Primary Fluid Inclusions In Quartz Research Articles

Multiphase-solid fluid inclusions in HP-LT eclogite facies rock (Zavkhan Terrane, Western Mongolia): evidence for the evolution from saline to hypersaline fluids during metamorphism in subduction zone

Fluid inclusions in high- and ultrahigh-pressure metamorphic rocks provide direct information on the composition of the fluids that evolved during metamorphism and fluid-rock interactions in deep subduction zones. We investigate the fluid inclusions in the Khungui eclogite of the Zavkhan Terrane, Central Asian Orogenic Belt. Fluid inclusions are observed in garnet and quartz in the eclogite samples that underwent metamorphism during subduction. The primary fluid inclusions in quartz are composed of liquid and vapor with high salinities (15.7–16.4 wt.% NaCl eq.), whereas the secondary fluid inclusions in quartz are classified as: relatively high salinity (Type I:12.5–16.3 wt.% NaCl eq.) and low salinity (Type II:6.7–10.6 wt.% NaCl eq.). The garnet shows compositional zoning from Ca-poor cores to Ca-rich rims, and the rims that grew during the eclogite-stage metamorphism (2.1–2.2 GPa at 580–610 °C) preferentially contain numerous primary fluid inclusions. The primary fluid inclusions in garnet are commonly bi-phases (liquid and vapor); however, some are multiphase-solid fluid inclusions composed of fluids (liquid and vapor) and combinations of several minerals (halite, quartz, apatite, calcite, biotite, chlorite, and actinolite). Bi-phase fluid inclusions preferentially occur in the inner parts of the Ca-rich garnet rim, whereas multiphase-solid fluid inclusions occur along the margins of the Ca-rich rim. We hypothesize that the multiphase-solid fluid inclusions are formed via interactions between trapped fluids, trapped minerals, and the host garnet during exhumation. By combination of FIB–SEM and synchrotron X-ray CT analyses, the detailed occurrences, volumes, and compositions of the solid phases in the fluid inclusion was analyzed. We then conduct mass balance analysis to reconstruct accurate fluid compositions using data from the FIB–SEM and synchrotron X-ray CT images of the multiphase-solid fluid inclusion. The results of these analyses reveal that (1) fluid changed from an H2O-dominated saline fluid (13–16 wt. % NaCl eq.) at the prograde to the earlier eclogite stage to H2O–CO2-dominated hypersaline fluid at later eclogite stage (~ 32 wt. % NaCl eq., 7.3 wt. % CO2 and ~ 19 molal dissolved cations); (2) a variety of mineral assemblages in multiphase-solid fluid inclusions are produced by post-entrapment reactions between the trapped hypersaline fluid, trapped minerals and the fluid host mineral. The evolution of fluids from saline to hypersaline during the eclogite facies stage is probably caused by the formation of hydrous minerals (i.e., barroisite) under a near-closed system.

Genesis of the world-class Dachang gold deposit, northern Qinghai-Tibet Plateau: A multiproxy approach

Dachang is a world-class gold deposit that occurs in the Bayan Har Terrane, northern Qinghai-Tibet Plateau, with Au resources of 121 t at an average grade of 3.0 g/t. Although this deposit was investigated in some previous studies, its genesis remains debated. The deposit is controlled by the Gande-Maduo anticlinorium and thrust fault zone, and the orebodies occur in subsidiary interlayer faults or fractures in thin-bedded, stratatoid, lenticular and vein forms. The host strata belong to the Lower Triassic Bayan Har Shan Group consisting of metasandstone, metasiltstone, slate, and carbonaceous slate interlayered with phyllite. The mineralization of Dachang can be divided into the following four stages. Stage 1 gold-sulfide-quartz veins, stage 2 gold-sulfide-carbonate-quartz veins, stage 3 stibnite-quartz veins and stage 4 quartz-carbonate veins. There are two types of ores: altered rocks and quartz veins. The main ore minerals are pyrite, arsenopyrite, stibnite and native gold, with trace amounts of tetrahedrite, chalcopyrite, galena and sphalerite. The wallrock alteration comprises mainly sulfidation, silicification, carbonatization, and sericitization. The element association is As, Sb, Au, Te, Bi, Ag, W and Pb in order of enrichment factors, whereas Cu, Mo, and Sn are depleted relative to their crustal abundances.In this study, we identified three types of primary fluid inclusions in quartz from veins of the four mineralization stages as follows: type 1 liquid-dominated two-phase aqueous inclusions; type 2 aqueous-carbonic inclusions, including aqueous-dominated (type 2a) and carbonic-dominated inclusions (type 2b); and type 3 two-phase hydrocarbon inclusions. Microthermometric measurements of fluid inclusion assemblages (FIAs) and bulk fluid inclusion analyses show that the ore-forming fluids had similar low-salinity compositions (1.7 to 6.9 wt% NaCl equiv.) in the H2O-CO2-N2 system with trace amounts of hydrocarbons. The mineralization temperatures range from 150 to 200 °C, and estimated depths are approximately 5 to 6.5 km. Fluid/wallrock reactions and immiscibility or boiling during mineralization in response to pressure fluctuations in a brittle faults were the main triggers for gold precipitation at Dachang.The hydrogen, oxygen, carbon and He-Ar isotopic compositions indicate that the ore-forming fluids originated from metamorphic devolatilization of immediate- and deep-level country rocks and no meteoric water or mantle volatiles were involved. The organic carbon proportions account for 42 to 53% of the total carbon in ore-forming fluid, and carbon fractions from sedimentary carbonates range from 47 to 58%. The gold mineralization followed immediately after the regional metamorphism (240–225 Ma) and occurred when the host rocks had exhumated into a brittle environment. The Dachang deposit is an epizonal orogenic gold deposit.

Fluid Inclusion Studies of Ore and Gangue Minerals from the Piaotang Tungsten Deposit, Southern Jiangxi Province, China

AbstractThe Piaotang deposit is one of the largest vein‐type W‐polymetallic deposits in southern Jiangxi Province, South China. The coexistence of wolframite and cassiterite is an important feature of the deposit. Based on detailed petrographic observations, microthermometry of fluid inclusions in wolframite, cassiterite and intergrown quartz was undertaken. The inclusions in wolframite were observed by infrared microscope, while those in cassiterite and quartz were observed in visible light. The fluid inclusions in wolframite can be divided into two types: aqueous inclusions with a large vapor‐phase proportion and aqueous inclusions with a small vapor‐phase ratio. The homogenization temperature (Th) of inclusions in wolframite with large vapor‐phase ratios ranged from 280°C to 390°C, with salinity ranging from 3.1 to 7.2 wt% NaCl eq. In contrast, the Th values of inclusions with small vapor‐phase ratios ranged from 216°C to 264°C, with salinity values ranging from 3.5 to 9.3 wt% NaCl eq. Th values of primary inclusions in cassiterite ranged from 316°C to 380°C, with salinity ranging from 5.4 to 9.3 wt% NaCl eq. Th values for primary fluid inclusions in quartz ranged from 162°C to 309°C, with salinity values ranging from 1.2 to 6.7 wt% NaCl eq. The results show that the formation conditions of wolframite, cassiterite and intergrown quartz are not uniform. The evolutionary processes of fluids related to these three kinds of minerals are also significantly different. Intergrown quartz cannot provide the depositional conditions of wolframite and cassiterite. The fluids related to tungsten mineralization for the NaCl‐H2O system had a medium‐to‐high temperature and low salinity, while the fluids related to tin mineralization for the NaCl‐H2O system had a high temperature and medium‐to‐low salinity. The results of this study suggest that fluid cooling is the main mechanism for the precipitation of tungsten and tin.

Late Miocene magmatic‐hydrothermal system and related Cu mineralization of the Arakawa area, Akita, Japan

AbstractThe Northeast Japan arc hosts a number of hydrothermal vein‐type copper deposits associated with Neogene felsic intrusions. The Arakawa area is underlain by Cretaceous granites and Tertiary sedimentary rocks, which were intruded by the Miocene Ushizawamata dacite. Zircon grains from the dacite intrusion yield a 206Pb/238U intercept age (laser ablation inductively coupled plasma mass spectrometry) of 8.10 ± 0.30 Ma, consistent with a previously reported K‐Ar illite age (8.1 ± 0.4 Ma) of the Ushizawamata lead and zinc prospect in the Arakawa area. The dacite intrusion and the surrounding Miocene sedimentary rocks were altered by hydrothermal activity on the surface, classified into four alteration zones: (1) biotite‐chlorite, (2) illite, (3) chlorite and (4) smectite, centered on the intrusion. About 20 major vertical sub‐parallel copper‐bearing quartz veins occur in the chlorite alteration zone on the west side of the dacite. The first vein stage is composed of chalcopyrite and chamosite with a minor amount of quartz in brecciated wall rocks, and the second‐stage is characterized by the presence of hematite in addition to the first‐stage mineral assemblage. The third‐stage consists of comb‐shaped quartz veins with a minor amount of chalcopyrite, sphalerite, galena and pyrite, and the fourth‐stage of barite and apatite present in druse in the third‐stage veins. Primary fluid inclusions in quartz of the first‐ and third‐stages are all liquid‐rich and two‐phase. Homogenization temperature and salinity of first‐stage quartz are 263–277°C and 5.7–7.5 wt% NaCl equivalent (eq.); in quartz of the third‐stage, 251–270°C and 2.7–4.2 wt% in the inner zone and 207–250°C and 2.7–3.7 wt% in comb‐shaped quartz on the vein margin. The fluid inclusions in quartz phenocrysts of the Ushizawamata dacite show two distinct assemblages, halite‐bearing polyphase inclusions that coexist with vapor‐phase inclusions and/or vapor‐rich two‐phase inclusions, and liquid‐rich two‐phase inclusions. Homogenization temperature and salinity of the polyphase inclusions are higher than 401°C and 46.7 wt%, respectively, and those of the vapor‐rich two‐phase inclusions report 393–419°C and 2.6–3.7 wt% NaCl, whereas the liquid‐rich two‐phase inclusions returned 344–403°C and 8.0–9.3 wt%, respectively. These results indicate that the ore forming fluid was slightly cooler and lower in salinity than the late‐stage hydrothermal fluid in the Ushizawamata intrusion. The spatial and temporal proximity between the Ushizawamata dacite and the hydrothermal veins indicates that the dacitic magma was genetically related to the vein copper mineralization in the Arakawa area.

Progressively released gases from fluid inclusions reveal new insights on W-Sn mineralization of the Yaogangxian tungsten deposit, South China

The gentle stepwise crushing technique permits progressive extraction of gases from large, irregular, secondary fluid inclusions (SFIs) along micro-cracks to small, regular, liquid-rich primary fluid inclusions (PFIs). Previous studies have verified this general gas release pattern in different hydrothermal minerals based on the agreements between 40Ar/39Ar ages of the released gases from different crushing stages and the most likely corresponding geological events dated with other methods; and based on the excellent agreements between gas chemistry measured by QMS (quadrupole mass spectrometer) during stepwise crushing and the expected characteristics of SFIs and PFIs in the same samples acquired using Raman spectroscopy. In this study, we applied this QMS-based gentle stepwise crushing technique on ore and gangue minerals from the world-class Yaogangxian tungsten deposit in South China. Gases released from PFIs in cassiterite, wolframite, and quartz from ore-bearing veins show high CH4, N2, C3H8, and low CO2, C4H10 contents, low CO2/CH4 (≪1) and high N2/Ar (327–861 for ore minerals and 60.7–943 for quartzs from ore veins) ratios, along with multiple organic gaseous species. Their compositions resemble those of gases released from PFIs in quartzs from the Yaogangxian granites which represent magmatic-hydrothermal fluids that exsolved from the granitic magma. However, compared with these exsolved magmatic-hydrothermal fluids, ore mineral PFIs have relatively high He contents which indicate contributions from non-magmatic crustal fluids. This is consistent with high Ca2+, low F− concentrations, and low K+/Na+ (0.21–0.32) ratios observed in fluids in the quartz samples, as well as their relatively low water hydrogen isotopes (δD-H2O = −102 to −59‰). These chemical characteristics, coupled with their magmatic sulfur (δ34S-molybdenite and arsenopyrite = −0.2–1.4‰), methane hydrogen (δD-CH4 = −58 to −42‰), and fluid oxygen (δ18O-H2O = 3.9–5.1‰) isotopic compositions, indicate both magmatic and non-magmatic crustal components in the mineralizing fluids. Based on these observations and the low methane carbon isotope values (δ13C-CH4) in the studied quartz samples, we propose that fluid-rock interactions between the exsolved magmatic-hydrothermal fluids and the metasedimentary wall rocks played an important role in promoting W-Sn precipitation in this ore deposit. This study demonstrates the power of combining QMS-based stepwise crushing technique with traditional geochemical techniques in investigating ore-forming processes.

Ore Geology, Fluid Inclusions, and (H-O-S-Pb) Isotope Geochemistry of the Sediment-Hosted Antimony Mineralization, Lyhamyar Sb Deposit, Southern Shan Plateau, Eastern Myanmar: Implications for Ore Genesis

The Lyhamyar deposit is a large Sb deposit in the Southern Shan Plateau, Eastern Myanmar. The deposit is located in the Early Silurian Linwe Formation, occurring as syntectonic quartz-stibnite veins. The ore body forms an irregular staircase shape, probably related to steep faulting. Based on the mineral assemblages and cross-cutting relationships, the deposit shows two mineralization stages: (1) the pre-ore sedimentary and diagenetic stage, and (2) the main-ore hydrothermal ore-forming stage (including stages I, II, and III), i.e., (i) early-ore stage (stage I) Quartz-Stibnite, (ii) late-ore stage (stage II) Quartz-calcite-Stibnite ± Pyrite, and (iii) post-ore stage (stage III) carbonate. The ore-forming fluid homogenization temperatures from the study of primary fluid inclusions in quartz and calcite indicate that the ore-forming fluid was of a low temperature (143.8–260.4 °C) and moderate to high-salinity (2.9–20.9 wt. % NaCl equivalent). Hydrogen and oxygen isotopes suggest that the ore-forming fluids of the Lyhamyar deposit were derived from circulating meteoric water mixed with magmatic fluids that underwent isotopic exchange with the surrounding rocks. Sulfur in Lyhamyar was dominated by thermochemical sulfate reduction (TSR) with dominant magmatic source sulfur. The lead isotope compositions of the stibnite indicate that the lead from the ore-forming metals was from the upper crustal lead reservoir and orogenic lead reservoir. On the basis of the integrated geological setting, ore geology, fluid inclusions, (H-O-S-Pb) isotope data, and previous literature, we propose a new ore-deposit model for the Lyhamyar Sb deposit: It was involved in an early deposition of pyrite in sedimentary and diagenetic stages and later Sb mineralization by mixing of circulating meteoric water with ascending magmatic fluids during the hydrothermal mineralization stage.

Mineralogy of the Tumanny Au-Ag-Te-Hg epithermal veins, Western Chukchi Peninsula, Russia

The Tumanny Au-Ag-Te-Hg epithermal prospect, Chukchi Peninsula, Russia is located in the near contact zone of the large Vuknei pluton, part of the Early Cretaceous Egdykgych Complex, which is related to the Peschanka porphyry copper deposit and the Nakhodka porphyry-epithermal ore district located 45 km north of Tumanny. Mineralization at Tumanny is characterized by an early carbonate-base metal and a late gold-silver-sulfosalt mineral assemblage. Pyrite of the first assemblage is unzoned and As-poor, and tennantite-tetrahedrite solid solutions (Ttrs) are characterized by a weak oscillatory zoning caused by variable contents of Sb and As, low Ag content, and the widely variable Sb/(Sb + As) values. Pyrite of the second assemblage is zoned and As-rich (up to 5.2 wt%) and Ttrs is weakly zoned and Ag-rich (up to 10.35 apfu) and has a comparatively narrow range of Sb/(Sb + As) value. Electrum, Au-Ag alloy (“küstelite”), Au-Ag-S phases, hessite, polybasite, and imiterite recognized at the prospect belong to the second mineral assemblage. The homogenization temperature of primary fluid inclusions in quartz ranges from 340 to 210 °C and decreased during the mineralizing process; fluid salinity varying from 0.8 to 5.2 wt% NaCl equiv is not evolved during the process. Cores of quartz crystals formed from a boiling fluid, whereas crystal rims appear to have formed from a lower temperature non-boiling liquid. LogfS2 decreased from −8 at the formation of carbonate-base metal assemblage to −16 at the deposition of gold-silver-sulfosalt assemblage as temperature decreased from 340 to 200 °C. Bulk ICP-OES analysis of mineralized samples indicated up to: 38 ppm Au, 908 ppm Ag, 154 ppm Mo, 106 ppm Te, 53 ppm Se, 9246 ppm As, 1284 ppm Sb, and 23 ppm Bi. Cluster analysis of the data obtained identified three geochemical associations corresponding the carbonate-base metal the gold-silver-sulfosalt, and to the overlapped mineral assemblages. The whole-ore composition is controlled by mineralogy of these assemblages. Taking into account the geological setting, mineralogy, composition of minerals, and fluid inclusion data, the Tumanny Au-Ag-Te-Hg prospect is characterized as intermediate sulfidation epithermal style.

Geology and ore genesis of the late Paleozoic Heijianshan Fe oxide–Cu (–Au) deposit in the Eastern Tianshan, NW China

The Heijianshan Fe–Cu (–Au) deposit, located in the Aqishan-Yamansu belt of the Eastern Tianshan (NW China), is hosted in the mafic–intermediate volcanic and mafic–felsic volcaniclastic rocks of the Upper Carboniferous Matoutan Formation. Based on the pervasive alteration, mineral assemblages and crosscutting relationships of veins, six magmatic–hydrothermal stages have been established, including epidote alteration (Stage I), magnetite mineralization (Stage II), pyrite alteration (Stage III), Cu (–Au) mineralization (Stage IV), late veins (Stage V) and supergene alteration (Stage VI). The Stage I epidote–calcite–tourmaline–sericite alteration assemblage indicates a pre-mineralization Ca–Mg alteration event. Stage II Fe and Stage IV Cu (–Au) mineralization stages at Heijianshan can be clearly distinguished from alteration, mineral assemblages, and nature and sources of ore-forming fluids.Homogenization temperatures of primary fluid inclusions in quartz and calcite from Stage I (189–370 °C), II (301–536 °C), III (119–262 °C) and V (46–198 °C) suggest that fluid incursion and mixing probably occurred during Stage I to II and Stage V, respectively. The Stage II magmatic–hydrothermal-derived Fe mineralization fluids were characterized by high temperature (>300 °C), medium–high salinity (21.2–56.0 wt% NaCl equiv.) and being Na–Ca–Mg–Fe-dominated. These fluids were overprinted by the external low temperature (<300 °C), medium–high salinity (19.0–34.7 wt% NaCl equiv.) and Ca–Mg-dominated basinal brines that were responsible for the subsequent pyrite alteration and Cu (–Au) mineralization, as supported by quartz CL images and H–O isotopes. Furthermore, in-situ sulfur isotopes also indicate that the sulfur sources vary in different stages, viz., Stage II (magmatic–hydrothermal), III (basinal brine-related) and IV (magmatic–hydrothermal). Stage II disseminated pyrite has δ34Sfluid values of 1.7–4.3‰, comparable with sulfur from magmatic reservoirs. δ34Sfluid values (24.3–29.3‰) of Stage III Type A pyrite (coexists with hematite) probably indicate external basinal brine involvement, consistent with the analytical results of fluid inclusions. With the basinal brines further interacting with volcanic/volcaniclastic rocks of the Carboniferous Matoutan Formation, Stage III Type B pyrite–chalcopyrite–pyrrhotite assemblage (with low δ34Sfluid values of 4.6–10.0‰) may have formed at low fO2 and temperature (119–262 °C). The continuous basinal brine–volcanic/volcaniclastic rock interactions during the basin inversion (∼325–300 Ma) may have leached sulfur and copper from the rocks, yielding magmatic-like δ34Sfluid values (1.5–4.1‰). Such fluids may have altered pyrite and precipitated chalcopyrite with minor Au in Stage IV. Eventually, the Stage V low temperature (∼160 °C) and low salinity meteoric water may have percolated into the ore-forming fluid system and formed late-hydrothermal veins.The similar alteration and mineralization paragenetic sequences, ore-forming fluid sources and evolution, and tectonic settings of the Heijianshan deposit to the Mesozoic Central Andean IOCG deposits indicate that the former is probably the first identified Paleozoic IOCG-like deposit in the Central Asian Orogenic Belt.

Re-Os and U-Pb Geochronology of the Doña Amanda and Cerro Kiosko Deposits, Bayaguana District, Dominican Republic: Looking Down for the Porphyry Cu-Mo Roots of the Pueblo Viejo-Type Mineralization in the Island-Arc Tholeiitic Series of the Caribbean

Hosted in the Early Cretaceous bimodal tholeiite volcanic series of the Los Ranchos Formation, the Dona Amanda and Cerro Kiosko deposits in the Bayaguana district represent significant Au, Cu, and Ag resources in the Cordillera Oriental of the Dominican Republic. At Dona Amanda, a dense stockwork of quartz-sulfide veins is hosted by volcanic rocks with intense transitional phyllic-advanced argillic and silicic hydrothermal alteration assemblages, indicating a high-sulfidation environment. Wavy quartz veins with central sutures and rims of pyrite + enargite + molybdenite + fahlore (B veins) are cut by planar quartz-pyrite D veins. Primary fluid inclusions in quartz from B veins (Th: 160°–>400°C; salinity: 7.9–16.4 wt % NaCl equiv) are interpreted as porphyry-type fluids. Inclusion fluids in quartz of quartz-pyrite veins (Th: 125°–175°C; salinity: 4.8–12.2 wt % NaCl equiv), quartz from silicic altered wall rocks (Th: 150°–175°C; salinity: 8.3–13.9 wt % NaCl equiv), and late, distal calcite veins (Th: 120°–160°C; salinity: 5.0–13.3 wt % NaCl equiv) indicate limited mixing with more dilute fluids and rule out mixing with fresh meteoric water. In Cerro Kiosko, a swarm of fault-controlled massive chalcopyrite + enargite + bornite + fahlore D veins and lodes are hosted by rocks with pervasive kaolinite alteration after sericite. δ 34S values of vein sulfides from both deposits are all close to −2‰ and consistent with a predominance of magmatic sulfur and sulfide deposition from an oxidizing magmatic fluid. These data are consistent with a transitional environment between a deeper porphyry Cu(-Mo) and an overlying high-sulfidation epithermal deposit. An Re-Os age (112.6 ± 0.4 Ma) for molybdenite from the Dona Amanda deposit places the porphyry-epithermal mineralization as Early Cretaceous, coeval with the Los Ranchos Formation host rocks and with the Pueblo Viejo deposit. New sensitive high-resolution ion microprobe U-Pb ages on zircons from plagioclase-phyric rhyolite domes in the Bayaguana district are consistent with porphyry–high-sulfidation epithermal mineralization occurring along the Los Ranchos Formation during tonalite batholith emplacement in the basaltic island-arc basement at ca. 118 to 112 Ma and finalization of felsic volcanism at ca. 110 to 107 Ma.

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 0.45million tons of proven ore reserves with grades of 1.03wt.% Cu and 1.83wt.% Zn. This deposit is composed of stratiform, massive sulfide ore bodies, which contain more than 60vol.% 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 5.7wt.% 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 −505m 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.

Crustal-Extension Ag-Pb-Zn Veins in the Xiong'ershan District, Southern North China Craton: Constraints from the Shagou Deposit

The Shagou vein-type Ag-Pb-Zn deposit in the Xiong’ershan district, southern margin of the North China craton, is hosted within amphibolite facies metamorphic rocks of the Late Archean to early Paleoproterozoic Taihua Group. The Ag-Pb-Zn veins are localized in NE- to NNE-trending brittle faults and typically display symmetrical zoning consisting of siderite, quartz + sphalerite, galena, and quartz + calcite from the margin toward the center of each vein. Ore-related hydrothermal alteration is well developed on both sides of the veins, dominated by silicification, sericitization, chloritization, and carbonatization. Sericite separates extracted from a major Ag-Pb-Zn vein yield a 40 Ar/ 39 Ar plateau age of 140.0 ± 1.0 Ma (1 σ ) and isochron age of 141.1 ± 1.6 Ma (1 σ ), indicating that mineralization occurred at the beginning of Early Cretaceous. Field and textural relationships indicate four hydrothermal stages marked by assemblages of quartz + siderite (stage I), quartz + sphalerite + ankerite (stage II), quartz + galena + silver minerals + ankerite (stage III), and quartz + calcite (stage IV), respectively. Silver minerals are abundant in all veins and are composed of, in paragenetic order, argentiferous tetrahedrite, polybasite, jalpaite, argentite, and native silver. These silver minerals commonly occur as replacements of galena, chalcopyrite, and other sulfides, or as fillings of microfractures in sulfides and quartz. Microthermometric measurements of primary fluid inclusions in quartz, carbonates, and sphalerite from various hydrothermal stages indicate that ore minerals were deposited at intermediate temperatures (267°–157°C) from aqueous-carbonic to aqueous fluids with moderate salinities (7.2–15.9 wt % NaCl equiv). Coexisting galena-sphalerite pair yields sulfur isotope equilibrium temperatures of 205° to 267°C, consistent with the overall homogenization temperatures of fluid inclusions. The microthermometric data also indicate that both fluid mixing and fluid-rock interaction were important mechanisms for ore precipitation. Carbonate minerals (siderite, ankerite, calcite) spanning the entire mineralization history have δ 13 C V-PDB values of −5.2 to −1.4‰ and δ 18 O V-SMOW of 10.9 to 15.0‰, corresponding to calculated values for the ore fluids of −6.5 to −1.8‰ and 1.4 to 5.4‰, respectively. δ 34 S V-CDT values of sulfide minerals (pyrite, sphalerite, galena) range from 1.1 to 5.5‰, consistent with a deep-seated sulfur source. Galena separates have 206 Pb/ 204 Pb ratios of 17.472 to 17.813, 207 Pb/ 204 Pb ratios of 15.411 to 15.498, and 208 Pb/ 204 Pb ratios of 38.178 to 38.506. The isotope data, together with geological and geochronological evidence, favor a primary metamorphic source for sulfur and other components in the ore fluids. A synthesis of available data suggests that the Shagou deposit is a typical vein-type Ag-Pb-Zn deposit that formed under an extensional geodynamic setting associated with tectonic reactivation of the North China craton during the late Mesozoic, a time period that is manifested by pervasive magmatism, widespread formation of metamorphic core complexes, and development of faulted basins throughout the eastern part of the craton. Metamorphic devolatilization of the Meso-Neoproterozoic marine sedimentary rocks previously subducted beneath the Xiong’ershan district, facilitated by extensive magmatism and elevated heat flow due to lithospheric extension, could have provided large amounts of ore fluids responsible for the Ag-Pb-Zn mineralization. The NE- to NNE-trending faults affiliated with the transcrustal Machaoying fault may have acted as pathways for the upward migration of deep-seated metamorphic fluids. Mixing of the metamorphically derived fluids with meteoric waters ultimately resulted in deposition of the Ag-Pb-Zn veins in brittle extensional structures.

Subepithermal Au-Pd Mineralization Associated with an Alkalic Porphyry Cu-Au Deposit, Mount Milligan, Quesnel Terrane, British Columbia, Canada

At the Mount Milligan Cu-Au porphyry deposit, Quesnel terrane, British Columbia, Canada, barren and weakly mineralized, late-stage hydrothermal veins occur in volcanic rocks adjacent to zones of Cu-Au porphyry mineralization, and have overprinted the porphyry-stage veins. The earliest of the late-stage hydrothermal veins are barren and consist of quartz ± pyrite ± carbonate ± chlorite ± tourmaline. These veins are similar to “transitional” to late-stage hydrothermal veins in other alkaline porphyry Cu-Au deposits, and we consider these to be the equivalent of transitional (post-porphyry, pre-epithermal) quartz-sericite-pyrite veins in calc-alkaline porphyry environments. A later generation of volumetrically minor, mineralized veins are composed of pyrite (Hg- and As-bearing) ± quartz ± carbonate ± chlorite and contain early electrum, arsenopyrite, tetra-hedrite-tennantite, platinum-group element (PGE) tellurides, galena, sphalerite, barite, and chalcopyrite as inclusions in pyrite, and a later assemblage of electrum, PGE tellurides, arsenides and antimonides, galena, sphalerite, chalcopyrite, and various Au-Ag-Te-Bi minerals in annealed fractures and open-space infillings in quartz and pyrite. Metal precipitation in these veins was temporally and spatially associated with the deposition and later recrystallization of pyrite. Primary fluid inclusions in quartz in the barren and weakly mineralized veins are two-phase (L+V), homogenize to liquid over a narrow range in T (~170°–270°C; n = 96, 12 veins), and show a wide range in salinity (4.2 wt % NaCl equiv to 28.7 wt % CaCl2 equiv) when all samples are considered. However, individual veins show narrow ranges in salinity and homogenization temperature. LA-ICP-MS analyses indicate that the fluids were highly enriched in As (to 2,260 ppm), Sb (to 230 ppm), B (to 5,400 ppm), Au (~1–2 ppm) and Pd (~0.5–1 ppm) but depleted in Cu ( 80 ppm) compared to typical porphyry-stage fluids. Metal ratios in the fluids overlap with bulk rock metal ratios in the mineralized veins. The inclusions are interpreted to contain a contracted magmatic vapor (produced by boiling) that lost Cu during the formation of porphyry stage veins at depth. Fluids show decreasing B, As, Sb, and increasing Sr, Ca, and salinity with time. Stable C, O, and H isotope analyses of vein minerals indicate that mixing of this magmatic fluid with meteoric water was not responsible for metal deposition. Rather, metal precipitation was possibly the result of mixing of the magmatic-derived fluid with a heated saline groundwater. The precious and accessory metal mineralogy of the hydrothermal veins is similar to that found in low- to intermediate-sulfidation epithermal systems. Fluid inclusion microthermometry and chlorite thermometry constrain the approximate formation conditions of the veins between ~200 and 1,500 bars and ~240° and 280°C. After the formation of the mineralized veins, circulation of low salinity, metal-depleted fluids occurred. These latest stage fluids may have formed by mixing of the saline magmatic fluid-groundwater hybrid with meteoric water. The results of this study suggest a genetic link between porphyry-stage events and the deposition of Au and PGE in late-stage veins in an alkalic igneous environment. Recognition of hydrothermal processes involving the transport of Au-PGE-As-Sb-Bi-Te-B-rich fluids in the “subepithermal” regimes implies that low-sulfidation epithermal Au deposits may have been present in the shallower parts of the magmatic-hydrothermal complex and that there is potential for the discovery of PGE-rich epithermal veins in less deeply exhumed terranes. On the other hand, the formation of high-grade, low-sulfidation epithermal Au-PGE deposits may be prohibited if porphyry-epithermal transitional fluids precipitate ore metals through mixing with groundwater prior to reaching the level where meteoric water mixing and epithermal boiling normally occur.

Ore Geology, Fluid Inclusion, and S‐ and Pb‐Isotopic Constraints on the Genesis of the Chitudian Zn‐Pb Deposit, Southern Margin of the North China Craton

AbstractThe Chitudian Zn‐Pb ore deposit, Luanchuan, Henan province, was recently discovered in the southern margin of the North China Craton. The Zn‐Pb orebodies are hosted in the Proterozoic Guandaokou and Luanchuan Groups, occurring as veins in interbedding fracture zones mainly in a WNW‐ and partially in a NS‐direction. The Zn‐Pb ores are characterized by banded, massive, and breccia structures, coarse crystal grains, and a simple mineral composition mainly of galena, sphalerite, pyrite, quartz, dolomite, and calcite. In addition to the vein type orebodies, there are Mo‐ and Zn‐bearing skarn orebodies in the northwest of the Chitudian ore field. Four types of primary fluid inclusions in quartz and calcite were recognized in the Chitudian Zn‐Pb ores, including aqueous, aqueous‐CO2, daughter‐mineral‐bearing aqueous, and daughter‐mineral‐bearing aqueous‐CO2 inclusions, with aqueous inclusion being most common. The homogenization temperatures of the fluid inclusions from the main mineralization stage are from 290°C to 340°C, and the salinities mainly from 3.7 to 14.8 wt% NaCl equivalent. In addition to CO2, CH4 and H2S were detected in the vapor phase and HS‐ in the liquid phase of the fluid inclusions by Laser Raman spectroscopy. The δ34SV‐CDT values of ore sulfides from the Chitudian deposit range from −0.32‰ to 8.30‰, and show two modal peaks in the histogram, one from 1‰ to 4‰, and the other from 5‰ to 7‰. The former peak is similar to that of porphyry‐type Mo‐W deposits in the area, whereas the latter is relatively close to the sulfur in the strata. The ore sulfur may have been derived from both the magma and the strata. The Pb‐isotopic compositions of the ore minerals from Chitudian, with 206Pb/204Pb from 17.005 to l7.953, 207Pb/204Pb from 15.414 to 15.587, and 208Pb/204Pb from 37.948 to 39.036, are similar to those of Mesozoic porphyries in the Chitudian ore field, suggesting that the ore‐forming metals were mainly derived from the Mesozoic magmatic intrusions. The Chitudian Zn‐Pb deposit is interpreted to be a distal hydrothermal vein‐type deposit, which was genetically related to the proximal, skarn‐type Mo ore deposits in the region.

Ag‐rich Tetrahedrite in the El Zancudo Deposit, Colombia: Occurrence, Chemical Compositions and Genetic Temperatures

AbstractChemical compositions of tetrahedrite—Ag‐rich tetrahedrite—freibergite solid solutions (Ag‐rich tetrahedritess) and homogenization temperatures of fluid inclusions in quartz and carbonates of seventeen samples from nine veins in the El Zancudo deposit, Antioquia, Colombia, were investigated to reveal the origin of silver in Ag‐rich tetrahedritess, to derive their crystallization temperatures and to examine the relationship between chemical compositions of Ag‐rich tetrahedritess and their crystallization temperatures. The ores consist of arsenopyrite, pyrite, sphalerite, Ag‐rich tetrahedritess, galena, boulangerite, andorite, owyheeite, diaphorite, jamesonite, miargyrite, bournonite, chalcopyrite, and electrum. Ag‐rich tetrahedritess forms about 10 volume % of the total ores and is one of the most common and widely distributed sulfosalts in this deposit. Ag‐rich tetrahedritess is rich in Ag (1.13 to 31.02 wt%) and Sb (22.93 to 29.82 wt%), and poor in As (0.06 to 2.43 wt%), consistent with the reported incompatibilities of Ag and As in Ag‐rich tetrahedritess. The Zn/(Zn + Fe)‐, Ag/(Ag + Cu)‐ and Sb/(Sb + As + Bi)‐atomic ratios exhibit some variations among the veins. Ag‐rich tetrahedritess with higher Ag/(Ag + Cu) ratios coexist with diaphorite, whereas those with lower ratios are not associated with this sulfosalt. Ag‐rich tetrahedritess in the assemblages of Ag‐rich tetrahedritess+ sphalerite and of Ag‐rich tetrahedritess+ bournonite + galena shows no Zn ↔ Fe and Cu ↔ Ag variations between core and rim, respectively, negating the possibility of solid state reaction during cooling. Ag‐rich tetrahedritess is thus regarded as primary phase. Homogenization temperatures of primary fluid inclusions in quartz and carbonates co‐existing with Ag‐rich tetrahedritess define the mineralization temperatures of 134 to 263°C. Independent crystallization temperatures of Ag‐rich tetrahedrite estimated based on Zn/(Zn + Fe) and Ag/(Ag + Cu) ratios of the Ag‐rich tetrahedritess associated with silver minerals such as miargyrite, andorite and diaphorite using Sack's thermochemical database lie in a range between 170 and ∼250°C. Both results are thus in good agreement.

Silver‐Bearing and Associated Minerals in El Zancudo Deposit, Antioquia, Colombia

AbstractThe occurrence and the chemical compositions of ore minerals (especially the silver‐bearing minerals) and fluid inclusions of the El Zancudo mine in Colombia were investigated in order to analyze the genetic processes of the ore minerals and to examine the genesis of the deposit. The El Zancudo mine is a silver–gold deposit located in the western flank of the Central Cordillera in Antioquia Department. It consists mainly of banded ore veins hosted in greenschist and lesser disseminated ore in porphyritic rocks. The ore deposit is associated with extensive hydrothermally altered zones. The ores from the banded veins contain sphalerite, pyrite, arsenopyrite, galena, Ag‐bearing sulfosalts, Pb‐Sb sulfosalts, and minor chalcopyrite, electrum, and native silver. Electrum is included within sphalerite, pyrite, and arsenopyrite, and is also partially surrounded by pyrite, arsenopyrite, sphalerite, and tetrahedrite. Native silver is present in minor amounts as small grains in contact with Ag‐rich sulfosalts. Silver‐bearing sulfosalts are argentian tetrahedrite–freibergite solid solution, andorite, miargyrite, diaphorite, and owyheeite. Pb‐Sb sulfosalts are bournonite, jamesonite, and boulangerite. Two main crystallization stages are recognized, based on textural relations and mineral assemblages. The first‐stage assemblage includes sphalerite, pyrite, arsenopyrite, galena and electrum. The second stage is divided into two sub‐stages. The first sub‐stage commenced with the deposition and growth of sphalerite, pyrite, and arsenopyrite. These minerals are characterized by compositional growth banding, and seem to have crystallized continuously until the end of the second sub‐stage. Tetrahedrite, Pb‐Cu sulfosalts, Ag‐Sb sulfosalt, and Pb‐Ag‐Sb sulfosalts crystallized from the final part of the first sub‐stage and during the whole second sub‐stage. However, one Pb‐Ag‐Sb sulfosalt, diaphorite, was formed by a retrograde reaction between galena and miargyrite. The minimum and maximum genetic temperatures estimated from the FeS content of sphalerite coexisting with pyrite and the silver content of electrum are 300°C and 420°C, respectively. These estimated genetic temperatures are similar to, but slightly higher than the homogenization temperatures (235–350°C) of primary fluid inclusions in quartz. The presence of muscovite in the altered host rocks and gangue suggest that the pH of the hydrothermal solutions was close to neutral. Most of the sulfosalts in this deposit have previously been attributed as the products of epithermal mineralization. However, El Zancudo can be classified as a xenothermal deposit, in view of the low pressure and high temperature genetic conditions identified in the present study, based on the mineralogy of sulfosalts and the homogenization temperatures of the fluid inclusions.

Hydrothermal Alteration and Mineralization of Middle Jurassic Dexing Porphyry Cu‐Mo Deposit, Southeast China

AbstractThe Dexing deposit is located in a NE‐trending magmatic belt along the southeastern margin of the Yangtze Craton. It is the largest porphyry copper deposit in China, consisting of three porphyry copper orebodies of Zhushahong, Tongchang and Fujiawu from northwest to southeast. It contains 1168 Mt of ores with 0.5% Cu and 0.01% Mo. The Dexing deposit is hosted by Middle Jurassic granodiorite porphyries and pelitic schist of Proterozoic age. The Tongchang granodiorite porphyry has a medium K cal‐alkaline series, with medium K2O content (1.94–2.07 wt%), and low K2O/(Na2O + K2O) (0.33–0.84) ratios. They have high large‐ion lithophile elements, high light rare‐earth elements, and low high‐field‐strength elements. The hydrothermal alteration at Tongchang is divided into four alteration mineral assemblages and related vein systems. They are early K‐feldspar alteration and A vein; transitional (chlorite + illite) alteration and B vein; late phyllic (quartz + muscovite) alteration and D vein; and latest carbonate, sulfate and oxide alteration and hematite veins. Primary fluid inclusions in quartz from phyllic alteration assemblage include liquid‐rich (type 1), vapor‐rich (type 2) and halite‐bearing ones (type 3). These provide trapping pressures of 20–400 ´ 105 Pa of fluids responsible for the formation of D veins. Igneous biotite from least altered granochiorite porphyry and hydrothermal muscovite in mineralized granodiorite porphyry possess δ18O and δD values of 4.6‰ and −87‰ for biotite and 7.1–8.9‰, −71 to −73‰ for muscovite. Stable isotopic composition of the hydrothermal water suggests a magmatic origin. The carbon and oxygen isotope for hydrothermal calcite are −4.8 to −6.2‰ and 6.8–18.8‰, respectively. The δ34S of pyrite in quartz vein ranges from −0.1 to 3‰, whereas δ34S for chalcopyrite in calcite veins ranges from 4 to 5‰. These are similar to the results of previous studies, and suggest a magmatic origin for sulfur. Results from alteration assemblages and vein system observation, as well as geochemical, fluid inclusion, stable isotope studies indicate that the involvement of hydrothermal fluids exsolved from a crystallizing melt are responsible for the formation of Tongchang porphyry Cu‐Mo orebodies in Dexing porphyry deposit.

Dawsonite: An inclusion mineral in quartz from the Tin Mountain pegmatite, Black Hills, South Dakota

Dawsonite -NaAl(CO 3 )(OH) 2 -was identified in primary fluid inclusions in quartz from the Li-rich Tin Mountain pegmatite, Black Hills, South Dakota, by petrography, SEM-EDS analysis, and Raman spectroscopy. This is the first report of dawsonite as an inclusion mineral in a pegmatite. The presence of dawsonite in the inclusions is evidence for the existence of carbonate ions in the complex pegmatite melt and/or exsolved magmatic fluid. The lack of dawsonite as a macroscopic mineral is attributed to its high solubility in the late pegmatite fluids and to the small fraction of carbonate ions in the melt. However, its common occurrence, along with other carbonate and borate minerals in fluid inclusions, suggests that carbonate and borate complexes play an important role in petrogenesis of pegmatites..

Evolution of fluids associated with metasedimentary sequences from Chaves (North Portugal)

In order to identify and characterise fluids associated with metamorphic rocks from the Chaves region (North Portugal), fluid inclusions were studied in quartz veinlets, concordant with the main foliation, in graphitic-rich and nongraphitic-rich lithologies from areas with distinct metamorphic grade. The study indicates multiple fluid circulation events with a variety of compositions, broadly within the C–H–O–N–salt system. Primary fluid inclusions in quartz contain low salinity aqueous–carbonic, H 2O–CH 4–N 2–NaCl fluids that were trapped near the peak of regional metamorphism, which occurred during or immediately after D2. The calculated P– T conditions for the western area of Chaves (CW) is P=300–350 MPa and T∼500 °C, and for the eastern area (CE), P=200–250 MPa and T=400–450 °C. A first generation of secondary fluid inclusions is restricted to discrete cracks at the grain boundaries of quartz and consists of low salinity aqueous–carbonic, H 2O–CO 2–CH 4–N 2–NaCl fluids. P– T conditions from the fluid inclusions indicate that they were trapped during a thermal event, probably related with the emplacement of the two-mica granites. A second generation of secondary inclusions occurs in intergranular fractures and is characterised by two types of aqueous inclusions. One type is a low salinity, H 2O–NaCl fluid and the second consists of a high salinity, H 2O–NaCl–CaCl 2 fluid. These fluid inclusions are not related to the metamorphic process and have been trapped after D3 at relatively low P (hydrostatic)– T conditions ( P<100 MPa and T<300 °C). Both the early H 2O–CH 4–N 2–NaCl fluids in quartz from the graphitic-rich lithologies and the later H 2O–CO 2–CH 4–N 2–NaCl carbonic fluid in quartz from graphitic-rich and nongraphitic-rich lithologies seem to have a common origin and evolution. They have low salinity, probably resulting from connate waters that were diluted by the water released from mineral dehydration during metamorphism. Their main component is water, but the early H 2O–CH 4–N 2–NaCl fluids are enriched in CH 4 due to interaction with the C-rich host rocks. From the early H 2O–CH 4–N 2–NaCl to the later aqueous–carbonic H 2O–CO 2–CH 4–N 2–NaCl fluids, there is an enrichment in CO 2 that is more significant for the fluids associated with nongraphitic-rich lithologies. The aqueous–carbonic fluids, enriched in H 2O and CH 4, are primarily associated with graphitic-rich lithologies. However, the aqueous–carbonic CO 2-rich fluids were found in both graphitic and nongraphitic-rich units from both the CW and CE studied areas, which are of medium and low metamorphic grade, respectively.

The evolution of aqueous–carbonic fluids in the Amba Dongar carbonatite, India: implications for fenitisation

Primary and pseudosecondary fluid inclusions hosted by apatite in calciocarbonatite at Amba Dongar, India, and primary fluid inclusions in quartz of the surrounding fenitised sandstones provide an important opportunity to reconstruct the evolution of fluids in a carbonatite setting and determine the composition of the fenitising fluids. Five types of fluid inclusions are present in apatite, namely aqueous liquid–vapour (LV), two-phase aqueous–carbonic (VL), three phase aqueous–carbonic (LLV), one or two solid-bearing aqueous (LVS), and multisolid-bearing aqueous (LVMS) fluid inclusions. Electron microprobe analyses of the solids in opened inclusions suggest that they comprise, calcite, strontianite, ankerite, arcanite (K 2SO 4), albite, K-feldspar and, possibly, pyrite, and hematite. Fluid inclusions in calcite of the calciocarbonatite are either secondary or of indeterminate origin and comprise LV, VL, and LLV types. Quartz-hosted inclusions are either secondary in detrital grains or primary, where they occur in overgrowths, and are mainly of the LV type; small proportions of VL and LLV inclusions are also present. Inclusions were rarely observed in K-feldspar (fenite), and where present are all secondary and similar to those in quartz. LV, LVS, and LVMS inclusions in apatite homogenise at temperatures from 142 to 342, 406 to 594 and 422 to 628 °C, respectively, while LV inclusions in calcite, quartz (primary), and K-feldspar homogenise at temperatures of 120 to 192, 122 to 262 and 183 to 228 °C, respectively; homogenisation temperatures of quartz-hosted inclusions increase with decreasing proximity to the carbonatite, i.e., with increasing intensity of fenitisation. Homogenisation temperatures were not determined for VL or LLV inclusions. All fluid inclusion types in apatite have similar ranges of salinity, from approximately 5 to 15wt.% NaCl eq. Those in calcite have a slightly different range of salinity, from 2 to 13 wt.% NaCl. eq., while inclusions in quartz and K-Feldspar have salinities of 7 wt.% NaCl eq. or lower. The carbonic phase of VL and LLV inclusions is CO 2-dominated, with the contents of other gases (CH 4 and/or N 2), being highest and lowest in apatite- and quartz-hosted inclusions, respectively. A carbonic phase is also present in some LVS inclusions, and contains the highest proportions of CH 4 and/or N 2. Analyses of decrepitate residues indicate that LVS inclusions have low Na/Na+K and high S/S+Cl ratios and contain minor concentrations of Fe. Residues of LVMS inclusions also have low Na/Na+K and high S/S+Cl ratios, albeit higher and lower, respectively, than those of LVS inclusions, but differ from LVS inclusions mainly in having high apparent concentrations of Si and Al, which together with Ca and Sr, likely represent contributions from daughter minerals or trapped solids. LV inclusion residues have relatively high Na/Na+K and low S/S+Cl ratios, particularly in quartz. The trend of Na/Na+K vs. S/S+Cl defined by fluid inclusion residues suggests that the fluids evolved in the sequence LVS⇒(LVMS)⇒LV (ap)⇒LV (qtz). This is supported by isochoric projections, which indicate that LVS fluids and accompanying carbonic fluids were the first to be trapped (at temperatures of >700 °C, the carbonatite solidus, and pressures>4 kilobars (kb)), LVMS fluids were trapped at similar temperatures, but pressures<2 kb, LV (ap) inclusions were trapped at lower temperature and LV (qtz) inclusions were trapped at temperatures of <250 °C and a pressure of about 100 bars. A model is proposed in which a CO 2–CH 4-bearing, S- and K-rich aqueous fluid, preserved as LVS fluid inclusions, exsolved from the calciocarbonatite at a depth of >10 km and a temperature≥700 °C. As this fluid rose to higher crustal levels, it exsolved its carbonic component, which was trapped as LLV and VL fluid inclusions, and mixed with oxidising meteoric or formational waters. The LVS fluid became more dilute and was trapped as LV fluid inclusions in apatite, while the carbonic fluid evolved by converting its CH 4 to CO 2. When the carbonatite magma reached a depth of 3–5 km, it exsolved its last aliquots of aqueous fluid, which were solute-rich, had intermediate K/Na and S/Cl ratios and probably contained significant concentrations of Ca, Al, and Si. These fluids, which were trapped as LVMS inclusions, invaded the surrounding sandstones and replaced quartz with K-feldspar producing potassic fenite. During the waning stages of hydrothermal activity, the system was dominated by a low salinity (NaCl) meteoric or formation water, which redeposited silica as overgrowths on corroded quartz relicts in the fenites.

Multiple Hydrothermal Processes in Footwall Units of the North Range, Sudbury Igneous Complex, Canada, and Implications for the Genesis of Vein-Type Cu-Ni-PGE Deposits

Systematic regional sampling along the contact zone of the 1.85 Ga age Sudbury Igneous Complex with the Archean Levack Gneiss footwall in the North Range of the Sudbury structure revealed textural, mineralogical, and fluid inclusion systematics of post-Sudbury Igneous Complex hydrothermal processes that affected the various lithologies. At late stages of emplacement of the Sudbury Igneous Complex and formation of magmatic Fe-Ni-Cu sulfide deposits a partial melt from the Levack Gneiss invaded the contact zone. This partial melt is revealed by small microdikes and irregular bodies of granophyric quartz-plagioclase with megacrysts of hornblende, clinopyroxene, and titanite. Plagioclase-hornblende equilibrium was established at 750° to 800°C and 1.10 to 1.55 kbars pressure, corresponding to about a 4- to 6-km depth under lithostatic conditions. F/Cl wt percent ratios around 15 for accessory apatite and hornblende indicate that a Cl-rich fluid phase separated during the crystallization of the footwall granophyre: this is expressed as miarolites. The Cl-rich fluid (50 wt % NaCl equiv salinity) was trapped as primary fluid inclusions in quartz at around 480°C minimum temperature and around 1.1 kbars minimum pressure during supercooling. These inclusions are rich in Ca, Fe, Mn, and K, as well as Na chloride. This fluid may have interacted with earlier primary magmatic sulfide, causing remobilization and reprecipitation of Cu-Ni-platinum-group elements (PGE) in veins and disseminations in the footwall. These veins are parallel to the Sudbury Igneous Complex-footwall contact and are characterized by chalcopyrite, pentlandite, millerite, magnetite, stilpnomelane, ferropyrosmalite, epidote, and chlorite. Michenerite, moncheite, merenskyite, froodite, insizwaite, sobolevskite, gold, and Bi-Ni sulfides are associated with sulfides. Epidote, quartz, actinolite, and chlorite are common in the alteration selvages of the veins. Compositions and assemblages of platinum-group minerals (PGM) indicate their precipitation below 575° to 485°C. Primary, highly saline fluid inclusions (about 40 wt % NaCl equiv) in quartz indicate crystallization at 400° to 480°C and around 1.6 kbars minimum T and P, respectively. Late carbonate-epidote-actinolite-chlorite veins and alteration overprinted the earlier assemblages at about 300° to 400°C, with some bornite, millerite, native silver, and other Bi sulfides. The second stage of hydrothermal activity is characterized by regional carbonic-aqueous (NaCl-CO 2 -CH 4 -H 2 O-type) immiscible fluids that were trapped as secondary inclusions in quartz from various lithologies. No mineralization is related to this stage along the North Range. Boiling of these fluids took place during a pressure drop from lithostatic to hydrostatic conditions during uplift from about a 5- to 6- to a 3- to 4-km depth at 300° to 350°C. High- (20–26 wt % NaCl equiv) and low-salinity (6–12 wt % NaCl equiv) fluids coexisted with different carbonic species contents. The predominant north-south and northwest-southeast orientations of fluid inclusion planes reveal that mobilization of carbonic-aqueous solutions may be related to the northwesterly oriented thrusting in the Sudbury structure during the late stages (1.8–1.7 Ga) of the Penokean orogeny and to predominantly north-south faulting in the North Range. The third stage of fluid mobilization also took place mostly along northerly and northwesterly oriented fractures. Aqueous fluids were Ca-rich brines (20–40 wt % salinity) with temperatures between 150° and 250°C. Some of these fluids also circulated in fractures parallel to the Sudbury Igneous Complex-footwall contact (NE-SW); thus, they locally may have interacted with the earlier sulfide assemblages and are also responsible for formation of late veinlets with chalcopyrite-epidote-quartz-chlorite assemblages. Mobilization of these fluids took place under litho- and hydrostatic conditions at about a 4- to 6-km depth. This hydrothermal activity is related to a regional tectonothermal event during emplacement of northwesterly oriented Sudbury dikes (1.24 Ga). Regional-scale fluid inclusion studies along the North Range highlight the role of multiple hydrothermal processes in the metallogenic evolution of the Sudbury Igneous Complex and especially in the distribution of copper and precious metals in the footwall units.