Dolomite Host Rock Research Articles

Genesis of As-Pb-Rich Supergene Mineralization: The Tazalaght and Agoujgal Cu Deposits (Moroccan Anti-Atlas Copperbelt)

Abstract In the Moroccan Anti-Atlas, sulfide deposits hosted by Neoproterozoic to Cambrian formations underwent significant weathering, leading to the formation of supergene profiles. In the Tazalaght Cu-As deposit, three mineralogical steps are distinguished: (1) the replacement of hypogene sulfides (chalcopyrite, pyrite, tennantite) by supergene sulfides (bornite, chalcocite) in the large cementation zone; (2) the formation of oxidized minerals (malachite, azurite, olivenite, and chenevixite, mainly) in a more oxidizing and neutral environment; and (3) the precipitation of goethite, hematite, and quartz in the gossan. In the Cu-As-Pb-V deposit of Agoujgal, the mineralogical units are spatially less confined than at Tazalaght. The narrow cementation zone hosts chalcocite, resulting from the weathering of hypogene chalcopyrite, pyrite, tennantite and galena, while the much more extended and diversified oxidized zone is rich in Cu and Pb carbonates, arsenates, sulfates, phosphates, vanadates, and oxides. Goethite, hematite, mottramite, and late calcite occur in the gossan. Both deposits are characterized by As-rich secondary ores that were formed through similar processes, despite some mineralogical and chemical variations highlighting the influence of the host rocks on weathering. The restricted oxidized mineralization at Tazalaght and the Agoujgal cementation zone most likely arise from the contrasting omnipresence of quartzite at Tazalaght that could not enable a fast and effective neutralization of the fluid’s acidity, and the large amounts of dolomitic host rocks that could be dissolved at Agoujgal. At both sites, the weathering of tennantite through a boxwork texture records the transition from the cementation zone (chalcocite), the oxidized zone (arsenates), and the gossan, and reflects the fluids evolution with time.

Emittance Spectroscopy and Broadband Thermal Remote Sensing Applied to Phosphorite and Its Utility in Geoexploration: A Study in the Parts of Rajasthan, India

The contrast in the emissivity spectra of phosphorite and associated carbonate rock can be used as a guide to delineate phosphorite within dolomite. The thermal emissivity spectrum of phosphorite is characterized by a strong doublet emissivity feature with their absorption minima at 9 µm and 9.5 µm; whereas, host rock dolomite has relatively subdued emissivity minima at ~9 µm. Using the contrast in the emissivity spectra of phosphorite and dolomite, data obtained by the thermal bands of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) sensor were processed to delineate phosphorite within dolomite. A decorrelation stretched ASTER radiance composite could not enhance phosphorite rich zones within the dolomite host rock. However, a decorrelation stretched image composite of selected emissivity bands derived using the emissivity normalization method was suitable to enhance large surface exposures of phosphorite. We have found that the depth of the emissivity minima of phosphorite gradually has increased from dolomite to high-grade phosphorite, while low-grade phosphate has an intermediate emissivity value and the emissivity feature can be studied using three thermal bands of ASTER. In this context, we also propose a relative band depth (RBD) image using selected emissivity bands (bands 11, 12, and 13) to delineate phosphorite from the host rock. We also propose that the RBD image can be used as a proxy to estimate the relative grades of phosphorites, provided the surface exposures of phosphorite are large enough to subdue the role of intrapixel spectral mixing, which can also influence the depth of the diagnostic feature along with the grade. We have validated the phosphorite pixels of the RBD image in the field by carrying out colorimetric analysis to confirm the presence of phosphorite. The result of the study indicates the utility of the proposed relative band depth image derived using ASTER TIR bands for delineating Proterozoic carbonate-hosted phosphorite.

Groundwater and potentially toxic elements in a dismissed mining area: Thallium contamination of drinking spring water in the Apuan Alps (Tuscany, Italy)

Thallium is among the most toxic elements to humans. Environmental regulations of the European Community adopted by Italy pose a maximum concentration level for Tl in groundwater of 2 μg/L. However, at present drinkable water must not abide any concentration threshold for thallium and this element is not routinely monitored. In the past-mining area nearby the Valdicastello Carducci village (northern Tuscany, Italy), characterized by Tl-bearing ores, water from a spring that has been tapped for human consumption contains thallium in the range 4.3–27.8 μg/L. The challenge of this study is to highlight the mechanisms leading to Tl contamination in groundwater, by crossing hydrochemical and O-H isotopic data with hydrogeological information. We propose that the Tl-contamination in the spring results from a binary mixing that occurs within a karst system involving an uncontaminated Ca-HCO3 water (mainly interacting with limestone) and a component originated by the interaction of acid drainages (generated by the weathering of Tl-bearing mineral phases in the area) with dolomitic host rocks. In this process, most of the potentially toxic elements released during the weathering are scavenged by the carbonate neutralization of the acid plume due to adsorption onto the iron and manganese oxyhydroxides that formed during pH changes. Nevertheless, the aquifer has little or no attenuation capacity for thallium. Moreover, O-H isotopic data and hydrogeological constraints are compatible with a spring feeding mechanism related to a piston displacement effect.

Ground-based hyperspectral imaging as a tool to identify different carbonate phases in natural cliffs

ABSTRACTRecent research has shown hyperspectral imaging to be a powerful tool to distinguish carbonate phases with slight compositional differences on quarry cliff faces. The traditional remote sensing set-up uses an optimal short distance between the hyperspectral camera mounted on a tripod and a quarry wall characterized by a planar, mostly unweathered surface. Here we present results of a modified workflow geared to the application of ground-based hyperspectral imaging of rough and weathered cliff faces in order to map large scale dolomite bodies from a distance of up to several kilometres. The goal of the study was to determine unique spectral properties of fracture-controlled dolomite bodies in order to be able to distinguish them from a dolomitic host rock. In addition, the impact of weathering on carbonate phases and thus, the modification of the spectral signature between altered and unaltered carbonates is assessed. The spectral analysis is complemented by ICP-AES (inductively coupled plasma atomic emission spectroscopy) measurements of the spectrally measured powders. Furthermore, we examined the detection limits and characterisation potential of dolomite bodies from hyperspectral images captured at varying distances from cliff faces in the study area. Hyperspectral images of 10 natural cliffs distributed across the Central Oman Mountains were obtained with a Push broom scanner system. The high resolution of 5.45 nm (288 bands in total) enabled the visualization of small-scale changes in the near infrared continuous spectrum of all present lithofacies types. The determination of dolomite bodies of varying sizes (metre to hundreds of metres) on natural cliffs was achieved with the hyperspectral mapping approach and mapping results have been tested with the position of visually defined dolomite bodies on field panoramas. Spectra of natural cliffs contain a strong absorption peak indicative for iron which is absent in spectra of unweathered sample powders. However, ICP-AES analysis of powders revealed relatively low contents of iron of 12,392 ppm. The strong peaks in field images are interpreted as linked to intensive weathering associated with the precepitation of goethite, hematite, specularite and manganese as well as intensive dedolomitization. Dedolomitization is indicated by calcitic spectra derived from the dolomite bodies. The spectral difference of laboratory and field spectra interferes significantly the application of laboratory spectra of powdered samples for the identification of dolomite bodies in the field. Furthermore, the process of late dedolomitization puts an additional challenge on the determination of dolomite bodies. Due to these strong spectral variations between laboratory and field spectra, we recommend that the mapping approaches should not solely rely on spectral algorithms but also consider normal light field panoramas and representative outcrop analysis. We also note that the quality of resolution is too low for the determination of small-scale variations of diagenetic phases at distances larger than 4 km. However, when the limitations mentioned are taken into account, hyperspectral imaging proves to be a powerful tool that helps in the determination of the distribution of diagenetic phases, even in challenging conditions.

Mineralogical and geochemical characterization of supergene Cu–Pb–Zn–V ores in the Oriental High Atlas, Morocco

In the Moroccan High Atlas, two sulfide deposits hosted by Jurassic dolostones underwent significant weathering. In the Cu deposit of Jbel Klakh, several stages of supergene mineralization are distinguished: (1) the replacement of hypogene sulfides in the protolith (chalcopyrite) by secondary sulfides in the cementation zone (bornite, digenite, chalcocite, covellite), (2) the formation of oxidized minerals in the saprolite (malachite, azurite, brochantite) where the environment becomes more oxidizing and neutral, and (3) the precipitation of late carbonates (calcite) and iron (hydr-)oxides in the laterite. The precipitation of carbonates is related to the dissolution of dolomitic host rocks, which buffers the fluid acidity due to the oxidation of sulfides. In the Jbel Haouanit Pb–Zn deposit, the mineral assemblage is dominated by typical calamine minerals, Cu minerals (chalcocite, covellite, malachite), and a Cu–Pb–Zn vanadate (mottramite). Galena is successively weathered in anglesite and cerussite. Sphalerite is weathered in smithsonite, which is rapidly replaced by hydrozincite. Late iron (hydr-)oxides are mainly found at the top of both deposits (laterite). Both deposits are thus characterized by specific mineral zoning, from laterite to protolith, related to variations in the mineralogy and ore grades and probably caused by varying Eh-pH conditions.

Hypogene speleogenesis in dolomite host rock by CO2-rich fluids, Kozak Cave (southern Austria)

A growing number of studies suggest that cave formation by deep-seated groundwater (hypogene) is a more common process of subsurface water–rock interaction than previously thought. Fossil hypogene caves are identified by a characteristic suite of morphological features on different spatial scales. In addition, mineral deposits (speleothems) may provide clues about the chemical composition of the paleowater, which range from CO2-rich to sulfuric acid-bearing waters. This is one of the first studies to examine hypogene cave formation in dolomite. Kozak Cave is a fossil cave near the Periadriatic Lineament, an area known for its abundance of CO2-rich springs. The cave displays a number of macro-, meso- and micromorphological elements found also in other hypogene caves hosted in limestone, marble or gypsum, including cupolas, cusps, Laughöhle-type chambers and notches. The existance of cupolas and cusps suggests a thermal gradient capable of sustaining free convection during a first phase of speleogenesis, while triangular cross sections (Laughöhle morphology) indicate subsequent density-driven convection close to the paleowater table. Notches mark the final emergence of the cave due to continued rock uplift and valley incision. Very narrow shafts near the end of the cave may be part of the initial feeder system, but an epigene (vadose) overprint cannot be ruled out. Vadose speleothems indicate that the phreatic phase ended at least about half a million years ago. Drill cores show no evidence of carbon or oxygen isotope alteration of the wall rock. This is in contrast to similar studies in limestone caves, and highlights the need for further wall-rock studies of caves hosted in limestone and dolomite.

Genetic constraints on world-class carbonate- and siliciclastic-hosted stratabound fluorite deposits in Burgundy (France) inferred from mineral paragenetic sequence and fluid inclusion studies

Stratabound fluorite deposits occur at the unconformity between the Variscan crystalline basement and the Mesozoic sandstone, conglomerate, limestone, and dolomite rocks of the Morvan Massif in central Burgundy. This study describes their petrographic characteristics in an attempt to determine the nature and temperature of mineralizing fluids in order to better understand the fluid migrations that led to massive stratabound fluorite deposition. The general paragenesis encompasses two major mineralizing events causing a succession of fluorite, barite, and quartz in all deposits. The two mineralizing events were preceded by two corrosion (dissolution or karstification) events affecting both the dolomite host rock at Pierre-Perthuis and Marigny-sur-Yonne and the limestone host rock at Courcelles-Frémoy with the creation of 1–10m cavities and microscopic vugs. At Antully, the blocky calcite initially cementing the sandstone was partially dissolved. Microthermometric data on aqueous two-phase inclusions attest to CaCl2-rich fluids giving rise to fluorite deposition in the Pierre-Perthuis, Courcelles-Frémoy, and Antully deposits. Homogenization temperatures range from 80 to 100°C at Pierre-Perthuis and Courcelles-Frémoy, with sporadically higher temperatures. The range of CaCl2 contents is 6.5–15wt.% at Pierre-Perthuis, 1.7–9.4wt.% at Courcelles-Frémoy, and 1.6–16.3wt.% at Antully. The thermal history of the northwestern Morvan, compiled from organic matter, clay minerals and apatite fission track data indicates that the temperatures in fluorite and barite are higher than the maximum temperature recorded in sediments. This implies deep ascendant hydrothermal brine circulation during the Early Cretaceous. The impermeable cap rock retained the ascendant hydrothermal brine and allowed the deposition of massive fluorite stratabound mineralizations.

Rock pulverization and localization of a strike-slip fault zone in dolomite rocks (Salzach–Ennstal–Mariazell–Puchberg fault, Austria)

Detailed investigations of dolomite fault rocks, formed at shallow crustal depths along the Salzach–Ennstal–Mariazell–Puchberg (SEMP) fault system in the Northern Calcareous Alps, revealed new insights into cataclasite formation. The examined Miocene, sinistral strike-slip faults reveal grain size reduction of dolomite host rocks by tensile microfracturing at a large range of scales, producing rock fragments of centimetre to micrometre sizes. In situ fracturing leads to grain size reduction down to grain sizes <25 μm, producing mosaic breccias and fault rocks which have previously been described as “initial/embryonic” and “intermediate” cataclasites. At all scales, grain fragments display little to no rotation and no or minor evidence of shear deformation. The observed microstructures are similar to those previously described in studies on pulverized rocks. Microstructural investigations of cataclasites and mosaic breccias revealed aggregations of small dolomite grains (<50 μm) that accumulated on top of large fragments or as infillings of V-shaped voids between larger grains and show constant polarity throughout the investigated samples. Fabrics indicate deposition in formerly open pore space and subsequent polyphase cementation. The newly described tectonic geopetal fabrics (geopetal-particle-aggregates, GPA) prove that these faults temporarily passed through a stage of extremely high porosity/permeability prior to partial cementation.

Hydrothermal Dolomite in the Upper Sinian (Upper Proterozoic) Dengying Formation, East Sichuan Basin, China

Hydrothermal Dolomite (HTD) is present in the Upper Sinian (Upper Proterozoic) Dengying Formation, east Sichuan Basin, China. The strata are comprised by primary dolomite. The HTD has various textures, including zebra dolomite, subhorizontal sheet-like cavities filled by saddle dolomite and breccias cemented by saddle dolomites as well occur as a fill of veins and fractures. Also co-occur MVT type lead-zinc ores in the study area. The δ13C and δ18O isotopes of HTD in the Upper Sinian Dengying Formation are lighter than those of the host rocks, while 87Sr/87Sr is higher. The apparent difference in carbon, oxygen and strontium isotopes, especially the large difference in 87Sr/86Sr isotopes ratio indicate crystallization from hot basinal and/or hydrothermal fluids. Saddle dolomite was precipitated at temperatures of 270–320° C. The diagenetic parasequences of mineral assemblage deposited in the Dengying Formation are: (1) dolomite host rock sphalerite-galena-barite-fluorite; (2) dolomite host rock saddle dolomite quartz; (3) dolomite host rock saddle dolomite bitumen; (4) dolomite host rock saddle dolomite barite. The mean chemical composition of the host dolomite matrix and HTD didn't change much during hydrothermal process. The fluids forming the HTDs in the Dengying Formation were mixtures of freshwater from the unconformity at the top of Sinian, fluids from diagenetic compaction and hydrocarbon generation & expulsion from the Lower Cambrian Niutitang Formation mudstones or the Doushantuo Formation silty mudstones, and hydrothermal fluids from the basement The hydrocarbon reservoirs associated with the HTD were mostly controlled by the basement faults and fractures and karsting processes at the unconformity separating Sinian and Cambrian strata. The hydrocarbon storage spaces of HTD included dissolved cavities and intercrystalline pores. Dissolution cavities are extensive at the top of Dengying Formation, up to about 46m below the unconformity between Sinian and Cambrian and were generated mainly during karstification. Hydrothermal alteration enhanced the reservoir property of the Dengying Formation dolomites with 3%–5% increase in porosity. No agreement has been reached why zebra dolomite occurs only in the Upper Sinian strata, which would indicate that HTD mineralization occurred during two different periods, each of them related to major extensional tectonic event. The early one related to the Xingkai taphrogenesis (Z2−€1) and the later one to the Emei taphrogenesis (D2-T2). But, all the data from saddle dolomite suggest that the predominant crystallization occurred during the latter event.

The Jabali nonsulfide Zn–Pb–Ag deposit, western Yemen

The Jabali Zn–Pb–Ag deposit is located about 110km east of Sana'a, the capital of Yemen, along the western border of the Marib-Al-Jawf/Sab'atayn basin. The economic mineralization at Jabali is a nonsulfide deposit, consisting of 8.7milliontons at an average grade of 9.2% zinc, derived from the oxidation of primary sulfides. The rock hosting both primary and secondary ores is a strongly dolomitized carbonate platform limestone of the Jurassic Shuqra Formation (Amran Group). The primary sulfides consist of sphalerite, galena and pyrite/marcasite. Smithsonite is the most abundant economic mineral in the secondary deposit, and is associated with minor hydrozincite, hemimorphite, acanthite and greenockite. Smithsonite occurs as two main generations: smithsonite 1, which replaces both host dolomite and sphalerite, and smithsonite 2, occurring as concretions and vein fillings in the host rock. At the boundary between smithsonite 1 and host dolomite, the latter is widely replaced by broad, irregular bands of Zn-bearing dolomite, where Zn has substituted for Mg. The secondary mineralization evolved through different stages: 1) alteration of original sulfides (sphalerite, pyrite and galena), and release of metals in acid solutions; 2) alteration of dolomite host rock and formation of Zn-bearing dolomite; 3) partial dissolution of dolomite by metal-carrying acid fluids and replacement of dolomite and Zn-bearing dolomite by a first smithsonite phase (smithsonite 1). To this stage also belong the direct replacement of sphalerite and galena by secondary minerals (smithsonite and cerussite); 4) precipitation of a later smithsonite phase (smithsonite 2) in veins and cavities, together with Ag- and Cd-sulfides.The δ18O composition of Jabali smithsonite is generally lower than in other known supergene smithsonites, whereas the carbon isotope composition is in the same range of the negative δ13C values recorded in most supergene nonsulfide ores. Considering that the groundwaters and paleo-groundwaters in this area of Yemen have negative δ18O values, it can be assumed that the Jabali smithsonite precipitated in different stages from a combination of fluids, possibly consisting of local groundwaters variably mixed with low-temperature hydrothermal waters. The carbon isotope composition is interpreted as a result of mixing between carbon from host rock carbonates and soil/atmospheric CO2.The most favorable setting for the development of the Jabali secondary deposit could be placed in the early Miocene (~17Ma), when supergene weathering was favored by major uplift and exhumation resulting from the main phase of Red Sea extension. Low-temperature hydrothermal fluids may have also circulated at the same time, through the magmatically-induced geothermal activity in the area.

The Kipushi Cu–Zn deposit (DR Congo) and its host rocks: A petrographical, stable isotope (O, C) and radiogenic isotope (Sr, Nd) study

Near the city of Kipushi, located in the southern part of the Central African Copperbelt, a major vein-type Cu–Zn ore deposit occurs. A combination of petrographic techniques and both stable (O, C) and radiogenic (Sr, Nd) isotope analysis is used to investigate the influence of the mineralisation on the Neoproterozoic dolomite host rocks. A quantification of the abundance and size of the different host rock constituents (dolomite types, quartz, phyllosilicates) revealed a lithostratigraphical controlled variation, without trends towards the ore body. The bulk oxygen isotopic composition of the host rock varies between −2.54‰ and −9.64‰ V-PDB, with most values within the range of Neoproterozoic marine dolomite. Samples with more positive δ18O all originate from the same stratigraphic interval and are interpreted as the result of reflux dolomitisation by an evaporated brine. Few samples with depleted δ18O signatures could indicate the influence of a depleted or high temperature fluid, but are not related to the ore deposit. Moreover, the presence of the ore body cannot be traced through the host rock oxygen isotopic composition. δ18O of gangue dolomite is significantly depleted in comparison with the host rocks and ranges between −7.67‰ and −12.46‰ V-PDB. For an estimated mineralisation temperature of 310°C, this implies a δ18Ofluid between 10.7‰ and 15.6‰ V-SMOW. This is a significant enrichment compared to Neoproterozoic seawater, indicating that the mineralising fluid underwent significant fluid–rock interactions. δ13C of both host rock and gangue dolomite are in range of Neoproterozoic marine dolomites. However, a limited stratigraphic interval has clearly more negative δ13C signatures, due to in situ maturation of carbonaceous material. At the time of mineralisation (450Ma), the host rock dolomite has a strontium isotopic composition partly more radiogenic than Neoproterozoic marine carbonates (0.70793<87Sr/86Sr<0.71167). Nevertheless, the signatures show no relation to the ore body. The gangue dolomite is significantly more radiogenic (0.71061<87Sr/86Sr<0.71332) than the host rock. The radiogenic signature may be due to the interaction of formational and mineralising fluids with Neoproterozoic siliciclastic-rich dolomites, e.g. the top the Kakontwe Supérieur which has 87Sr/86Sr values up to 0.72575 at 450Ma. Alternatively, the mineralising fluid could have interacted with basement rocks. According to εNd450 values between −5.4 and −0.9 for gangue dolomites, this basement was mafic in nature. However, mafic rocks also occur in the Roan Group near the Kipushi deposit. Taken into account previous fluid inclusion data, the mineralising fluid most likely derived metals both from mafic and felsic basement units and possibly interacted with Roan rocks.

Polaris as a guide to northern exploration: Ore textures, paragenesis and the origin of the carbonate-hosted Polaris Zn–Pb Mine, Nunavut, Canada

The Polaris Zn–Pb Mine in Nunavut, Canada was one of the largest single Mississippi Valley-type ore deposits in the world. Over 20Mt of sphalerite (ZnS) and galena (PbS) was hosted in brecciated carbonate rocks of the Upper Ordovician Thumb Mountain Formation. Three paragenetic stages are recognized: 1) early dolomite and marcasite; 2) main stage sulphide and dolomite; and 3) late calcite, marcasite and barite. Ore mineral textures range from discrete crystals to massive crystal aggregates and formed as replacements of the dolomite host rock or as fracture- and open space-filling mineralization. Zinc concentration is highest in the core of the deposit where botryoidal aggregates predominate, whereas iron is concentrated in the upper part. Observations of temperature and in situ sulphur isotope fractionation support a genetic model for the Polaris deposit in which thermochemical sulphate reduction occurred within the deposit, with locally generated hydrocarbons acting as a reducing agent. Information from the Polaris Mine indicates that hydrothermal alteration including dolomite, marcasite and barite; complex paragenesis with numerous ore textures; Th values >100°C associated with organic-rich strata; and a geochemical signature that includes in situ sulphur fractionation are effective predictors for determining which showings are prospective in the vast central Arctic Pb–Zn district.

Effects of host rock mineralogical composition and sedimentary facies on development of geochemical halos in Shahmirzad Pb-Zn deposits, central Alborz, Iran

Strata-bound carbonate hosted PbZn deposits or Mississippi Valley-type (MVT) deposits represent an economically important class of ore that received much attention in terms of defining effects of host rock petrography and sedimentary facies on development of geochemical halos. However there are few works on relation between sedimentary facies and MVT mineralizations in Iran. The Cretaceous carbonate rocks in the central Alborz (CA), northern Iran, host numerous small PbZn deposits and occurrences. The Shahmirzad PbZn deposits, located in the south of CA, geographically can be split into two groups, the north east (NEDs) and north west (NWDs) deposits. The Upper Cretaceous sequence in the NWDs area is composed of massive light gray limestone without any signs of later changes such as dolomitization and silicification. Additionally, three principal micro-facies are present: skeletal mudstone/wackestone (F1), biolithoclastic packstone (F2) and allochthonous bioclastic floutstone (F3). In contrast, the Cretaceous sequence hosting the NEDs is composed of massive brown limestone with six different types of dolomite (Rd1, Rd2, Rd3, Rd4, Cd1 and Cd2, the two last types are related to ore mineralization and open space filling) distinguished based on crystal size, distribution, and crystal boundary shape. The sedimentary succession in the NEDs indicates that the sedimentation took place in inner, mid and outer ramp environments, resulting in three different carbonate facies. In this area 15 carbonate facies were identified. The permeability and mineral compositions of these areas have obviously influenced the quantity of major and minor elements such as Fe (in average 0.24% in the NWDs and 0.12% in the NEDs), Si (in average 1.86% in the NWDs and 0.44% in the NEDs), Al (in average 0.50% in the NWDs and 0.13% in the NEDs), K (in average 0.24% in the NWDs and 0.06% in the NEDs), Sr (in average 685ppm in the NWDs and 204ppm in the NEDs), Na (in average 0.03% in the NWDs and 0.01% in the NEDs), Zn (in average 46ppm in the NWDs and 188ppm in the NEDs) and Pb (in average 3.3ppm in the NWDs and 83ppm in the NEDs). Not only are minor and major elements affected by changes in host rock mineral composition, but REEs decreased in concentration in dolomitic host rocks of NEDs. Some of the noticeable differences between the NWDs and the NEDs REEs data are i.e. La (in average 3.52ppm in the NWDs and 1.47ppm in the NEDs), Ce (in average 5.20ppm in the NWDs and 2.21ppm in the NEDs) and Eu (in average 0.13ppm in the NWDs and 0.03ppm in the NEDs). The combination of these data and host rock characteristics indicate that host rock compositions and textures effectively controlled these variations. We believe that perhaps the lack of dolomitization and low permeable facies in the NWDs precluded migration of diagenetic fluids and hydrothermal solutions in the carbonate host rocks. On the other hand, the presence of broad dolomitization and permeable facies in the NEDs carbonate host rocks may create appropriate conditions for diagenetic and hydrothermal fluid migration and interaction with the host rocks.

Genesis and environment characteristics of dolomite-hosted quartz and its significance for hydrocarbon exploration, in Keping Area, Tarim Basin, China

A great amount of quartz was found filling vugs of the dolomite strata of Upper Cambrian-Lower Ordovician in Keping (柯坪) outcrop area of Tarim basin, China, which is widely distributed with obvious impact on dolomite reservoirs. The quartz’s genesis and environment were studied based on careful field observation and indoor comprehensive analysis. The abnormal high temperature of the primary saline aqueous inclusions of quartz minerals might reach as high as 578 °C, showing the characteristics of basin abnormal heat events. The quartz-rare earth element (REE) pattern is similar to those of the granite, diabase, and basalt of Permian in Tabei (塔北) area. According to the regional geological setting and quartz’s distribution as well as its occurrence, the genesis of quartz is attributed to Permian magmatic activity-related hydrothermal liquid instead of the primary marine sedimentary genesis or regional metamorphic genesis. The δ13C values of CO2 in the quartz inclusions range from −1.4‰ to −4.6‰, which might derive from the mantle, while δ18O of SiO2 indicates a hot fluid genesis of meteoric precipitation, which consists of the meteoric precipitation diagenetic setting of C and O isotopes in the dolomite host rocks and the low-medium salinity freshwater condition in the inclusions. REEs must have experienced clear differentiation, since the values of LREE/HREE are all over 1, clearly indicating the characteristic of enrichment of LREE and decrement of HREE. Ce has no obvious anomaly and Eu has clear negative anomalies, which suggests a reducing environment when the quartz crystallized. After the comprehensive analysis, we believe that quartz is a product of siliceous hydrothermal liquids in combination with meteoric precipitation under a reducing environment. Additionally, the precipitation hydrothermal fluid mixture might have also mixed thoroughly with the formation water and exchanged material with the host dolomites. The significance of siliceous hydrothermal fluids for dolomite reservoirs as well as oil-gas exploration is finally discussed in this article.

Stable-isotope geochemistry of syntectonic veins in Paleozoic carbonate rocks in the Livingstone Range anticlinorium and their significance to the thermal and fluid evolution of the southern Canadian foreland thrust and fold belt

The Livingstone Range anticlinorium (LRA) marks a major hanging-wall ramp where the Livingstone thrust cuts approximately 1000 m (3281 ft) between regional decollement in the upper part of the Devonian Palliser Formation and the Jurassic Fernie Formation. Prethrusting and folding jasper fluorite sphalerite veins with halos of altered dolomitic host rock with high 87Sr/86Sr ratios (0.7094–0.7101) relative to most Paleozoic carbonate rocks (0.7081–0.7091) record percolation of fluids along basement faults that may also have contributed anomalously radiogenic strontium to diagenetically altered Paleozoic carbonate rocks throughout the Western Canada sedimentary basin. Fluid flow that occurred during thrust-propagation folding is recorded by dolomite calcite veins, with 18O values that are similar to those of host rocks (–7.92 to –1.08 Peedee belemnite). Anomalously high equilibrium temperatures (250 50C) as determined by oxygen-isotope thermometry and slightly higher 87Sr/86Sr ratios relative to adjacent host rocks indicate that they formed from formation fluids and hot basement fluids in a rock-dominated system. Calcite veins with very low 18O values (–18 to –9) precipitated along faults that were active while the LRA was transported eastward by underlying thrust faults, uplifted, and rapidly cooled by infiltrating meteoric water. Thrusting created heating in the foreland basin ahead of the deformation because of the influx of thick insulating foreland basin sediments, causing thermal maturation of hydrocarbons. As thrusting deformation advanced through the rocks, infiltrating meteoric waters cooled the rocks and hydrocarbon maturation stopped. Structural traps accumulated hydrocarbons only if they were juxtaposed over both thermally favorable and hydrocarbon-favorable source rocks.

Sulfide-Bearing Veinlets Throughout the Stratiform Mineralization of the Central African Copperbelt: Temporal and Genetic Implications

The Central African Copperbelt of Zambia and Democratic Republic of Congo, along with its broad time equivalents in Botswana and Namibia (the Kalahari Copperbelt), is the world’s largest sediment-hosted, stratiform copper province. The economically dominant stratiform copper ± cobalt or silver mineralization, accepted by many as at least partly early diagenetic in origin, is ubiquitously accompanied by sulfide-bearing quartz-carbonate veins and veinlets. Traditionally, this veining is considered to be the product of either metamorphic lateral secretion or discrete late-diagenetic to synorogenic mineralization events. This view is challenged because identical sulfide assemblages and textures characterize both the disseminated and accompanying veinlet mineralization throughout the zoned sulfide assemblages that constitute many of the stratiform orebodies, a situation that is taken to imply contemporaneity of the two styles. Consequently, during the mineralization, the siltstone, sandstone, or dolomite host rocks must have been sufficiently competent to undergo widespread brittle fracturing, implying that the disseminated mineralization cannot have been introduced by passive fluid infiltration during early diagenesis. This conclusion is further supported by the local occurrence of minor disseminated and veinlet mineralization hosted by mafic sills and dikes. Both the disseminated and veinlet mineralization styles lack ductile deformation features in many Congolese deposits, but are variably metamorphosed and deformed in some deposits in Zambia, Botswana, and Namibia. In conjunction, these observations suggest that the stratiform copper orebodies were the results of massive saline fluid expulsion by hydraulic fracturing after the mafic magmatism (~765–<715 Ma), most probably spanning peak Damara-Lufilian metamorphism and ductile deformation (~530 Ma) and continuing in places until at least ~500 Ma. Thus, basin inversion, contractional tectonism, and associated uplift and exhumation, and not the earlier extension and rifting, seem more likely to have been the ultimate drivers for the fluid mobilization and expulsion.

Origin and evolution of formation water at the Jujo–Tecominoacán oil reservoir, Gulf of Mexico. Part 2: Isotopic and field-production evidence for fluid connectivity

Abstract The chemical and isotopic characterization of formation water from 18 oil production wells, extracted from 5200 to 6100 m b.s.l. at the Jujo–Tecominoacan carbonate reservoir in SE-Mexico, and interpretations of historical production records, were undertaken to determine the origin and hydraulic behavior of deep groundwater systems. The infiltration of surface water during Late Pleistocene to Early Holocene time is suggested by 14C-concentrations from 2.15 to 31.86 pmC, and by 87Sr/86Sr-ratios for high-salinity formation water (0.70923–0.70927) that are close to the composition of Holocene to modern seawater. Prior to infiltration, the super-evaporation of seawater reached maximum TDS concentrations of 385 g/L, with lowest δ18O values characterizing the most hypersaline samples. Minor deviations of formation water and dolomite host rocks from modern and Jurassic 87Sr/86Sr-seawater composition, respectively, suggest ongoing water–rock interaction, and partial isotopic equilibration between both phases. The abundance of 14C in all sampled formation water, 87Sr/86Sr-ratios for high-salinity water close to Holocene – present seawater composition, a water salinity distribution that is independent of historic water-cut, and a total water extraction volume of 2.037 MMm3 (1/83–4/07) excludes a connate, oil-leg origin for the produced water of the Jurassic–Cretaceous mudstone-dolomite sequence. Temporal fluctuations of water chemistry in production intervals, the accelerated migration of water fronts from the reservoir flanks, and isotopic mixing trends between sampled wells confirms the existence of free aquifer water below oil horizons. Vertical and lateral hydraulic mobility has probably been accelerated by petroleum extraction. The combination of interpreting historical fluctuations of salinity and water percentage in production wells with chemical-isotopic analysis of formation water resulted in a successful method to distinguish four groundwater bodies, stratified vertically within the Jujo–Tecominoacan reservoir. Two with low TDS from 10 to 23 g/L are preserved in the upper reservoir section, mainly in Lower Cretaceous and Kimmeridgian strata. For the deeper part of the reservoir, 87Sr/86Sr trends indicate an affiliation of most samples to two independent mixing trends between “intermediate saline” (TDS ∼200 g/L) and hypersaline (>350 g/L) groundwater end-members.

Origin and evolution of formation water at the Jujo–Tecominoacán oil reservoir, Gulf of Mexico. Part 1: Chemical evolution and water–rock interaction

The origin and evolution of formation water from Upper Jurassic to Upper Cretaceous mudstone–packstone–dolomite host rocks at the Jujo–Tecominoacán oil reservoir, located onshore in SE-Mexico at a depth from 5200 to 6200 m.b.s.l., have been investigated, using detailed water geochemistry from 12 producer wells and six closed wells, and related host rock mineralogy. Saline waters of Cl–Na type with total dissolved solids from 10 to 23 g/L are chemically distinct from hypersaline Cl–Ca–Na and Cl–Na–Ca type waters with TDS between 181 and 385 g/L. Bromine/Cl and Br/Na ratios suggest the subaerial evaporation of seawater beyond halite precipitation to explain the extreme hypersaline components, while less saline samples were formed by mixing of high salinity end members with surface-derived, low salinity water components. The dissolution of evaporites from adjacent salt domes has little impact on present formation water composition. Geochemical simulations with Harvie-Mφller-Weare and PHRQPITZ thermodynamic data sets suggest secondary fluid enrichment in Ca, HCO3 and Sr by water–rock interaction. The volumetric mass balance between Ca enrichment and Mg depletion confirms dolomitization as the major alteration process. Potassium/Cl ratios below evaporation trajectory are attributed to minor precipitation of K feldspar and illitization without evidence for albitization at the Jujo–Tecominoacán reservoir. The abundance of secondary dolomite, illite and pyrite in drilling cores from reservoir host rock reconfirms the observed water–rock exchange processes. Sulfate concentrations are controlled by anhydrite solubility as indicated by positive SI-values, although anhydrite deposition is limited throughout the lithological reservoir column. The chemical variety of produced water at the Jujo–Tecominoacán oil field is related to a sequence of primary and secondary processes, including infiltration of evaporated seawater and original meteoric fluids, the subsequent mixing of different water types and the formation of secondary minerals by water–rock interaction. A best fit between measured and calculated reservoir temperatures was obtained with the Mg–Li geothermometer for high salinity formation water (TDS > 180 g/L), whereas Na–K, Na–Ka–Ca and quartz geothermometers are partially applicable for less salinite water (TDS < 23 g/L).

Track of fluid paleocirculation in dolomite host rock at regional scale by the Anisotropy of Magnetic Susceptibility (AMS): An example from Aptian carbonates of La Florida, Northern Spain

The present study aims to apply the AMS method (Anisotropy of Magnetic Susceptibility) at a regional scale to track the fluid circulation direction that has produced an iron metasomatism within pre-existing dolomite host rock. The Urgonian formations hosting the Zn–Pb mineralizations in La Florida (Cantabria, northern Spain) have been taken as target for this purpose. Sampling was carried out, in addition to ferroan dolomite host rock enclosing the Zn–Pb mineralizations, in dolomite host rock and limestone to make the comparison possible between magnetic signals from mineralized rocks, where fluid circulation occurred, and their surrounding formations. AMS study was coupled with petrofabric analysis carried out by texture goniometry, Scanning Electron Microscopy (SEM) observations and also Shape Preferred Orientation (SPO) statistics. SEM observations of ferroan dolomite host rock illustrate both bright and dark grey ribbons corresponding respectively to Fe enriched and pure dolomites. SPO statistics applied on four images from ferroan dolomite host rock give a well-defined orientation of ribbons related to the intermediate axis of magnetic susceptibility K 2. For AMS data, two magnetic fabrics are observed. The first one is observed in ferroan dolomite host rock and characterized by a prolate ellipsoid of magnetic susceptibility with a vertical magnetic lineation. The magnetic susceptibility carrier is Fe-rich dolomite. These features are probably acquired during metasomatic fluid circulations. In Fe-rich dolomite host rock, ‹ c› axes are vertical. As a rule, (0001) planes (i.e. planes perpendicular to ‹ c› axes) are isotropic with respect to crystallographic properties. So, the magnetic anisotropy measured in this plane should reflect crystallographic modification due to fluid circulation. This is confirmed by the texture observed using the SEM. Consequently, AMS results show a dominant NE–SW elongation interpreted as the global circulation direction and a NW–SE secondary elongation that we have considered as sinuosities of the fluid trajectory. The second type of magnetic fabric is essentially observed in the limestone and characterized by an oblate form of the ellipsoid of magnetic susceptibility, a horizontal magnetic foliation and mixed magnetic susceptibility carriers. It is interpreted as a sedimentary fabric.

THE REE MINERALS OF THE BASTNAS-TYPE DEPOSITS, SOUTH-CENTRAL SWEDEN

On the basis of characteristic assemblages, the Bastnas-type Fe–(Cu–)REE skarn deposits in the Bergslagen mining region of south-central Sweden can be divided into two subtypes: one almost exclusively with LREE enrichment (subtype 1, mainly in the Riddarhyttan–Bastnas area), and another showing enrichment of both LREE and Y + HREE (subtype 2, Norberg District). New data have been collected on twenty REE species including three unnamed ones, using electron-microprobe analysis, X-ray diffraction and Mossbauer spectroscopy. Cerite-(Ce) is, together with ferriallanite-(Ce), the most important REE mineral at Bastnas, but less common at the subtype-2 deposits. Compared to other occurrences worldwide, the present samples of cerite-(Ce) are poor in Ca and enriched in Mg and F. Calcium and (REE + Y) in the samples are negatively correlated, and the upper limit for Ca is close to 1 apfu. Iron ranges from 0.01 to 0.30 apfu, and is mainly in the trivalent state. Cerite-(Ce) is a major carrier of Y (up to 3.5 wt.% Y2O3). Fluorbritholite is found only in the subtype-2 deposits, and is low in P, Na and the actinides compared to other major occurrences in the world. Significant inter- and intrasample variations in REE and Y occur, ranging from fluorbritholite-(Ce) to the unaccredited member fluorbritholite-(Y). Fluorbritholite is non-stoichiometric, with Ca 3 apfu. Vastmanlandite-(Ce) is another important host for the REE in subtype 2. By means of the substitutions Mg2+ + F– Fe3+ + O2– and Mg2+ Fe2+, it forms solid solutions with an unnamed Fe-dominant member that is found in the subtype-1 deposits. Dollaseite-(Ce), which forms a partial solid-solution series with dissakisite-(Ce), is restricted to subtype 2. Gadolinite is very rare at the subtype-1 deposits, and more widespread in the Norberg District. All samples have a significant hingganite component in solid solution (0.10–0.35 molar fraction). Single crystals are commonly zoned with respect to REE and Y, with compositions corresponding to gadolinite-(Ce), gadolinite-(Y) and an Nd-dominant member. The substantial fractionation of REE and Y on a local scale within the Bastnas-type deposits is dependent both on crystal-chemical factors and on fluctuations in fluid composition during crystallization. The initial precipitation of REE silicates in type-1 and type-2 deposits (mainly cerite and fluorbritholite, respectively) was a reaction between relatively acidic solutions carrying major amounts of REE complexed mainly by ligands of F and Si, and dolomitic host-rocks.