Inclusion Assemblages Research Articles

Geology and genesis of the Chomalu polymetallic deposit, NW Iran

The vein-type Chomalu polymetallic epithermal deposit, located in the Tarom-Hashtjin Metallogenic Province (THMP), western Alborz, formed in two separate mineralized zones controlled by fault zones within andesite and olivine basalt host rocks. The deposit occurs adjacent to the Varmarziar quartz monzonite, quartz monzodiorite, and monzosyenite intrusions. Ore mineralization consists of three stages; (i) an early stage dominated by gold-bearing breccia veins mainly consisting of quartz, pyrite, arsenopyrite, chalcopyrite, minor magnetite and bornite assemblages, (ii) a main stage, which is divided in three sub-stages I, II and III. Sub-stage I is dominated by Pb-Zn-Cu-Ag-rich mineral assemblages including galena, Fe-poor sphalerite, minor chalcopyrite, tetrahedrite, and pyrite. Sub-stage II is only represented by galena and Fe-poor sphalerite. Sub-stage III is defined by Fe-Mn-Zn-Ca-Mg carbonate minerals. The late supergene stage (iii) is characterized by malachite, covellite, chalcocite, cerussite, and Fe-oxide and hydroxides. Microthermometry of primary fluid inclusion assemblages indicate homogenization temperatures (Th) of 253° to 305 °C and salinities of 3.5–4.82 wt% NaCl equivalent in the early stage. The main stage indicates homogenization temperatures (Th) of 200° to 280 °C and 180° to 235 °C with salinities of 2.18–4.96 and 0.53–2.90 wt% NaCl equivalent for the sub-stages I and II, respectively. Ore precipitation was possibly caused by simultaneous dilution and cooling of hydrothermal fluids. The epithermal mineralization in the Chomalu deposit may be classified as an E-type intermediate sulfidation epithermal deposit in the extensional back-arc regime.

Entrapment history of aqueous fluids in Archean cherts from the Barberton Greenstone Belt, South Africa

We performed fluid inclusion microthermometric analyses of four cherts and one silicified anhydrite from various successions of the Mesoarchean Barberton Greenstone Belt, South Africa, and constrained the entrapment conditions and provenance of fluids that were incorporated during different magmatic or tectono-thermal events. Major saline aqueous fluid inclusion assemblages in cherts from the 3.2 Ga old Mendon Formation (Barite Valley Syncline, Power Line Road) and from the 3.42–3.45 Ga old uppermost Hooggenoeg Formation (Buck Reef Chert) correspond to fluids incorporated during metamorphism. Their documented P-T conditions compare well with reported petrologic constraints. In Buck Reef chert we also found a distinctive suite of inclusions with high salinities and low homogenization temperatures (61–118 °C). This is in agreement with a hydrothermal formation of these fluids at subseafloor conditions. Finally, we investigated quartz in a silicified anhydrite and its sandstone host from the Stolzburg Syncline (Moodies Group, ca. 3.2 Ga). One major inclusion assemblage in the evaporite and in adjacent quartz within the embedding sandstone share the P-T range observed for the cherts from the Mendon Fm. Its incorporation is postdating the silicification process. We also found two higher temperature inclusion types that favor a magmatic origin. In particular, one of these inclusion suites is only observed in quartz within the matrix and likely mirrors trapping of brines associated with magmatic fluids predating sedimentation of the sandstone, e.g., from the potential magmatic source region of the quartz. Our results indicate the presence of coexisting aqueous fluids in cherts derived from distinct source environments, spanning an age range of several 100 Ma on a grain scale. The earliest hydrothermal aqueous inclusions were documented in a Buck Reef chert (3.42–3.45 Ga), whereas the majority of aqueous fluids are metamorphic in origin and likely related to the documented 3.23 Ga and 3.1 Ga tectono-thermal events. Our findings demonstrate that constraining environmental conditions by the volatile record of Archean cherts requires analyses on inclusion, not grain scale.

The Formation of Magmatic-Hydrothermal Features in Sn-Mineralized and Barren Tasmanian Intrusions, Southeast Australia: Insights from Quartz Textures, Trace Elements, and Microthermometry

Abstract Tasmania is the most important tin province in Australia, having been endowed with >0.65 Mt Sn. Some granitic intrusions in western Tasmania have distinctive tourmaline- and quartz-rich magmatic-hydrothermal features, whether they are mineralized (e.g., Heemskirk Granite) or barren (Pieman Heads Granite). The Devonian Heemskirk and Pieman Heads plutons crop out on the western coast of Tasmania and are characterized by similar mineralogical and geochemical compositions and ages. The magmatic-hydrothermal textural features include tourmaline patches, tourmaline orbicules, and tourmaline-muscovite veins, as well as miarolitic cavities and quartz-fluorite-sulfide veins in the Heemskirk Granite. Cathodoluminescence (CL) imaging, laser ablation-inductively coupled plasma-mass spectrometry, and microthermometric analyses of quartz have revealed the physicochemical evolution of the magmatic-hydrothermal fluids from which these tourmaline- and quartz-bearing assemblages precipitated. High Ti quartz (20–28 ppm) in tourmaline patches, orbicules, and cavities typically have homogeneous CL-bright intensity, whereas CL-dark fractures have cut and/or offset the CL-bright and -gray domains that characterize low Ti quartz (3.4–8.5 ppm) from the tourmaline veins. The earliest fluid inclusion assemblages in the quartz-tourmaline orbicules and cavities have a salinity range from 3 to 14 wt % NaCl equiv with intermediate density and were probably trapped at lithostatic pressures of 1.57 ± 0.2 kbar and temperatures of 550° to 570°C, suggesting a depth of 5.9 ± 0.8 km. Prolonged depressurization and cooling may have led to the evolution of a brine (~39 wt % NaCl equiv salinity) from the primary magmatic liquid, which formed halite-bearing hypersaline inclusions in the tourmaline orbicules. Continuous pressure decrease explains the intense brittle failure and fluid migration outward from the apical portions of the pluton, where magmatic fluids partially mixed with and were cooled by external meteoric water. These mechanisms triggered the formation of tourmaline-muscovite-quartz veins and local cassiterite-bearing greisens from a moderate-salinity fluid (~12 wt % NaCl equiv) at temperatures of ~300°C and hydrostatic pressures of 120 bars. Retrograde dissolution textures evident from CL-bright quartz cores surrounded by oscillatory growth zones with gray CL response characterize the low Ti (<1 ppm) and high Al (500–1,000 ppm) quartz from the fluorite-sulfide veins that precipitated from a low-salinity (5.7 wt % NaCl equiv) acidic fluid at temperatures of 200° ± 25°C and hydrostatic pressures of <50 bars. High Sb concentrations (up to 80 ppm) in quartz may be an indicator of low-temperature base metal mineralization related to granitic intrusions. Abundant fluid percolation, protracted fractional crystallization, and high tin concentrations in exsolved hydrothermal fluids may explain why the Heemskirk Granite is well endowed in Sn and base metal deposits, whereas the Pieman Heads Granite is barren.

Magmatic-Hydrothermal Processes of Vein-Type Haman-Gunbuk-Daejang Copper Deposits in the Gyeongnam Metallogenic Belt in South Korea

Haman, Gunbuk, and Daejang deposits are neighboring vein-type hydrothermal Cu deposits located in the SE part of the Korean Peninsula. These three deposits are formed by magmatic-hydrothermal activity associated with a series of Cretaceous granodioritic intrusions of the Jindong Granitoids, which have created a series of veins and alterations in a hornfelsed shale formation. The copper deposits have common veining and alteration features: 1) a pervasive chlorite-epidote alteration, cut by 2) Cu-Pb-Zn-bearing quartz veins with a tourmaline-biotite alteration, and 3) the latest barren calcite veins. Chalcopyrite, pyrite, and pyrrhotite are common ore minerals in the three deposits. Whereas magnetite is a dominant mineral in the Haman and Gunbuk deposits, no magnetite is present, but sphalerite and galena are abundant in the Daejang deposit. Ore-bearing quartz veins have three types of fluid inclusions: 1) liquid-rich, 2) vapor-rich, and 3) brine inclusions. Hydrothermal temperatures obtained from the brine inclusion assemblages are about 340–600, 250–500, and 320–460°C in the Haman, Gunbuk, and Daejang deposits, respectively. The maximum temperatures (from 460 to 600°C) recorded in the fluid inclusions of the three deposits are higher than those of the Cu ore precipitating temperature of typical porphyry-like deposits (from 300 to 400°C). Raman spectroscopy of vapor inclusions showed the presence of CO2 and CH4 in the three deposits, which indicates relatively reduced hydrothermal conditions as compared with typical porphyry deposits. The Rb/Sr ratios and Cs concentrations of brine inclusions suggest that the Daejang deposit was formed by a later and more fractionated magma than the Haman and Gunbuk deposits, and the Daejang deposit has lower Fe/Mn ratios in brine inclusions than the Haman and Gunbuk deposits, which indicates contrasting redox conditions in hydrothermal fluids possibly caused by an interaction with a hosting shale formation. In brines, concentrations of base metals do not change significantly with temperature, which suggests that significant ore mineralization precipitation is unlikely below current exposure levels, especially at the Haman deposit. Ore and alteration mineral petrography and fluid inclusions suggest that the Haman deposit was formed near the top of the deep intrusion center, whereas the Gunbuk deposit was formed at a shallower intrusion periphery. The Daejang deposit was formed later at a shallow depth by relatively fractionated magma.

A New Approach to Characterizing Deposit Type Using Mineral Inclusion Assemblages in Gold Particles

Abstract Mineral inclusions within native gold are features of lode gold occurrences that are preserved in detrital particles. Inclusion assemblages in populations of gold particles in placers from specific localities are revealed through inspection of polished sections, and assimilation of robust data sets permits reconstruction of the lode source mineralogy. Inclusion assemblages differ considerably according to the source deposit type, and various approaches have been employed to graphically represent inclusion mineralogy. We present a simple method for depicting and comparing inclusion assemblages using a single standardized radar diagram template that illustrates the proportions of 11 metal and five nonmetal (and metalloid) elements in each inclusion assemblage. The Canadian Cordillera hosts many different gold-bearing deposit types and is an ideal terrane in which to develop a globally applicable methodology. Although placer gold is widespread, the location and nature of source mineralization is commonly unclear. This study is based on the inclusion suites recorded in 37 sample sets of gold particles from both placer and lode localities. Radar diagrams describing inclusion assemblages show clear generic differences according to deposit type. Diagnostic signatures have been established and act as templates against which samples of unknown origin may be compared. This approach permits differentiation between populations of gold particles formed in different magmatic systems (low-sulfidation epithermal, calc-alkalic porphyry, and alkalic porphyry), which may all be distinguished from gold formed in orogenic (amagmatic) mineralization. Metallic element signatures are most useful in differentiating gold from different magmatic hydrothermal systems, whereas nonmetallic elements allow for classification of orogenic gold subtypes. Comparisons of mineral inclusion signatures from gold in the Canadian Cordillera to samples from similar geologic settings worldwide suggest that this approach to gold fingerprinting is globally applicable. Therefore, the geochemical signatures of inclusion assemblages provide a robust indication of deposit type and may be applied in exploration to illuminate regional metallogeny in areas where relationships between placer deposits and their source(s) may be unclear.

Genesis and mineralization style of gold occurrences of the Lower Lom Belt, Bétaré Oya district, eastern Cameroon

The Bétaré Oya district has a substrate composed of the Neoproterozoic metavolcanic-metasedimentary rocks of the Lom Belt in eastern Cameroon. The district is well-known for alluvial gold mining activities, however, the primary gold mineralization has received little attention. In the current study, we newly report geological, ore mineralogy, fluid inclusion microthermometric, laser Raman spectroscopy, and stable isotope data from auriferous veins of the Lom Belt to characterize the style of gold mineralization and to constrain the origin of the ore-forming fluids. The auriferous quartz veins are laminated, fractured, N- to NE-trending, and spatially associated with the Bétaré Oya Shear Zone. Fieldwork coupled with microscopic examination and the textural relationships of ore minerals revealed two stages of mineralization. The first stage is characterized by the presence of pyrite, sphalerite, galena, chalcopyrite, pyrrhotite, hematite, petzite, hessite, wolframite, electrum, and gold, while the second stage is characterized by the presence of a later deposition of galena and pyrite, as well as minor greenockite. The gangue minerals are quartz, sericite, muscovite, chlorite, calcite, ankerite, and barite, whereas the supergene assemblage (stage 3) includes goethite, hematite, covellite, and enargite. Two fluid inclusion assemblages containing three types of inclusions are identified in mineralized quartz veins. Assemblage 1 consists of gold-related types 1 and 2 with daughter minerals consisting of nahcolite, magnesite, and arsenolamprite (black native arsenic), whereas assemblage 2 consists of post-gold type 3 fluid inclusions. Type 1 has H2O-NaCl-CO2 ± N2 ± CH4 primary fluid inclusions (FI), with a TmCO2 ranging from −59.8 to −56.6 °C, salinities from 0.5 to 10.8 wt% NaCl eq., densities from 0.87 to 1.00 g.cm−3, and total homogenization temperatures between 280 and 360 °C. Type 2 contains CO2 (±H2O-NaCl) ± N2 ± CH4 FI that exhibit TmCO2 ranging between −60.0 and −56.7 °C, ThCO2 from 13 to 25 °C, and densities between 0.73 and 0.85 g.cm−3. Type 3 shows H2O-NaCl FI with salinities between 0.2 and 10.1 wt% NaCl eq., densities between 0.82 and 0.98 g.cm−3, and total homogenization temperatures from 160 to 235 °C. Measured δ18O for gold-bearing quartz (+11.5 to +16.0‰), δD from FI (−50.6 to −21.8‰), δ13C from FI (−5.8 to −5.5‰), and δ34S from galena and pyrite grains (+5.3‰ and +8.2‰, respectively) suggest a metamorphic source as most likely for the ore-forming fluids and sulfur, although a mantle CO2 contribution cannot be ruled out.The gold deposition probably took place by fluid-rock interaction and fluid unmixing at ∼310 °C and at a depth of about 6–9 km. The ore-forming fluid was a low salinity (∼6.2 wt% NaCl eq.) H2O-NaCl-CO2 ± N2 ± CH4 solution. These data revealed that the Lom Belt gold mineralization is best classified as a mesozonal orogenic gold deposit.

Multi-Stage Metamorphism of the South Altyn Ultrahigh-Pressure Metamorphic Belt, West China: Insights into Tectonic Evolution from Continental Subduction to Arc–Backarc Extension

Abstract The South Altyn ultrahigh-pressure (UHP) metamorphic belt is claimed to host the deepest subducted continental crust based on the discovery of former stishovite, and thus can provide unique insights into the tectonic evolution from deep continental subduction and exhumation to arc–backarc extension. In this article, we present detailed studies of petrography, mineral chemistry, phase equilibria modelling and zircon U–Pb dating for three representative samples involving garnet amphibolite (A1531 & A1533) and associated garnet-biotite gneiss (A1534) from the UHP belt. Three phases of metamorphism are inferred for the rocks. The first phase high-pressure (HP)–UHP-type eclogite facies is represented by the mineral assemblages of garnet and phengite inclusions in zircon and garnet cores with the high grossular (XGrs = 0.33–0.34). The Si contents of 3.40–3.53 and 3.24–3.25 p.f.u. in phengite inclusions yield pressure conditions of >1.7–2.3 GPa for A1533 and 2.5–2.55 GPa for A1534 at a fixed temperature of 770°C. The second phase medium-pressure (MP)-type overprinting of garnet amphibolite facies shows P–T conditions of 0.8–1.2 GPa/750–785°C based on the stability fields of corresponding mineral assemblages, the measured isopleths of Ti contents in biotite and amphibole cores, and XGrs in garnet. The third phase low-pressure (LP) type overprinting includes early-stage heating to peak granulite facies followed by cooling towards a late-stage amphibolite facies. The peak granulite facies is represented by the high Ti amphibole mantle, high Zr titanite and the intergrowths of clinopyroxene + ilmenite in A1533 & A1531, with P–T conditions of 800–875°C/0.80–0.95 GPa. The late-stage is defined by the solidus assemblages, giving P–T conditions of 0.5–0.7 GPa/720–805°C. U–Pb geochronology on metamorphic zircons from A1533 and A1534 gives three ages of c. 500 Ma, c. 482 Ma and c. 460 Ma. They are interpreted to represent the HP-UHP, MP and LP types of metamorphism respectively, based on cathodoluminescence (CL) images, mineral inclusions and trace element patterns. Combining the regional geology and metamorphic evolution from the Altyn Orogen, a tectonic model is inferred, including the following tectonic scenarios. The small Altyn Microcontinent (AM) was subducted to great mantle depths with dragging of the surrounding vast oceanic lithosphere to undergo the HP-UHP eclogite facies metamorphism during the early subduction stage (c. 500 Ma) of the Proto-Tethys Ocean. Then, the subducted slabs were exhumed to a thickened crust region to be overprinted by the MP-type assemblages at c. 482 Ma. Finally, an arc–backarc extension was operated within the thickened crust region due to the retreat of subduction zones. It caused evident heating and the LP-type metamorphic overprinting at c. 460 Ma, with a fairly long interval of 30–40 Myr after the HP-UHP metamorphism, distinct from the short interval of <5–10 Myr in the Bohemian Massif.

Saline fluids drive Cu mineralization in Precambrian metasediments: Evidence from the Trans-North China Orogen

Epigenetic hydrothermal Cu-(Co) deposits are widely distributed in the Zhongtiao Mountains within the Paleoproterozoic Trans-North China Orogen in the North China Craton. Here we investigate the Henglingguan Cu deposit hosted in metasediments to gain insights on the fluid evolution characteristics associated with the ore formation. This Cu deposit is characterized by early disseminated mineralization and late quartz-vein type mineralization. The negative sulfur isotope values (about −10‰) of the vein sulfides suggests a sedimentary rock source for sulfur associated with the hydrothermal Cu mineralization. The coexistence of CO2-rich carbonic inclusions and halite-bearing inclusions within a single fluid inclusion assemblage suggests fluid immiscibility in the H2O-NaCl-CO2 system. Based on the fluid phase analysis, our data show that the parent ore fluid was characterized by X(NaCl) of 10 to 20 mol.% (salinity of around 23 to 41 wt% NaCl) and X(CO2) of 10 to 15 mol.% before fluid immiscibility. The estimated temperature–pressure conditions of 320 to 340 °C and 3.5 to 4.0 kbar based on the carbonic fluid inclusions is suggested to be close to the conditions of fluid immiscibility. In-situ composition analyses of fluid inclusions by LA-ICP-MS show that the ore fluids contained Na, Ca, Fe and Mn, and evolved towards Sr-Ba enrichment. The data are consistent with a metamorphic brine derived from the sedimentary rocks. The brine-rich metamorphic fluids were likely derived from the dehydration and decarbonation of evaporitic carbonate rocks developed on the basin margin or a passive continental margin. The decompression during post-collisional exhumation triggered fluid immiscibility resulting in the quartz vein-type Cu mineralization.

Deciphering early Neoproterozoic and Cambrian high-grade metamorphic events in the Archean/Mesoproterozoic Rauer Group, East Antarctica

A U–Pb geochronological and rare earth element (REE) geochemical study of zircon, monazite and garnet was carried out on rocks of Mesoproterozoic and Archean crustal domains in the Rauer Group of East Antarctica. The zircon and monazite U–Pb age spectra define concordia intercepts mainly at ca 1200, 990–910, and 530–500 Ma, suggesting that the Mesoproterozoic crustal domain is a significant part of the Rayner Complex that also underwent early Neoproterozoic and Cambrian high-grade metamorphism. The age data, mineral inclusion assemblages in zircon, and REE features for zircon and garnet indicate that all the granulite facies mineral assemblages in this domain might have formed during early Neoproterozoic metamorphism. Some zircon and monazite grains or domains have experienced complete U–Pb isotopic resetting during Cambrian reworking, which did not result in new zircon and monazite growth. The Archean crustal domain consists mainly of Paleo–Mesoarchean orthogneisses interleaving with Neoproterozoic paragneisses that contain inherited metamorphic zircon domains with ages of ca 1330 and 970 Ma. The mineral assemblages in these gneisses formed during a single Cambrian granulite facies metamorphic event. Garnet-bearing and -free rocks cooled to solidus temperatures at ca 527 and 517 Ma, respectively, whereas the isotopic system of early-crystallized zircon was completely reset during the growth of new zircon. As such, all the zircon domains in the same sample could have the same concordant or weighted mean age. The 511 Ma monazite and 506 Ma zircon overgrowths in a paragneiss have REE contents in equilibrium with garnet, implying that later modification and isotopic resetting of zircon and monazite might have resulted in younger U–Th–Pb ages and, in this case, establishing the age–mineral assemblage relationship based on REE partition coefficients between zircon/monazite and garnet may be invalid. Overall, the available data support the notion that different crustal components of the Rauer Group were juxtaposed in the Cambrian as a consequence of the Gondwana assembly.

Olivine as an indicator of polygenic assemblage of inclusions from Late Cenozoic volcanic rocks in the Tunka Valley, Baikal Rift Zone

Research subject. Olivine of deep-seated inclusions from Late Cenozoic volcanic rocks of the Tunka valley.Materials and methods. Electron probe microanalysis of minerals with determination of calcium in olivine by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). A complex of methods of quantitative chemical analysis for determination of major oxides in rocks.Results. Deep-seated nodules from Late Cenozoic volcanic rocks in the Tunka Valley characterize the root part of the Slyudyanka metamorphic subterrane, formed during collisional accretion of the Khamar-Daban terrane to the Siberian paleocontinent in the Early Paleozoic and reactivated in the Late Cenozoic. From bulk compositions of deep-seated nodules, groups of xenoliths (restites, rocks closed to the primitive mantle composition, and metasomatites) and cognate rocks (magmatic and magmatic-metasomatic genesis) were distinguished. The polygenetic character of this assemblage is designated by variations of major and trace-element abundances in olivine. In olivine of xenoliths, forsterite (Fo) ranges from 86 to 91% under abundances of NiO – 0.2–0.5, MnO – 0.1–0.2, and CaO < 0.16 wt %. In olivine of magmatic rocks and those of mixed (magmatic-metasomatic) genesis, Fo decreases to 64% with a decrease in NiO (Ni/Mg ratio) and an increase in MnO and CaO. Magmatic olivine shows 0.16–0.21 wt % MnO and about 0.1 wt % CaO, in contrast to olivine of magmatic-metasomatic genesis, enriched in these oxides to 0.5 wt % and 0.45 wt %, respectively, with an increase in the Mn/Fe and Ca/Fe ratios. Olivine trends of the shallow polygenetic assemblage of inclusions are emphasized when compared with those of the deeper ones from volcanic rocks of the Vitim and Oka plateaus that represent a transition from the garnet to the spinel-pyroxene facies in mantle peridotites.Conclusion. Olivine compositions are indicative of the shallow origin of peridotite and pyroxenite nodules from the crust and crust–mantle transition in the root part of the Slyudyanka collision zone reactivated in the Late Cenozoic.

Fluid evolution along the Patterson Lake corridor in the southwestern Athabasca Basin: constraints from fluid inclusions and implications for unconformity-related uranium mineralization

The Patterson Lake corridor (PLC) in the southwestern margin of the Athabasca Basin hosts several high-grade uranium deposits. These deposits are located in the basement up to 900 m below the unconformity surface, raising questions about their affiliation with typical unconformity-related uranium (URU) deposits elsewhere in the basin. Based on cross-cutting relationships four pre- and three syn- to post-mineralization quartz generations were identified. Fluid inclusion analyses indicate that pre-mineralization fluids have salinities ranging from 0.2 to 27.2 wt% NaCl equiv. (avg. 9.0 wt%), whereas syn-mineralization fluids have salinities ranging from 8.8 to 33.8 wt% NaCl + CaCl 2 (avg. 25.4 wt%), with NaCl- and CaCl 2 -rich varieties. The homogenization temperatures ( T h ) of fluid inclusions from pre-mineralization quartz range from 80 to 244°C (avg. 147°C), and from syn-mineralization quartz range from 64 to 248°C (avg. 128°C). Fluid boiling is indicated by the co-development of liquid-dominated and vapour-dominated fluid inclusions within individual fluid inclusion assemblages from the syn-mineralization quartz and is related to episodic fluid pressure drops caused by reactivation of basement faults. Our results indicate that composition and P – T conditions of the ore fluids in the PLC are comparable to those of typical URU deposits in the Athabasca Basin, indicating that the uranium deposits in the PLC formed under similar hydrothermal conditions. Episodic reactivation of basement faults was an important driving force to draw uraniferous fluids from the basin and reducing fluids from the basement to the mineralization sites, forming deep basement-hosted deposits. Supplementary material: Table 1, microthermometric results of type-2 and -5 fluid inclusion assembladges and isolated inclusions from the Patterson Lake corridor, and table 2, microthermometric results of type-6 fluid inclusions from the Patterson Lake corridor are available at https://doi.org/10.6084/m9.figshare.c.5510179 Thematic collection: This article is part of the Uranium Fluid Pathways collection available at: https://www.lyellcollection.org/cc/uranium-fluid-pathways

Metallogeny of the Dagangou Au-Ag-Cu-Sb Deposit in the Eastern Kunlun Orogen, NW China: Constraints from Ore-Forming Fluid Geochemistry and S-H-O Isotopes

A study of ore-forming fluid geochemistry and S-H-O isotopes has been conducted to reveal the metallogeny of the Dagangou Au-Ag-Cu-Sb deposit, NW China. Three mineralization stages are identified within Dagangou, including ankerite-quartz-pyrite (Ank-Qz-Py), quartz-Au-sulfide (Qz-Au-Sul), and quartz-calcite (Qz-Cal) stages. Four types of primary fluid inclusions (FIs) and various fluid inclusion assemblages (FIAs) are identified by petrographic observations, including pure CO2, CO2-bearing, vapour-liquid and halite-bearing FIs, most of which are distributed randomly or along growth zones. Raman spectroscopy suggests that the fluid vapour is mainly H2O and CO2 with minor N2, CH4, and C2H6. The homogenization temperatures of primary FIs from the Ank-Qz-Py to Qz-Au-Sul to Qz-Cal stages are 208.7–313.4°C, 184.3–251.7°C, and 167.3–224.2°C, with salinities of 3.4–13.4 wt.%, 5.4–39.5 wt.% and 4.9–11.4 wt.% NaClequiv.. Sulfide S isotopes show a predominant source of sedimentary sulfur with minor magmatic sulfur, as evidenced by δ34S values of -1.4–5.1‰. The FI H-O isotopes reveal that the primary fluid was derived from magmatic waters mixed with minor sedimentary water (δD=−90.0to -86.8‰,δ18OH2O=6.7–7.4‰) and then was significantly diluted by meteoric water in the Qz-Cal stage (δD=−97.2to -95.7‰,δ18OH2O=3.9–4.3‰). Thus, the ore-forming fluid geochemistry and S-H-O isotopes indicate that fluid boiling occurred in the Qz-Au-Sul stage due to a pressure reduction from 85 MPa to 70 MPa, which led to the phase separation of fluid, loss of CO2, increase in residual fluid salinity, and formation of immiscible FIAs and halite-bearing FIs in the Qz-Au-Sul stage. This indicates a system moving away from a primary equilibrium, resulting in the deposition of metallic elements. The Dagangou Au-Ag-Cu-Sb deposit, therefore, is an orogenic gold-polymetallic deposit with a mesozonal depth of 5.9–7.5 km.

P-T-t-d evolution of Brattnevet Peninsula, Larsemann Hills, East Antarctica

Brattnevet Peninsula, Larsemann Hills, comprises metamorphosed acidic rocks, sedimentary rocks and basic igneous suite belonging to timespan of 920-478 Ma and have experienced three phases of deformation and metamorphism. The first and second phase of deformation gave rise to moderately plunging isoclinal and open to closed folds, respectively and the third phase resulted into broad warping. M1 prograde (amphibolite) and M2 peak (granulite) and M3 retrograde (decompression) events have been inferred. In pyroxene granulite, inclusion assemblage of quartz, biotite, plagioclase and ilmenite in hornblende correspond to the relict M1 amphibolite grade event associated with D1-D2 deformation events (occurred at ~920 Ma; 538–692 °C at ~6kb). However, the presence of enstatite, diopside, quartz, biotite, ilmenite and plagioclase in pyroxene granulite and sillimanite, biotite and almandine garnet and plagioclase in metapelite indicates granulite facies during M2 event (~530 Ma; 868–920 °C at ~6.7 kb). The retrograde M3 event (500-480 Ma; ~620 °C at ~5kb) is largely depicted in pyroxene granulite and metapelite by coronal and symplectite cooling textures. The study shows a clockwise P-T path and suggests the crustal evolution of Larsemann Hills area have commenced at the wanning stages of Grenvillian Orogeny and imprinted by the Pan-African orogenic event in the terrane.

A Preliminary Study on the Aghbolaq (Fe, Cu) Skarn Deposit, Oshnavieh, NW Iran: Constraints on Metasomatic Fluid Evolution

AbstractThe Aghbolaq skarn deposit is located in the Urumieh‐Golpayegan plutonic belt, NW Iran. The garnetite skarn (stage I) has been intensely cross‐cut by the magnetite‐garnet skarn (stage II) which were, in turn, cut and offset by the ore‐hosting quartz veins/veinlets (stage III). The predominance of andradite (Adr82.5–89.1) and its high Fe3+/Al ratio (up to 1685) apparently supports the high fO2, salinity and prevalence of magmatic/hydrothermal fluids involved, rather than meteoric waters, during the magnetite‐garnet skarn formation. Two major groups of fluid inclusions, namely aqueous (LV, LVS) and aqueous–carbonic (LVC, LLCVC), were recognized in garnet and quartz veins that, especially in growth zones and along intra‐granular trails, better display fluid inclusion assemblages (FIAs) than those in clusters. The prograde magnetite‐garnet skarn was formed by the metasomatic fluid at relatively high Th (209–374°C), under a lithostatic pressure of ∼200 bars. The retrograde mineralized quartz veins were formed at temperatures ranging from 124°C to 256°C, by dilute and less saline (2.57–11.93 wt% NaCl eq.) hydrothermal fluids under a hydrostatic pressure of ∼80 bars. The fluid evolution of the Aghbolaq skarn began with an earlier simple cooling of metasomatic fluid during the prograde stage, followed by the later influx of low salinity meteoric fluids during the retrograde stage.

Multiphase dolomitization and hydrothermal alteration of the Upper Cambrian-Lower Ordovician carbonates in the Gucheng uplift, Tarim Basin (NW China)

The thick Upper Cambrian-Lower Ordovician carbonates of the Gucheng uplift (southeastern Tarim Basin, northwestern China) have been extensively dolomitized, leading to the formation of massive dolomite reservoirs. The origins of different dolomites and their original fluids were studied based on an integrated analysis of petrology, geochemical data (C, O, and Sr isotopes) and fluid inclusion assemblages (FIAs). The D1 (very finely to finely crystalline planar-e/s dolomite) and the D2 (finely to medium crystalline planar-s/a dolomite) dolomites were formed from the mesosaline to penesaline seawater in the subsurface to shallow burial stage. The increasing temperatures and changes of dolomitizing fluid properties led to the formation of different textures and distributions. Both D1 and D2 dolomites, compared to the matrix limestone, show no obvious improvements in porosity, which indicates that early dolomitizations did not create or preserve the primary pores. CD1 (finely to medium crystalline nonplanar-e/s dolomite) and CD2 (medium to coarsely crystalline nonplanar saddle dolomite) dolomites are dolomite cements of hydrothermal alteration at the deep burial stage. Due to changes in temperature and Mg2+ concentrations of the hydrothermal fluids, the precipitation of CD1 and CD2 dolomites exhibited different textures and filling sequences. In the limited space of matrix dolomite, hydrothermally altered dolomite was dominated by D3 (medium to coarsely crystalline nonplanar-a dolomite). The late calcite fillings were precipitated on account of decreasing Mg2+ concentration at the end of the hydrothermal alteration. Hydrothermal fluids, depleted in Mg2+ and enriched in silica, could be deep-basinal brines driven by the strongly transpressional faults from the Late Caledonian to Early Hercynian. Compared to D1 and D2 dolomites, the higher porosities in hydrothermally altered dolomite indicate that hydrothermal fluids are constructive to the formation of reservoirs. Hydrothermal alteration products showed obvious differences between the dolomite and limestone, and led to different performance in physical characteristics. Hence, the hydrothermally altered reservoirs in the eastern Tarim Basin were mostly distributed in the dolomite but were less developed in the limestone. This study provides new insights into the evolution of dolomitizing fluids that are responsible for multiple dolomitization events and the formation of the hydrtothermal dolomite in the Tarim Basin and elsewhere around the world.

Fluid diversity in the gold-endowed Archean orogenic systems of the Abitibi greenstone belt (Canada) I: Constraining the PTX of prolonged hydrothermal systems

Although many aspects of orogenic gold systems are well understood, considerable debate surrounds the chemistry of the ore-forming fluids and their relevance to mineralization. This may in part relate to the lack of both regional-scale studies and a uniform approach in studying fluid inclusions in these complex and protracted hydrothermal ore systems. In order to address this problem, over 30 orogenic-type deposits covering a variety of geological settings were systematically studied using the fluid inclusion assemblage (FIA) protocol integrated with microthermometry in the gold-endowed Abitibi greenstone belt, Canada. A diversity of fluid inclusion types are documented (i.e., carbonic CO2-CH4, aqueous-carbonic, aqueous), in addition to complex textural features, in particular decrepitation clusters which are rarely reported in the literature or the deposits studied; together these data provide new insight to these ore systems. Furthermore, we note that the fluid composition is significantly affected by the proximity of carbon-rich sedimentary units which are capable of producing a carbonic-only fluid. In later stages of hydrothermal evolution, variable XCO2 in aqueous-carbonic inclusions reflects variable degrees of fluid-rock interaction. Pressure cycling is an integral part of these systems, but petrographic evidence of in situ fluid unmixing is lacking, and cannot be accounted for gold mineralization. Vein-style gold is not related to fluid chemistry and, where best constrained with petrography (i.e., visible Au near fluid inclusions), P-T estimates based on fluid inclusion microthermometry link the mineralization to the later, low-pressure (<1.5 kbar) stages of deposit evolution. Accordingly, gold is coeval with quartz vein formation in epizonal deposits whereas it postdates quartz in the ore dominant mesozonal ore systems.

Microthermometry and noble gas isotope analysis of magmatic fluid inclusions in the Kerman porphyry Cu deposits, Iran: constraints on the source of ore-forming fluids

This paper reports microthermometric and noble gas isotope data for fluid inclusion assemblages (FIAs) with evidence of phase separation, i.e. coexisting vapor-rich and halite-saturated inclusions, hosted in the early-formed quartz stockwork veins and post-magmatic quartz eye crystals in two economic porphyry Cu deposits (PCDs; Sar Cheshmeh and Miduk) and two sub-economic prospects (Sar Kuh and Abdar) from the Kerman porphyry copper belt (KPCB), Iran. The multiphase halite-saturated inclusions (i.e., Type I) in all studied PCDs and prospects had the highest homogenization temperature (Th = 525–594 °C) and salinities (63–73 wt% NaClequiv), whereas vapor-rich inclusions (Type II) had lower Th (362–460 °C). Fluid inclusion data show that like economic PCDs, the sub-economic prospects were formed in a fertile hydrothermal system and benefited from a mineralizing fluid, which evolved from a primary hot (mostly > 400 °C), metal-rich and oxidized fluid (as evidenced by the presence of opaque- and hematite-bearing fluid inclusions) of unknown salinity, which underwent a phase separation process to form both brine and vapor phases in the early stage of mineralization. The helium abundance and its isotopic composition document a mantle-derived magmatic source for the primary ore fluid in the formation of the studied PCDs and prospects (3He/4He ratios ranging from 0.46 to 2.8 Ra, corresponding to a mantle He contribution in ore fluids between ~ 7 and 45%). However, subsequent hydrothermal processes, i.e., vapor–brine phase separation, fluid-rock interaction with crustal rocks, and mixing with meteoric pore water containing dissolved atmospheric (e.g., Ne and Xe) and some crustal noble gases (e.g., Ar), changed the initial noble gas composition of the magmatic ore fluid to predominantly atmospheric- and crustal-like compositions. A significant proportion of mantle-derived He (up to 45%) in high-temperature (513–594 °C) and high-salinity (61.5–73 wt% NaClequiv) FIAs may indicate the existence of buried, economic, porphyry Cu mineralization in the Abdar prospect; therefore, it is suggested to be a possible target for further exploration. Comparing the He and Ar noble gas isotope composition in porphyry copper systems of different size and economic importance in this study showed that the ore-forming fluids of the outsized PCD (i.e., Sar Cheshmeh) have higher contributions of crustal-derived fluids characterized by predominantly radiogenic noble gas signatures (4He and 40Ar) than the smaller PCDs. This could have been achieved by a prolonged hydrothermal circulation in a large volume of crustal rocks containing radiogenic noble gases under a long-lived heat regime resulting from a deeply emplaced and slowly cooled composite intrusive body.

Metapelite from the high‐ to ultrahigh‐pressure terrane of the Eastern Alps (Pohorje Mountains, Slovenia)—New pressure, temperature and time constraints on a polymetamorphic rock

AbstractThe Austroalpine nappe stack of the Pohorje Mountains (Mts.) in northeastern Slovenia comprises a suite of eclogite facies metamorphic rocks that were partially assigned to Eo‐Alpine ultrahigh‐pressure metamorphism (UHPM). We selected a micaschist, which was previously related to this metamorphism, for a detailed study including the chemical zonation of garnet and potassic white mica, the identification of mineral inclusion assemblages, pseudosection modelling with PERPLE_X, and monazite in‐situ dating with the electron microprobe. Polymetamorphism was revealed by (at least) two generations of garnet and phengite and four populations of monazite yielding ages of 283.6 ± 6.1 (2σ), 94.1 ± 3.7, 47.9 ± 10.8 and 26.2 ± 2.8 Ma. The Permian monazite population is characterized by relatively high Y contents (~1.15 wt% Y) and low La/Gd mass ratios (8.7) indicating its formation before the growth of porphyroblastic garnet. The Eo‐Alpine population, however, grew synchronously with garnet based on low Y contents (~0.05 wt%) and high La/Gd ratios (21.4). The older Tertiary population (48 Ma) shows also high Y contents (1.1 wt%) and low La/Gd ratios (10.6) whereas the younger Tertiary population is characterized by low Y contents. The Permian P–T conditions of 7.5–10 kbar at 600–650°C were obtained using the inclusion assemblage of staurolite+rutile+biotite in porphyroblastic garnet. High pressure (HP) but no UHPM was reconstructed for both Eo‐Alpine coarse phengite (Si = 3.22 per formula unit = pfu) and small Tertiary garnet+fine‐grained phengite (Si = 3.27 pfu) at peak pressures ~16 kbar and 18.5–23 kbar respectively. Maximum temperatures close to 650°C were likely reached during the Eo‐Alpine HP event, whereas those of the Tertiary HP event were probably ~580°C. These HP metamorphic events suggest that the Pohorje Mts. experienced both an Eo‐Alpine and a Tertiary subduction–exhumation history, the latter of which was mainly reported for underlying Penninic nappes so far.

Fluid inclusion evidence for hydrothermal evolution of the Sadaigoumen porphyry Mo deposit on the northern margin of the north China Craton

The Sadaigoumen porphyry Mo deposit is located on the northern margin of the North China Craton (NCC). In this study, in situ fluid-inclusion analyses including microthermometry, laser Raman spectroscopy, and LA–ICP–MS microanalysis, as well as Quartz H–O isotopes are present to precisely constrain the hydrothermal evolution of the Sadaigoumen deposit. Five stages of hydrothermal activity are identified, with Stage 1 characterized by the development of magnetite with rare sulfide minerals; Stage 2 by K-feldspar as the typical vein mineral; Stage 3 by molybdenite mainly in veins; Stage 4 by the development of fluorite–quartz and quartz–pyrite veins; and Stage 5 by barren carbonate–quartz veins. Fluid inclusion types were distinguished based on petrographic observations and microthermometric analysis, and fluid compositions were detected by LA–ICP–MS. Microthermometric analyses indicate decreasing homogenization temperatures in primary fluid inclusions from Stage 1 to Stage 3. Intermediate-density fluids of stages 1 and 2 are characterized by high homogenization temperatures (420 °C–470 °C), moderate salinities (6.3–11.2 wt%), and high Mo contents (52–541 ppm). Hydrothermal evolution during Mo mineralization in Stage 3 is characterized by immiscible inclusion assemblages that formed through phase separation from intermediate-density fluid. Fluid inclusions of Stage 3 were trapped at pressures fluctuating around the two-phase fluid surface, leading to at least two fluid immiscible phases (373 °C–414 °C and 311 °C–341 °C). The Mo contents of fluid inclusions vary greatly in Stage 3, with Mo tending to be enriched in brine. In stages 4 and 5, fluids display characteristics that indicate mixing with meteoric water, with homogenization temperatures (<299 °C) and salinities (<5.8 wt%) decreasing together with Mo content. The Cs/(Na + K + Mn + Fe) ratios of fluid inclusions are stable from stages 1 to 4, indicating a constant supply of ore-forming fluids from an underlying, geochemically uniform magma chamber. The fluid inclusion data suggest that Mo and Cu in the initial exsolved fluids of the Sadaigoumen porphyry Mo deposit display different migration and precipitation progresses, which is of great significance to the study of the delicate ore-forming progress of porphyry Mo (–Cu) deposits in the northern NCC.

Formation and evolution of the calcic-magnesian Saheb Fe (Cu) skarn deposit from the Sanandaj-Sirjan Belt, NW Iran: Evidence for multistage boiling in episodes of magnetite saturation

The Saheb Fe (Cu) skarn deposit is located along the northwestern end of the Sanandaj-Sirjan Zone. This deposit occurs over an approximately 2 km distance along the contact zone between the Upper Cretaceous causative granite and the Upper Permian calcareous host rock, with several iron orebodies occurring as high-grade lenticular massive magnetite bodies (with a length of up to 90 m and a width of 25 m).Following the formation of pyroxene hornfels facies, four main stages of skarn formation have been recognized: (I) early prograde stage (type I garnet + clinopyroxene + olivine): fluid inclusions in the type I garnet yielded a high temperature and salinity (average of 506 °C and 57 wt% NaCl equiv.), which may represent the composition of initial magmatic fluids. Higher salinities at this early stage can be associated with orthomagmatic differential fluid exsolution. (II) late prograde stage (type II and III garnets ± clinopyroxene ± disseminated magnetite): the coexistence of vapour-rich and halite-bearing fluid inclusions in type II garnet and their similar homogenization temperatures (420°–480 °C) presumably signify that fluid immiscibility from a supercritical fluid occurred at lithostatic pressures of 300 to 500 bar, corresponding to a depth of 1.5–2.0 km. (III) early retrograde stage (massive magnetite + pyrite + chalcopyrite + epidote + quartz + type I calcite + amphibole + serpentine + talc): hypersaline liquid and low-salinity vapour phases trapped within the epidote, quartz, and type I calcite are coexisting, indicating phase separation of hydrothermal fluids at temperatures of 300°–400 °C and pressures ranging from 310 to >100 bar under hydrostatic conditions, equivalent to a depth of 1.5 to 2.0 km. Hypersaline inclusions in the quartz and type I calcite show Td(NaCl) > Th(L-V), deduced to have been trapped by the over-saturated fluids or under over-pressured conditions before the hydrofracturing (up to 2200 bars). (IV) Late retrograde stage (type II calcite ± quartz ± chlorite), which is characterized by moderate temperature and salinity and non-boiling fluid inclusion assemblage (with average of 172 °C and 17 wt% NaCl equiv.).The first boiling event is documented in the type II garnet of the late prograde stage and provoked the ore-bearing fluids to precipitate the disseminated iron ore. The main deposition of magnetite has plausibly been triggered by the second episode of fluid boiling, recorded in epidote, quartz, and type I calcite during the early retrograde stage. Two boiling events mean producing more fractured and brecciated zones, which can act as conduits for the iron-bearing hydrothermal fluids. This seems to be why there are dozens of isolated magnetite orebodies in the Saheb skarn deposit, which extend over an area of greater than two kilometers (in an E-W direction). Comparably higher salinities even in the type II calcite of the late retrograde stage (25 wt% NaCl equiv.) may reveal that significant mixing of saline magmatic fluids with external, dilute, and cold fluids is almost implausible, despite the presence of brecciated zones. Boiling, therefore, can be pondered as the most salient factor in controlling the ore precipitation at the Saheb Fe (Cu) skarn deposit. However, mixing with meteoric fluids and shifting in pH values could also facilitate the alteration and mineralization.