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Identifying the source, origin and time of emplacement of copper and gold deposits in the Neoproterozoic volcano-sedimentary and plutonic series of southern Brazil is a longstanding research goal. In this geological context, the Camaquã and Lavras do Sul areas are reported to be a post-collisional domain related to the Braziliano–Pan-African Orogenic Cycle. This study focused on the Uruguay open-pit mine through a detailed assessment of the mineralogy, geochemistry and (S, O, H and C) stable isotopes of pyrite, chalcopyrite, clay minerals and carbonates as veins or disseminated sulfides in sedimentary rocks. The results indicate that the Camaquã mine is a low-sulfidation epithermal-type deposit with sulfide precipitations related to a mixture of magmatic (δ34S=1 to 4‰; δ18O: 3 to 9‰) and meteoric waters (δ18O=−4 to −12‰) and a temperature varying from 350 to 80°C. Ore precipitation is associated with sequential sulfide crystallization exhausting the S magmatic source. [ABSTRACT FROM AUTHOR]

The Sierra Huautla (Morelos State, Mexico) is a biological reserve with historical mines of Ag and Pb. In this area, waters used by inhabitants are contaminated by arsenic (As). An integrated environmental survey was realized both in waters and sediments to better constrain the source and the mobility of As and other trace elements. Two areas of interest were selected: (1) the Nexpa River ecosystem to determine the local geochemical background, and (2) the Huautla area, affected by past mining activities. This study allowed the definition of the local geochemical baseline in sediments or in waters, demonstrated uncontaminated by TE in the Nexpa area, except for As in the dissolved phase or for Cd in Suspended Particulate Matters (SPM). In the Huautla area, TE contents in water were higher than the World Health Organization (WHO) limits for Al, As and Mn in unfiltered waters, and only for As in the dissolved phase. Speciation analyses revealed arsenic to be present only as the toxic-inorganic arsenate species, As(+V). In SPM, Ag, As, Cd and Zn concentrations were higher than Sediment Quality Guidelines (SQG). The different geochemical indice (EF: 5, PLI: 3, EF: Igeo: 5–3) demonstrated that SPM were significantly contaminated and consistute an health risk for Huautla inhabitants exposed to As-contaminated waters and TE-rich SPM. The combination of mineralogy, chemistry, C and S stable isotopes with thermodynamic modeling indicate dissolutions of minerals from local geological formations, sorption-desorption phenomena from clays and oxy-hydroxides, and the weathering responsible for the transport of the TE-rich SPM (1.8 wt% for 17% of the total TE concentration). Moreover, the past mining activity would be a source of the contamination only for As in waters from flooded mines. Unlabelled Image • Dissolved arsenate species are the main contaminant in waters. • Suspended particles are a potential secondary source of water contamination. • Organic contaminations occur in drinking waters. • The As contamination may originate from the dissolution of sulfides by oxidation. • TE contaminations in SPM due to oxi-hydroxides and sorption onto clays. [ABSTRACT FROM AUTHOR]

The Buracão area, Central Brazil, comprises a group of structurally–controlled vein–type gold prospects, hosted in the metasedimentary Traíras Fm. of the Araí Group. This group records all tectonic stages of the Statherian taphrogenesis in Central Brazil and represents part of the Brasília Fold Belt (BFB) developed during the Neoproterozoic Brasiliano Orogeny. Three lithostratigraphic units were identified at the study area: two phyllitic and one composed of intercalation of quartzite and carbonaceous/sericite phyllites that hosted the Au mineralized vein systems. These units were metamorphosed to the biotite zone, greenschist facies. The high–grade mineralization (up to 31 g/t) occurs as subvertical zoned or massive veins with quartz, white mica, Fe–sulfides and Fe–oxides oriented in four main directions (i.e. N50°E; N°20E; N–S and E–W). Two deformational phases, each subdivided in two stages (D 1a , D 1b , D 2a and D 2b ) were recognized in the study area. These deformational phases comprise different sets of structures, such as foliations, folds, faults, fractures and veins that represent regional deformational events on subordinated scale. The D 1a and D 1b stages represent the regional N–S trending thrust fronts and propagation folding developed during the evolution of W–E shortening, respectively. The D 2a and D 2b were correlated to the development and reactivation of regional–scale N– and NE–trending transpressive shear systems at different crustal levels, respectively. The gold–bearing quartz veins were developed during D 2a stage as a complex shear fracture set according to a Riedel system, representing subordinated structures of the regional transpressive system. The structurally–controlled gold occurrences of the Buracão area is an example of orogenic vein–type gold mineralization developed during the Brasiliano–Pan African Orogeny at the BFB in Central Brazil. Our work suggests that this poorly studied part of BFB has good potential for additional high-grade orogenic gold deposits. [ABSTRACT FROM AUTHOR]

This research explores the phosphatization process in the São Pedro and São Paulo Archipelago (SPSPA), located in the Equatorial Atlantic, by analyzing the geochemical interactions between guano coatings and the underlying rocks. The study also examines the geochemical characteristics of the substrates, assessing the behavior of major, minor, trace, and rare earth elements, and determines the degree of weathering degradation of guano crusts, speleothems, peridotites and carbonate sedimentary rocks affected by environmental factors as rain, wind and halmyrolysis. The distribution of chemical elements in water-soluble, adsorbed, acid-extractable/exchangeable, readily acid and reducible, acid oxidizable, and residual phases were investigated. The principal findings of this method show that elevated Na levels are leached in water-soluble fractions, thereby highlighting the occurrence of rapid and active geochemical cycling, which is influenced by seawater. In contrast, in more acidic environments (pH close 4.8 to 2.0), considerable amounts of macroelements like P, Fe, Mg, Al, and Mn are found in association with remaining phases, indicating a slower and more incremental geochemical cycle of mature guano in the input and output systems of these nutrients in the archipelago. The mineralogy, as indicated by the presence of mafic minerals such as olivine, augite, and Cr-spinel, as well as phosphates including fluorapatite, collinsite, and spheniscidite, also supports this finding. The findings underscore the complex interplay between biological activities and geochemical processes, which influence the elemental dynamics in the phosphate-rich rocks of the SPSPA at depth. This study not only enhances our understanding of phosphatization processes but also offers important insights into the sustainable management of marine resources in environmentally sensitive areas. The research underscores the necessity of considering biological and environmental variables when examining geochemical cycles, particularly in regions exhibiting distinctive geological and biological interactions that challenge the prevailing models, such as those identified in the ASPSP. • Bird guano affects geochemical patterns and element nutrient cycling. • High sodium leached at neutral pH from seawater influence. • Acidic conditions slow the cycling of key macroelements. • Results show deep interplay of biology and geochemistry in oceanic islands. [ABSTRACT FROM AUTHOR]

Piaba is an orogenic gold deposit (~ 3.5 Moz) of the São Luís cratonic fragment in north-northeastern Brazil. The deposit is epizonal–mesozonal and associated with the development of a subvertical strike–slip fault that cut across Rhyacian metavolcano-sedimentary rocks and granitoids. The metavolcano-sedimentary sequence comprises carbon-bearing schists, andesite/dacite, ultramafic rocks and felsic tuffs formed at 2240 ± 5 Ma to 2227 ± 33 Ma that were intruded at 2214 ± 3 Ma by fine-grained, subvolcanic granophyric granodiorite. Whole-rock geochemistry indicates that the hosting metavolcano-sedimentary sequence formed in a subduction-related (arc, back-arc?) setting, which has previously been interpreted as the early-arc stage of an accretionary orogen (2240–2214 Ma) that was followed by voluminous subduction-related calc-alkaline arc magmatism (2168–2145 Ma) and by a collisional phase (2100 ± 15 Ma) that produced several bodies of peraluminous granites. The hydrothermal alteration of the host rocks produced early, distal carbonization and hematitization and proximal, ore-related chlorite–sericite–carbonate–sulfide alteration. Two styles of mineralization, occurring in association, are present: (1) a network of thin quartz ± sulfide veinlets and subordinate narrow breccia veins with refractory- to free-milling gold dissemination in hydrothermally-altered host rocks, and (2) subordinate low-sulfide Au-quartz veins with free-milling gold. Fluid inclusions and stable isotopes characterize the mineralizing solution as a relatively reduced (log f O 2 = − 30 to − 35), neutral to slightly alkaline (pH 5 to 6.2), low-salinity (average 5% NaCl), aqueous-carbonic ( X CO 2 = 5 to 59 mol%, X H 2 O = 40 to 93 mol%, X N 2 and X CH 4 < 1 mol%) metamorphic fluid (δ 18 O H2O = + 5.5 to + 8.9‰, δD H2O = − 8 to − 69‰), and suggest that gold mineralization occurred at 250° to 330 °C and 1.25 to 2.8 kbar in response to phase separation and fluid–rock interactions that produced sulfidation and minor carbonatization. Sulfur (δ 34 S = − 2.7 to − 3.8‰) is from an undetermined source, probably magmatic (oxidized) or sedimentary. Lead isotope compositions of gold, ore-related pyrite and K-feldspar from regional rocks suggest that collision-related granitoids of ca. 2100 ± 15 Ma are the most likely sources for Pb in the ore, although mantle and lower crustal sources are also observed. 40 Ar/ 39 Ar in hydrothermal sericite yielded a plateau age of 1980 ± 13 Ma, which is similar to the Pb–Pb model age of gold of 1959 Ma. These are considered the minimum age for gold mineralization, whereas the maximum age of 2009 ± 11 Ma is given by a small gold-hosting intrusion in the same region. [ABSTRACT FROM AUTHOR]

Abstract: Ain Allega and El Aguiba are among the largest barite–celestite deposits attributed to the Triassic period in Tunisia. They are located in the flysch zone on the eastern edge of the Triassic diapir of Jebel Hamra. The ore body consists of dolomite intensely brecciated and surrounded by marls, clay, gypsum and dolomite which forms the hanging wall of the deposit, and rimmed by Paleocene marls. The ore minerals show a cap-rock type mineralization with various forms and types, in particular impregnation in dolomite, breccia cement, replacement of carbonate cap-rock and open space-filling in the dissolution cavities and fractures. Ore minerals include sphalerite, galena, marcasite and pyrite. Principal gangue minerals are barite, celestite, calcite, dolomite and quartz. The ore minerals are hosted by the Triassic carbonate rocks, which show hydrothermal alteration, dissolution and brecciation. The study of barite–celestite solid-solution shows a bimodal distribution in which the composition is not continuous. Some intermediate compositions are missing; 20–80% SrSO4 for the El Aguiba ore deposit and 50–70% SrSO4 for the Ain Allega ore deposit. X-ray diffraction peaks in the BaSO4–SrSO4 series display considerable broadening at intermediate compositions. The unit cell volume varies from 346.97Å3 in barite (100% BaSO4) to 308.29Å3 in celestite (100% SrSO4). This variation in cell parameters is attributed to the degree of substitution of barium by strontium since there is less variation between ionic radii of these elements. Consequently, these substitutions caused variation in morphologies, position and d-spacing of peaks (200, 011, 113, 312,122, 112, and 111). Some barite crystals have small-scale, strontium compositional banding which suggests that the crystals responded to regular fluctuations in physiochemical conditions during deposition. The degree of supersaturation in Ba2+ and Sr2+ controls the growth mechanism and consequently the surface structure, which in turn controls the mechanism of adsorption and incorporation of minor and trace elements into the growing crystal. For a solid solution, the transitional supersaturation for different growth mechanisms can be significantly different for the two end members. One of the most interesting and intriguing phenomena observed in natural crystals of the (Ba,Sr)SO4 solid solution from this study area is the development of compositional oscillatory zoning, consisting of alternating Ba-rich and Sr-rich layers. The sulfur isotope analyses show significant variability for barite and celestite (from 16.2 to 23‰). These values are interpreted as the result of the mixing of two sulfur end-members in the mineralizing fluids, corresponding to thermochemical sulfate reduction of Messinian seawater, together with Triassic sulfate, as sulfur sources. Fluid inclusion studies of celestite show that the BaSO4–SrSO4 solid-solution in both deposits was precipitated from hot saline solution (Th =190±20°C; 16.37wt.% NaCl equivalent in Ain Allega and 8.2wt.% NaCl equivalent in El Aguiba). Consequently, the precipitation could be from mixing of basinal brines with magmatic–meteoric fluid. [Copyright &y& Elsevier]

Abstract: Type I and Type II eclogite xenoliths from the Roberts Victor kimberlite (South Africa) show marked differences in terms of microstructures, mineralogy, major- and trace-element compositions and oxygen-isotope compositions. The unequilibrated microstructures of Type I eclogites, their typical accessory assemblages (phologopite, diamond, sulphides, fluid inclusions) and the ubiquitous presence of “melt pockets” in garnets provide strong evidence of metasomatism. Type II eclogites systematically lack such features and are microstructurally more equilibrated. Type I eclogites are more magnesium-rich than most Type II (mean Mg#=0.56 vs. 0.46), while Type II eclogites are generally more Ca-rich (mean CaO=9 vs. 12wt%) and Fe-rich (mean FeO=10 vs. 12wt%). Type I eclogites are systematically enriched in LREE, Sr, Ba, alkali elements, HFSE, Th and U compared to the more depleted Type II eclogites. Calculated trace-element patterns of fluids in equilibrium with Type I eclogites are closely similar to those of volatile-rich small-volume mantle melts in the carbonatite–kimberlite spectrum commonly inferred to be responsible for mantle metasomatism. Although oxygen isotopes are often used to argue for a subduction origin of mantle eclogites, correlations between δ18O of garnet and typical metasomatic tracers suggest that the metasomatic process also has shifted the oxygen-isotope compositions of the Type I eclogites toward heavier values. Roberts Victor Type I eclogites thus carry the imprint of a metasomatic process that strongly modified their major-element, trace-element and isotopic compositions, while the more pristine Type II eclogites escaped this modification. Therefore, attempts to constrain the origin of Roberts Victor eclogites should not be based on the much more abundant Type I eclogites, which retain little geochemical memory of their protoliths. The most suitable materials for such investigations may be the less metasomatised, but more rare, Type II eclogites. [Copyright &y& Elsevier]