Evaporite Source Research Articles

Erosional modulation of the balance between alkalinity and acid generation from rock weathering

The balance between alkalinity generation by carbonate and silicate weathering and sulfuric acid generation by sulfide weathering controls the effect of weathering on atmospheric pCO2 over geologic timescales. How this balance varies across environmental gradients remains poorly constrained. Here, we analyze this balance across an erosional gradient of two orders of magnitude in the Three Rivers (Yangtze, Mekong, and Salween Rivers) Headwater Region (TRHR), eastern Qinghai-Tibet Plateau (QTP). By employing major element chemistry and multiple isotopes (δ34SSO4, δ18OSO4, and δ18OH2O) coupled with forward and inverse approaches, we unmix contributions of silicates, carbonates, evaporites, and sulfides to the total weathering budget. Across the TRHR, riverine SO42– is derived mainly from a mixture of an evaporite source with uniform values of δ34SSO4 and δ18OSO4, and a sulfide source that contributes highly variable values of δ34S (−12.2 ‰ to +4.1 ‰) and δ18O (−17.7 ‰ to −1.6 ‰). Contributions of sulfide oxidation to riverine SO42– vary from 16 % to 94 %, and sulfuric acid consumes 6 % to 63 % of the alkalinity produced by weathering. The fractions of weathering alkalinity derived from carbonate weathering range from 36 % to 98 % relative to silicate weathering. The combination of silicate, carbonate, and sulfide weathering suggests that the instantaneous weathering fluxes of most sampled catchments in the TRHR act as a sink of atmospheric CO2 over timescales shorter than marine carbonate burial (∼104 years), but as a carbon source over timescales longer than carbonate burial and shorter than sulfide burial (∼107 years). The spatial variability of the balance between alkalinity and acid generation, and, thus, the relationship between chemical weathering and atmospheric pCO2, are largely dependent on lithology. However, within comparable lithologic settings, sulfide and carbonate weathering rates rise with increasing erosion, whereas silicate weathering rates remain constant. Consequently, plateau weathering shifts from a sink to a source of atmospheric CO2 with increasing erosion. These findings suggest that sulfide weathering is more sensitive to erosion than carbonate and silicate weathering, and that it could play an important role in the long-term carbon cycle during mountain building.

Genetic features of tourmaline from the magnesian skarns of Kuhilal deposit, Southwestern Pamir

Background. Although the Kuhilal deposit of Mg skarns has been thoroughly studied, tourmaline in their composition has never been considered [5, 6, 16]. Uvite, a tourmaline variety, was mentioned in the list of rare minerals only with its crystallochemical formula [17]. L.N. Shabynin, a great connoisseur of skarns, noted the rarity of tourmaline in Mg skarns of various deposits. Tourmaline of this type is characterized by a strong variation in ferruginosity and a significant content of Ca. At the same time, Ca:Na varies from 2:3 to 20:1 [15]. According to N.N. Pertsev, another major specialist in skarns, tourmaline is a rare mineral of Mg skarn deposits [11]. The boron-iron ore deposit of Tayozhnoe, Aldan shield, is an exception. This deposit belongs to skarns, where a widespread development of tourmaline was noted [12]. Tourmaline (dravite) of this deposit was found only in silicate metamorphic rocks containing marbles and Mg skarns.Aim. To determine the paragenesis, crystal morphology, chemical composition, and genetic characteristics of tourmaline from Kuhilal Mg skarns.Materials and methods. Large aggregates (more than 10 cm across) and crystals of apple-green tourmaline from the Mg skarns of the Kuhilal deposit were investigated. Samples of Mg skarns containing tourmaline were collected and documented by the authors during fieldwork research at the Kuhilal deposit in 2017. The samples were examined by mineralogical and petrographic methods, X-ray diffraction analysis using a DRONE-3M (analyst A.V. Fedorov, Sergo Ordzhonikidze Russian State University for Geological Prospecting), microprobe analysis using a Cameca SX 100 in 15 kV 30 nA shooting mode (analyst V.I. Taskaev, IGEM RAS), and X-ray fluorescence analysis using an AXIOS advanced spectrometer with an X-ray tube equipped with a 3 kW Rh anode and Philips PW-2400 with a sensitivity of 10-4% (analyst A.I. Yakushev, IGEM RAS). The majority of studies were conducted at the Department of Mineralogy and Gemology of the Sergo Ordzhonikidze Russian State University for Geological Prospecting.Results. Tourmaline from the Mg skarns of the Kuhilal deposit has been studied for the first time. This mineral is rare for skarn rocks. Two tourmaline mineral associations were identified: with spinel and forsterite, and with chlorite and serpentine. In terms of chemical composition, tourmaline isattributed to fluorine-containing uvite with a close to zero ferruginosity. Uvite is characterized by a lenticular shape with a rarely observed, underdeveloped prism. Its mineralogical properties, chemical composition, and formation conditions were analyzed. Tourmaline can be used to make inexpensive jewelry inserts.Conclusion. Tourmaline crystallization occurred under the conditions of granulite facies of regional metamorphism. For boron formation, an apo-sedimentary, evaporite source is assumed.

Nature of the Skarn–Borosilicate Deposit Giants of the Sikhote-Alin and Pamir Mountains

It is shown that the giant Sikhote-Alin and Pamir deposits of boron are associated with fragments of the boron-bearing evaporite strata of paleoceanic atolls in the subduction melange. Ore formation relates both to in situ transformation of boron-bearing evaporite strata at the Ak-Arkhar deposit and to boron mobilization and redeposition some distance away from the evaporite source in course of intensive post-accretionary fluid-magmatic reworking of the subduction melange at the Dalnegorskoe deposit.

An Approach to Genesis of Sepiolite and Palygorskite in Lacustrine Sediments of the Lower Pliocene Sakarya and Porsuk Formations in the Sivrihisar and Yunusemre-Biçer Regions (Eskişehir), Turkey

AbstractThe Lower Pliocene lacustrine sediments of the Sakarya and Porsuk Formations in the Sivrihisar and Yunusemre-Biçer regions consist of claystone, argillaceous carbonate, carbonate, and evaporites. No detailed studies of paleoclimatic conditions have been performed previously. The present study aimed to determine the depositional environment and paleoclimatic conditions for the formation of these economically important sepiolite/palygorskite/carbonate/evaporite deposits based on detailed mineralogical, geochemical, and isotopic studies. Samples from various lacustrine sediments were examined using polarized-light microscopy, X-ray diffraction, scanning electron microscopy, and chemical and isotopic analysis methods. Dolomites are predominantly of micrite, which is partly recrystallized to dolomicrosparite/dolosparite close to desiccation fractures. The presence of ostracods and dacycladecean algae in the carbonates reflects a restricted depositional environment. The formation of sepiolite and palygorskite fibers, either as cement between/enclosing dolomite and/or as calcite crystals, reflects occasional changes in physicochemical conditions provided by fluctuations in the lake-water level and influx of groundwater in relation to climatic changes during and after dolomite precipitation. The positive correlations of ΣREE with Al2O3, Nb, high-field-strength elements, and transition elements are due to alteration of feldspar and hornblende in the volcanic units. The high values of Ba and Sr relative to Cr, Co, Ni, and V also indicate that felsic rather than ophiolitic rocks were the parent material. The crossplot of whole-rock SiO2vs. Al2O3+K2O+Na2O and V/Cr ratio suggests deposition of carbonate-dolomitic sepiolite-sepiolitic dolomite under arid climate and oxic conditions, whereas the Ni/Co and V/(V+Ni) ratios of the sediments indicate deposition of organic-bearing sepiolite/palygorskite under anoxic-dysoxic conditions. An enrichment in δ13C and δ18O values of dolomite with respect to calcite is probably due to differences in mineral fractionations. The δ34S and δ18O values and 87Sr/86Sr isotope ratios for gypsum suggest an intensely evaporitic lacustrine environment fed by an older marine evaporitic source. The Si, Al, Mg, Ca, and enhanced TOT/C required for periodic precipitation of organic-rich brown sepiolite/palygorskite characterize deposition in a swampy environment, while dolomitic sepiolite and sepiolitic dolomite formed in ponds by partial drying of the main alkaline lake.

Discriminating fluid source regions in orogenic gold deposits using B-isotopes

The genesis of orogenic gold deposits is commonly linked to hydrothermal ore fluids derived from metamorphic devolatilization reactions. However, there is considerable debate as to the ultimate source of these fluids and the metals they transport. Tourmaline is a common gangue mineral in orogenic gold deposits. It is stable over a very wide P–T range, demonstrates limited volume diffusion of major and trace elements and is the main host of B in most rock types. We have used texturally resolved B-isotope analysis by secondary ion mass spectrometry (SIMS) to identify multiple fluid sources within a single orogenic gold ore district. The Loulo Mining District in Mali, West Africa hosts several large orogenic gold ore bodies with complex fluid chemistry, associated with widespread pre-ore Na- and multi-stage B-metasomatism. The Gara deposit, as well as several smaller satellites, formed through partial mixing between a dilute aqueous-carbonic fluid and a hypersaline brine. Hydrothermal tourmaline occurs as a pre-ore phase in the matrix of tourmalinite units, which host mineralization in several ore bodies. Clasts of these tourmalinites occur in mineralized breccias. Disseminated hydrothermal and vein hosted tourmaline occur in textural sites which suggest growth during and after ore formation. Tourmalines show a large range in δ11B values from −3.5 to 19.8‰, which record a change in fluid source between paragenetic stages of tourmaline growth. Pre-mineralization tourmaline crystals show heavy δ11B values (8–19.8‰) and high X-site occupancy (Na±Ca; 0.69–1apfu) suggesting a marine evaporite source for hydrothermal fluids. Syn-mineralization and replacement phases show lighter δ11B values (−3.5 to 15.1‰) and lower X-site occupancy (0.62–0.88apfu), suggesting a subsequent influx of more dilute fluids derived from devolatilization of marine carbonates and clastic metasediments. The large, overlapping range in isotopic compositions and a skew toward the opposing population in the δ11B data for both tourmaline groups reflects continual tourmaline growth throughout mineralization, which records the process of fluid mixing. A peak in δ11B values at ∼8‰ largely controlled by tourmalines of syn- to post-ore timing represents a mixture of the two isotopically distinct fluids. This paper demonstrates that B-isotopes in tourmaline can be instrumental in interpreting complex and dynamic hydrothermal systems. The importance of B as an integral constituent of orogenic ore forming fluids and as a gangue phase in orogenic gold deposits makes B-isotope analysis a powerful tool for testing the level of source region variability in these fluids, and by extension, that of metal sources.

Mineralization of the South Pennine Orefield, UK—A Review

This review of the South Pennine Orefield (SPO) draws together the findings from many years of underground field observations and petrographical study. Mineralization is of the Mississippi Valley-type (MVT) and is concentrated within an area of some 200 km 2 , mainly along the eastern margins of a large inlier, the Derbyshire High, in Carboniferous platform carbonate host rocks. The inlier covers some 390 km 2 , forms an up-dip promontory of a larger structure, the East Midlands Shelf, and is surrounded by shales and sandstones of the Millstone Grit and Pennine Coal Measures groups. Mineralization probably began during the late Westphalian (Moscovian, Mid Pennsylvanian), when subsidence due to thermal sag resulted in the limestone being buried to depths of c. 4 km beneath younger strata. A palaeohydraulic reconstruction is presented from analysis of mineralized palaeokarst features, which are interpreted as representing hypogenic or deep-seated karst formed by the interstratal circulation of hydrothermal water in a mostly confined hydrodynamic setting. It is reasoned that Variscan inversion of N–S faults to the east of the SPO resulted in erosion of Namurian and Westphalian (Upper Mississippian–Middle Pennsylvanian) rocks and created a hydraulic gradient inclined towards the south-west. Acidic F-Ba-Pb-Zn enriched fluid evolved in the Namurian basinal rocks and migrated into fractured limestone. The resultant wall-rock dissolution along existing wrench faults led to the formation of a maze of stratiform mineral deposits (flats) and more irregular spongework-shaped structures (pipes). The presence of hydrocarbon accumulations in the limestones and evidence from fluid inclusions indicates that the mineralizing fluids were chloride/fluoride-rich and compositionally typical of oilfield brine. Isotope evidence demonstrates a sulphate evaporite source of sulphur, mainly from the Chadian (Visean, Middle Mississippian) Middleton Anhydrite Formation. By the late Cenozoic, karstification of exposed carbonate rocks began and the current pattern of epigenic karst drainage started to develop as the regional hydraulic gradient reversed, assuming its present eastward inclined attitude. The mineralized hypogenic karst was overprinted by later drainage systems as the hydraulic gradient changed, and placer deposits were formed from the erosion of existing mineralization. This was accompanied by circulation of meteoric water and resulted in the supergene weathering of the sulphide ore minerals. Eastward underflow of meteoric groundwater also exploited the same mineralization flow paths. There is evidence that pre-mineralization hypogenic karst was also significant in the formation of orebodies in the North Pennine Orefield and the Halkyn–Minera Orefield of NE Wales.

Fluid evolution and mineralogy of Mn-Fe-barite-fluorite mineralizations at the contact of the Thuringian Basin, Thüringer Wald and Thüringer Schiefergebirge in Germany

Abstract Numerous small deposits and occurrences of Mn-Fe-fluorite-barite mineralization have developed at the contact of the Thuringian Basin, Thüringer Wald and Thüringer Schiefergebirge in central Germany. The studied mineralizations comprise the assemblages siderite+ankerite-calcite-fluorite-barite and hematite-Mn oxides-calcite-barite, with the precipitation sequence in that order within each assemblage. A structural geological analysis places the origin of the barite veins between the Middle Jurassic and Early Cretaceous. Primary fluid inclusions contain water vapour and an aqueous phase with NaCl and CaCl2 as the main solutes, with salinities mostly between 24–27 mass. % CaCl2 eq. Th measurements range between 85 °C and 160 °C in barite, between 139 °C and 163 °C in siderite, and between 80 °C and 130 °C in fluorite and calcite. Stable isotopes (S, O) point to the evaporitic source of sulphur in the observed mineralizations. The S,C,O isotopic compositions suggest that barite and calcite could not have precipitated from the same fluid. The isotopic composition of the fluid that precipitated barite is close to the sea water in the entire Permo–Mesozoic time span whereas calcite is isotopically distinctly heavier, as if the fluids were affected by evaporation. The fluid evolution in the siliciclastic/volcanic Rotliegend sediments (as determined by a number of earlier petrological and geochemical studies) can be correlated with the deposition sequence of the ore minerals. In particular, the bleaching of the sediments by reduced Rotliegend fluids (basinal brines) could be the event that mobilized Fe and Mn. These elements were deposited as siderite+ankerite within the Zechstein carbonate rocks and as hematite+Mn oxides within the oxidizing environment of the Permian volcanic and volcanoclastic rocks. A Middle-Jurassic illitization event delivered Ca, Na, Ba, and Pb from the feldspars into the basinal brines. Of these elements, Ba was deposited as massive barite veins.

A NEW OCCURRENCE OF THE BOROSILICATE SERENDIBITE IN TOURMALINE-BEARING CALC-SILICATE ROCKS, PORTAGE-DU-FORT MARBLE, GRENVILLE PROVINCE, QUEBEC: EVOLUTION OF BORON ISOTOPE AND TOURMALINE COMPOSITIONS IN A METAMORPHIC CONTEXT

Abstract Serendibite, Ca 1.93 Na 0.08 Mg 2.58 Al 5.03 B 1.53 Si 2.71 O 20 , was recently discovered in a lens of B-rich calc-silicate rock metamorphosed at 6–7 kbar, 650–700 °C in the Central Metasedimentary Belt, Grenville Province. Based on paragenesis and tourmaline composition, which were used to monitor changes in fluid composition, the following stages of mineralization are recognized: (1) a prograde assemblage consisting of K-feldspar, tourmaline [T1 with X Ca = Ca/(Ca + Na) = 0.54, T2 with X Ca = 0.71], and calcite, inferred from relicts in scapolite; (2A) a peak metamorphic assemblage of aluminous diopside, serendibite, lesser phlogopite, and local scapolite (Me 62 ); (2B) continued formation of phlogopite around serendibite in calcite pockets although serendibite was stable; and (3) high-temperature breakdown of serendibite to uvite (T3; X Ca = 0.82) + spinel + calcite, and of aluminous diopside to pargasite. Three generations of idiomorphic magnesian tourmaline that crystallized in calcite pockets (T4 to T6) recorded fluid evolution as follows: (A) X Ca ≈ 0.5 and relatively moderate Ti concentrations (generation T4); (B) increase in Na relative to Ca, X Ca ≈ 0.3 (generation T5), with the rare occurrence of oligoclase ( X Ca = 0.17) and relatively low Ti concentrations; and (C) X Ca ≈ 0.5, relatively high Ti concentrations (generation T6). The final stage is localized, low-temperature alteration to fine-grained phyllosilicates. The boron isotopic composition of peak metamorphic serendibite (δ 11 B = +4.3 ± 1.7‰) is lower than that of prograde tourmaline (δ 11 B = +10.6 ± 2.3‰) from which it formed. The fractionation difference between serendibite and metamorphic fluid (with B derived from prograde tourmaline), Δ 11 B Srd−Tur = δ 11 B Srd − δ 11 B Tur = −6.3‰, is similar to the ab initio calculated fractionation factor between serendibite and fluid (Δ 11 B Srd−fluid = −7.6 ± 1.4‰ at 727 °C). Tetrahedral boron coordination in serendibite is largely responsible for this fractionation. High-temperature uvite replacement of serendibite is significantly higher in δ 11 B relative to serendibite (+10.6‰ versus +4.3‰), which is attributed to buffering of boron isotopes by serendibite in local equilibrium with uvite. Tourmalines formed in calcite pockets range from +8‰ to almost +12‰ on average, while the core of a T4 tourmaline has δ 11 B = +3.4‰. This low δ 11 B value is attributed to a temporary departure from equilibrium due to fluid influx. The boron isotope signature of prograde tourmaline suggests a marine evaporite source.

Boron isotopic composition of tourmaline, prismatine, and grandidierite from granulite facies paragneisses in the Larsemann Hills, Prydz Bay, East Antarctica: Evidence for a non-marine evaporite source

A unique assemblage of B-rich rocks (<2 wt.% B) containing the borosilicate minerals tourmaline (Tur), prismatine (Prs) and grandidierite (Gdd) is exposed in the ca. 1000Ma Brattstrand Paragneiss in the Larsemann Hills, East Antarctica. The B isotope composition of these three minerals in 21 paragneisses and 6 anatectic pegmatites were measured in situ using secondary ion mass spectrometry. δ11B ranges from −2.8 to −14.4‰ in tourmaline, from −9.6 to −17.8‰ in prismatine and from–2.8 to −8.7‰ in grandidierite (weighted uncertainties mostly ±1–2‰ per sample). In most cases, average δ11B increases in a given sample Prs<Tur<Gdd. Whether the observed B-isotope distribution has attained equilibrium were assessed using two criteria: microstructural equilibrium and regular distribution of Mg and Fe. In samples for which the two criteria are met, we assume the measured distribution of isotopes Δ11BA–B (=δ11BA−δ11BB) represents equilibrium: Δ11BTur-Prs= +5.0±1.4‰ (5 of 12 pairs), Δ11BTur -Gdd=−3.3±0.8‰ (3 of 5 pairs), and Δ11BGdd-Prs= +7.2±1.3‰ (2 of 2 pairs), consistent with the preference of 10B for tetrahedral sites (prismatine) and 11B for trigonal sites (tourmaline, grandidierite). Ab initio computations of B isotope fractionation factors, Δ11BTur-Prs= +6.4±1.3‰, Δ11BTur -Gdd=−1.8±1.1‰, and Δ11BGdd-Prs= +8.2±1.1‰, provide confirmation of the measured fractionations. The computed, relaxed ionic structure of grandidierite gave a shorter (1.368Å) B–O bond length than in case of dravite (1.385Å), which explains the measured enrichment of grandidierite in 11B relative to tourmaline. The precursor of the B-rich rock least changed by metamorphism, tourmaline metaquartzite (δ11B(Tur)=−8.6‰ to −5.9‰), is interpreted to be a product of pre-metamorphic, hydrothermal B-metasomatism. Boron sources consistent with this δ11B range include oceanic crust, clastic sediments, volcanic rocks and non-marine evaporite borate, but not marine evaporite. However, dominance of metasediments in the Brattstrand Paragneiss, major and trace element compositions of the B-rich rocks and presence of abundant B are consistent with non-marine evaporite being the most important source of B for the B rich lithologies of the Larsemann Hills. A possible scenario for precursors of these rocks is a succession of clastic sediments, in part tuffaceous, intercalated with B-rich evaporite deposits in a continental rift basin, in which circulating hydrothermal fluids leached B from the evaporite and precipitated it as tourmaline in the associated clastic rocks to form the precursors to the tourmaline metaquartzites.

The Cihai diabase in the Beishan region, NW China: Isotope geochronology, geochemistry and implications for Cornwall-style iron mineralization

Diabase dykes in Cihai, Beishan region, NW China are spatially and temporally associated with ‘Cornwall-type’ iron deposits. U–Pb dating of zircons from a diabase dyke using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) yields an age of 128.5±0.3Ma, indicating an Early Cretaceous crystallization age. Most of the diabases show low Mg-numbers, suggesting evolved magmas. The diabase dykes show typical ophitic or sub-ophitic textures, and are dominantly composed of phenocrysts of plagioclase (40–50%) and clinopyroxene (30–45%), with minor and varying amounts of biotite and hornblende (1–5%), and minor disseminated magnetite (∼5%). Their mineralogy reflects magma differentiation under relatively low oxygen fugacity conditions. The diabase dykes are characterized by minor variation in SiO2 (44.67–49.76wt.%) and MnO (0.14–0.26wt.%), but show a marked range of Al2O3 (10.66–14.21wt.%), total Fe2O3 (9.52–13.88wt.%), TiO2 (0.66–2.82wt.%) and relatively high MgO (4.87–9.29wt.%) with an Mg# value [atomic Mg/(Mg+Fe2+)] of up to 66. The Cihai diabases possibly experienced fractional crystallization of olivine+clinopyroxene and minor crustal contamination during the differentiation process. Prominent negative Nb, Ta and Ti anomalies suggest derivation from subduction-modified mantle. Furthermore, the rocks have relatively unradiogenic Sr- and Nd-isotopic ratios. These characteristics probably reflect partial melting of a subduction component in the source mantle lithosphere through heat input from an upwelling asthenospheric mantle. Such processes probably occurred within an extensional setting during the Early Cretaceous in the Beishan area. The iron-rich fluids were derived from deep sources, and the iron ores were concentrated through a convection cell driven by temperature gradients established by the intrusion of the diabase sills. The combined processes of subduction-related enrichment in the source, shallow depth of emplacement, and the involvement of large-scale circulation of basinal brines from an evaporitic source are inferred to have contributed to the formation of the ‘Cornwall-type’ mineralization in Cihai.

Geochemistry of Evaporites in Lacustrine Basin, Dongying Depression, Bohai Bay Basin, China

The middle and lower parts of the Fourth Member of the Shahejie Formation in the Dongying Depression are representatives of evaporites and mud rocks in Eastern China. To investigate hydrocarbon potential, some deep wells, such as the Hk1 well and Fs2 well, were intensively cored and evaluated by geochemistry. The total salinity of the formation waters is very high and is dominated by CaCl2, suggesting favorable seal development and preservation of petroleum. The content of Al, Mn, and Ba and the ratios of V/Ni and (Fe+Al)/(Ca+Mg) were low, whereas the content of Na, Ca, and Mg and the ratios of Sr/Ba, Fe/Mn, and Sr/Ca were high in the evaporites and mudstones of the Fourth Member compared with those in the mudstones of the Third Member. Sr content was remarkably high. The co-sedimentary system of evaporites and mudstones displays a heavier isotope of carbon. The values of δ13C for different group compositions and n-alkanes ranged from -28‰ to -21‰ and -18‰ to -32‰, the fractionation of δ13C between group compositions was no greater than -2‰, and δ13C of C17 and C18 significantly fluctuated. δ13C for high-maturity evaporite source rocks was significantly increased by thermal evolution and thermochemical sulfate reduction. High content of gammacerane and low Pr/Ph was exhibited in the evaporite system compared with that in the non-evaporite system. Different responses of biomarker parameters for the different sedimentary systems were exhibited, such as C19/(C19+C23) terpanes, C29/(C27+C28+C29) steranes, and C24/C23 and C22/C21 tricyclic terpanes. The reverse trend of the maturation parameters at depths corresponding to abundant carbonate minerals indicates that carbonate retards the thermal evolution of organic matter. Evaporites and mudstones can generate and expel hydrocarbons. The tested samples were mostly types I and II1 of organic matter, and their original generating capacity can reach 40 and 20 mg/g rock, respectively. The efficiency of hydrocarbon expulsion reached 60%, but the distribution of organic matter and its generative potential were highly variable. In general, the mudstones show greater generative potential than the evaporites.

Hydrothermal fluid source constrained by Co/Ni ratios in coexisting arsenopyrite and tourmaline: the auriferous lode of Passagem, Quadrilátero Ferrífero of Minas Gerais, Brazil

The auriferous lode of Passagem de Mariana is characterised by abundant tourmaline, which is intergrown with arsenopyrite. Spot measurements using laser ablation–inductively coupled plasma–mass spectrometry show that Co and Ni are the most abundant trace elements in the arsenopyrite (45–538 ppm Co, 246–828 ppm Ni), with Co/Ni ratios consistently <1. The coexisting tourmaline also has Co/Ni <1, with Co and Ni contents that are ~2 orders of magnitude lower than those in the arsenopyrite. The Co/Ni ratios of tourmaline and arsenopyrite are tightly distributed along a positive linear trend, the angular coefficient of which represents the Co/Ni of the hydrothermal fluid from which these minerals precipitated. The fluid Co/Ni ratio is close to the average Co/Ni value for the upper continental crust. In conjunction with the abundance of lode tourmaline and its B-isotope data (from the literature), the Co/Ni ratios of tourmaline and arsenopyrite fingerprint a continental evaporitic source of B.

Boron and lithium isotopic compositions as provenance indicators of Cu-bearing tourmalines

AbstractThe Li and B isotopic compositions of gem-quality Cu-bearing tourmalines were used (1) to distinguish among Paraiba tourmalines from Brazil and Cu-bearing tourmalines from Nigeria and Mozambique; and (2) to identify the likely source of Li and B for these gem-quality tourmalines. The δ11B values of tourmaline from Paraiba, Brazil, range from –42.4‰ to –32.9‰, whereas the δ11B values of Cu-bearing tourmaline from Nigeria and Mozambique range from –30.5‰ to –22.7‰ and –20.8‰ to –19.1‰ respectively. Tourmalines from each locality have relatively homogeneous δ11B values and display no overlap. There is slight overlap between δ7Li values of Paraiba tourmaline (+24.5‰ to +32.9‰) and Cu-bearing tourmaline from Nigeria (+32.4‰ to +35.4‰), and δ7Li values of Cu-bearing tourmaline from Nigeria and Mozambique (+31.5‰ to +46.8‰). Nevertheless, Cu-bearing tourmalines from each locality can be fingerprinted using a combination of their δ11B and δ7Li values. The very small δ11B values are consistent with a non-marine evaporite source, and are among the smallest reported for magmatic systems, expanding the global range of B isotopicc omposition for tourmaline by 12‰. The corresponding large δ7Li values are among the largest reported, although they are less diagnostic of the source of the Li. The large δ7Li values in conjunction with the small δ11B values suggest a non-marine evaporite or brine as a source for Li and B, either as constituent(s) of the magma source region or, by assimilation during magma ascent. The large range in δ11B and δ7Li values suggests that B and Li isotope fractionation occurred during magmatic degassing and late-stage magmatic-hydrothermal evolution of the granite-pegmatite system.

Selected ore deposits of Brazil

This series of papers on a variety of ore deposits in Brazil is the outgrowth of the “First Brazilian Symposium on Metallogeny” held in Gramado, Rio Grande do Sul, in May 2005. The selection of papers mirrors quite well the range of commodities and fascinating diversity of ore deposit types Brazil has to offer. The thematic issue starts with a paper by Monteiro et al. who use detailed petrography to establish a consistent paragenetic sequence of alteration and mineralization throughout the Sossego iron–oxide–copper–gold (IOCG) deposits in the giant Carajas Mineral Province. Combined with stable isotope data, it allows the authors to constrain the spatial and temporal zoning of hydrothermal alteration and mineralization. They conclude that the Pista-SequerinhoBaiano and Sossego-Curral ore bodies formed at a deep and high structural level of an IOCG system, respectively. Significantly, the copper–gold mineralization was late in the alteration history and broadly synchronous in both ore bodies. Another contribution to the ore deposit geology of the Carajas Mineral Province is by Dreher et al. who use new geologic, fluid inclusion and stable isotope data to establish that the Igarape Bahia Cu–Au deposit displays both characteristic syngenetic VHMS and epigenetic IOCG style geological features. This controversial paper provides evidence via recent B isotope data on tourmaline that there is the distinct possibility of an involvement of a marine evaporitic source in the hydrothermal system of Igarape Bahia. The authors carefully evaluate evidence for both genetic models and correctly agree that more detailed work needs to be conducted to resolve this fascinating genetic conundrum. Souza Neto et al. provide an overview of W–Au skarns in the Serido mobile belt in the Borborema Province in northeast Brazil. Special emphasis is put on the reduced Bonfim Au–Bi–Te skarn deposit. The majority of skarns in the Serido mobile belt are oxidized tungsten skarns even though the Itajubatiba and Bonfim gold-bearing skarns display reduced features, such as pyrrhotite as predominant sulfide, garnet with high almandine plus spessartite component, and elevated gold content. PIXE data revealed that significant amounts of F (up to 9,490 ppm) are present in prograde titanite, and retrograde clinozoisite–zoisite and gold-related epidote, suggesting significant F activity in the skarn forming hydrothermal fluids. Beurlen et al. in a second paper on mineralization in the Borborema Province use Nb–Ta–(Ti–Sn)–oxide mineral chemistry as tracer of rare-element granitic pegmatite fractionation in the pegmatite-rich Borborema Province. The Borborema Pegmatite Province, with over 750 registered mineralized rare-element granitic pegmatites, is a historically important tantalum province and is currently well known for top-quality gems, such as the Paraiba Elbaite. Detailed electron microprobe analyses of pegmatite Miner Deposita (2008) 43:127–128 DOI 10.1007/s00126-007-0174-y

New geologic, fluid inclusion and stable isotope studies on the controversial Igarapé Bahia Cu–Au deposit, Carajás Province, Brazil

The Igarape Bahia Cu–Au deposit in the Carajas Province, Brazil, is hosted by steeply dipping metavolcano-sedimentary rocks of the Igarape Bahia Group. This group consists of a low greenschist grade unit of the Archean (∼2,750 Ma) Itacaiunas Supergroup, in which other important Cu–Au and iron ore deposits of the Carajas region are also hosted. The orebody at Igarape Bahia is a fragmental rock unit situated between chloritized basalt, with associated hyaloclastite, banded iron formation (BIF), and chert in the footwall and mainly coarse- to fine-grained turbidites in the hanging wall. The fragmental rock unit is a nearly concordant, 2 km long and 30–250 m thick orebody made up of heterolithic, usually matrix-supported rocks composed mainly of coarse basalt, BIF, and chert clasts derived from the footwall unit. Mineralization is confined to the fine-grained matrix and comprises disseminated to massive chalcopyrite accompanied by magnetite, gold, U- and light rare earth element (LREE)-minerals, and minor other sulfides like bornite, molybdenite, cobaltite, digenite, and pyrite. Gangue minerals include siderite, chlorite, amphibole, tourmaline, quartz, stilpnomelane, epidote, and apatite. A less important mineralization style at Igarape Bahia is represented by late quartz–chalcopyrite–calcite veins that crosscut all rocks in the deposit area. Fluid inclusions trapped in a quartz cavity in the ore unit indicate that saline aqueous fluids (5 to 45 wt% NaCl + CaCl2 equiv), together with carbonic (CO2 ± CH4) and low-salinity aqueous carbonic (6 wt% NaCl equiv) fluids, were involved in the mineralization process. Carbonates from the fragmental layer have δ13C values from −6.7 to −13.4 per mil that indicate their origin from organic and possibly also from magmatic carbon. The δ34S values for chalcopyrite range from −1.1 to 5.6 per mil with an outlier at −10.8 per mil, implying that most sulfur is magmatic or leached from magmatic rocks, whereas a limited contribution of reduced and oxydized sulfur is also evident. Oxygen isotopic ratios in magnetite, quartz, and siderite yield calculated temperatures of ∼400°C and δ18O-enriched compositions (5 to 16.5 per mil) for the ore-forming fluids that suggest a magmatic input and/or an interaction with 18O-rich, probably sedimentary rocks. The late veins of the Igarape Bahia deposit area were formed from saline aqueous fluids (2 to 60 wt% NaCl + CaCl2 equiv) with δ18Ofluid compositions around 0 per mil that indicate contribution from meteoric fluids. With respect to geological features, Igarape Bahia bears similarity with syngenetic, volcanic-hosted massive sulfide (VHMS)-type deposits, as indicated by the volcano-sedimentary geological context, stratabound character, and association with submarine volcanic flows, hyaloclastite, and exhalative beds such as BIF and chert. On the other hand, the highly saline ore fluids and the mineral assemblage, dominated by magnetite and chalcopyrite, with associated gold, U- and LREE-minerals and scarce pyrite, indicate that Igarape Bahia belongs to the Fe oxide Cu–Au (IOCG) group of deposits. The available geochronologic data used to attest syngenetic or epigenetic origins for the mineralization are either imprecise or may not represent the main mineralization episode but a later, superimposed event. The C, S, and O isotopic results obtained in this study do not clearly discriminate between fluid sources. However, recent B isotope data obtained on tourmaline from the matrix of the fragmental rock ore unit (Xavier, Wiedenbeck, Dreher, Rhede, Monteiro, Araujo, Chemical and boron isotopic composition of tourmaline from Archean and Paleoproterozoic Cu–Au deposits in the Carajas Mineral Province, 1° Simposio Brasileiro de Metalogenia, Gramado, Brazil, extended abstracts, CD-ROM, 2005) provide strong evidence of the involvement of a marine evaporitic source in the hydrothermal system of Igarape Bahia. Evaporite-derived fluids may explain the high salinities and the low reduced sulfur mineral paragenesis observed in the deposit. Evaporite-derived fluids also exclude a significant participation of magmatic or mantle-derived fluids, reinforcing the role of nonmagmatic brines in the genesis of Igarape Bahia. Considering this aspect and the geological features, the possibility that the deposit was generated by a hydrothermal submarine system whose elevated salinity was acquired by leaching of ancient evaporite beds should be evaluated.

Carbonation of Cl‐rich scapolite boudins in Skallen, East Antarctica: evidence for changing fluid condition in the continental crust

AbstractSpectacular reaction textures in poikiloblastic scapolitite boudins, within marbles in the continental crust exposed in the Lützow–Holm Complex, East Antarctica, provide insights into the changing fluid composition and movement of fluid along grain boundaries and fractures. Petrographic and geochemical features indicate scapolite formation under contrasting fluid compositions. Core composition of scapolite poikiloblasts (ScpI) are marialitic (Cl = 0.7 apfu) whereas rims in contact with biotite or clinopyroxene are meionite rich. Fine‐grained recrystallized equigranular scapolite (ScpII) shows prominent chemical zoning, with a marialitic core and a meionitic rim (Cl = 0.36 apfu). Scapolite poikiloblasts are traversed by ScpIII reaction zones along fractures with compositional gradients. Pure CO2 fluid inclusions are observed in healed fractures in scapolite poikiloblasts. These negative crystal‐shaped fluid inclusions are moderately dense, and are believed to be coeval with ScpIII formation at temperatures &gt;600 °C and a minimum pressure of c. 3.8 kbar. Grain‐scale LA‐ICPMS studies on trace and rare earth elements on different textural types of scaplolites and a traverse through scapolite reaction zone with compositional gradient suggest a multistage fluid evolution history. ScpI developed in the presence of an internally buffered, brine‐rich fluid derived probably from an evaporite source during prograde to peak metamorphism. Recrystallization and grain size reduction occurred in the presence of an externally sourced carbonate (CaCO3)‐bearing fluid, resulting in the leaching of Cl, K, Rb and Ba from ScpI along fractures and grain boundaries. Movement of fluids was enhanced by micro‐fracturing during the transformation of ScpI to ScpIII. Fractures in fluorapatite are altered to chlorapatite proving evidence for the pathways of escaping Cl‐bearing fluids released from ScpI. The present study thus provides evidence for the usefulness of scapolite in fingerprinting changing volatile composition and trace element contents of fluids that percolate within the continental crust.

Stratigraphy and sedimentology of a dry to wet eolian depositional system, Burns formation, Meridiani Planum, Mars

Outcrop exposures of sedimentary rocks at the Opportunity landing site (Meridiani Planum) form a set of genetically related strata defined here informally as the Burns formation. This formation can be subdivided into lower, middle, and upper units which, respectively, represent eolian dune, eolian sand sheet, and mixed eolian sand sheet and interdune facies associations. Collectively, these three units are at least 7 m thick and define a “wetting-upward” succession which records a progressive increase in the influence of groundwater and, ultimately, surface water in controlling primary depositional processes. The Burns lower unit is interpreted as a dry dune field (though grain composition indicates an evaporitic source), whose preserved record of large-scale cross-bedded sandstones indicates either superimposed bedforms of variable size or reactivation of lee-side slip faces by episodic (possibly seasonal) changes in wind direction. The boundary between the lower and middle units is a significant eolian deflation surface. This surface is interpreted to record eolian erosion down to the capillary fringe of the water table, where increased resistance to wind-induced erosion was promoted by increased sediment cohesiveness in the capillary fringe. The overlying Burns middle unit is characterized by fine-scale planar-laminated to low-angle-stratified sandstones. These sandstones accumulated during lateral migration of eolian impact ripples over the flat to gently undulating sand sheet surface. In terrestrial settings, sand sheets may form an intermediate environment between dune fields and interdune or playa surfaces. The contact between the middle and upper units of the Burns formation is interpreted as a diagenetic front, where recrystallization in the phreatic or capillary zones may have occurred. The upper unit of the Burns formation contains a mixture of sand sheet facies and interdune facies. Interdune facies include wavy bedding, irregular lamination with convolute bedding and possible small tepee or salt-ridge structures, and cm-scale festoon cross-lamination indicative of shallow subaqueous flows marked by current velocities of a few tens of cm/s. Most likely, these currents were gravity-driven, possibly unchannelized flows resulting from the flooding of interdune/playa surfaces. However, evidence for lacustrine sedimentation, including mudstones or in situ bottom-growth evaporites, has not been observed so far at Eagle and Endurance craters. Mineralogical and elemental data indicate that the eolian sandstones of the lower and middle units, as well as the subaqueous and eolian deposits of the Burns upper unit, were derived from an evaporitic source. This indirectly points to a temporally equivalent playa where lacustrine evaporites or ground-water-generated efflorescent crusts were deflated to provide a source of sand-sized particles that were entrained to form eolian dunes and sand sheets. This process is responsible for the development of sulfate eolianites at White Sands, New Mexico, and could have provided a prolific flux of sulfate sediment at Meridiani. Though evidence for surface water in the Burns formation is mostly limited to the upper unit, the associated sulfate eolianites provide strong evidence for the critical role of groundwater in controlling sediment production and stratigraphic architecture throughout the formation.

Application of geochemical parameters for classification of crude oils from Egypt into source-related types

Seven crude oils representing the different petroleum-bearing basins in the Western Desert were characterized by a variety of biomarker and nonbiomarker parameters. For comparison, one crude oil from the Gulf of Suez region has also been studied. The oils have been analyzed for geochemical biomarkers using GC and GC-MS techniques. The results reveal significant differences within the oils that suggest five oil types. Type 1 oils from the Gulf of Suez show C 29/C 30 17α(H) hopane ratio >1, high C 35 homohopane index, and the presence of considerable amounts of gammacerane indicating a marine saline carbonate or evaporite source rock and highly reducing (low Eh) conditions. Type 2 oils from Matruh basin and type 3 oils from Shushan basin are very similar and show relatively high Pr/Ph ratios, low sulfur and metal contents, paucity of C 30 steranes and the presence of high relative abundance of 17α(H) diahopane (C 30*) suggesting that they probably originated from source rocks containing a significant proportion of higher plant material. Another related feature of these oils is the absence of 18α(H)-oleanane which suggests a source age older than Cretaceous. Type 4 oils from Abu-Elgharadig basin show terpane distribution dominated by C 24 tricyclic, absence of C 30*-diahopane and medium diasterane/sterane ratios which reflect generation from marine siliciclastic source rocks. Type 5 oils from Alamein basin possess source biomarkers indicating a mixed contribution of terrestrial and marine sources. The presence of measurable amounts of oleanane in this type of oils suggests source rocks deposited in deltaic or near shore environment in Post-Cretaceous basin.

Major element, rare earth element, and sulfur isotopic composition of a high‐elevation firn core: Sources and transport of mineral dust in central Asia

The arid region extending from the Caspian Sea into China contains some of the most extensive dust aerosol production areas in the world. Major element (Al, Fe, Ca, and S), rare earth element (REE), and isotopic (δ34S and δ18O) data from a 14.4‐m‐long firn core recovered on the Inilchek Glacier (Tien Shan Mountains, Kyrgyzstan; 5100‐m elevation) are presented and used to investigate the composition, sources, and transport of mineral dust to this central Asian site. The combination of geochemical compositions points to three distinct types of mineral deposits (loess, calcium carbonate, and gypsum), which do not covary in a linear fashion during the period recorded in the firn core, (1992–1998). Loess, identified by its REE composition, is characterized by distinct large‐scale deposition events and by ubiquitous background deposition at other times. Significant interannual‐scale variability is evident, with the two largest loess events occurring within a single accumulation year, (1997) and background conditions prevailing during 1994–1996. REE compositions, specifically Nd/Yb ratios, point to multiple sources and transport directions of loess to Inilchek Glacier. Nonloess sources provide a significant fraction of the total Ca and S deposited at the Inilchek Glacier. Hence the downcore profiles of Ca and S reflect variability in the deposition of calcium carbonate and gypsum. Differences in the sulfur isotopic composition (δ34S) of high dust period (+15.0‰) and low (background) dust period (+5.4‰) samples provide strong evidence for two sources of sulfate. The former isotopic composition reflects a marine evaporite source of which there are many to the west of the Tien Shan Mountains. The latter composition indicates that a major portion of the atmospheric SO42− was derived from anthropogenic emissions. While the deposition of non‐loess Ca (calcite) and non‐loess S (gypsum) is also high during the periods of high loess deposition, there is pronounced Ca and S variability during background loess periods, (1994–1996). We suggest that the observed geochemical variability in the firn core can be explained by changes in local‐ to regional‐scale atmospheric circulation and associated transport from multiple dust sources (western Kyrgyzstan/Kazakhstan versus the Taklimakan Desert, China).

Metasomatic alteration associated with regional metamorphism: an example from the Willyama Supergroup, South Australia

The Olary Domain, part of the Curnamona Province, a major Proterozoic terrane located within eastern South Australia and western New South Wales, Australia, is an excellent example of geological region that has been significantly altered by metasomatic mass-transfer processes associated with regional metamorphism. Examples of metasomatically altered rocks in the Olary Domain are ubiquitous and include garnet–epidote-rich alteration zones, clinopyroxene- and actinolite-matrix breccias, replacement ironstones and albite-rich alteration zones in quartzofeldspathic metasediments and intrusive rocks. Metasomatism is typically associated with formation of calcic, sodic and/or iron-rich alteration zones and development of oxidised mineral assemblages containing one or more of the following: quartz, albite, actinolite–hornblende, andradite-rich garnet, epidote, magnetite, hematite and aegerine-bearing clinopyroxene. Detailed study of one widespread style of metasomatic alteration, garnet–epidote-rich alteration zones in calc-silicate host rocks, provides detailed information on the timing of metasomatism, the conditions under which alteration occurred, and the nature and origin of the metasomatic fluids. Garnet–epidote-bearing zones exhibit features such as breccias, veins, fracture-controlled alteration, open space fillings and massive replacement of pre-existing calc-silicate rock consistent with formation at locally high fluid pressures and fluid/rock ratios. Metasomatism of the host calc-silicate rocks occurred at temperatures between ∼400°C and 650°C, and involved loss of Na, Mg, Rb and Fe 2+, gain of Ca, Mn, Cu and Fe 3+ and mild enrichment of Pb, Zn and U. The hydrothermal fluids responsible for the formation of garnet–epidote-rich assemblages, as well as those involved in the formation of other examples of metasomatic alteration in the Olary Domain, were hypersaline, oxidised, and chemically complex, containing Na, Ca, Fe 3+, Cl, and SO 4 2−. Sm–Nd geochronology indicates that the majority of garnet–epidote alteration occurred at 1575±26 Ma, consistent with field and petrographic observations that suggest that metasomatism occurred during the retrograde stages of a major amphibolite-grade regional metamorphic event, and prior to the latter stages of regional-scale intrusion of S-type granites at 1600±20 Ma. The fluids responsible for metasomatism within the Olary Domain are inferred to have been derived from devolatilisation of a rift-related volcano-sedimentary sequence, perhaps containing oxidised and evaporitic source rocks at deeper structural levels, during regional metamorphism, deformation and intrusion of granites. At the present structural level, there is no unequivocal evidence for the fluids to have been directly sourced from granites.