Low-salinity Fluids Research Articles

Origin and Characteristics of Ore-Forming Fluids of the Abu Gaharish Orogenic Gold Deposits, Central Eastern Desert, Egypt: Evidence from Alteration Geochemistry, Mineral Chemistry and Fluid Inclusions

AbstractThe Abu Gaharish mineralization comprises Au-rich NNE-trending quartz veins, and their surrounding shear zone is found hosted within a Neoproterozoic Sn-W-bearing monzogranite pluton and intersects the metavolcano-sedimentary assemblage at its eastern contact. The ore mineralogy consists of pyrite, galena, chalcopyrite, sphalerite, and gold, with secondary vanadinite occurring after galena and hematite after pyrite. Paragenesis reveals two generations of pyrite and galena, where gold formed mainly during the early sulfidation stage and mobilized upon later deformation events and incipient recrystallization. Mass balance calculations on the alteration zones reveal that the inputs upon alteration included Al2O3, Fe2O3, K2O, MgO, P2O5 W, Rb, Cu, Zn, Pb, V, Cr, Mo, Co, Sn, As, and U, whereas the outputs comprised SiO2, Na2O, CaO, MnO, Sr, Ba, Zr, Y, Th, Nb, and Ni, respectively, in decreasing order. The low contents of granitophile elements (such as Mo, Sn, and W) in the quartz veins at Abu Gaharish suggest that the gold-sulfide-quartz veins have no linkage with granitoid intrusion. However, the enrichment of these elements in the proximal alteration zone is attributed to their leaching by hydrothermal fluids from anomalous zones. The fluid inclusion data observed in quartz showcases the existence of CO2-bearing low-salinity fluids mixed with Mg, Na, and K chlorides under mesothermal conditions during the ore deposition process. This aligns with the formation of orogenic gold deposits as a consequence of the metamorphic devolatilization of the adjacent metavolcano-sedimentary succession. The characteristics of mineralizations at Abu Gaharish display resemblances to orogenic gold deposits and refute the intrusion-related model.

Fluid flow in the Katanga Supergroup: From Lufilian brittle tectonic stages to the post-Lufilian period (Democratic Republic of Congo)

The metasedimentary rock succession of the Neoproterozoic-Cambrian Katanga Supergroup in the Central Africa Copperbelt shows evidence of several complex tectonic events. The deformation of this supergroup started from the tectonic inversion at about 570 Ma and lasted up to today, but reached paroxysm at ∼550 Ma. This long period was characterized by folding and faulting throughout multiple compressive and extensional events, which controlled the regional fluid flow on the one hand, and played an important role during formation of the stratiform to stratabound Cu-Co (Ni, U) deposits and the polymetallic Cu-Zn-Pb (Ag, Ge, Mo, Cd) vein type deposits on the other hand. Based on the structural analysis and paleostress reconstruction, coupled with fluid inclusion characterization from mineralized structures in rocks from the Nguba, Kundelungu and Biano Groups, this study demonstrates that the composition of hydrothermal fluids changed during brittle tectonic deformation during the Lufilian orogeny and subsequent uplift and post-Lufilian faulting.During early brittle tectonic deformation along strike slip faults with sinistral and dextral movement related to a NE-SW transpression, the Cu-mineralizing fluid was hypersaline (27.9–31.1 eq. wt% NaCl) with moderate temperatures (Th = 128–216 °C). The subsequent Cu or Cu (Zn, Pb) mineralization formed within an E-W extensional stress regime, related to the late Lufilian orogenic collapse. The fluid inclusions present in the gangue minerals associated with this latter mineralization show a large range in Th (50–264 °C) and salinity (26.7–36.0 eq. wt% NaCl). The decrease in temperature is interpreted to be due to migration of the fluids at shallower depth in the subsurface after uplift and erosion of the orogen. The increased salinity of the fluid is related to the dissolution of evaporites, mainly NaCl. A second H2O-NaCl-CaCl2 fluid with a homogenization temperature below 55 °C has also been found associated with this brittle stage and mineralization phase, but only in rocks belonging to the Kundelungu Group. A third mineralization phase, also characterized by Cu or Cu (Zn, Pb), formed during the post-Lufilian period within a NW-SE transpressional inversion regime. The fluid inclusion in the gangue minerals of this mineralization phase have a smaller range in homogenization temperature (Th = 37–172 °C) and the largest range in salinity (0.71–30 eq. wt% NaCl), compared to the earlier fluid inclusions generations. This large range in salinity may be explained by the mixing of a high salinity fluid, already present during the earlier tectonic stages in the sedimentary basin, with meteoric water. During the more recent rift-related extension, a fluid with again a large and higher range in homogenization temperatures (Th = 47–257 °C) and with a typical low salinity (<10 eq. wt% NaCl) has been recognized in minerals filling NNE-SSW to NE-SW oriented faults and fractures. The upward migration of a relatively low-salinity fluid from deeper parts in the subsurface explains the variation in the temperatures observed with this tectonic event.

Geochemistry of coastal geothermal systems from southern Baja California peninsula (Mexico): Fluid origins, water-rock interaction and tectonics

The Baja California peninsula forms the western margin of the Gulf of California (GC) rift system, which is an active tectonic setting manifested by seismicity and numerous geothermal sites. The present study examines the geochemistry of thermal fluids (major ions and gas species, δ18O-δD, δ13C, 3He/4He) from the southern tip of the peninsula (Los Cabos Block, LCB). Sampling was mostly focused on the coastal thermal manifestations in the towns of Buenavista and El Sargento, but other sites further inland are also included for broadening the scope of the study. The main objectives include: (i) characterize water-rock interactions and other processes controlling the fluid composition, (ii) constrain solute geothermometry estimates through multi-step geochemical modeling, and (iii) discuss the fluid origins in terms of tectonics and regional shear-wave velocity anomalies in the upper lithosphere. The geothermal systems in the area have a tectonic origin and result from fluid circulation along regional faults that penetrate the upper crust through the granitic basement. Mixing between the thermal fluids and seawater at the coast is clearly illustrated through major ions and δ18O-δD relationships. Reconstruction of the pre-mixing chemical compositions indicates a low-salinity fluid at Buenavista (Cl = 104–109 mg L−1) and a saline fluid at El Sargento (Cl = 7169 mg L−1). The geochemical modeling allows us to validate these endmember compositions and to address a common issue when using solute geothermometry, which is the uncertainty on the state of equilibrium of the thermal fluid with respect to wall-rock minerals. The corresponding results reveal contrasting thermal regimes at depths (Buenavista: 101–122 °C, El Sargento: 212–220 °C), likely associated with differences in geothermal gradient and fluid circulation depth. Our gas samples have among the lowest He isotopic ratios (0.07–0.95 Ra) in the Baja California peninsula, indicating that the origin of helium is mainly crustal. Shear wave tomography demonstrates that the study area is associated with two low velocity regions, one in the crust and the other one in the upper mantle. Our results favor the hypothesis that the heating of fluids is produced by lower-crustal flow driven by strong topographic gradients along the rifted margin. This study provides new insights into the origin of the thermal anomalies in the LCB and the adjacent GC rift system.

Mineralogy and genesis of the Pb-Zn-Sb-Ag vein H32A in the Příbram uranium and base-metal district, Bohemian Massif, Czech Republic

A detailed paragenetic, electron microprobe, fluid inclusion and S,C,O-isotope study was conducted on Pb-Zn-Sb-Ag mineralization of the vein H32A, Háje deposit, Příbram uranium and base-metal district, Czech Republic. The vein is hosted by folded Neo-Proterozoic sandstones and siltstones of the Teplá-Barrandian unit in exo-contact of the Central Bohemian Plutonic Complex of the Variscan age (354–335 Ma). The ore zone is close to a crustal-scale tectonic zone dividing two distinctly different basement blocks of the Bohemian Massif. A rich mineral assemblage consisting of 44 mineral phases formed during two major mineralization stages (siderite-sulphidic and calcite-sulphidic) widespread in the Příbram area. A complex history of the vein is evidenced from the presence of seven mineralization sub-stages and widespread repeated metasomatic replacement and dissolution of older vein during younger mineralizing events. The vein is dominated by carbonates (especially siderite-rhodochrosite, less frequently dolomite-ankerite-kutnohorite, rarely calcite), minor constituents are quartz, illite-muscovite, and chamosite (locally with up to 1.52 apfu Sb, 0.53 apfu Zn and 0.38 apfu Pb). Ore minerals are dominated by Ag-poor galena. The Fe-poor sphalerite, pyrrhotite, löllingite, arsenopyrite, Pb-Sb, Pb-Sb-Ag and Cu-Ag-Sb sulphosalts are subordinate phases and dyscrasite, native antimony and native arsenic are accessories. The fluid inclusion study revealed the wide ranges of homogenization temperatures (210–<100 °C) and salinities (0.0–30.5 wt% NaCl eq.) of exclusively aqueous fluid inclusions. General decrease of both parameters occurred during evolution of the vein. Sulphur isotope disequilibrium was found between co-existing galenas and sphalerites. The calculated fluid δ13C values range between −9.0 and −12.3 ‰ V-PDB, implying for a well-mixed source of carbon, or its significant re-cycling during processes of dissolution-reprecipitation of vein carbonates. The calculated fluid δ18O values are highly positive for early siderite (+7.2 to +9.6 ‰ V-SMOW), and near-zero for later dolomites and calcites (−2.9 to +3.2 ‰ V-SMOW). The siderite fluids likely represent deeply circulated waters, which underwent a significant isotope exchange with hot crustal rocks. Late low-salinity fluids were likely sourced from the Permian partly evaporated freshwater piedmont basins. The high-salinity Ca-Na-Cl brines rarely recorded in secondary fluid inclusions hosted by late calcite likely represent either local “shield brines” or basinal brines of marine provenance. The study site covers a metallogenetically significant portion of the Late Variscan fluid history within an active crust-scale tectonic zone hosted by basement of the Bohemian Massif. A pronounced Ag enrichment of this vein is mainly caused by widespread occurrence of inclusions of late-stage Ag minerals (dyscrasite) in the vein, which form rich concentrations in the shallower parts of the Háje deposit.

Timing and origin of skarn-, greisen-, and vein-hosted tin mineralization at Geyer, Erzgebirge (Germany)

AbstractThis contribution presents new insights into the origin and age relationships of the Geyer tin deposit in the Erzgebirge, Germany. Tin mineralization occurs in skarns, greisen, and in cassiterite-bearing fluorite-quartz veins. Skarn alteration replaces marble layers of the Cambrian Jáchymov Group and occurs in two clearly distinct stages. The first skarn stage forms skarnoid textured assemblages of clinopyroxene, garnet, and wollastonite with no tin phases recognized. Garnet U-Pb ages of this skarn stage (~322 Ma) relate the earlier skarn stage to the emplacement of the Ehrenfriedersdorf granite (~324 to 317 Ma). The second stage of skarn alteration is marked by the occurrence of malayaite and cassiterite associated with garnet recording ages of 307 to 301 Ma. Greisen- and skarn-hosted cassiterite-bearing veins provide U-Pb ages in the range of 308 to 305 Ma, relating greisenization and vein formation to the same magmatic-hydrothermal event as the second skarn stage. This suggests that tin mineralization at Geyer is related to a distinctly younger magmatic-hydrothermal event, clearly postdating the Ehrenfriedersdorf granite, which was previously assumed as the source of the tin-rich fluids. Fluid inclusions show salinities in the range of 1.0 to 31.5 % eq. w(NaCl±CaCl2) and homogenization temperatures between 255 and 340 °C. Cassiterite-associated fluid inclusions show indications for heterogeneous entrapment and dilution of hydrothermal with meteoric fluids. Dilution of high-salinity fluids with low-salinity fluids and cooling of the system was probably a decisive process in the precipitation of cassiterite in the Geyer Sn system.

Structure–Fluid–Mineralization System of Jiadi Gold Deposit in Southwestern Guizhou, China

The Jiadi gold deposit, situated in the Southwestern region of Guizhou, is a large-scale, recently discovered, basalt-hosted, and fine-grained disseminated gold deposit. This study has unveiled that the tectonic deformation of the Jiadi gold deposit can be categorized into four stages: the Late Paleozoic crustal uplift and brittle deformation (D1-deformation) stage; the Early Yanshanian NW-trending compressive ductile deformation (D2-deformation) stage; the Late Yanshanian NS-trending strike-slip fault (D3-deformation) stage; and the Himalayan EW-trending nappe structure (D4-deformation) stage. The outcomes of the C-O isotope analysis revealed that the fluid responsible for ore formation exhibits the attributes of magmatic water blended with meteoric water, and the metallogenic constituents primarily originate from a deep source. The formation of the Jiadi gold deposit occurred during the D2-deformation stage and extended to the D3-deformation stage. The D2-deformation stage induced hydrothermal activity and rock devolatilization, leading to the generation of CO2-rich and low-salinity fluids. The D3-deformation stage, which is intimately associated with mineralization, can be classified into three stages: the pyrite stage; the smoky quartz stage; and the sulfide stage. The findings obtained from laser Raman spectroscopy indicate that the inclusions typically comprise CO2, CH4, N2, and SO2. These deep fluids ascended along the fault to the interlayer fracture zone during the D3-deformation stage. The alteration of the geochemical environment was accompanied by fluid immiscibility or boiling, resulting in the rapid precipitation of metallogenic materials.

In-situ texture and geochemistry of apatite from the Jinling and Zhangjiawa iron skarn deposits, eastern North China Craton: Implications for ore-forming processes and formation of high-grade ores

Apatite commonly contains abundant halogens and trace elements, which can occur in both the magmatic and hydrothermal stages, becoming favorable for recording the magmatic-hydrothermal properties and processes. Skarn-type iron deposits associated with high-Mg diorites are widespread in the Luxi Block, eastern North China Craton. Their ore-forming processes, especially how the different wallrocks (limestones and dolomites) controlled the formation of high-grade iron ores, have been poorly constrained. To unravel the mechanisms, in this contribution, in-situ textural, geochronological (U-Pb dating) and geochemical (halogens and trace elements) analyses by SEM, EPMA and LA-ICP-MS were conducted on apatite from the representative Fe skarn deposits (Jinling and Zhangjiawa deposits) in the Luxi Block. Three generations of apatite were identified in the Jinling deposit, which occur in the feldspathization (Ap1), garnet–pyroxene skarns (Ap2) and massive magnetite ores (Ap3), respectively. One generation of apatite occurring in the massive magnetite ores (Ap-3) was identified in the Zhangjiawa deposit. The Ap2 and Ap-3 apatite grains show patchy textures composed by residual bright zones (Ap2a and Ap-3a) and newly-formed grey to dark zones (Ap2b and Ap-3b), which indicate fluid metasomatism. LA-ICP-MS U-Pb dating shows that the Ap1, Ap2a and Ap3 apatite in the Jinling deposit formed at 130.4 ± 0.9 Ma (2σ), 128.5 ± 2.4 Ma and 128.3 ± 9.2 Ma, respectively, whereas the Ap-3a in the Zhangjiawa deposit formed at 128.1 ± 4.2 Ma. These ages are well consistent with the intrusive ages of the spatially associated high-Mg diorites within errors, corroborating the ore-forming fluids originating from the high-Mg rocks. All the studied apatite grains show F/Cl ratios higher than 1, especially in the early-stage apatite (e.g., 5–19 in Ap1), indicating that the originally exsolved fluids were enriched in F. The high F contents probably played a significant role in leaching Fe from the high-Mg rocks by enhancing rock porosities. The Eu/Eu* ratios of apatite increase from Ap2a to Ap3 while the Ce/Ce* ratios decrease. This suggests an increase of oxygen fugacity. The Sr concentrations in Ap3 are much higher than those in Ap-3a, correlating well with the wallrocks that limestones are developed in the former while dolomites in the latter. The above features indicate that fluid-rock interaction likely led to the increase of oxygen fugacity, which controlled the massive Fe deposition. In the Jinling deposit, the altered apatite grains (Ap2b) show much lower REE + Y but higher F contents than those of the unaltered (Ap2a), not only indicating the REEs being easily mobilized during metasomatism, but also suggesting that the high F contents likely contributed to the formation of high-grade ores by leaching additional Fe from the previously formed Fe-rich skarns. In the Zhangjiawa deposit, the altered apatite grains (Ap-3b) shows lower Cl contents than those of the unaltered (Ap-3a). This probably indicates the mixing of low-salinity fluids (e.g., recycling meteoric water), leading to the loss of Fe and thus lowering the Fe grade. The above results indicate that metasomatism is common in the skarn deposits, which can either elevate or lower the metal grade. The processes can be well recorded by apatite.

Trace element variations of pyrite in orogenic gold deposits: Constraints from big data and machine learning

Orogenic gold deposits represent key global gold resources, yet its internal differences are poorly constrained. Pyrite is ubiquitous in orogenic gold ores, and its trace element geochemistry has significant implications for the ore-forming processes. However, the diffusion regularity and controlling factors of pyrite trace element geochemistry of orogenic gold deposits lack effective constraints. To address these issues, pyrite trace element geochemical data (4092 sets of data) from 67 orogenic gold deposits were compiled for big- data and machine learning analyses. The principal component analysis (PCA) divides the pyrite data into four groups according to trace element correlations: I) Au-As; II) Sb-Cu; III) Ag-Pb-Bi; and IV) Co-Ni-Te-Se-Mo. This division is likely ascribed to the element occurrence in pyrite. Partial least squares-discriminant analysis (PLS-DA) and Random Forests classification were used to identify pyrite trace elemental variations among orogenic deposits of different mineralization ages, wallrocks, and tectono-metallogenic settings. The results show that the influence on pyrite geochemistry from temporal variation is not obvious, and only Triassic and Jurassic orogenic-type auriferous pyrite shows enrichments of Cu, Pb, and Sb, which may reflect the multiple mineralization events in these deposits as Pb commonly occurs as cation in the fluids, while Sb usually occurs in the melt. The spatial difference is more distinct with less overlapping of confidence circles in PLS-DA and higher accuracy in Random Forests, which may have been associated with fluid source variation influenced by the geodynamic setting (mantle model, supposed by distinctive geodynamic variation from metallogenic belts in China) and source strata (metamorphic model, supposed by Archean deposits mostly hosted in green-schist phase besides China). Pyrite in orogenic gold ores hosted in different wallrocks can be differentiated by the different As, Pb, and Cu contents, as the contents of these elements vary widely in the wallrocks and participate in pyrite lattice through fluid-rock reactions. Random Forests regression reveals that pyrite As-Sb contents in pyrite show distinct negative correlations with ore-forming temperature, but Se content shows a non-linear influence. This suggests that the pyrite As-Sb-Se contents in orogenic gold ores can be used as a geothermometer. Furthermore, we suggest that the pyrite Co-Ni contents are not good indicators for the ore-forming temperature, as the low salinity fluid in orogenic gold systems makes Co-Ni difficult to form chloride complexes which is sensitive to temperature variation.

Geology, geochemistry, fluid inclusion data, stable isotope characteristics, and ore genesis of the Barout Aghaji gold deposit, NW Zanjan, Iran

The Barout Aghaji gold deposit is located ∼90 km northwest of Zanjan, within the Takab-Takht-e-Soleyman subzone of the Sanandaj-Sirjan metamorphosed-deformed zone. Ore-bearing quartz veins are hosted by Neoproterozoic amphibolite and Eocene to Oligocene granitic gneisses. Oligo-Miocene Upper Red Formation unconformably overlies the amphibolite and granitic gneisses. Field observations and petrographic studies show that two deformation stages occurred in this area. The first deformation stage was ductile, producing mylonitic and proto-mylonitic microstructures, but the second one was brittle, represented by sheeted quartz veins and veinlets. In the first stage, barren milky quartz veins occurred containing minor sulfide minerals, but dark to light gray ore-bearing quartz veins and veinlets are formed in the latter stage. The mineralized veins appear as massive microcrystalline quartz cut by sheeted quartz veins with comb, druse, and crustiform textures. The gold-bearing quartz veins contain as much as 3% sulfide minerals. Pyrite is the main sulfide mineral and is associated with minor chalcopyrite. Sulfides are commonly altered to hematite, goethite, and rarely malachite. Hydrothermal alteration around the quartz veins consists of silicification, pyritization, and sericitization. The whole-rock geochemistry of the collected samples from the granitic gneisses and quartz veins shows that Au is enriched in the quartz veins (average of 114 ppb) relative to host rocks (average of 22.5 ppb). Au shows strong positive correlations with As, Ba, Mo, Pb, Sc, Tl, Ag, and negative correlations with Cu, Bi, Se, and Te in the granitic gneisses. It also shows strong positive correlations with S, Hg, Th, Co, Bi, Pb, and Ag and negative correlations with P, V, Te, W, Sc, Zn in quartz veins. Four types of primary fluid inclusions were identified, including type I, two-phase aqueous-rich fluid inclusions (liquid > vapor; LV); type II, two-phase vapor-rich fluid inclusions (gas > liquid; VL); type III, three-phase fluid inclusions containing CO2 with clathrate formation (L1L2V); and type IV three-phase fluid inclusions (aqueous, vapor, and solid; LVS). The homogenization temperatures of fluid inclusions in auriferous quartz veins range from 199 −446 with a mode of 270–300 °C. Salinities range from 0.8 to 49.02 wt% NaCl Equiv. with two distinct populations at 0.8–8.5 and 31.1–49.02 wt% NaCl Equiv. The large variations in the homogenization temperatures and salinities can be attributed to the cooling and isothermal mixing of fluids. The δ34S values for four pyrites separated from auriferous quartz veins range from +2.9 to +7.1‰, with an average of 4.5‰. δ34S values of fluids in equilibrium with pyrite were calculated from +3.5 to +7.3‰, with an average of 5.4‰, indicating a metamorphic source for the sulfur using temperatures estimated from the fluid inclusion study. The Field observations, vein textures, mineralogy, ore geochemistry, fluid inclusion studies, and sulfur isotope data indicate that gold mineralization in the Barout Aghaji area has many similarities to orogenic and intrusion-related gold deposits, such that low salinity fluids derived from metamorphic rocks are mixed with high salinity fluid inclusions possibly derived from granitic gneisses during syn to post tectonic magmatism.

Multi-stage alteration at Nifty copper deposit resolved via accessory mineral dating and trace elements

The sediment hosted Nifty prospect is one of the most prominent Cu deposits in the Neoproterozoic Paterson Province, which girdles the eastern margin of the Archean Pilbara Craton in Western Australia. The timing of mineralization at Nifty has proved challenging to constrain despite several attempts to date it using a range of isotopic methods, including muscovite 40Ar/39Ar (total fusion) and solution ICP-MS apatite U–Pb geochronology. The region preserves a protracted and complex geological history, with potential for several generations of fluid flow/mineralisation, which necessitates a texturally-controlled geochronology approach. Here, we report in situ apatite and monazite U–Pb isotopes complemented with trace elements from both mineralized veins and matrix of the sedimentary host-rock collected via laser ablation split stream inductively coupled plasma mass spectrometry (LASS-ICP-MS). Apatite grains from pyrite- and quartz-bearing veins are enriched in middle rare earth elements (MREE) with prominent convex-upward chondrite-normalized REE profiles. This chemical signature is similar to hydrothermal apatite from the Olympic Dam high-grade bornite Cu deposit, commonly associated with MREE-enriched, lower salinity fluids, and alkaline pH conditions capable of mobilizing Cu. Hydrothermal apatite from pyrite-bearing veins associated with euhedral, concentrically zoned pyrite yields lower intercept ages of ca 810 Ma, whereas hydrothermal apatite from quartz-pyrite veins associated with pyrite replacement microstructures have variable apparent ages. Monazite grains on the margins of pyrite-bearing veins (along micro-cracks) and monazite associated with chalcopyrite in veinlets yield a weighted mean 238U/206Pb age of ca 640 Ma. These results necessitate at least two distinct hydrothermal/fluid flow events related to the formation of the Nifty Cu deposit, with the former being temporally associated with ca 830 Ma mafic intrusions. Evidence for the latter hydrothermal event is cryptic, being highly localised at a grain scale, but is broadly coeval with ca 640 Ma granitic magmatism linked to Cu–Au mineralization elsewhere in the Paterson Orogen (e.g., Telfer & Winu deposits).

Helvine Mineralization in Devonian-Carboniferous Skarns of the Oriental Pampean Ranges, Córdoba, Argentina: Mineralogy and Nature of Ore-Forming Fluids

AbstractHelvine [Be3Mn2+4(SiO4)3S] occurs in less than one meter-sized spotty concentrations in some calcic skarns of Córdoba province, Argentina. The local geology, mineral chemistry, paragenetic mineral associations, and the evolution of helvine mineralizing fluids were studied from two selected localities (i.e., the Chingolo scheelite mine and Casa la Plata). Helvine from the Chingolo scheelite mine occurs in idiomorphic crystals up to 15 cm long, partially intergrown with spessartine-rich garnet, and in part or totally included in vug-filling spar calcite in the prograde garnet-vesuvianite skarn zone developed between replaced aplite-pegmatite dikes and calcic phlogopite-bearing marbles. Helvine from Casa la Plata occurs abundantly in vein-like, fluorite-rich garnet-vesuvianite skarn associations, where tetrahedrons up to 2 cm long occur preferentially included in fluorite in an illite-sericite-chlorite strongly replaced schist. The composition of helvine from both localities does not differ from other worldwide known compositions. Associated garnet from both localities is enriched in molar subcalcic garnet, largely as spessartine. Clinochlore is a conspicuous phase that occurs both as late infills and/or replacing phases in both the Chingolo scheelite mine and Casa la Plata. Primary fluid inclusions from helvine of both localities suggest that helvine deposited from moderate to high temperature and salinity aqueous fluids of likely magmatic origin. In both localities a late influx of gas-rich, CO2-bearing, moderate temperature, and moderate to low salinity fluid was trapped as secondary fluid inclusions. At the Chingolo scheelite mine, CO2-bearing fluids likely originated from decarbonation along the skarnification reaction front distanced from the prograde zone at lower temperature. In both studied areas, the latest trapped secondary fluids were of lower temperature and lower salinity. Pressure-corrected homogenization temperatures between 1.8 and 2.0 kbar sustain trapping temperatures for primary fluid inclusions within the 510–610 °C range for both localities. In the Chingolo scheelite mine, helvine formed at the contact zone between Ordovician aplite-pegmatites and Cambrian marbles that differentially reacted with infiltrated distal metasomatic-hydrothermal Be-bearing fluids fractionated from evolved granitic facies and/or pegmatites of the neighboring Devonian to Carboniferous Achala Batholith. Calculated δ18OH2O in equilibrium with garnet at ∼575 °C yielded δ18OH2O = 11.1 or 12.2‰. These heavy δ18OH2O values may derive from a magmatic fluid source possibly enriched in 18O from marbles or any other metasedimentary country rock during skarnification. This interpretation is supported by δ18OH2O values (8.3 to 10.1‰) and heavy δD values (−21.6 to −20.6‰) from retrograde epidote of other neighboring scheelite mines related to the Chingolo scheelite mine. The presence of beryl partially replaced by bertrandite + K-feldspar, non-paragenetically associated with helvine, confirms that early prograde crystallization conditions switched from a high temperature, subaluminous environment to a lower temperature, aluminous and increasingly acidic environment toward the retrograde skarn stage. At Casa la Plata, Cambrian schists and marble-amphibolite were also skarnified after the circulation of Be-bearing fluids derived from an epizonal Carboniferous granite (Capilla del Monte pluton) and its pegmatite dike swarm, which share parental links with the Achalian magmatism. The major Be supply for both studied localities should be attributed to Devonian and Carboniferous postorogenic to extensional peraluminous A-type granitic magmatism, which is the major source of beryllium not only for the Pampean Ranges but for the whole country.

A fluid inclusion and stable-isotope study of hydrothermal vein mineralization, Schwarzwald district, Germany

A combined fluid inclusion and stable isotope study has been carried out on over 180 individual samples from 89 post-Variscan hydrothermal veins (Pb-Zn-Cu-bearing fluorite-barite-quartz veins, Co-Ni-Ag-Bi-U-bearing barite-fluorite-quartz veins and barren barite-fluorite-quartz veins) from the Schwarzwald district, Germany. The salinities of fluid inclusions in post-Variscan primary fluorite, calcite, barite and quartz are in the range of 22–25 wt.% equivalent (eqv.) NaCl, and the eutectic temperatures range between –57 and –45°C, indicating the presence of H2O-NaCl-CaCl2 fluids. Homogenization temperatures vary from 130 to 180°C. A low-salinity fluid (0 to 15 wt.% eqv. NaCl) was observed in some late stage fluorite, calcite and quartz samples, which were trapped similar temperature, range of high salinity fluids.&#x0D; Raman microprobe analyses show that the only detectable volatile in the vapour is CO2. Almost all δ18O (n=86) measurements of quartz from the fluorite-bearing post-Variscan veins range between +11.1 and +20.9 ‰. The calculated δ18OH2O values are between –11.0 and +4.4 ‰, using known quartz-water fractionation and fluid inclusion homogenization temperatures. The δ18OH2O values of directly extracted fluid inclusion water of fluorites range from –11.6 to +1.1 ‰, very consistent with the calculated values. The δD values of fluid inclusion water in calcites (extracted from primary and late calcite samples) lie in a narrower range between –26 and –15 ‰. The extracted fluid inclusion water from quartz samples has significantly more variable δD values between –63 and +9 ‰. The δ13C and δ18O values of fluid inclusion gas (CO2) range between –21.4 and –6.7 ‰ and between –16.3 to –7.1 ‰, respectively.&#x0D; Calculations for fluorite-barite-quartz veins combining oxygen isotope equilibria with microthermometric data result in quartz precipitation temperatures of 120–170°C at pressures between 0.3 to 0.5 kbar. The δ18OH2O and δD data, particularly the observed wide range in hydrogen isotopic compositions, indicate that the hydrothermal mineralization formed through large-scale mixing of a basement-derived saline NaCl-CaCl2 brine with meteoric water.

The role of salt dissolution on the evolution of petrophysical properties in saline-lacustrine carbonate reservoirs: Pore structure, porosity–permeability, and mechanics

The invasion of low-salinity fluids into saline-lacustrine reservoirs may induce salt dissolution, resulting in significant changes in petrophysical properties. Accurate characterization of these changes due to salt dissolution is critical for efficient drilling and development. Here, low-salinity core flooding experiments and several core analysis technologies were employed to investigate the changes in pore structure, porosity–permeability evolution, and mechanical properties. Ion compositions in outlet fluids analyzed in real-time to determine the dissolved salt mineral compositions, mainly include halite (1.66 %), anhydrite (7.32 %), and glauberite (0.98 %). Salt dissolution produces many dissolution pores, especially with sizes ranging from 0.01 to 10 μm, broadens the seepage channels, and enhances the connectivity between pores, which significantly improves rock porosity close to 1.80 times the initial one and permeability to 70 times initial one. During the salt dissolution process, the stability of clay minerals is weakened to induce dispersion, migration, and swelling behavior, leading to the reduction of porosity and permeability. A semi-empirical power law relating expressing porosity and permeability for saline-lacustrine reservoirs is proposed, allowing for salt dissolution and clay swelling during the invasion process of low-salinity fluids. Although salt dissolution could slightly relieve stress sensitivity damage by 56.54 %, it significantly weakens rock strength by reduction of elastic modulus (43.51 %) and compressive strength (61.54 %). The positive and negative effects of salt dissolution on petrophysical properties are critical to guide the development of formation damage control measures for saline-lacustrine reservoirs.

Sulfur isotope, fluid inclusions, and quartz solubility model decipher multistage hydrothermal fluids evolution and ore precipitation mechanism at the Nantai porphyry Mo deposit, Qinling, Central China

Qinling orogenic belt hosts many porphyry Mo deposits, which were formed by ore-forming fluids exsolved from oxidized magmas. Detailed processes of ore fluids evolution and metal deposition mechanisms of porphyry Mo deposits remain unclear. The quartz solubility model and cathodoluminescence (CL) images combined with fluid inclusions (FIs) and isotope analyses at the Nantai Mo deposit provide new insights into processes and physicochemical conditions for Mo mineralization. Based on field and microscopy of ore and alteration assemblages together with CL images, four types of hydrothermal veins (i.e., A, B, C, and D veins) have been identified. The quartz in A veins intergrown with K-feldspar precipitated from high-temperature and low- to intermediate-salinity fluids under near-lithostatically conditions. B vein comprises quartz and K-feldspar and forms at 336–466 °C, pH from 6.30 to 11.02, and oxygen fugacity (fO2) of −30.81 to −24.26. Hydrothermal fluids during this stage underwent phase separation under ductile to brittle transition. C veins include muscovite and most molybdenite without quartz, indicating that they formed in the retrograde quartz solubility field at 353 to 398 °C. Thermodynamic calculation suggests molybdenite mainly precipitated under fO2 from −32.37 to −29.05 and pH from 2.68 to 6.54. Quartz in D veins formed from low-temperature and low-salinity fluids. Sulfur isotopic analyses suggest that the ore components are derived from the old crust. The main factors controlling Mo mineralization at Nantai are the decreases in fO2 and pH values of hydrothermal fluids.

Polymetallic Sulfide–Quartz Vein System in the Koudiat Aïcha Massive Sulfide Deposit, Jebilet Massif, Morocco: Microanalytical and Fluid Inclusion Approaches

The Koudiat Aïcha Zn-Cu-Pb deposit (3–Mt ore @ 3 wt.% Zn, 1 wt.% Pb, 0.6 wt.% Cu) in the Jebilet massif (Morocco) comprises stratabound lenticular orebodies and crosscutting sulfide-bearing quartz ± carbonate veins in the lower Carboniferous Sarhlef volcano sedimentary succession. The veins are characterized by abundant pyrrhotite, sphalerite, subordinate chalcopyrite and galena and rare Ag and Au minerals. The stratabound massive sulfide ores are attributed to a “VMS” type, whereas the origin of the sulfide–quartz ± carbonate veins remains poorly understood. New mineralogical and microanalytical data (SEM, EPMA and LA-ICP-MS) combined with fluid inclusion results point to two-stage vein formation. The early stage involved C–H–O–N Variscan metamorphic fluids which percolated through fractures and shear zones and deposited pyrite at &gt;400 °C, followed by the formation of pyrrhotite and sphalerite (300 ± 20 °C) in quartz veins and in banded and breccia ores. The pyrrhotite–sphalerite mineralization was overprinted by aqueous brines (34 to 38 wt% eq. NaCl + CaCl2) that precipitated carbonate and Cu-Pb sulfides (±Ag-Au) at ~180–210 °C through mixing with low-salinity fluids during tectonic reworking of early-formed structures and in late extension fractures. The latter ore fluids were similar to widspread post-Variscan evaporitic brines that circulated in the Central Jebilet. Overlapping or successive pulses of different ore fluids, i.e., metamorphic fluids and basinal brines, led to metal enrichment in the quartz–carbonate veins compared to the massive sulfide ores. These results underscore that even a single deposit may record several distinct mineralizing styles, such that the ultimate metal endowment may be cumulative over multiple stages.

Hydrothermal evolution from porphyry- to epithermal-style mineralization in the Naozhi deposit, NE China

The Naozhi deposit in the Yanbian district of northeastern China contains polymetallic epithermal veins and subeconomic porphyry-style Cu mineralization within 2 km of each other. These two mineralization styles are genetically associated with 130–126 Ma andesites and dioritic–granitic intrusions. Isotopic ages determined on sericite (128.2 ± 2.9 Ma; 40Ar–39Ar plateau age) and sulfide (124.6 ± 2.7 Ma; Rb–Sr isochron) associated with the epithermal mineralization constrain the hydrothermal activity to at approximately 128 Ma. Three types of fluid inclusions, including halite-bearing, vapor-rich, and liquid-rich inclusions were trapped in quartz from porphyry-style veinlets associated with an assemblage of K-feldspar–biotite–magnetite. Based on microthermometry and the homogenization behavior of halite-bearing inclusions, they were trapped at lithostatic pressures of > 500 bars and at temperatures of ≥ 350 °C, whereas the vapor- and liquid-rich inclusions were trapped at pressures and temperatures of 110 ± 35 bars and ≤ 350 °C.Epithermal mineralization comprises four vein stages, which, from oldest to youngest, are quartz–pyrite, quartz–base metal sulfide, quartz–precious metal, and quartz–carbonate. The ore assemblage precipitated from low-salinity fluids (<10 wt% NaCl equiv.) at temperatures and pressures of ≤ 350 °C and ≤ 100 bars. The δ18OH2O (2.5 to −2.4 ‰) and δDH2O (−85 to −114 ‰) compositions of fluids related to porphyry- and epithermal-style mineralization indicate a mixture of magmatic and meteoric waters. The δ34SH2S (−3.1 to 4.5 ‰) values of fluids become isotopically heavier from the quartz–pyrite stage to the quartz–base metal sulfide stage, reflecting continuous reduction during epithermal mineralization. The mineral assemblage and chemical composition of sulfide minerals indicate a transition from oxidizing, high sulfidation porphyry-style fluids to reducing, intermediate–low sulfidation epithermal fluids, with log ƒO2 values ranging from −31 to > −33, and log ƒS2 values ranging from −7.0 to ≤ −13.4 during the epithermal veining stage. Metal deposition in the Naozhi magmatic–hydrothermal system resulted from cooling, sulfidation, neutralization, and boiling of predominantly magmatically sourced hydrothermal fluids that mixed with meteoric water.

Spontaneous imbibition characteristics of shale oil reservoir under the influence of osmosis

The spontaneous imbibition (SI) process in shale oil reservoirs is not only influenced by capillary force, but also by the osmotic pressure between the fracturing fluid and formation water in the nanopores media. In this study, experimental methods are used to investigate the mechanisms of osmosis in the SI, taking into account the presence of initial formation water in shale oil reservoirs. To investigate the effect of osmosis, SI experiments were performed on the fine-grained felsic shale of the Qikou sag of Dagang oilfield. Low-field NMR testers and high-precision electronic balances are utilized for the measuring of oil–water migration. The results show that, when Sw ≠ 0, high-salinity fluid SI can be divided into four stages: initial imbibition stage, drainage stage, secondary imbibition stage and stationary stage; when Sw = 0, there is no drainage stage of high-salinity fluid SI; when Sw ≠ 0 or Sw = 0, low-salinity fluid SI can be called the “osmosis-enhanced SI”; and we have found that “newly formed pores or microfractures” as well as reducing salinity can promote SI. This article presents a systematic study of SI of shale oil reservoirs under the influence of osmosis, which provide useful information for reservoir numerical simulation and development program design.

A method for the direct analysis of quenched, magmatic-hydrothermal fluids recovered from high-pressure, high-temperature experiments

A method for analyzing low-salinity fluids (up to ~11 wt% NaCl equivalent) that have been equilibrated with felsic melts in high-pressure, high-temperature experiments is reported. Experiments were performed at a pressure of 200 MPa and 800 °C in internally-heated pressure vessels, wherein magmatic fluids were quenched and recovered in a multi-step process for analyses of major (Na, K, Ca, Cl, and F) and trace (Li, Be, Mn, Cu, Zn, Rb, and Sr) elements by ion chromatography and inductively coupled plasma mass spectrometry (ICP-MS). The quenched melts were analyzed for major and trace elements by electron microprobe and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Such analyses permitted the composition of the fluid to be directly measured without the need for mass balance calculations or trapping fluids within fluid inclusions while still enabling the precise determination of partition coefficients. Mass balance calculations tend to overestimate Na, K, Ca, and Be; underestimate Cu, Zn, and F; and are in agreement for Li, Mn, Rb, Sr, and Cl when compared to direct measurements. Additionally, a comparison of partition coefficients calculated from measured and mass balance derived concentrations to available literature data is consistent with mass balance calculations being less accurate than measured values. Thus, direct measurements of quenched fluids by ion chromatography and ICP-MS offer a robust method for characterizing the composition of magmatic-hydrothermal fluids that have been equilibrated with silicate melts.

VEINS SYSTEM AND THEIR MINERALOGICAL AND MICROTHERMOMETRIC CHARACTERISTICS WITHIN THE HUMPA LEU EAST PORPHYRY COPPER-GOLD MINERALIZATION AT HU'U DISTRICT, SUMBAWA ISLAND, INDONESIA

The study area, i.e Humpa Leu East is a porphyry prospect located in Hu'u district, Sumbawa Island, Indonesia. This study is aimed to understand the characteristics of veins, the distribution of veins and mineralogy, microthermometric conditions of ore fluids, distribution of elements, and their implications for exploration and deposit model. The Hu'u district is a paleo-volcano member of the Miocene to Plio-pleistocene volcanic rocks. Hydrothermal alteration evolved to get out from the tonalitic body, consists of potassic, propyllitic and overprinted by phyllic and advanced argillic. The mineralization is dominated by chalcopyrite, associated with quartz±anhydrite veins, and hydrothermal breccia. Hydrothermal fluids temperature measured at the value of 109.9 °C - 525.3 °C and &gt; 550 ° C with an averaging range of Th is 296.5 -329.7 °C, and salinity of 10.9 wt% NaCl eq. Quartz veins occur as a package or series of porphyry type veins designated as EDM-M1-A1, A2-A3-Apsb-anh, A3-A2, M2-Apsb-A2-A3, M1-B-C, C-D-anhydrite, and epithermal veins. Hydrothermal fluids possibly have mixed by high-temperature hyper-saline fluids to medium temperature low saline fluids. The Humpa Leu East can be identified as a 'push-up porphyry system' that still remains more extensive system underneath or in the side area. These results can be used for understanding the texture of veins as a vectoring to ore, metal distribution in veins and rocks. Early type veins such as type A have more intensive metal content and are followed by type B, C and weaken until the latest stage.

The solubility of Cu, Ag and Au in magmatic sulfur-bearing fluids as a function of oxygen fugacity

Magma-derived fluids containing chlorine and sulfur are critical for the transport of ore metals to porphyry ore-forming environments. However, experimental data on the speciation and the solubility of ore metals in these fluids at conditions relevant to the arc magmatism are scarce. In particular, the effect of redox conditions on ore metal speciation and solubilities in sulfur-bearing fluids has not yet been experimentally constrained. We performed experiments to determine the effect of oxygen fugacity (fO2) on the solubility of Cu, Ag and Au in high-temperature, low- density, low-salinity fluids and hypersaline brine. The experiments were conducted at T = 900 °C, P = 2000 bar varying fO2 in 7 steps between 0.5 log units below the Ni-NiO buffer (NNO − 0.5) to NNO + 2.5. A prototype rapid-quench Molybdenum-Hafnium Carbide (MHC) externally heated pressure vessel assembly was used, which was fitted with a Shaw membrane for precise control of fO2. The fluid phase was sampled as synthetic fluid inclusions (SFI) by in situ fracturing of quartz chips during the experiments. As capsule material, Au97Ag2Cu1 alloy was used, which imposed activities of 0.962, 0.0082 and 0.0097 for Au, Ag and Cu, respectively. The apparent solubility of Cu and Ag at the imposed metal activities increases by a factor of 7 in the H2O-NaCl-KCl-S low-salinity fluid with fO2 increasing from NNO − 0.5 to NNO + 2.5. The addition of 0.198 m HCl increases the overall solubility of Cu by a factor of 1.2–2.4. The apparent solubility of Au decreases by a factor of 9 as fO2 changes from NNO − 0.5 to NNO + 2.5. The relationship between the logarithms of the apparent Cu and Ag solubilities and fO2 is linear and the slope of the fitted line corresponds to 1+ oxidation state of these metals indicating that they are dominantly complexed by ligands that are S-free (e.g., chloride). Thermodynamic model calculations indicate that the dominant species for Cu and Ag are NaCuCl2 and NaAgCl2, respectively. For Au, the dominant species is predicted to be NaAu(HS)2 at low and intermediate fO2 conditions, and AuCl and NaAuCl2 at oxidizing conditions. The measured gold solubilities at intermediate fO2 do not indicate significant Au complexation with S species containing S in intermediate oxidation states. Considering previous studies on silicate melts and our experimental data for volatiles, we conclude that Cu and Ag likely have constant fluid/melt partition coefficients in the typical fO2 range of arc magmatism because they dissolve in the same oxidation state in the fluid and the melt (1+) and are dominantly chloride complexed in the fluid.