Sediment-hosted Stratiform Copper Deposits Research Articles

Fluid Flow, Alteration, and Timing of Cu-Ag Mineralization at the White Pine Sediment-Hosted Copper Deposit, Michigan, USA

Abstract White Pine, located in Michigan’s Upper Peninsula, is an archetypal sediment-hosted stratiform copper deposit. The Midcontinent rift system is one of only seven basins globally that host a giant sediment-hosted stratiform copper deposit. Despite many similarities with other deposits of this type, White Pine displays some important differences, including the late Mesoproterozoic age, a thick basalt sequence, an apparent lack of evaporites, and a lacustrine depositional setting. This study analyzes paleofluid flow related to the formation of White Pine and places a particular emphasis on structural and diagenetic fluid pathways. Most of the ore is located in a 30-m-wide zone spanning the Copper Harbor Formation red beds and the overlying Nonesuch Formation shales. Sedimentation of these units was accompanied by subtle synsedimentary faulting. Premineralization phases include calcite concretions and nodules, illite and hematite grain coatings, isopachous chlorite rims, emplacement of liquid petroleum (now pyrobitumen), and bleaching. Mineralization introduced native copper into the footwall sandstones and a zoned suite of native copper and sulfur-poor copper sulfide minerals across a migrating redox front in the overlying shales where copper minerals nucleated on authigenic and detrital chlorite grains. Postmineralization phases include quartz cement, calcite cement, and calcite veins that partially overlapped inversion of synsedimentary faults. Contrary to previous studies, we identified evidence for only one phase of mineralization. An Re-Os chalcocite age of 1067 ± 11 Ma places mineralization 11 to 17 m.y. after host-rock deposition. Sulfide δ34S values of –14.0 to 29.9‰ suggest an important contribution from sour gas and thermochemical sulfate reduction of seawater. Carbon (δ13C) and oxygen (δ18O) isotope compositions of five calcite generations range from –15.1 to –1.3‰ and 10.4 to 41.3‰, respectively, and record early meteoric pore water displaced by later seawater. White Pine is both a sediment-hosted stratiform copper deposit and a paleo-oil field. Synsedimentary faults controlled the sedimentological character of the upper Copper Harbor Formation, and together these imparted a strong control on fluid flow and later diagenetic processes. Early oxidized meteoric fluids were displaced by liquid petroleum and sour gas, which were in turn succeeded by metal-rich but sulfate-poor oxidized seawater. Burial compaction during deposition of the overlying Freda Formation drove fluids through White Pine due to its situation on a paleotopographic high near the basin margin. Mineralization occurred at ~125°C at depths of ~2.0 km and spanned incipient basin inversion related to the distal effects of Grenvillian orogenesis. The hightenor copper mineral assemblage is the product of an abundant supply of metal from basaltic volcanic detritus in the Copper Harbor Formation and low seawater sulfate concentrations in late Mesoproterozoic oceans. This demonstrates that viable sediment-hosted stratiform copper systems can form when a readily leachable metal source rock is present, even if hypersaline and sulfate-rich brines are not.

Comparison of main and accompanying metals distribution patterns in newly documented deposits of the Northern Copper Belt in Poland

Three new world-class sediment-hosted stratiform copper deposits have recently been documented in SW Poland, in the deep parts of the Fore-Sudetic Monocline: Nowa Sól, Sulmierzyce North and Mozów. Along with the adjacent prognostic areas, these deposits form an extensive west–east-trending belt, distant from the well-known deposits of the Sieroszowice–Lubin area and referred to as the Northern Copper Belt. This major discovery was a result of an exploration program performed by the Canadian Miedzi Copper Corporation, which involved studies of archival data and the company's own drilling program. Investigation of the core material included microscopic observations of thin sections and chemical analyses using inductively coupled plasma (ICP) mass spectrometry and/or ICP optical emission spectrometry. The aim of this study was to compare available geochemical data from the documented deposits and determine the distribution patterns of the main and accompanying metals. Particular attention was paid to Cu, Ag, Pb, Zn, Co, Ni, Mo, V and Au. Each of the studied deposits is characterized by a different position of the ore mineralization in the lithological profile. Average concentrations of the analysed elements and their distribution in the ore series, both vertically and laterally, indicate that distinct mineralizing systems were responsible for the formation of each orebody. Among the main factors influencing metal distribution in the analysed deposits are: the spatial range of the epigenetic, oxidized Rote Fäule facies; composition and thickness of the source rocks for the mineralizing brines; and tectonics.

Co–Mn Mineralisations in the Ni Laterite Deposits of Loma Caribe (Dominican Republic) and Loma de Hierro (Venezuela)

Cobalt demand is increasing due to its key role in the transition to clean energies. Although the main Co ores are the sediment-hosted stratiform copper deposits of the Democratic Republic of the Congo, Co is also a by-product of Ni–Co laterite deposits, where Co extraction efficiency depends, among other factors, on the correct identification of Co-bearing minerals. In this paper, we reported a detailed study of the Co mineralisation in the Ni–Co laterite profiles of Loma Caribe (Dominican Republic) and Loma de Hierro (Venezuela). Cobalt is mainly associated with Mn-oxyhydroxide minerals, with a composition between Ni asbolane and lithiophorite, although a Co association with phyllosilicates has also been recorded in a Loma de Hierro deposit. In Loma Caribe, Co-bearing Mn-oxyhydroxide minerals mainly developed colloform aggregates, and globular to spherulitic grains, while in Loma de Hierro, they displayed banded colloform, fibrous or tabular textures. Most of the compositional analyses of Mn-oxyhydroxides yielded 20 and 40 wt.% Mn, with Ni and Co up to 16 and 10 wt.%, respectively. In both profiles, Mn-bearing minerals were mainly found in the transition from the oxide horizon to the saprolite, as observed in other laterite profiles in the world, where the precipitation of Mn-bearing minerals is enhanced because of the pore solution saturation and pH increase.

New perspective on trace element (Re, Ge, Ag) hosts in the Cu-Ag Kupferschiefer deposit, Poland: Insight from a LA-ICP-MS trace element study

The Kupferschiefer deposit in SW Poland provides an opportunity to study an ore system where besides factors such as temperature, salinity and pH, redox conditions play a major role in controlling trace element incorporation in sulphides. Chalcocite, djurleite, bornite, chalcopyrite, sphalerite, covellite and pyrite from the Cu-Ag deposit in the Lubin-Sieroszowice district were analysed with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to study the distribution of trace elements and to identify the main carriers of by-products such as silver and rhenium. Trace element anomalies are not only associated with certain minerals but also linked to specific mineralization styles. Chalcopyrite in epigenetic veins has significantly higher content of Ge, As and Sb while sulphides adjacent to hematitic “red spots” are enriched in redox sensitive elements: djurleite is enriched in V, Se, Re, Au, Hg and Tl, chalcopyrite in V, Se, Hg and Tl, and pyrite in Tl in comparison with bulk Cu-Ag mineralization. Elevated and substantially more variable data for V, Mn, Ni, Co, Ga, Mo, Sn in shale hosted sulphides can be attributed to submicron inclusions of Kupferschiefer silicate matrix which might skew low concentration values even if careful petrographic observations and precise spot selection are applied. Silver concentrations form a linear pattern from chalcocite/djurleite > bornite > chalcopyrite > sphalerite. Silver distribution in the deposit on a broad scale could be linked to the general Fe3+-Cu-Pb-Zn-Fe2+ zonation as well as mineral sequence (hematite-chalcocite-bornite-chalcopyrite-galena-sphalerite-pyrite) observed in the majority of sediment hosted stratiform copper deposits. Epigenetic sulphide-calcite veins associated with local faults and tectonic zones contain chalcopyrite exceptionally rich in germanium (up to 4806 μg/g of Ge) suggesting that the potential of Kupferschiefer Cu-Ag ores in terms of Ge grades and resources could be underestimated. The majority of analysed sulphides do not contain rhenium above detection limits (0.1 μg/g Re) with measurable quantities found almost exclusively in djurleite (up to 3.9 μg/g Re). This indicates that copper sulphides, in particular djurleite, could be a preferred host and an important carrier of rhenium in the deposit.

Ore Minerals and Metal Distribution in Tailings of Sediment-Hosted Stratiform Copper Deposits from Poland and Kazakhstan

This study, carried out in tailings from two sediment-hosted stratiform copper deposits in the Lublin-Głogów Copper District in Poland (Kupferschiefer-type deposit) and Zhezkazgan (cupriferous sandstone-type deposit) in Kazakhstan, analysed the mineralogy of copper, zinc, and lead minerals as related to metal accumulation in sediments. Microscopic study in reflected light and SEM–EDS (Scanning Electron Microscope—Energy Dispersive Spectrometer) analysis, as well as chemical diversity in the used INAA (Instrumental Neutron Activation Analysis), ICP (Inductively Coupled Plasma), and AAS (Atomic Absorption Spectroscopy) methods in 35 samples from Kazakhstan and 35 from Poland were examined due to their diversity. In both tailing deposits in Kazakhstan and Poland, heavy fractions were dominated by copper sulphides: chalcopyrite (CuFeS2), bornite (Cu5FeS4). and chalcocite (Cu2S). Moreover, sphalerite, galena, and cerussite have been recognized as a carriers of Zn and Pb. Their geochemistry was dominated by Cu, showing a mean content of 2500 ppm, in both Poland and Kazakhstan. Zinc and lead also occurred, showing a content of approximately 200 ppm and 500 ppm in Poland, and 1500 ppm Zn and 2500 ppm Pb in Kazakhstan, respectively. Grain size analysis indicated that the dominant grain size in both districts corresponded to the silt and fine sands fractions. Copper, zinc and lead sulphides accumulated in fine fractions in tailings from Kazakhstan (in sandstones and quartz grains), and mainly in coarse fractions in Poland (within carbonates, sandstones, and black shales). Mineralogical and geochemical features should be taken into consideration when assessing potential metal sources of technogenic materials.

The prospecting strategy for a deep Cu-Ag ore deposit in Poland – An anatomy of success

The article presents the prospecting strategy implemented by Miedzi Copper Corporation in search for new Cu-Ag ore deposits of the Kupferschiefer type. Sediment-hosted stratiform copper deposits in Poland occur mostly in the Fore-Sudetic Monocline, in the contact zone between the continental red beds (Rotliegend) and Zechstein marine sediments. The ore-bearing series consists of white sandstone (Weissliegend), copper-bearing shale (Kupferschiefer) and carbonates (Zechstein Limestone). The southern-central part of the Fore-Sudetic Monocline is a well-known Lubin-Sieroszowice mining area, where mining operations are conducted at the depths up to 1300 m below surface. However, due to significant depletion of shallower reserves, deep Cu-Ag mineralisation have recently become a subject of interest. Several basic problems had to be resolved in order to reach for such deposits: finding indicators suggesting the presence of rich mineralisation; more precise identification of the current geothermal field in the area of the Fore-Sudetic Monocline; and the availability of modern technologies which would allow safe and economic operation of a deep underground mine. In 2011, Miedzi Copper Corporation initiated its exploration programme focused on deep Cu-Ag ore deposits, based on known prognostic areas in the vicinity of palaeorises and contacts between oxidised and reduced facies, as copper mineralization occurs at the rims of oxidised areas, on the reduced side of the redox boundary. Exploration programme has led to the documentation of three new deep deposits: Nowa Sól, Sulmierzyce North and Mozów, two of which were identified in areas previously described as prospective by the Polish Geological Institute.

The Kupferschiefer Deposits and Prospects in SW Poland: Past, Present and Future

Polish sediment-hosted stratiform copper deposits associated with Zechstein sediments are one of the country’s most valuable natural resources and the basis for its copper industry. The paper presents the history of their research and current exploration. Although documented deposits and their identified resources are described and characterized, much attention is paid to areas located outside of them, including those where geological exploration is currently underway. The gradual depletion of shallow Cu–Ag reserves in the Lubin-Sieroszowice district and elsewhere in the world requires exploration for deeper-seated deposits. To expand resources, time span and scale of copper production in Poland, exploration and definition of new prospects is of great importance. Thirty-five prospective areas with hypothetical and speculative resources have been delineated in SW Poland, including the most prospective areas adjacent to the Cu–Ag Lubin-Sieroszowice deposit. The paper also focuses on those parts of the Fore-Sudetic Monocline where new copper deposits were recently identified. Their resources are described along with the methodology of establishing their boundaries, which differs from recommended Polish threshold parameters not taking into account the depths exceeding 1500 m. Intelligent modern mining and ore processing technologies are considered as a prerequisite for future profitable development of deposits at deeper levels.

Sediment Hosted Stratiform Copper (SSC) Mineralization in Bhudoli-Basari Area, North Delhi Fold Belt, Mesoproterozoic Delhi Supergroup, Rajasthan

The rocks of the Kushalgarh Formation of the Delhi Supergroup in northeast Rajasthan, host low-temperature prospect generally known as sediment hosted stratiform copper (SSC) mineralization in Bhudoli-Basari area, district Sikar, Rajasthan. Mineralization is about 3km in length with variable width of up to 600m and sulphides occur mainly as fine dissemination as well as veinlets within the steeply dipping carbonate rocks, which were remobilized along the quartz-calcite veins during compaction and subsequent deformation. The prospect is product of low-temperature aqueous brines. The brines leached, transported, and deposited copper (and associated metals) as per physical and chemical stabilities. The mineralization in the Bhudoli-Basari SSC consists mainly of bedding-parallel disseminations of fine-grained sulphides, typically zoned. Copper is leached from poorly sorted first-cycle clastic sediments of footwall redbeds, associated meta/volcanics of the Raialo Group and basement rocks during their progressive diagenetic alteration by moderate-temperature oxygen rich brines. Ore mineral zoning from chalcocite to pyrite in the Bhudoli-Basari area is archetypal to other SSC deposits viz. the Kupferschiefer of Poland-Germany and the Central African Copper belt of Zambia.

Stable isotope and fluid inclusion study of sediment-hosted stratiform copper deposits from the Neuquén Basin, Argentina

The Los Chihuidos and El Porvenir deposits of the Neuquen Basin are examples of sediment-hosted stratiform Cu deposits generated by interaction of hydrocarbons and formation water with host red beds and metal-charged basinal brines. During early diagenesis of red beds precipitated hematite, kaolinite at 60 °C from meteoric water followed by calcite 1 (δ13C − 8 to − 3.3‰) and barite (δ34S + 4.1‰) with increasing temperature (75–85 °C) from an evolved surface fluid (δ18Ofluids + 0.9 to + 2.7‰). During the Tertiary Andean orogeny, hydrocarbons and formation water migrated and reacted with the red beds resulting in bleaching of the sandstone. Smectite (fluid δ18O − 2.2‰ and δD − 73.7‰), chlorite-smectite mixed-layer minerals (fluid δ18O − 6.9‰ and δD − 84‰), pyrite (δ34S + 10.2‰), and calcite 2 (δ13C − 12.9 to − 6.8‰) formed as by-product of the redox reactions. Calcite 2 formed from low-salinity fluids (0.4 to 5.9 wt% NaCl equiv.) at slightly higher temperatures (125–145 °C) at El Porvenir and at Los Chihuidos deposit (80–105 °C). During subsequent uplift, metal-charged basinal brines flowed into the bleached sandstone and precipitated chalcopyrite-bornite (δ34S + 12.3‰) followed by chalcocite-spionkopite (δ34S − 64 to + 4.1‰). Calcites 3a (δ13C − 19 to − 10.1‰) and 3b (δ13C − 31.4 to − 9.5‰) that accompanied Fe-Cu and Cu sulfides, respectively, formed from saline fluids (up to 21.3 wt% NaCl equiv.) at temperatures of 159–70 °C. The δ18O values (+ 5.6 to + 11.4‰) of fluids in equilibrium with calcite 3a are similar to oilfield and basinal brines. Fluids in equilibrium with calcite 3 in Los Chihuidos and calcite 3b in El Porvenir show much lighter δ18O values (− 7.3 to − 4.6‰ and − 0.6 to + 2.3‰, respectively), which suggests the involvement of methane as the main reductant and possibly meteoric water interaction.

Physiochemical Restrictions of Mineral Zoning of Sediment-Hosted Stratiform Copper Deposit in SW China

The Chuxiong basin, located in southwest China, is well known as a mineralization area of red-bed type copper deposits in China. These deposits are characterized by mineral zoning, which is especially true for the Dayao deposits. The mineral zoning is consistent for both horizontal and vertical zoning; from the base (center) of the ore body to the top (outermost), the mineral zones are from hematite, chalcocite, chalcocite + bornite, and bornite + chalcopyrite to pyrite. We studied the mineral zoning in detail using a thermodynamic phase diagram method, such as log⁡fO2-log⁡fS2, pH-log⁡fO2, and pH-Eh, and discussed the constraints on the order of the minerals precipitation under different physiochemical conditions. It is indicated that changes in temperature have little effect on pH and Eh in the formation of minerals. S2− is stable only below 473 K, and the forming temperature of chalcocite must be below 473 K. In this paper, we also explain the mineral zoning formation mechanism and propose that the main controlling factor of mineral zoning is pH. Because this mineral zoning is widespread in sediment-hosted deposits, studies on this mechanism can considerably promote better understanding of the genesis of ore deposits in order to guide the exploration.

In Situ Monazite Dating of Sediment-Hosted Stratiform Copper Mineralization in the Redstone Copper Belt, Northwest Territories, Canada: Cupriferous Fluid Flow Late in the Evolution of a Neoproterozoic Sedimentary Basin

AbstractThe 300-km-long Redstone copper belt in the Mackenzie Mountains, Northwest Territories, Canada, is composed of a series of sediment-hosted stratiform copper (SSC) deposits hosted in Neoproterozoic fault-bounded intracontinental rift basins. Mineralization at Coates Lake, the largest of these deposits, is concentrated within microbial laminite layers in the transition zone between underlying continental red beds of the Redstone River Formation and overlying marine carbonates of the Coppercap Formation. Disseminated cupriferous sulfides (chalcopyrite, bornite, and chalcocite) form part of a late diagenetic mineral association with dolomite, K-feldspar, albite, quartz, monazite, apatite, and pyrite that partially replaced detrital and early diagenetic minerals, including calcite cements, sulfate, and earlier generations of pyrite. Bornite (± minor chalcopyrite), calcite, dolomite, quartz, K-feldspar, and albite were also deposited in rare bedding-parallel veins adjacent to the lowermost mineralized microbial laminite layer in the transition zone.The absolute timing of mineralization was constrained by in situ U-Th-Pb chemical dating of monazite from four samples hosting disseminated SSC-type mineralization. The monazite have rounded, Th-U-heavy rare earth element-rich, detrital cores surrounded by Th-U-poor, light rare earth element-S-Sr-rich rims. The rim stage of monazite growth is intergrown with and enveloped by cupriferous sulfide and is paragenetically constrained as being part of the disseminated SSC-type mineralizing event. Eleven detrital cores yielded dates between 1843 and 1025 Ma, older than the depositional age of transition zone strata previously constrained to be between 775 and 732 Ma. Ten monazite rims yielded dates between 661 and 607 Ma. A weighted average date of 635 ± 13 Ma provides a maximum estimate, and is our preferred interpretation, for the absolute age of all copper mineralization at the Coates Lake deposit. Mineralization formed approximately 100 m.y. after deposition of the host rocks, during the thermal sag phase of continental rifting.Stratigraphic reconstructions, coupled with estimates of sediment compaction, indicate that at 635 Ma the transition zone was buried by ~4 km of sediments and overlaid another ~1.7 km of sediments that formed the Redstone River and Thundercloud Formations. Mudstone and carbonate-rich units above the transition zone acted as low permeability caps that led to suprahydrostatic fluid pressures in the underlying sediments. The bedding-parallel veins indicate transient supralithostatic fluid pressures. Free convection of pore fluids began within the transition zone and underlying units once they became hydrologically isolated from overlying strata. Mineralization formed as oxidized saline pore fluids circulated through the red beds (± underlying basaltic flows and basal sedimentary detritus), stripping copper and carrying it up into the transition zone. The salinity of the pore fluids may have, at least in part, originated from cryogenic brines generated by the Sturtian (717–662 Ma) global glaciation event.

Geology and geochemistry of the sediment-hosted Cheshmeh-Konan redbed-type copper deposit, NW Iran

The several-hundred-m-thick Miocene Upper Red Formation in northwestern Iran hosts stratiform and fault-controlled copper mineralization. Copper enrichment in the percent range occurs in dm-thick carbonaceous sandstone and shale units within the clastic redbed sequence and consists of fine-grained disseminated copper sulfides (chalcopyrite, bornite, chalcocite) and supergene alteration minerals (covellite, malachite and azurite). The copper mineralization formed after calcite cementation of the primary rock permeability. Copper sulfides occur mainly as replacement of diagenetic pyrite, which, in turn, replaced organic matter. Electron microprobe analysis on bornite, chalcocite and covellite identifies elevated silver contents in these minerals (up to 0.12, 0.72 and 1.21wt%, respectively), whereas chalcopyrite and pyrite have only trace amounts of silver (<0.26 and 0.06wt%, respectively). Microthermometric data on fluid inclusions in authigenic quartz and calcite indicate that the Cu mineralization is related to a diagenetic fluid of moderate-to low temperature (Th=96–160°C) but high salinity (25–38wt% CaCl2 equiv.). The range of δ34S in pyrite is −41.9 to −16.4‰ (average −31.4‰), where framboidal pyrite shows the most negative values between −41.9 and −31.8‰, and fine-grained pyrite has relatively heavier δ34S values (−29.2 to −16.4‰), consistent with a bacteriogenic derivation of the sulfur. The Cu-sulfides (chalcopyrite, bornite and chalcocite) show slightly heavier values from −14.6 to −9.0‰, and their sulfur sources may be both the precursor pyrite-S and the bacterial reduction of sulfate-bearing basinal brines. Carbonates related to the ore stage show isotopically light values of δ13CV-PDB from −8.2 to −5.1‰ and δ18OV-PDB from −10.3 to −7.2‰, indicating a mixed source of oxidation of organic carbon (ca. −20‰) and HCO3− from seawater/porewater (ca. 0‰). The copper mineralization is mainly controlled by organic matter content and paleopermeability (intragranular space to large fracture patterns), enhanced by feldspar and calcite dissolution. The Cheshmeh-Konan deposit can be classified as a redbed-type sediment-hosted stratiform copper (SSC) deposit.

Sulfide mineral chemistry investigation of sediment-hosted stratiform copper deposits, Nahand-Ivand area, NW Iran

Copper mineralization in Nahand-Ivand area, NW Iran, appears as disseminated copper sulfides along a redox boundary between gray sandstone and microconglomerate and hematitic sandstones, siltstones and shaly marl. Geochemical analyses of the Nahand-Ivand deposits show as much 35wt.% Cu and 730ppm Ag in the gray sandstone. Electron probe micro-analysis (EPMA) was used to determine the mineralogical composition and distribution patterns of copper and silver-bearing phases mainly in pyrite and Cu-bearing sulfides. The EPMA data were essential for evaluating the distribution and partitioning of potential economically-valuable components between co-existing minerals. Furthermore, they contribute to a better understanding of the genesis of the Nahand­Ivand copper deposits and will guide further exploration in the region.The EPMA results from different types of pyrite reveal Cu contents as high as 0.32wt.%, 1.10wt.%, and 2.88wt.% for framboidal pyrite (PyI), overgrowths on framboidal pyrite (PyII), and diagenetic pyrite (PyIII), respectively. This successive increase of copper from PyI to PyIII is attributed to a continuous supply of copper that replaces framboidal pyrite in turn by the more copper-rich digenetic pyrite. Because of hydrothermal overprinting, pyrite has been replaced by djurleite, roxbyite, and other nonstoichiometric Cu–S minerals that include covellite. The EPMA study indicates that covellite contains significant concentrations of Ag (locally >1wt.%). In contrast. only trace amounts of silver have been detected in pyrite and other copper sulfides, indicating that covellite is the major Ag-carrier in the ore. According to textural relationships, such open space filling, impregnation, and replacement textures, and the EPMA results, later stage copper-bearing fluids were responsible for the silver enrichment in the Nahand-Ivand deposits.

Fluid evolution of the Paleoproterozoic Hujiayu copper deposit in the Zhongtiaoshan region: Evidence from fluid inclusions and carbon–oxygen isotopes

The Zhongtiaoshan region, located in the southern margin of the North China Craton, is the host to a number of metamorphosed sediment-hosted stratiform copper deposits. These deposits are hosted by dolomitic marble and silicic albitite of the mid-Paleoproterozoic Zhongtiao Group and contain economically significant copper and cobalt. The formation of these deposits is considered to be closely associated with the evolution of the Paleoproterozoic “Zhongtiao” rift. The Hujiayu deposit, the second largest Cu deposit of this type, is mainly hosted in silicic albitite and dolomitic marble in the upper part of the Bizigou Formation but locally extends into carbonaceous shales at the bottom of the Yujiashan Formation. Mineralization of the Hujiayu Cu deposit can be divided into an early stage (diagenetic stage) with disseminated to veinlet sulfides and a late stage (metamorphism stage) with coarse-vein sulfides. Mineral assemblages are similar for the two stages, with major minerals as chalcopyrite, pyrite, and pyrrhotite and main gangue minerals as dolomite and quartz. Sulfide veinlets formed in the early stage are thin and discrete, and have irregular boundaries with the host rocks; whereas the ore-bearing veins of the late stage are controlled by fractures.Five types of fluid inclusions are recognized in the Hujiayu Cu deposit and they are: (1) pure vapor and vapor-rich inclusion (V-type), (2) pure CO2 inclusion (PC-type), (3) CO2–H2O inclusion (C-type), (4) liquid-rich inclusion (L-type), and (5) daughter mineral-bearing inclusion (S-type). Microthermometric analysis shows that the ore-forming fluids of the early mineralization stage are characterized by high salinity (22–40wt.% NaCl equiv.) and moderate temperature (120–280°C). The ore-forming fluids of the late mineralization stage are characterized by CO2 enrichment, high salinity, high temperature and underwent significant unmixing at a temperature interval of 240–480°C. Compositions of the ore-forming fluids in the early and late stages are interpreted to be mainly basinal brine and metamorphic hydrothermal solution, respectively. Carbon and oxygen isotope compositions suggest possible carbon isotope exchange between the ore-forming fluids and organic-rich carbonaceous shales during the early stage. In the late mineralization stage, both degassing of CO2 and isotopic exchange with organic carbon may have contributed to the formation of the more negative δ13CV-PDB values of mineralized carbonates.The early stage mineralization of the Hujiayu Cu deposit may have occurred via interaction of oxidized Cu-bearing brines from the red-bed in the lower segment of the Bizigou Formation with the overlying reductive carbonaceous shales. Late stage mineralization at Hujiayu was mainly triggered by CO2 escaping from metamorphic hydrothermal solutions.

Paragenesis of cobalt and nickel in the Black Butte shale-hosted copper deposit, Belt Basin, Montana, USA

The Black Butte copper deposits (formerly known as Sheep Creek) are a group of sediment hosted, laterally extensive Cu–(Co–Ag) deposits hosted in dolomitic shale of the mid-Proterozoic Newland Formation. Copper–cobalt mineralization occurs in zones of massive, laminated pyrite that were locally reworked and infiltrated by Cu-rich fluids during early diagenesis. Cobalt, along with substantial nickel and arsenic, mainly occurs as impurities within early, porous pyrite, or as minute grains of sulpharsenides (i.e., cobaltite, glaucodot, and/or alloclasite). Later thermal events remobilized the Co, Ni, and As to form intergrowths of siegenite (Co,Ni)3S4 and tennantite. The temperature of this later event is constrained by the mineralogical assemblage to have been relatively low, between 125 and 225 °C. Although many of the characteristics of SEDEX-type deposits are present at Black Butte (e.g., laterally extensive massive pyrite horizons, interbedded black shales, abundant barite and local phosphate horizons, and rifted continental margin setting), the lack of economic Pb and Zn mineralization in the main deposits, and the abundance of Cu with high Co, is more typical of sediment-hosted stratiform copper deposits. The Neihart Formation, a hematitic quartz sandstone resting below the base of the Belt Supergroup, may have been an important source bed for Cu–Co–Ni–Ag fluids. It is speculated that these fluids, ideal for forming Cu deposits, were expelled along growth faults near the margin of the Belt Basin and deposited metals on or just below the sea floor in a setting that is typical of SEDEX deposits. This unique mineral deposit model may have applications to other districts where Cu–Co-rich sulfides are deposited in an exhalative setting.

Mount Isa copper orebodies: improving predictive discovery

The most likely source for oxidised brines that produced the Mount Isa copper–cobalt deposits is the hematitic meta-sediments of the <1500 Ma South Nicholson Group or its lateral equivalents. Downwards movement of the brines into relatively impermeable basement rocks at about 1370 Ma is suggested to have been a product of reactivation of NNW–SSE faults (e.g. Transmitter fault) during post-Isan inversion. Evidence for the involvement of such structures includes spatial association with major orebodies and mica–dickite–hematite alteration indicative of the passage of hot, acidic and oxidised fluid. Copper depletion and K enrichment in metabasalt of the ca 1830 Ma Eastern Creek Volcanics along such structures has previously been documented. Anhydrite-rich rocks preserved at the northern end of the copper orebodies represent a zone of high fluid input, where the characteristics of the original fluid are best preserved. The most plausible chemical process generating economic Cu is reduction of oxidised brine during reaction with carbonaceous meta-sediments of the Urquhart Shale. Many lithologies in the region would have had the chemical capacity to extract Cu from hydrothermal solutions. The availability of suitable reductant rocks was therefore not a limiting factor on Cu deposition. Rather, the distribution of porosity and permeability would have controlled deposit location. The Urquhart Shale is a particularly favourable host unit on account of pervasive mechanical anisotropy produced by regular interlayering of ductile metapelite and brittle metasiltstone. Strain partitioning during post-Isan inversion resulted in pervasive permeability development in metasiltstone, particularly when steeply dipping. An implication of this research is that rocks of the South Nicholson Group and equivalents could be targetted for sediment-hosted stratiform copper (SSC) deposits.

Geodynamic and climate controls in the formation of Mio–Pliocene world-class oxidized cobalt and manganese ores in the Katanga province, DR Congo

The Katanga province, Democratic Republic of Congo, hosts world-class cobalt deposits accounting for ~50% of the world reserves. They originated from sediment-hosted stratiform copper and cobalt sulfide deposits within Neoproterozoic metasedimentary rocks. Heterogenite, the main oxidized cobalt mineral, is concentrated as "cobalt caps" along the top of silicified dolomite inselbergs. The supergene cobalt enrichment process is part of a regional process of residual ore formation that also forms world-class "manganese cap" deposits in western Katanga, i.e., the "black earths" that are exploited by both industrial and artisanal mining. Here, we provide constraints on the genesis and the timing of these deposits. Ar­Ar analyses of oxidized Mn ore and in situ U­Pb SIMS measurements of heterogenite yield Mio­Pliocene ages. The Ar­Ar ages suggest a multi-phase process, starting in the Late Miocene (10­5 Ma), when the metal-rich substratum was exposed to the action of meteoric fluids, due to major regional uplift. Further oxidation took place in the Pliocene (3.7­2.3 Ma) and formed most of the observed deposits under humid conditions: Co- and Mn-caps on metal-rich substrata, and coeval Fe laterites on barren areas. These deposits formed prior to the regional shift toward more arid conditions in Central Africa. Arid conditions still prevailed during the Quaternary and resulted in erosion and valley incision, which dismantled the metal-bearing caps and led to ore accumulation in valleys and along foot slopes.

Formation of Sedimentary Rock-Hosted Stratiform Copper Deposits through Earth History

Sedimentary rock-hosted stratiform copper deposits form by movement of oxidized, copper-bearing fluids across a reduction front that results in the precipitation of copper sulfides. Large-scale production of such oxidized fluids, as well as the formation of mobile hydrocarbons (oil) has probably been common since the formation of the first red beds in the Paleoproterozoic, and deposits of this type occur in rocks from the Paleoproterozoic to the Tertiary. However, supergiant deposits are currently recognized in only three basins: the Paleoproterozoic Kodaro-Udokan basin of Siberia, the Neoproterozoic Katangan basin of south-central Africa, and the Permian Zechstein basin of northern Europe. The paucity of data regarding the Udokan deposit makes understanding this system difficult in terms of Earth history events. Both the Neoproterozoic and the Permian were times of supercontinent breakup with major landmasses at low latitudes. This global tectonic framework favored the formation of failed rifts that subsequently became significant intracratonic basins with basal, synrift red-bed sequences overlain by marine and/or lacustrine sediments and, in some basins located at low latitudes, by thick evaporitic strata. The intracratonic setting of these basins allowed the development of a hydrologically closed basinal architecture in which highly oxidized and saline, moderate-temperature basinal brines were produced that were capable of supplying reduction-controlled sulfide precipitation over very long time periods (tens to hundreds of millions of years). The length of time available for the mineralizing process may be the key factor necessary to form supergiant deposits. However, examination of the absolute ages for the Kupferschiefer (Zechstein basin) and Katangan deposits allows speculation that other factors may also have been important. Both the Neoproterozoic and Permian were times of major glacial events. Glaciation may also be conducive for the formation of supergiant sediment-hosted stratiform copper deposits. Glacial periods correspond to magnesium- and sulfate-rich oceans that could have been responsible for additional sulfur in basinal brines developed during evaporite formation and would then be available during the long mineralization process.

A PROCESS-BASED APPROACH TO ESTIMATING THE COPPER DERIVED FROM RED BEDS IN THE SEDIMENT-HOSTED STRATIFORM COPPER DEPOSIT MODEL

Most estimates of the total tonnage of copper metal leached from footwall red beds to form sediment-hosted stratiform copper deposits are based on the amounts of copper at ppm levels that might have been derived from the volume of red beds underlying given sediment-hosted stratiform copper deposits. That static calculation is replaced here with a process-based approach in which the release of copper results from the progressive diagenetic alteration of the footwall aquifers by oxygen-rich meteoric water driven by topographic recharge in highlands adjacent to the intracontinental rift basins hosting these deposits. The total amount of liberated copper depends on the volume of meteoric water, its oxygen content, and the duration of the diagenetic leaching of copper. Estimates suggest that adequate amounts of copper may be derived from footwall red beds within reasonable periods of early, and possibly more advanced, diagenesis.

Fluid speciation controls of low temperature copper isotope fractionation applied to the Kupferschiefer and Timna ore deposits

Mass-balance among reduced and oxidized solution species exerts a strong control on the isotopic composition of sulphide minerals. We explore this issue for copper isotopic fractionation in sedimentary copper sulfide deposits through fluid speciation-isotopic Eh–pH calculations based on the thermodynamic stability of copper chloride aqueous solution complexes and experimentally measured copper isotope fractionation factors. Applying these speciation diagrams to MC-ICP-MS copper isotope data on the Kupferschiefer (SW part of the Lubin-Sieroszowice Copper District) and Timna (S. Israel) Sediment-hosted Stratiform Copper deposits (SSC), identifies differences in ore formation redox conditions. Timna Valley copper sulfides have light Cu-isotopic compositions of δ 65Cu = − 2.04 ± 0.44‰ 2σ (relatively to SRM 976 copper standard), which are shown to correspond to Eh values of 0.5 to 0.6 V at formation conditions (T = 40 °C; pH < 6). These Eh values indicate precipitation at relatively oxidized conditions where the Cu(I) solution complex (CuCl 3 2−) is < 10% of the total solution species. In contrast, the Kupferschiefer Cu-sulfides analyzed in this work dominantly show significantly higher δ 65Cu values = − 0.39 ± 0.36‰, corresponding to Eh values of 0.4 to 0.5 V at formation conditions (T = 100 °C; pH = 6.3). In these Eh conditions, most copper in solution occurs as CuCl 3 2− complexes (~ 80%). The above observations are in accord with field relations showing that Cu(II) minerals dominate the Timna system, but Cu-sulfides are the major minerals of the Kupferschiefer deposits. Fluid speciation–isotopic calculations show that copper isotopes can be used as effective tracers of redox conditions and this approach can be potentially applied to various hydrothermal ore deposits, such as black smokers and volcanic-hosted massive sulphide deposits, and to other metallic isotope systems. However, the isotopic fractionation may also be strongly influenced by the types of ligand bonding of copper ions and these effects need to be fully evaluated before the isotope geochemistry of copper ores can be fully understood.