Sediment-hosted Base Metal Deposits Research Articles

Stratiform Host-Rock Replacement via Self-Sustaining Reactions in a Clastic-Dominated (CD-type) Zn Deposit

Abstract Stratiform to stratabound replacement of a mixed siliciclastic-carbonate host rock is a defining characteristic of many sediment-hosted base metal deposits. Mineralized rocks in clastic-dominated (CD-type) Zn-Pb ore deposits, which represent our highest value base metal resources, are generally thin (101 m), laterally extensive (103 m), and stratiform to stratabound in fine-grained siltstone and mudstone facies. At the recently discovered Teena CD-type Zn-Pb deposit (Proterozoic Carpentaria province, Australia), the host rock was undergoing burial diagenesis when altered and mineralized by hydrothermal fluids that moved up to 2 km lateral to the fluid input conduit (growth fault) through intraformational intervals. In much of the deposit, carbonate dissolution was an important reaction permeability control, although significant amounts of mineralization also occur in carbonate-free siliciclastic beds. In this study, transmission electron microscopy (TEM) data has been generated on a drill core sample that preserves a sharp reaction front between mineralized and unmineralized domains of the fine-grained siliciclastic compositional end member (carbonate free). Petrographic and mineralogical data provide evidence that oxidized hydrothermal fluids moved through the protolith via reaction permeability that developed from feldspar dissolution. The nature of reactive fluid flow was determined by reactions that took place at the fluid-mineral interface. Pyrite formation during the earliest stage of the hydrothermal paragenesis increased the mineral reactive surface area in the protolith. Acidity was then generated in situ via self-sustaining reactions involving pyrite oxidation, transient Fe sulfate formation, and sphalerite precipitation, which provided positive feedbacks to enhance porosity creation and further fluid infiltration and mineralization. In the absence of carbonate, however, ore fluid pH was buffered by K-feldspar dissolution (~4.5), thereby ensuring sphalerite precipitation was not inhibited under more acidic conditions. All CD-type deposits in the Carpentaria province are hosted by a protolith comprising carbonate, K-feldspar, pyrite, and organic matter; these phases set the boundary conditions for the development of self-sustaining reactions during ore formation. Importantly, these self-sustaining reactions represent a Goldilocks zone for ore formation that is applicable to other sediment-hosted deposits that formed via replacement of mixed siliciclastic-carbonate host rocks (e.g., stratiform Cu).

Signals of low grade organic matter alteration in the Upper Permian Kupferschiefer (Spremberg area, Eastern Germany) – A by-product of copper mineralization?

Organic matter (OM) is known to be an important reductant in many sediment-hosted base metal deposits. However, the precise nature of the interactions between OM and hydrothermal fluids is still debated, as is just how such interconnected reactions develop over geological timescales. We investigated the Kupferschiefer mineralization in two drill cores from the Spremberg-Graustein field in Eastern Germany focusing on the inventory of organonitrogen, -sulfur and -oxygen (NSO) compounds, which are most affected by organic–inorganic interactions with mineral surfaces, metals and aqueous fluids. Rather homogeneous deposition of type II marine organic matter, prevailing photic zone euxinia and an early oil window thermal maturity were documented using conventional hydrocarbon analysis. Furthermore, a slight level of hydrothermal alteration was detected in the lower part of drill core 121 h/72, in proximity to the epigenetic hematitic alteration zone known as ‘Rote Fäule’. A minor loss of extractable organic matter and especially long-chain n-alkanes was accompanied by significant enrichment in oxygenated compounds and increased molecular oxygen content detected by ultra-high resolution mass spectrometry (FT–ICR–MS). A strong enrichment of deuterium observed in the n-alkanes (up to −36‰) and isoprenoids (up to −20‰) indicates that hydrogen exchange with an evaporitic fluid occurred in the Spremberg Kupferschiefer over geological timescales at maximum temperatures not exceeding 110 °C. We therefore conclude that FT–ICR–MS, combined with determination of stable hydrogen isotope compositions, can provide valuable information about interaction time scales and the origin of mineralizing hydrothermal fluids.

Base metal mobility linked to brine and hydrocarbon migration at the Huincul High in the Neuquén Basin, Argentina: Implications for the formation of sediment-hosted base metal deposits

This contribution discusses the discovery of PbZn mineralization in hydrocarbon exploration drill holes hosted in siliciclastic Jurassic rocks below the Cretaceous sediment hosted Cu mineralization located north of the Huincul High. In the Early Cretaceous, hydrocarbon and basinal water expulsion took place from the oil source rocks (Los Molles Formation) into the reservoir rocks Lajas Formation (Middle Jurassic), after the formation of diagenetic quartz overgrowths. Marcasite, pyrite and illite + illite-smectite mixed layer minerals then formed together with hydrocarbons. Marcasite (δ34S −3.5 and −2.2‰) and pyrite (δ34S −11.6 to 18.1‰) precipitated via bacterial sulfate reduction from fluids showing temperatures <80 °C and fluctuating pH conditions. During the Paleogene, Andean tectonism triggered migration of oxidized basinal brine containing dissolved leached Zn and Pb from the Los Molles Formation source rocks and S from the evaporites of the basin. This brine entered in the hydrocarbon reservoir of the Lajas Formation, resulting in the precipitation of sphalerite (δ34S +10.4 to +10.8‰) and galena (δ34S +5.2‰) by TSR at temperatures of 119–123 °C together with siderite. The Andean orogeny may also have triggered the migration of hydrocarbons + brines from the shallowest source rocks (Vaca Muerta Formation) and reservoir rocks (Mulichinco, Lajas and Lotena formations) into the red beds of the upper Cretaceous Neuquén Group, which became bleached as a result. Marcasite (δ34S −36.4 to 18.2‰) and pyrite (δ34S −60.2 to −24‰) precipitated by BSR, while calcite (δ13C −12.3 to −5.6‰) formed due to redox reactions accompanying formation of quartz overgrowths, clay mineral coatings (δD −92.4‰ to −82.8‰ and δ18O 16.2‰ to 18.40‰), and barite (δ34S −5.9‰). Quartz overgrowths and barite precipitated from brines (1.5–10 wt% NaCl equiv.) at temperatures of 91–120 °C, with barite precipitating close to the feeder zones. Calcite cements are more widespread and generated from several pulses of brine and hydrocarbons up flow, evidenced by the wide range of homogenization temperatures (100–185 °C) and diversity of UV fluorescence colors of hydrocarbon-rich fluid inclusions. In subsequent Miocene tectonic events, basinal brines were able to leach Cu from the underlying thick red beds of the basin, entering the Neuquén Group oil reservoir traps or carrier beds where they became reduced. Chalcopyrite and bornite precipitated at temperatures >100 °C close to the feeder zones at the expense of barite as a local sulfur source. Chalcocite group minerals (δ34S −21.3‰ to −7.3‰) precipitated outwards in pore spaces of the sandstones at lower temperatures (<100 °C) through BSR of sulfate in the mineralizing brines. Final exhumation of the Neuquén Group (Upper Miocene-Pliocene) promoted the infiltration of the meteoric water, the oxidation of the sulfides and formation of supergene FeCu, V and U minerals. Isotope geochemistry and fluid inclusion results combined with alteration and ore mineralogy document the processes controlling the distribution of base metals in the Huincul High region that can be applied to evaluate the metal potential in other areas of the Neuquén Basin and further extrapolated to similar geological systems elsewhere.

Carbonate Replacement as the Principal Ore Formation Process in the Proterozoic McArthur River (HYC) Sediment-Hosted Zn-Pb Deposit, Australia

AbstractThe McArthur River (HYC) Zn-Pb-Ag deposit in the Carpentaria Zn belt, northern Australia, is one of the world’s largest and most studied sediment-hosted base metal deposits, owing to its lack of deformation and preservation of sedimentary and ore textures. However, the ore formation process (syngenetic vs. epigenetic) is still a subject of controversy. In this paper we focus on key characteristics of the HYC deposit that remain unexplained: preservation of sedimentary carbonate (dolomite) and its association with Zn, and the role of thallium (Tl) and manganese (Mn) distribution in the orebody.Our findings demonstrate a sequence of events during ore formation: Tl is hosted almost exclusively within euhedral pyritic overgrowths around early diagenetic pyrite; sphalerite mineralization occurred after Tl-bearing pyrite overgrowths, in association with acid dissolution (replacement) of laminated and nodular dolomite across the subbasin; and outer rims are enriched in Mn on preserved dolomite at the dissolution reaction front in contact with sphalerite. New thermodynamic fluid chemistry modeling demonstrates the metal distribution and paragenesis can be explained by acidic, oxidized ore fluids entering the pyrite-dolomite host lithology, allowing reduction and pH buffering by acid carbonate dissolution, resulting in stepwise metal deposition in an evolving fluid.We argue this represents strong evidence for epigenetic ore formation at HYC. Furthermore, the primary control on ore deposition is not synsedimentary faulting in the subbasin; rather, the chemical potential of sedimentary carbonate within reduced, sulfidic lithologies appears to be of critical importance to precipitation of sphalerite.

Global distribution of sediment-hosted metals controlled by craton edge stability

Sustainable development and the transition to a clean-energy economy drives ever-increasing demand for base metals, substantially outstripping the discovery rate of new deposits and necessitating dramatic improvements in exploration success. Rifting of the continents has formed widespread sedimentary basins, some of which contain large quantities of copper, lead and zinc. Despite over a century of research, the geological structure responsible for the spatial distribution of such fertile regions remains enigmatic. Here, we use statistical tests to compare deposit locations with new maps of lithospheric thickness, which outline the base of tectonic plates. We find that 85% of sediment-hosted base metals, including all giant deposits (>10 megatonnes of metal), occur within 200 kilometres of the transition between thick and thin lithosphere. Rifting in this setting produces greater subsidence and lower basal heat flow, enlarging the depth extent of hydrothermal circulation available for forming giant deposits. Given that mineralization ages span the past two billion years, this observation implies long-term lithospheric edge stability and a genetic link between deep Earth processes and near-surface hydrothermal mineral systems. This discovery provides an unprecedented global framework for identifying fertile regions for targeted mineral exploration, reducing the search space for new deposits by two-thirds on this lithospheric thickness criterion alone. Major sediment-hosted base metal deposits are located within 200 km of the border between thick and thin lithosphere, according to statistical comparisons between global lithospheric thickness and known deposit locations.

New titanite U–Pb and molybdenite Re–Os ages for a hydrothermal vein-type Cu deposit in the Lanping Basin, Yunnan, SW China: constraints on regional metallogeny and implications for exploration

The small Kedengjian volcanic rock-hosted hydrothermal vein-type Cu deposit (100,000 t Cu metal content at about 3% Cu) is located in the Lanping Basin, one of several Mesozoic–Cenozoic continental sedimentary basins in the northern part of the Indochina block and in the eastern part of the North Qiangtang block. Many sediment-hosted base metal ore deposits with ages of < 65 Ma, which coincide with the Indian–Asian continental collision or the Himalayan orogeny, are present in these basins and are collectively referred to as the Sanjiang sediment-hosted base metal metallogenic belt. The Kedengjian volcanic rock-hosted hydrothermal vein-type Cu deposit has been commonly regarded as part of the Cenozoic Sanjiang sediment-hosted base metal metallogenic belt. We determined the age of mineralization for the Kedengjian deposit using hydrothermal titanite and molybdenite. The U–Pb ages of the first and second generations of titanite are 107.4 ± 5.6 Ma and 100.1 ± 3.4 Ma (2 σ), respectively. The associated molybdenite yields a Re–Os isochron age of 98.2 ± 2.2 Ma (2 σ) and a mean age of 98.3 ± 2.2 Ma, which are identical (within analytical error) to the U–Pb age of the younger titanite. Our new results indicate that the Kedengjian deposit formed at ~ 100 Ma, much older than the Cenozoic sediment-hosted base metal deposits in the Lanping Basin. This significant age difference dismisses the Kedengjian deposit as a member of the Cenozoic Sanjiang sediment-hosted base metal metallogenic belt. The Kedengjian deposit formed during the Late Cretaceous subduction of the Neo-Tethys oceanic plate beneath the previously amalgamated Indochina and North Qiangtang blocks of the Asian continent. On the basis of this tectonic correlation, we suggest that there is potential for pre-Cenozoic hydrothermal Cu mineralization in the region.

The mineralization process of the Lanuoma Pb-Zn-Sb deposit in the Sanjiang Tethys region: Constraints from in situ sulfur isotopes and trace element compositions

The Lanuoma deposit, located in the Sanjiang Tethyan metallogenic domain, shows a characteristic assemblage of Pb-Zn-Sb mineralization, distinguished from those in other sediment-hosted base metal deposits of this region. The ore textures reveal two mineralization stages: the early stage is characterized by Zn mineralization with occurrences of porous pyrite (Py2) and yellowish-brown sphalerites (Sp1), and the late stage is characterized by Sb-Pb mineralization with occurrences of subhedral-euhedral pyrite (Py3), yellowish-white sphalerites (Sp2), sulfosalts consisting of boulangerite, zinkenite, sorbyite and plagionite and calcite veins. The framboidal pyrites (Py1) predate Pb-Zn-Sb mineralization. Py2 has higher Zn contents (16.97–621.67 ppm, mean: 181.17 ppm) than Py3 (Zn: 0.76–323.44 ppm, mean: 29.58 ppm) by up to one order of magnitude. The Se contents of Py2 (Se: 5.46–87.82 ppm, mean: 50.96 ppm) are also one order of magnitude higher than those of Py3 (Se: 1.64–10.99 ppm, mean: 4.35 ppm). Sp1 is characterized by one order of magnitude higher contents of Fe (381.99–9046.64, mean: 4875.50 ppm) and Se (4.02–72.51 ppm, mean: 34.70 ppm) than Sp2 (Fe: 287.53–584.02 ppm, mean: 416.75 ppm; Se: 2.89–10.80 ppm, mean: 6.20 ppm). However, the average content of Cd (4256.38 ppm) in Sp2 is almost twice that in Sp1 (2635.74 ppm). The higher Se contents in Py2 coupled with higher Se and Fe contents in Sp1 indicate that Py2 and Sp1 had higher precipitation temperatures than Py3 and Sp2. Sulfur isotope compositions of pyrites and sphalerites acquired by NANO-SIMS and LA-MC-ICP-MS suggest two origins: the sulfur of Py2 (δ34S: −3.0 to 2.3‰), Sp1 (δ34S: 0.2–0.9‰) and Sp2 (δ34S: 0.7–2.5‰) was mainly of magmatic source, and sulfur reduced via thermochemical sulfate reduction participated in the formation of Py3 (δ34S: 0.2–6.0‰) and S-Sb-Pb minerals intergrown with Py3. The sulfur isotope compositions of Py1 (δ34S: −31.8 to −19.4‰) indicate a biogenic origin associated with bacterial sulfate reduction. A fluid mixing model is suggested to interpret the genesis of the Lanuoma Pb-Zn-Sb deposit: mixing of a reduced S-bearing fluid with a metalliferous basinal brine resulted in the precipitation of sulfides. Temperature was the most important factor controlling metal precipitation in the Lanuoma deposit.

Mineral footprints of the Paleoproterozoic sediment-hosted Abra Pb-Zn-Cu-Au deposit Capricorn Orogen, Western Australia

The minerals exploration undercover relies on the understanding of the ore-forming processes and the associated alteration characteristics that are used in vectoring towards a deposit. Previously, the proposed process for forming the 1590 Ma Abra base-metal deposit, hosted at 250 m depth in the Paleoproterozoic Edmund Basin of the Western Australian Capricorn Orogen, was through exhalation to the seafloor (SEDEX-style). Lacking any visible signs of alteration, sediments overlying the Abra deposit were perceived as younger, unaltered, and cutting the mineralisation and associated alteration comprised of siderite, chlorite and minor sericite. However, a recent study delineated a phengitic (i.e. Al-poor, Mg-, Fe-, Si-rich) white mica footprint in regolith above the deposit from hyperspectral field and remote sensing data. This study links the surficial mineral footprint to the deposit below through detailed analysis of hyperspectral data from drill core. The hyperspectral data processing results were validated using optical microscopy, micro-XRF and SEM-EDS analysis, and compared with geochemical, lithological and structural drill core data. Hyperspectral short-wave infrared (SWIR) data revealed a vertically zoned alteration footprint from the deposit to the surface, where very fine-crystalline phengitic white mica (i.e. sericite) and chlorite are most abundant a few tens of metres above the deposit. The abundance of chlorite gradually decreases, and the distal phengitic alteration is cut by the present erosional surface, but only moderately affected by weathering, confirming the surficial footprint detected in the previous study. Similarly, a vertically zoned carbonate alteration was delineated from thermal infrared (TIR) reflectance spectra from drill cores up to 2 km away from the deposit. Carbonate alteration from proximal to distal comprised of Mn-rich siderite, ankerite and ferroan dolomite, which replace the dolomite matrix in coarse-grained stratigraphic sequences. The replacement of dolomite matrix suggests that these permeable horizons acted as fluid conduits, which channelled hydrothermal fluids into sedimentary strata from a feeder structure. The presence of a vertically zoned carbonate and chlorite-phengite footprint in the overlying sediments advocates for their deposition before the ore formation at Abra, and a sub-seafloor replacement within a permeable sedimentary sequence as the ore-forming process. Comparable alteration footprints have been described around some world-class sediment-hosted base-metal deposits, including the Sullivan in the British Columbia, which is considered as a classic SEDEX-style by some and an exhalative-replacement hybrid by others. Also, identical footprints are common around the volcanic-hosted massive sulphide and are associated with the carbonate replacement-style deposits – an overlap which is an interesting future research space where hyperspectral technologies could provide answers. Validated hyperspectral data combined with geochemical, lithological and structural information enabled the identification of a subtle mineral footprint above the Abra deposit, and the established method can be used for vectoring towards other buried mineralisations – even through weathering.

Isotopic constraints on fluid evolution and ore precipitation in a sediment-hosted Pb-Ag-Ba-Zn-Cu-Au deposit in the Capricorn Orogen, Western Australia

The Abra Pb-Ag-Ba-Zn-Cu-Au deposit in the Capricorn Orogen, Western Australia is primarily a lead and silver resource currently estimated at 47.8 Mt (indicated and inferred) of 7.3–10.1% Pb and 18–28 gt-1 Ag, although significant Cu-Au zones are also identified. The deposit is unique within sediment-hosted Pb-Zn deposits for its low Zn content, significant Cu-Au zone and high Fe content, providing a case study where the source of fluid and ore-forming processes are contentious. The combination of whole-rock hydrogen and oxygen isotope data, in situ oxygen isotope data in quartz, and in situ sulphur isotope data of pyrite and chalcopyrite, has been used to reconstruct a complex history of overprinting, involving stages of sedimentation, diagenesis and hydrothermal activity. The host sedimentary rocks consist of detrital quartz (δ18O ∼11–18‰) and whole rock δ18O values (∼9–16‰) reflecting the combined composition of detrital and authigenic minerals, diagenetic-metamorphic exchange, chlorite and iron content. Quartz in recrystallised chemical sedimentation, quartz cementation, and quartz-barite veins at low temperatures (∼100–250 °C) involved predominantly surface and formation fluids with a wide range of fluid δ18O values between ∼ -5‰ and 2.6‰. Quartz in chloritized host rock with disseminated pyrite and chalcopyrite-galena veins at 250–320 °C reflect exchange with fluids(s) having a narrow range of δ18O values (∼5–9‰), most likely formation fluids. The fluid responsible for iron oxide, pyrite and polymetallic carbonate veins appears to be a mixture of formation and lighter surface fluids, with a range of fluid δ18O values (∼0.8–5.5‰). In situ sulphur isotopes are consistent with reduced seawater sulphate source in all samples, therefore it is likely that metal-rich formation fluids have interacted with reduced sulphate in the host sediments to precipitate as sulphide. Mineralisation and associated alteration at Abra has caused whole rock δ18O values to decrease in the deposit which may be useful as a tool for exploration in similar sediment-hosted base-metal deposits. We have shown the combination of different isotopic systems, and utilisation of in situ techniques, can constrain the sources and evolution of fluid and sulphur involved in basin formation, hydrothermal alteration and base metal mineralisation. Isotopic values can be directly related to different mineral populations within a relative temporal framework and can be used to distinguish fluids between multiple events.

Plate tectonic aspects of the Triassic carbonate-hosted stratiform-stratabound base-metal deposits in the Western Balkan, NW Bulgaria

The Triassic carbonate-hosted stratiform-stratabound base-metal deposits in the Western Balkan, NW Bulgaria, have well defined regional geological and tectonic settings, style ofmineralisation, mineralogical, geochemical and isotopic data. Their genesis, however, remains controversial in the framework of plate tectonic models, and is not supported by comparative analysis of possible counterpart candidates within the NW Tethyan domain. Besides, the crucial missing information is the geochronology of ore mineralisation relative to the Triassic carbonate host rocks. The article stresses the ambiguities due to the lack ofmineralisation age dating, and emphasises the necessity for building a convincing plate tectonic model based on the relevant geodynamic setting. A prospective direction of researchis the critical comparison with equivalent sediment-hosted base-metal deposits within the continental margin environments such as those in the Alps and Dinarides as well as others in the Alpine-Himalayan orogen and worldwide.

Thrust-controlled, sediment-hosted, Himalayan Zn–Pb–Cu–Ag deposits in the Lanping foreland fold belt, eastern margin of Tibetan Plateau

Cenozoic economic Zn–Pb–Cu–Ag ore deposits occur in the Lanping foreland fold belt (Lanping basin) of the Eastern Indo-Asian collision zone (EIACZ), eastern margin of the Tibetan plateau. They are sediment-hosted and controlled by two Cenozoic thrust–nappe systems developed in the western and eastern segments of the Tertiary basin, respectively, where thrusting was towards the centre of the basin in an approximately E–W direction and juxtaposes Triassic–Cretaceous over Tertiary strata. In the western system, Cu deposits occur as veins in the root zone, hosted by ~ NS-striking faults in Jurassic intense folds. Zn deposits occur in its front zone, controlled by second-order faults of major thrust faults and thrust-related strike-slip faults. In the eastern system, Zn–Pb–Cu–Ag deposits are located in the front zone, controlled by an E-dipping thrust fault and its second-order faults. The giant Jinding Zn–Pb deposit is controlled by a trap structure related to regional thrusting. Homogenization temperatures of fluid inclusions from these deposits are predominately < 200 °C, with salinities of 1.6 to 27.7 wt.% NaCl equiv. Ore sulfides have Pb isotopes similar to the rocks in the basin and δ 34S CDT values ranging from − 30.4‰ to 11.2‰. H–O isotopes of fluid inclusions are similar to those of basinal brines, and C–O isotopes of hydrothermal carbonate minerals are partly close to those of carbonates and sedimentary organisms. These data suggest that the ore-forming fluids and materials mainly derive from the fluids and various rocks of the basin, respectively. The sediment-hosted Zn–Pb–Cu–Ag deposits in the Lanping foreland fold belt share some similarities with MVT-, SST-, SSC-, and SEDEX-type deposits in that there is a lack of any igneous affinity and instead must relate to activities of basinal fluids. The deposits show, however, a set of unique features: (1) they were formed in a strongly deformed foreland basin within a continent collision orogen and (2) closely related to regional thrust–nappe structure; (3) they are fault-controlled, commonly without strong selection for type of host rocks; and (4) contain a variety of metals including Zn, Pb, Cu, Ag, and minor Sr, Co, etc. It is suggested that the Zn–Pb–Cu–Ag deposits controlled by the thrust systems in the Lanping foreland belt are a new sub-type within the family of sediment-hosted base metal deposits.

9: Processes of tectonism, magmatism and mineralization: Lessons from Europe

Metallogenic provinces in Europe range in age from the Archaean to the Neogene. Deposit types include porphyry copper and epithermal Cu–Au, volcanic-hosted massive sulphide (VMS), orogenic gold, Fe-oxide–Cu–Au, anorthosite Fe–Ti-oxide and sediment-hosted base-metal deposits. Most of them formed during short-lived magmatic events in a wide range of tectonic settings; many can be related to specific tectonic processes such as subduction, hinge retreat, accretion of island arcs, continental collision, lithosphere delamination or slab tear. In contrast, most sediment-hosted deposits in Europe evolved in extensional, continental settings over significant periods of time. In Europe, as elsewhere, ore formation is an integral part of the geodynamic evolution of the Earth's crust and mantle. Many tectonic settings create conditions conducive to the generation of water-rich magma, but the generation of ore deposits appears to be restricted to locations and short periods of change in temperature and stress, imposed by transitory plate motions. Crustal influence is evident in the strong structural controls on the location and morphology of many ore deposits in Europe. Crustal-scale fault–fracture systems, many involving strike-slip elements, have provided the fabric for major plumbing systems. Rapid uplift, as in metamorphic core complexes, and hydraulic fracturing can generate or focus magmatic–hydrothermal fluid flow that may be active for time spans significantly less than a million years. Once a hydrologically stable flow is established, ore formation is strongly dependent on the steep temperature and pressure gradients experienced by the fluid, particularly within the upper crust. In Europe, significant fracture porosity deep in the crystalline basement (∼1%) is not only important for magmatic–hydrothermal systems, but allows brines to circulate down through sedimentary basins and then episodically upward, expelled seismically to produce sediment-hosted base-metal deposits and Kupferschiefer copper deposits. Emerging research, stimulated by GEODE, can improve the predicting power of numerical simulations of ore-forming processes and help discover the presence of orebodies beneath barren overburden.

Apparent stable isotope heterogeneities in gangue carbonates of the Mississippi Valley-type Zn-Pb deposit of San Vicente, central Peru

The aim of the present communication is to emphasize that some variations of the measured ∂13C and ∂180 values are apparent, and due to analytical interferences caused by the presence of sulfur and organosulfur compounds in the analyzed carbonates. This is particularly relevant for isotopic studies on carbonate-hosted mineral deposits, where the nearly ubiquitous association of the host carbonates with organic matter and sulfides can certainly affect the metallogenetic interpretations. In this work two methods were used to overcome the disturbing effects of sulfides and organic matter: (1) sample pretreatment following the method proposed by Charef and Sheppard (1984), combining the oxidation of organic matter with sodium hypochlorite and trapping of the sulfur species with silver phosphate; and (2) laser-based microprobe extraction. Apparent isotopic variations in sparry dolomite from a single hand sample of zebra ore from the MVT Zn-Pb deposit, San Vicente, central Peru, are as large as 6 per mil ∂13C and 4per mil ∂180. These variations are reduced to several tenths of a per rail when the samples are pretreated. A careful examination of the effects of treatment with NaOC1 and/or AgaPO4 in relation to the concentration of sulfide inclusions indicates that the main disturbing effects for ∂13 C values are the presence of sulfur species and organic matter, whereas the 6180 values are mainly affected by the presence of sulfides. Fine- and medium-grained replacement carbonates from MVT and other sediment-hosted base metal deposits are potentially the most affected during isotope analysis, due to the common presence of organic matter and sulfides. Using in situ laser microprobe techniques, it is possible to determine isotopic variations at a sub-millimeter scale. Our results show that laser extraction analysis allows a more precise sampling of the carbonate minerals, and minimizes contamination of the sample with sulfides and to some extent with intergrown organic matter. However, there is an isotopic shift associated with the laser extraction technique, of the order of 0.5-1%o for ∂13C and ∂180 values.