Implications of the 2740 Ma Cote Gold Au(-Cu) deposit for Archean gold metallogeny and porphyry Au deposit formation in the Archean Swayze greenstone belt, northern Ontario

International audienceOre-deposit models, which are fundamental to mineral exploration, result from integrating robust field observations with high-quality mineral-chemical-isotopic data. New discoveries departing from current models demand explanation, and in some cases new deposit models result (e.g., IOCG). The recent (2009/2010) discovery of the Cote Gold Au(-Cu) deposit in the Archean Swayze greenstone belt, northern Ontario, challenges the age-restricted view of porphyry Cu-Au models. The +8 Moz Au deposit is hosted by a subvolcanic tonalite-diorite intrusive complex where magmatic-hydrothermal breccia bodies occur. Gold mineralization, of disseminated-, fracture-and vein-types, is spatially associated with hydrothermal biotite and muscovite alteration that is similar to potassic and phyllic alteration, respectively, in porphyry-type settings. A robust program involving core logging and field mapping with structural analysis integrated with detailed petrographic studies, geochronology, lithogeochemistry, isotopes, and fluid inclusion studies (microthermometry and evaporate mound SEM-EDS analysis) was used to assess the deposit's origin. That the age of host rocks (U-Pb zircon, titanite) and timing of alteration (U-Pb titanite) and mineralization (Re-Os molybdenite) centre on 2740 Ma and pre-dates the age of regional deformation (ca. 2680 Ma), in addition to the co-spatial nature of mineralization and alteration, suggests a magmatic-hydrothermal connection. Structural analysis of auriferous quartz veins also indicates a relationship to the intrusive centre versus regional deformation, which is further supported by a 2740 Ma molybdenite age for one such gold-mineralized vein. Fluid-chemical data indicates fluid mixing with ?34S (py, cpy, moly = 0 ± 1‰) and fluid inclusions (mixed H2O-CO2 fluid (XCO2=0.10); Na-K-CaFe -Mn-Cl-F-S chemistry) suggesting a magmatic fluid reservoir whereas ?18O (qtz = 7-12‰) also indicates a possible seawater contribution. These observations are best reconciled with a deposit model involving Au(-Cu) mineralization originating from exsolution of magmatic fluids from a high-level, hydrous intermediate magma in the same manner as models for younger porphyry analogues

Paleomagnetism indicates that primary magnetite in zircon records a strong Hadean geodynamo.

Determining the age of the geomagnetic field is of paramount importance for understanding the evolution of the planet because the field shields the atmosphere from erosion by the solar wind. The absence or presence of the geomagnetic field also provides a unique gauge of early core conditions. Evidence for a geomagnetic field 4.2 billion-year (Gy) old, just a few hundred million years after the lunar-forming giant impact, has come from paleomagnetic analyses of zircons of the Jack Hills (Western Australia). Herein, we provide new paleomagnetic and electron microscope analyses that attest to the presence of a primary magnetic remanence carried by magnetite in these zircons and new geochemical data indicating that select Hadean zircons have escaped magnetic resetting since their formation. New paleointensity and Pb-Pb radiometric age data from additional zircons meeting robust selection criteria provide further evidence for the fidelity of the magnetic record and suggest a period of high geomagnetic field strength at 4.1 to 4.0 billion years ago (Ga) that may represent efficient convection related to chemical precipitation in Earth's Hadean liquid iron core

Fluid events associated with gold, uranium, and REE deposits in the proterozoic of northern Saskatchewan, Canada

The Contact Lake lode gold deposit is situated in northern Saskatchewan within the Proterozoic La Ronge Domain of the Trans-Hudson Orogen ($\sim$1.9-1.7 Ga). The deposit is associated with a high angle, reverse shear zone, which crosscuts the central granitic and granodioritic phases of the composite Little Deer Lake pluton. Four temporally distinct stages of veining within the shear zone have been identified on the basis of mineralogical, textural and crosscutting relationships. The four discernable vein types (two devoid of gold and two containing gold mineralization) permit the detailed study of fluids responsible for gold mineralization versus barren fluids, which is critical to gold exploration. High grade gold mineralization is related to stage IV veins. The interaction between the auriferous stage IV vein fluid and the granitic and granodioritic phases of the pluton produced distinct alteration mineral assemblages. The Proterozoic Athabasca Basin in northern Saskatchewan, Canada, hosts several of the largest and richest uranium deposits in the world. The uranium deposits occur at the intersection between Hudsonian-age faults and the unconformity between Athabasca Group sandstones and Aphebian metasediments and Archean gneisses. Uranium mineralization formed at ca. 200°C from the mixing between the oxidizing, uranium bearing basinal brine and the reducing basement fluid which issued along the faults. High concentrations of REE-rich phosphates are associated with uranium mineralization in the Athabasca Basin. The relations among REE-rich minerals, diagenetic clays and uranium mineralization in the basin and basement rocks indicate extensive REE mobility during diagenesis of the Athabasca Basin. The REEs and U most likely were derived from detrital fluor-apatite and zircon in the sandstone, and garnets in the basement rocks. These relations indicate that REEs, and possibly the uranium, were transported as F-complexes. The three stages of ore formation and associated alteration minerals permit the detailed study of fluids responsible for uranium deposition and alteration, which is critical to understanding and refining the model for the genesis of unconformity-type uranium deposits. In addition, the high concentration of REEs associated with uranium mineralization may render these deposits viable sources of REEs in addition to uranium. (Abstract shortened by UMI.

Multistage mineralization in the Haoyaoerhudong gold deposit, Central Asian Orogenic Belt: Constraints from the sedimentary-diagenetic and hydrothermal sulfides and gold

The Central Asian Orogenic Belt, as one of world-class gold economic belts, preserves a number of giant, large black shale-hosted gold deposits, while it is still debated for origin of sulfides and gold mainly due to lack of identification for multiple stages of sulfides. The Haoyaoerhudong gold deposit is hosted in a sequence of Mesoproterozoic carbonaceous and pyritic slate, phyllite, and schist that form a tight syncline along the north margin of the North China Craton. Detailed petrography of the host rocks and mineralization have defined five stages of pyrites. The earliest form of pyrite (Py1) occurs as fine-grained dispersed pyrite in black carbonaceous slate and medium- to coarse-grained disseminated pyrite in pyrite-rich layers, contains relative low gold and high arsenic content, indicating a syn-sedimentary or diagenetic in origin. Stage II pyrite (Py2) occurs with garnet and quartz inclusions and Py3 occurs as pyrite veins, contains higher gold and lower As content, and are interpreted to have formed from the dissolution-reprecipitation of Py1 during the peak metamorphism or post-peak metamorphism. Stage IV pyrite (Py4) from the pyrite-quartz veins crosscut the metamorphic garnet, contains the highest gold concentrations and other trace elements, and is considered to have formed post-peak metamorphism. Abundant native gold, electrum, and maldonite occur as inclusions within Py4 and monazite and in fractures that crosscut garnet. While, Py5 with typical remobilized feature is thought to be a product of melting of former pyrites (Py1 to Py4) triggered by the large-scale Hercynian magmatism. The sedimentary/diagenetic Py1 have δ34S values that range from +12.4‰ to +16.2‰. Later generations of sulfides, including Py2 to Py5, and Ccp2 to Ccp3, have δ34S values from +9.5‰ to +12.7‰. Monazite with maldonite inclusions from quartz-pyrite veins yielded an intercept age of 341.3 ​± ​6.6 ​Ma, while coarse grained monazite associated biotite along fractures in the reefs yielded an intercept age of 254.6 ​± ​8.2 ​Ma.The paragenetic, textural, chemical, and isotopic data suggest three distinct gold producing episodes at Haoyaoerhudong gold deposit. Gold and arsenic were clearly initially concentrated in organic muds, and enriched along the structures of diagenetic arsenic-rich pyrite. Subsequently, accompanying metamorphism and deformation, gold was liberated from the dissolution of diagenetic pyrites to form the pyrite veins. Finally, accompanying transformation of pyrite into pyrrhotite, gold was released into the metamorphic fluids to become concentrated as native gold, electrum, and maldonite in pyrite-quart veins. Monazite with age of 341 ​Ma from quartz-pyrite veins suggests that the third major gold mineralizing event in Haoyaoerhudong occurred before the Hercynian magmatism, suggesting that the Haoyaoerhudong deposit is a typical orogenic gold deposit rather than intrusion-related deposit

Isotopic composition of lead for raw material tracing: the case of Roman lead-glazed ceramic

Numerous studies of glazed coatings on ancient ceramics aimed at understanding the ancient production process, how it changed and developed through the centuries (Tite et al., 1998; Özçatal et al., 2014). In addition, investigating the provenance of the raw materials involved in glaze production is important to reconstruct the main trade routes that disseminated this technology. Stable lead isotope analyses, using secondary ion mass spectrometry (SIMS), was applied to trace the provenance of lead minerals involved in the production of Roman lead-glazed ceramics. The fragments analyzed here were unearthed in different archaeological contexts of ancient Rome (Testaccio Market, Magna Mater sanctuary and Domus Tiberiana on the Palatine Hill, Forum of Caesar and Forum of Nerva), spanning the period from the 2nd Century AD to the Middle Age. The ceramics analyzed clustered within the distribution patterns of several deposits including Bulgaria, Italy (Sardinia), Greece and Britain. However, the comparison with the lead isotope data of ores by statistical treatment of the data (PCA) shows that the deposits from England were important sources of lead used for the production of glazed ceramics since the second Century AD. A comparison of our lead isotopic results obtained using SIMS with data collected from previous works by ICP-MS and TIMS (Wolf et al., 2003; Walton & Tite, 2010) shows that SIMS can be used for lead isotope provenance research. In addition, SIMS micro-analytical technique includes easy sample preparation, quick acquisition times, and is minimally destructive, which are advantages in the field of cultural heritage (Hull et al., 2014). Furthermore, the acquisition of accurate and precise isotopic measurements on a micrometer scale with the possibility to choose the location of each analysis point avoids the inclusions especially in the case of ceramic material

MR diffusion imaging in mediastinal masses the differentiation between benign and malignant lesions

Background: Diffusion-weighted imaging is a fundamental tool integrated in MR protocols useful in differentiating benign from malignant mediastinal masses, assessing mediastinal lymphadenopathy and investigating central bronchogenic carcinoma. This method is an excellent alternative to CT or PET/CT in the investigation of mediastinal masses. Current applications of diffusion MRI in malignancies include monitoring the treatment response and detecting recurrent cancer. Aim of the work: This study aims to assess the value of using MRI diffusion in differentiating benign and malignant mediastinal masses, differentiating central masses from post obstructive collapse and differentiating lymphoma versus sarcoidosis. Patients and methods: This study included 30 patients; 16 males and 14 females in the period from June 2013 to July 2014. The mean age was 49.3 ± 16.85 (range: 22â82 years).Cases were referred for MRI assessment and were approved by the ethical committee in our department.The complaints varied between dyspnea, chest pain, cough, hemoptysis, fatigue and loss of weight. A superconducting 1.5 T MRI machine with a four-channel body phased-array coil was used for the examination. Biopsy and histopathological assessment was done after that. Results: MRI examination with diffusion imaging was able to differentiate between benign and malignant mediastinal and hilar lesion confirmed by the biopsy and histopathology. Conclusion: MRI with diffusion weighted images can detect and stage lung cancer, differentiate benign from malignant mediastinal masses and differentiate lymphoma from sarcoidosis in mediastinal/hilar lymphadenopathy. Keywords: Magnetic resonance imaging, Diffusion, Sarcoidosis, Lymphoma, Bronchogenic carcinom

Uranium-Series Disequilibria in the Groundwater of the Shihongtan Sandstone-Hosted Uranium Deposit, NW China

Uranium (U) concentration and the activities of 238U, 234U, and 230Th were determined for groundwaters, spring waters, and lake water collected from the Shihongtan sandstone-hosted U ore district and in the surrounding area, NW China. The results show that the groundwaters from the oxidizing aquifer with high dissolved oxygen concentration (O2) and oxidation-reduction potential (Eh) are enriched in U. The high U concentration of groundwaters may be due to the interaction between these oxidizing groundwaters and U ore bodies, which would result in U that is not in secular equilibrium. Uranium is re-precipitated as uraninite on weathered surfaces and organic material, forming localized ore bodies in the sandstone-hosted aquifer. The 234U/238U, 230Th/234U, and 230Th/238U activity ratios (ARs) for most water samples show obvious deviations from secular equilibrium (0.27–2.86), indicating the presence of water-rock/ore interactions during the last 1.7 Ma and probably longer. The 234U/238U AR generally increases with decreasing U concentrations in the groundwaters, suggesting that mixing of two water sources may occur in the aquifer. This is consistent with the fact that most of the U ore bodies in the deposit have a tabular shape originati from mixing between a relatively saline fluid and a more rapidly flowing U-bearing meteoric water

Luminescence of uranium-bearing opals: Origin and use as a pH record

International audienceFluorescence of minerals has been long used for U exploration. It is proposed here that opal fluorescence can be used as a probe of the pH of the formation solution, bringing constraints for modeling U speciation and sequestration at the Earth's surface. We present a study of fluorescence spectroscopy of U in opals resulting from low-temperature (29 degre) alteration of mineralized rhyolitic lavas (Nopal I U deposit, Sierra Peña Blanca, Mexico). These opals show green fluorescence with a concentric distribution at microscopic scale, which is unambiguously assigned to uranyl groups (oxidized form of U). Spectra appear typical of uranyl in opal as reported in literature for other localities according to peak positions. When considering also the lifetime of fluorescence spectra (with time resolved laser fluorescence spectroscopy), fingerprinting indicates that uranyl occurs either as phosphate or hydroxo-polynuclear complexes trapped onto the opal internal surface. Data are indicative of a pH of opal formation around 8, as derived from both conditions of laboratory experiments and reference to calculated diagrams of speciation. This pH value is consistent with the Nopal geological formations that were potentially in contact with the aquifer involved in the hydrothermal process. In addition, the microscopic zoning of U suggests that the speciation has been stable since opal formation more than 50 ka ago, which allows the record of pH that prevailed at that time