Fluid Inclusion Studies Research Articles

Complex carbonate-sulfate brines in fluid inclusions from carbonatites: Estimating compositions in the system H2O-Na-K-CO3-SO4-Cl

Studies of fluid inclusions in carbonatitic rocks are essential for understanding physicochemical processes involved in carbonatite-related hydrothermal ore mineralization and fenitization. However, the composition of many carbonatite-derived fluids is challenging to quantify, which hampers their detailed interpretation. Here, we present a systematic study of microthermometry of fluid inclusions found in carbonatites from the Kaiserstuhl (SW Germany), and a simple numerical model to estimate the compositions of such fluids, which are typical of numerous carbonatites worldwide.Four types of fluid inclusions have been identified in the Kaiserstuhl carbonatites: (I) vapor-poor H2O-NaCl fluids with <50 wt.% salinity; (II) vapor-rich H2O-NaCl-CO2 fluids with <5 wt.% salinity; (III) multi-component fluids with high salinity and high CO2 contents; and (IV) multi-component fluids with high salinity but little to no CO2. At present, it is only possible to quantify fluid compositions for types I and II. For the complex types III and IV, we conducted predictive modeling of the liquidus surface based on the Margules equations. The results suggest that carbonatite melts predominantly exsolve Na-K-sulfate-carbonate/bicarbonate-chloride brines (types III or IV). Such fluid inclusions may represent immiscible fluids that were trapped after segregation by boiling from a parental highly saline brine (type I). Fluid boiling, in turn, was probably triggered by a rapid pressure release during melt ascent. The present model enables quantification of fluid compositions associated with carbonatitic magmatism.

Mineralized Granitic Porphyry of the Yangla Copper Deposit, Western Yunnan, China: Geochemistry of Fluid Inclusions and H-O, S, and Pb Isotopes

The Yangla copper deposit (YCD) is located in the central part of the Jinshajiang tectonic belt (Jinshajiang metallogenic belt) and is one of the most important copper deposits which has the large-scale copper reserves of the northwestern Yunnan, China. The ore bodies are strictly controlled by the stratum, pluton, and structure, which are layered, lens, and vein-like within the contact or fracture zone of the pluton and surrounding rock. At Yangla, two styles of mineralization occur at the brecciated contact zone between the pluton (granodiorite and granitic porphyry) and carbonaceous wall rock and include strata bound/lens-shaped replacement of carbonate rocks (skarn style) and porphyry-style sulfide-quart-calcite veins. But, the granitic porphyry mineralization have received less attention; the isotope and fluid inclusion studies are relatively scarce for limited porphyry ore bodies that have been discovered at the YCD. Quartz-hosted fluid inclusions from the recently discovered granitic porphyry have homogenization temperature averaging around 180±20°C and 300±20°C with salinities ranging from 4 to 22 wt.% NaCleq, pointing toward the contribution of medium temperature-medium salinity and low temperature-low salinity fluids during the metallogenesis. These fluid inclusions have δ18OH2O values ranging between -1.91‰ and -1.02‰ and δD values ranging between -143.10‰ and -110‰, suggesting that the ore-forming fluid was a mix of magmatic and meteoric water. Ore-related pyrite/chalcopyrite have δ34SV-CDT values ranging from -1.0‰ to 1.0‰ and whole rocks have δ34SΣS = 0.34, suggesting that sulfur mainly derived from magmatic rocks of the Yangla mining area. The sulfides 208Pb/204Pb ranged from 38.8208-38.9969, 207Pb/204Pb from 15.7079-15.7357, and 206Pb/204Pb from 18.5363-18.7045, indicating that the lead mainly originated from the upper crust. It is demonstrated that the evolution of ore-forming fluid is continuous from the skarn ore body (SOB) stage to the porphyritic ore body stage and belong to the products of the same ore-forming fluid system, and the unisothermal mixing and cooling actions were maybe the main mechanism at the metallic minerals precipitation in mineralized granitic porphyry (MGP). A model is proposed according to the early stage, a magmatic fluid reacted and replaced with the surrounding carbonate rocks and then formed skarn-type ore bodies. The magmatic-hydrothermal fluid subsequently deposited porphyry-type quartz-calcite veins, veinlets, and stockwork mineralization.

Geosciences in Central South University: A state-of-the-art review

In recent decades, colleagues working in the Discipline of Geological Resources and Geological Engineering at Central South University made significant progress in theoretic study and application of geophysics, ore deposit and shale gas geology, 3D predictive modeling of concealed resources, and geological engineering. In geophysics, world-class progress was achieved in the development, data processing, equipment, and scientific survey of electromagnetic method in onshore and offshore environments and the tectonic evolution of the Tibetan Plateau. Especially, advanced wide-field electromagnetic exploration method and equipment as a highlight technique won the first prize of National Science and Technology Invention of China. In ore geology, progressive and complex characteristics of most nonferrous ore deposits and the geodynamic relationship between crust-mantle reaction and mineralization in south China were revealed. Progressive metallogenic models of certain typical ore deposits were established based on the study of fluid inclusion and geochemistry. According to characteristics of complex metallogenic system of polygenetic ore deposit, key ore-controlling factors were found and summarized. The investigation on unconventional resources advanced China???s shale gas resource evaluation system. In 3D predictive modelling of metallogenic, a large-scale location prediction model was established for exploration of crisis mines and concealed ore deposits. Our developed 3D predictive modeling techniques for concealed orebodies were widely used to explore deep mineral resources in China. In geological engineering area, the key technologies for deep drilling into complex formations were developed. Especially the drilling fluid and tools were utilized in the fieldwork. The rock and soil mechanics analysis method and anchor technologies were also established and applied to engineering practice.

SEM-CL Study of Quartz Containing Fluid Inclusions in Wangjiazhuang Porphyry Copper (-Molybdenum) Deposit, Western Shandong, China

The Wangjiazhuang porphyry copper (-molybdenum) deposit is located at Zouping volcanic basin in Shandong Province, East China and hosted in the Wangjiazhuang intrusive complex emplaced along a late volcanic conduit. There are two types of ores in this deposit: early disseminated and stockwork ores in the ore-bearing intrusion, and late massive sulfide-quartz veins above brecciated quartz monzonite. The ore minerals are mainly pyrite, chalcopyrite, and subordinately magnetite, tennantite, molybdenite with minor bornite, enargite, galena and sphalerite, etc., and gangue minerals including K-feldspar, biotite, quartz, muscovite-sericite, chlorite and calcite. Combined with fluid inclusion study, the scanning electron microscope-cathodoluminescence (SEM-CL) study of quartz in the deposit and wall rocks shows significant differences between the two types of quartz in the ores. In addition, four types of primary-pseudosecondary fluid inclusions in the quartz have been recognized. They are one- or two-phase aqueous inclusions with vapor/liquid ratios less than 30% to 40% (type I); gas-rich inclusions with vapor/ liquid ratios more than 50% (type II), some of which contain some small opaque minerals, probably chalcopyrite; multiphase fluid inclusions with daughter minerals of halite±anhydrite±opaque (chalcopyrite)± sylvite±hematite±unknown crystal (type III); and mica-bearing fluid inclusions (type IV). Quartz containing abundant muscovite-bearing and halite-bearing fluid inclusions in the mineralized quartz monzonite with potassic-silicic alteration, have better oscillatory growth zoning with CL-colors from bright in the core to darker in the rim, indicating variations of element concentrations in the fluid media from which quartz grew during the later period of magmatic-hydrothermal process. In contrast, the quartz in the sulfide-quartz veins contains mainly fluid inclusions of low-to-medium salinities and does not show oscillatory zoning, indicating that there was less fluctuation in composition and element concentrations of the hydrothermal fluids. However, the quartz containing halite-bearing fluid inclusions and being associated with copper-molybdenum mineralization in the sulfide-quartz veins shows zoning in its rims, indicating variations in composition and element concentrations of the hydrothermal fluids.

Hydrochronology of a proposed deep geological repository for low- and intermediate-level nuclear waste in southern Ontario from U–Pb dating of secondary minerals: response to Alleghanian events

U–Pb ages have been measured on secondary dolomite and silica cements in Cambrian sandstone at the base of an 840 m thick sub-horizontally bedded sedimentary sequence beneath the Bruce nuclear site in southern Ontario to document the history of fluid movement. Results show an average U–Pb age of 320 ± 10 Ma. The initial common Pb end member is slightly, but distinctly, enriched in 206Pb compared with that in older and younger calcite cements elsewhere within the sedimentary section. Combined with previous hydro-geochemical and fluid inclusion studies of the same rocks, the age is interpreted to record episodic migration of a saturated hydrothermal brine. Previously dated calcite cement in sub-horizontal fractures about 500 m higher in the stratigraphic section near the base of the Silurian sequence records similar U–Pb ages of 318 ± 10 Ma by laser ablation inductively coupled plasma mass spectrometry and 313 ± 1 Ma by isotope dilution thermal ionization mass spectrometry. We suggest that the subhorizontal fractures were generated by slumping that resulted from dissolution of underlying evaporite deposits. These ages overlap with the peak of plutonism in the Alleghanian mountains, which were being uplifted contemporaneously 500 km to the southeast. The results suggest the transport of hydrothermal brine from areas of crustal melting through the deep Cambrian sandstone aquifer, while at higher crustal levels meteoric water was also driven over equally large distances by hydraulic gradients from the Alleghanian mountains.

Geochemistry of skarn and porphyry deposits in relation to epithermal mineralization in the Arasbaran metallogenic zone, NE Tabriz, Iran

The Arasbaran metallogenic zone in northern Iran is part of the Alborz–Azerbaijan magmatic zone, which developed along the southern margin of Eurasia during the Early Mesozoic–Late Cenozoic. This region hosts precious and base metal mineralization, including porphyry, skarn, and epithermal copper, molybdenum, and gold deposits. Rare earth element variations across all the deposits are similar, indicating a similar source for these elements. The north-west trending belt comprising the Nabijan to the Sonajil deposits consistently shows chiefly alkaline conditions of formation. Fluid inclusion studies indicate that both high and low temperature hydrothermal fluids participated in the formation of all of the deposits. The mineralization age decreases from north to south and east to west and, although metal zonation is complex, the Cu–Au association post-dated the Cu–Mo mineralization reflecting that the ore fluid evolved in terms of both cooling and chemical changes due to fluid–fluid and fluid–rock interactions. In this region most deposits record a concentric zonation, with the centres preserving porphyry and skarn deposits and deposits becoming progressively epithermal toward the outer parts of the mineralizing system. According to this, the mineralization age decreases from the porphyry and skarn deposits to the epithermal deposits. The homogenization temperature and salinity both decrease from the centre to the outer zone. The pattern of homogenization temperature zonation, which is concordant with salinity zonation, suggests that fluids migrated up-dip and towards the margins of the zonation system.

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.

Geology, geochronology, and fluid inclusion studies of the Xiaorequanzi volcanogenic massive sulphide Cu–Zn deposit in the East Tianshan Terrane, China

The Xiaorequanzi Cu–Zn deposit is in the westernmost part of East Tianshan Terrane in northwestern China. The deposit is unique in the region being a volcanogenic massive sulphide (VMS) deposit located near a zone (or belt) containing giant late Paleozoic porphyry Cu deposits. Aiming to better understand the genesis of the mineral deposits in the terrane and their tectonic setting, we report our findings of detailed studies on fluid inclusion microthermometry, Re–Os dating of chalcopyrite from the massive ore, and U–Pb dating of zircons from the host volcanic rocks. There are two sulphide stages with early pyrite succeeded by chalcopyrite–sphalerite, which are hydrothermally overprinted and supergene enriched. The hydrothermal overprinting is characterised by quartz–sulphide veins crossed by carbonate-rich quartz veins. Quartz from the chalcopyrite–sphalerite stage is characterised by primary fluid inclusions containing H2O–NaCl(–CO2) and homogenise at 228–392 °C with a salinity of 2.2–13.3 wt.% NaCl equiv. Secondary fluid inclusions related to the hydrothermal overprinting homogenise at 170–205 °C with a salinity of 2.7–12.1 wt.% NaCl equiv. Fluid inclusions in the quartz–sulphide stage of the hydrothermal overprinting contain H2O–NaCl with homogenisation temperatures of 164–281 °C and salinities in ranging from 2.9 to 12.4 wt.% NaCl equiv. Fluid inclusion in the quartz–calcite stage contain H2O–NaCl with homogenisation temperatures of 122–204 °C with salinities of 1.4–12.4 wt.% NaCl equiv. These characteristics are like those of the secondary fluid inclusions in the VMS mineralisation. Combining these findings with H–O isotopic data from previous studies, we propose that the primary mineralising fluid is magmatic in origin. Tuff hosting the mineralisation yields a SHRIMP U–Pb zircon age of 352 ± 5 Ma, which is interpreted as the age of the tuff, and a porphyritic felsite dyke intruding the tuff yields a SHRIMP U–Pb zircon date of 345 ± 6 Ma, interpreted as the emplacement age of the dyke. Chalcopyrite from the main orebody at Xiaorequanzi yields a Re–Os isochron age of 336 ± 13 Ma with an initial 187Os/188Os ratio of 0.25 ± 0.55 (MSWD = 12). Given that the VMS deposit is a syngenetic deposit, we regard the upper ca. 349 Ma limit of the Re–Os date as the approximate age of the chalcopyrite. The three dates are the same within error, and the upper limit of the Re–Os date of ca. 349 is taken as the age of the volcanic, dyke, and mineralisation. The volcanic rocks around the Xiaorequanzi deposit have been previously classified as calc–alkaline to high-K calc–alkaline enriched in large-ion lithophile elements and depleted in high-field-strength elements, which are characteristics indicative of a forearc setting. It is suggested that VMS mineralisation formed in a forearc setting related to the north-directed subduction of the Palaeo-Kangguer or North Tianshan oceanic plates.

Geology, fluid inclusions, C–O–S–Pb isotopes and genesis of the Ahangaran Pb-Ag (Zn) deposit, Malayer-Esfahan Metallogenic Province, western Iran

The Malayer-Esfahan Metallogenic Province (MEMP) of western Iran hosts many Pb-Zn deposits and occurrences, largely hosted within a sequence of clastic detrital and carbonate host rocks. A well-known example is Ahangaran which is composed of several hypogene orebodies with estimated resources of ~1.5 Mt sulfide ore grading 4 wt% Pb, 0.5 wt% Zn and 200 g/t Ag, plus a supergene iron-bearing oxyhydroxide ore with 1 Mt reserve and grading 30–35 wt% FeO. The sulfide orebodies are primarily hosted in Lower Cretaceous dolostone and sandstone along the marginal-southern part of the NW–SE trending Kolehbid Syncline. Galena and pyrite, together with chalcopyrite and sphalerite in lesser amounts, comprise the primary sulfide ores. Minor sulfosalt minerals including bournonite, tetrahedrite, freibergite, famatinite and meneghinite have also been identified by electron probe microanalysis. Three paragenetic stages are recognized: (1) pre-sulfide stage (oxide stage); (2) main sulfide stage (including three sulfide sub-stages); and (3) supergene stage. Fluid inclusion studies conducted on pre- and main-sulfide stages identified two end-member fluids: a hot (~290 °C) and saline fluid (~20 wt% NaCl eq.) and a cooler (<150 °C), lower salinity fluid (~7 wt% NaCl eq.). C–O–S isotope studies suggest that these fluids originated as an 18O-enriched, heated basinal brine that carried Pb, Ag, and Ba, and a low salinity, SO42−- and H2S-bearing fluid derived from Mesozoic seawater. Sulfide required for ore formation was generated by thermochemical sulfate reduction and the sulfur of sulfate minerals was derived directly from dissolved SO42−. The Pb isotope compositions are homogeneous with 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios ranging from 18.458 to 18.501, 15.623 to 15.708 and 38.512 to 38.715, respectively. These ratios are indicative of a continental crustal reservoir as the source of lead and presumably other metals. Taken together, our observations suggest that the Ahangaran sediment-hosted Pb-Ag (Zn) deposit is an epigenetic, structurally-controlled and strata-bound deposit with fluids and metals derived from the Paleozoic basement rocks. This model for formation of the Ahangaran deposit is different to magmatic-hydrothermal, sedimentary-exhalative or Mississippi Valley-types, and to some extent is similar to Irish-type deposits.

Multiple fluid migration events and REE+Th mineralisation during Alpine metamorphism in the Sopron micaschist from the Eastern-Alps (Sopron area, Western Hungary)

Four fluid migration events were recorded during the Alpine metamorphism in the Sopron micaschist from the Grob gneiss series of the Lower Austroalpine Unit of the Eastern Alps near Sopron, using mineral chemistry data, geothermo-barometry and fluid inclusion studies.1. Tourmaline mineralisation in quartz veins and to some extent in the host rock. Similar mineral compositions in the quartz-tourmaline veins and in the host rock show equilibrium between fluid and the host rock. Geothermo-barometry gives 560-610oC temperature and 950-1230 MPa pressure for the formation of quartz-tourmaline veins which is the same as the determined P-T peak (T=560 and 600°C p= 840-1230 MPa).2. Fluids causing Mg-metasomatism in the shear zones. The result of this fluid invasion was the formation of leucophyllite in the shear zones and Mg-enrichment of some minerals (chlorite, muscovite, garnet) in the close vicinity of the shear zone. The effect of this fluid was confined to the shear zones and the neighbouring host rock.3. The rock was infiltrated along the shear zones and quartz veins with CO2-bearing hypersaline fluids during retrograde metamorphism. The presence of this fluid is evidenced by secondary CO2 inclusions and hypersaline aqueous fluid inclusions ± CO2. The aqueous fluid had high concentrations of Na, Ca, Fe, Al, Cl and contained moderate amounts of Mg, Zn, Ti, K, Mn, S and P. This fluid was the carrier of the REE and Th and locally precipitated florencite, monazite, allanite, apatite, thorite and thorianite in the shear zone. Traces of this mineralisation are found in quartz-tourmaline veins, postdating the tourmaline mineralisation.4. Late retrograde metamorphic fluid represented by two phase (liquid+vapor) aqueous inclusions of the NaCl-CaCl2-H2O system with total salinity between 25 and 28.5% and homogenisation temperatures between 229.6 and 322oC

Isotopic Signatures of Carbon in the ‘Los Pobres’ Graphite Mine, Ronda, Spain

AbstractGraphite formation temperatures in the ‘Los Pobres’ mine within the Ronda peridotite, Spain, previously reported to be between 770 and 820 °C, have been reinterpreted based on new temperature measurements using Raman spectroscopy. Additional in situ and bulk stable carbon isotopic measurements and fluid inclusion studies contributed to improved understanding of parts of the graphite formation process. Raman spectroscopy revealed that the formation of the ‘Los Pobres’ graphite extends to temperatures as low as 500 °C, indicating a broader temperature range than previously reported. Stable carbon isotopes and temperature estimates suggest two different crystallization events, followed by a late hydrothermal alteration of the host rock. The first event occurred at temperatures higher than ∼600°C, in which crystalline graphite was formed with a mixed 13C composition as a result of the mixing of two different carbon-bearing sources. The second graphite formation event took place below ∼600°C, within the same system, but with lower purity and crystallinity of the graphite. In the third event, the temperature decreased to less than 550 °C, and hydrothermal fluids altered the host rock, precipitating silica and iron oxides in veins penetrating both the host rock and the deposited graphite.

An improved equation of state of binary CO2–N2 fluid mixture and its application in the studies of fluid inclusions

An equation of state (EOS) in the form of Helmholtz free energy for fluid mixtures was improved by using four mixing parameters. It can calculate the pressure-volume-temperature-composition (PVTx) and vapor-liquid equilibrium (VLE) properties of the CO2–N2 fluid mixture. This EOS is based on highly accurate EOSs of pure CO2 and N2 fluids and contains a simple generalized departure function of Lemmon and Jacobsen (1999). This EOS can reproduce experimental PVTx and VLE data available up to 673 K and 1000 bar, with an accuracy within or close to experimental uncertainties. The EOS of CO2–N2 fluid mixture and an updated Gibbs free energy model of solid CO2, can be combined to determine the compositions and molar volumes of CO2–N2 fluid inclusions with the melting temperature of solid CO2 and the vapor-liquid homogenization temperature which can be directly obtained from experimental microthermometric analysis. Isochores of CO2–N2 inclusions can be easily calculated from this EOS.

Paragenetic and petrographic study of uranium mineralization along the Midwest Trend, Northeastern Athabasca Basin

The Athabasca Basin in northern Saskatchewan is host to many world-class uranium deposits associated with the unconformity between the Paleoproterozoic sandstone of the basin and the underlying crystalline basement (Jefferson et al., 2007). While the style and tonnage of these deposits vary, the current genetic model for unconformity-related uranium deposits has been a practical tool for exploration in the Athabasca Basin. However, the factors which control the location and formation of these deposits is still not fully understood. A paragenetic and petrographic study of mineralization along the Midwest Trend, located on the northeastern margin of the Athabasca Basin, aims to refine the current model and to address the general problem: What are the factors which control mineralization and non-mineralization? The Midwest Trend will be used as a "modèle réduit" for uranium mineralization, as it displays many features characteristic of unconformity type deposits. The Midwest Trend comprises three mineral leases that encompass two uranium deposits, the Midwest Main and Midwest A (Allen et al., 2017a, b). Mineralization occurs along a NE-trending graphitic structure, and is hosted by the sandstone, at the unconformity, and in much lesser amounts in the underlying basement rocks. Petrographic observations aided by the use of RAMAN spectroscopy and SEM-EDS, have been used to create a paragenetic sequence of mineralization (Fig.1). Future work will focus on fluid inclusion studies using microthermometry, LA-ICP-MS, and mass spectrometry of contained gases.&#x0D;

Geochemical and fluid inclusion studies of Kalchuyeh Cu-Au epithermal mineralization in the central part of the Urumieh-Dokhtar magmatic arc

Geochemical and fluid inclusion studies of Kalchuyeh Cu-Au epithermal mineralization in the central part of the Urumieh-Dokhtar magmatic arc

Tectono-metamorphic evolution of an evaporitic décollement as recorded by mineral and fluid geochemistry: The “Nappe des Gypses” (Western Alps) case study

Evaporites play a major role on the structuration of collisional orogens especially when they act as décollement units. However, their exact pressure-temperature-deformation (P-T-d) paths are poorly documented. In this study, the first direct P-T-d constraints of the “Nappe des Gypses” formation (western French Alps) have been established. An innovative association of structural geology, petrography, crystallochemistry, and detailed study of both fluid inclusions and stable isotopes (C, O) analysis has been applied to this evaporitic facies. Geochemical analysis shows that the “Nappe des Gypses” formation has recorded the three typical metamorphic and deformational events of the Alps (namely D1, D2 and D3). These different constraints allow the determination of the first P-T path determination for this unit. Metamorphic peak conditions of the “Nappe des Gypses” are at 16.6 ± 2.3 kbar and 431 °C ± 28 °C. This formation was buried at similar conditions than the oceanic units. During the exhumation path, the D1-D2 transition is reached at 350 °C ± 20 °C and 6.5 ± 1.8 kbar and the D2-D3 transition is assumed to be at 259 °C ± 24 °C and 2.0 ± 1.0 kbar (Strzerzynski et al., 2012). Peak P-T conditions overlap those of the median Liguro-Piemontese units but are different from those of the Briançonnais units. It implies 1) an active and crucial role of the “Nappe des Gypses” during the exhumation of the Alpine oceanic complex, and 2) confirms the allochthonous and more distal origin of the European Tethyan passive margin of the “Nappe des Gypses” formation. Consideration of sulfates dehydration probably between 15.0 and 16.6 kbar and 200 and 300 °C, allows to discuss pore pressure excess and its mechanical consequences on the exhumation process. This process is very likely to amplify the “décollement” effect of the evaporites and allow the nappe stack formation.This illustrates the role of this formation as a décollement surface. This difference of evolution highlights the major role of the evaporitic formations on the exhumation and structuration of a collisional chain. Such methodology could contribute to decipher the role of evaporites in the structural context of other collisional chains such as Himalaya, Pyrenees or Zagros.

Fluid Inclusion Study of the Penjom, Tersang, and Selinsing Orogenic Gold Deposits, Peninsular Malaysia

Ore-forming fluids in the auriferous district of the Central gold belt in Peninsular Malaysia were studied for their temperature, salinity, and relationship to the surrounding geology. Microthermometric analysis carried out showed homogenisation temperatures range from 210 to 348 °C (Tersang), between 194 and 348 °C (Selinsing), and from 221 to 346 °C (Penjom). Salinities range from 2.41 to 8.95 wt % NaCl equiv (Tersang), between 1.23 and 9.98 wt % NaCl equiv (Selinsing), and from 4.34 to 9.34 wt % NaCl equiv (Penjom). Laser Raman studies indicated that at the Tersang gold deposit, most inclusions are either pure or nearly pure CO2-rich (87–100 mol %), except for one inclusion, which contains CH4 gas (13 mol %). In addition, at Selinsing, most inclusions are CO2-rich (100 mol %). However, an inclusion was found containing CO2 (90 mol %), with minor N2 and CH4. Additionally, at the Penjom gold deposit, most fluid inclusions are CO2-rich (91–100 mol %), whereas one fluid inclusion is N2-rich (100 mol %) and another one has minor N2 and CH4. At a basin scale, homogenisation temperatures against salinity suggests an isothermal mixing of fluids. Most fluids are CO2-rich and are interpreted to be of metamorphic origin. The evidence further indicates involvement of magmatic fluids that is supported by the association of sandstone and carbonaceous black shales with magmatic rocks, such as rhyolite, rhyolite-dacite, and trachyte-andesite at the Tersang and Penjom orogenic gold deposits.

Stable isotope and fluid inclusion studies of early Zn-Cu-(Ni-Co)-rich ores, lower ore zone of Brushy Creek mine, Viburnum Trend MVT district, Missouri, U.S.A.: Products of multiple sulfur sources and metal-specific fluids

Mississippi Valley-type (MVT) deposits of the Viburnum Trend are typically lead-dominant and occur in the upper portion of the Cambrian Bonneterre Dolomite. Unusual zinc- and copper-rich ores, with notable enrichments in Ni, Co, and Ag, have been discovered within the lower Bonneterre Dolomite, >30 m below the district’s main ore-bearing horizon. These ores appear to be localized along fault/fracture zones within the Lamotte Sandstone, which promoted extreme dissolution of the base of the overlying Bonneterre Dolomite and resulted in pronounced vertical zoning of early Ni-Co, Cu, Zn and Pb ores above the sandstone/dolomite contact.Stable isotope and fluid inclusion studies were undertaken to assess whether the deep ores resulted from a single fluid reservoir that evolved from Cu-Ni-Co-rich to Zn-rich to Pb-rich or are instead products of distinct fluids unrelated to the stratigraphically higher, main Pb-rich ores of the district. Sphalerite-hosted fluid inclusions from the lower orebody appear to be geochemically distinct from those that formed the upper orebodies. They record the highest K/Na and Mg/Na ratios in the southeast Missouri MVT district, suggesting dominance of siliceous lithologies (arkose and/or felsic igneous rocks) over limestones along their ore-fluid pathways. Paragenetic trends indicate that distinct fluid compositions are associated with specific generations of sphalerite, which likely reflects the presence of multiple fluids.The δ34S values of ore minerals (early pyrite and chalcopyrite, −7 to +5‰; early sphalerite, +6 to +15‰; main sphalerite, +7 to +17‰ V-CDT) indicate deposition from fluids that utilized isotopically distinct sulfide reservoirs. Low-δ34S sulfur sources for early ores likely included sulfide in local brines within the Lamotte Sandstone and diagenetic sulfide minerals within the basal units of the Bonneterre Dolomite. A trend of increasing δ34S values of ore sulfides (from −5 toward +17‰) with vertical distance above the Lamotte Sandstone-Bonneterre Dolomite contact indicates that as the deep ore fluid system worked its way upward, it breached less permeable units in the lower Bonneterre Dolomite, allowing incorporation of high-δ34S sulfide from brines present higher in the stratigraphic section. A concomitant up-section pattern of decreasing δ18O values of dolomite cement and recrystallized remnant host rock (from −3 toward −9‰ V-PDB) is also consistent with a localized, deep mineralizing fluid system, which interacted progressively with the stratigraphically higher, regional-scale, Pb-rich fluid system.The episodic nature of metal sulfide deposition in the lower ore zone points to a system in which fluid mixing would have been highly variable, both temporally and spatially. Reaction path models require mixing of multiple, ore metal-specific and sulfide-bearing fluids in order to form the orebody. The sequence and concentrations of metal sulfides are inconsistent with ore formation from a single, evolving metal-bearing fluid.This study indicates that faults localized early metal- and sulfide-bearing fluids, facilitating mixing of fluids that resulted in the accumulation of high-grade ores near the Bonneterre Dolomite-Lamotte Sandstone contact. Recognition of fault/fracture patterns beneath more typical Pb-rich orebodies in the Viburnum Trend may be a viable exploration strategy for similar Zn-Cu-Co-Ni-rich deposits.

Fluid signature of the shear zone–controlled Veio de Quartzo ore body in the world-class BIF-hosted Cuiabá gold deposit, Archaean Rio das Velhas greenstone belt, Brazil: a fluid inclusion study

The world-class Cuiaba gold deposit of the Archaean Rio das Velhas greenstone belt in Brazil is hosted in banded iron formation containing carbonaceous matter and carbonate, within the reclined, isoclinal Cuiaba fold. Mineralised quartz veins are hosted in andesite in the stratigraphic footwall of the banded iron ores and form some of the more recently discovered ore bodies. Fluid inclusion data of the quartz vein–associated “Veio de Quartzo” ore body are obtained from four quartz types (Qz1, Qz2, Qz3, Qz5) in gold-mineralised V1 shear vein and V2 extensional veins, barren V3 extensional vein array, and V4 breccia-style veins, all developed during the Archaean D1 event. Three fluid types are distinguished: (i) aqueous fluids of low salinity (1.8–3.8 wt% NaCl equiv), homogenisation (into liquid) at 220 to 230 °C; (ii) aqueous fluids of moderate salinity (5.3–12.7 wt% NaCl equiv), and homogenisation at 250 to 290 °C; and (iii) aqueous-carbonic fluids of moderate salinity (6.0–15.1 wt% NaCl equiv), with 30–91 mol% CO2, 8–41 mol% CH4 and up to 28 mol% N2 and decrepitation (into vapour) at 280 to 310 °C. Based on an independent pressure estimate, a pressure correction was applied to aqueous fluid inclusions, resulting in minimum trapping temperatures at 360 °C for V1 veins, 330 °C for V2 veins, 300 °C for V3 veins and 270 °C for the late-stage V4 veins. Ion chromatography analyses reveal a Br/Cl ratio of 0.7 × 10−3 in Qz1-V1, from 1.4 to 1.5 × 10−3 in Qz2-V2, 0.3 to 0.4 × 10−3 in Qz3-V3 and 0.7 to 0.9 × 10−3in Qz5-V4 veins. Zinc, Pb and Cu are relatively enriched with ~ 100 to 1000 ppm in aqueous and aqueous-carbonic fluid inclusion assemblages in all vein and quartz types, which is similar to other orogenic gold deposits hosted in the Rio das Velhas greenstone belt. The fluid inclusion data are consistent with a model invoking a metamorphic origin for the mineralising fluid. A two-step model of hydrothermal fluid flow and gold enrichment is suggested to have developed during the Archaean D1 event, with an early, aqueous-carbonic fluid pulse of relatively high temperature (from V1 up to V3) and an evolved, aqueous-carbonic fluid pulse of lower temperature (V4, breccia-style veins). The Rio das Velhas greenstone belt is dominated by regionally metamorphosed metasedimentary rocks, resulting in a complex hydrothermal fluid evolution and related gold mineralisation such as the shear zone–controlled Veio de Quartzo ore body.

Fluid Inclusion Characteristics of the Kışladağ Porphyry Au Deposit, Western Turkey

The deposit occurs in a mid-Miocene monzonite magmatic complex represented by three different intrusions, namely Intrusion 1 (INT#1), Intrusion 2 (INT#2, INT #2A), and Intrusion 3 (INT#3). Gold mineralization is hosted in all intrusions, but INT#1 is the best mineralized body followed by INT#2. SEM-CL imaging has identified two different veins (V1 and V2) and four distinct generations of quartz formation in the different intrusions. These are: (i) CL-light gray, mosaic-equigranular quartz (Q1), (ii) CL-gray or CL-bright quartz (Q2) that dissolved and was overgrown on Q1, (iii) CL-dark and CL-gray growth zoned quartz (Q3), and (iv) CL-dark or CL-gray micro-fracture quartz fillings (Q4). Fluid inclusion studies show that the gold-hosted early phase Q1 quartz of V1 and V2 veins in INT#1 and INT#2 was precipitated at high temperatures (between 424 and 594 °C). The coexisting and similar ranges of Th values of vapor-rich (low salinity, from 1% to 7% NaCl equiv.) and halite-bearing (high salinity: &gt;30% NaCl) fluid inclusions in Q1 indicates that the magmatic fluid had separated into vapor and high salinity liquid along the appropriate isotherm. Fluid inclusions in Q2 quartz in INT#1 and INT#2 were trapped at lower temperatures between 303 and 380 °C and had lower salinities between 3% and 20% NaCl equiv. The zoned Q3 quartz accompanied by pyrite in V2 veins of both INT#2 and INT#3 precipitated at temperatures between 310 and 373 °C with a salinity range from 5.4% to 10% NaCl eq. The latest generation of fracture filling Q4 quartz, cuts the earlier generations with fluid inclusion Th temperature range from 257 to 333 °C and salinity range from 3% to 12.5% NaCl equiv. The low salinity and low formation temperature of Q4 may be due to the mixing of meteoric water with the hydrothermal system, or late-stage epithermal overprinting. The separation of the magmatic fluid into vapor and aqueous saline pairs in the Q1 quartz of the V1 vein of the INT#1 and INT#2 and CO2-poor fluids indicates the shallow formation of the Kışladağ porphyry gold deposit.

Infrared microthermometry of fluid inclusions in transparent to opaque minerals: challenges and new insights

Infrared (IR) studies of fluid inclusions in opaque minerals provide direct insights into the ore-forming fluids. However, the challenge posed by the “warming effect” of IR light during microthermometry remains unresolved. Here we address this problem and show that the deviation in temperature of phase changes within fluid inclusions caused by IR light is more common than what was thought before. Our results reveal that transparent to translucent minerals (e.g., quartz, sphalerite) also absorb IR energy. Since IR absorption is influenced by the host mineral, the fluid inclusions hosted by different minerals exhibit different degrees of deviation in temperature during microthermometry. The Fourier transform infrared (FTIR) spectra do not display a consistent correlation between the band gap energy of a mineral and its absorption of IR energy. Minerals with low band gap energy, such as pyrite, absorb limited IR energy, resulting in small deviations of fluid inclusion data. In contrast, this deviation could be significant for fluid inclusions hosted in minerals with a relatively high band gap energy, such as iron-rich sphalerite and wolframite. Substitution of iron increases the absorption of IR energy in these minerals. The substitution of trace elements may also play a role. Our quantitative analyses confirm that using the lowest possible IR light intensity with the smallest diaphragm minimizes the “warming effect” of IR light. We also propose an improved cycling method as a better option where high IR light intensity is required.