Host Rock Minerals Research Articles

The baryte-bearing beryl-phosphate pegmatite Plössberg—A missing link between pegmatitic and vein-type barium mineralization in NE Bavaria, Germany

Pegmatites and aplites enriched in P, Be, Nb, Ta and Li occur in the high-temperature metamorphic lithological units of the NE Bavarian Basement, SE Germany. They are accompanied by Ba mineralization, in vein-type deposits in the basement as well as in its foreland. Locally, Ba minerals are encountered in the late Variscan pegmatites and aplites too. The shallow discordant stock-like pegmatites (Hagendorf-type) are barren as to Ba, but in the tabular, concordant aplites and pegmatites Ba was concentrated (Plössberg-type). These concordant pegmatites and aplites are supposed to be the root zone of the intrusive pegmatites. In the rare case of low sulfur fugacity, Ba forms Ba–Zr–K–Sc phosphates/silicates in the pegmatites (transition of magmatic into the hydrothermal stages I/II). Under high sulfur fugacity, Ba is accommodated within the same stages in the structure of baryte. Barium is not accommodated in the lattice of phosphates during or in the immediate aftermaths of the emplacement of these Be–P–Nb–Ta pegmatites (stage III). This element shows up again in APS minerals during supergene alteration under acidic conditions (stage IV). Considering the host rocks of baryte mineralization, the Sr contents of baryte increased from the early Paleozoic to the Late Triassic. The Sr contents of baryte are a function of the depth below ground in the vein-type deposits and in the shear-zones bounding the tabular concordant pegmatites. Beryl is not only a marker mineral for the shear-zone-hosted pegmatites but can also be used as a tool for the geodynamic positioning of these pegmatites using its oxygen isotopes. A subdivision of the pegmatites into intrusive and shear-zone hosted may be achieved by its REE and minor elements.

Internal and External Fluid Sources for Eclogite-facies Veins in the Monviso Meta-ophiolite, Western Alps: Implications for Fluid Flow in Subduction Zones

To contribute to our understanding of the mechanisms and pathways of fluid movement through deeply subducted crust, we investigate high-pressure veins cutting eclogite-facies (� 2· 0G Pa and � 6008C) metagabbros of the Monviso Ophiolite, Italian Western Alps. The veins consist mainly of omphacite with minor garnet, rutile, talc and accessory zircon. Most of the vein minerals have major and trace element compositions that are comparable with the host-rock minerals, and vein and host-rock zircons have similar Hf isotopic compositions.These observations support the conclusions of previous studies that these veins largely formed from a locally sourced hydrous fluid during prograde or peak metamorphism. However, the bulk-rock Cr and Ni contents of the veins are significantly higher than those of the surrounding host eclogites. We also document distinct Cr-rich (up to weight per cent levels) zones in omphacite, garnet and rutile in some vein samples. Vein garnet and talc also have relatively high MgO and Ni contents. X-ray maps of vein garnet and rutile grains reveal complex internal zoning features, which are largely defined by micrometre-scale variations in Cr content. Some grains have concentric and oscillatory zoning in Cr, whereas others feature a chaotic fracture-like pattern. These Cr-rich zones are associated with high concentrations of Ni, B, As, Sb, Nb, Zr and high ratios of light rare earth elements (LREE) to middle REE (MREE) compared with low-Cr vein and host-rock minerals. Petrological and mass-balance constraints verify that the Cr-rich zones in the veins were not derived from internally sourced fluids, but represent precipitates from an external fluid.The external source that is consistent with the distinctive trace element characteristics of the vein components is antigorite serpentinite, which forms the structural basement of the high-pressure metagabbros.We propose at least two separate growth mechanisms for the Monviso veins. Most vein infillings were formed during progressive prograde metamorphism from locally derived fluid. Influx of the serpentinitederived or other external fluid was transient and episodic and was probably achieved via brittle fractures, which preferentially formed along the pre-existing vein structures.The dehydration of serpentinite at high pressures in subduction zones may provide crucial volatiles and trace elements for arc magmas. Our results indicate that the movement of these fluids through subducted oceanic crust is likely to be highly channeled and transient so the progressive development of vein systems in mafic rocks may also be crucial for forming channelways for long-distance fluid flow at depth in subduction zones.

Iodide interaction with natural pyrite

129I is one of the major dose-determining nuclides in the safety analysis of deep storage of radioactive waste. Iodine forms anionic species that hardly sorb on the surfaces of common host-rock minerals. Recently, interest has arisen on the role of pyrite, an accessory mineral capable of binding anionic selenium. Whereas the interaction of selenium with pyrite is well documented, corresponding results on iodine sorption are still scarce and controversial. Pyrite is present in argicilleous rocks which are being considered in many countries as potential host rocks for a radioactive waste repository. The uptake of iodide (I−) on natural pyrite was investigated under nearly anoxic conditions (O2 < 5 ppm) over a wide concentration range (10−11–10−3 M total I−) using 125I as the radioactive tracer. Weak but measurable sorption was observed; distribution coefficients (Rd) were less than 0.002 m3 kg−1 and decreased with increasing total iodide concentration. Iodide sorption was connected to the presence of oxidized clusters on the pyrite surface, which were presumably formed by reaction with limited amounts of dissolved oxygen. The results obtained indicated that pyrite cannot be considered as an effective scavenger of 129I under the geochemical conditions prevailing in underground radioactive waste geologic storage.

Geochemical tracing and hydrogeochemical modelling of water–rock interactions during salinization of alluvial groundwater (Upper Rhine Valley, France)

In the southern Upper Rhine Valley, groundwater has undergone intensive saline pollution caused by the infiltration of mining brines, a consequence of potash extraction carried out during the 20th century. Major and trace elements along with Sr and U isotopic ratios show that groundwater geochemical characteristics along the saline plumes cannot reflect conservative mixing between saline waters resulting from the dissolution of waste heaps and one or more unpolluted end-members. The results imply the occurrence of interactions between host rocks and polluted waters, and they suggest that cationic exchange mechanisms are the primary controlling process. A coupled hydrogeochemical model has been developed with the numerical code KIRMAT, which demonstrates that cationic exchange between alkalis from polluted waters and alkaline-earth elements from montmorillonite present in the host rock of the aquifer is the primary process controlling the geochemical evolution of the groundwater. The model requires only a small amount of montmorillonite (between 0.75% and 2.25%), which is in agreement with the observed mineralogical composition of the aquifer. The model also proves that a small contribution of calcite precipitation/dissolution takes places whereas other secondary mineral precipitation or host rock mineral dissolution do not play a significant role in the geochemical signature of the studied groundwater samples. Application of the model demonstrates that it is necessary to consider the pollution history to explain the important Cl, Na and Ca concentration modifications in groundwater samples taken over 2 km downstream of waste heaps. Additionally, the model shows that the rapidity of the cationic exchange reactions insures a reversibility of the cation fixation on clays in the aquifer.

Chemical heterogeneity in an enderbite-hosted pseudotachylite, eastern India: evidence for syn-deformation multi-reaction melting in pseudotachylite

The origin of chemical and mineralogical heterogeneity in tens-of-microns wide layers and domains in enderbite-hosted couple-of-centimeters wide pseudotachylite vein is examined based on the results of BSE and X-ray element imaging, and electron probe microanalyses of major elements of host-rock minerals, clasts, microphenocrysts, and pseudotachylite matrix. The pseudotachylite layers and domains containing variable proportions of orthopyroxene and magnetite microphenocrysts continue as mantles around quartz, K-feldspar, plagioclase and garnet clasts. The clasts are chemically modified along margins and intra-clast pseudotachylite injections. The chemical modifications are extensive in smaller clasts <5 μm diameter. At least three chemically distinct layers and domains in the pseudotachylite, and their fine-grained matrices, plot in sharply defined, well-segregated and non-overlapping fields in FeO + MgO−Al2O3–SiO2, FeO–CaO–MgO and CaO–Na2O–K2O and FeO vs. FeO/FeO + MgO diagrams. The compositions of the layers and domains—smeared between a feldspar + quartz component and a ferromagnesian component of garnet + Fe–Ti oxides (±orthopyroxene)—possibly correspond to fractionated quenched melts, or admixtures of microphenocrysts that cannot be resolved by the microbeam techniques employed. The compositional variations are incompatible with deformation-driven crystal fractionation in melt. Instead the layers and domains possibly are crystal-melt mushes produced by syn-deformation ultra-high temperature (1,250–1,375°C) melting reactions involving variable proportions of host-rock minerals determined by time-transient local phase aggregates experiencing strain. The similar element variation trends in pseudotachylite examined here and those reported from anorthosite, metapelite and charnockite elsewhere suggests local phase aggregate controlled multi-reaction melting is a phenomenon commoner than hitherto realized in pseudotachylites.

Lithogeochemical localisation of disseminated gold in the White River area, Yukon, Canada

Gold mineralisation in the White River area, 80 km south of the highly productive Klondike alluvial goldfield, is hosted in amphibolite facies gneisses in the same Permian metamorphic pile as the basement for the Klondike goldfield. Hydrothermal fluid which introduced the gold was controlled by fracture systems associated with middle Cretaceous to early Tertiary extensional faults. Gold deposition occurred where highly fractured and chemically reactive rocks allowed intense water–rock interaction and hydrothermal alteration, with only minor development of quartz veins. Felsic gneisses were sericitised with recrystallisation of hematite and minor arsenic mobility, and extensively pyritised zones contain gold and minor arsenic (ca 10 ppm). Graphitic quartzites (up to 5 wt.% carbon) caused chemical reduction of mineralising fluids, with associated recrystallisation of metamorphic minerals (graphite, pyrrhotite, pyrite, chalcopyrite) in host rocks and veins, and introduction of arsenic (up to 1 wt.%) to form arsenopyrite in veins and disseminated through host rock. Veins have little or no hydrothermal quartz, and up to 19 wt.% carbon as graphite. Late-stage oxidation of arsenopyrite in some graphitic veins has formed pharmacosiderite. Gold is closely associated with disseminated and vein sulphides in these two rock types, with grades of up to 3 ppm on the metre scale. Other rock types in the White River basement rocks, including biotite gneiss, hornblende gneiss, pyroxenite, and serpentinite, have not developed through-going fracture systems because of their individual mineralogical and rheological characteristics, and hence have been little hydrothermally altered themselves, have little hydrothermal gold, and have restricted flow of fluids through the rock mass. Some small post-metamorphic quartz veins (metre scale) have been intensely fractured and contain abundant gold on fractures (up to 40 ppm), but these are volumetrically minor. The style of gold mineralisation in the White River area is younger than, and distinctly different from, that of the Klondike area. Some of the mineralised zones in the White River area resemble, mineralogically and geochemically, nearby coeval igneous-hosted gold deposits, but this resemblance is superficial only. The White River mineralisation is an entirely new style of Yukon gold deposit, in which host rocks control the mineralogy and geochemistry of disseminated gold, without quartz veins.

Native aluminum: Does it exist?

Several papers reporting exotic native elements have been published within the last few decades. The “native” occurrences described are rather dubious in view of the lack of solid proof of their relationships with the host-rock minerals. Consequently, the genetic models proposed ranging from bio-reduction to the influence of deep-mantle, strongly reduced fluids, are somewhat speculative. Here we present data for a unique Al0 flake protruding from the phlogopite matrix of a rock specimen collected from a desilicated pegmatite vein. The geologic setting suggests two processes that might have played a key role in the Al0 formation: (1) desilication of pegmatite, resulting in its Al residual enrichment; and (2) serpentinization of an ultramafic body, providing a strongly reduced front (H2 and hydrocarbons) toward the serpentinite/pegmatite contact. These processes have presumably led to the reduction of Al to Al0 at discrete sites of alumina-rich minerals.

Geochemistry of the Permian Kalatongke Mafic Intrusions, Northern Xinjiang, Northwest China: Implications for the Genesis of Magmatic Ni-Cu Sulfide Deposits

The Kalatongke Cu-Ni sulfide deposit in northern Xinjiang, northwest China, is located on the southern side of the regional Irtysh fault zone, which is the boundary between the early Paleozoic Altai orogenic belt to the north and the late Paleozoic Junggar terrane to the south. In the Kalatongke region there are 11 mafic intrusions which were emplaced in Lower Carboniferous strata. Economic Ni-Cu sulfide ores are found within three of these intrusions, which are well differentiated, and compositionally zoned. Four rock types are recognized which, from top to base of the three mineralized intrusions, include biotite diorite, biotite-hornblende norite, biotite-hornblende-olivine norite, and biotite hornblende-dolerite. The biotite-hornblende olivine norites and biotite-hornblende norites are the most favorable host rocks for Ni-Cu mineralization. The initial (87Sr/86Sr)t (t = 280 Ma) ratios of the intrusions vary from 0.70375 to 0.70504, and eNd(t) from 6.3 to 8.2, implying that the magmas originated from depleted asthenospheric mantle. However, the strong enrichment of lithophile elements such as K, Rb, Th, U, and LREE, the negative Nb and Ta anomalies, and the high δ18O values of whole rocks (5.4‐10.6‰) suggest significant crustal contamination. These patterns are present in almost all samples from the intrusions, indicating that the contamination took place before the magma was emplaced at its present level in the crust. Crustal contamination is interpreted to have driven the magma to S-saturation and brought orthopyroxene onto the liquidus. Convection, or perhaps flow differentiation of the crystal-bearing magma during ascent, caused the dense sulfide melt with entrained olivine and orthopyroxene crystals to become concentrated in the center of the intrusions, whereas the fractionated magmas formed other, less mafic intrusions, which intrude the nearby strata.

Preface: thematic issue on uranium deposits

This thematic issue comprises five papers covering various aspects of the genesis of different types of uranium deposits from Australia, Canada, India, and central Europe. Three papers are on Mesoproterozoic uranium deposits: Valhalla from Queensland, Australia, related to sodium metasomatism, by Polito et al., the unconformity related deposits of the Athabasca Basin, Saskatchewan, by Alexandre et al., and an iron oxide–copper–gold (IOCG) type of uranium mineralization from the Singhbhum Shear Zone, India, by Pal et al. Two other papers concern hydrothermal uranium veins hosted by Early Paleozoic metamorphic rocks of the Variscan Bohemian Massif (Dolnicek et al.; Kribek at al.). There are numerous uranium deposits related to Na metasomatism, but the two largest and presently mined ones are those of the Central Ukrainian Province (Belevtsev and Koval 1968) and the Lagoa Real district in Brazil. The geotectonic setting of this type of mineralization and the origin of the mineralizing fluids is debated (Lobato et al. 1983; Turpin et al. 1988). The paper by Polito et al., besides a detailed mineralogical description of the alteration and ore minerals, provides U–Pb isotopic ages on the uranium minerals and Ar/Ar ages on alteration minerals, allowing to link these processes to a very-low-grade metamorphic event at about 1,550 Ma, the Isan orogeny. The temperature of alteration and mineralization (340–380°C) at Valhalla is, however, significantly lower than in the Ukrainian and Brazilian occurrences (>500°C to 300°C). Fluids associated with the Valhalla deposit would be either basinal brines expulsed during the metamorphic event or magmatic fluids, whereas the fluids associated with the Lagao Real deposits have a seawater composition (Lobato et al. 1983). The age of the primary uranium deposition in unconformity-related uranium deposits has been continuously revised since the first determinations during the 1970s. With improvement of the dating techniques, older and older ages have been obtained from about 900 Ma to nearly 1,600 Ma for the latest ages obtained by Kurt Kyser’s group at Queen’s University and reviewed by Alexandre et al. in the present volume. Uranium oxides are easily remobilized by highto low-temperature fluid percolations and tend to lose their radiogenic lead because of the much larger diameter of Pb compared to U. The use of in situ dating techniques by secondary ion mass spectroscopy or laser ablation inductively coupled plasma mass spectroscopy allows analyzing the less disturbed domains of uranium oxide minerals. Alexandre et al. also associate, the first time for these deposits, in situ laser step heating Ar/Ar dating of metamorphic host rock minerals and syn-ore alteration clay minerals. The events younger than 1.6 to 1.5 Ga are interpreted as remobilization of the primary uranium mineralization in response to far-field tectonic or magmatic events. The possibility that some of these events may represent new uranium deposition events, upgrading the uranium deposits and thus explaining their extremely high grade, is excluded. With a 1.6 Ga deposition age, the timing of the formation of the Athabasca deposits becomes very close to the age of the uranium deposits of the Kombolgie Basin, Northern Territory, Australia, which formed in the same P-T-X conditions (Derome et al. 2003, 2005). Miner Deposita (2009) 44:1–2 DOI 10.1007/s00126-008-0218-y

Postorogenic Origin of the Stratiform Cu Mineralization at Lufukwe, Lufilian Foreland, Democratic Republic of Congo

The Central African Copperbelt, which stretches across the border between Zambia and the Democratic Republic of Congo, is one of the largest sediment-hosted stratiform Cu-Co provinces in the world. The triangular-shaped Lufilian foreland is located to the northeast of the Copperbelt. Recently sediment-hosted stratiform copper occurrences have been explored in the foreland, including the Kinkumbi prospect in the Lufukwe anticline. Although the stratiform copper mineralization in the Copperbelt is mainly concentrated in the Roan Group sedimentary rocks, where it is commonly associated with cobalt, the Lufilian foreland copper mineralization occurs in sedimentary rock units of the Nguba and Kundelungu Groups where it is mainly associated with silver. This paper examines the metallogenesis of the stratiform copper mineralization at Lufukwe. The Lufukwe anticline is situated in the eastern part of the Lufilian foreland and is composed of Neoproterozoic sedimentary rocks belonging to the Katanga Supergroup. The Kinkumbi prospect, a sediment-hosted stratiform copper-silver occurrence, is located in the northern part of the western flank of the anticline. This part of the anticline is characterized by disseminated copper-silver mineralization hosted in the lower 10 to 15 m of the Monwezi Sandstone (Nguba Group). A comparison between the location of high copper grades in surface samples and boreholes and the location of structural lineaments visible on ASTER images indicates that the mineralization is spatially related to northeast-southwest to east-northeast‐west-southwest strike-slip faults. These faults are nearly perpendicular to the strike of the host rock and postdate both the Lufilian folding and deposition of the entire Katanga Supergroup. The Monwezi Sandstone was subjected to strong compaction and silica cementation (authigenic quartz overgrowths), followed by intense feldspar dissolution, which resulted in a well-developed secondary porosity represented by dissolution cavities. Copper sulfides are mainly concentrated in these cavities and partially replace the detrital grains. The copper mineralization is both hypogene (chalcopyrite, bornite, and chalcocite) and supergene (digenite, covellite, and minor native copper), with malachite and chrysocolla as the main oxidation products. Point counting and grain size measurements demonstrate that the sandstone horizons with high copper content (>1.25% Cu) are those with a detrital grain size larger than 175 µm, more than 35 percent altered feldspars and little or no fine-grained matrix. Microthermometry of fluid inclusions indicates that the authigenic quartz overgrowths precipitated from a moderate-temperature (80°‐130°C), high-salinity (18.8‐23.4 wt % CaCl2 equiv) H2O-NaCl-CaCl2 fluid. The hypogene copper-silver mineralization was deposited from a hot (120°‐180°C) and low- to moderate-salinity (1.9‐7.7 wt % NaCl equiv) H2O-NaCl fluid with a general trend of increasing homogenization temperatures with increasing salinities. The interpretation of the available structural, stratigraphic, petrographic, and fluid inclusion microthermometric data constrain the timing of the mineralization to a time after the Lufilian folding and deposition of the entire Katanga Supergroup. The data presented support a postorogenic fluid-mixing model in which the mineralization is related to the mixing of a copper-rich mineralizing fluid with a temperature ≥180°C and salinity ≥7.7 wt percent NaCl equiv, likely migrating upward along northeast-southwest‐ to east-northeast‐west-southwest‐oriented strike-slip faults, with a colder, low-salinity, reducing fluid in the Monwezi Sandstone. The location and distance between the northeast-southwest to east-northeast‐west-southwest faults strongly influenced the spatial distribution of the copper mineralization in the anticline, and the variability in grain size and composition of the Monwezi Sandstone caused the preferential lateral migration of the mineralizing fluid through the lower 10 to 15 m. Copper precipitation was possibly induced by reduction by preexisting noncopper sulfides (pyrite and arsenopyrite) and hydrocarbons and by the drop in fluid temperature and salinity. Based on this research the areas with dominant northeast-southwest to east-northeast‐west-southwest structural lineaments are considered favorable sites for further copper exploration in the Lufilian foreland, especially where these lineaments cut previously deformed Katanga sediments. Coarse-grained sandstones with no fine matrix that underwent intense feldspar dissolution are the most promising host rocks for the late disseminated stratiform copper mineralization in the Lufilian foreland. Airborne gamma-ray spectrometric surveys could be used as a powerful exploration tool for targeting ore-related, K-depleted zones in these sandstones.

Grenville-age pseudotachylite in the Lewisian: laserprobe 40 Ar/ 39 Ar ages from the Gairloch region of Scotland (UK)

Infrared laserprobe 40 Ar/ 39 Ar dating has been used to date pseudotachylite and host-rock minerals from a crush belt in the Lewisian basement of Scotland. It has revealed complexity in the pseudotachylite data that is attributable to the presence of refractory host-rock clasts and mineral fragments in the pseudotachylite. In conjunction with the host-rock mineral laserprobe 40 Ar/ 39 Ar data it has been possible to simplify the pseudotachylite data for the samples, and the preferred ages for these are: 980 ± 39 Ma, 999 ± 31 Ma and 1024 ± 30 Ma (2σ). These ages are the first record of Grenville-aged brittle deformation in the Lewisian. Further, this study serves to illustrate the complexity of dating pseudotachylites, and the advantages and limitations of the IR laserprobe applied to such materials.

Behavior of uranium under conditions of interaction of rocks and ores with subsurface water

The behavior of uranium during interaction of subsurface water with crystalline rocks and uranium ores is considered in connection with the problem of safe underground insulation of spent nuclear fuel (SNF). Since subsurface water interacts with crystalline rocks formed at a high temperature, the mineral composition of these rocks and uranium species therein are thermodynamically unstable. Therefore, reactions directed toward the establishment of equilibrium proceed in the water-rock system. At great depths that are characterized by hindered water exchange, where subsurface water acquires near-neutral and reducing properties, the interaction is extremely sluggish and is expressed in the formation of micro- and nanoparticles of secondary minerals. Under such conditions, the slow diffusion redistribution of uranium with enrichment in absorbed forms relative to all other uranium species is realized as well. The products of secondary alteration of Fe- and Ti-bearing minerals serve as the main sorbents of uranium. The rate of alteration of minerals and conversion of uranium species into absorbed forms is slow, and the results of these processes are insignificant, so that the rocks and uranium species therein may be regarded as unaltered. Under reducing conditions, subsurface water is always saturated with uranium. Whether water interacts with rock or uranium ore, the equilibrium uranium concentration in water is only ≤10−8 mol/l. Uraninite ore under such conditions always remains stable irrespective of its age. The stability conditions of uranium ore are quite suitable for safe insulation of SNF, which consists of 95% uraninite (UO2) and is a confinement matrix for all other radionuclides. The disposal of SNF in massifs of crystalline rocks at depths below 500 m, where reducing conditions are predominant, is a reliable guarantee of high SNF stability. Under oxidizing conditions of the upper hydrodynamic zone, the rate of interaction of rocks with subsurface water increases by orders of magnitude and subsurface water is commonly undersaturated with uranium. Uranium absorbed by secondary minerals, particularly by iron hydroxides and leucoxene, is its single stable species under oxidizing conditions. The impact of oxygen-bearing water leads to destruction of uranium ore. This process is realized simultaneously at different hypsometric levels even if the permeability of the medium is variable in both the lateral and vertical directions. As a result, intervals containing uranyl minerals and relics of primary uranium ore are combined in ore-bearing zones with intervals of completely dissolved uranium minerals. A wide halo of elevated uranium contents caused by sorption is always retained at the location of uranium ore entirely destroyed by weathering. Uranium ore commonly finds itself in the aeration zone due to technogenic subsidence of the groundwater table caused by open-pit mining or pumping out of water from underground mines. The capillary and film waters that interact with rocks and ores in this zone are supplemented by free water filtering along fractures when rain falls or snow is thawing. The interaction of uranium ore with capillary water results in oxidation of uraninite, accompanied by loosening of the mineral surface, formation of microfractures, and an increase in solubility with enrichment of capillary water in uranium up to 10−4 mol/l. Secondary U(VI) minerals, first of all, uranyl hydroxides and silicates, replace uraninite, and uranium undergoes local diffusion redistribution with its sorption by secondary minerals of host rocks. The influx of free water facilitates the complete dissolution of primary and secondary uranium minerals, the removal of uranium at the sites of groundwater discharge, and its redeposition under reducing conditions at a greater depth. It is evident that the conditions of the upper hydrodynamic zone and the aeration zone are unfit for long-term insulation of SNF and high-level wastes because, after the failure of containers, the leakage of radionuclides into the environment becomes inevitable.

Geochronology of unconformity-related uranium deposits in the Athabasca Basin, Saskatchewan, Canada and their integration in the evolution of the basin

The importance of geochronology in the study of mineral deposits in general, and of unconformity-type uranium deposits in particular, resides in the possibility to situate the critical ore-related processes in the context of the evolution of the physical and chemical conditions in the studied area. The present paper gives the results of laser step heating 40Ar/39Ar dating of metamorphic host-rock minerals, pre-ore and syn-ore alteration clay minerals, and laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) U/Pb dating of uraninite from a number of basement- and sediment-hosted unconformity-related deposits in the Athabasca Basin, Canada. Post-peak metamorphic cooling during the Trans-Hudson Orogen of rocks from the basement occurred at ca 1,750 Ma and gives a maximum age for the formation of the overlying Athabasca Basin. Pre-ore alteration occurred simultaneously in both basement- and sandstone-hosted mineralizations at ca 1,675 Ma, as indicated by the 40Ar/39Ar dating of pre-ore alteration illite and chlorite. The uranium mineralization age is ca 1,590 Ma, given by LA-ICP-MS U/Pb dating of uraninite and 40Ar/39Ar dating of syn-ore illite, and is the same throughout the basin and in both basement- and sandstone-hosted deposits. The mineralization event, older than previously proposed, as well as several fluid circulation events that subsequently affected all minerals studied probably correspond to far-field, continent-wide tectonic events such as the metamorphic events in Wyoming and the Mazatzal Orogeny (ca 1.6 to 1.5 Ga), the Berthoud Orogeny (ca 1.4 Ga), the emplacement of the McKenzie mafic dyke swarms (ca 1.27 Ga), the Grenville Orogeny (ca 1.15 to 1 Ga), and the assemblage and break-up of Rodinia (ca 1 to 0.85 Ga). The results of the present work underline the importance of basin evolution between ca 1.75 Ga (basin formation) and ca 1.59 Ga (ore deposition) for understanding the conditions necessary for the formation of unconformity-type uranium deposits.

Hypogene Zn carbonate ores in the Angouran deposit, NW Iran

The world-class Angouran nonsulfide Zn–Pb deposit is one of the major Zn producers in Iran, with resources estimated at about 18 Mt at 28% Zn, mainly in the form of the Zn carbonate smithsonite. This study aims to characterize these carbonate ores by means of their mineralogy and geochemistry, which has also been extended to the host rocks of mineralization and other local carbonate rock types, including the prominent travertines in the Angouran district, as well as to the local spring waters. Petrographical, chemical, and stable isotope (O, H, C, Sr) data indicate that the genesis of the Zn carbonate ores at Angouran is fairly distinct from that of other “classical” nonsulfide Zn deposits that formed entirely by supergene processes. Mineralization occurred during two successive stages, with the zinc being derived from a preexisting sulfide ore body. A first, main stage of Zn carbonates (stage I carbonate ore) is associated with both preexisting and subordinate newly formed sulfides, whereas a second stage is characterized by supergene carbonates (Zn and minor Pb) coexisting with oxides and hydroxides (stage II carbonate ore). The coprecipitation of smithsonite with galena, pyrite and arsenopyrite, as well as the absence of Fe- and Mn-oxides/hydroxides and of any discernible oxidation or dissolution of the sphalerite-rich primary sulfide ore, shows that the fluids responsible for the main, stage I carbonate ores were relatively reduced and close to neutral to slightly basic pH with high fCO2. Smithsonite δ18OVSMOW values from stage I carbonate ore range from 18.3 to 23.6‰, while those of stage II carbonate ore show a much smaller range between 24.3 and 24.9‰. The δ13C values are fairly constant in smithsonite of stage I carbonate ore (3.2–6.0‰) but show a considerable spread towards lower δ13CVPDB values (4.6 to −11.2‰) in stage II carbonate ore. This suggests a hypogene formation of stage I carbonate ore at Angouran from low-temperature hydrothermal fluids, probably mobilized during the waning stages of Tertiary–Quaternary volcanic activity in an environment characterized by abundant travertine systems throughout the whole region. Conversely, stage II carbonate ore is unambiguously related to supergene weathering, as evidenced by the absence of sulfides, the presence of Fe-Mn-oxides and arsenates, and by high δ18O values found in smithsonite II. The variable, but still relatively heavy carbon isotope values of supergene smithsonite II, suggests only a very minor contribution by organic soil carbon, as is generally the case in supergene nonsulfide deposits.

Occurrences of Ore Minerals and Fluid Inclusion Study on the Kingking Porphyry Copper–Gold Deposit, Eastern Mindanao, Philippines

AbstractThe Kingking deposit is a gold‐rich porphyry copper deposit and the southernmost deposit at the eastern Mindanao mineralized belt, Philippines. It is underlain by Cretaceous–Paleogene sedimentary and volcanic rocks that are intruded by mineralized Miocene diorite porphyries and by barren Miocene–Pliocene dacite and diorite porphyries. The main alteration zones in the deposit are the inner potassic zone and the outer propylitic zone. The biotite‐bearing diorite and hornblende diorite porphyries are the primary host rocks of mineralization. Two dominant copper minerals, bornite and chalcopyrite, which usually occur as fracture fillings, are associated with fine crystalline quartz veinlet stockworks in the mineralized diorites. Minor secondary covellite, chalcocite and digenite are also observed. The primary Cu‐Fe sulfide phases initially deposited from ore fluids consisted of bornite solid solution (bnss) and intermediate solid solution (iss), which decomposed to form the bornite and chalcopyrite. Peculiar bornite pods that are different from dissemination and are associated with volcanic rock xenoliths in biotite‐bearing diorite porphyry are noted in a drill hole. These pods of bornite are not associated with quartz veinlet stockworks. Fluid inclusion analyses show three types of inclusions contained in Kingking samples: two‐phase fluid‐rich and vapor‐rich inclusions and polyphase hypersaline inclusions from porphyry‐type quartz veinlet stockworks. The liquid–vapor homogenization temperatures (TH) and the dissolution temperature of halite daughter crystals (TM) from the polyphase hypersaline inclusions predominantly range from 400°C up to &gt;500°C. The wide range of TH and TM may be due to heterogeneous trapping of variable ratios of vapor and brine. For some inclusions, TH &gt; TM and in some cases, TH &lt; TM, indicating that some of the brine was supersaturated or saturated with NaCl at the time of entrapment. Calculated salinity of the polyphase hypersaline inclusions ranges from 40 to 60% NaCl equivalent. Temperature and vapor pressure of mineralized fluid were estimated to be 400°C and 16 MPa.

Hydrothermal alteration and magnetic properties of rocks in the Carolina de Michilla stratabound copper district, northern Chile

In the Carolina de Michilla district, northern Chile, stratabound copper mineralization is hosted by Jurassic volcanic rocks along the trace of the Atacama fault system. In this study, we present the overall effects of hydrothermal alteration on the magnetic properties of rocks in this district. Two types of metasomatic alteration associations occur, one of regional extent and the other of local hydrothermal alteration associated with copper mineralization (e.g., Lince–Estefania–Susana). Regional alteration is interpreted as a low-grade “propylitic association” characterized by an epidote–chlorite–smectite–titanite–albite–quartz–calcite association. The local hydrothermal alteration is characterized broadly by a quartz–albite–epidote–chlorite–calcite mineral assemblage. The most pervasive alteration mineral is albite, followed by epidote and, locally, actinolite. These minerals contrast sharply against host rock minerals such as chlorite, calcite, zeolite, prehnite, and pumpellyite, but alteration is constrained to mineralized bodies as narrow and low contrast alteration halos that go outwards from actinolite–albite to epidote–albite, to epidote–chlorite, and finally to chlorite. Hydrothermal alteration minerals, compared to regional alteration minerals, show iron-rich epidotes, a lower chlorite content of the chlorite–smectite series, and a nearly total albite replacement of plagioclase in the mineralized zones. Opaque minerals associated with regional alteration are magnetite and maghemite, and those associated to hydrothermal alteration are magnetite, hematite, and copper sulphides. We present paleomagnetic results from nine sites in the Michilla district and from drill cores from two mines. Local effects of hydrothermal alteration on the original magnetic mineralogy indicate similar characteristics and mineralogy, except for an increase of hematite that is spatially associated with the Cu–sulphide breccias with low magnetic susceptibilities. Results indicate that it is impossible to magnetically differentiate mineralized bodies from unmineralized lavas, except for pyrite-rich hydrothermal breccias. In conclusion, for stratabound copper deposits of the Michilla type, the overall effect of hydrothermal alteration on the paleomagnetic properties of rocks is of low contrast, not clearly discernable even at a small scale. From an exploration point of view, magnetic exploration surveys should not discern mineralized bodies of Cu–sulphide breccias except in detailed ground surveys due to the small size of contrasting bodies. Unoriented drill cores with primary ore mineralization record a characteristic remanent magnetization of reverse polarity. Taking into account the azimuth and dip of the drill cores, we were able to compare the magnetization of the mineralized bodies with the characteristic directions from sites drilled in situ from Late Jurassic–Early Cretaceous intrusives mostly. The characteristic direction recorded by the Pluton Viera is similar to the magnetization of the ore bodies of the Estefania mine. If copper mineralization mostly postdates the tilt of the volcanic flows, the low paleomagnetic inclinations suggest an age for the mineralization near 145 Ma, the time of the lowest paleolatitude for the South American plate during the Mesozoic.

Application of lithogeochemistry to gold exploration in the St Ives goldfield, Western Australia

Lithogeochemistry is being routinely applied to gold exploration programmes at St Ives, Kambalda. Sampling of fresh rock for multi-element analysis is undertaken in all ongoing exploration drilling, and supplementary sampling of archived drill hole pulp samples has also been implemented. Samples are digested by a four-acid digest (hydrofluoric, nitric, perchloric and hydrochloric acids) in Teflon tubes using a specially designed microprocessor controlled digester and analysed by ICP-MS/OES for 36 elements (Ag, As, Ba, Bi, Ce, Cs, Hf, La, Li, Mo, Nb, Pb, Rb, Sb, Ta, Te, Th, W, Zr, Al, Ca, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, P, S, Sc, Sr, Ti, V, Zn). Multi-element results undergo two stages of assessment: (a) utilization of trace elements to characterize lithological and stratigraphic domains, and (b) utilization of major and pathfinder elements to characterize samples that are altered or enriched above background levels. Bi-plots of trace elements are used to constrain rock type and stratigraphy. Diagnostic inter-element ratios are apparent, which enable clear differentiation of Kambalda Komatiite, Devon Consols Basalt and Paringa Basalt. Condenser Dolerite can be differentiated from other mafic units except Lunnon Basalt. Fractionated dolerite sills are highly prospective host rocks for gold mineralization in the St Ives field. In all cases the fractionated portion of dolerite sills is readily identified by a Ti concentration greater than 6000 ppm. Unaltered and altered ultramafic, mafic, felsic–intermediate rocks have been assessed using molar Na/Al and K/Al alteration indices to define areas of sodic, potassic and strong potassic alteration. The distribution of potassic and sodic altered samples favourably correlates with the distribution of observed biotite and albite alteration and known gold mineralization. Pathfinder elements (As, Bi, Sb, W, Mo) have been assessed at the scale of the whole field. Antimony is decoupled from Mo and Bi. Bismuth displays good association with existing gold deposits and areas of known intrusions. Areas of As enrichment correlate spatially with Sb although they are not as widely distributed. Tungsten is the most constrained of the pathfinders and displays excellent correlation with major structures and existing gold deposits.

Shock implantation of Martian atmospheric argon in four basaltic shergottites: A laser probe 40Ar/ 39Ar investigation

Spatially resolved argon isotope measurements have been performed on neutron-irradiated samples of two Martian basalts (Los Angeles and Zagami) and two Martian olivine-phyric basalts (Dar al Gani (DaG) 476 and North West Africa (NWA) 1068). With a ∼50 μm diameter focused infrared laser beam, it has been possible to distinguish between argon isotopic signatures from host rock (matrix) minerals and localized shock melt products (pockets and veins). The concentrations of argon in analyzed phases from all four meteorites have been quantified using the measured J values, 40Ar/ 39Ar ratios and K 2O wt% in each phase. Melt pockets contain, on average, 10 times more gas (7–24 ppb 40Ar) than shock veins and matrix minerals (0.3–3 ppb 40Ar). The 40Ar/ 36Ar ratio of the Martian atmosphere, estimated from melt pocket argon extractions corrected for cosmogenic 36Ar, is: Los Angeles (∼1852), Zagami (∼1744) and NWA 1068 (∼1403). In addition, Los Angeles shows evidence for variable mixing of two distinct trapped noble gas reservoirs: (1) Martian atmosphere in melt pockets, and (2) a trapped component, possibly Martian interior ( 40Ar/ 36Ar: 480–490) in matrix minerals. Average apparent 40Ar/ 39Ar ages determined for matrix minerals in the four analyzed meteorites are 1290 Ma (Los Angeles), 692 Ma (Zagami), 515 Ma (NWA 1068) and 1427 Ma (DaG 476). These 40Ar/ 39Ar apparent ages are substantially older than the ∼170–474 Ma radiometric ages given by other isotope dating techniques and reveal the presence of trapped 40Ar. Cosmic ray exposure (CRE) ages were measured using spallogenic 36Ar and 38Ar production. Los Angeles (3.1 ± 0.2 Ma), Zagami (2.9 ± 0.4 Ma) and NWA 1068 (2.0 ± 0.5 Ma) yielded ages within the range of previous determinations. DaG 476, however, yielded a young CRE age (0.7 ± 0.25 Ma), attributed to terrestrial alteration. The high spatial variation of argon indicates that the incorporation of Martian atmospheric argon into near-surface rocks is controlled by localized glass-bearing melts produced by shock processes. In particular, the larger (mm-size) melt pockets contain near end-member Martian atmospheric argon. Based on petrography, composition and argon isotopic data we conclude that the investigated melt pockets formed by localized in situ shock melting associated with ejection. Three processes may have led to atmosphere incorporation: (1) argon implantation due to atmospheric shock front collision with the Martian surface, (2) transformation of an atmosphere-filled cavity into a localized melt zone, and (3) shock implantation of atmosphere trapped in cracks, pores and fissures.

Structural geometry of uraniferous rocks in the Umra area, Aravalli Supergroup, India

Uranium mineralization occurs in Palaeoproterozoic metasediments of the Aravalli Supergroup in the Umra area, Udaipur district, Rajasthan, India. The host rocks for mineralization were deposited in a shallow marine shelf type environment over an Archaean basement gneiss, the Banded Gneissic Complex. A reducing environment at the depositional site, associated with large-scale biogenic activity, was responsible for precipitation of uranium bearing minerals. Our structural analysis shows that the rocks of the mineralized area have been involved in four phases of deformation. The carbonaceous phyllite unit near Umra shows a ‘mirror-image’ pattern, resulting from the superposition of upright F 2 folds with NE–SW striking axial planes over early ( F 1) isoclinal folds. The exploration block is located on one limb of the regional F 2 fold.

Numerical simulation of CO 2 disposal by mineral trapping in deep aquifers

Carbon dioxide disposal into deep aquifers is a potential means whereby atmospheric emissions of greenhouse gases may be reduced. However, our knowledge of the geohydrology, geochemistry, geophysics, and geomechanics of CO 2 disposal must be refined if this technology is to be implemented safely, efficiently, and predictably. As a prelude to a fully coupled treatment of physical and chemical effects of CO 2 injection, the authors have analyzed the impact of CO 2 immobilization through carbonate mineral precipitation. Batch reaction modeling of the geochemical evolution of 3 different aquifer mineral compositions in the presence of CO 2 at high pressure were performed. The modeling considered the following important factors affecting CO 2 sequestration: (1) the kinetics of chemical interactions between the host rock minerals and the aqueous phase, (2) CO 2 solubility dependence on pressure, temperature and salinity of the system, and (3) redox processes that could be important in deep subsurface environments. The geochemical evolution under CO 2 injection conditions was evaluated. In addition, changes in porosity were monitored during the simulations. Results indicate that CO 2 sequestration by matrix minerals varies considerably with rock type. Under favorable conditions the amount of CO 2 that may be sequestered by precipitation of secondary carbonates is comparable with and can be larger than the effect of CO 2 dissolution in pore waters. The precipitation of ankerite and siderite is sensitive to the rate of reduction of Fe(III) mineral precursors such as goethite or glauconite. The accumulation of carbonates in the rock matrix leads to a considerable decrease in porosity. This in turn adversely affects permeability and fluid flow in the aquifer. The numerical experiments described here provide useful insight into sequestration mechanisms, and their controlling geochemical conditions and parameters.