, ages of 92–81 Ma for the trench‐fill sediments, are indicated, whereas the age of ductile deformation and metamorphism of garnet and chlorite zones are 74–72 Ma and 65 Ma, respectively. Our results and occurrence of c. 150 Ma Besshi‐type deposits formed at mid‐ocean ridge suggest that the 60‐Myr‐old Izanagi Plate was subducted beneath the Eurasian Plate at c. 90 Ma, and this observation is consistent with recent plate reconstructions. For the Koboke unit, the depositional ages of the trench‐fill sediments and the dates for the termination of ductile deformation and metamorphism are estimated at c. 76–74 and 64–62 Ma, respectively. In the Ikeda district, the depositional ages generally become younger towards lower structural levels in the Sanbagawa metamorphic complex. Our results of U–Pb and K–Ar dating show that the circulation of material from the deposition of the Minawa and Koboke units at the trench through an active high‐P metamorphic domain to the final exhumation from the domain occurred continuously throughout c. 30 Myr (from c. 90 to 60 Ma).

The evolution of quartz veins during the tectonometamorphic development of the Brusque Metamorphic Complex, Brazil

Microstructural analysis and characterization of quartz veins hosted in rocks from the Brusque Metamorphic Complex in the region of Brusque, State of Santa Catarina, have been performed to establish the relations between fluid regimes, tectonic styles and deformation/recrystallization mechanisms. The analysis was based on observations of structural overprinting, spatial structural relations, the origin of the quartz veins and microtectonics studies. Five types of veins were identified: meter-long veins parallel to the regional foliation (V1-veins); massive, meterwide veins present in thermal aureoles (V2-veins); millimeter-wide, erratic veins, also present in thermal aureoles (V3-veins); tabular, undeformed and NW striking (V4-veins); millimeter-wide erratic veins restricted to brittle reactivation of strike-slip shear zones (V5-veins). The regional foliation, developed under garnet zone metamorphic conditions, was more effective in vein production when compared to the steeply dipping mylonitic foliation corridors developed under chlorite zone conditions. Pressure solution was the main deformation mechanism during the regional foliation development. However, granoblastic and decussate textures in hornfels reveal an influence of grain boundary area reduction mechanism. The granoblastic texture in hornfels would have inhibited fluid circulation during dehydration reactions, increasing fluid pressure and promoting massive (V2-veins) and erratic hydraulic fracture-related (V3) veins. In dextral NEtrending strike-slip shear zones, tabular quartz veins (V4-veins) are parallel to tension gashes. Reactivation of strike-slip shear zones under brittle conditions has produced local brecciated millimeter-wide quartz veins (V5-veins). This study underscores the important role of fluids during orogenic evolution near the brittle-plastic transition of the crust, and demonstrates how combined vein and microtextural analysis can reveal the tectonometamorphic history of lowgrade metamorphic rocks.

High-pressure medium-temperature metamorphism of semi-pelitic rocks in the Scotia Metamorphic Complex, Powell Island, South Orkney Islands, Antarctica

The Antarctic continent constituted the southwestern margin of Gondwana until its break-up in the early Cretaceous, when new margins were created along the separating fragments of South America and Antarctica, forming the Scotia Arc. In the Jurassic, part of this passive continental margin became active with subduction of oceanic lithosphere, leading to the introduction of ocean floor material into the accretionary wedge accompanied by deformation and metamorphism. One of these margins is preserved in the South Orkney Microcontinent, and crops out at the South Orkney Islands. At Powell Island, situated in the center of the South Orkney Islands, a gradual transition from very low-grade metarenite, interlayered with metasiltite and slate of the Greywacke Shale Formation, in the south, to biotite-garnet schist, in the north, belonging to the Scotia Metamorphic Complex, is present. The metamorphic map presents from south to north a pumpellyite muscovite chlorite zone, a garnet zone, a biotite-garnet zone and an abundant biotite-garnet zone. Thermobarometric calculations yielded for the garnet and garnet-biotite zones temperatures between 498 and 517 °C with pressures of 9–11 kbar and for the abundant biotite-garnet zone temperatures between 522 and 550 °C and pressures between 11.8 and 13 kbar. These results align well with earlier obtained data for the lower grade rocks and confirm the idea of metamorphism in an accretionary wedge. The relatively high-pressure is interpreted to be responsible for the inversion of the biotite and garnet isograds, for the albitic composition of plagioclase and for the relatively Ca-rich garnet. P-T conditions fall in the transitional field between greenschist, amphibolite, blueschist and eclogite facies.

Erosion of the Southern Alps of New Zealand during the last deglaciation

Research Article| October 08, 2018 Erosion of the Southern Alps of New Zealand during the last deglaciation Ruohong Jiao; Ruohong Jiao 1Institute of Earth Surface Dynamics, University of Lausanne, 1015 Lausanne, Switzerland2Department of Geological and Environmental Sciences, Ben-Gurion University of the Negev, Beer-Sheva 8410501, Israel3Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany Search for other works by this author on: GSW Google Scholar Frédéric Herman; Frédéric Herman 1Institute of Earth Surface Dynamics, University of Lausanne, 1015 Lausanne, Switzerland Search for other works by this author on: GSW Google Scholar Olivier Beyssac; Olivier Beyssac 4Institut de Minéralogie, de Physique des Matériaux, et de Cosmochimie, UMR CNRS 7590, Sorbonne Universités–Université Pierre et Marie Curie, 75005 Paris, France Search for other works by this author on: GSW Google Scholar Thierry Adatte; Thierry Adatte 5Institute of Earth Sciences, University of Lausanne, 1015 Lausanne, Switzerland Search for other works by this author on: GSW Google Scholar Simon C. Cox; Simon C. Cox 6GNS Science, Private Bag 1930, Dunedin 9054, New Zealand Search for other works by this author on: GSW Google Scholar Faye E. Nelson; Faye E. Nelson 7Department of Marine Science, University of Otago, P.O. Box 56, Dunedin 9054, New Zealand Search for other works by this author on: GSW Google Scholar Helen L. Neil Helen L. Neil 8National Institute of Water and Atmospheric Research, Private Bag 14901, Wellington 6021, New Zealand Search for other works by this author on: GSW Google Scholar Author and Article Information Ruohong Jiao 1Institute of Earth Surface Dynamics, University of Lausanne, 1015 Lausanne, Switzerland2Department of Geological and Environmental Sciences, Ben-Gurion University of the Negev, Beer-Sheva 8410501, Israel3Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany Frédéric Herman 1Institute of Earth Surface Dynamics, University of Lausanne, 1015 Lausanne, Switzerland Olivier Beyssac 4Institut de Minéralogie, de Physique des Matériaux, et de Cosmochimie, UMR CNRS 7590, Sorbonne Universités–Université Pierre et Marie Curie, 75005 Paris, France Thierry Adatte 5Institute of Earth Sciences, University of Lausanne, 1015 Lausanne, Switzerland Simon C. Cox 6GNS Science, Private Bag 1930, Dunedin 9054, New Zealand Faye E. Nelson 7Department of Marine Science, University of Otago, P.O. Box 56, Dunedin 9054, New Zealand Helen L. Neil 8National Institute of Water and Atmospheric Research, Private Bag 14901, Wellington 6021, New Zealand Publisher: Geological Society of America Received: 17 May 2018 Revision Received: 02 Aug 2018 Accepted: 06 Sep 2018 First Online: 08 Oct 2018 Online Issn: 1943-2682 Print Issn: 0091-7613 © 2018 Geological Society of America Geology (2018) 46 (11): 975–978. https://doi.org/10.1130/G45160.1 Article history Received: 17 May 2018 Revision Received: 02 Aug 2018 Accepted: 06 Sep 2018 First Online: 08 Oct 2018 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Ruohong Jiao, Frédéric Herman, Olivier Beyssac, Thierry Adatte, Simon C. Cox, Faye E. Nelson, Helen L. Neil; Erosion of the Southern Alps of New Zealand during the last deglaciation. Geology 2018;; 46 (11): 975–978. doi: https://doi.org/10.1130/G45160.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract During the Quaternary, periodic glaciations transformed mountain landscapes. However, characterizing the way in which mountain erosion changes between glacier- and river-dominated conditions has been elusive. Here, using samples from an offshore sedimentary core, we estimated the spatial distribution of erosion in the southern part of the Southern Alps of New Zealand during a full transition from the Last Glacial Maximum (LGM), ca. 20 ka, to the last millennium. Raman spectroscopy analyses of carbonaceous material revealed a marked change in the sediment provenance, which we interpreted to reflect the evolving erosion pattern of the mountain range. Over the Holocene, since at least ca. 9 ka, erosion was focused on the chlorite zone schist within the upper reaches of the valleys (>15–20 km distance from the mountain front), possibly dominated by large-magnitude landslides. During the last glaciation, the proportion of sediments from the biotite schist and higher-grade metamorphic rocks in the lower-lying areas closer to the mountain front (<15–20 km) was relatively higher, probably as a result of glacier carving. Our results suggest that glacier retreat during the last deglaciation caused an upstream localization of the high erosion rates, which is consistent with the snowline records in the Southern Alps and regional and global climate histories. 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40Ar/39Ar chronological constraints on syn- and post-Variscan biotite porphyroblasts from the Iberian Chains, NE Spain

We perform a multidisciplinary study of biotite porphyroblasts and veinlet infills hosted in Cambrian strata of the hanging walls from the NW-SE-trending Datos, Jarque and Daroca thrusts (Iberian Chains). Stratigraphic and microstructural crosscutting features indicate that a biotite isograd runs parallel to the southeastern transects of the three thrusts. The metamorphic grade reached in the southeastern edge of the Iberian Chains is clearly distinct from both the Cadomian epizonal metamorphism, exclusively recorded in the Ediacaran–basal Cambrian Paracuellos Group, and the Variscan anchizone metamorphism recorded throughout Cambrian–Devonian strata. During post–Variscan negative inversion tectonics, renewed K-metasomatism along the same thrusts led to crystallisation of geochemically similar biotites in parallel fissures and veins. 40Ar/39Ar ages are (i) latest Westphalian–to–Guadalupian (285.82 ± 23.75 Ma) for metamorphic biotite porphyroblasts affected by diffusion loss of Ar, including the Carboniferous–Permian transition within error; and (ii) Early Triassic (246.87 ± 5.36 Ma) for biotite occlusion in post–Variscan veinlets. The Iberian Chains represent the southeastern prolongation of the Cantabrian and West Asturian-Leonese zones. The latter displays a uniform eastward decrease in metamorphic grade, whereas the Iberian Chains exhibits a heterogeneous distribution of low-grade metamorphic conditions (chlorite zone). The local presence of the biotite isograd is linked to the higher tectonic gradients associated with the Daroca, Jarque and Datos thrusts, quite similar to the metamorphic isogrades recognized both in the northeastern edge of the Demanda Massif and the Novellana-Pola de Allande-Degana Belt of the West Asturian-Leonese Zone. This distribution allows the identification of a broad NW-SE belt of biotite-in isograd-related synkinematic metamorphism, following the western contact of the Narcea Antiform, the Anguiano Thrust and the southeastern edges of the Daroca, Jarque and Datos Thrusts in the Iberian Chains. Based on Variscan structural style and metamorphic grades, the Datos Thrust links with the contact that separates the Cantabrian and West Asturian-Leonese zones of the Iberian Massif.

Degassing of organic carbon during regional metamorphism of pelites, Wepawaug Schist, Connecticut, USA

A comprehensive understanding of the degassing systematics of organic carbon (OC) during regional metamorphism is necessary to evaluate the role that metamorphism plays in the global carbon cycle. In this study, weight percentages and δ13C values of OC were measured in 70 samples of metapelites from the Wepawaug Schist, Connecticut, where classic Barrovian metamorphism occurred and graphitic OC is widespread. Relative to low-grade chlorite + biotite zone rocks, our mass balance analysis shows that OC in the metapelites underwent progressive loss from −14% (−0.06 g OC per 100 g rock) in the garnet zone, through −21% (−0.09 g/100 g) in the staurolite zone, to −26% (−0.11 g/100 g) in the kyanite zone. The average δ13C values in different metamorphic zones (ranging from −14.74‰ to −16.24‰) are all much higher than normal organic material in marine sediments, and increase slightly from the chlorite + biotite zone to the garnet zone and decrease slightly at higher metamorphic grades. Organic carbon degassing in the form of CH4 during the late stage of diagenesis or in the earliest stages of metamorphism could produce this significant 13C enrichment. Under the assumption that the 13C enrichment is caused by graphite degassing during the lowest-grade metamorphism (chlorite zone or lower), the degassing profile of OC during the regional metamorphism is reconstructed by combining the δ13C and OC mass change data. The computed results indicate that graphitic OC in the Wepawaug Schist probably underwent considerable loss at lowest-grade metamorphic conditions, ranging from ~−40% to ~−90% (or from −0.23 g OC per 100 g rock to −2.8 g OC per 100 g rock), and remained relatively inert at higher grades. Based on the mass balance analysis, δ13C systematics, and exploratory modeling results, this study argues that the lowest-grade or pre-metamorphic stages would be the more efficient OC liberators, and that the degassing potential of OC in the major stages of Barrovian metamorphism appears to be much more restricted. Additional independent studies are required to decipher the early degassing of OC after the deposition of organic matter, which could in turn help better constrain the degassing of OC during regional metamorphism.

Prograde Barrovian metamorphism along Darjeeling–Tista transect, Eastern Himalaya, India: constraints from textural relationship, phase equilibria and geothermobarometry

The Darjeeling–Tista transect, located in Lesser Himalaya, is a part of the North‐Eastern Himalaya. Along the transect, polyphase deformation and prograde Barrovian metamorphism have been delineated. The time relations between the phases of deformations (D1, D2 and D3) and metamorphic crystallization reveal a single major prograde metamorphic event that initiated with the D1 deformation and finally outlasted it. The earlier phase of this metamorphism is essentially regional syntectonic low grade, which may be designated (M1a). This was followed by regional static metamorphism (M1b) in the post‐tectonic phase between the D1 and D2 deformations. This M1 metamorphism is superposed by later retrogressive metamorphism (M2) during the D2 and D3 deformations. The different parageneses of pelitic rocks containing chlorite, muscovite, biotite, garnet, staurolite, kyanite, sillimanite, K‐feldspar and plagioclase show various textures that resulted from the continuous and discontinuous reactions in the different zones. The metamorphic zones and isograds delineated on the basis of specific metamorphic reactions, namely reaction isograd or isoreaction grad, and by critical mineral assemblages are as follows: (a) chlorite–biotite zone, (b) garnet–chlorite zone, (c) staurolite–biotite–chlorite zone, (d) kyanite–biotite–staurolite zone and (e) sillimanite–biotite–staurolite zone. Chemographic relations and inferred reactions for different zones are portrayed in the AKF and AFM projections. A sequence of metamorphic reactions at different isograds has been deduced through textural relations. The prograde P‐T evolution of the study area has been constrained through the use of internally consistent winTWQ programme and Perple_X software in the MnNCKFMASHTO model system. The study has the potential to instigate future researches focusing on Himalayan metamorphic evolutionary trends using modern approaches. Copyright © 2017 John Wiley &amp; Sons, Ltd.

Ore deposits and epithermal evidences associated with intra-magmatic faults at Aïn El Araâr-Oued Belif ring structure (NW of Tunisia)

Hydrothermal ore deposits at Ain El Araâr-Oued Belif location are classified as epithermal deposits type. The ore bodies are hosted by upper Turonian (8–9 M.y) volcanic rhyodacitic complex. Polymetallic sulfide orebodies are mainly concentrated within intra-magmatic faults. Petrographic, XRD, and TEM–STEM investigations revealed that ore minerals are essentially, arsenopyrite, pyrite, chalcopyrite, pyrrhotite, hematite, goethite and magnetite with Au, Ag and Pt trace metals. Gangue minerals are mainly adularia, quartz, sericite, alunite, tridymite, chlorite, phlogopite and smectite. Epithermal alteration is well zoned with four successive characteristic zones: (1) zone of quartz–adularia–sericite and rare alunite; (2) zone of kaolinite and plagioclase albitization; (3) intermediate zone of illite–sericite; (4) sapropelic alteration type zone of chlorite–smectite and rare illite. This can be interpreted as a telescoping of two different acidity epithermal phases; low sulfidation (adularia–sericite) and high sulfidation (quartz–alunite), separated in time or due to a gradual increase of fluids acidity and oxicity within the same mineralization phase. Brecciated macroscopic facies with fragments hosting quartz–adularia–sericite minerals (low-sulfidation phase) without alunite, support the last hypothesis. Geodynamic context and mineral alteration patterns are closely similar to those of Maria Josefa gold mine at SE of Spain which exhibit a volcanic-hosted epithermal ore deposit in a similar vein system, within rhyolitic ignimbrites, altered to an argillic assemblage (illite–sericite abundant and subordinate kaolinite) that grades outwards into propylitic alteration (Sanger-von Oepen et al. (1990)). Mineralogical and lithologic study undertaken in the volcanic host rock at Ain El Araâr-Oued Belif reveals a typical epithermal low-sulfidation and high-sulfidation ore deposits with dominance of low-sulfidation. Host rocks in these systems range from silicic to intermediate for adularia–sericite type (low sulfidation) to rhyodacite for quartz–alunite type (high sulfidation).

A two-stage fluid history for the Orocopia Schist and associated rocks related to flat subduction and exhumation, southeastern California

ABSTRACTStable isotopes combined with pre-existing 40Ar/39Ar thermochronology at the Gavilan Hills and Orocopia Mountains in southeastern California record two stages of fluid–rock interaction: (1) Stage 1 is related to prograde metamorphism as Orocopia Schist was accreted to the base of the crust during late Cretaceous–early Cenozoic Laramide flat subduction. (2) Stage 2 affected the Orocopia Schist and is related to middle Cenozoic exhumation along detachment faults. There is no local evidence that schist-derived fluids infiltrated structurally overlying continental rocks. Mineral δ18O values from Orocopia Schist in the lower plate of the Chocolate Mountains fault and Gatuna normal fault in the Gavilan Hills are in equilibrium at 490–580°C with metamorphic water (δ18O = 7–11‰). Phengite and biotite δD values from the Orocopia Schist and upper plate suggest metamorphic fluids (δD ~ –40‰). In contrast, final exhumation of the schist along the Orocopia Mountains detachment fault (OMDF) in the Orocopia Mountains was associated with alteration of prograde biotite and amphibole to chlorite (T ~ 350–400°C) and the influx of meteoric-hydrothermal fluids at 24–20 Ma. Phengites from a thin mylonite zone at the top of the Orocopia Schist and alteration chlorites have the lowest fluid δD values, suggesting that these faults were an enhanced zone of meteoric fluid (δD < –70‰) circulation. Variable δD values in Orocopia Schist from structurally lower chlorite and biotite zones indicate a lesser degree of interaction with meteoric-hydrothermal fluids. High fluid δ18O values (6–12‰) indicate low water–rock ratios for the OMDF. A steep thermal gradient developed across the OMDF at the onset of middle Cenozoic slip likely drove a more vigorous hydrothermal system within the Orocopia Mountains relative to the equivalent age Gatuna fault in the Gavilan Hills.

Hyperspectral interpretation of selected drill cores from orogenic gold deposits in central Victoria, Australia

ABSTRACTHyLogger hyperspectral data obtained from seven orogenic gold deposits in central Victoria, including Bendigo, Ballarat, Maldon, Fosterville, Costerfield, Castlemaine and Wildwood, are presented. The data demonstrate that fresh diamond drill core displays substantial mineralogical variation that can be attributed to the effects of cryptic hydrothermal alteration that might not otherwise be recognised. The most significant hyperspectral response lies in the white mica compositions, which vary in a systematic manner between high-Al muscovitic zones (Al–OH absorption around 2208 nm) that define a phyllic alteration halo around mineralised structures, and low-Al phengitic–chlorite zones (Al–OH absorption >2014 nm) inferred to represent either more distal alteration or possibly regional metamorphic background. An extensive ferroan dolomite alteration halo overlaps the phyllic and sulfidic alteration zones and extends beyond the sampled core in most instances. This ferroan dolomite halo has previously been defined petrographically, geochemically and using carbonate staining techniques, and is further characterised using thermal infrared hyperspectral data in drill core from the Ballarat goldfield. The mineralogical trends identified by the hyperspectral data are best developed in diamond drill core from the Costerfield, Fosterville and Ballarat goldfields, and are less pronounced at the other deposits. At Bendigo and Castlemaine the reasons for this are not immediately clear, but may be related to the close timing of gold mineralisation relative to peak metamorphism. The Maldon area lies within the contact aureole of the Harcourt Batholith and so has been thermally overprinted leading to the recrystallisation of earlier hydrothermal assemblages. The Wildwood deposit is similar to the Magdala deposit at Stawell and differs from the other goldfields in its geological setting, host rock lithologies and style of hydrothermal alteration, with the development of Fe-rich chlorite closely associated with gold mineralisation. The results demonstrate how hyperspectral data can be used to define large hydrothermal alteration footprints associated with orogenic gold mineralisation in central Victoria that are of direct benefit to mineral explorers, as well as independently characterising lithological variations in drill core.

Hydrothermal Alteration and Mineralization of the Randu Kuning Porphyry Cu-Au and Intermediate Sulphidation Epithermal Au-Base Metals Deposits in Selogiri, Central Java, Indonesia

The Randu Kuning Porphyry Cu-Au prospect area is situated in the Selogiri district, Wonogiri regency, Central Java, Indonesia, about 40 km to the South-East from Solo city, or approximately 70 km east of Yogyakarta city. The Randu Kuning area and its vicinity is a part of the East Java Southern Mountain Zone, mostly occupied by both plutonic and volcanic igneous rocks, volcaniclastic, silisiclastic and carbonate rocks. Magmatism-volcanism products were indicated by the abundant of igneous and volcaniclastic rocks of Mandalika and Semilir Formation. The Alteration zones distribution are generally controlled by the NE–SW and NW–SE trending structures. At least eight types of hydrothermal alteration at the Randu Kuning area and its vicinity had been identified, i.e. magnetite + biotite ± K-feldspar ± chlorite (potassic), chlorite + sericite + magnetite ± actinolite, chlorite + magnetite ± actinolite ± carbonate (inner propylitic), chlorite + epidote ± carbonate (outer propylitic), sericite + quartz + pyrite (phyllic), illite + kaolinite ± smectite (intermediate argillic), illite + kaolinite ± pyrophyllite ± alunite (advanced argillic) and quatz + chlorite (sillisic) zones. The Randu Kuning mineralization at Selogiri is co existing with the porphyry Cu-Au and intermediate sulphidation epithermal Au-base metals. Mineralization in the porphyry environment is mostly associated with the present of quartz-sulphides veins including AB, C, carbonate-sulphides veins (D vein) as well as disseminated sulphides. While in the epithermal prospect, mineralization is particularly associated with pyrite + sphalerite + chalcopyrite + carbonate ± galena veins as well as hydrothermal breccias. The Randu Kuning porphyry prospect has copper gold grade in range at about 0.66–5.7 gr/t Au and 0.04–1.24 % Cu, whereas in the intermediate sulphidation epithermal contain around 0.1–20.8 gr/t Au, 1.2–28.1 gr/t Ag, 0.05–0.9 % Zn, 0.14–0.59 % Pb and 0.01–0.65 % Cu.

Phyllosilicates geochemistry and distribution in the Altar porphyry Cu-(Au) deposit, Andes Cordillera of San Juan, Argentina: Applications in exploration, geothermometry, and geometallurgy

Biotite, chlorite, muscovite, illite, and kaolinite from the Altar porphyry Cu-(Au) deposit of the Andean Main Cordillera of San Juan Province (Argentina) were constrained using X-ray diffraction, electron microprobe, and infrared spectroscopy analyses to map compositional variations.Magmatic and hydrothermal biotites from the andesite-dacite mineralized porphyries have higher XMg, K, and F contents and lower Fe/(Fe+Mg) ratios compared to the magmatic biotites from the andesite-dacite barren porphyries of the district Hydrothermal biotites from deep levels with potassic alteration and high Cu grades have the highest XMg ratios and high F contents. The similarity of the log fH2O/fHF, log fHF/fHCl, and log fH2O/fHCl fugacity ratios of biotites from Altar mineralized porphyries and from the neighbouring Los Pelambres porphyry copper deposit suggests that these parameters may be a function of the magmatic source. Chlorite crystals associated with Cu mineralization (0.2 to 1.2% Cu) show lower Fe and Mn and higher Mg contents than chlorite from shallow and distal zones. Potassic dioctahedral phyllosilicates are the most abundant phyllosilicates in the Altar deposit, occur in the phyllic and chloritic zones, and are superimposed on potassic alteration. In zones of high copper grades (>0.8% Cu), potassic dioctahedral phyllosilicates have total Al (apfu) between 2.4 and 2.8 and intermediate compositions between muscovite, phengitic muscovite, and illite, whereas those with higher and lower Al contents come from zones with lower Cu grades.Temperatures obtained from XMg-Ti equilibria in biotite (691–800°C) and IVAl occupancy in chlorite (214–340°C), agree with previous temperature estimates based on Ti in quartz and fluid inclusion microthermometry. Muscovite is stable at temperatures higher than ~300°C, whereas phengitic muscovite indicates temperatures between 280 and 400°C and higher K+/H+ conditions (less acidic environment) compared to muscovite. Illite represents a younger and cooler (220 to 310°C) hydrothermal alteration event, and kaolinite in late veins halos reflects a decrease of the temperature (<200°C) of late hydrothermal fluids.Our study demonstrates that variations in phyllosilicate composition have the potential to be used as vectors in ore exploration and to differentiate between barren and fertile intrusions. A detailed analysis of type and proportion of phyllosilicates, as well as the presence of ore minerals in fine fractions, should be undertaken to optimize metal recoveries during the upcoming benefaction of these ores.

Two-stage fluid flow and element transfers in shear zones during collision burial-exhumation cycle: Insights from the Mont Blanc Crystalline Massif (Western Alps)

The Mont-Blanc Massif was intensely deformed during the Alpine orogenesis: in a first stage of prograde underthrusting at c. 30 Ma and in a second stage of uplift and exhumation at 22-11 Ma. Mid-crustal shear zones of 1 mm–50m size, neighbouring episyenites (quartz-dissolved altered granite) and alpine veins, have localised intense fluid flow, which produced substantial changes in mineralogy and whole-rock geochemistry. Four main metamorphic zones are oriented parallel to the strike of the massif: (i) epidote, (ii) chlorite, (iii) actinolite-muscovite±biotite and (iv) muscovite±biotite. In addition, phlogopite-bearing shear zones occur in the chlorite zone, and calcite-bearing shear zones are locally found in the muscovite zone. The initial chemical composition of the granitic protolith is relatively constant at massif scale, which allows investigating compositional changes related to shear zone activity, and subsequent volume change and elements mobility. The variations of whole-rock composition and mineral chemistry in shear zones reflect variations in fluid/rock ratios and fluid’s chemistry, which have produced specific mineral reactions. Estimated time-integrated fluid fluxes are of the order of 106m3/m2. The mineral assemblages that crystallised upon these fluid-P-T conditions are responsible for specific major and trace element enrichments. The XFe (Fe/Fe+Mg) pattern of shear zone phyllosilicates and the δ13C pattern of vein calcite both show a bell-type pattern across the massif with high values on the massif rims and low values in the centre of the massif. These low XFe and δ13C values are explained by down temperature up-flow of a Fe-Mg-CO2-rich and silica-depleted fluid during stage 1, while the massif was underthrusting. These produced phlogopite, chlorite and actinolite precipitation and quartz hydrolysis, resulting in strong volume losses. In contrast, during stage 2 (uplift), substantial volume gains occurred on the massif rims due to the precipitation of quartz, epidote and muscovite from a local fluid hosted in the Helvetic cover. These two fluids advocate for the presence of an upper-crustal scaled fluid convection cell, with up-going fluids through the lower crust and likely down-going fluids in the 15km upper crust.

The Chahnaly Low-Sulfidation Epithermal Gold Deposit, Western Makran Volcanic Arc, Southeast Iran

The Chahnaly low-sulfidation epithermal Au deposit and nearby Au prospects are located northwest of the intermittently active Bazman stratovolcano on the western end of the Makran volcanic arc, which formed as the result of subduction of the remnant Neo-Tethyan oceanic crust beneath the Lut block. The arc hosts the Siah Jangal epithermal and Kharestan porphyry prospects, near Taftan volcano, as well as the Saindak Cu-Au porphyry deposit and world-class Reko Diq Cu-Au porphyry deposit, near Koh-i-Sultan volcano to the east-northeast in Pakistan. The host rocks for the Chahnaly deposit include early Miocene andesite and andesitic volcaniclastic rocks that are intruded by younger dacitic domes. Unaltered late Miocene dacitic ignimbrites overlie these rocks. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb zircon geochronology data yield ages between 21.8 and 9.9 Ma for the acidic-intermediate regional volcanism. The most recent volcanic activity of the Bazman stratovolcano involved extrusion of an olivine basalt during Pliocene to Quaternary times. Interpretation of geochemical data indicate that the volcanic rocks are synsubduction and calc-alkaline to subalkaline. The lack of a significant negative Eu anomaly, a listric-shaped rare earth element pattern, and moderate La/Yb ratios of host suites indicate a high water content of the source magma. Gold and electrum are temporally and spatially related to a series of structurally controlled, 030°-trending, subvertical hydrothermal breccias with chalcedony-adularia that cut porphyritic andesite and andesitic volcaniclastic rocks. Gold is associated with pyrite, a siliceous matrix of hydrothermal breccia, and previously formed vein clasts, as well as with iron oxides and hydroxides in oxidized zones. Rare silver minerals include Ag-bearing electrum and naumannite, iodargyrite, an unnamed silver diiodide, and hessite. Hydrothermal alteration is generally well developed surrounding the ore-bearing hydrothermal breccia. The main types of alteration in the area include an inner ~0.5- to 20-m-thick gold-bearing hydrothermal breccia composed of quartz-chalcedonyadularia-illite-pyrite, a ~5- to 50-m-thick zone of quartz, chalcedony, pyrite, illitic phengite, phengite, illitic muscovite, illite, illitic paragonite, paragonite, muscovite, montmorillonite and, rarely, siderite, and a 30- to 70-m outer propylitic zone of Fe-Mg chlorite, calcite, ankerite, dolomite, epidote, palygorskite, and pyrite. The Chahnaly Au deposit formed during the early stages of magmatism. LA-ICP-MS zircon U-Pb geochronology of host andesite and 40Ar/39Ar dating of two samples of gold-associated adularia show that the ore-stage adularia (19.83 ± 0.10 and 19.2 ± 0.5 Ma) is younger, by as much as 1.5 million years, than the volcanic host rock (20.32 ± 0.4 Ma). Therefore, either hydrothermal activity continued well after volcanism or a second magmatic event rejuvenated hydrothermal activity. This second magmatic event may be related to eruption of porphyritic andesite at ~20.32 ± 0.40 Ma, which is within error of ~19.83 ± 0.10 Ma adularia. The new LA-ICP-MS zircon U-Pb host rock and vein adularia 40Ar/39Ar ages suggest that early Miocene magmatism and mineralization in the Bazman area is of a similar age to that of the Saindak porphyry and Tanjeel porphyry center of the giant Reko Diq deposit. This confirms the existence of early Miocene arc magmatism and mineralization along the Iranian part of the Makran volcanic arc. Ore, alteration mineralogy, and alteration patterns indicate that the Chahnaly deposit is a typical low-sulfidation epithermal Au deposit, located in a poorly explored part of the Makran volcanic arc in Iran.

Geology of metamorphic rocks deriving from paleohydrothermal systems in the Mesoproterozoic Serra do Itaberaba Group, São Paulo State, southeastern Brazil

ABSTRACTIn the central portion of the Ribeira fold belt, southeastern Brazil, the Mesoproterozoic volcanosedimentary Serra do Itaberaba Group was deposited in an ocean basin having normal mid-ocean ridge basalt type basalts that evolved to a back-arc environment. This succession was affected by two medium-grade regional metamorphic events and a third low-grade retrometamorphic event. This geological map covers an area of approximately 16 km2, between 23°16′41.824″ and 23°18′47.744″ latitudes S, and 46°20′57.056″ and 46°23′18.933″ longitudes W, as a scale of 1:5000. It encompasses the metamorphic products of tectonically deformed paleohydrothermal systems that developed in a back-arc environment, which are spatially and genetically linked to small metamorphosed andesitic-rhyolitic bodies. These systems were responsible for lixiviation of Ca and alkali in deeper parts, carbonatization in shallower parts, and a first large chloritic alteration zone (CZ1) that was crosscut by small chloritic (CZ2), silicification, and advanced argillic alteration zones. The metamorphic products of CZ1 are cummingtonite/anthophyllite rocks, whereas those related to CZ2 are Mg-rich chloritites. The metamorphic products of silicification and advanced argillic alteration zones are rocks composed of quartz ± specularite and of corundum, margarite, sericite, tourmaline, rutile, and Ca-plagioclase, respectively. Those associated with Ca and alkali depletions are garnet-hornblende amphibolites, whereas those related to carbonatization zones are composed of diopside, hornblende, tremolite/actinolite, carbonate, clinozoisite/epidote, plagioclase, and quartz. Cummingtonite/anthopyllite rocks and Mg-rich chloritites are similar to those associated with metamorphosed Kuroko-type volcanogenic massive sulfide deposits, whereas rocks composed of corundum, margarite, sericite, tourmaline, rutile, and Ca-plagioclase are genetically associated with oceanic high-sulfidation magmatic-hydrothermal gold mineralization.

The Baklia Fault Zone – a regional strike-slip zone splitting Prins Karls Foreland (Svalbard). [abstract

The Baklia Fault Zone – a regional strike-slip zone splitting Prins Karls Foreland (Svalbard). [abstract

The Effect of Dolerite Intrusions on the Hydrocarbon Potential of the Lower Permian Whitehill Formation (Karoo Supergroup) in South Africa and Southern Namibia: A Preliminary Study

Recent interest in the shale gas potential in the main Karoo Basin of South Africa has focused attention mainly on the carbonaceous Whitehill Formation (Ecca Group, Karoo Supergroup). To unravel the metamorphic effect of the Lower Jurassic Karoo igneous intrusions on the Lower Permian Whitehill Formation, outcrop samples were taken at various distances (ranging from 1 to >30 m) from the intrusive contacts of dolerite sills in the western part of the main Karoo Basin (Northern Cape, South Africa) and in the Karasburg Basin (southern Namibia). Furthermore, to establish the baseline petrological and geochemical composition of the Whitehill Formation, samples have also been collected at localities unaffected by dolerites in the southern main Karoo Basin. Mineral assemblages from all sample localities plot within the chlorite zone and show very low grade metamorphism. Mineral assemblages and illite crystallinity data indicate that the Whitehill Formation has undergone (a) anchizone metamorphism (250 to 300°C) in the southern main Karoo basin as well as in areas close to the dolerite intrusions, and (b) moderate burial diagenesis (~180°C) in areas away from the immediate effect of dolerites in the Northern Cape and southern Namibia. Regional evaluation of unconventional gas potential of the Karoo basins of southern Africa ought, therefore, to consider the subsurface thickness and spacing of Karoo intrusive bodies within the target carbonaceous formations.

Ankerite grains with dolomite cores: A diffusion chronometer for low- to medium-grade regionally metamorphosed clastic sediments

Ankerite grains with dolomite cores occur in marls, pelites, and psammites from a Buchan terrain in Maine and a Barrovian terrain in Vermont (U.S.A.). Dolomite cores are typically ≤20 μm in diameter, have sharp but irregular contacts with ankerite, and have the same crystallographic orientation as ankerite rims. Ankerite grains with dolomite cores are common in the chlorite zone, less abundant in the biotite and garnet zones, and rare (Vermont) or absent (Maine) at higher grades. The texture and crystallographic orientation of dolomite and ankerite and the sharpness of the dolomite-ankerite contact are consistent with partial replacement of detrital dolomite by ankerite by solution-reprecipitation. Metamorphic biotite is in Fe-Mg exchange equilibrium with ankerite rims but not with dolomite cores, implying that ankerite did not form long after biotite (biotite has no phlogopite cores). Possible sources of iron for the formation of ankerite are reduction of ferric iron hydroxide or the smectite-to-illite reaction during diagenesis. The sharpness of the dolomite-ankerite contact is a diffusion chronometer that constrains timescales of metamorphic process. Relatively low spatial resolution analyses of Fe/Mg across the contact with a NanoSIMS instrument and a FEG TEM give upper bounds on the thickness of the transition from ankerite to dolomite of ~2 and ~0.5 μm, respectively. Higher resolution analysis of BSE grayscale contrast with a FEG SEM gives a thickness ~100 nm. Fit of the grayscale profile to a model of one-dimensional diffusion across an infinite plane gives Dt = 10^(−15) m^2 (± a factor of 5), where D is the effective Fe-Mg interdiffusion coefficient and t is the duration of diffusion. Using the published experimental determination of D, upper bounds on the residence time of ankerite grains with dolomite cores at peak T = 400–500 °C, on the duration of linear cooling from peak T to 100 °C, and on the duration of linear heating from 100 °C to peak T followed by linear cooling to 100 °C are all 100 °C. The question is what explains the occurrence of ultrasteep composition gradients between dolomite and ankerite. Regional metamorphism on a timescale of a year or less is unrealistic. No barrier to diffusion at the dolomite-ankerite contact was observed in TEM images. Post-metamorphic formation of ankerite at very low temperature is ruled out by Fe-Mg exchange equilibrium between biotite and ankerite but not dolomite. It is unlikely that the steep composition gradients were preserved by intracrystalline pressure gradients. Alternatively, the steep composition gradients would be consistent with timescales of metamorphic process ~10^6 years or longer if D values during metamorphism were approximately six orders of magnitude or more smaller than those measured in the laboratory. The error of measurement is much less, approximately ± a factor of 2. A correction to D for the difference in P between measurements (0.1 MPa) and metamorphism (350–800 MPa) is likely an order of magnitude or less. Oxygen activity (a_(O2)), however, was 17–20 orders of magnitude larger during the laboratory measurements than during metamorphism. A correction to measured D for the difference in a_(O2_ between experiment and metamorphism appears to be the likeliest way to reconcile the steep composition gradients with realistic timescales of metamorphism. Before ankerite grains with dolomite cores are fully realized as a useful diffusion chronometer for low- and medium-grade metamorphic rocks, the rates of Fe-Mg interdiffusion in ankerite and dolomite need to be calibrated as a function of a_(O2).

Permo-carboniferous paleoclimate of the Congo Basin: Evidence from lithostratigraphy, clay mineralogy, and stable isotope geochemistry

Approximately 1000m of strata in the Upper Paleozoic Lukuga Formation in the Dekese core in the central Congo Basin provide lithostratigraphic, mineralogical, and isotopic evidence for substantial climatic variation within a long-lived lacustrine basin. Lithostratigraphic indicators of cold climate include polymictic strata (dropstone deposits) and coupled laminations of fine clay-size material and coarse silt (glacial varves). Dropstones are concentrated in three stratigraphic zones in the lower ~425m of the Lukuga Formation, and varved strata occur in two broad stratigraphic zones in the lower ~700m of the formation. These sedimentological indicators suggest that the lower ~2/3 of the Lukuga Formation was strongly influenced by frigid conditions and glacial-like processes.The clay-size fraction of 97 samples is dominated by detrital minerals, including quartz, feldspar, chlorite, illite, and poorly ordered expansible 2:1 phyllosilicates. Based on variation in the mineralogy of these samples, the Lukuga Formation is divisible into three Clay Mineral Zones (CMZs), numbered in ascending stratigraphic order. CMZ 1 and CMZ 3 include several horizons of expansible 2:1 phyllosilicates that represent warmer/wetter intervals. Intervening CMZ 2 is a long (~500m) zone of chlorite and illite with no expansible phyllosilicates and is interpreted as a continuous cold/frigid interval.There are numerous calcite-cemented layers, including spar-filled veins that cross depositional bedding and represent postburial alteration, radiaxial fibrous cements that displace detrital grains, and micrite that crystallized near the time of deposition. Eighty-two stable isotope analyses of micrite yield δ13C values that range from –44.6‰ to –4.1‰ and δ18O values that range from –20.0‰ to 5.0‰ (VPDB). The carbon isotope data likely reflect a range of local carbon sources derived from bacterial activity and are unrelated to paleoclimatic conditions. In contrast, stratigraphic patterns in the oxygen isotope data suggest five or six intervals of frigid conditions conducive to glacial processes.