New geologic, fluid inclusion and stable isotope studies on the controversial Igarapé Bahia Cu–Au deposit, Carajás Province, Brazil

The Igarapé Bahia Cu–Au deposit in the Carajás Province, Brazil, is hosted by steeply dipping metavolcano-sedimentary rocks of the Igarapé Bahia Group. This group consists of a low greenschist grade unit of the Archean (∼2,750 Ma) Itacaiúnas Supergroup, in which other important Cu–Au and iron ore deposits of the Carajás region are also hosted. The orebody at Igarapé Bahia is a fragmental rock unit situated between chloritized basalt, with associated hyaloclastite, banded iron formation (BIF), and chert in the footwall and mainly coarse- to fine-grained turbidites in the hanging wall. The fragmental rock unit is a nearly concordant, 2 km long and 30–250 m thick orebody made up of heterolithic, usually matrix-supported rocks composed mainly of coarse basalt, BIF, and chert clasts derived from the footwall unit. Mineralization is confined to the fine-grained matrix and comprises disseminated to massive chalcopyrite accompanied by magnetite, gold, U- and light rare earth element (LREE)-minerals, and minor other sulfides like bornite, molybdenite, cobaltite, digenite, and pyrite. Gangue minerals include siderite, chlorite, amphibole, tourmaline, quartz, stilpnomelane, epidote, and apatite. A less important mineralization style at Igarapé Bahia is represented by late quartz–chalcopyrite–calcite veins that crosscut all rocks in the deposit area. Fluid inclusions trapped in a quartz cavity in the ore unit indicate that saline aqueous fluids (5 to 45 wt% NaCl + CaCl₂ equiv), together with carbonic (CO₂ ± CH₄) and low-salinity aqueous carbonic (6 wt% NaCl equiv) fluids, were involved in the mineralization process. Carbonates from the fragmental layer have δ¹³C values from −6.7 to −13.4 per mil that indicate their origin from organic and possibly also from magmatic carbon. The δ³⁴S values for chalcopyrite range from −1.1 to 5.6 per mil with an outlier at −10.8 per mil, implying that most sulfur is magmatic or leached from magmatic rocks, whereas a limited contribution of reduced and oxydized sulfur is also evident. Oxygen isotopic ratios in magnetite, quartz, and siderite yield calculated temperatures of ∼400°C and δ¹⁸O-enriched compositions (5 to 16.5 per mil) for the ore-forming fluids that suggest a magmatic input and/or an interaction with ¹⁸O-rich, probably sedimentary rocks. The late veins of the Igarapé Bahia deposit area were formed from saline aqueous fluids (2 to 60 wt% NaCl + CaCl₂ equiv) with δ¹⁸O_fluid compositions around 0 per mil that indicate contribution from meteoric fluids. With respect to geological features, Igarapé Bahia bears similarity with syngenetic, volcanic-hosted massive sulfide (VHMS)-type deposits, as indicated by the volcano-sedimentary geological context, stratabound character, and association with submarine volcanic flows, hyaloclastite, and exhalative beds such as BIF and chert. On the other hand, the highly saline ore fluids and the mineral assemblage, dominated by magnetite and chalcopyrite, with associated gold, U- and LREE-minerals and scarce pyrite, indicate that Igarapé Bahia belongs to the Fe oxide Cu–Au (IOCG) group of deposits. The available geochronologic data used to attest syngenetic or epigenetic origins for the mineralization are either imprecise or may not represent the main mineralization episode but a later, superimposed event. The C, S, and O isotopic results obtained in this study do not clearly discriminate between fluid sources. However, recent B isotope data obtained on tourmaline from the matrix of the fragmental rock ore unit (Xavier, Wiedenbeck, Dreher, Rhede, Monteiro, Araújo, Chemical and boron isotopic composition of tourmaline from Archean and Paleoproterozoic Cu–Au deposits in the Carajás Mineral Province, 1° Simpósio Brasileiro de Metalogenia, Gramado, Brazil, extended abstracts, CD-ROM, 2005) provide strong evidence of the involvement of a marine evaporitic source in the hydrothermal system of Igarapé Bahia. Evaporite-derived fluids may explain the high salinities and the low reduced sulfur mineral paragenesis observed in the deposit. Evaporite-derived fluids also exclude a significant participation of magmatic or mantle-derived fluids, reinforcing the role of nonmagmatic brines in the genesis of Igarapé Bahia. Considering this aspect and the geological features, the possibility that the deposit was generated by a hydrothermal submarine system whose elevated salinity was acquired by leaching of ancient evaporite beds should be evaluated.     

Hydrothermal alteration,fluid inclusions and stable isotope systematics of the Alvo 118 iron oxide–copper–gold deposit,Carajás Mineral Province (Brazil): Implications for ore genesis

The Alvo 118 iron oxide–copper–gold (IOCG) deposit (170 Mt at 1.0 wt.% Cu, 0.3 g/t Au) lies in the southern sector of the Itacaúnas Shear Belt, Carajás Mineral Province, along a WNW–ESE-striking, 60-km-long shear zone, close to the contact of the ~2.76-Ga metavolcano-sedimentary Itacaiúnas Supergroup and the basement (~3.0 Ga Xingu Complex). The Alvo 118 deposit is hosted by mafic and felsic metavolcanic rocks and crosscutting granitoid and gabbro intrusions that have been subjected to the following hydrothermal alteration sequence towards the ore zones: (1) poorly developed sodic alteration (albite and scapolite); (2) potassic alteration (biotite or K-feldspar) accompanied by magnetite formation and silicification; (3) widespread, pervasive chlorite alteration spatially associated with quartz–carbonate–sulphide infill ore breccia and vein stockworks; and (4) local post-ore quartz–sericite alteration. The ore assemblage is dominated by chalcopyrite (~60%), bornite (~10%), hematite (~20%), magnetite (10%) and subordinate chalcocite, native gold, Au–Ag tellurides, galena, cassiterite, F-rich apatite, xenotime, monazite, britholite-(Y) and a gadolinite-group mineral. Fluid inclusion studies in quartz point to a fluid regime composed of two distinct fluid types that may have probably coexisted within the timeframe of the Cu–Au mineralizing episode: a hot (>200°C) saline (32.8‰ to 40.6 wt.% NaCl eq.) solution, represented by salt-bearing aqueous inclusions, and a lower temperature (<200°C), low to intermediate salinity (<15 wt.% NaCl eq.) aqueous fluid defined by two-phase (L_H2O + V_H2O) fluid inclusions. This trend is very similar to those defined for other IOCG systems of the Carajás Mineral Province. δ ¹⁸O_H2O values in equilibrium with calcite (−1.0‰ to 7.5‰ at 277°C to 344°C) overlap the lower range for primary magmatic waters, but the more ¹⁸O-depleted values also point to the involvement of externally derived fluids, possibly of meteoric origin. Furthermore, sulphide δ ³⁴S values (5.1‰ to 6.3‰), together with available boron isotope and Cl/Br–Na/Cl data provide evidence for a significant component of residual evaporative fluids (e.g., bittern fluids generated by seawater evaporation) in this scenario that, together with magma-derived brines, would be the main sources of the highly saline fluids involved in the formation Alvo 118 IOCG deposit. The restricted high temperature sodic alteration, the pervasive overprinting of the potassic alteration minerals by chlorite proximal to the ore zones, ore breccias with open-space filling textures in brittle structures, microthermometric and stable isotope data indicate, collectively, that the Alvo 118 IOCG system developed at structurally high levels and may be considered the shallower representative of the IOCG systems of the CMP.     

Mesoarchean (3.0 and 2.86 Ga) host rocks of the iron oxide–Cu–Au Bacaba deposit,Carajás Mineral Province: U–Pb geochronology and metallogenetic implications

The Bacaba iron oxide–copper–gold deposit, situated within a WNW–ESE-striking shear zone in the Carajás Domain, Carajás Mineral Province, is hosted by the Serra Dourada Granite, the Bacaba Tonalite, and crosscutting gabbro intrusions, which were intensely affected by sodic (albite–scapolite), potassic, chloritic, and hydrolytic hydrothermal alteration. This deposit is located 7 km northeast of the world-class Sossego iron oxide–copper–gold deposit and might represent a distal and deeper portion of the same or related hydrothermal system. The U–Pb laser ablation inductively coupled plasma–mass spectrometry data for zircon from a sodically altered sample of the Serra Dourada Granite yielded a 2,860±22 Ma (MSWD=11.5) age. Three samples from the Bacaba Tonalite, including one with potassic alteration and two with Cu–Au mineralization, rendered the 3,001.2±3.6 Ma (MSWD=1.8), 2,990.9±5.8 Ma (MSWD=1.9), and 3,004.6±9 Ma (MSWD=2.2) ages, respectively. The ca. 2.86 and ca. 3.0 Ga ages are interpreted as the timing of the igneous crystallization of the Serra Dourada Granite and the Bacaba Tonalite, respectively, and represent the oldest magmatic events recognized in the Carajás Domain. The Serra Dourada Granite and the Bacaba Tonalite are interpreted to greatly predate the genesis of the Bacaba deposit. A genetic link is improbable in the light of the similarities with the Sossego deposit, which is also hosted by younger ca. 2.76 Ga metavolcano-sedimentary units of the Itacaiúnas Supergroup. In this context, the iron oxide–copper–gold deposits in the southern sector of the Carajás Domain could be mainly controlled by important crustal discontinuities, such as a regional shear zone, rather than be associated with a particular rock type. These results expand the potential for occurrences of iron oxide–copper–gold deposits within the Mesoarchean basement rocks underlying the Carajás Basin, particularly those crosscut by Neoarchean shear zones.     

Spatial and temporal zoning of hydrothermal alteration and mineralization in the Sossego iron oxide–copper–gold deposit,Carajás Mineral Province,Brazil: paragenesis and stable isotope constraints

The Sossego iron oxide–copper–gold deposit (245 Mt @ 1.1% Cu, 0.28 g/t Au) in the Carajás Mineral Province of Brazil consists of two major groups of orebodies (Pista–Sequeirinho–Baiano and Sossego–Curral) with distinct alteration assemblages that are separated from each other by a major high angle fault. The deposit is located along a regional WNW–ESE-striking shear zone that defines the contact between metavolcano–sedimentary units of the ∼2.76 Ga Itacaiúnas Supergroup and tonalitic to trondhjemitic gneisses and migmatites of the ∼2.8 Ga Xingu Complex. The deposit is hosted by granite, granophyric granite, gabbro, and felsic metavolcanic rocks. The Pista–Sequeirinho–Baiano orebodies have undergone regional sodic (albite–hematite) alteration and later sodic–calcic (actinolite-rich) alteration associated with the formation of massive magnetite–(apatite) bodies. Both these alteration assemblages display ductile to ductile–brittle fabrics. They are cut by spatially restricted zones of potassic (biotite and potassium feldspar) alteration that grades outward to chlorite-rich assemblages. The Sossego–Curral orebodies contain weakly developed early albitic alteration and very poorly developed subsequent calcic–sodic alteration. These orebodies contain well-developed potassic alteration assemblages that were formed during brittle deformation that resulted in the formation of breccia bodies. Breccia matrix commonly displays coarse mineral infill suggestive of growth into open space. Sulfides in both groups of deposits were precipitated first with potassic alteration and more importantly with a later assemblage of calcite–quartz–epidote–chlorite. In the Sequeirinho orebodies, sulfides range from undeformed to deformed; sulfides in the Sossego–Curral orebodies are undeformed. Very late, weakly mineralized hydrolytic alteration is present in the Sossego/Currral orebodies. The sulfide assemblage is dominated by chalcopyrite with subsidiary siegenite, and millerite. Pyrrhotite and pyrite are minor constituents of ore in the Sequerinho orebodies while pyrite is relatively abundant in the Sossego–Curral bodies. Oxygen isotope partitioning between mineral pairs constrains temperatures in the deposit spatially and through time. In the Sequeirinho orebody, the early sodic–calcic alteration stage was characterized by temperatures exceeding 500°C and values for the alteration fluid of 6.9 ± 0.9‰. Temperature declines outward and upward from the zone of most intense alteration. Paragenetically later copper–gold mineralization displays markedly lower temperatures (<300°C) and was characterized by the introduction of ¹⁸O-depleted hydrothermal fluids −1.8 ± 3.4‰. The calculated δD_H2O and values suggest that the fluids that formed the early calcic–sodic alteration assemblage were of formational/metamorphic or magmatic origin. The decrease of values through time may reflect influx of surficially derived waters during later alteration and mineralization events. Influx of such fluids could be related to episodic fluid overpressure, resulting in dilution and cooling of the metalliferous fluid, causing deposition of metals transported as metal chloride complexes.     

Stable isotope and fluid inclusion studies on the Mount Black lead‐zinc deposit,southern New South Wales

The Mount Black Pb‐Zn deposit is a quartz‐galena‐sphalerite replacement body in the Silurian Cooleman Limestone. Fluid inclusion homogenisation temperatures range from 120° to 170°C for paragenetically early sphalerite, to 210° to 315°C for late quartz, and 245° to 320°C for calcite from contiguous recrystallised limestone. Fluid salinities increased with rising temperature, during deposition of the minerals, and the fluid composition changed from NaCl‐rich to possibly CaCl₂‐NaCl (‐?MgCl₂)‐rich brines.

δ³⁴S values of sphalerite and galena range from —8.1 to —2.7 per mil, and —13,5 to —4.4 per mil respectively. Although a magmatic source for sulphur is not excluded, it is suggested that most probably the sulphur was derived by biogenic reduction of sea‐water sulphate during diagenesis. Carbon and oxygen isotope data for the Cooleman Limestone range from compositions typical of Silurian marine carbonate in samples distant from the deposit, to fluctuating, but ¹²C‐ and ¹⁶O‐enriched in recrystallised material adjacent to the quartz‐sulphide rocks. ¹²C‐enrichment probably reflects organic carbon oxidation during karst formation, continuing later during limestone recrystallisation and accompanied by ¹⁶O‐enrichment during the action of saline formation waters.

The process of formation of the Mount Black deposit may have been analogous to that of Mississippi Valley‐type deposits, but modified by and/or resulting from, an increasing geothermal gradient caused by nearby synchronous intrusions.     

Geological,fluid inclusion and isotopic studies of the Yinshan Cu–Au–Pb–Zn–Ag deposit,South China: Implications for ore genesis and exploration

The Yinshan Cu–Au–Pb–Zn–Ag deposit is located in Dexing, South China. Ore bodies are primarily hosted in low-grade phyllite of the Neoproterozoic Shuangqiaoshan Group along EW- and NNW-striking fault zones. Pb–Zn–Ag mineralization is dictated by Jurassic rhyolitic quartz porphyries (ca. 172 Ma), whereas Cu–Au mineralization is associated with Jurassic dacite porphyries (ca. 170 Ma). The main ore minerals are pyrite, chalcopyrite, galena, sphalerite, tetrahedrite–tennatite, gold, silver, and silver sulphosalt, and the principal gangue minerals are quartz, sericite, calcite, and chlorite. Two-phase liquid-rich (type I), two-phase vapor-rich (type II), and halite-bearing (type III) fluid inclusions can be observed in the hydrothermal quartz-sulfides veins. Type I inclusions are widespread and have homogenization temperatures of 187–303 °C and salinities of 4.2–9.5 wt.% NaCl equivalent in the Pb–Zn–Ag mineralization, and homogenization temperatures of 196–362 °C and salinities of 3.5–9.9 wt.% NaCl equivalent in the Cu–Au mineralization. The pervasive occurrence of type I fluid inclusions with low-moderate temperatures and salinities implies that the mineralizing fluids formed in epithermal environments. The type II and coexisting type III inclusions, from deeper levels below the Cu–Au ore bodies, share similar homogenization temperatures of 317–448 °C and contrasting salinities of 0.2–4.2 and 30.9–36.8 wt.% NaCl equivalent, respectively, which indicates that boiling processes occurred. The sulfur isotopic compositions of sulfides (δ³⁴S = −1.7‰ to +3.2‰) suggest a homogeneous magmatic sulfur source. The lead isotopes of sulfides (²⁰⁶Pb/²⁰⁴Pb = 18.01–18.07; ²⁰⁷Pb/²⁰⁴Pb = 15.55–15.57; and ²⁰⁸Pb/²⁰⁴Pb = 38.03–38.12) are consistent with those of volcanic–subvolcanic rocks (²⁰⁶Pb/²⁰⁴Pb = 18.03–18.10; ²⁰⁷Pb/²⁰⁴Pb = 15.56–15.57; and ²⁰⁸Pb/²⁰⁴Pb = 38.02–38.21), indicating a magmatic origin for lead in the ore. The oxygen and hydrogen isotope compositions (δ¹⁸O = +7.8‰ to +10.5‰, δD = −66‰ to −42‰) of inclusion water in quartz imply that ore-forming fluids were mainly derived from magmatic sources. The local boiling process beneath the epithermal Cu–Au ore-forming system indicates the possibility that porphyry-style ore bodies may exist at even deeper zones.     

Reflectance spectroradiometry applied to a semi-quantitative analysis of the mineralogy of the N4ws deposit,Carajás Mineral Province,Pará, Brazil

Quantifying the abundance and physicochemical properties of minerals using reflectance spectroradiometry in the visible, near infrared and shortwave infrared (400–2500 nm) regions is an important tool in mineral exploration. In this study, the reflectance spectra of drill cores from the world-class N4WS iron deposit located in the Carajás Mineral Province, Brazil, were obtained. These spectra were validated using X-ray fluorescence (XRF) geochemical analyses and thin sections. The reflectance spectra were collected using a FieldSpec 3 spectroradiometer (ASD, Boulder, Colorado, USA) in 10 drill cores. The mineralogy of the deposit is mainly hematite, with lesser amounts of magnetite, goethite, quartz, kaolinite, gibbsite, smectite, talc, carbonate and chlorite. The mineralogy of the iron deposit was extracted from the spectral data using the geometry (depth and wavelength) of absorption features across the reflectance spectrum removed from the continuum. The depth of the absorption features is proportional to the mineral abundance, and the wavelength is proportional to the mineral chemical composition. The diagnostic absorption features of each mineral were used to determine the mineral abundance and composition. The final products include the abundance of iron (hydro) oxide (11.6% root-mean-square error [RMSE] Fe₂O₃); abundance of aluminous clays (RMSE 6% Al₂O₃); abundance of talc (8% RMSE MgO); identification of clay type (kaolinite, montmorillonite or gibbsite); composition of carbonate (dolomite vs. calcite); and composition of chlorite (Mg vs. Fe). The mineral abundance and composition results provided an effective characterisation of the ore, protore and host rocks and showed variations within the ore body.     

Geology,geochronology, fluid inclusion and H–O isotope geochemistry of the Luoboling Porphyry Cu–Mo deposit,Zijinshan Orefield,Fujian Province,China

The Luoboling Cu–Mo deposit in the Zijinshan Orefield, Fujian province, southeastern China, is a large porphyry deposit hosted by the Sifang granodiorite and the Luoboling granodiorite porphyry. The largest Cu–Mo orebody is saddle-shaped with various types of hydrothermal veinlets. Intensive hydrothermal alteration in the deposit is characterized by outward zoning from potassic, overprinted by phyllic alteration, to phyllic and alunite–dickite alteration. Based on the mineral assemblages and crosscutting relationships of veins, the ore-forming process can be divided into three stages, namely: an early-stage K-feldspar + quartz ± magnetite ± molybdenite veins associated with potassic alteration; a middle-stage quartz + molybdenite + chalcopyrite + pyrite veins in phyllic zone; and a late-stage quartz ± gypsum veins in the phyllic and alunite–dickite alteration zones. Six molybdenite separates yield a Re−Os isochron age 104.6 ± 1.0 Ma, which is identical to the age of emplacement of the Sifang and Luoboling granodiorite porphyries. Three types of fluid inclusions (FIs) were observed at the Luoboling deposit: 1) NaCl–H₂O (aqueous), 2) daughter mineral-bearing and 3) CO₂–H₂O fluid inclusions. FIs of the early and middle stages are predominantly vapor-rich aqueous and daughter mineral-bearing inclusions, together with minor CO₂-rich and liquid-rich aqueous inclusions; whereas the late-stage minerals only contain liquid-rich aqueous inclusions. Homogenization temperatures and salinities of FIs trapped in the early-stage minerals range from 420 to 540 °C and 0.4 to 62.9 wt.% NaCl equiv., respectively. FIs of the middle-stage yield homogenization temperatures of 340 to 480 °C and salinities of 0.5 to 56.0 wt.% NaCl equiv. CO₂ content and the oxygen fugacity (indicated by daughter minerals) of FIs trapped in middle-stage minerals are lower than those in the early stage. The liquid-rich aqueous inclusions of the late-stage homogenize at temperatures of 140 to 280 °C, yielding salinities of 0.4 to 8.4 wt.% NaCl equiv. The minimum estimated pressures of the three stages are 30–70 MPa, 10–40 MPa and 1–10 MPa, respectively, corresponding to minimum ore-forming depths of 1–2.8 km. Fluids trapped in early, middle and late stages yield δD values of − 67‰ to − 54‰, − 54‰ to − 70‰, and − 62‰, and δ¹⁸O values of 5.4‰ to 6.7‰, 2.8‰ to 4.2‰, and − 2.1‰, respectively. Fluid boiling, which resulted in the formation of stockworks and the precipitation of sulfides, occurred in the early and middle stages. The fluids subsequently evolved into a low temperature, low salinity system in the late stage, along with an input of meteoric water. The Luoboling porphyry Cu–Mo system was developed in a transition from continental arc to back-arc extension region, which was related to the westward subduction of the paleo-Pacific plate beneath the Huanan Orogen.     

Ore Control Factors of the High-grade BIF of the Carajás Iron Province in Brazil

Please refer to the attachment(s) for more details.     

Geochemical exploration challenges in the regolith dominated Igarapé Bahia gold deposit,Carajás,Brazil

The Igarapé Bahia, situated in the Carajás Mineral Province, is a world-class example of a lateritic gold deposit. It has developed under tropical weathering conditions since at least the Eocene and resulted in a regolith cover of at least 100 m thickness. The regolith is dominated by ~ 80 m thick ferruginous saprolite containing gossan bodies that constitute the main Au ore. Above saprolite the regolith stratigraphy has been established considering two distinct domains. One composed of residual materials and the other transported materials deposited over palaeochannels. In the residual domain the ferruginous saprolite grades upwards into a fragmental duricrust, interpreted as a collapsed zone, and then into different types of ferruginous duricrusts. Over palaeochannel the ferruginous saprolite is truncated by poorly sorted ferruginous sediment of variable composition that grades upwards into the ferruginous duricrusts formed over transported materials. Lateritization took place during a marked period that transformed the colluvium of the residual domain, and the transported materials accumulated in the channel depressions, into the ferruginous duricrust units. A later bauxitization event has overprinted all duricrust types but has mostly affected the duricrusts over the palaeochannel forming gibbsitic nodules. All duricrusts were finally covered by a transported layer of latosol which flattened the whole landscape in the Carajás region. Gold shows a depletion trend across the regolith but is enriched in the fragmental duricrust below the ferruginous duricrust from which gold is leached. Gold is also chemically dispersed laterally into the fragmental duricrust, but lateral Au dispersion in the ferruginous duricrusts of the residual domain is probably also influenced by colluvial transport. Metals associated with Au mineralization (Cu, U, Mo, Pb, Ag, LREE, Sn, W, Bi, Sb and P) are generally depleted in the saprolite but most of them are still anomalous. The fragmental and ferruginous duricrusts are more leached but the tests performed to estimate the dispersion potential of metals contained in the ferruginous duricrust show that some metals are still significantly anomalous especially Au, Ag and Cu. However, if ferruginous duricrusts are used as an exploration sample media their environment of formation must be considered. Metal depletion is generally more advanced in the ferruginous duricrusts developed in the vicinities of palaeochannels as oppose to those developed in residual domain. On the contrary, Au over palaeochannel areas is enriched in the upper bauxitized ferruginous duricrusts and in their gibbsitic nodules as a result of lateral chemical transport that is more widespread than in the colluvium over residual domain. The latosol is highly depleted in most metals due to its transported nature. However, the nodular fractions of the latosol show the greatest dispersion potential especially for Au, Ag, W, U, Bi and Sn. It can incorporate magnetic nodules that bring a rich suit of metals associated to the magnetic gossans, and non-magnetic nodules, classified as concretion and pisolites, which bring metals enriched or dispersed in the ferruginous duricrusts. This suggests that Lag constitutes a promising sample medium for geochemical exploration in the lateritic terrains of the Carajás region.     

Geology,fluid inclusion,and stable isotope study of the skarn-related Pb–Zn (Cu–Fe–Sn) polymetallic deposits in the southern Great Xing’an Range,China: implications for deposit type and metallogenesis

In recent decades, several skarn-related deposits have been found and explored in the southern Great Xing’an Range of China. To get a clear understanding of the characteristics and genetics of this type of deposit in this area, three of the largest, most typical, and most famous skarn-related deposits (Haobugao Pb–Zn deposit, Huanggang Sn–Fe polymetallic deposit, and Baiyinnuoer Pb–Zn deposit) are selected for systematically metallogenic study in this paper. The results of ore geology, fluid inclusion, and stable isotopes indicate that (1) most of the ore bodies of each deposit, occurred in the outer contact zone of the magma intrusion and Permian strata, fine vein disseminated mineralization within the intrusions were also found in this study. Mineralization of these deposits all show closely temporal, spatial, and genetic relationships with skarns. (2) Fluid inclusion petrography and microthermometry results show that the fluid inclusion assemblages developed in the different mineralization stages of each deposit changed from Type-S (daughter mineral-bearing three-phase fluid inclusions) + Type-V (vapor-rich fluid inclusions) + Type-L (liquid-rich fluid inclusions) to Type-V + Type-L and eventually evolved into L-Type. Correspondingly, the ore-forming fluids changed from medium to a high-temperature, high-salinity, and boiling fluid system and then to a low-temperature, low-salinity, and uniform fluid system. The types of fluid inclusions in garnets are consistent with those in quartz phenocrysts of Mesozoic granites, indicating that the formation of skarns is directly related to Mesozoic magmatic activity. (3) The δ³⁴S values of ores from the above three deposits all exhibit a narrow variation range (changes are mainly around 0‰) and greatly differ from the SEDEX-type deposits in China. The lead isotope compositions of the sulfide minerals are also consistent with those of Mesozoic granites. These previous characteristics suggest that both of the ore-forming fluids and the ore-forming materials were of magmatic origin. Consequently, the Haobugao, Baiyinnuoer, and Huanggang deposits are all skarn-type deposits, which are related to Mesozoic magmatic activities in terms of ore geology features, ore-forming fluids, and ore-forming material.     

Genesis of the Fuxing porphyry Cu deposit in Eastern Tianshan,China: Evidence from fluid inclusions and C–H–O–S–Pb isotope systematics

The Fuxing porphyry Cu deposit is a recently discovered deposit in Eastern Tianshan, Xinjiang, northwestern China. The Cu mineralization is associated with the Fuxing plagiogranite porphyry and monzogranite, mainly presenting as various types of hydrothermal veins or veinlets in alerted wall rocks, with potassic, chlorite, phyllic, and propylitic alteration developed. The ore-forming process can be divided into four stages: stage I barren quartz veins, stage II quartz–chalcopyrite–pyrite veins, stage III quartz–polymetallic sulfide veins and stage IV quartz–calcite veins. Four types of fluid inclusions (FIs) can be distinguished in the Fuxing deposit, including hypersline (H-type), vapor-rich two-phase (V-type), liquid-rich two-phase (L-type), and trace amounts of pure vapor inclusions (P-type), but only the stage I quartz contains all types of FIs. The stages II and III quartz have two types of FIs, with exception of H- and P-types. In stage IV quartz minerals, only the L-type inclusions can be observed. The FIs in quartz of stages I, II, III and IV are mainly homogenized at temperatures of 357–518 °C, 255–393 °C, 234–322 °C and 145–240 °C, with salinities of 1.9–11.6 wt.% NaCl equiv., 1.6–9.6 wt.% NaCl equiv., 1.4–7.7 wt.% NaCl equiv. and 0.9–3.7 wt.% NaCl equiv., respectively. The ore-forming fluids of the Fuxing deposit are characterized by high temperature, moderate salinity and relatively oxidized condition. Carbon, hydrogen and oxygen isotopic compositions of quartz indicate that the ore-forming fluids were gradually evolved from magmatic to meteoric in origin. Sulfur and lead isotopes suggest that the ore-forming materials were derived from a deep-seated magma source. The Cu mineralization in the Fuxing deposit occurred at a depth of ~ 1 km, and the changes of oxygen fugacity, decompression boiling, and local mixing with meteoric water were most likely critical for the formation of the Fuxing Cu deposit.     

Geology, mineralization, stable isotope geochemistry, and fluid inclusion characteristics of the Novogodnee–Monto oxidized Au–(Cu) skarn and porphyry deposit, Polar Ural, Russia

The Novogodnee–Monto oxidized Au–(Cu) skarn and porphyry deposit is situated in the large metallogenic belt of magnetite skarn and Cu–Au porphyry deposits formed along the Devonian–Carboniferous Urals orogen. The deposit area incorporates nearly contemporaneous Middle–Late Devonian to Late Devonian–Early Carboniferous calc-alkaline (gabbro to diorite) and potassic (monzogabbro, monzodiorite- to monzonite-porphyry, also lamprophyres) intrusive suites. The deposit is represented by magnetite skarn overprinted by amphibole–chlorite–epidote–quartz–albite and then sericite–quartz–carbonate assemblages bearing Au-sulfide mineralization. This mineralization includes early high-fineness (900–990?‰) native Au associated mostly with cobaltite as well as with chalcopyrite and Co-pyrite, intermediate-stage native Au (fineness 830–860?‰) associated mostly with galena, and late native Au (760–830?‰) associated with Te minerals. Fluid inclusion and stable isotope data indicate an involvement of magmatic–hydrothermal high-salinity (>20 wt.% NaCl-equiv.) chloride fluids. The potassic igneous suite may have directly sourced fluids, metals, and/or sulfur. The abundance of Au mineralization is consistent with the oxidized character of the system, and its association with Co-sulfides suggests elevated sulfur fugacity.     

Geology,fluid inclusion and isotope constraints on ore genesis of the post-collisional Dabu porphyry Cu–Mo deposit,southern Tibet

The Dabu Cu-Mo porphyry deposit is situated in the southern part of the Lhasa terrane within the post-collisional Gangdese porphyry copper belt (GPCB). It is one of several deposits that include the Qulong and Zhunuo porphyry deposits. The processes responsible for ore formation in the Dabu deposit can be divided into three stages of veining: stage I, quartz–K-feldspar (biotite) ± chalcopyrite ± pyrite, stage II, quartz–molybdenite ± pyrite ± chalcopyrite, and stage III, quartz–pyrite ± molybdenite. Three types of fluid inclusions (FIs) are present: liquid-rich two-phase (L-type), vapor-rich two-phase (V-type), and solid bearing multi-phase (S-type) inclusions. The homogenization temperatures for the FIs from stages I to III are in the ranges of 272–475 °C, 244–486 °C, and 299–399 °C, and their salinities vary from 2.1 to 49.1, 1.1 to 55.8, and 2.9 to 18.0 wt% NaCl equiv., respectively. The coexistence of S-type, V-type and L-type FIs in quartz of stage I and II with similar homogenization temperatures but contrasting salinities, indicate that fluid boiling is the major factor controlling metal precipitation in the Dabu deposit. The ore-forming fluids of this deposit are characterized by high temperature and high salinity, and they belong to a H₂O–NaCl magmatic–hydrothermal system. The H–O–S–Pb isotopic compositions indicate that the ore metals and fluids came primarily from a magmatic source linked to Miocene intrusions characterized by high Sr/Y ratios, similar to other porphyry deposits in the GPCB. The fluids forming the Dabu deposit were rich in Na and Cl, derived from metamorphic dehydration of subducted oceanic slab through which NaCl-brine or seawater had percolated. The inheritance of ancient subduction-associated arc chemistry, without shallow level crustal assimilation and/or input of the meteoric water, was responsible for the generation of fertile magma, as well as CO₂-poor and halite-bearing FIs associated with post-collisional porphyry deposits. The estimated mineralization depths of Qulong, Dabu and Zhunuo deposits are 1.6–4.3 km, 0.5–3.4 km and 0.2–3.0 km, respectively, displaying a gradual decrease from eastern to western Gangdese. Deep ore-forming processes accounted for the generation of giant-sized Qulong deposit, because the exsolution of aqueous fluids with large fraction of water and chlorine in deep or high pressure systems can extract more copper from melts than those formed in shallow systems. However, the formation of small-sized Dabu deposit can be explained by a single magmatic event without additional replenishment of S, metal, or thermal energy. In addition, the ore-forming conditions of porphyry Cu–Mo deposits in GPCB are comparable to those of porphyry Cu ± Au ± Mo deposits formed in oceanic subduction-related continental or island arcs, but differ from those of porphyry Mo deposit formed in the Dabie-Qinling collisional orogens. The depth of formation of the mineralization and features of primary magma source are two major controls on the metal types and ore-fluid compositions of these porphyry deposits.     

Spinifex-textured komatiites in the south border of the Carajas ridge,Selva Greenstone belt,Carajás Province,Brazil

Spinifex-textured komatiites in the Selva greenstone belt are the first unequivocal examples of komatiites in the Transition Subdomain of the Carajás Mineral Province. Outcrops of spinifex-textured komatiites, located ∼1.5 km to the south of the Carajás ridge, were discovered during regional exploration for Ni–Cu–(PGE) sulfide deposits by VALE. They are associated with a 3.8 km long unit consisting of variable types of ultramafic rocks (talc schist, serpentinite and spinifex-textured komatiite). This ultramafic unit follows the steep dipping NW–SE trending Selva greenstone belt composed mainly by quartz-chlorite schists (interpreted as metasediments) and chlorite-actinolite schists (interpreted as metabasalts). Greenschist facies metamorphic parageneses characterize all rock types in the Selva greenstone belt.The komatiitic rocks in the Selva belt comprise a sequence of flows consisting of an upper spinifex-textured layer and a lower olivine cumulate layer. Although the spinifex and cumulus textures are well preserved in the field, the primary mineralogy of the komatiites has been completely replaced by greenschist facies metamorphic minerals. Platy olivine spinifex texture, consisting of an array of roughly parallel olivine plates, and random spinifex texture, consisting of randomly oriented olivine plates, are the most common primary volcanic textures in komatiites in the Selva greenstone belt. Platy and random spinifex texture is defined by former plates of olivine replaced by serpentine with minor actinolite, chlorite and magnetite, alternating with former matrix replaced by abundant actinolite and minor chlorite, talc, serpentine, and magnetite. The domains between olivine plates in both platy and random spinifex-textured rocks contain irregular arrays of fine-grained parallel crystals, representing primary fine-grained “quench” clinopyroxene crystals replaced by actinolite.Spinifex-textured komatiites have MgO contents bracket between 22.8 and 26.9 wt.%, and cumulate textured komatiites have MgO contents up to 40.6 wt.%. When plotted vs MgO contents, most major and minor elements fall on well-defined linear trends indicating control by olivine fractionation or accumulation. Komatiites from the Selva and Seringa (located in the Rio Maria Domain) belts are Al-undepleted with Al₂O₃/TiO₂ ratios close to 20. Results for CaO, Na₂O, and REE suggest that these elements were mobile and their abundances have been modified during metasomatic alteration. REE contents in some samples are very high (up to 40 times primitive mantle values) and REE patterns vary from flat (La/Yb_MN ∼ 1) to highly enriched in LREE (La/Yb_MN up to ∼ 10). The REE mobility may be related to hydrothermal alteration associated to Cu–Au mineralization in the region.The identification of spinifex-textured komatiites close to the Carajás Basin suggests the continuation of 3.0–2.9 Ga greenstone belts of the Rio Maria Domain within the Transition Subdomain, and enlarges the area with potential to host komatiite-associated Ni–Cu–PGE deposits.     

The Jiguanshan porphyry Mo deposit in the Xilamulun metallogenic belt,northern margin of the North China Craton,U–Pb geochronology,isotope systematics,geochemistry and fluid inclusion studies: Implications for a genetic model

The Jiguanshan porphyry Mo deposit is located in the southern part of Xilamulun metallogenic belt at the northern margin of the North China Craton (NCC). In the Jiguanshan mining district, two stages of granitoids intrusions have been recognized: a pre-ore granite porphyry with stockworks and veins of Mo mineralization, and a granite porphyry with disseminated Mo mineralization. Zircon U–Pb data and Hf isotope analyses show that the dissemination-mineralized granite porphyry yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 156.0 ± 1.3 Ma, with a crustal ε_Hf(t) values from − 5.6 to + 0.2, and that the main group of magmatic zircons from the pre-ore granite porphyry have a weighted mean ²⁰⁶Pb/²³⁸U age of 167.7 ± 1.7 Ma with ε_Hf(t) values from − 3.2 to + 1.0. Combined with groundmass Ar–Ar age data of the granite porphyry and molybdenite Re–Os age, it is suggested that the Mo mineralization of Jiguanshan deposit was formed in the late Jurassic (153 ~ 155 Ma) during tectonic and magmatic events that affected northeast China. The Mo mineralization was a little bit later than the host granite porphyry. Besides disseminated in the host granite porphyry, Mo mineralization also presents in middle Jurassic pre-ore granite porphyry, Jurassic fine-grained diabase, Triassic quartz porphyry, and in rhyolitic volcanic rocks as well as syenite of Devonian age.The Jiguanshan mining district was affected by the tectonic events associated with the Paleo-Asian Ocean closure, and later by far-field tectonism, related to subduction of the Paleo-Pacific plate (Izanagi) in the Jurassic-Cretaceous. The tectonic and thermal events linked with the latter are commonly referred to as Yanshanian tectono-thermal event, and consists of a series of geodynamic, magmatic and ore-forming processes, which in the mining district area included the intrusion of the pre-ore granite porphyry, the host granite porphyry, Mo mineralization, and fine-grained diabase. Major and trace element analyses show that the host granite porphyry is characterized by high silica abundances (SiO₂ = 77.16 to 77.51%), high Rb/Sr ratios (13.57 to 14.83), high oxidation (Fe₂O₃/FeO = 34.25 to 62.00) and high alkalies (Na₂O + K₂O = 8.21 to 8.38%). Petrographic and microthermometry studies of the fluid inclusions from Mo mineralized veins, characterized by plenty of daughter mineral-bearing inclusions, showed that the predominant homogenization temperatures range from 250 to 440 °C. Combined with Laser Raman analysis of the fluid inclusions, it is indicated that Mo mineralization is related to a high-temperature, hypersaline and high-oxygen fugacity H₂O–NaCl fluid system, with high F contents.Based on geology, geochronology, isotope systematics, geochemistry and fluid inclusion studies as well as regional geology, we propose, for the first time, a genetic model for the Jiguanshan porphyry Mo deposit. During the Jurassic geodynamic evolution of northeast China, high silicic, high oxidized and alkaline-rich granitic magma probably derived from partial melting of the lower crust, episodically intruded along faults into the country rocks. This fluid system, fractionating from the highly differentiated granitic magma and bearing Mo with minor Cu metals, migrated upwards and interacted with the older wall rocks and associated fractures, in which the ore minerals precipitated, resulting in the development of what we refer to as the “Jiguanshan-type” porphyry Mo deposit.     

Petrographic,fluid inclusion and isotopic study of the Dikulushi Cu–Ag deposit,Katanga (D.R.C.): implications for exploration

The Dikulushi Cu–Ag vein-type deposit is located on the Kundelungu Plateau, in the southeastern part of the Democratic Republic of Congo (D.R.C.). The Kundelungu Plateau is situated to the north of the Lufilian Arc that hosts the world-class stratiform Cu–Co deposits of the Central African Copperbelt. A combined petrographic, fluid inclusion and stable isotope study revealed that the mineralisation at Dikulushi developed during two spatially and temporally distinct mineralising episodes. An early Cu–Pb–Zn–Fe mineralisation took place during the Lufilian Orogeny in a zone of crosscutting EW- and NE-oriented faults and consists of a sequence of sulphides that precipitated from moderate-temperature, saline H₂O–NaCl–CaCl₂-rich fluids. These fluids interacted extensively with the country rocks. Sulphur was probably derived from thermochemical reduction of Neoproterozoic seawater sulphate. Undeformed, post-orogenic Cu–Ag mineralisation remobilised the upper part of the Cu–Pb–Zn–Fe mineralisation in an oxidising environment along reactivated and newly formed NE-oriented faults in the eastern part of the deposit. This mineralisation is dominated by massive Ag-rich chalcocite that precipitated from low-temperature H₂O–NaCl–KCl fluids, generated by mixing of moderate- and low-saline fluids. The same evolution in mineralisation assemblages and types of mineralising fluids is observed in three other Cu deposits on the Kundelungu Plateau. Therefore, the recognition of two distinct types of (vein-type) mineralisation in the study area has a profound impact on the exploration in the Kundelungu Plateau region. The identification of a Cu–Ag type mineralisation at the surface could imply the presence of a Cu–Pb–Zn–Fe mineralisation at depth.