H2O NaCl Inclusions Research Articles

Mineralization and genesis of the orogenic gold system in the Kalamaili area, East Junggar, Xinjiang, northwestern China

There are many lode gold deposits and occurrences in the Kalamaili area of the East Junggar, northwestern China. However, little is known about the specifics of ore geology and geochemistry of these gold deposits because of very limited exploration and research work in this region. Field geology, fluid inclusions, stable isotopes, trace elements in quartz, and hydrothermal zircon U-Pb dating are combined in this study to constrain the nature and source of ore fluids, the timing of mineralization, and the mechanism of gold precipitation in Kalamaili. The gold deposits are confined to a narrow zone between two regional NW- to NWW-trending shear zones and are structurally controlled by secondary, high-angle faults of the shears. The orebodies occur in the Middle Devonian and Lower Carboniferous, zeolite to lower greenschist facies clastic sedimentary and pyroclastic rocks. Gold mineralization occurs as auriferous quartz-sulfide ± tourmaline veins/veinlets and disseminated ores in the immediate altered wall rocks. The ore mineralogy is relatively simple and dominated by quartz with minor to trace amounts of sulfides (pyrite and arsenopyrite), sericite, calcite, and native gold. Quartz of various generations contains three types of fluid inclusions, including predominant H2O-CO2-NaCl inclusions and subordinate H2O-NaCl inclusions and CO2 ± CH4 ± N2 inclusions. The mineralizing fluid is uniformly characterized by a medium to high homogenization temperature (mostly 240−330 °C), low salinity (typically <6 wt% NaCl equivalent), reduced, and CO2-rich-H2O-NaCl ± CH4 fluid. The hydrogen and oxygen isotope data (δ18OH2O = +5.6‰ to +14.3‰, δDH2O = −99‰ to −62‰) indicate a metamorphic origin for the mineralizing fluid. The majority of sulfide δ34S values range between 0‰ and +10‰, indicative of a largely sedimentary rock reservoir of sulfur in the ore-forming fluids. Geochemically, the auriferous quartz is characterized by low concentrations of most trace elements including Ti, Al, Li, Ge, and Sb. Laser ablation−inductively coupled plasma−mass spectrometry U-Pb isotope dating of hydrothermal zircons from the auriferous quartz vein yielded a weighted mean 206Pb/238U age of ca. 313 Ma. Phase separation of ore fluids and fluid-rock interaction are suggested as key mechanisms for the gold precipitation. Integrated geological and geochemical evidence indicates that formation of the orogenic gold system in Kalamaili is related to the transition from compressional to transcurrent deformation during the Late Carboniferous. Target gold exploration in this region should focus on the northeast side of the Kalamaili fault zone, where there exist suitable structural and stratigraphic trap sites with high fluid flux and potential gold mineralization.

Fluid inclusion and stable isotope (O, H, C) constraints on the genesis of the Pedra Branca gold deposit, Troia Massif, Borborema Province, NE Brazil: An example of hypozonal orogenic gold mineralization

At the Archean-Paleoproterozoic Troia Massif, NE Brazil, two major Paleoproterozoic greenstone belts are documented (Algodões and Serra das Pipocas). They share similar lithostratigraphic characteristics with other 2.2–2.1 Ga greenstone belts of the surrounding cratonic domains (e.g. Guiana shield and West Africa craton). Gold mineralization in the Serra das Pipocas greenstone belt is associated with a regional NE-trending shear zone. In the mineralized area (the Pedra Branca gold deposit) the main gold mineralization is found in association with quartz veins, calc–silicate alteration (e.g. diopside, K-feldspar, amphibole, titanite, biotite, pyrite, albite, magnetite ± carbonates) and albitization. Pyrrhotite is commonly found as inclusions in pyrite, and late magnetite replaces pyrite, suggesting progressive oxidation of the ore–forming fluid. Free-milling gold commonly precipitates at the late stages of alteration, in close association with magnetite and tellurides. The main tellurides found are hessite, altaite, sylvanite, calaverite and tellurobismuthite. Two fluid inclusion assemblages were identified from mineralized quartz veins. Assemblage 1 is characterized by pseudo-secondary trails that show the coexistence of CO2–rich and low salinity (0–8 wt% NaCl equiv.) CO2–H2O–NaCl and H2O–NaCl inclusions, suggesting formation during phase separation (fluid immiscibility). Assemblage 2 is represented by late secondary low–temperature (Th < 200 °C) H2O–NaCl inclusions, probably unrelated to gold mineralization. The δ18O, δD and δ13C values of hydrothermal minerals (quartz, calcite, biotite, hornblende and magnetite) and fluid inclusions are compatible with a magmatic–hydrothermal ore-forming fluid that underwent fluid–rock interaction with the greenstone sequence. The calculated fluid δ18O values range from +8.3 to +11.0‰ (n = 59), fluid δD from −98 to –32‰ (n = 24) and δ13C values of calcite from −6.35 to −9.40‰ (n = 3). Oxygen isotope thermometry for quartz–magnetite pairs yields temperatures from 467 to 526 °C (n = 7, average 503 °C). This probably represents the temperature of gold deposition, because gold is generally associated with magnetite in the ore mineral assemblage. The estimated PT conditions of the gold mineralization range from 467 °C and 1.93 kbar (7 km) to 526 °C and 3.48 kbar (13 km), repectively, and they were obtained from the intersection of isochores with the temperature range derived from the quartz–magnetite isotope geothermometer. This PT range is akin to hypozonal orogenic gold deposits. The association of gold with magnetite and tellurides implies an ore–forming fluid derived from oxidized magmas, similar to those interpreted as ‘orogenic oxidized intrusion-related gold deposits’ in other Precambrian greenstone belts (e.g. Abitibi and Eastern Goldfields).

Involvement of variably-sourced fluids during the formation and later overprinting of Paleoproterozoic Au-Cu mineralization: Insights gained from a fluid inclusion assemblage approach

Au-Cu mineralized quartz veins of the Reef Deposit, Wisconsin, USA, originally formed prior to or early in the Paleoproterozoic Penokean orogeny in central North America as either the root zone of a gold-rich VMS deposit, or in an orogenic gold setting. Nearly 400 m.y. later, magmatic hydrothermal fluids associated with a continental scale anorogenic magmatic event remobilized mineralization within the veins. And still later, during the Paleozoic, remobilization and perhaps upgrading of Au and other metals occurred in response to circulation of fluids associated with Mississippi Valley-type deposits found in overlying supracrustal rocks. Fluid constraints on the formation of the Reef Deposit and later overprinting over a 1.5 b.y. timeframe are examined using the fluid inclusion assemblage (FIA) approach. Distinctive assemblages are examined on individual merit, each offering individual stories to aid in our understanding of the protracted development of the Reef Deposit.Presumably primary fluid inclusions in Au- and Cu-bearing quartz-sulfide veins that formed during the Paleoproterozoic event show H2O-CO2-CH4 ± NaCl compositions based on microthermometry and laser Raman spectroscopy. Possibly primary H2O-NaCl and secondary H2O-CO2-CH4 -NaCl FIA formed prior to regional deformation and are recognized as discrete relict and neonate clusters. The intersection of isochores from the highest density neonate inclusions of either fluid composition indicates re-equilibration at lower amphibolite conditions during regional deformation. Quartz hosted secondary fluid inclusion assemblages characterized by H2O-CH4 and CO2-CH4 compositions formed at high temperatures and have not been re-equilibrated and are interpreted to post-date regional metamorphism. These FIA occur along healed fractures with chalcopyrite mineralization. These inclusions are likely associated with fluid circulation driven by emplacement of the adjacent Wolf River Batholith (ca. 1.45 Ga), that altered pyrite to pyrrhotite and remobilized or emplaced chalcopyrite at temperatures as high as 700 °C. This reduced fluid was in equilibrium with locally observed graphitic sediments. Secondary lower temperature (<50–210 °C) calcite and quartz hosted H2O-NaCl inclusions show variable homogenization temperatures and salinities. H2O-NaCl inclusions in quartz homogenize at 78–210 °C, with lower salinities of 3.0–13.9 wt% NaCl equivalent, and are observed to crosscut CH4 bearing FIA. H2O-NaCl inclusion assemblages in calcite have maximum homogenization temperatures of 103 °C and salinities of ~24 wt% NaCl equiv. Lower temperature H2O-NaCl inclusions are most likely related to the Paleozoic-aged MVT fluid overprint that remobilized and possibly emplaced gold mineralization.

Metallogeny of the Handagai skarn Fe–Cu deposit, northern Great Xing'an Range, NE China: Constraints on fluid inclusions and skarn genesis

The newly discovered Handagai skarn Fe–Cu deposit is located in the northern Great Xing'an Range of NE China and is hosted by the Ordovician Luohe Formation. The orebodies that form the deposit are generally concordant with the bedding within these sediments, and are spatially related to areas of skarn development. The Fe–Cu mineralization in this area records four stages of paragenesis, namely prograde skarn, retrograde skarn, quartz–sulfide, and quartz–carbonate stages. The Handagai deposit is a calcic skarn that is dominated by an andradite–diopside–epidote–actinolite assemblage. The mineralogy and geochemistry of the skarn indicate that it formed from a hydrothermal fluid that altered the carbonate units in this area to a garnet (And42–95Grs4–53) and pyroxene (Di71–78Hd22–29Jo0–2) bearing skarn. The epidote within the skarn has an epidote end-member composition, with the chlorite in the skarn dominantly Fe-rich, indicating that these minerals formed in an Fe-rich environment. The petrographic, microthermometric, and Raman spectroscopic analysis of fluid inclusions within garnet, epidote, actinolite, quartz, and calcite precipitated at different stages of formation of the Handagai deposit indicate that mineralization-related fluid inclusions are either liquid-rich two-phase H2O–NaCl (type I), gas-rich two-phase H2O–NaCl (type II), three-phase (liquid+vapor+solid) H2O–NaCl (type III), or CO2–H2O–NaCl inclusions (type IV). The early stages of mineralization are associated with all four types of inclusion, whereas the later stages of mineralization are only associated with type I and II inclusions. Inclusion homogenization temperatures vary between the four stages of mineralization (370°C–530°C and >600°C, 210°C–290°C, 190°C–270°C, and 150°C–230°C, from early to late, respectively), with salinities also varying between the earlier and later stages of mineralization (11–18 and >45, 7–15, 6–9, and 3–7wt.% NaCl equivalent (equiv.), respectively). The majority of the inclusions within the Handagai deposit have homogenization temperatures and salinities of 200°C–350°C and 4–14wt.% NaCl equiv., respectively, indicating that this is a medium–high temperature and medium–low salinity type deposit. The fluid inclusions were trapped at pressures of 11 to 72MPa, corresponding to depths of 0.4 to 2.9km. The geology, mineralogy, geochemistry, and fluid inclusions microthermometry indicate that the Handagai deposit formed as a result of contact infiltration metasomatism, with the deposition of ore minerals resulting from a combination of factors that include boiling as a result of reduced pressure, cooling, and fluid mixing.

Compositions and Pressure–Temperature Conditions of Metamorphic Fluids Overprinting the Talate VMS Pb–Zn Deposit, Southern Altay, China

The Talate Pb–Zn deposit, located in the east of the NW–SE extending Devonian Kelan volcanic‐sedimentary basin of the southern Altaides, occurs in the metamorphic rock series of the upper second lithological section of the lower Devonian lower Kangbutiebao Formation (D1k12). The Pb–Zn orebodies are stratiform and overprinted by late sulfide–quartz veins. Two distinct mineralization periods were identified: a submarine volcanic sedimentary exhalation period and a metamorphic hydrothermal mineralization period. The metamorphic overprinting period can be further divided into two stages: an early stage characterized by bedding‐parallel lentoid quartz veins developed in the chlorite schist and leptite of the ore‐bearing horizon, and a late stage represented by pyrite–chalcopyrite–quartz veins crosscutting chlorite schist and leptite or the massive Pb–Zn ores. Fluid inclusions in the early metamorphic quartz veins are mainly CO2–H2O–NaCl and carbonic (CO2±CH4±N2) inclusions with minor aqueous inclusions. The CO2–H2O–NaCl inclusions have homogenization temperatures of 294–368° C, Tm,CO2 of –62.6 to –60.5°C, Th,CO2 of 7.7 to 29.6°C (homogenized into liquid), and salinities of 5.5–7.4 wt% NaCl eqv. The carbonic inclusions have Tm,CO2 of –60.1 to –58.5°C, and Th, CO2 of –4.2 to 20.6°C. Fluid inclusions in late sulfide quartz veins are also dominated by CO2–H2O–NaCl and CO2±CH4 inclusions. The CO2–H2O–NaCl inclusions have Th,tot of 142 to 360°C, Tm,CO2 of –66.0 to –56.6°C, Th,CO2 of –6.0 to 29.4°C (homogenized into liquid) and salinities of 2.4–16.5 wt% NaCl eqv. The carbonic inclusions have Tm, CO2 of –61.5 to –57.3°C, and Th, CO2 of –27.0 to 28.7°C. The aqueous inclusions (L‐V) have Tm,ice of –9.8 to –1.3°C and Th,tot of 205 to 412°C. The P–T trapping conditions of CO2‐rich fluid inclusions (100–370 MPa, 250–368°C) are comparable with the late‐ to post‐regional metamorphism conditions. The CO2‐rich fluids, possibly derived from regional metamorphism, were involved in the reworking and metal enrichment of the primary ores. Based on these results, the Talate Pb–Zn deposit is classified as a VMS deposit modified by metamorphic fluids. The massive Pb–Zn ores with banded and breccia structures were developed in the early period of submarine volcanic sedimentary exhalation associated with an extensional subduction‐related back‐arc basin, and the quartz veins bearing polymetallic sulfides were formed in the late period of metamorphic hydrothermal superimposition related to the Permian–Triassic continental collision.

Hydrothermal alteration and Cu–Ni–PGE mobilization in the charnockitic rocks of the footwall of the South Kawishiwi intrusion, Duluth Complex, USA

In the Neoarchean (~2.7Ga) contact metamorphosed charnockitic footwall of the Mesoproterosoic (1.1Ga) South Kawishiwi intrusion of the Duluth Complex, the primary metamorphic mineral assemblage and Cu–Ni–PGE sulfide mineralization is overprinted by an actinolite+chlorite+cummingtonite+prehnite+pumpellyite+quartz+calcite hydrothermal mineral assemblage along 2–3cm thick veins. In calcite, hosted by the hydrothermal alteration zones and in a single recrystallized quartz porphyroblast, four different fluid inclusion assemblages are documented; the composition of these fluid inclusions provide p–T conditions of the fluid flow, and helps to define the origin of the fluids and evaluate their role in the remobilization and reprecipitation of the primary metamorphic sulfide assemblage.Pure CO2 fluid inclusions were found as early inclusions in recrystallized quartz porphyroblast. These inclusions may have been trapped during the recrystallization of the quartz during the contact metamorphism of the footwall charnockite in the footwall of the SKI. The estimated trapping pressure (1.6–2.0kbar) and temperature (810–920°C) conditions correspond to estimates based on felsic veins in the basal zones of the South Kawishiwi intrusion.Fluid inclusion assemblages with CO2–H2O–NaCl and CH4–N2–H2O–NaCl compositions found in this study along healed microfractures in the recrystallized quartz porphyroblast establish the heterogeneous state of the fluids during entrapment. The estimated trapping pressure and temperature conditions (240–650bar and 120–150°C for CO2–H2O–NaCl inclusions and 315–360bar and 145–165°C for CH4–N2–H2O–NaCl inclusions) are significantly lower than the p–T conditions (>700°C and 1.6–2kbar) during the contact metamorphism, indicating that this fluid flow might not be related to the cooling of the Duluth Complex and its contact aureole. The presence of chalcopyrite inclusions in these fluid inclusions and in the trails of these fluid inclusion assemblages confirms that at least on local scale these fluids played a role in base metal remobilization. No evidences have been observed for PGE remobilization and transport in the samples. The source of the carbonic phase in the carbonic assemblages (CO2; CH4) could be the graphite, present in the metasedimentary hornfelsed inclusions in the basal zones of the South Kawishiwi intrusion.The hydrothermal veins in the charnockite can be characterized by an actinolite+cummingtonite+chlorite+prehnite+pumpellyite+calcite (I–II)+quartz mineral assemblage. Chlorite thermometry yields temperatures around 276–308°C during the earliest phase of the fluid flow. In the late calcite (II) phase, high salinity (21.6–28.8 NaCl+CaCl2 equiv. wt.%), low temperature (90–160°C), primary aqueous inclusions were found. Chalcopyrite (±sphalerite±millerite), replacing and intersecting the early hydrothermal phases, are associated to the late calcite (II) phase. The composition of the formational fluids in the Canadian Shield is comparable with the composition of the studied fluid inclusions. This suggests that the composition of the fluids did not change in the past 2Ga and base metal remobilization by formational fluids could have taken place any time after the formation of the South Kawishiwi intrusion.Sulfur isotope studies carried out on the primary metamorphic (δ34S=7.4–8.9‰) and the hydrothermal sulfide mineral assemblage (δ34S=5.5–5.7‰) proves, that during the hydrothermal fluid flow the primary metamorphic ores were remobilized.

Fluid Inclusion Study of the Higher Himalayan Quartzitic Pelites, Garhwal Himalaya, India: Implications for Recrystallization History of Metasediments

Abstract Quartzitic pelites forms a part of Higher Himalayan Crystalline of higher geotectonic zone in Garhwal Himalaya. Quartzitic pelites (locally known as Pandukeshwar Quartzite) in Garhwal Himalaya is sandwiched between high grade metamorphic rocks of Central Crystallines and Badrinath Formation. Fluid inclusion studies are carried out on the detrital, and recrystallized quartz grains of quartzitic pelites to know about the fluid phases present during recrystallization processes at the time of maximum depth of burial. The quartzitic pelite (Pandukeshwar Quartzite) essentially consists of recrystallised quartz with accessory minerals like mica and feldspar. Fluid microthermometry study reveals the presence of three types of fluids: (i) high-salinity brine, (ii) CO2- H2O and (iii) H2O-NaCl. These fluids were trapped during the development of grain and recrystallization processes. The high saline brine inclusions and CO2- H2O fluid with the density of 0.90 to 0.97 gm/cm3 are remnants of provenance area. CO2 density in detrital quartz grains characterise the protolith of the sandstone as granite or metamorphic rock. The H2O–NaCl fluids involved in the recrystallization processes at temperature-pressure of 430-350°C; 4.8 to 0.5 Kbars as constrained by fluid isochores of CO2- H2O and H2O–NaCl inclusions and bulging and sub- grain development during recrystallization processes. The re-equilibration of the primary fluid due to elevated internal and confining pressure is evident from features like ‘C’ shaped cavities, stretching of the inclusions, their migration and decrepitation clusters. The observed inclusion morphology revealed that the rocks were exhumed along an isothermal decompression path.

Marble-Hosted Submicroscopic Gold Mineralization at Asimotrypes Area, Mount Pangeon, Southern Rhodope Core Complex, Greece

Gold mineralization at Asimotrypes, Mount Pangeon, Greece, occurs within amphibolite facies rocks of the Southern Rhodope Core Complex, one of the largest metamorphic core complexes in the world. Exhumation of the complex resulted from middle Eocene to middle Miocene northeast-southwest–oriented extension in the northern Aegean and was controlled by the Kerdylion detachment zone. Host rocks are mylonitic, impure dolomite marbles of marine provenance ( δ 13C = 1.9 – 2.9‰), which are intercalated with paramica schists, and amphibolites, and intruded by early Miocene syntectonic granitoids. In the Asimotrypes area, metamorphic rocks and granitoids exhibit flat mylonite-type ductile fabrics with consistent top-to-the-southwest sense of shear, as does the entire Complex. Two mineralogically similar and spatially coincident gold mineralization styles, with a supergene overprint, exist: (1) strata-bound replacement bodies that contain up to 17 ppm gold, concentrated at and controlled by the intersections of several hydrothermally altered top-to-the-southwest marble mylonites, with crosscutting northwest-southeast to east-west–trending high-angle brittle normal faults and fractures, and (2) structurally controlled quartz veins, pods, and lenses with 4 to 13 ppm gold, which occur along the northwest-oriented subvertical brittle structures; the latter crosscut the ductile shear foliation of the host rocks, together with the granitoids, and dip steeply to the southwest. Gold-bearing replacement bodies typically occur as discordant wide halo adjacent to the centrally located brittle structures. Ductile top-to-the-southwest shear zones and cross-cutting brittle structures are considered contemporaneous within the regional extensional deformation and exhumation history of the core complex host rocks. The distribution of gold mineralization is related to the geometry of the brittle structures, strongly suggesting that faults acted as major fluid feeder conduits during gold mineralization. Hydrothermal alteration associated with the auriferous bodies and veins, overprinting the metamorphic rocks at Asimotrypes, consists of quartz, muscovite (sericite), chlorite, calcite (dedolomite), and sulfide minerals (locally, as much as 35 vol %). Gangue quartz occurs as (1) peripheral banded quartz with mylonitic texture, (2) dominant gold-bearing jasperoidal quartz, and (3) late, fine drusy quartz. The gold assemblage consists of arsenopyrite (41.7–43 wt % As; electron microprobe analyses) and arsenian pyrite encompassed by jasperoidal quartz; chalcopyrite, galena, tetrahedrite-tennantite, and sphalerite are trace phases. Secondary ion mass spectrometry (SIMS) spot analyses revealed that gold in replacement ore is submicroscopic and occurs in arsenopyrite and arsenian pyrite (As = 0.01–3.8 wt %), ranging from 0.5 to 29 ppm (mean = 11.6 ppm), and 0.14 to 11 ppm (mean = 2.3 ppm), respectively. Supergene overprint includes colloidal goethite + hematite containing native gold grains (size range, 20–40 μ m) and lesser covellite, malachite, cerrussite, and chrysocolla. SIMS analysis also shows a primary, inhomogeneous distribution pattern for gold that appears to mimic finely banded growth twinning lamellae or growth zones. The form of the vein-related gold is unknown. Fluid inclusions trapped during gold mineralization are low to medium salinity (<10 wt % NaCl equiv), aqueous-carbonic H2O-CO2-NaCl inclusions with highly variable carbonic to aqueous contents and naturally decrepitated or reequilibrated inclusions, interpreted to represent decompression by concurrent exhumation of the Southern Rhodope Core Complex. Trail-bound aqueous H2O-NaCl inclusions are interpreted as a late extensional brittle deformation-related hydrothermal event unrelated to gold-bearing sulfide precipitation. Trapping temperatures of the H2O-CO2-NaCl inclusions and oxygen isotope equilibrium temperatures between quartz and muscovite (sericite) indicate formation of gold-bearing arsenopyrite, arsenian pyrite, and quartz bodies at ~270°C, with pressures of 1,800 to 2,000 bars corresponding to depths between 6 and 9 km under lithostatic load. Measured hydrogen and calculated oxygen isotope compositions of the aqueous-carbonic mineralizing fluids ( δ Dwater = −125 to −105‰, δ 18Owater = 11.9 to 13.7 ‰ at 250°C, and 13.9 and 15.7 ‰ at 300°C) indicate an evolved meteoric origin. Water/rock calculations indicate these values could derive from heated meteoric water that had undergone isotope exchange during circulation through marbles at low water/rock ratio. Gold-associated sulfide minerals have δ 34S values between 2.2 and 3.1 per mil, interpreted as leaching of the core complex Paleozoic marine metasedimentary (marble), and interlayered mafic metavolcanic (amphibolites) and Miocene granitic rocks through fluid/rock interaction. Gold was probably precipitated from an H2S-rich fluid when the host sulfide minerals were stabilized by pyritization of marble wall rock. Asimotrypes gold mineralization is interpreted as postmetamorphic and postmagmatic, and linked to late stages of exhumation of the Complex by middle Eocene to middle Miocene extension in the northern Aegean. The location and shape of gold orebodies are controlled by interactive ductile-then-brittle deformation, fluid flow and fluid-marble interaction processes. High paleogeothermal gradients associated with extension may have driven the dominantly meteoric fluid circulation downward within the exhuming Complex. Asimotrypes may represent a potential new gold mineralization style for the Rhodope region.

Structural controls, temperature–pressure conditions and fluid evolution of orogenic gold mineralisation at the Betam mine, south Eastern Desert, Egypt

The Betam gold deposit, located in the southern Eastern Desert of Egypt, is related to a series of milky quartz veins along a NNW-trending shear zone, cutting through pelitic metasedimentary rocks and small masses of pink granite. This shear zone, along with a system of discrete shear and fault zones, was developed late in the deformation history of the area. Although slightly sheared and boudinaged within the shear zone, the auriferous quartz veins are characterised by irregular walls with a steeply plunging ridge-in-groove lineation. Shear geometry of rootless intra-folial folds and asymmetrical strain shadows around the quartz lenses suggests that vein emplacement took place under a brittle–ductile shear regime, clearly post-dating the amphibolite-facies regional metamorphism. Hydrothermal alteration is pervasive in the wallrock metapelites and granite including sericitisation, silicification, sulphidisation and minor carbonatisation. Ore mineralogy includes pyrite, arsenopyrite and subordinate galena, chalcopyrite, pyrrhotite and gold. Gold occurs in the quartz veins and adjacent wallrocks as inclusions in pyrite and arsenopyrite, blebs and globules associated with galena, fracture fillings in deformed arsenopyrite or as thin, wire-like rims within or around rhythmic goethite. Presence of refractory gold in arsenopyrite and pyrite is inferred from microprobe analyses. Clustered and intra-granular trail-bound aqueous–carbonic (LCO2 + Laq ± VCO2) inclusions are common in cores of the less deformed quartz crystals, whereas carbonic (LCO2 ± VCO2) and aqueous H2O–NaCl (L + V) inclusions occur along inter-granular and trans-granular trails. Clathrate melting temperatures indicate low salinities of the fluid (3–8 wt.% NaCl eq.). Homogenisation temperatures of the aqueous–carbonic inclusions range between 297 and 323°C, slightly higher than those of the intra-granular and inter-granular aqueous inclusions (263–304°C), which are likely formed during grain boundary migration. Homogenisation temperatures of the trans-granular H2O–NaCl inclusions are much lower (130–221°C), implying different fluids late in the shear zone formation. Fluid densities calculated from aqueous–carbonic inclusions along a single trail are between 0.88 and 0.98 g/cm3, and the resulting isochores suggest trapping pressures of 2–2.6 kbar. Based on the arsenopyrite–pyrite–pyrrhotite cotectic, arsenopyrite (30.4–30.7 wt.% As) associated with gold inclusions indicates a temperature range of 325–344°C. This ore paragenesis constrains f S2 to the range of 10−10 to 10−8.5 bar. Under such conditions, gold was likely transported mainly as bisulphide complexes by low salinity aqueous–carbonic fluids and precipitated because of variations in pH and f O2 through pressure fluctuation and CO2 effervescence as the ore fluids infiltrated the shear zone, along with precipitation of carbonate and sericite. Wallrock sulphidation also likely contributed to destabilising the gold–bisulphide complexes and precipitating gold in the hydrothermal alteration zone adjacent to the mineralised quartz veins.

Fluid inclusion and stable isotope (O, H, C, and S) constraints on the genesis of the Serrinha gold deposit, Gurupi Belt, northern Brazil

The Serrinha gold deposit of the Gurupi Belt, northern Brazil, belongs to the class of orogenic gold deposits. The deposit is hosted in highly strained graphitic schist belonging to a Paleoproterozoic (∼2,160 Ma) metavolcano-sedimentary sequence. The ore-zones are up to 11 m thick, parallel to the regional NW-SE schistosity, and characterized by quartz-carbonate-sulfide veinlets and minor disseminations. Textural and structural data indicate that mineralization was syn- to late-tectonic and post- metamorphic. Fluid inclusion studies identified early CO2 (CH4-N2) and CO2 (CH4-N2)-H2O-NaCl inclusions that show highly variable phase ratios, CO2 homogenization, and total homogenization temperatures both to liquid and vapor, interpreted as the product of fluid immiscibility under fluctuating pressure conditions, more or less asso- ciated with postentrapment modifications. The ore-bearing fluid typically has 18-33mol% of CO2, up to 4mol% of N2, and less than 2mol% of CH4 and displays moderate to high densities with salinity around 4.5wt% NaCl equiv. Mineralization occurred around 310 to 335°C and 1.3 to 3.0 kbar, based on fluid inclusion homogenization temperatures and oxygen isotope thermometry with estimated oxygen fugacity indicating relatively reduced conditions. Stable isotope data on quartz, carbonate, and fluid inclusions suggest that veins formed from fluids with δ 18 OH2O and δDH2O (310-335°C) values of +6.2 to +8.4‰