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.     

The Transfiguration continental red-bed Cu–Pb–Zn–Ag deposit,Quebec Appalachians,Canada

The Transfiguration Cu–Pb–Zn–Ag deposit, enclosed within reduced grey sandstone, is associated with continental red beds of the Lower Silurian Robitaille Formation in the Quebec Appalachians, Canada. The Robitaille Formation rests unconformably on foliated Cambro-Ordovician rocks. The unconformity is locally cut by barite veins. The basal unit of the Robitaille Formation comprises green wacke and pebble conglomerate, which locally contain calcite nodules. The latter have microstructures characteristic of alpha-type calcretes, such as “floating” fabrics, calcite-filled fractures (crystallaria) and circumgranular cracks. Massive, grey sandstone overlies the basal green wacke and pebble conglomerate unit, which is overlain, in turn, by red, fine-grained sandstone. Mineralisation occurred underneath the red sandstone unit, chiefly in the grey sandstone unit, as disseminated and veinlet sulphides. Chalcopyrite, the most abundant Cu sulphide, replaced early pyrite. Calcrete, disseminated carbonate and vein carbonate have stable isotope ratios varying from −7.5‰ to −1.1‰ δ¹³C and from 14.7‰ to 21.3‰ δ¹⁸O. The negative δ¹³C values indicate the oxidation of organic matter in a continental environment. Sulphur isotope ratios for pyrite, chalcopyrite and galena vary from −19‰ to 25‰ δ³⁴S, as measured on mineral concentrates by a conventional SO₂ technique. Laser-assisted microanalyses (by fluorination) of S isotopes in pyrite show an analogous range in δ³⁴S values, from −21‰ to 25‰. Negative and positive δ³⁴S values are compatible with bacterial sulphate reduction (BSR) in systems open and closed with respect to sulphate. We interpret similarly high δ³⁴S values for sulphide concentrates (25.1‰) and for vein barite (26.2‰) to result from rapid and complete thermochemical reduction of pore-water sulphate. Two early to late diagenetic stages of mineralisation best explain the origin of the Transfiguration deposit. The first stage was characterised by the ponding of groundwater over the Taconian unconformity, recorded by calcrete and early pyrite formation via BSR in grey sandstone. Early pyrite contains up to 2 wt.% Pb, which is consistent with Pb fixation by sulphate-reducing bacteria. The second stage (II) is defined by the replacement of early pyrite by chalcopyrite, as well as by sulphide precipitation via either BSR or thermochemical sulphate reduction (TSR) in grey sandstone. This event resulted from the synsedimentary fault-controlled percolation and mixing of (1) an oxidising, sulphate-bearing cupriferous fluid migrating per descensum from the red-bed sequence and (2) a hydrocarbon-bearing fluid migrating per ascensum from the Cambro-Ordovician basement. Mixing between the two fluids led to sulphate reduction, causing Cu sulphide precipitation. The positive correlation between Cu and Fe³⁺/Fe²⁺ bulk rock values suggests that Fe acted as a redox agent during sulphate reduction. Stage II diagenetic fluid migration is tentatively attributed to the Late Silurian Salinic extensional event.     

The Jabali nonsulfide Zn–Pb–Ag deposit,western Yemen

The Jabali Zn–Pb–Ag deposit is located about 110 km east of Sana'a, the capital of Yemen, along the western border of the Marib-Al-Jawf/Sab'atayn basin. The economic mineralization at Jabali is a nonsulfide deposit, consisting of 8.7 million tons at an average grade of 9.2% zinc, derived from the oxidation of primary sulfides. The rock hosting both primary and secondary ores is a strongly dolomitized carbonate platform limestone of the Jurassic Shuqra Formation (Amran Group). The primary sulfides consist of sphalerite, galena and pyrite/marcasite. Smithsonite is the most abundant economic mineral in the secondary deposit, and is associated with minor hydrozincite, hemimorphite, acanthite and greenockite. Smithsonite occurs as two main generations: smithsonite 1, which replaces both host dolomite and sphalerite, and smithsonite 2, occurring as concretions and vein fillings in the host rock. At the boundary between smithsonite 1 and host dolomite, the latter is widely replaced by broad, irregular bands of Zn-bearing dolomite, where Zn has substituted for Mg. The secondary mineralization evolved through different stages: 1) alteration of original sulfides (sphalerite, pyrite and galena), and release of metals in acid solutions; 2) alteration of dolomite host rock and formation of Zn-bearing dolomite; 3) partial dissolution of dolomite by metal-carrying acid fluids and replacement of dolomite and Zn-bearing dolomite by a first smithsonite phase (smithsonite 1). To this stage also belong the direct replacement of sphalerite and galena by secondary minerals (smithsonite and cerussite); 4) precipitation of a later smithsonite phase (smithsonite 2) in veins and cavities, together with Ag- and Cd-sulfides.The δ¹⁸O composition of Jabali smithsonite is generally lower than in other known supergene smithsonites, whereas the carbon isotope composition is in the same range of the negative δ¹³C values recorded in most supergene nonsulfide ores. Considering that the groundwaters and paleo-groundwaters in this area of Yemen have negative δ¹⁸O values, it can be assumed that the Jabali smithsonite precipitated in different stages from a combination of fluids, possibly consisting of local groundwaters variably mixed with low-temperature hydrothermal waters. The carbon isotope composition is interpreted as a result of mixing between carbon from host rock carbonates and soil/atmospheric CO₂.The most favorable setting for the development of the Jabali secondary deposit could be placed in the early Miocene (~ 17 Ma), when supergene weathering was favored by major uplift and exhumation resulting from the main phase of Red Sea extension. Low-temperature hydrothermal fluids may have also circulated at the same time, through the magmatically-induced geothermal activity in the area.     

A model of boiling for fluid inclusion studies: Application to the Bolaños Ag–Au–Pb–Zn epithermal deposit,Western Mexico

Boiling can be inferred from fluid inclusion microthermometry studies when a progressive increase in apparent salinity is observed along with a decrease of homogenization temperature (T_H) and depth, thus reflecting the partitioning of non-volatile solutes into the liquid phase during steam loss. We propose a model for fluid evolution during boiling based on mass and heat balance equations, which establishes paths in the T_H-salinity space that can be compared with fluid inclusion data to confirm or discard boiling. Additionally, the model allows calculating paleo-depths, for which the effect of steam bubbles lowering the hydrostatic pressure is taken into account.     

Mineralization and fluid evolution of the Jiyuan polymetallic Cu–Ag–Pb–Zn–Au deposit,eastern Tianshan,NW China

The newly discovered Jiyuan Cu–Ag–(Pb–Zn–Au) deposit is located in the southern section of the eastern Tianshan orogenic belt, Xinjiang, northwestern China. It is the first documented deposit in the large Aqikekuduke Ag–Cu–Au belt in the eastern Tianshan orogen. Detailed field observations, parageneses, and fluid inclusion studies suggest an epithermal ore genesis for the main Cu–Ag mineralization, accompanied by a complicated hydrothermal alteration history most likely associated with the multi-stage tectonic evolution of the eastern Tianshan. The Jiyuan Cu–Ag ore bodies are located along the EW-striking, south-dipping Aqikekuduke fault and are hosted by Precambrian marble and intercalated siliceous rocks. Early-stage skarn alteration occurred along the contact zone between the marble layers and Early Carboniferous diorite–granodiorite and monzogranite intrusions; the skarns are characterized by diopside–tremolite–andradite–pyrite–(magnetite) assemblages. Local REE-enriched synchysite–rutile–arsenopyrite–(clinochlorite–microcline–albite) assemblages are related to K–Na alteration associated with the monzogranite intrusions and formed under conditions of high temperature (310°C) and high salinity (19.9 wt.% NaCl). Subsequent hydrothermal alteration produced a series of quartz and calcite veins that precipitated from medium- to low-temperature saline fluids. These include early ‘smoky’ quartz veins (190°C; 3.0 wt.% NaCl) that are commonly barren, coarse-grained Cu–Ag mineralized quartz veins (210°C; 2.4 wt.% NaCl), and late-stage unmineralized calcite veins (140°C; 1.1 wt.% NaCl). Tremolite and Ca-rich scapolite veins formed at an interval between early and mineralized quartz veins, indicating a high-temperature, high-salinity (>500°C; 9.5 wt.% NaCl) Ca alteration stage. Fluid mixing may have played an important role during Cu–Ag mineralization and an external low-temperature Ca-rich fluid is inferred to have evolved in the ore-forming system. The Jiyuan auriferous quartz veins possess fluid characteristics distinct from those of the Cu–Ag mineralized quartz veins. CO₂-rich fluid inclusions, fluid boiling, and mixing all demonstrate that these auriferous quartz veins acted as hosts for the orogenic-type gold mineralization, a common feature in the Tianshan orogenic belt.     

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.     

The mangazeya Ag–Pb–Zn vein deposit hosted in sedimentary rocks,Sakha-Yakutia,Russia: Mineral assemblages,fluid inclusions,stable isotopes (C,O, S), and origin

The succession of mineral assemblages, chemistry of gangue and ore minerals, fluid inclusions, and stable isotopes (C, O, S) in minerals have been studied in the Mangazeya silver–base-metal deposit hosted in terrigenous rocks of the Verkhoyansk Fold–Thrust Belt. The deposit is localized in the junction zone of the Kuranakh Anticlinorium and the Sartanga Synclinorium at the steep eastern limb of the Endybal Anticline. The deposit is situated at the intersection of the regional Nyuektame and North Tirekhtyakh faults. Igneous rocks are represented by the Endybal massif of granodiorite porphyry 97.8 ± 0.9 Ma in age and dikes varying in composition. One preore and three types of ore mineralization separated in space are distinguished: quartz–pyrite–arsenopyrite (I), quartz–carbonate–sulfide (II), and silver–base-metal (III). Quartz and carbonate (siderite) are predominant in ore veins. Ore minerals are represented by arsenopyrite, pyrite, sphalerite, galena, fahlore, and less frequent sulfosalts. Three types of fluid inclusions in quartz differ in phase compositions: two- or three-phase aqueous–carbon dioxide (FI I), carbon dioxide gas (FI II), and two-phase (FI III) containing liquid and a gas bubble. The homogenization temperature and salinity fall within the ranges of 367–217°C and 13.8–2.6 wt % NaCl equiv in FI I; 336–126°C and 15.4–0.8 wt % NaCl equiv in FI III. Carbon dioxide in FI II was homogenized in gas at +30.2 to +15.3°C and at +27.2 to 29.0°C in liquid. The δ³⁴S values for minerals of type I range from–1.8 to +4.7‰ (V-CDT); of type II, from–7.4 to +6.6‰; and of type III, from–5.6 to +7.1‰. δ¹³C and δ¹⁸O vary from–7.0 to–6.7‰ (V-PDB) and from +16.6 to +17.1 (V-SMOW) in siderite-I; from–9.1 to–6.9‰ (V-PDB) and from +14.6 to +18.9 (V-SMOW) in siderite-II; from–5.4 to–3.1‰ (V-PDB) and from +14.6 to +19.5 (V-SMOW) in ankerite; and from–4.2 to–2.9‰ (V-PDB) and from +13.5 to +16.8 (V-SMOW) in calcite. The data on mineral assemblages, fluid inclusions, and ratios of stable isotopes allow us to speak about the formation of the Mangazeya deposit in relation to the activity of the hydrothermal–magmatic system. The latter combines emplacement of subvolcanic granitic stocks and involvement of fluids variable in salinity and temperature in ore deposition zone. The fluids released from crystallizing felsic magma and were formed in a convective cell by heating of meteoric and marine waters. The mechanism of ore deposition is related to phase separation (boiling) and mixing of fluids.     

A combined fluid inclusion and S–Pb isotope study of the Neoproterozoic Pingshui volcanogenic massive sulfide Cu–Zn deposit,Southeast China

The Pingshui Cu–Zn deposit is located in the Jiangshan–Shaoxing fault zone, which marks the Neoproterozoic suture zone between the Yangtze block and Cathaysia block in South China. It contains 0.45 million tons of proven ore reserves with grades of 1.03 wt.% Cu and 1.83 wt.% Zn. This deposit is composed of stratiform, massive sulfide ore bodies, which contain more than 60 vol.% sulfide minerals. These ore bodies are hosted in altered mafic and felsic rocks (spilites and keratophyres) of the bimodal volcanic suite that makes up the Neoproterozoic Pingshui Formation. Metallic minerals include pyrite, chalcopyrite, sphalerite, tennantite, tetrahedrite and magnetite, with minor galena. Gangue minerals are quartz, sericite, chlorite, calcite, gypsum, barite and jasper. Three distinct mineralogical zones are recognized in these massive sulfide ore bodies: a distal zone composed of sphalerite + pyrite + barite (zone I); an intermediate zone characterized by a pyrite + sphalerite + chalcopyrite assemblages (zone II); and a proximal zone containing chalcopyrite + pyrite + magnetite (zone III). A thin, layer of exhalative jaspilite overlies the sulfide ore bodies except in the proximal zone. The volcanic rocks of the Pingshui Formation are all highly altered spilites and keratophyres, but their trace element geochemistry suggests that they were generated by partial melting of the depleted mantle in an island arc setting. Homogenization temperatures of the primary fluid inclusions in quartz from massive sulfide ores are between 217 and 328 °C, and their salinities range from 3.2 to 5.7 wt.% NaCl equivalent. Raman spectroscopy of the fluid inclusions showed that water is the dominant component, with no other volatile components. Fluid inclusion data suggest that the ore-forming fluids were derived from circulating seawater. The δ³⁴S values of pyrite from the massive sulfide ores range from − 3.6‰ to + 3.4‰, indicating that the sulfur was primarily leached from the arc volcanic rocks of the Pingshui Formation. Both pyrite from the massive sulfide ores and plagioclase from the spilites have similar lead isotope compositions, implying that the lead was also derived from the Pingshui Formation. The low lead contents of the massive sulfide ores and the geochemistry of their host rocks are similar to many VMS Cu–Zn deposits in Canada (e.g., Noranda) and thus can be classified as belonging to the bimodal-mafic subtype. The presence of magnetite and the absence of jaspilite and barite at the − 505 m level in the Pingshui deposit suggest that this level is most likely the central zone of the original lateral massive sulfide ore bodies. If this interpretation is correct, the deep part of the Pingshui Cu–Zn deposit may have significant exploration potential.     

Origin of epithermal Ag–Au–Cu–Pb–Zn mineralization in Guanajuato, Mexico

The Guanajuato epithermal district is one of the largest silver producers in Mexico. Mineralization occurs along three main vein systems trending dominantly northwest–southeast: the central Veta Madre, the La Luz system to the northwest, and the Sierra system to the east. Mineralization consists dominantly of silver sulfides and sulfosalts, base metal sulfides (mostly chalcopyrite, galena, sphalerite, and pyrite), and electrum. There is a broad zonation of metal distribution, with up to 10 % Cu+Pb+Zn in the deeper mines along the northern and central portions of the Veta Madre. Ore occurs in banded veins and breccias and as stockworks, with gangue composed dominantly of quartz and calcite. Host rocks are Mesozoic sedimentary and intrusive igneous rocks and Tertiary volcanic rocks. Most fluid inclusion homogenization temperatures are between 200 and 300 °C, with salinities below 4 wt.% NaCl equivalent. Fluid temperature and salinity decreased with time, from 290 to 240 °C and from 2.5 to 1.1 wt.% NaCl equivalent. Relatively constant fluid inclusion liquid-to-vapor ratios and a trend of decreasing salinity with decreasing temperature and with increasing time suggest dilution of the hydrothermal solutions. However, evidence of boiling (such as quartz and calcite textures and the presence of adularia) is noted along the Veta Madre, particularly at higher elevations. Fluid inclusion and mineralogical evidence for boiling of metal-bearing solutions is found in gold-rich portions of the eastern Sierra system; this part of the system is interpreted as the least eroded part of the district. Oxygen, carbon, and sulfur isotope analysis of host rocks, ore, and gangue minerals and fluid inclusion contents indicate a hydrothermal fluid, with an initial magmatic component that mixed over time with infiltrating meteoric water and underwent exchange with host rocks. Mineral deposition was a result of decreasing activities of sulfur and oxygen, decreasing temperature, increasing pH, and, in places, boiling.     

Geology,mineralogy and fluid inclusion study of Oued Jebs Pb–Zn–Sr deposit; comparison with the Bou Grine deposit (diapirs zone,Tunisian atlas)

The Oued Jebs Pb–Zn–Sr deposit is situated on the south edge of the Mourra Triassic diapir, in the Diapir Zone of the Tunisian Atlas. Tow orebody-type are recognized: (1) lens-chapped orebodies hosted in the Dolomitic cap rock that marks the transition zone between the Triassic gypsum cap rock and the overlaying Late Cretaceous series. Mineralization is composed of epigenetic celestite and minor Pb–Zn sulfides. (2) Vein-type and massive-type orebodies crosscutting the Late Cenomanian and Turonian limestone. Mineralization is composed of high-grade ore ranging from 10 to 25 % combined Pb–Zn. Fluid inclusion data for celestite indicate that deposition took place between 70 and 100 °C, or more cooler conditions as indicated by the presence of single-phase inclusions, from mixed NaCl–CaCl₂-bearing brines (12–19 wt% NaCl equiv). For the vein-type and massive-type fluid inclusion, data recorded in sphalerite indicate that sulfide deposition took place between 125 and 130 °C mixed NaCl–CaCl₂-bearing brines (10–15 wt% NaCl equiv). At least three dilution and cooling trends are also observed that indicate the involvement of more than one fluid in the Oued Jebs hydrothermal system. The epigenetic character of the ores, the host rock nature and the fluid inclusion together permitted to include the Oued Jebs deposit in the large class of MVT deposits and preciously in the sub-class of MVTs associated with salt diapirs environment. The new discovered Oued Jebs deposit is similar in many aspects to the economic Bou Grine deposit. This may point to significant other potential for economic Pb–Zn concentrations that may be located at depth alongside or above many other unexplored Triassic diapirs in the Diapirs zone of the Tunisian Atlas.     

Modification of a Palaeoproterozoic porphyry-like system: Integration of structural,geochemical, petrographic,and fluid inclusion data from the Aitik Cu–Au–Ag deposit,northern Sweden

The Aitik Cu–Au–Ag deposit in the Gällivare area in northern Sweden is Sweden's largest sulphide mine with an annual production of 35 Mt of ore, and the biggest open pit operation in northern Europe. It is proposed in the present study that the Aitik deposit represents a Palaeoproterozoic, strongly metamorphosed porphyry copper deposit that was affected ca. 100 Ma later by a regional IOCG-type hydrothermal event. Consequently, the Aitik deposit might represent a mixed ore system where an early copper mineralisation of porphyry type has been overprinted by later regional IOCG mineralisation.Several attempts have previously been made to genetically classify the Aitik Cu–Au–Ag deposit as a distinct ore type. New geochemical, petrographic, structural, and fluid inclusion results combined with published data have provided the opportunity to present new ideas on the genesis and evolution of the Aitik Cu–Au–Ag deposit. The emplacement of a ca. 1.9 Ga quartz monzodiorite that host the ore at Aitik was related to subduction processes and volcanic arc formation, and synchronous with quartz vein stockwork formation and porphyry copper mineralisation. Highly saline aqueous (38 wt.% NaCl) fluid inclusions in the stockwork veins suggest entrapment at 300 °C and a pressure of nearly 3 kbar, a high pressure for a typical porphyry copper ore, but consistent with conditions at associated deep root zones of intrusion-related magmatic–hydrothermal systems. The highly saline fluid formed disseminated and vein-type ore of mainly chalcopyrite and pyrite within comagmatic volcaniclastic rocks, and caused potassic alteration (biotite, microcline) of the host rocks. The early porphyry copper mineralising event was followed, and largely overprinted, by CO₂ and aqueous medium- to high-salinity (16–57 wt.% salts) fluids related to a ca. 1.8 Ga tectonic and metamorphic event (peak conditions 500–600 °C and 4–5 kbar). Extensive deformation of rocks and redistribution of metals occurred. Magnetite enrichment locally found within late veins, and late amphibole–scapolite and K feldspar alterations within the deposit, are some of the features at Aitik implying that aqueous fluids responsible for IOCG-mineralisation (200–500 °C and ~ 1 kbar) and extensive Na–Ca alteration in the region during the 1.8 Ga tectonic event also affected the Aitik rocks, possibly leading to addition of copper ± gold.     

The Palaeocene-early Oligocene Zacatecas conglomerate,Mexico: sedimentology,detrital zircon U–Pb ages,and sandstone provenance

ABSTRACT

This article presents detailed mapping results and the first U–Pb zircon dating and sedimentological characterization of the Zacatecas Conglomerate, which belongs to the Palaeogene red beds of central Mexico, deposited in fault-bounded basins during the Late Cretaceous to Eocene Laramide orogeny. The conglomerate was divided into five depositional facies associations according to their clast-type abundances and interlayered volcanic rocks. The lowermost member has a maximum depositional age based on young zircon grain ages varying from ca. 63 to 81 Ma. It is unconformably overlain by a continuous sequence characterized by a conglomerate rich in granite clasts at the bottom, with an interlayered tuff dated at 37.64 ± 0.36 Ma. Near the top, another tuff was dated at 30.84 ± 0.47 Ma, and a sandstone has a maximum depositional age of ca. 31.5 Ma. Normal grading, massive textures, channels, channel-form sandstone bodies, and upward-finning successions suggest that the Zacatecas Conglomerate is of fluvial origin. Late Jurassic to Early Cretaceous ages from zircons in plutonic rocks and sandstones bracket possible source regions for the Zacatecas Conglomerate. One possible source is Late Jurassic-Early Cretaceous granite derived from the Alisitos-Guerrero arc of western Mexico. Another possible source is the Tuna Manza Diorite, now exposed 250 km southeast of the study area. The lack of pre-Jurassic grains implies that possible sources such as the Nazas arc or the Potosí fan were not cropping out at that time, or at least that these areas were not affected by the fluvial system feeding the Zacatecas Conglomerate. It is possible that during the Palaeocene-early Oligocene the fluvial systems drained from west to east and from southeast to north, according to the above-mentioned constraints.     

Metamorphosed Pb–Zn–(Ag) ores of the Keketale VMS deposit,NW China: Evidence from ore textures,fluid inclusions,geochronology and pyrite compositions

The Keketale Pb–Zn deposit is located in the Devonian volcanic-sedimentary Maizi basin of the Altay orogenic belt. The mineralization at Keketale is hosted in marbles and deformed volcanic tuffs and biotite–garnet–chlorite schists, folded into a series of overturned synclines formed in multiple deformation events. Keketale contains economic amounts of Pb (0.89 Mt @ 1.51 wt.%), Zn (1.94 Mt @ 3.16 wt.%) and Ag (650 t @ 40 g/t).Detailed petrographic studies have defined two main generations of sulfide development. The banded pyrite of the early Stage A is commonly stratiform, with minor galena, sphalerite and chalcopyrite. Stage B is characterized by a large amount of polymetallic sulfides including pyrrhotite, chalcopyrite, sphalerite and galena, with minor pyrite hosted in quartz veins.Three types of fluid inclusions (FIs), including mixed carbonic-aqueous (C-type), pure carbonic (PC-type) and aqueous (W-type), have been recognized in quartz of stage B. The C-type FIs have homogenization temperatures of 150–326 °C and salinities of 0.2–16.6 wt.% NaCl equivalent. The PC-type FIs are dominated by CO₂ with minor CH₄ and N₂ and have initial ice-melting temperatures of − 57.5 to − 56.7 °C, CO₂ homogenization temperatures of 11–14.1 °C. The W-type primary FIs were completely homogenized at temperatures of 124–359 °C with salinities of 5.0–14.6 wt.% NaCl equivalent. Such CO₂-rich fluid inclusions are consistent with those discovered in orogenic-type deposits in the Altay area and elsewhere.Muscovite separates from the polymetallic quartz veinlets of stage B yield a well-defined ⁴⁰Ar/³⁹Ar isotopic plateau age of 259.33 ± 2.56 Ma, with an isochron age of 259.62 ± 2.65 Ma. This age is coeval with the closure of the Paleo-Asia Ocean and reactivation of the Ertix Fault system.LA-ICP-MS analyses of two generations of pyrite indicate that the banded pyrite of stage A is relatively depleted in metallic elements and contains low contents of Cu (0.39 ppm), Ag (0.20 ppm), Au (below the detection limits), Pb (17.43 ppm) and Zn (14.38 ppm); whereas the pyrite in quartz–polymetallic sulfide veinlets of the stage B is relatively rich in metallic elements, e.g., Cu (2.56 ppm), Ag (3.07 ppm), Au (0.01 ppm), Pb (1047 ppm) and Zn (1136 ppm). The trace amounts of Cu, Pb, Zn, Au and Ag are interpreted to have been initially locked in the lattice of type-A pyrite, and then liberated and precipitated as micromineral inclusions with type-B pyrite during subsequent metamorphism and deformation.Two key factors are considered vital to the formation of economic ores of the Keketale Pb–Zn deposit, namely the original Devonian banded pyrite formed in a VMS system and subsequent Permian deformation and metamorphic processes that liberated Cu, Pb, Zn, Au and Ag from the lattice of type-A pyrite to form galena, sphalerite and chalcopyrite with minor muscovite in quartz veinlets. The model provides a new interpretation of VMS Pb–Zn deposit occurring in back-arc basin environments followed by collision, and new insights into the unique regional Fe–Cu–Pb–Zn–Au mineralization in the Altay orogenic belt.     

Geology,geochemistry and fluid inclusions of the Bianjiadayuan Pb–Zn–Ag deposit,Inner Mongolia,NE China: Implications for tectonic setting and metallogeny

The Bianjiadayuan Pb–Zn–Ag deposit in the Southern Great Xing'an Range consists of quartz-sulfide vein-type and breccia-type mineralization related to granite. Vein orebodies are localized in NW-trending extensional faults. Hydrothermal alteration is well developed and includes silicification, potassic alteration, chloritization and sericitization. Three stages of mineralization are recognized based on field evidence and petrographic observation and are marked by assemblages of quartz–arsenopyrite–pyrite (stage I), quartz–pyrrhotite–chalcopyrite–sphalerite (stage II) and quartz–galena–silver minerals (stage III). The granite, with a zircon age of 143.2 ± 1.5 Ma (n = 14, MSWD = 0.93), is subalkaline, peraluminous and is classified as A2-type granite originating in a post-orogenic extensional setting during the opening of suture zone between the North China Craton and the Siberia Craton from the Late Jurassic to the Early Cretaceous. The δ³⁴S_CDT values of sulfides, ranging from 3.19 to 10.65‰, are not consistent with the majority of magmatic hydrothermal deposits in the SGXR, possibly implying accessory source in addition to magmatic source. Microthermometric measurements show that ore minerals were deposited at intermediate temperatures (347.8–136.4 °C) with moderate salinities (2.9–14.4 wt.% NaCl). Ore-forming fluids were derived largely from magmatic hydrothermal processes, with the addition of meteoric water in late stage. Successive precipitation of Pb, Zn and Ag occurred with changes of physicochemical conditions. Overall considering mineralization features, ore-forming fluids and materials and tectonic setting and comparing with adjacent deposits, the Bianjiadayuan deposit is a mesothermal magmatic hydrothermal vein-type Pb–Zn–Ag deposit controlled by fractures and related to A2-type granite in response to the tectonic/magmatic/hydrothermal activity in late Jurassic. Besides, the explosive breccias in the west area require more attention in future exploration.     

Numerical heat and fluid flow modeling of the Hercynian Draa Sfar polymetallic (Zn–Pb–Cu) massive sulfide deposit,Central Jbilets,Morocco

Draa Sfar is a polymetallic (Zn–Pb–Cu) volcanogenic massive sulfide deposit with an actual resource of 13 Mt at 4.0% Zn and 1.3% Pb. It is part of the central Jbilets area known for its several Cu–Zn ore deposits. The ore is hosted in the upper Visean-Namurien sedimentary formation. Owing to the complexity of the geology of the ore deposits, numerical simulation approach was attempted to shed light into the temperature distribution, the circulation of the hydrothermal fluid and the genesis of massive sulfide ore bodies by evaluating the permeability, porosity, and thermal conductivity. On the basis of this simulation approach, the ore is predicted to be deposited at a temperature ranging between 230 and 290 °C. This temperature range is dependent on the pre-existing temperature of the discharge area where a metal-rich fluid precipitated the ore. The duration of the Draa Sfar ore body formation is predicted to be 15, 000 to 50, 000 years. Based on geological studies of Draa Sfar deposit together with the aforementioned results of the simulation approach, an ore genetic model for the massive sulfide ore bodies is proposed. In this model, the supply of ore-forming fluids is ensured by the combination of seawater and magmatic waters. Magma that generated rhyodacite dome acted as the heat source that remobilized the circulation of these ore-bearing fluids. The NW-SE trending faults acted as potential pathways for both the downward and upward migration of the ore-forming fluids. Due to their high permeability, the ignimbritic facies, host rocks of Draa Sfar ore bodies, have favored the circulation of the fluids. The mixing between the ore-forming fluids of magmatic origin and the descending seawaters and/or in situ pore waters led to the formation the ore bodies in 35,000 years. The position and size of the ore body, determined by the simulation approach, is consistent with the actual field geological data.     

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.     

Thermobarometry in the Sarvian Fe-skarn deposit (Central Iran) based on garnet–pyroxene chemistry and fluid inclusion studies

The Sarvian Fe skarn deposit is located in the Urumieh–Dokhtar magmatic arc, western Iran. The Sarvian quartz diorite intruded the surrounding Permian to Tertiary limy formation, culminated in thermal metamorphism as well as skarnification in which the Sarvian deposit formed. Microthermometry studies in the Sarvian skarn deposit reveal two distinct inclusion groups; group A with medium–high temperature and hypersaline and group B with low–medium temperature and low salinity. Group A inclusions which are entrapped during formation of prograde are thought to be derived from the magmatic source. Fluid boiling and subsequent developing of hydraulic fracturing led to inflow and/or mixing of early magmatic fluids (group A) with circulating groundwater culminated in formation of low salinity and low temperature fluid inclusions (group B) during the formation of retrograde assemblage. Fluid inclusion thermometry reveals the formation temperature and the salinity of 300–370 °C and 31–33 wt% NaCl for the prograde stage and 180–230 °C and 1–15 wt% NaCl for the retrograde stage of skarnification at Sarvian skarn rocks. Fe-mineralization as well as hydrothermal minerals occurred during retrograde metasomatism. The estimated depth and pressure of occurrence for prograde stage are 1000–1200 m and 100–150 bars, and for retrograde stage, these are about 200 m and 50 bars, respectively. Garnet and pyroxene, as the main constituent minerals of prograde stage, are the most informative minerals offering a suitable tool to constrain the skarnification conditions. Garnets in the Sarvian deposit are mainly grossular and andradite, showing both normal and inverse zoning as the result of variation in their chemical composition. Such types of zoning represent alternation of high acidity oxidation and low acidity oxidation conditions that were prevailed on skarnification in the Sarvian prograde assemblage. Also, chemical composition of the Sarvian pyroxenes shows an alternation of high oxygen fugacity and low oxygen fugacity conditions for their formation. This is also supported by fluctuation of the ratios of andradite to grossular and diopside to hedenbergite, denoting to an obvious shifting that was prevailed between oxidizing and redox conditions during formation of prograde assemblage in the Sarvian. Garnet–pyroxene thermometry determines the formation temperature of prograde assemblage between 370 and 550 °C at Sarvian skarn rocks which is verified also by fluid inclusion thermometry. Decomposition of limestone by reaction of high-temperature hydrothermal fluids with carbonate host rock resulted in injection of CO₂ into the Sarvian system that caused oxidation, changing Fe⁺² to Fe⁺³ culminated in the magnetite deposition.