Meteoric Fluids Research Articles

Mineralogy, Geochemistry and Fluid Inclusion Data from the Tumanpınarı Volcanic Rock-Hosted Fe-Mn-Ba Deposit, Balıkesir-Dursunbey, Turkey

The Tumanpınarı mineralization is a volcanic rock-hosted epithermal Fe-Mn-Ba deposit located in the southwestern part of Dursunbey, Balıkesir, Turkey. The deposit constitutes one of the most important deposits of the Havran-Dursunbey metallogenic sub-province in which numerous Early Miocene Fe-Mn-Ba deposits are distributed. The ore occurs as open-space fillings in faults, fractures, and breccias in the andesite. Early hydrothermal activity was responsible for four types of hypogene alteration in decreasing intensity: silicification, sericitization, hematization and argillic alteration. The mineral assemblage includes pyrolusite, psilomelane, hematite, and barite as well as minor magnetite, manganite, poliannite, limonite, braunite, bixbyite, galena, pyrite, and goethite. Mineralogically, three ore types are recognized as pyrolusite + psilomelane + hematite + barite ore, pyrolusite + psilomelane + poliannite ore and barite + pyrolusite + psilomelane + hematite ore (barite-dominant ore). In addition to Fe, Mn and Ba, the ore contains substantial quantities of Pb, Zn, As. Chemically, the transition from fresh to altered rocks has little effect on the elemental levels for Si, Al, Fe, Ca, Mg, K, Rb, Sr and H2O. The homogenization temperature of fluid inclusions hosted in the main stage quartz and barite ranged from 113 to 410 °C with salinities ranging from 0.4 to 14.9 eq. wt % NaCl, respectively. Overall, the available data suggest that the deposits formed as the result of the interaction of two aqueous fluids: a higher-salinity fluid (probably magmatic) and a dilute meteoric fluid.

Paleofluids circulation associated with the Gerês late-orogenic granitic massif, northern Portugal

The Hercynian late-orogenic granites from the Gerês massif, northern Portugal, underwent intense hydrothermal activity along tectonic structures striking N-S and NE-SW. The first hydrothermal stage is characterized by the albitization of feldspars (primary K-feldspar and plagioclase) followed by the dissolution of magmatic quartz, the chloritization of biotite, and the muscovitization of magmatic biotite and feldspars and occasionally of authigenic albite. Whole-rock geochemistry shows a decrease of SiO2, K2O and Rb and an increase of Na2O, Al2O3 and Sr amounts during the albitization. The second hydrothermal stage is characterized by a mineral assemblage consisting of secondary quartz, albite, chlorite, hematite, apatite, muscovite, epidote, sphene and carbonates, which infilled the cavities produced by the early quartz leaching.Mass balance calculations demonstrate a noticeable element mobilization during the granite alteration. The average ∑REE of the Gerês granite is nearly constant at 154ppm, but ranges from 91 to 163ppm in the altered rocks. A relative LREE depletion and a slight HREE enrichment associated with a negative Eu anomaly characterize the feldspathization process of Gerês granite. The average ∑REE of the Carris granite is at 159ppm but in the altered epidote+chlorite+hematite assemblage it reaches about 201ppm with a slight HREE increase.Two different fluids were involved in the hydrothermal alteration. A first aqueous fluid (Lw1), with a low to intermediate salinity (<10wt.% eq. NaCl) circulated along the main structures (N-S, NE-SW and NW-SE), characterized by entrapment temperature lower than 350°C and maximum pressure of 115MPa followed by a later colder and more saline fluid (Lw2) under a temperature of 220°C and a pressure of about 27MPa.Oxygen isotope data obtained on magmatic quartz indicate δ18Oquartz of+9.3 and+11.0‰, pointing to an interaction of magmatic fluids with others of meteoric origin. For the secondary quartz, δ18Owater of +0.5 and +0.7‰ were calculated for a temperature of 250°C, suggesting a meteoric fluid with a possible seawater signature.The K-Ar data of K-feldspar from the altered rocks confirm an age of 273.6±11.7 Ma, attributed to the first alteration process. Younger K-Ar ages between 155.8±6.7 Ma and 124±5.3 Ma were also obtained in the feldspathized rocks, confirming the late hydrothermal activity.Albitization and quartz dissolution of granitic rocks from the Gerês massif occurred at depths shallower than 5km, induced by the circulation of fluids along brittle structures, during the orogenic uplift and extensional tectonics which affected the Iberian Massif in the Early Permian. A second major event is attributed to late hydrothermal circulations of aqueous brines until a depth of about 3km, presumably derived from interaction with sub-surface evaporites throughout Late Jurassic to Early Cretaceous. These late hydrothermal events probably reflect the rifting episodes and the rising of geothermal gradient, associated with the opening of the Atlantic Ocean and Gulf of Biscay, respectively.

Meteoric fluid infiltration in crustal-scale normal fault systems as indicated by δ18O and δ2H geochemistry and40Ar/39Ar dating of neoformed clays in brittle fault rocks

Both the sources and pathways of fluid circulation are key factors to understanding the evolution of low-angle normal fault (LANF) systems and the distribution of mineral deposits in the upper crust. In recent years, several reports have shown the presence of meteoric waters in mylonitic LANF systems at mid-crustal conditions. However, a mechanism for meteoric water infiltration to these mid-crustal depths is not well understood. Here we report paired δ 18 O and δ 2 H isotopic values from dated, neoformed clays in fault gouge in major detachments of the southwest United States. These isotopic values demonstrate that brittle fault rocks formed from exchange with pristine to weakly evolved meteoric waters at multiple depths along the detachment. 40 Ar/ 39 Ar dating of these same neoformed clays constrains the Pliocene ages of fault-gouge formation in the Death Valley area. The infiltration of ancient meteoric fluids to multiple depths in LANFs indicates that crustal-scale normal fault systems are highly permeable on geologic timescales and that they are conduits for efficient, coupled flow of surface fluids to depths of the brittle-plastic transition.

Basinal Brines at the Origin of the Imiter Ag-Hg Deposit (Anti-Atlas, Morocco): Evidence from LA-ICP-MS Data on Fluid Inclusions, Halogen Signatures, and Stable Isotopes (H, C, O)

The Imiter Ag-Hg deposit is located in the Precambrian volcano-sedimentary formations of the Saghro inlier (eastern part of the Anti-Atlas Mountains, Morocco). The orebodies consist of northeast-southwest to east-west veins and lenses hosted by Cryogenian black shales and gray-wackes and Neoproterozoic conglomerates, and are controlled by an east-west fault network, the so-called Imiter fault. Mineralogical and paleo-fluid geochemistry investigations (microthermometry, Raman spectroscopy, LA-ICP-MS on individual inclusions, bulk crush-leach analyses, and stable isotope data (O, H)) show that the main Ag ore stage is related to circulation of deep-basinal sedimentary brines (Na-K-(Mg) (salinity = 16.7 to ≥26 wt % NaCl equiv, molar Cl/Br = 330, δ 18 O = 2.15–2.35‰ SMOW , and δ D = −53.8 to −65.5‰ SMOW ), at temperatures of about 180° to 220°C and hydrostatic pressures. The main driving mechanism for silver ore deposition is the dilution of ore-bearing brines by a low-salinity meteoric fluid containing a low-density volatile component (N 2 > CH 4 > CO 2 ), T h = 180° to 220°C, δ 18 O = −1.4‰ SMOW , and δ D of about −28.2‰ SMOW . Silver content of the brines ranges from 2 to 30 mmol/kg solution (up to 3,200 ppm Ag, avg Ag concentration about 900 ppm), whereas the maximum Ag content found in dilute waters is about 0.4 mmol/kg solution (40 ppm). The ore-forming model proposed for the Imiter deposit is (1) Ag extraction from the basement by the penetration of deep-basinal brines, and (2) deposition in a structural trap through fluid mixing with recharge fluids. This model is comparable to that described worldwide for the origin of Pb-Zn, F, Ba, and U deposits near unconformities between basement and sedimentary basins. Similarities among the major Ag deposits from the Anti-Atlas (Imiter, Zgounder, Bou Azzer) strongly suggest that they resulted from a unique event, likely related to the opening of the Atlantic Ocean. The silver ores are superimposed on the same lineament as a preexisting uneconomic Pan-African Co-Ni-As system linked to magmatic intrusions, but Ag ores have no genetic relationship with them.

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.

High-strain deformation and fluid infiltration diachronism of the middle crust: New Devonian–Permian Alice Springs ages (365–290 Ma) of shear zones in the Strangways Metamorphic Complex, Central Australia

The Proterozoic granulite-facies Strangways Metamorphic Complex (SMC) in the Arunta Region of Central Australia was partially hydrated during the Devonian–Carboniferous (380–320Ma) Alice Springs Orogeny by amphibolite-greenschist-facies high-strain deformation. Stable and radiogenic isotope analyses on coeval muscovite and biotite from shear-zone samples of the SMC were performed to analyse the composition and date the timing of fluid circulation and cooling.Stable isotope data (δ18O and δD) indicate that in the south of the SMC, the source of fluids appear to be the thick successions of Neoproterozoic Heavitree Quartzite, while another source in the north and central part of the SMC may be related to meteoric fluids. The heterogeneous δ18OFluid and homogenous δDFluid values for muscovite reflect changes in water/rock ratios with initial high values in the south and lower in the north, possibly suggesting a migration of fluids from south to north (as indicated by the evolving δ18OFluid and homogenous δDFluid values). Rb/Sr muscovite ages of 365–340Ma approximate the time of crystallization and fluid infiltration. They also indicate that the shear zones were active at different times during the Alice Springs Orogeny and that some were active prior to the peak Carboniferous amphibolite-facies metamorphism. 40Ar/39Ar muscovite ages of 350–325Ma are mostly interpreted as cooling ages. Rb/Sr biotite ages of 335–290Ma are interpreted as cooling ages at ~300°C (Sr closure temperature for biotite) and extend the activity of the Alice Springs Orogeny until the Permian.Cooling histories of the Wallaby Knob and Harry Creek shear zones, which serve as the tectonic boundaries of the SMC to the north and south respectively, indicate slow cooling (<3.5°CMa−1) for c. 30Ma (360–330Ma) at both boundaries continuing until 290Ma in the western portion of the Harry Creek shear zone. This slow cooling is associated with passive exhumation (i.e. erosion). Rapid cooling (20–10°CMa−1) then occurred over c. 10Ma (330–320Ma) in the Wallaby Knob shear zone and in the east portion of the Harry Creek shear zone. This change in cooling rate is associated with the combined initiation in the Carboniferous of dextral and thrusting deformation by the Pinnacles Bore and Erontonga shear zones.

Carbonates and sources of fluids in ores and metasomatites of the Ermakovka fluorite-bertrandite-phenacite deposit (western Transbaikalia)

We present results of isotope-geochemical study of the Ermakovka F-Be deposit, including data on the oxygen and carbon isotope compositions in dolomite and calcite marbles and in carbonates accompanying skarns, of early and late stages of ore formation and of post-ore parageneses. To elucidate the sources of fluids participated in the ore formation, we calculated the oxygen isotope composition in water and the hydrogen isotope composition in hydroxyl-containing minerals. Phlogopite in marbleized dolomites, vesuvianite and amphibole in skarns, eudidimite and bertrandite in ore parageneses, and bavenite formed during post-ore processes are analyzed. Most of the ore-stage minerals are depleted in heavy oxygen. Their 518O values are lower than 5-6%c (SMOW). Oxygen in carbonate minerals of the initial stage (dolomite and bastnaesite) is heavier (1.3-4.9%c) than that in calcite (+2 to -3.7%c). The 518O values of water in equilibrium both with carbonate and with silicate minerals (-4 to -14%c) suggest the contribution of meteoric water to the mineral formation. A magmatic fluid (518O from +6 to +9%c) participated in the skarn formation at the initial stage, and a meteoric fluid, at the final stage (518O from -1 to -9%c). A meteoric source is confirmed by the depleted hydrogen isotope composition in minerals (5D from -119 to -192%c).

Reconstruction of Holocene paleohydrological and sedimentological characteristics of Umm Ar-Rimam basin (northern Kuwait)

The paleohydrological and sedimentological characteristics of a playa lake in northern Kuwait (Arabian Gulf) are reconstructed using sedimentological, geochemical, and isotopic techniques. The sequence consists of up to 8 cycles of S-poor, alluvial sediments capped by a thin organic soil interbedded with gravity-fall calcrete sediments. The succession is locally derived from mainly Quaternary sediments and is regressive with upsection filling of the subsiding basin by cycles of sheetwash flow in response to climatic change. There is no natural, open-water lake water as indicated by low total organic carbon (TOC) data, but the presence of incised calcrete yardangs suggests that more extensive open-water conditions were operative in the past. Stable isotope (δ18O‰ and δ13C‰) values of the authigenic carbonates indicate the following three distinct processes: evaporation, meteoric fluid infiltration, and rapid per-descensum flow (rapid downward movement of water and playa sediment through pipes) through a porous, clastic sequence. Because evaporites are scarce, other factors besides evaporation action control chemical and isotopic compositions of the per-decensum lake fluids. Consequently, the isotopic composition cannot be interpreted exclusively as an indicator of salinity or evaporation ratio. The degassing of CO2 during groundwater discharge may explain the enriched carbon isotope values for the authigenic carbonates precipitated in the sediments. Hydrologically closed lake water bodies tend to show low negative carbonate oxygen and carbon isotopic signatures. Isotopically negative δ13C values imply a strong input of soil-zone carbon to the groundwater of the top 60 cm of the sediment. Lakes that are hydrologically closed and evaporate or equilibrate with atmospheric CO2 will tend to have low negative δ18O and δ13C values in the carbonates as reported by Talbot (Chem Geol: Isotope Geosci Sect 80(4):261–279, 1990). Biologically active lakes will tend toward lower δ13C of dissolved carbon due to the photosynthetic effects of 12C withdrawal as reported by Dunagan and Driese (J Sed Res 69:772–783, 1999). Increased biological activity during sedimentation may account for low carbon isotope values where plants were abundant, but in shrinkage-dominated systems (those of clay-rich soil subjecting to wet-dry conditions), carbon isotopes will be largely inherited from the calcretic limestones in the land extending landward of the coast and not influenced by coastal processes (known as hinterland), such as Umm Ar-Rimam depression. This basin does not fit the classic shallow playa-type basins of the Arabian Peninsula but rather the recharge playas of the southwestern USA.

Fluid inclusion geochemistry and 40Ar/39Ar geochronology constraints on the genesis of the Jianchaling Au deposit, China

The giant Jianchaling gold deposit is located in the Shaanxi Province, China. The mineralization is hosted by WNW-trending faults in the Mianxian-Lueyang-Yangpingguan (MLY) area. The mineralization can be divided into three stages based on mineralogical assemblages and crosscutting relationships of mineralized quartz veins. These stages, from early to late, are characterized by the mineral assemblage of: (1) quartz – coarse-grained pyrite – pyrrhotite – pentlandite – dolomite; (2) quartz – pyrite – gold – sphalerite – galena – carbonate – arsenopyrite – fuchsite; and (3) dolomite – calcite – quartz – fine-grained pyrite – realgar – orpiment.Three types of fluid inclusions have been recognized in this study based on petrographic and microthermometric measurements, including pure CO2 and/or CH4 (PC-type), NaCl-H2O (W-type), and NaCl-CO2-H2O (C-type) fluid inclusions. These fluid inclusion types are present in quartz from the Stage 1 and 2 assemblages, whereas the Stage 3 quartz only contains W-type fluid inclusions. The Stage 2 assemblage is associated with the mineralization at the Jianchaling deposit. Fluid inclusions of Stage 1 quartz homogenize mainly between 250° and 360°C, with salinities up to 15.6wt.% NaCl equiv., whereas the Stage 3 dolomite with homogenization temperatures of 160° – 220°C and salinities of 1.1–7.4wt.% NaCl equiv. This indicates that the ore fluid system evolved from CO2-rich, probably metamorphic hydrothermal to CO2-poor, meteoric fluid. All three types of fluid inclusions can be observed in the Stage 2 quartz, suggesting that this heterogeneous association was trapped from a boiling fluid system. These inclusions homogenized at temperatures of 200°–250°C and salinities of 1.2–12.4wt.% NaCl equiv. The estimated trapping pressures of the fluid inclusions are between 117 and 354MPa in Stage 1, suggesting an alternating lithostatic–hydrostatic fluid system, which was controlled by a fault-valve at the depth of ~12km.Two fuchsite samples collected from the Stage 2 polymetallic-quartz veins yielded well-defined 40Ar/39Ar isotopic plateau ages of 197±2 and 194±2Ma, and 39Ar/36Ar-40Ar/36Ar normal isochrones of 198±2 and 199±2Ma. This indicates that the mineralization at Jianchaling is Early Jurassic (ca. 198Ma) in age. We propose that Jianchaling is an orogenic gold deposit, and formed during continental collision related to the northward subduction of the Mian-Lue oceanic plate during the Early Jurassic. We also conclude that the beginning of the continental collision between the Yangtze and the North China Cratons took place around 200Ma.

A fluid inclusion and stable isotope study of the Pebble porphyry copper-gold-molybdenum deposit, Alaska

The hydrothermal fluid evolution in the supergiant Pebble porphyry Cu-Au-Mo deposit in southwest Alaska has been constrained using fluid inclusion and light stable isotope data. The deposit is related to ~90Ma granodiorite porphyry intrusions and mineralization occurs primarily in sodic-potassic, potassic, and advanced argillic alteration zones.Throughout the deposit vein quartz hosts intermediate-density fluid inclusion assemblages with an average salinity of 9.5wt% NaCl equivalent and homogenization temperatures of 377 to 506°C. Pressure corrections result in trapping temperatures with a maximum of 610°C. This early fluid exsolved from a crystallizing magma at depth and underwent phase separation under varying conditions during cooling and depressurization forming two distinct fluid compositions. The two fluids formed two hydrothermal alteration assemblages which have different metal signatures. In the hydrothermal core in the eastern part of the system, high-density quartz veins, potassic alteration and the most economically significant mineralization are related to ~52wt% NaCl equivalent brines that coexist with very low-density vapor. In the western part of the system peripheral to the hydrothermal core, lower quartz vein densities accompanied by sodic-potassic alteration and lower grade mineralization are associated with ~38wt% NaCl equivalent brines and low-density vapor. Sulfide mineralization precipitated late in this early alteration event at temperatures between 375 and 330°C.Fluids related to sodic-potassic alteration minerals have δ18O signatures from +6.0 to +10.5‰ VSMOW and δD values from −70 to −41‰ VSMOW, which are consistent with a magmatic fluid source. Fluids related to potassic alteration assemblages have δ18O of −1.7 to +2.3‰ VSMOW and δD of −68 to −59‰ VSMOW, which suggests that the magmatic fluids evolved to lighter oxygen isotope signatures due to quartz precipitation during alteration.Lower temperature (~280°C) sericite and illite alteration followed the high-temperature magmatic stage and formed from the rising vapor plume as it mixed with varying amounts of meteoric fluid. Sericite in quartz-sericite-pyrite alteration on the periphery of the deposit and illite from areas of illite±kaolinite alteration which overprints sodic-potassic alteration and forms quartz-illite-pyrite alteration within the deposit are related to low-salinity aqueous fluids that have a significant magmatic component (+2.1 to +4.1‰ δ18O and −76 to −67‰ δD). Illite alteration which overprints both potassic alteration and quartz-sericite-pyrite alteration formed from fluids with a large meteoric component (−9.1 to −4.8‰ δ18O and −101 to −90‰ δD). The higher vein density and therefore higher permeability in the core of the deposit facilitated ingress of larger amounts of meteoric fluid compared with the more peripheral region.Advanced argillic alteration and associated high-grade mineralization are related to a second pulse of magmatic fluid which did not intersect the two phase field during cooling and depressurization. The fluid contracted to form a 3wt% NaCl equivalent aqueous fluid that resulted in a sericite-pyrite-bornite-digenite bearing assemblage with a magmatic signature (4.6 to 6.5‰ δ18O and −108 to −102‰ δD) at 340°C. Cooling and mixing of this fluid with meteoric water resulted in a fluid that formed a pyrophyllite-quartz-sericite-chalcopyrite bearing assemblage (−8.9 to −0.6‰ δ18O and −110 to −96‰ δD) at 300°C. Compared to older potassic and sodic-potassic alteration, the advanced argillic alteration has a highly depleted δD signature which is attributed to degassing of a crystallizing intrusive fluid source.

Isotopic and geochemical constraints on lead and fluid sources of the Pb[sbnd]Zn[sbnd]Ag mineralization in the polymetallic Tighza-Jbel Aouam district (central Morocco), and relationships with the geodynamic context

The WAu, PbZnAg, and SbBa mineralizations of the polymetallic Tighza-Jbel Aouam district (central Meseta, Morocco), are hosted in Paleozoic rocks surrounding late-Carboniferous granitic stocks. The PbZnAg Tighza deposit formed at 254 ± 16 Ma, and is clearly disconnected from the late-Variscan WAu deposit (295-280 Ma). The PbZnAg mineralization precipitated from a complex hydrothermal fluid. It displays air-normalized 3He/4He ratio (0.018–0.103) typical of the upper crust. This crustal component is confirmed by the oxygen and carbon isotope compositions (δ18O = +19 to +25‰; δ13C = −3.6 to −11.2‰) and the ɛNd values (−4.84 to −9.01) of gangue carbonates, which show mixing of (i) fluids that have interacted with late-Carboniferous magmatic rocks, and (ii) fluids in equilibrium with the Paleozoic metasediments. In addition, the PbZnAg mineralization has 40Ar/36Ar values in the range 284–315 typical of a meteoric fluid. The radiogenic Pb isotopic compositions (207Pb/204Pb = 15.70–15.80 and 206Pb/204Pb = 18.30–18.50) suggest leaching of Pb from the surrounding Paleozoic metasediments and late-Variscan granites, whereas the low radiogenic signatures (207Pb/204Pb = 15.40 and 206Pb/204Pb = 18.05) provide evidence of a deeper source attributed to the lower crust.Crustal thinning related to extensional tectonics in late-Permian and Early-Triassic lead to high-K calc-alkaline to alkaline magmatic activity, which is evidenced by a dense SW-NE-trending dike network that pre-dated the Atlantic Ocean opening (early Liassic times). This magmatic event induced a regional heat flux increase that triggered the circulation of a complex hydrothermal fluid, which has a strong crustal component, but also a meteoric and a lower crustal components. The polymetallic district of Tighza-Jbel Aouam thus results from superposition of an intrusion related porphyry-gold mineralization (WAu, 286 Ma) followed by a PbZnAg epithermal mineralization (254 Ma), during two distinct magmatic-hydrothermal events.The proposed metallogenic model for the PbZnAg Tighza-Jbel Aouam deposit provides new constraints for the PbZnAg exploration in the Moroccan Meseta. Exploration targets must take into account the following geological features: (i) Permo-triassic high-K calk-alkaline to alkaline dikes, (ii) extensional tectonics and reactivation of ancient crust-scale faults and shear zones, and (iii) Paleozoic series containing organic matter (e.g., black shales) subjected to low grade metamorphism (e.g., greenschist facies).

Contrasting fault fluids along high-angle faults: a case study from Southern Apennines (Italy)

This work focuses on two fault-controlled deposits, the Atella and Rapolla travertines, which are associated with high-angle extensional faults of the Bradano Trough, southern Apennines (Italy). The Atella travertine is along a NW-SE striking, deep-seated extensional fault, already described in literature, which crosscuts both Apulian carbonates and the overlying foredeep basin infill. The Rapolla travertine is on top of a NE-SW striking, shallow-seated fault, here described for the first time, which is interpreted as a tear fault associated with a shallow thrust displacing only the foredeep basin infill. The results of structural, sedimentological, mineralogical, and C and O isotope analyses are here reported and discussed to assess the provenance of mineralizing fluids, and to evaluate the control exerted by the aforementioned extensional faults on deep, mantle-derived and shallow, meteoric fluids. Sedimentological analysis is consistent with five lithofacies in the studied travertines, which likely formed in a typical lacustrine depositional environment. Mineralogical analysis show that travertines mainly consist of calcite, and minor quartz, feldspar and clay minerals, indicative of a terrigenous supply during travertine precipitation. The isotope signature of the two studied travertines shows different provenance for the mineralizing fluids. At the Atella site, the δ13CPDB values range between +5.2 and +5.7‰ and the δ18OPDB values between −9.0 and −7.3‰, which are consistent with a mantle-derived CO2 component in the fluid. In contrast, at the Rapolla site the δ13CPDB values vary from −2.7 to +1.5‰ and the δ18OPDB values from −6.8 to −5.4‰, suggesting a mixed CO2 source with both biogenic-derived and mantle-derived fluids. The results of structural analyses conducted along the footwall damage zone of the fault exposed at the Rapolla site, show that the whole damage zone, in which fractures and joints likely channeled the mixed fluids, acted as a distributed conduit for both fault-parallel and cross-fault fluid migration.

Origin and role of fluids involved in the seismic cycle of extensional faults in carbonate rocks

We examine the potentially-seismic right-lateral transtensional–extensional Tre Monti Fault (central Apennines, Italy) with structural and geochemical methods and develop a conceptual evolutionary model of extensional faulting with fluid involvement in shallow (≤3 km depth) faults in carbonate rocks. In the analysed fault zone, multiscale fault rock structures include injection veins, fluidized ultracataclasite layers, and crackle breccias, suggesting that the fault slipped seismically. We reconstructed the relative chronology of these structures through cross-cutting relationship and cathodoluminescence analyses. We then used C- and O-isotope data from different generations of fault-related mineralizations to show a shift from connate (marine-derived) to meteoric fluid circulation during exhumation from 3 to ≤1 km depths and concurrent fluid cooling from ∼68 to <30 °C. Between ∼3 km and ∼1 km depths, impermeable barriers within the sedimentary sequence created a semi-closed hydrological system, where prevalently connate fluids circulated within the fault zone at temperatures between 60° and 75 °C. During fault zone exhumation, at depths ≤1 km and temperatures <30 °C, the hydrological circulation became open and meteoric-derived fluids progressively infiltrated and circulated within the fault zone. The role of these fluids during syn-exhumation seismic cycles of the Tre Monti Fault has been substantially passive along the whole fault zone, the fluids being passively redistributed at hydrostatic pressure following co-seismic dilatancy. Only the principal fault has been characterized, locally and transiently, by fluid overpressures. The presence of low-permeability clayey layers in the sedimentary sequence contributed to control the type of fluids infiltrating into the fault zone and possibly their transient overpressures. These results can foster the comprehension of seismic faulting at shallow depths in carbonate rocks of other fold-thrust belts involved in post-collisional seismogenic extensional tectonics.

Magmatic and hydrothermal behavior of uranium in syntectonic leucogranites: The uranium mineralization associated with the Hercynian Guérande granite (Armorican Massif, France)

Most of the hydrothermal uranium (U) deposits from the European Hercynian belt (EHB) are spatially associated with Carboniferous peraluminous leucogranites. In the southern part of the Armorican Massif (French part of the EHB), the Guérande peraluminous leucogranite was emplaced in an extensional deformation zone at ca. 310Ma and is spatially associated with several U deposits and occurrences. The apical zone of the intrusion is structurally located below the Pen Ar Ran U deposit, a perigranitic vein-type deposit where mineralization occurs at the contact between black shales and Ordovician acid metavolcanics. In the Métairie-Neuve intragranitic deposit, uranium oxide-quartz veins crosscut the granite and a metasedimentary enclave.Airborne radiometric data and published trace element analyses on the Guérande leucogranite suggest significant uranium leaching at the apical zone of the intrusion. The primary U enrichment in the apical zone of the granite likely occurred during both fractional crystallization and the interaction with magmatic fluids. The low Th/U values (<2) measured on the Guérande leucogranite likely favored the crystallization of magmatic uranium oxides. The oxygen isotope compositions of the Guérande leucogranite (δ18Owhole rock=9.7–11.6‰ for deformed samples and δ18Owhole rock=12.2–13.6‰ for other samples) indicate that the deformed facies of the apical zone underwent sub-solidus alteration at depth with oxidizing meteoric fluids. Fluid inclusion analyses on a quartz comb from a uranium oxide-quartz vein of the Pen Ar Ran deposit show evidence of low-salinity fluids (1–6wt.% NaCl eq.), in good agreement with the contribution of meteoric fluids. Fluid trapping temperatures in the range of 250–350°C suggest an elevated geothermal gradient, probably related to regional extension and the occurrence of magmatic activity in the environment close to the deposit at the time of its formation. U-Pb dating on uranium oxides from the Pen Ar Ran and Métairie-Neuve deposits reveals three different mineralizing events. The first event at 296.6±2.6Ma (Pen Ar Ran) is sub-synchronous with hydrothermal circulations and the emplacement of late leucogranitic dykes in the Guérande leucogranite. The two last mineralizing events occur at 286.6±1.0Ma (Métairie-Neuve) and 274.6±0.9Ma (Pen Ar Ran), respectively. Backscattered uranium oxide imaging combined with major elements and REE geochemistry suggest similar conditions of mineralization during the two Pen Ar Ran mineralizing events at ca. 300Ma and ca. 275Ma, arguing for different hydrothermal circulation phases in the granite and deposits. Apatite fission track dating reveals that the Guérande granite was still at depth and above 120°C when these mineralizing events occurred, in agreement with the results obtained on fluid inclusions at Pen Ar Ran.Based on this comprehensive data set, we propose that the Guérande leucogranite is the main source for uranium in the Pen Ar Ran and Métairie-Neuve deposits. Sub-solidus alteration via surface-derived low-salinity oxidizing fluids likely promoted uranium leaching from magmatic uranium oxides within the leucogranite. The leached out uranium may then have been precipitated in the reducing environment represented by the surrounding black shales or graphitic quartzites. As similar mineralizing events occurred subsequently until ca. 275Ma, meteoric oxidizing fluids likely percolated during the time when the Guérande leucogranite was still at depth. The age of the U mineralizing events in the Guérande region (300–275Ma) is consistent with that obtained on other U deposits in the EHB and could suggest a similar mineralization condition, with long-term upper to middle crustal infiltration of meteoric fluids likely to have mobilized U from fertile peraluminous leucogranites during the Late Carboniferous to Permian crustal extension events.

Hydrothermal mineralization in the sandstone–hosted Hangjinqi uranium deposit, North Ordos Basin, China

Tabular–type uranium ore deposits (the Hangjinqi and Daying deposits) have recently been found in the Middle Jurassic Zhiluo Formation, north of the Ordos Basin, China. Petrographic observations, the chemical composition of U minerals determined by EMPA and fs–LA–ICP–MS, whole rock geochemistry and the microthermometric study of fluid inclusions have been integrated to characterize the genetic conditions of the U mineralization in the Hangjinqi sandstone–hosted deposit. Two different groups of U minerals have been identified. One group includes coffinite(I) associated with vanadium–rich micas. Coffinite(I) is enriched in vanadium (V) and devoid of iron (Fe) and yttrium (Y) and has a LREE–enriched chondrite–normalized REE pattern. The U minerals of this group are similar to meteoric fluid infiltration related deposits. The second group has coeval coffinite(II) and coarsely crystalline calcite cement. Coffinite(II) is enriched in Y and Fe and depleted in V and is marked by a flat chondrite–normalized REE pattern, which is compatible with typical hydrothermal genetic deposits with high–salinity mineralizing fluids. The temperature and salinity of the primary aqueous inclusions in the ore–stage calcite are 120–180°C and 8.00–16.34% (eq. wt% NaCl), respectively. These mineral assemblages, temperatures and salinities indicate that the Hangjinqi deposit was affected by two distinct types of ore–bearing fluids: low–salinity meteoric waters and high–salinity hydrothermal fluids. The meteoric fluids event began at 97±5Ma with the titling of the northern Ordos Basin and the uplift of the Hetao region to the north. Hydrothermal U mineralization occurred since 39±2Ma with the rifting of the Hetao graben. Thus, the previous biogenic model for the U mineralization should be modified in the uraniferous region of the north Ordos Basin.

Molecular evidence for abiotic sulfurization of dissolved organic matter in marine shallow hydrothermal systems

Shallow submarine hydrothermal systems are extreme environments with strong redox gradients at the interface of hot, reduced fluids and cold, oxygenated seawater. Hydrothermal fluids are often depleted in sulfate when compared to surrounding seawater and can contain high concentrations of hydrogen sulfide (H2S). It is well known that sulfur in its various oxidation states plays an important role in processing and transformation of organic matter. However, the formation and the reactivity of dissolved organic sulfur (DOS) in the water column at hydrothermal systems are so far not well understood. We investigated DOS dynamics and its relation to the physicochemical environment by studying the molecular composition of dissolved organic matter (DOM) in three contrasting shallow hydrothermal systems off Milos (Eastern Mediterranean), Dominica (Caribbean Sea) and Iceland (North Atlantic). We used ultra-high resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) to characterize the DOM on a molecular level. The molecular information was complemented with general geochemical data, quantitative dissolved organic carbon (DOC) and DOS analyses as well as isotopic measurements (δ2H, δ18O and F14C). In contrast to the predominantly meteoric fluids from Dominica and Iceland, hydrothermal fluids from Milos were mainly fed by recirculating seawater. The hydrothermal fluids from Milos were enriched in H2S and DOS, as indicated by high DOS/DOC ratios and by the fact that >90% of all assigned DOM formulas that were exclusively present in the fluids contained sulfur. In all three systems, DOS from hydrothermal fluids had on average lower O/C ratios (0.26–0.34) than surrounding surface seawater DOS (0.45–0.52), suggesting shallow hydrothermal systems as a source of reduced DOS, which will likely get oxidized upon contact with oxygenated seawater. Evaluation of hypothetical sulfurization reactions suggests DOM reduction and sulfurization during seawater recirculation in Milos seafloor. The four most effective potential sulfurization reactions were those exchanging an O atom by one S atom in the formula or the equivalent+H2S reaction, correspondingly exchanging H2O, H2 and/or O2 by a H2S molecule. Our study reveals novel insights into DOS dynamics in marine hydrothermal environments and provides a conceptual framework for molecular-scale mechanisms in organic sulfur geochemistry.

Mineralogical and geochemical characterization of the Riópar non-sulfide Zn-(Fe-Pb) deposits (Prebetic Zone, SE Spain)

The present paper reports the first detailed petrological and geochemical study of non-sulfide Zn–(FePb) deposits in the Riópar area (Prebetic Zone of the Mesozoic Betic Basin, SE Spain), constraining the origin and evolution of ore-forming fluids. In Riópar both sulfide and non-sulfide Zn–(FePb) (“calamine”) ores are hosted in hydrothermally dolomitized Lower Cretaceous limestones. The hypogene sulfides comprise sphalerite, marcasite and minor galena. Calamine ores consist of Zn-carbonates (smithsonite and scarce hydrozincite), associated with abundant Fe-(hydr)oxides (goethite and hematite) and minor Pb-carbonates (cerussite). Three smithsonite types have been recognized: i) Sm-I consists of brown anhedral microcrystalline aggregates as encrustations replacing sphalerite; ii) Sm-II refers to brownish subhedral aggregates of rugged appearance related with Fe oxi-hydroxides in the surface crystals, which replace extensively sphalerite; and iii) Sm-III smithsonite appears as coarse grayish botryoidal aggregates in microkarstic cavities and porosity. Hydrozincite is scarce and appears as milky white botryoidal encrustations in cavities replacing smithsonite. Also, two types of cerussite have been identified: i) Cer-I cerussite consists of fine crystals replacing galena along cleavage planes and crystal surfaces; and ii) Cer-II conforms fine botryoidal crystals found infill porosity. Calcite and thin gypsum encrustations were also recognized. The field and petrographic observations of the Riópar non-sulfide Zn–(FePb) revealed two successive stages of supergene ore formation under meteoric fluid processes: i) “gossan” and “red calamine” formation in the uppermost parts of the ore with deposition of Fe-(hydr)oxides and Zn- and Pb-carbonates (Sm-I, Sm-II and Cer-I), occurring as direct replacements of ZnPb sulfides; and ii) “gray calamine” ore formation with deposition of Sm-III, Cer-II and hydrozincite infilling microkarst cavities and porosity. The stable isotope variation of Riópar smithsonite is very similar to those obtained in other calamine-ore deposits around the world. Their CO isotope data (δ18O: +27.8 to +29.6‰ V-SMOW; δ13C: −6.3 to +0.4‰ V-PDB), puts constrains on: i) the oxidizing fluid type, which was of meteoric origin with temperatures of 12 to 19°C, suggesting a supergene weathering process for the calamine-ore formation under a temperate climate; and ii) the carbon source, that resulted from mixing between two CO2 components derived from: the dissolution of host-dolomite (13C-enriched source) and vegetation decomposition (13C-depleted component).

Magmatic and meteoric fluid flow in the Bitterroot extensional detachment shear zone (MT, USA) from ductile to brittle conditions

The Bitterroot shear zone developed as a rolling-hinge detachment system where a syntectonic granodiorite in the footwall was progressive exhumed beneath a detachment shear zone, providing a record of deformation and fluid-rock interaction during progressive exhumation and cooling. The shear zone displays a high strain gradient over ∼1km of structural section from the relatively undeformed footwall, where the granodiorite contains a magmatic foliation and lineation, through a mylonite sequence that culminates upward in the fine interlayering of ultramylonite layers and the development of a breccia zone. We measured the stable isotope composition of quartz, muscovite, biotite, chlorite, and epidote across the shear zone and estimated equilibrium temperatures using oxygen isotope thermometry based on mineral pairs. We also measured the hydrogen isotope ratios of hydrous minerals and of quartz fluid inclusions. The main results are: (1) The relatively undeformed footwall granodiorite interacted with magmatic fluids at 500–600°C; (2) the mylonitic fabric defined by muscovite, biotite, and chlorite developed between 500 and 300°C and interacted with a fluid system that was connected to the Earth’s surface (meteoric fluids), as indicated by the low δD values of hydrous mineral phases, including muscovite; (3) the fluxes of surface fluids were not sufficient to shift the δ18O values of muscovite significantly, but were sufficient to control the δ18O composition of biotite and chlorite during deformation-induced recrystallization and chloritization, and (4) the isotopic composition of fluid inclusions in quartz as well as the δD values of late quartz veins track the mixing of fluid sources between the magmatic and meteoric reservoirs. The distribution of stable isotope compositions in the various tectonites of the granodioritic Bitterroot shear zone, from ductile to brittle, provides a rich spatial and temporal record of the interaction between deformation and fluid flow in a crustal-scale detachment.

Constraints on paleofluid sources using the clumped-isotope thermometry of carbonate veins from the SAFOD (San Andreas Fault Observatory at Depth) borehole

The San Andreas Fault Observatory at Depth (SAFOD), near Parkfield, California, is a borehole drilled through two active deforming zones of the San Andreas fault, the Southwest Deforming Zone (SDZ) and the Central Deforming Zone (CDZ). These zones accommodate displacement by seismic slip and aseismic creep. Elevated fluid pressures and fluid–rock interactions have been proposed to explain the low apparent strength and aseismic creep observed, but the origin of the fluids and existence of high fluid pressures remains uncertain. We use clumped-isotope thermometry and δ18O–δ13C compositions of calcite in veins to constrain the origin of paleofluids and compare these results to the isotopic composition of modern-day pore fluids from the SAFOD borehole and nearby areas. We observe that: (1) calcite vein temperatures vary from 81 to 134°C, which overlaps the current ambient borehole temperatures of 110–115 °C at sampled depths; (2) vein calcite is not in carbon isotope equilibrium with modern-day pore fluids; (3) the δ18O values of paleofluids close to the SDZ and CDZ, calculated from vein δ18O and temperature data, are not in equilibrium with local modern-day pore waters but approach equilibrium with modern pore waters far from these zones; and (4) syntectonic vein calcite is only in C- and O-isotopic equilibrium with their host rocks within the SDZ and CDZ. Spatial patterns of δ18O and δ13C show little evidence for across-fault fluid-flow. Clumped isotope temperatures are consistent with locally-derived fluid sources, but not with continuous or episodic replenishment of fluids from shallow sedimentary brines or deep fluid sources. Our findings are compatible with flow of meteoric fluids from the southwestern damage zone into the SDZ and CDZ, which would have favored the formation of weak phyllosilicates and contributed to the present day weakness of the two actively deforming zones.

The Itataia phosphate-uranium deposit (Ceará, Brazil) new petrographic, geochemistry and isotope studies

The Itataia phosphate-uranium deposit is located in Santa Quitéria, in central Ceará State, northeastern Brazil. Mineralization has occurred in different stages and involves quartz leaching (episyenitization), brecciation and microcrystalline phase formation of concretionary apatite. The last constitutes the main mineral of Itatiaia uranium ore, namely collophane. Collophanite ore occurs in massive bodies, lenses, breccia zones, veins or episyenite in marble layers, calc-silicate rocks and gneisses of the Itataia Group.There are two accepted theories on the origin of the earliest mineralization phase of Itataia ore: syngenetic (primary) – where the ore is derived from a continental source and then deposited in marine and coastal environments; and epigenetic (secondary) – whereby the fluids are of magmatic, metamorphic and meteoric origin. The characterization of pre- or post-deformational mineralization is controversial, since the features of the ore are interpreted as deformation.This investigation conducted isotopic studies and chemical analyses of minerals in marbles and calc-silicate rocks of the Alcantil and Barrigas Formations (Itataia Group), as well as petrographic and structural studies. Analysis of the thin sections shows at least three phosphate mineral phases associated with uranium mineralizaton: (1) A prismatic fluorapatite phase associated with chess-board albite, arfvedsonite and ferro-eckermannite; (2) a second fluorapatite phase with fibrous radial or colloform habits that replaces calcium carbonate in marble, especially along fractures, with minerals such as quartz, chlorite and zeolite also identified in calc-silicate rocks; and (3) an younger phosphate phase of botryoidal apatite (fluorapatite and hydroxyapatite) related with clay minerals and probably others calcium and aluminum phosphates. Detailed isotopic analysis carried out perpendicularly to the mineralized levels and veins in the marble revealed significant variation in isotopic ratios. Mineralized zones exhibit a decrease in δ13C and δ18O isotope values and a higher 87Sr/86Sr ratio toward the center of the vein. In conjunction with petrographic studies, these changes contesting the hypothesis of a sedimentary origin for uranium and suggest a radiogenic Sr input by alkaline to peralkaline fluids from fertile granites of the end of Brasiliano/Pan-African orogeny, located outside the deposit. The origin of the phosphorous is associated with phosphorite deposits in the same depositional environment of the neoproterozoic supracrustal quartz-pelite-carbonate sediments of the Itataia Group.Considering the studies conducted here and available geological data, three main mineralizing events can be identified in Itataia: (1) an initial high temperature event connected with a sodium metasomatism-related uranium episode, taking place in Borborema Province and its African counterpart; (2) a second lower temperature stage, consisting of a multiphase cataclastic/hydrothermal event limited to fault and paleokarst zones; and (3) a third and final event, developed in frankly oxidizing conditions. The last two involving mixing of hydrothermal and meteoric fluids.