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Diatreme-hosted fluorite mineralization in S-Namibia – A tale of cryogenic brine formation and fluid mixing below an unconformity in the context of Pangea rifting

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Abstract

The Garub fluorite deposit in south Namibia is hosted by a Neoproterozoic tuffisite diatreme which is cut by a hydrothermal vein with base metal mineralization. Petrographic observations indicate a single hydrothermal event producing a paragenesis of fluorite-baryte with base metal sulfides, quartz and carbonates. Microthermometry data from fluid inclusions show that hydrothermal fluorite and baryte were precipitated from a high-salinity (16.9–22.7 wt% NaCl + CaCl2), moderate-temperature brine (180–210 °C). This brine formed by mixing between a warmer, Ba- and F-rich endmember likely representing a deep-seated basement brine (chemically equilibrated with Namaqua-Natal-Metamorphic Complex basement rocks) and a cooler, SO4- and Ca-rich endmember that likely equilibrated with Nama Group limestone. Chlorine/Br mass ratios between 152 and 612 indicate the mixing of a fluid with a halite dissolution brine signature (endmember 1) and an equally saline brine derived from Nama Group limestone (endmember 2). Rubidium/Cesium (Rb/Cs) ratios between 2.2 and 20.5 provide evidence for significant fluid interaction with clay minerals. Since clay minerals are abundant in the tuffisite of the Garub diatreme, these Rb/Cs ratios indicate that fluids migrated along the diatreme-gneiss contact along a zone of weakness. Late-stage quartz and calcite likely reflect cooling and induced a shutdown of the hydrothermal system. The sulfide phases in this hydrothermal vein are strongly depleted in trace elements compared to other hydrothermal vein districts, which is interpreted as an inherited signature of the fluid source. There is no record of evaporites in the sedimentary rocks of the Nama Group. Based on the fluid chemistry of the mixed fluid, which clearly indicates halite dissolution and water-rock interaction with basement rocks, it is concluded that the basement brine fluid endmember is likely generated through cryogenic brine formation during the large scale Dwyka glaciation event. As a farfield consequence of Pangea breakup fluid pathways were established that acted as host structures for the Garub fluorite deposit. This could be the first evidence for unconformity-related hydrothermal fluorite deposit formation in the context of Dwyka cryogenic brine formation in southern Africa.

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The Paleoproterozoic Athabasca Basin (Canada) hosts numerous giant unconformity-related uranium deposits. The scope of this study is to establish the pressure, temperature, and composition (P-T-X conditions) of the brines that circulated at the base of the Athabasca Basin and in its crystalline basement before, during and after UO2 deposition. These brines are commonly sampled as fluid inclusions in quartz- and dolomite-cementing veins and breccias associated with alteration and U mineralization. Microthermometry and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) data from five deposits (Rabbit Lake, P-Patch, Eagle Point, Millennium, and Shea Creek) complement previously published data for the McArthur River deposit. In all of the deposits investigated, fluid inclusion salinity is between 25 and 40 wt.% NaCl equiv., with compositions displaying a continuum between a “NaCl-rich brine” end-member (Cl > Na > Ca > Mg > K) and a “CaCl2-rich brine” end-member (Cl > Ca ≈ Mg > Na > K). The CaCl2-rich brine has the highest salinity and shows evidence for halite saturation at the time of trapping. The continuum of compositions between the NaCl-rich brine and the CaCl2-rich brine end-members combined with P-T reconstructions suggest anisothermal mixing of the two brines (NaCl-rich brine, 180 ± 30 °C and 800 ± 400 bars; CaCl2-rich brine, 120 ± 30 °C and 600 ± 300 bars) that occurred under fluctuating pressure conditions (hydrostatic to supra-hydrostatic). However, because the two brines were U bearing and therefore oxidized, brine mixing was probably not the driving force for UO2 deposition. Several scenarios are put forward to account for the Cl-Na-Ca-Mg-K composition of the brines, involving combinations of seawater evaporation, halite dissolution, mixing with a halite-dissolution brine, Mg/Ca exchange by dolomitization, Na/Ca exchange by albitization of plagioclase, Na/K exchange by albitization of K-feldspar, and Mg loss by Mg-rich alteration. Finally, the metal concentrations in the NaCl-rich and CaCl2-rich brines are among the highest recorded compared to present-day sedimentary formation waters and fluid inclusions from basin-hosted base metal deposits (up to 600 ppm U, 3000 ppm Mn, 4000 ppm Zn, 6000 ppm Cu, 8000 ppm Pb, and 10,000 ppm Fe). The CaCl2-rich brine carries up to one order of magnitude more metal than the NaCl-rich brine. Though the exact origin of major cations and metals of the two brines remains uncertain, their contrasting compositions indicate that the two brines had distinct flow paths and fluid-rock interactions. Large-scale circulation of the brines in the Athabasca Basin and Basement was therefore a key parameter for metal mobility (including U) and formation of unconformity-related U deposits.
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Hydrothermal Ag-Co-Ni-Bi-As (five-element vein type) ore deposits show very conspicuous textures of the native elements silver, bismuth, and arsenic indicating formation from a rapid, far-from-equilibrium process. Such textures include up to dm-large tree- and wire-like aggregates overgrown by Co-Ni-Fe arsenides and mostly carbonates. Despite the historical and contemporary importance of five-element vein type deposits as sources of silver, bismuth, and cobalt, and despite of spectacular museum specimens, their process of formation is not yet understood and has been a matter of debate since centuries. We propose, based on observations from a number of classical European five-element vein deposits and carbon isotope analyses, that “natural fracking,” i.e., liberation of hydrocarbons or hydrocarbon-bearing fluids during break up of rocks in the vicinity of an active hydrothermal system and mixing between these hydrocarbons (e.g., methane and/or methane-bearing fluids) and a metal-rich hydrothermal fluid is responsible for ore precipitation and the formation of the unusual ore textures and assemblages. Thermodynamic and isotope mixing calculations show that the textural, chemical, and isotopic features of the investigated deposits can entirely be explained by this mechanism.
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High purity fluorite veins (acid grade > 97 % CaF 2) are generally rare in a global context. The fluorite-(±quartz-calcite) veins in the Aukam Valley mining district in SW Namibia have not previously been studied but due to their excellent exposure they are an ideal natural laboratory to study such mineralization. These veins cross-cut Mesoproterozoic high-grade gneisses and granites (Namaqua-Natal Metamorphic Province) close to the unconformity with the overlying late-Neoproterozoic Nama Group sedimentary rock cover. Based on the regional context, their genesis has been assumed to be related to nearby fluorite-bearing pegmatites. However, the present contribution provides new microthermometry data from vein fluorite (with 25.0-25.6 wt% NaCl and T h of 270-300°C) that indicate precipitation from a mixed fluid with two chemically contrasting end-members for the early paragenetic fluorite stage. However, it was not possible to identify the chemical composition of the end-members. Based on the chemical composition of the fluid mixture and the regional geological context, we propose that these end-members are F-rich brines sourced from the Namaqua basement and Ca-rich limestone-derived formation fluids of the Nama Group. A fluorite-overgrowing quartz precipitation is likely the effect of post-mixing fluid cooling as the fluid inclusions hosted in quartz show lower homogenization temperatures (T h 230-260°C) with similar salinity. Locally, late-stage calcite is recognized with fluid inclusion compositions (0.2-2.7 wt% NaCl and T h of 60-92°C) different from those observed in the fluorite and quartz. With significantly lower homogenization temperatures, they likely represent precipitates formed during the shutdown and collapse of the hydrothermal system. The fluorite veins are hosted in post-Cambrian N-trending brittle structures that probably formed during the opening of the South Atlantic in late Mesozoic times. Interestingly, similar massive fluorite mineralization on both sides of the Atlantic Ocean is also likely associated with Pangea rifting and known from numerous mining districts operating on unconformity-related hydrothermal veins in Central and North America, northern Africa and Europe.
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Carbonatites exist as intrusive and extrusive rocks, with the former dominating the rock record. The geochemical link between intrusive and extrusive equivalents and the processes during eruption of carbonatite melt are essentially unknown. This contribution aims at providing new insights into the transition from intrusive to extrusive carbonatites with special emphasis on the trace element budget. The Gross Brukkaros natural laboratory in central Namibia was chosen to study the interface between fine-grained dolomite-carbonatite dykes and associated diatremes. Whole rock geochemistry combined with micro-scale petrography via BSE imaging and microXRF mapping provide evidence that the carbonatite dykes contain significant amounts of silica and aluminium, which is inherited from crustal xenoliths by resorption and leaching. At the transition from carbonatite dyke to diatreme, the CO2 component (likely together with most of the other volatiles e.g. Cl, H2O, F, S) of the carbonatitic brine-melt vaporised and was released into the atmosphere. During this process the Si, Ca, Mg and Fe transported together with a high amount of trace elements became precipitated from a resulting primarily magmatic fluid as a mixture of cryptocrystalline quartz (quartz I) and aegirine-augite (plus minor magnetite) with frequently occurring microcrystalline quartz I grains. This mineral assemblage forms the matrix in the diatreme breccia and precipitated by rapid temperature drop in the course of decompression from the hydrothermal fluid. In the post-eruption stage the influx of meteoric waters (into the system), which mixed with the remaining magmatic fluid, caused precipitation of a second quartz generation (quartz II; euhedral grains), which is depleted in trace elements. All measured trace elements show significantly higher contents in quartz I compared to quartz II (with exception of Li). The application of the TitaniQ geothermometer indicates that Ti incorporation in quartz is kinetically controlled for quartz I. During quartz II formation the system thermodynamically equilibrates and ends up with realistic precipitation temperatures of 290-350°C. Therefore, this study shed light on the trace element behaviour at the interface between feeder dykes and diatreme roots of a strongly contaminated carbonatite melt.
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This work focuses on an exceptionally complex natural laboratory, the Triassic Latemar isolated platform in the Dolomite Mountains of northern Italy. It explores spatial and temporal gradients in processes and products related to contact metamorphism, dolomitization, and the dedolomitization of marine limestones. Rock samples were studied using dual fluid-inclusion thermometry and clumped-isotope thermometry. Independent of the spatial position at Latemar, Δ47 clumped-isotope and fluid-inclusion data provide contrasting paleotemperature estimates. An apparent lack of systematic patterns in fluid-inclusion data (homogenization temperature, salinity, density) results from analyses of micrometer-sized growth zones within a single crystal. The composition of the individual fluid inclusions represents a “snapshot” of fluid mixing with variable endmember elemental ratios. The bulk crush-leach data and slopes in Caexcessversus Nadeficit diagrams indicate different water–rock interactions and fluid signatures with evaporation sequences and crystalline rocks. The presence of three fluid types (crystalline basement brine, halite-dissolution brine, seawater) in all carbonates suggests that all fluids coexisted during contact metamorphism and dolomitization of Latemar carbonates. Non-equilibrium processes overruled thermodynamic controls on the precipitation of diagenetic phases. Fluid mixing resulted in the precipitation of two complex carbonate successions. The Δ47 data represent bulk temperatures, averaging the mixing ratio of fluids with different temperatures and their respective volume. Fluid-inclusions record patterns of remarkable complexity and shed light on the complexity of a multi-fluid system. Data shown here provide answers to the controversial interpretation of dolomitizing fluid temperature in the Latemar and exemplify the strengths of multi-proxy paleotemperature studies.
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A study of the NW Kakamas Domain in South Africa/Namibia provides a new, unified lithostratigraphy and evolutionary history, applicable to the whole Namaqua Sector. The Mesoproterozoic history ranges from ∼1350 Ma to 960 Ma, but isotopic evidence suggests it was built upon pre-existing Paleoproterozoic continental crust that extended west from the Archaean Craton. In eastern Namaqualand, early rift-related magmatism and sedimentation at ∼1350 occurred in a confined ocean basin. Subsequent tectonic reversal and subduction at ∼1290–1240 Ma led to establishment of the Areachap, Konkiep and Kaaien Domains. In the Kakamas Domain, widespread deposition of pelitic sediments occurred at v1220 Ma (Narries Group). These contain detrital zircons derived from proximal crust with ages between ∼2020 Ma and 1800 Ma (western Palaeoproterozoic domains) and 1350-1240 Ma (eastern early Namaqua domains), suggesting pre-sedimentation juxtaposition. The pelites underwent granulite grade metamorphism at ∼1210 Ma (peak conditions: 4.5–6 kbar and 770–850◦C), associated with voluminous, predominantly S-type granitoid orthogneisses between ∼1210 Ma and 1190 Ma (Eendoorn and Ham River Suites) and low-angle ductile (D2) deformation which continued until ∼1110 Ma, interspersed with periods of sedimentation. This enduring P-T regime is inconsistent with the expected crustal over-thickening associated with the generally-accepted collision-accretion Namaqualand model. Rather, we propose the Namaqua Sector is a ‘hot orogen’ developed in a wide continental back-arc with subduction west of the present-day outcrop. The observed high geotherm resulted from thinned back-arc lithosphere accompanied by an influx of mantle melts. Ductile D2 deformation resulted from “bottom-driven” tectonics and viscous drag within the crust by convective flow in the underlying asthenospheric mantle. This extended tectonothermal regime ceased at ∼1110 Ma when SW-directed thrusting stacked the Namaqua Domains into their current positions, constrained in the Kakamas Domain by late- to post-tectonic I-type granitoids intruded between ∼1125 Ma and 1100 Ma (Komsberg Suite). The thermal peak then shifted west to the Bushmanland and Aus Domains, where voluminous granites (1080–1025 Ma) were associated with high-T/low-P granulite facies thermal metamorphism and mega-scale open folding (D3). Unroofing of the Namaqua Sector is marked by large-scale, NW-trending, sub-vertical transcurrent dextral shear zones and associated pegmatites and leucogranites at ∼990 Ma.
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The microscale mechanisms of hydrocarbons movement along faults and fault zones, the potential carriers of hydrocarbons toward the productive geological traps, remain largely unknown. The majority of previous studies inferred the hydraulic behavior of faults with respect to hydrocarbon movements without providing meso and microstructural observations from faults permeated by hydrocarbons. To fill this gap, we document meso-structures together with the first fossil microstructural portraits of hydrocarbon flow and pathways along two hydrocarbon-bearing carbonate-hosted normal faults exposed in the central Apennines, Italy. In particular, we show that hydrocarbons cyclically move within tectonically active carbonate normal faults possibly during interseismic and coseismic phases of the seismic cycle. Channelized structures, injection features, and clast-cortex grains suggest the occurrence of transient and localized pulses of pressurized hydrocarbons during coseismic slip. In particular, clast-cortex grains are very similar to microstructures that develop along fault planes for slip velocities between 0.0001 and 1 m/s. Patches of hydrocarbon-bearing breccias/cataclasites with lobate and irregular boundaries, crackle breccias filled by hydrocarbons, and hydrocarbons arrested against hydrocarbon-free discrete fault planes suggest hydrocarbon flow during the interseismic phases with hydrocarbon (over)pressure dissipation after coseismic phases. Fault permeability is created during the coseismic phase and hydrocarbons penetrate the most permeable and uncemented uncohesive fault rocks within the damage zone and arrest against and/or within low-permeability and cemented fault cores. Results are consistent with previous numerical simulations on hydrocarbon movements along faults and time-lapse seismic-reflection imaging, suggesting that the movement of hydrocarbon occurs during interseismic periods within tectonically active faults and along permeable zones cyclically created by seismic activity. Results from this paper are crucial for modelling the hydraulic behavior of carbonate fault damage zone, which have progressively gained popularity as targets of hydrocarbon exploration and production and for CO2 or H storage.
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The main ore stage of three similar unconformity-related vein systems in the Schwarzwald, SW Germany spanning a period of activity of ~150 Ma, was investigated to understand the details of fluid penetration from overlying sediments/marine environment into the basement, their evolution as well as the processes involved in vein formation. To investigate temporal and spatial variations of the hydrothermal fluids responsible for mineralization, over 1650 fluid inclusions were analyzed by microthermometry. Of these, a total of 108 fluid inclusions (mainly in fluorite) were successfully analyzed by LA-ICP-MS. The fluid inclusions reveal a binary mixing trend between a CaCl2- and a NaCl-rich endmember. Independent of major element composition, the fluids are metal-bearing (e.g., up to ~100 mg/kg Ba, Pb, Zn, Ni and up to 10 mg/kg Ag), show high As (up to 1000 mg/kg) and low S (below the detection limit in most analyses). Over time, the mixed fluid shows a gradual decrease in CaCl2 and increase in NaCl with slightly decreasing total salinity. Based on earlier studies and geochemical arguments, the veins formed by anisothermal binary fluid mixing of two fluids, which both were originally derived from seawater and chemically modified through interaction with the basement and sedimentary rocks in different ways. This produced a gradual stratified basement fluid reservoir comprising a modified bittern/halite dissolution brine. The fluids involved in the vein formation are sourced from different depths of this modified bittern/halite dissolution basement brine reservoir: fluid A, a CaCl2-dominated, KCl-poor, deeper seated brine with a salinity of ~25 wt% CaCl2 + NaCl, and fluid B, an NaCl-dominated and KCl-richer brine situated at shallower depths in the crystalline basement with salinities of ~22 wt% NaCl+CaCl2. Based on the Na-Ca-K and NaK thermometers and on Rb/Cs systematics, fluid A records alteration of the Na-, K- and Ca-bearing feldspars of the host rocks; progressive alteration led to consumption of mainly Ca-rich plagioclase in contact with these basement brines. Accordingly, fluid B that subsequently entered the basement was only in equilibrium with alkali feldspars and clay minerals. This scenario produced a gradual change of fluid composition with depth that was pushed to greater depth over time. The source temperatures are estimated to ~250 °C while vein formation occurred at 100–170 °C, based on fluid inclusion homogenization temperatures. Thus, significant fluid cooling without abundant fluid mixing (and without major mineralization) must have occurred during fluid ascent (3–7 km, depending on the assumed geothermal gradient). Fluid mixing then resulted in the formation of the major gangue minerals fluorite, quartz, barite and calcite, while the locally confined sulfides must have formed due to an influx of sulfide into the binary mixed fluid. The process of fluid mixing is a rapid and turbulent process. This is recorded by a great diversity of fluid compositions (including ones close to the mixing endmembers) trapped within the same crystal. Compositional variations are even visible within individual fluid inclusion trails.
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The crush-leach technique is a frequently used method to determine the bulk composition of fluid inclusions trapped in a range of geological samples. We present a modified crush-leach technique combining ion chromatography (IC) and total reflection X-ray fluorescence spectroscopy (TXRF) which allows to determine a range of major, minor and trace elements out of one leachate. To date, trace element detection by means of TXRF is barely used in geosciences, although it combines the advantages of low to very low detection limits (μg/L to ng/L range), small sample amount needed (μL-range) and a fast and inexpensive analytical procedure. Previously described problems of adsorption of polyvalent cations at sample surfaces have been overcome by using acidified water as a leachate. Instead, it has been demonstrated that, for example, the syringe filter type used for IC measurements influences contamination and/or adsorption for a number of elements. The proposed method combination was evaluated for accuracy, reproducibility and system blanks and subsequently applied to quartz samples from hydrothermal vein deposits of the Schwarzwald ore district, SW Germany.
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A significant proportion of the remaining oil reserves worldwide is concentrated in dolostone rock bodies. The present paper focuses on Tournaisian and Visean limestones and geologically younger diagenetic dolomite units in two mature oil fields of the Volga-Ural Basin, Russia. In contrast to many dolostone-hosted hydrocarbon reservoirs worldwide, the Tournaisian and Visean dolostones in the Demkinskoye and the Onbiyskoye oil fields plot near the low end of the porosity-permeability range. Replacive and porosity-destructive burial dolomitization has deteriorated the reservoir properties of these units. Dolomitization is related to the ascent of hot (170-215 oC) and saline (22-24 wt.%) fluids as based on fluid inclusion data. This temperature range points to an abnormally high temperature background thermal regime of the Volga-Ural basin. The onset of burial dolomitization predates oil charge as documented by the fact that oil-bearing inclusions are only present in the outermost zones of porosity-occluding dolomite cements. Isotope geochemical data, dolomite petrography and fluid inclusion data reveal important similarities between hydrothermal dolostone units in the Demkinskoye and the Onbiyskoye oil fields. Based on these data and the detailed cement stratigraphy presented here, it is argued that dolomite bodies in both oil fields formed as the result of one main, Alpine tectonic event pushing hot and saline dolomitizing brines ahead of ascending hydrocarbons.
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Studies of fluid inclusions in carbonatitic rocks are essential for understanding physicochemical processes involved in carbonatite-related hydrothermal ore mineralization and fenitization. However, the composition of many carbonatite-derived fluids is challenging to quantify, which hampers their detailed interpretation. Here, we present a systematic study of microthermometry of fluid inclusions found in carbonatites from the Kaiserstuhl (SW Germany), and a simple numerical model to estimate the compositions of such fluids, which are typical of numerous carbonatites worldwide. Four types of fluid inclusions have been identified in the Kaiserstuhl carbonatites: (I) vapor-poor H2O-NaCl fluids with <50 wt.% salinity; (II) vapor-rich H2O-NaCl-CO2 fluids with <5 wt.% salinity; (III) multi-component fluids with high salinity and high CO2 contents; and (IV) multi-component fluids with high salinity but little to no CO2. At present, it is only possible to quantify fluid compositions for types I and II. For the complex types III and IV, we conducted predictive modeling of the liquidus surface based on the Margules equations. The results suggest that carbonatite melts predominantly exsolve Na-K-sulfate-carbonate/bicarbonate-chloride brines (types III or IV). Such fluid inclusions may represent immiscible fluids that were trapped after segregation by boiling from a parental highly saline brine (type I). Fluid boiling, in turn, was probably triggered by a rapid pressure release during melt ascent. The present model enables quantification of fluid compositions associated with carbonatitic magmatism.
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Hydrothermal fluorite-hematite veins represent a relatively rare vein type, which is sometimes associated with more common, fluorite-barite-quartz‐‑carbonate veins containing base metal mineralization. The origin and significance of the fluorite-hematite veins, and how they relate to the more common expression of mineralization, is essentially unknown. Here, we present new data illuminating the formation conditions of these "“sulfide-barren"” fluorite-hematite veins in a mining district known for hydrothermal Pb + Zn + Ag ores for which formation conditions are well understood: the Schwarzwald district in SW Germany. The exemplary Ödsbach-Hesselbach vein was chosen to study the fluids involved in fluorite-hematite precipitation through the first, direct investigation of fluid inclusions hosted in hydrothermal hematite using combined infra-red light microscopy, microthermometry and LA-ICPMS. The results indicate that the hematite-precipitating fluids are nearly identical to those observed in the associated fluorite, showing consistent salinities of 24.6‐–25.0 wt.% (NaCl+CaCl2) and homogenization temperatures of 150 to 155 °C. Moreover, the trace element abundances determined by the LA-ICPMS analyses show a composition similar to other inclusions observed in the common mineralization in the mining district, including elevated Pb (>5000 μg/g) and Zn (>2000 μg/g) concentrations at low Cl/Br mass ratios, and sulfur contents mostly below the limits of detection. The results suggest fluid mixing between a continental basement brine and a redbed-derived fluid from the Triassic Buntsandstein. Therefore, the formation of the rare fluorite-hematite veins was probably triggered by a change of the sedimentary fluid endmember representing a shift from the Middle Triassic Muschelkalk fluid, which drove the more common Pb + Zn + Ag mineralization, towards fluids derived from the Buntsandstein. Furthermore, the absence of sulfur is interpreted to have precluded base metal sulfide precipitation, and the formation of hematite points to unusually oxidized conditions and hence towards a lack of a reducing agent, which would also have been required to generate a mineralization of base metal sulfides.
Article
The hydrothermal native element-arsenide mineralization comprises Ni-, Co-, and Fe-arsenides and –sulfarsenides, which typically form characteristic overgrowth textures on skeletal native silver and/or bismuth aggregates. Native arsenic, native antimony, antimony-arsenic-silver alloys, and uraninite are present in some of these deposits. Famous deposits of this type include Cobalt, Ontario; Bou Azzer, Morocco; Kongsberg, Norway; Jáchymov, Czech Republic; and Schneeberg, Germany. As the physico-chemical conditions of formation (e.g., p, T, fluid composition) and the host rocks are variable, the only unifying feature for their crystallization is a geologically fast reduction process of a Ni-, Co-, Fe-, As bearing fluid e.g., by methane, graphite or Fe2+-bearing minerals. The present contribution combines a comprehensive literature review with novel calculated stability relations of native elements (Ag, Bi, As), Ni-, Co- and Fe-mono-, di- and sulfarsenides, and sulfides/sulfosalts to understand the details of this formation mechanism and to explain the mineralogical and textural diversity observed in five-element assemblages. The characteristic sequence of Ni- → Co- → Fe-diarsenides is ubiquitous and can be explained by continuous reduction of an arsenic- and metal-bearing aqueous solution. This unique succession is largely independent of the metal ratios in the fluid, as three orders of magnitude differences between Fe, Co, and Ni concentrations are needed to change this sequence at neutral pH. At more basic conditions, the diarsenide sequence changes to Co → Ni → Fe, which has been observed at only two localities (Valais and Pirineos). Also, the prevalence of mono- versus diarsenides is mainly pH-dependent. Furthermore, differences in reduction agent, initial pH, fluid/rock ratio, and the crystallization in contained microenvironments all produce visible differences in mineralogy and/or textures that record the details of the formation processes. The stability of dissolved sulfide plays a crucial role in five-element mineralogy, as Co- and Fe-arsenides would not form in the presence of appreciable amounts of sulfide. The absence of large quantities of sulfide in the arsenide stage can be attributed to a lack of sulfur or a thermodynamic disequilibrium between sulfate and sulfide; both cases occur in nature. While the fluid prior to reduction must have been oxidized, slow reaction kinetics for the sulfate-sulfide compared to the arsenite/arsenate-arsenide conversion favor the formation of arsenides and native metals. Their formation is, hence, kinetically controlled, which is supported by the commonly skeletal textures. Sulfarsenides and/or sulfides of Co, Ni, Fe, Pb, and Cu appear only late in the five-element assemblages, when sulfide reduction progresses, and the system re-attains thermodynamic equilibrium or sufficient influx of sulfide has occurred.
Article
There is a disputable theory that hydrocarbon can be injected into the reservoir, and migrate downward in an oil reservoir with an upper oil source, which is still questioned now. However, although some examples are previously found, questions still remain about the theory, regarding oil sources, oil migration pathways, and migration forces that related to downward hydrocarbon migration. For this reason, the Chezhen Graben in the Bohai Bay Basin is selected to study all the problems above. Geo-chemical evidence show that lower Es3 source rock (T6 in seismic profiles) has a close genetic relationship with the underlying Es4 (T7 in seismic profiles) and Ordovician oil. Therefore, the “upper source-lower reservoir” type can be formed inside the lower gentle slopes and sag zones, and inside the Cambrian and Permian reservoirs. The fluid inclusion experiment shows that the lower E S 3 source rock underwent 3 over-pressure evolution stages. Meanwhile, there is a huge pressure difference between the lower E S 3 source rock and underlying E S 4 reservoirs, which is the migration force of downward migration. In the lower areas of gentle slopes and sags, the early-generated and early-ended faults are terminated at the source rock of Es3 Member, and can be re-opened under the over-pressure influence of the source rock and regional tensile stress. Then they become the pathways of the so-called downward migration. The distance of downward migration can be calculated through the accumulation dynamic, which is obtained by hydrocarbon pressure subtracting the reservoir hydrostatic pressure, pressure attenuation, and displacement pressure. The research result on downward migration process is of great significance to broaden the hydrocarbon exploration fields.
Article
Six mineralogically exemplary barren and mineralized hydrothermal veins (with Pb and Zn ores) of Jurassic-Cre-taceous and Cenozoic age in the Schwarzwald mining district, SW Germany were chosen to shed light on the origin of their mineralogical diversity. The selection of the veins was guided by the fact that they represent the largest number of veins in the region, are very well known mineralogically and geochemically, and they provide nice examples of barren and mineralized veins of similar age. Fluid inclusion data from the individual veins overlap implying their diverse mineralogy is not caused by different fluid compositions participating during fluid mixing. LA-ICPMS data of single fluid inclusions indicate no systematic variations in major elements, but significant changes in fluid mixing ratios which caused variable trace element concentrations of different fluid inclusion assemblages in one sample. We suggest that different ore-gangue-assemblages can be produced by mixing of identical fluid endmembers, but variable mixing ratios. LA-ICPMS analyses of single fluid inclusions in barren and mineralized veins record similar base metal and sulfur concentrations. Hence, sulfide mineralization in the veins appears not to be controlled by metal solubility. Thermodynamic modeling based on the fluid data indicates that the presence of a reducing phase during fluid mixing is required for sulfide mineralization to precipitate. LA-ICPMS trace element analyses of feldspars, biotites, chlorites and clay minerals in granites and parag-neisses which are the source of the metal content in the ore-forming fluids demonstrate that the dominant prove-nance for Pb, Zn, As, Sb, Ba, Tl, Mo, Fe and Mn are probably paragneisses, while Co, Cu and Ni are probably sourced from S-type granite. A rough quantification indicates that <1 m ⁠ 3 paragneiss or granite has to be altered (10% alteration) to supply sufficient Zn to 1 l of hydrothermal fluid reaching a concentration of 2 g/l Zn. Hence, this study confirms that ore fluids can be produced from any lithology in the upper crust that contains at least some trace metals (μg/g level is sufficient).
Article
Understanding the physical basics of tectonic events, related fluid flow and ore deposition represents one of the great challenges in modern geosciences. In this contribution, Sm-Nd ages of hydrothermal fluorites and U-Pb ages of carbonates and iron oxides from unconformity-related vein type deposits in the Schwarzwald next to the Upper Rhinegraben rift in SW Germany are used to distinguish different pulses of hydrothermal fluid activity and to understand their relation to large-scale tectonics and magmatism. While the fluorites are of Early Jurassic to Oligocene age, carbonate (calcite, dolomite, siderite) and hematite dated by U-Pb small-scale isochrons records formation from Permian to Quaternary with a clear culmination in the Neogene. These new age-data in combination with microthermometry data of primary fluid inclusions from growth zones in the fluorites and carbonates are used to constrain the timing of fluid signatures. This contribution shows that the ages are correlated with changes in fluid properties and/or tectonic events in the evolving continental crust. Comparison with published thermochronological data, apparent ages of URG-related volcanic rocks and the tectono-sedimentary evolution of the Upper Rhinegraben rift show clear correlations between the intensity of hydrothermal mineralization (and, hence, the intensity of fracture-bound fluid flow) with the U-Pb carbonate ages. This method accordingly provides an excellent tool to date rift-related processes like fluid flow, ore deposition and tectonic activation or reactivation of fractures. Fluid properties changed after the deposition of Middle Triassic Muschelkalk evaporites from low salinity, high temperature (1 - 6 wt.% NaCleq, 200-270°C, cooling late-metamorphic basement fluids) to high salinity, moderate temperature (20 - 26 wt.% (NaCl+CaCl2), 50-170°C, mixture of a modified bittern brine (“basement brine”) with halite dissolution brine). This change led to large scale ore deposition (fluorite-barite-quartz with Pb-Zn-Cu, Bi-Co-Ni-Ag-U, Fe-Mn ores). During the Paleogene and Neogene, previously separated aquifers from various sedimentary units were connected by juxtaposition of the fluid source rocks during Rhinegraben rifting, which resulted in variable salinity and temperature fluids (1 – 26 wt. % (NaCl+CaCl2), 50-350°C) by a multi-component fluid mixing process. Typical mineralization related to this shows barren or Pb-Zn-Cu veins with large amounts of barite.
Article
Hydrothermal five-element veins (Ag-Co-Ni-Bi-As) are mineral successions of native metals, encapsulated by Fe-Co-Ni arsenides and carbonates. Recent studies focused on the evolution from ordinary base-metal systems (sulfide-rich) to five-element veins (sulfide-poor) and revealed the importance of hydrocarbon-dominated fluids as essential redox agent in these systems. Although mineral successions reveal the natural subdivision into native As-, native Ag/Bi- or arsenide-dominated vein types and suggested processes explain the mineralogical variation to a certain degree, the Ni-Co-Fe-variations among the arsenides are not well understood yet. This is the first case study explaining compositional, mineralogical, and textural features of five-element veins by changing metal contents, arsenic/sulfur activities, pH and temperatures, applied to a multi-stage vein mineralization in the Penninic Alps, Switzerland. Textural relationships, mineral chemistry, fluid inclusion compositions (microthermometry and Raman spectroscopy), stable S-isotopes and in-situ U-Pb age dating of carbonates, magnetite and multi-mineral isochrons were investigated. U-Pb ages of paragenetic mineral fractions constrain a primary formation of löllingite-skutterudite-dolomite-dominated ores at 233 ± 10 Ma and niccolite-gersdorffite-skutterudite-ankerite-dominated ores at 188 ± 32 Ma, which links their formation to crustal thinning caused by the breakup of the Meliata ocean and Alpine Tethys. As secondary processes during the Alpine Orogeny, a in-situ remobilization of the ores occurred as mostly ternary Fe-Co-Ni sulfarsenides at ∼73–24 Ma due to continent-continent collision and neoformations of safflorite-cobaltite-skutterudite-dominated ores at ∼29–16 Ma due to transtensional strike-slip tectonics. Ore textures indicate that the dissolution of primary siderite, oxidation of ferrous iron and its precipitation as magnetite was the redox couple to precipitate native Bi, arsenides and sulfarsenides (Bi⁰, As³⁻ and As⁻¹) from their oxidized aqueous species Bi³⁺Cl4⁻, and As³⁺(OH)3 at temperatures between 200 and 300 °C. Ni and Co signatures of the arsenides/sulfarsenides show increasing mobilization from the host rocks during the successive evolution of the hydrothermal system in the Triassic, Jurassic and during the Alpine Orogeny. Stable S-isotopes and As/S signatures of the sulfarsenides indicate an increasing mobilization of host rock sulfides (i.e. fahlbands) from the primary to the secondary ores. Fluid inclusions and stable S-isotopes suggest involvement of fluid modification by water-rock interactions (i.e. cover rocks: carbonate, sulfate and halite dissolution; basement rocks: albitisation of plagioclase) during fluid descent.
Article
Fluid mixing is an important process in the formation of many hydrothermal vein-type deposits. Here, we present evidence from hydrothermal fluorite-barite-quartz veins with Pb-Zn-Cu-(Ag)-sulfides and associated mineralization, indicating that mineral precipitation was initiated by mixing of fluids derived from multiple sources, including mixing between more than two end-member fluid compositions. Based on our observations, we relate the diversity of the hydrothermal veins of the Schwarzwald mining district in terms of mineral assemblage and fluid inclusion chemistry to the disturbed and transient geological environment during ongoing rifting. Literature data on the regional geology, current groundwater reservoirs, formation processes and hydraulic features are augmented by new fluid inclusion analyses from post-Cretaceous, hydrothermal vein minerals including microthermometry, crush leach, Microraman and LA-ICP-MS analyses of individual fluid inclusions. Petrography and microthermometry of fluid inclusions show complex sequences of alternating fluid signatures within different growth zones of one crystal. High (20–26 wt% NaCl + CaCl2), moderate (5–20 wt% NaCl + CaCl2) and low salinity (< 5 wt% NaCl + CaCl2), sulfate- and/or CO2-bearing primary fluids were trapped during crystal growth. Such variations are commonly observed in minerals from different localities. Bulk crush leach analyses show significant variations in major element composition of the trapped fluids, within the overall Na-Ca-Cl-SO4-HCO3-system. These variations are caused by mixing of fluids from different aquifers and in various proportions. Ancient fluids show chemical similarities to modern groundwater aquifers that are available for direct sampling, such as granitic basement, Lower Triassic sandstones or Middle Triassic limestones and evaporites. Analyses of individual fluid inclusions by LA-ICP-MS support this interpretation and document the multi-component fluid mixing processes at individual localities recorded on the scale of single crystal growth zones. The latter data are used in a diffusion model to obtain the duration of mineral growth (before the fluid is homogenized), which implies very short-lived fluid events on the order of seconds to hours. By defining end member fluids and their proportions, we show that nearly all fluid mixtures are saturated with respect to barite. By contrast, fluorite-saturated fluids can only be modelled by mixing of a basement brine with fluids from Triassic sandstones. All fluid mixtures are strongly undersaturated with respect to galena, chalcopyrite and sphalerite, the most commonly observed ore minerals in the hydrothermal veins. As the calculated fluid mixtures are typically relatively oxidized and contain high sulfate/sulfide ratios, precipitation of sulfides was probably related to short-lived reduction events caused by an influx of hydrocarbons, by reactions with graphitic wall rocks in fractures by sulfidation related to fluid-rock reaction with the surrounding host rocks or an external influx of hydrocarbon-bearing fluids. The multi-aquifer fluid mixing processes involving aquifers of different chemical and physical constitution were triggered by brittle deformation related to rifting of the Rhine graben. This appears to be essential for the formation of a large number of mineralogically diverse hydrothermal ore deposits.
Article
The majority of hydrothermal vein systems of economic interest occur at relatively shallow crustal levels, although many of them formed at significantly greater depths. Their present position is a consequence of uplift and erosion. Although, many aspects of their formation are well constrained, the temporal chemical evolution of such systems during uplift and erosion is still poorly understood. These vein minerals comprise calcite, dolomite-ankerite, siderite-magnesite, anhydrite and gypsum forming the last gangue assemblages in Jurassic and Tertiary sulphide-fluorite-quartz-barite veins of the Schwarzwald mining district, SW Germany. Mineral textures of samples from nine localities reveal that in these sequences, mineral precipitation follows a recurring pattern: early calcite is followed by anhydrite or gypsum, siderite and/or dolomite. This succession may repeat up to three times. In-situ (LA-ICP-MS) U-Pb age dating of 15 carbonates from three subsequent generations of the late-stage vein assemblage yield robust ages between 20 to 0.6 Ma. Each mineral sequence forms in a distinctive period of about 2-5 Ma. These ages clearly relate these late-stage mineral phases to the youngest geological episode of the Schwarzwald, which is associated with the Cenozoic Rhine Graben rifting and basement uplift. Based on thermodynamic modelling, the formation of the observed mineral assemblages required an deeply sourced Mg-, Fe- and SO4- rich fluid (b), which was episodically mixed with a shallow crustal HCO3-rich fluid (a). As a consequence of fluid mixing, concentrations of Mg, Fe and SO4 temporarily increased and initiated the formation of the observed sulphate-carbonate mineral sequences. This discontinuous large-scale vertical fluid mixing was presumably directly related to episodes of active tectonics associated with the Cenozoic strike-slip regime of the Upper Rhine Graben. Analogously, episodic fluid mixing is a major key in the formation of older (Jurassic to early Tertiary) Pb-Zn-fluorite-quartz-barite assemblages in the same specific vein systems, albeit involving different fluid compositions. Late-stage hydrothermal (∼20-70 °C) vein assemblages reported in this study record the transition from deep (> 2 km) to very shallow (0-1 km) crustal conditions. As a consequence of successive uplift, increasing proportions of shallower and cooler (∼50-70 °C) fluids could take part in such mixing processes. Associated changes in the fluid composition caused the vein mineralogy to change from sulphide-quartz-fluorite-barite to calcite-anhydrite/gypsum-siderite-dolomite, as the system passively ascended closer to the surface.
Article
The Cretaceous Koegel Fontein igneous complex is situated on the west coast of South Africa, and has a high proportion of rocks with abnormally low δ¹⁸O values. The rocks with the lowest δ¹⁸O values (− 5.2‰) belong to intrusive matrix-supported breccia pipes and dykes, containing a variety of clast types. The breccia rocks range in SiO2 from 44 to 68 wt% and their whole-rock δ¹⁸O values vary between − 5.2‰ and + 1.8‰. The major and trace element composition of the breccia rocks is consistent with them containing variable proportions of clasts of Cretaceous intrusive rocks and basement gneiss and the matrix being fluidized material derived from the same source as the clasts. Based on the nature of the clasts contained in the breccia, it was emplaced just prior to intrusion of the main Rietpoort Granite at 134 Ma. All components of the breccia have low δ¹⁸O value and, at least in the case of the gneiss clasts, this predates incorporation in the fluidized material. Although the early Cretaceous appears to have been a period of cold climate, it is unlikely that the δ¹⁸O values of ambient precipitation (~− 10‰) would have been low enough to have generated the required ¹⁸O-depletion. The basement gneiss was probably ~ 2–3 km below the Cretaceous surface, minimizing the possibility of interaction with isotopically unmodified meteoric water, and there is no evidence for foundered blocks of cover rocks in the breccia. There is, therefore, no evidence for downwards movement of material. We favour a model where basement gneiss interacted with extremely ¹⁸O-depleted fluid during crustal reworking at ~ 547 Ma, a time of global glaciation. Low-δ¹⁸O metamorphic fluids produced by dehydration melting of ¹⁸O-depleted gneiss became trapped and, as the fluid pressure increased, failure of the seal resulted in explosive upwards movement of fluidized breccia. Migration was along pre-existing dykes, incorporating fragments of these dykes, as well as the country rock gneiss.
Article
Sulfate is among the most abundant ions in seawater and sulfate-bearing brines are common in sedimentary basins, among other environments. However, the properties of sulfate-bearing fluid inclusions during microthermometry are as yet poorly constrained, restricting the interpretation of fluid-inclusion compositions where sulfate is a major ion. The Schwarzwald mining district on the eastern shoulder of the Upper Rhinegraben rift is an example of a geologic system characterized by sulfate-bearing brines, and constraints on the anion abundances (chloride versus sulfate) would be desirable as a potential means to differentiate fluid sources in hydrothermal veins in these regions. Here, we use the Pitzer-type formalism to calculate equilibrium conditions along the vapor-saturated liquidus of the system H2O-Na-Ca-Cl-SO4, and construct phase diagrams displaying the predicted phase equilibria. We combine these predicted phase relations with microthermometric and crush-leach analyses of fluid inclusions from veins in the Schwarzwald and Upper Rhinegraben, to estimate the compositions of these brines in terms of bulk salinity as well as cation and anion loads (sodium versus calcium, and chloride versus sulfate). These data indicate systematic differences in fluid compositions recorded by fluid inclusions, and demonstrate the application of detailed low-temperature microthermometry to determine compositions of sulfate-bearing brines. Thus, these data provide new constraints on fluid sources and paleo-hydrology of these classic basin-hosted ore-forming systems. Moreover, the phase diagrams presented herein can be applied directly to compositional determinations in other systems.
Article
Late-stage pseudomorphous and perimorphous replacement of euhedral barite and, to a lesser extent fluorite and calcite by quartz is a common phenomenon in hydrothermal vein-type deposits. As a consequence of silicification, the primary mineral assemblage might be substantially altered and therefore, this process has a severe negative impact on the economic potential of mineral resources. Although these replacement textures are often reported and have a significant economic importance in mines producing barite and/or fluorite of chemical grade, the process that causes this silicification is surprisingly poorly understood. In the present contribution, more than 40 Jurassic-Cretaceous and post-Cretaceous hydrothermal veins from the Schwarzwald mining district including replacement textures of primary euhedral barite, fluorite and/or calcite were investigated with respect to their macro- and microscopic textures. It appears that barite is favourably replaced by pseudomorphs (bladed quartz) while fluorite and calcite are typically replaced by perimorphs. The textures indicate that the mode of replacement of the primary minerals happens continuously and after the initial vein formation. By combining these textural observations with calculated mineral solubilities, a detailed geochemical model has been developed. Existing fluid inclusion data indicate that substantial cooling of the hydrothermal solutions occurs after primary mineral formation. The calculated cooling path reveals opposing solubilities of quartz and the other gangue minerals (barite, fluorite and calcite) with decreasing temperature and explains the observed dissolution and precipitation textures. Furthermore, differences in temperature-solubility systematics between barite on the one hand and fluorite and calcite on the other hand are responsible for the differences observed in textures. This agrees with the occurrence of late-stage, low temperature barite crystals overgrowing primary barite assemblages. Conversely, analogous late-stage calcite and fluorite assemblages are only rarely observed. In summary, silicification is a typical cooling effect in various hydrothermal vein-type deposits but affects different gangue minerals in different ways depending on their temperature-dependent solubility.
Article
A series of theoretical corrections for particles of a variety of idealized geometric shapes was developed. Use of these corrections, along with careful preparation and analysis procedures, can yield routine and accurate quantitative analyses.
Article
Five-element veins (Ag, Bi, Co, Ni, As) have been valuable mineral deposits since medieval times. The characteristic occurrence of large aggregates of native metals (up to several dm) surrounded by a succession of arsenides makes this vein-type attractive for mining industry, natural history museums and private mineral collectors. Nevertheless, the exact formation process of these specific vein types has not been fully understood. This is the first case study applying a new model, which includes methane as a reducing agent to two typical examples of such mineralisations from the Odenwald, SW Germany. We analysed all mineralogical varieties (Ag-, As- and Bi-dominated) of the five-element veins in the Odenwald (SW Germany) in terms of ore textures, mineral chemistry, fluid inclusion compositions (microthermometry and Raman spectroscopy), stable isotopes (C, O and S) and in-situ U-Pb age dating of calcite and prehnite. A variety of Ag-, Bi- and As-dominated native metal-arsenide-calcite veins, sulphide-calcite veins and arsenide-free Ag-Hg-barite veins occurs in the Odenwald and has been examined in this study. All arsenide veins have in common that up to dm-sized, often dendritic native metals are overgrown by a succession of arsenides, followed by carbonate and finally sulphides. The succession of arsenides shows a distinct spatial and temporal chemical trend in their composition. This trend evolves from Ni- to Co and finally Fe-dominated compositions from the core to the rim. In contrast, spatially closely related Ag-Hg-barite veins consist of almost mono-mineralic amalgam inter-grown with barite. In-situ U-Pb age dating of low-U calcite and prehnite was applied to constrain the age of hydrothermal mineralization. The results imply that the five-element veins formed at 170-180 Ma from Na-Ca-Cl fluids at 290°C, salinities of ~27 wt.% and Ca/(Ca+Na) of 0.30 to 0.35 in the presence of methane. The age data clearly relate the relevant fluid migration to extension and crustal thinning caused by the opening of the North Atlantic. Ternary mixing of a deep-seated metal-rich basement brine (fluid A), a sulfide-bearing (H2S and HS-) basinal/sedimentary brine (fluid B) and methane-dominated fluid/gas (fluid C) induce ore formation. Mixing of such chemically contrasting fluids results in a strong chemical disequilibrium of the mixed fluid, which potentially leads to rapid precipitation of native metals and arsenides with these specific ore textures. In contrast, sulphide-bearing calcite veins formed under similar P-T-conditions due to mixing of fluid A and B, while fluid C was absent. Additionally, U-Pb analyses of post-ore calcite yields ages of ~60 Ma, which indicates that a second calcite formation event is associated with the onset of the Upper Rhine Graben rifting. Veins with amalgam and barite form as a consequence of post-ore oxidation of Ag2S at ~135°C. Conspicuously, this secondary silver II has Hg contents of up to ~30 wt.%, in contrast to Hg contents below 1 wt.% of primary silver I and Ag2S. The formation of amalgam was most likely related to a decrease of the S2-/SO42- ratio due to cooling of the late hydrothermal fluid resulting in the destabilisation of Hg-bisulfide complexes in this fluid.
Chapter
Morocco has been for the past two centuries one of the top ten Pb–Zn producers with two thirds of base-metal production derived from three major Mississippi Valley-type districts (Touissit-Bou Beker, Upper Moulouya, and Jbel Bou Dahar). Collectively, these districts have produced more than 100 Mt of ore at an average grade of ~3 wt% Zn and 4 wt% Pb. At the present time, none of the three districts is active. Economic orebodies are hosted by a succession of Lower to Middle Jurassic unmetamorphosed, platform carbonate rocks. The epigenetic and stratabound sulphide deposits occur as open-space fillings and metasomatic replacements of carbonate. Mineralization that fills open spaces (i.e., veins, interconnected cavities, solution-collapse breccias) accounts for most of the higher grade orebodies. Overall, the mineral paragenesis consists principally of variable proportions of sphalerite and galena, accompanied by different generations of saddle dolomite (Touissit-Bou Beker), calcite (Jbel Bou Dahar), or barite (Mibladen). In all of the districts, paleogeographic reconstructions indicate that the orebodies are located above basement topographic highs against which the Triassic and Early Jurassic formations pinch out. Regionally, ENE–WSW- and E–W-trending faults appear to have been a critical factor in ore genesis, having provided favorable fluid channels for metal-bearing brines into permeable host rocks and dissolution structures. The geometry of the orebodies that parallel the major alpine faults, coupled with lead isotopic constraints, suggest that the MVT mineralizing event occurred during midle Tertiary time (i.e., Cretaceous to Miocene) coincident with closing stages of the Alpine orogeny in the Atlasic orogenic belt. Alpine mineralization is thought to have been promoted by the mixing of older, high-temperature, rock-buffered, dense brines stored within the Paleozoic basement, and a downwelling, cooler fluid probably of meteoric origin. The resulting mixed brines were centered mainly on the basement high structure and its flanks, then flowed laterally away from the basement high and giving rise to the lower grade mineralization of the distal prospects. Fluid migration towards the ore districts could have been achieved either by a gravity-driven system (Touissit-Bou Beker, Mibladen, and Jbel Bou Dahar) or sediment compaction in the foredeep (Jbel Bou Dahar), or a combination of both. An alternative buoyancy-driven fluid convection model is proposed for the Touissit-Bou Beker MVT mineralization.
Article
We investigated the potential of common crystalline rocks to facilitate the geochemical evolution of continental basement brines and to serve as a metal source for hydrothermal ore deposits. We performed leaching experiments on typical crystalline basement rocks (granite and gneiss), a redbed sandstone and their mineral separates (feldspar, quartz and biotite) at variable T (25, 180, 275 and 350 °C), P (ambient pressure, 0.9, 1.4 and 1.9 kbar), grain-size fractions (< 0.01 mm, 0.063–0.125 and 2–4 mm) and variable fluid/rock ratios (10 to 1.1) with ultrapure water and 25 wt.% NaCl solution as solvents. The modification of the fluid chemistry during water–rock interaction strongly depends on grain-size: leachates (using pure H2O) of fine-grained rock powders have lower Na/Cl and Cl/Br ratios but much higher chlorinities (by a factor of up to 40) compared to leachates from coarse-grained rock powders. The Cl/Br ratios of all leachates are lower than that of their respective whole-rocks. Smaller grain-sizes of the starting materials yield element ratios (Cl/Br and Na/Cl) similar to those found in natural fluids, emphasizing the influence of cataclastic deformation on the fluid chemistry of crustal fluids. During our leaching experiments, Pb, Zn, Cu and W are released by felsic minerals, while biotite alteration releases Ni, As and additional Zn and Cu. Our experiments confirm that crystalline rocks may serve as metal source for hydrothermal ore deposits. Short-term water–rock interactions along cataclastic fault zones in the brittle crust may influence the geochemical evolution of upper crustal fluids. This is further suggested by low F/Cl and Cl/Br ratios in some of the leachates being very similar to halogen systematics in natural fluid samples.