Supergene Minerals Research Articles

Mineralizing fluids of the supergene-enriched Mashitu South Cu-Co deposit, Katanga Copperbelt, DRC

The sediment-hosted Mashitu South (Sud) Cu-Co deposit situated in the Katangan Copperbelt (DR Congo) hosts hypogene and supergene mineralization in Neoproterozoic age rocks of the Mines and Red R.A.T. Subgroups of the Neoproterozoic Katangan Supergroup. Detailed and systematic petrographic, fluid inclusion and stable isotope studies of minerals from various generations of stratabound metalliferous veins, crackle breccia, and alteration zones placed significant constraints on the intricate fluid dynamics, compositions as well as origins of ore-forming and ore-upgrading fluid systems at the Mashitu South deposit. Our data document large-scale migration of metalliferous (Cu, ±Co, Zn) high-salinity brines, sulfate-rich fluids and meteoric fluids, which can be regionally followed across the Katangan Copperbelt through deposits tens of kilometers apart from each other.The stratabound, hypogene, hydrothermal Cu-Co ore veins precipitated at high temperatures (∼247–316 °C) from mixed high-salinity Mg-rich/Na-K-Ca-Mg-Cl brines and Ba-K-Na-Ca ± Cl-SO42− fluids and locally operating thermochemical sulfate reduction-TSR as indicated by negative δ13CV-PDB ratios (down to −7.4‰). A number of these veins were subjected to dynamic recrystallization, under the influence of low salinity/meteoric fluids, during active deformation at the onset of the Lufilian Orogeny. Isotopic compositions of dolomites from veins containing spectacularly large carrollite, chalcopyrite, bornite and chalcocite crystals from the Kamoya South II deposit overlap with isotope data from hydrothermal veins from the Mashitu South deposit suggesting a similar nature of the parent fluids. Metalliferous (Cu, Zn) Na-K-Cl brines, which provided metals for giant crystals of carrollite in post-tectonic ore veins from the Kamoya South II deposit originated from seawater evaporation. Besides fluctuations of the water table the brecciation, alteration and dissolution/re-precipitation processes in the Red R.A.T. and the R.S.C. units were essential for deposition of supergene ores at Mashitu South. The quartz-specular hematite-dolomite veins sealing the crackle breccia, which resulted from hydraulic fracturing, precipitated from Na-Ca-Cl-SO42− fluids gaining salinity (<12.7 mass% NaCl equiv.) and compositions through pervasive interaction of low salinity meteoric fluids with formation waters stored in hematitic siliciclastic rocks of the Red R.A.T. Subgroup. Primary mono-phase liquid aqueous inclusions and C-O isotopic signatures of free-growing supergene minerals record T < 50 °C and precipitation from predominantly low salinity freshwaters, with local pulses of saline fluids during the formation of decoupled supergene Cu and Co mineralization.

A new unspiked K-Ar dating approach using laser fusion on microsamples.

Issues induced by neutron irradiation makes 40 Ar/39 Ar dating inapplicable in some cases. The first issue is 37 Ar and 39 Ar recoil effects during irradiation that affect fine-grained minerals (<50 μm), such as lunar rocks, glassy groundmass, supergene minerals (e.g., illite, glauconite, Mg-oxide, etc.). The second issue from neutron irradiation is the high radioactivity gain of iron-rich samples such as pyrite, and the third is the production of interference nuclides during irradiation. The inherent drawbacks of conventional K-Ar and current unspiked K-Ar dating make it difficult to assess the reliability of age results. A new approach is proposed using well-calibrated 40 Ar/39 Ar standard minerals to directly quantify 40 Ar, 38 Ar and 36 Ar. Fish Canyon sanidine (FCs), B4M muscovite and MMhb-1 hornblende, the widely used international standard minerals, were analyzed as unknowns to test the approach. Argon isotope analyses were carried out on a noble-gas mass spectrometer using laser fusion on microsamples (n × 0.01 to n × 0.2 mg). A new isochron - an "inverse isochron" for K-Ar dating - was designed. FCs and B4M yielded apparent and inverse isochron ages of 28.1 ± 0.1 and 28.0 ± 0.3 Ma, 18.2 ± 0.1 and 18.2 ± 0.5 Ma, which are consistent with the recommended ages, while the MMhb-1 presented lower apparent and inverse isochron ages (510.8 ± 4.8 and 512.3 ± 17.0 Ma) than the recommended ones. The initial argon compositions for the three standard minerals are 299.2 ± 205.3 (FCs), 294.0 ± 16.4 (B4M) and 290.9 ± 203.1 (MMhb-1), agreeing with that of air. The proposed approach potentially overcomes the issues of 40 Ar/39 Ar rising from irradiation and the drawbacks of K-Ar. By using laser fusion on multiple microaliquots from a same sample, this approach can produce accurate and precise K-Ar ages and give an inverse isochron. This new approach may provide an alternate dating method of geochronology based on argon isotopes.

Mineral characterisation of the non-sulphide Zn mineralisation of the Florida Canyon deposit, Bongará District, Northern Peru

ABSTRACTThe Florida Canyon deposit hosted by a carbonate-evaporite sequence of the Pucará Group, Peru, has both Zn–Pb sulphide and Zn non-sulphide mineralisations. The mineralogical characterisation of samples from the deposit enables the definition of a supergene assemblage dominated by smithsonite, in addition to hemimorphite, cerussite, and goethite. The main oxidation in the study area involved the direct replacement of primary MVT stratabound sulphide ore. In comparison with other non-sulphide deposits of the Bongará District, the Florida Canyon deposit can be correlated with the Cristal deposit by means of the original sulphides preserved in the supergene textures, the similar hosting dolostone layer, lack of hydrozincite, and depth of emplacement, which are characteristics that contrast with those of the Mina Grande deposit, also inserted in the Pucará Group. The Florida Canyon supergene mineral assemblage is a paleoclimatic indicator suggesting a humid tropical climate during the late Miocene in the Peruvian Amazon region.

Copper minerals from volcanic exhalations – a unique family of natural compounds: crystal-chemical review

The crystal-chemical characterization of oxysalts (sulfates, arsenates, vanadates, selenites, silicates, molybdates and borates), chlorides and oxides with species-defining Cu2+ formed in volcanic fumaroles (96 minerals representing 80 structure types; 81 species are endemic to fumarolic formation) is given. Copper minerals are known only from oxidizing-type fumaroles. The most diverse copper mineralization occurs at the Tolbachik volcano (Kamchatka, Russia). Copper minerals from fumarolic systems are subdivided into two genetic groups: Group I are minerals formed in the hot zones of fumaroles (&gt;473 K, mainly 673–973 K) and Group II are minerals formed in the moderately hot zones of fumaroles (&lt;473 K, mainly at 343–423 K). Group I includes 81 mineral species. Their most defining chemical feature is that all of them are hydrogen-free, and many of them contain the additional anion O2−. In comparison with minerals from other geological environments, in minerals of Group I the Cu2+ cation exhibits the strongest affinity for four- and fivefold coordinations and the strongest distortion of Cu2+-centred octahedra. Group II consists of 15 chlorides and sulfates including 13 H-bearing species. In these minerals the Cu2+ cation shows affinity for octahedral coordination, with OH− and/or H2O0 as ligands. In terms of crystal chemistry these minerals are closer to supergene minerals rather than to high-temperature fumarolic species. Temperature is the major factor governing the crystal chemistry of Cu2+ oxysalts and chlorides in low-pressure systems. The defining feature of fumarolic copper mineralization over this whole temperature range is the important role of alkali cations. The available data on complexes of Cu2+-centred polyhedra in the structures of natural oxysalts and halides are summarized and reviewed. Isomorphism in copper minerals from volcanic exhalations is discussed. The structures of high-temperature Cu oxysalts with additional O2− anions (i.e. O atoms non-bonded to S6+, Mo6+, As5+, V5+, Se4+ or B3+) are also interpreted using an approach based on oxocentred tetrahedra.

Geology, Re-Os age, S and O isotopic composition of the Lar porphyry Cu-Mo deposit, southeast Iran

The Lar porphyry Cu-Mo deposit is located 20 km northeast of Zahedan in the Sistan Suture Zone (SSZ), in the southeastern part of Iran. Lar is a sub-economic porphyry Cu-Mo deposit with average grades of 0.16% Cu, 0.01% Mo, and 0.66 ppb Au. The Cu-Mo mineralization developed in post-collisional shoshonitic syenitic to monzonitic intrusions in an Oligocene igneous complex known as the Lar Igneous Complex (LIC). The host and country rocks have undergone variable degrees of supergene and hypogene alteration including silicic, potassic, propylitic, phyllic, and argillic alteration. The Cu-Mo mineralization is mainly controlled by NNW striking faults and fractures, and occurs as quartz veins and veinlets, disseminated sulfides, and stockworks. The mineralization can be divided into two types: hypogene and supergene. Hypogene mineralization is characterized by chalcopyrite, pyrite, bornite, molybdenite and magnetite, whereas supergene mineralization is characterized by enargite, covellite, chalcocite, digenite, malachite, azurite, and iron hydroxides. Sulfur isotope values for hypogene sulfides range from 0.28 to −1.76‰ and suggest an igneous source/or mantle-derived origin for sulfur. The δ18O values for quartz in mineralized quartz veins range from 9.6 to 11.5‰. These values would be in equilibrium with waters having calculated δ18OH2O values of 9.3–11.2‰, suggesting magmatic water. Re-Os dating of two molybdenite samples from two different drill holes yields dates of 29.72 ± 0.11 and 31.95 ± 0.11, suggesting that the system was active for at least 2.0 m.y.

Chai Mat Kaolin–Bauxite Deposit (South Vietnam): Typomorphic Features of Kaolinite and Formation Mechanism of the Zonal Profile of the Bauxite-Bearing Weathering Crust of Granites

Analysis of the evolution of bauxite-bearing weathering crust in the geological history of Asia, as well as detailed mineralogical–geochemical and hydrogeochemical studies, has made it possible to consider supergene infiltration metasomatism as a single mechanism for the formation of the eluvial zonal profile, using the weathering crust of southern Vietnam weathering crust as an example. It is established that all weathering crust zones develop simultaneously throughout the existing fracturing as rocks interact with the solutions percolating through them. All supergene minerals are formed directly from original parent rock components rather than by staged development at each other’s expense. The resulting paragenetic assemblages of newly formed minerals result in zoning.

Effect of Chemical and Phase Compositions on Absorption and Flotation Properties of Tin–Sulphide Ore Tailings with Dibutyl Dithiocarbamate

—Using UV-visible spectrophotometry, it is found that old tailings of Solnechny Mining and Processing Plant are characterized with high absorptive capacity relative to sodium dibutyl dithiocarbamate (DBDTC) and require high consumption of collecting agents in flotation. The optical and scanning electron microscopy tools revealed gypsum-cemented finely dispersed fractions (smaller than 20 μm) coating larger mineral particles (60–80 μm). Minerals are represented by chalcopyrite, pyrrhotine, pyrite, sphalerite, jamesonite and cassiterite; rocks are quartz and silicates. A feature of these old tailings is supergene mineralization and sulphide oxidation products. Alongside with cassiterite, varlamovite is present as a typical product of stannite-group tin sulfosalts modification. New experimental data on flotation of Solnechny MPP tailings with DBDTC are obtained. The flotation experiments show that DBDTC addition to butyl xanthate (at ratio of 1: 3) enables increased recovery of copper, lead, zinc and silver in bulk concentrate and reduces loss of these minerals in slime.

Middlebackite, a new Cu oxalate mineral from Iron Monarch, South Australia: Description and crystal structure

AbstractMiddlebackite is a new supergene mineral formed in the upper levels of the Iron Monarch quarry, South Australia. It occurs as aggregates of blue, prismatic crystals up to 0.3 mm across comprising individual crystals up to 0.05 mm in length associated with atacamite and mottramite. Crystals are translucent with a vitreous lustre and have a pale blue streak. Middlebackite is brittle with one perfect cleavage and uneven fracture. Mohs hardness is ~2. The calculated density is 3.64 g cm–3. Crystals are biaxial (+) with α = 1.663(4), β = 1.748(4) and γ = 1.861(4) (measured in white light). The calculated 2V is 86.7°. Pleochroism isX= colourless,Y= very pale blue andZ= dark sky blue;Z&amp;gt;Y&amp;gt;X. The empirical formula unit, based on six oxygen atoms per formula unit is Cu2.00(C2O4)Cl0.02(OH)1.98. Middlebackite is monoclinic, space groupP21/c witha= 7.2597(15),b= 5.7145(11),c= 5.6624(11) Å, β = 104.20(3)°,V= 227.73(8) Å3andZ= 2. The five strongest lines in the powder X-ray diffraction pattern are [d(Å), (I), (hkl)]: 7.070 (16) (100), 3.739 (100) (11$\bar{1}$), 2.860 (18) (020), 2.481 (12) (12$\bar{1}$) and 2.350 (9) (300). The crystal structure was refined from synchrotron single-crystal X-ray diffraction data toR1= 0.0341 for 596 observed reflections withF0&amp;gt; 4σ(F0). The structure is based on sheets of edge- and corner-sharing octahedra parallel to thebcplane. Sheets link in theadirection via oxalate anions.

Atypical Cu mineralisation in the Cornwallis carbonate-hosted Zn district: Storm copper deposit, Arctic Canada

The metallogenetically important Cornwallis Zn district, in Canada’s Arctic islands, includes the past-producing Polaris mine and numerous base-metal showings, including Storm copper. Storm is unusual because it is near the southern limit of the district, is Cu- versus Zn-dominated, is overlain and underlain by red sandstone, and is hosted by Silurian strata (rather than Ordovician strata, as at Polaris). Mineralisation styles are primarily carbonate replacement and breccia, and consist of geerite, covellite, bornite, and chalcopyrite, all of which are associated with calcite and dolomite gangue. Multiple in situ micro-analytical techniques (fluid inclusion microthermometry, evaporate mound SEM-EDS, LA ICP-MS, SIMS), plus conventional Cu isotopic analysis were integrated to characterise the nature of the mineralising fluids and events. Fluid inclusions yielded low-temperature (Th <130 °C), moderate- to low-salinity (17.0 to 0.4 wt% NaCl equiv.) fluids. Evaporate mound SEM-EDS analysis, which quantified the solute chemistry, show a change from bimodal Na- and Na+K-dominated fluid mixture to a Na+K-dominated fluid. The δ18OH2O signature of the mineralising fluid involved reservoirs reflecting involvement of meteoric (<0%) and rock-equilibrated (8%) fluids, respectively. The δ34S values are nearly homogeneous for pre-ore and second generation sulphides at 0% and 10%, respectively, whereas the third generation (recrystallised) sulphides are more variable, with values from 8.8 to 23.4%. The majority of Cu sulphides have δ65Cu values near 0%, but second-generation chalcopyrite shows elevated δ65Cu values from +1.31% in the north to +3.44% in the south, suggesting a probable southward fluid flow. Positive Eu PAAS-normalised anomalies in early dolomite cements, which indicate basement involvement, are absent in succeeding cements, suggesting a change in fluid pathways and hence the mineralogical nature of the reservoir. Negative CeSN anomalies in main-stage calcite cement indicate an oxidised fluid, but the lack of similar Ce anomalies in the dolomite cements indicates reduced fluids. These data suggest that Storm copper mineralisation is related to basement-equilibrated fluids mobilised during the Ellesmerian orogeny (Devonian), which acquired heat and leached Cu from underlying Proterozoic red sandstone (Aston Formation) and were then focussed up faults where Cu sulphides precipitated. Prolonged meteoric fluid movement from the north, with intermittent episodes of basin-equilibrated fluid influx, produced hypogene and supergene mineralisation. Fluid movement resumed in the Cenozoic, when high-latitude fluids altered Cu minerals to atacamite. Storm copper mineralisation shares characteristics with both red-bed and Irish-type Cu deposit sub-types, but may also highlight a relationship with carbonate-replacement Zn-Pb deposits. Timing of mineralisation is comparable to that of Zn ore precipitation at Polaris, suggesting that Storm may be an integral part of the Cornwallis district, in spite of its unusual composition.

Mineralogy, Geochemistry, Fluid Inclusion and Oxygen Isotope Investigations of Epithermal Cu ± Ag Veins of the Khur Area, Lut Block, Eastern Iran

The Khur metallogenic district is located in a volcanic–plutonic belt in the central Lut Block (central eastern Iran). Mineralization occurs in Middle Eocene andesitic tuff and along four main vein systems trending northwest–southeast (Shurk, Mir‐e‐Khash, Shikasteh Sabz and Ghar‐e‐Kaftar veins). Microscopic studies reveal that the veins contain bornite, chalcocite, pyrite, tennantite together with minor sphalerite and chalcopyrite as hypogene minerals and chalcocite, digenite, covellite, valleriite, malachite, azurite, atacamite, hematite, and goethite as supergene minerals. The ore bodies are accompanied by narrow but intensely developed wall rock alterations of argillization, carbonatization and silicification. Copper content reaches 6.5, 2.4, 4.2 and 5% in Mir‐e‐Khash, Shikasteh Sabz, Ghar‐e‐Kaftar and Shurk, respectively. Microthermometric measurements of quartz‐ and calcite‐hosted fluid inclusions indicate that the mineralization might be derived from a moderately saline hydrothermal fluid at temperatures between 175–316°C. Calculated δ18O values of water in equilibrium with quartz and calcite for Khur veins suggest that the fluid might have had a magmatic source, but the 18O‐depletion was developed through mixing with meteoric water. Copper deposition in Khur veins is believed to have been largely caused by mixing, although wall rock reactions may also have occurred. The Khur veins are classified as volcanic‐subvolcanic hydrothermal‐related vein deposits.

Locating the Flood/post-Flood boundary using the relative dating of the weathering of ore deposits

Erosion at the end of and after the Flood exposed the tops of primary ore deposits to subsequent weathering and the formation of supergene minerals. The 40Ar/39Ar and (U-Th)/He radioisotope methods applied to these supergene minerals provide the dates for when these minerals formed. But given the documented problems with the radioisotope methods, they can only provide at best relative dates. When the Flood waters retreated, the ground surface was dry, but extra time was needed to allow the water table to drop, soil to form and plants to grow before Noah stepped off the Ark, which marked the end of the Flood event. The weathering front then progressed downwards during the early post-Flood decades for supergene minerals to start forming. Residual post-Flood catastrophism may have involved mountains still rising and ore deposits still forming, such as the porphyry copper deposits associated with granite intrusions as the Andes continued to rise. Erosion exposed those later-formed ore deposits to subsequent weathering to produce supergene minerals well into the early post-Flood era. Thus, the relative ages of the supergene iron oxides, and potassium-bearing sulfates and manganese oxides produced span the whole Cenozoic. It is proposed that relative dates for the first formation of supergene minerals can possibly be used as a criterion for determining the placement of the Flood/post-Flood boundary at the K-Pg boundary with a relative age of 66 Ma. The few slightly earlier relative ages likely resulted from weathering that commenced before the Flood event ended, though deep weathering would have required decades. The spread of relative ages through the Cenozoic thus represents the progressive formation of supergene minerals as primary ore deposits emplaced during, and maybe after, the Flood were subsequently exposed to weathering by residual catastrophism. Continuing investigation of this criterion for placement of the Flood/post-Flood boundary seems warranted.

Exhumation history and timing of supergene copper mineralisation in an arid climate: New thermochronological data from the Centinela District, Atacama, Chile

AbstractSupergene copper mineralisation (SCM) processes occur during the unroofing of porphyry copper deposits. However, the geomorphological stage during which the main mineralisation occurs is still under debate. Here, we present 24 new thermochronological data from Cenozoic intrusives and compare them with the evolution of supergene mineralisation from the Centinela Mining District in the Atacama Desert. Our results indicate a two‐step cooling path: a rapid Late Eocene exhumation followed by a slow denudation. Previously published supergene mineralisation ages cluster after the main Upper Oligocene exhumation period. Ours is the first study that establishes the relationship between exhumation and supergene processes on the scale of a single mining district. It confirms that SCM took place during pediplanation, likely a required condition for efficient SCM under arid climatic conditions, in contrast to wet tropical environments where SCM occurs during rapid relief growth but has limited preservation potential.

Abellaite, NaPb2 (CO3)2 (OH), a new supergene mineral from the Eureka mine, Lleida province, Catalonia, Spain

The new mineral abellaite (IMA 2014-111), ideally NaPb 2 (CO 3 ) 2 (OH), is a supergene mineral that was found in one of the galleries of the long-disused Eureka mine, in the southern Pyrenees (Lleida province), Catalonia, Spain. Abellaite is found as sparse coatings on the surface of the primary mineralization, it forms subhedral crystals not larger than 10 μm as well as larger pseudohexagonal platelets up to ∼30 μm. Individual crystals commonly have a tabular to lamellar habit and form fairly disordered aggregates. The mineral is associated with a large number of primary minerals (roscoelite, pyrite, uraninite, coffinite, ‘carbon’, galena, sphalerite, nickeloan cobaltite, covellite, tennantite and chalcopyrite) and supergene minerals (hydrozincite, aragonite, gordaite, As-rich vanadinite andersonite, cejkaite, malachite and devilline). Abellaite is colourless to white, with a vitreous to nacreous lustre. The mineral is translucent, has a white streak and is non-fluorescent. The aggregates of microcrystals are highly friable. The calculated density using the ideal formula is 5.93 g/cm 3 . The chemical composition of the mineral (the mean of 10 electron microprobe analyses) is Na 3.88, Ca 0.29, Pb 72.03, C 4.17, O 19.47 and H 0.17, total 100.00 wt% (H, C and O by stoichiometry assuming the ideal formula). On the basis of 7 O atoms, the empirical formula of abellaite is Na 0.96 Ca 0.04 Pb 1.98 (CO 3 ) 2 (OH). The simplified formula of the mineral is NaPb 2 (CO 3 ) 2 (OH). The mineral is hexagonal, space group P 6 3 mc , a = 5.254(2), c = 13.450(5) A, V = 321.5(2) A 3 and Z = 2. The strongest powder-diffraction lines [ d in A ( I ) ( h k l )] are: 3.193 (100) (0 1 3), 2.627 (84) (1 1 0), 2.275 (29) (0 2 0), 2.242 (65) (0 2 1, 0 0 6), 2.029(95) (0 2 3). Abellaite has a known synthetic analogue, and the crystal structure of the mineral was refined by using crystallographic data of the synthetic phase. The mineral is named in honour of the mineralogist and gemmologist Joan Abella i Creus (b. 1968), who has long studied the deposits of the Eureka mine and who collected the mineral.

Physicochemical Modeling of Sulfide Oxidation Processes in Tailing Dumps of the Kavalerovsky District and Their Impact on the Hydrosphere (Primorsky Krai)

Oxidation of tailings in the tailing dump of Vysokogorskoe deposit (Kavalerovsky district) in the temperature range from 0 to +45°C was simulated with successive exclusion of each of the sulfide minerals present therein (pyrite, pyrrhotite, galena, sphalerite, arsenopyrite, and chalcopyrite). The Eh-pH parameters for anthropogenic mineral formation were determined at different host rock–sulfide ratios (95: 5, 90: 10, 80: 20, 60: 40, 40: 60, 20: 80) in the tailings. Crystallization of supergene minerals was analyzed, qualitative and quantitative ionic compositions of solutions were determined, and their impact on the hydrosphere was assessed.

Supergene and exotic Cu mineralization occur during periods of landscape stability in the Centinela Mining District, Atacama Desert

AbstractThe Centinela Mining District (CMD), Atacama Desert (northern Chile), includes several mid‐late Eocene porphyry Cu deposits that contains supergene mineralization and provides access to a record of gravel deposits that host syn‐sedimentary exotic Cu mineralized bodies. By studying these gravels, we reconstruct the unroofing history and constrain the geomorphological conditions that produced supergene and exotic Cu mineralization. We present an integrated study based on stratigraphic and sedimentological data, lithology clast counts, 40Ar/39Ar and U/Pb ages from interbedded tuff layers and U/Pb detrital zircon geochronology data. To relate the gravel deposition episodes to the timing of the supergene mineralization, we provide in‐situ and exotic supergene mineral ages (40Ar/39Ar and K‐Ar). Six gravel units were deposited between the mid‐Eocene and the mid‐Miocene. The Esperanza gravels were deposited concurrently with the emplacement of porphyry Cu deposits at depth. The subsequent Tesoro I, II and III and Atravesado gravels register the unroofing of these deposits, from the advanced argillic zone to the sericitic and prophylitic hypogene zones. The Arrieros gravels register landscape pediplanation, that is, denudational removal and wear of the landscape to base level on a relatively stable tectonic regime, occurring roughly contemporaneous with supergene activity. The supergene mineral ages of the CMD define a time span (ca. 25–12 Ma) during which most of the supergene ages cluster in northern Chile. This time span corresponds with a period of warm and humid climate conditions in the southern hemisphere. We conclude that landscape pediplanation favours supergene mineralization and helps preserve the former supergene mineralized zones from significant erosion. Low erosion rates during pediplanation may constitute a necessary condition for the efficiency of the supergene processes in such semi‐arid climate.

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.

Development of partial extraction methods to estimate abundance of copper-iron sulphide minerals in the Escondida Norte porphyry copper deposit, Chile

Studies were conducted by Minera Escondida, Ltda. (MEL) between 1995 and 2003 to support an investment decision to open Escondida Norte, an enriched porphyry copper deposit adjacent to the Escondida mine and processing facilities. Due to the importance of copper-iron sulphide minerals in predicting metallurgical performance in both process alternatives, a quantitative spatial model of copper-iron sulphide mineral abundances was determined to be a critical study component. A suite of total and partial copper analyses already in common use was chosen to maximize relative selectivity among the major copper sulphide minerals in the mineral deposit. These analyses were merged with partial and total sulphur analysis and combined with a novel approach to interpret the analytical results as normative mineral estimates of chalcocite, covellite, chalcopyrite and pyrite content. Development and validation of the normative mineral estimates included verification on the analytical stability and reproducibility of the chemical analyses, determination of extraction rates on synthetic mineral standards, and comparison with modal mineral abundance on natural ore samples. Uncertainty of individual normative copper sulphide minerals is estimated at ±5% (95% confidence interval), while uncertainty on pyrite content is ±12%. The uncertainty of the normative mineral estimates are comparable to uncertainty of corresponding modal mineral estimates, and differences between modal and normative estimates are largely within the estimation uncertainty. While the methods were originally developed for a simple association based on supergene chalcocite ± covellite replacing and overprinting hypogene chalcopyrite + pyrite, subsequent work developed normative mineral models for bornite + chalcopyrite hypogene and mixed copper oxide-sulphide supergene mineralization.

Crystal structure of the (REE)-uranyl carbonate mineral shabaite-(Nd)

Shabaite-(Nd) is a rare supergene mineral formed during alteration-hydration weathering of uraninite; its structure has remained unknown until now. Based on single-crystal X-ray diffraction data, shabaite-(Nd) is triclinic, twinned (leading to a pseudo-monoclinic diffraction pattern), space group P-1, with a = 8.3835(5), b = 9.2766(12), c = 31.7519(3) angstrom, alpha = 90.058(3), beta = 89.945(4), gamma = 90.331(4)degrees, V = 2469.3(4) angstrom(3) and Z = 4. The structure was refined from diffraction data to R = 0.060 for 8434 unique observed reflections. The structure of shabaite-(Nd) is based upon finite clusters of polyhedra, the well-known uranyl-tricarbonate cluster (referred to as UTC), the Ca-polyhedra linked to UTC and infinite sheets of Nd-polyhedra with internal Nd-O-C linkages. The infinite sheets of Nd-polyhedra are stacked perpendicular to c; to the UTC are staggered approximately perpendicular to the Nd-based sheets (thus approx. parallel to c), forming electroneutral ({Nd-2(3+)(CO3)(2-)(2)(H2O)(2)}(2+){Ca(H2O)(5)}(2+)[(UO2)(CO3)(3)](4-))(0) layers. Adjacent layers are linked by hydrogen bonds. The ratio of elements in the chemical formula of shabaite-(Nd), obtained from the structure refinement, Nd2Ca[(UO2)(CO3)(3)] (CO3)(2)(H2O)(10.5), Z = 4, was confirmed by an electron-microprobe study. The resulting empirical formula for the average of six analyses is Ca-1.01(Nd0.64Ce0.32Sm0.28Gd0.19Y0.19Pr0.13Dy0.10La0.07Tb0.05Ho0.01)(Sigma 1.98)(UO2)(CO3)(4.98)(H2O)(10.5)(based on 1 U apfu). A discussion of the crystal-structural relationships of shabaite-(Nd) and chemically related minerals is given as well as discussion of complexities of their crystal structures.

The Karst-Hosted Mina Grande Nonsulfide Zinc Deposit, Bongará District (Amazonas Region, Peru)

The Mina Grande zinc nonsulfide deposit (Amazonas region, Peru) consists of several accumulations in karst cavities of Zn oxide minerals, derived from weathering of a Mississippi Valley-type deposit hosted in Mesozoic carbonate rocks of the Condorsinga Formation (Pucara Group). The karst cavities hosting the nonsulfide ores were developed in the Miocene along northwest-southeast faults (avg 230° dip, 70° dip angle) associated with regional structures, and locally along stratification joints (avg 8° strike, 278° dip, and 26° dip angle). Thus far, the Mina Grande Zn accumulations have been partially mined in two pits (named “Fase A” and “Fase B”) and explored in a third area, named “Fase C.” In 2008, the resources were as follows: Fase A = 160,000 metric tons (t) @ 21.2% Zn, Fase B = 36,400 t @ 28.9% Zn, and Fase C = 116,700 t @ 22.5% Zn. The nonsulfide mineral assemblage, consisting mostly of hydrozincite, smithsonite, and hemimorphite, is associated with few remnants of the hypogene ore (pyrite and sphalerite). Mineralogical and petrographic studies revealed several texturally distinct smithsonite and hydrozincite generations, which are characterized by extremely negative δ 13 C compositions, pointing to the prevailing contribution of an isotopically light carbon component from the oxidation of organic matter-derived carbon, and different δ 18 O isotope compositions: 26.9 to 27.2 and 26.0 to 26.3‰ δ 18 O VSMOW for smithsonite, and averages 24.6 and 23.7‰ δ 18 O VSMOW for hydrozincite. These distinct δ 18 O compositions indicate that smithsonite and hydrozincite precipitated at least after two depositional stages. These stages were probably related to several periods of uplift that occurred in the Bongara district since at least ~10 Ma, when the transition from the Pebas to Acre systems affected the Amazonas foreland basin in the Miocene. The karst activity, to which the supergene mineralization is related, was restricted to the late Miocene and early Pliocene periods. Climatic conditions reconstructed from the ecosystems persisting in the region from Late Tertiary to Recent time suggest that the Mina Grande supergene mineralization was associated with several weathering episodes that occurred under a climate resembling the present-day climatic conditions.

Relationship Between Hydrothermal Alteration Index and Geological Attributes at Chuquicamata Cu–Mo Porphyry Deposit, Chile

AbstractThe distribution of mineral zones, hydrothermal alteration assemblages and ore associations are important geological attributes of the Chuquicamata Porphyry Cu–Mo deposit. The development and morphology of these attributes were influenced by syn‐mineralization structures with later modification of post‐mineralization tectonism to yield the present day form. In this work we evaluate the elemental mass transfer index due to hydrothermalism by using Pearce Element Ratios, using a database of 1110 samples consisting of elemental concentrations by chemical analysis. We study the relationship of the aforementioned index with the geological attributes in the porphyry as well as evaluate the type of information that the index provides according to its distribution, magnitude and variability. The magnitudes of the alteration index are organized into deciles, each of which represents a source of information, or macrostate. Within these, the geological attribute, or microstate, has a probability to participate that depends on the possible categorization of a particular attributes. The amount of information or uncertainty that each geological attribute provides to each decile of the distribution of the alteration index is determined by measuring the value of entropy. The results indicate that this distribution is controlled by the geometric–kinematic properties of syn‐mineralization structures that generate the primary pattern related to potassic alteration that develops in the hanging wall of the East Shear Zone. Subsequently the shape of the potassic alteration is modified by the kinematic effect imposed by the Americana and Estanques Blancos faults, and this broadly defines the geometry of the Qz–Mo event and the shape of the phyllic alteration. In addition, the sinistral‐inverse shear of the West Fault Zone allows the construction of an enriched blanket and also generates the drag fold that is characteristically observed in the south wall of the mine. All of the above events combine to yield the final geometry of the geological attributes that controls the shape and magnitude of the alteration index in the porphyry. The magnitude of the alteration index is mainly driven by the potassic and phyllic alteration stages. Potassic alteration is dominant in the first four deciles, while phyllic alteration is dominant in the last six deciles. Supergene mineralization attributes contribute discreetly to the average magnitude of the alteration index for each decile. The distribution of the probability of participation of the geological attributes in the distribution of the alteration index suggests that in the potassic alteration, biotite and green‐gray sericite facies contribute to strengthen the index in an associated manner, while in the phyllic alteration, the Qz–Ser textural type is the most significant.