Primary Fluid Inclusion Assemblages Research Articles

Fracturing timing of Jurassic reservoirs in the Dibei-Tuziluoke gas field, Kuqa foreland basin: Evidence from petrography, fluid inclusions, and clumped isotopes

Determining the timing of fracturing is crucial for understanding reservoir evolution and hydrocarbon accumulation in foreland basins. Using fracturing data from cores, borehole images, and outcrops, combined with the clumped isotope (Δ47) and fluid inclusion analyses of carbonate minerals filled in pores and fractures, this study ascertained the fracturing timing of the Jurassic reservoirs in the Dibei-Tuziluoke Gas Field, Kuqa Foreland Basin. Data from outcrops and borehole images show two dominant fracture sets in the study area: W-E and NE-SW striking fractures. Some W-E striking fractures are carbonate-filled, while NE-SW striking fractures lack mineral fillings. Bitumen veins, not easy to be identified in borehole images, are prevalent in cores. The petrographic analysis reveals that these bitumen veins formed before the calcite cementation in pores and display high viscosity and low maturity. Homogenization temperatures (Th) from primary fluid inclusion assemblages in two representative calcite vein samples were notably lower than TΔ47 values from corresponding samples. This suggests the Δ47 signature underwent alteration due to partial reordering during burial. Thus, Δ47-derived temperatures (apparent temperatures) may not faithfully represent the mineral precipitation temperatures. When plotting these apparent temperatures vs. the burial history, only the possible latest ages of fracturing emerged. These ages were further refined by considering petroleum charging, tectonic evolution, and stress orientation. Bitumen-filled fractures likely resulted from the Late Cretaceous uplift, marking the migration of low-maturity hydrocarbons in the study area. Carbonate-filled E-W striking fractures emerged during the late Miocene (∼13–6.5 Ma) alongside fold development. NE-striking fractures that crosscut W-E ones possibly formed recently due to stress reorientation.

Geology and genesis of the North Narbaghi Cu-Ag deposit in the Urumieh-Dokhtar magmatic arc, Iran: Fluid inclusion and stable isotope constraints

The North Narbaghi Cu-Ag deposit in the central part of the Urumieh-Dokhtar magmatic arc (UDMA) of Iran is hosted by Eocene volcanic sequences and Oligo-Miocene intrusions of arc affiliation. The mineralization can be divided into five stages that are represented by barren hydrothermal breccias, Cu-Fe sulfides cementing in the breccias, sulfosalt and sulfide hydrothermal breccias, barren post-ore quartz veinlets, and barren post-ore calcite veinlets, respectively. The metallic minerals consist of pyrite, chalcopyrite, bornite, sphalerite, and tennantite-tetrahedrite, which are variably associated with gangue minerals including quartz, calcite, and sericite. The upper parts of the ores have been oxidized to form supergene minerals including malachite, azurite, chrysocolla, chalcocite, covellite, hematite, and goethite. The ore-related hydrothermal alteration includes silicification, pyritization, carbonatization, intermediate argillitization, and propylitic alteration. Primary fluid inclusion assemblages in quartz associated with main-stage sulfides and sulfosalts have homogenization temperatures ranging from 184° to 385 °C and calculated salinities between 12.0 and 29.7 wt% NaCl equiv. (avg. 21.4 wt% NaCl equiv.). The coexistence of fluid inclusion assemblages with highly variable volume percentages of vapor phase indicates that boiling commonly occurred in the hydrothermal system, contributing to Cu saturation. Sulfide minerals range in δ34S from −1.5 to 3.7 ‰, indicating a probable magmatic sulfur source. This view is further supported by the carbon and oxygen isotopes of hydrothermal carbonates, with δ13CPDB of −2.2 to −1.5 ‰ and δ18OSMOW between 5.30 and 11.75 ‰. Stable isotope and fluid inclusion data illustrate that boiling of the ore fluid and its mixing with meteoric water facilitated mineralization at the North Narbaghi. We suggest that the North Narbaghi Cu-Ag deposit can be best interpreted as an intermediate-sulfidation epithermal deposit formed due to post-Eocene magmatic-hydrothermal activity. The intersections of post-Eocene subvolcanic rocks with regional faults were the most favorable sites for focusing fluids responsible for epithermal mineralization along the UDMA.

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.

Using In Situ Monazite and Xenotime U-Pb Geochronology to Resolve the Fate of the “Missing” Banded Iron Formation-Hosted High-Grade Hematite Ores of the North China Craton

Abstract High-grade hematite mineralization is widely developed in banded iron formations (BIFs) worldwide. However, in the North China craton where Neoarchean-Paleoproterozoic BIFs are abundant, economic high-grade hematite ores are scarce. High-grade hematite ores hosted in the Paleoproterozoic Yuanjiacun BIFs represent the largest occurrence of this type of ore in the North China craton. The orebodies are fault controlled and show sharp contacts with lower greenschist facies metamorphic BIFs. In situ U-Pb geochronology of monazite and xenotime intergrown with microplaty hematite and martite in high-grade ore established two episodes of metamorphic-hydrothermal monazite/xenotime growth after deposition of the BIFs. The earlier episode at ca. 1.94 Ga is interpreted as the timing of lower greenschist-facies metamorphism, and the later episode at 1.41 to 1.34 Ga represents the timing of high-grade hematite mineralization. Petrography and microthermometry of primary fluid inclusion assemblages indicate that the high-grade hematite ore formed from hot (313°–370°C), CO2-rich, and highly saline (~20 wt % NaCl equiv) hydrothermal fluids. These fluids channeled along faults, which concentrated iron through interaction with the BIFs—a process similar to typical hematite mineralization elsewhere. The deposition of hematite was probably related to tectonic extension in the North China craton related to the breakup of the Columbia/Nuna supercontinent. Our results challenge a previously proposed model ascribing the scarcity of high-grade hematite ores in the North China craton to the lack of prolonged weathering conditions. Rather, we argue that the high-grade ore formed in lower metamorphic-grade BIFs at shallower depths than magnetite mineralization and was largely eroded during later exhumation and uplift of the craton.

Formation of the Majiayao gold deposit, Jiaodong Peninsula, eastern China: Constraints from fluid inclusions, H–O–S–Pb isotopes, and pyrite Rb–Sr age

The Majiayao gold deposit at Qixia in Shandong Province is a typical gold deposit of medium–low temperature hydrothermal veins. This gold deposit is one of the most important types in the Jiaodong Peninsula. Four stages have been identified based on the cross‐cutting relationships and mineralogical and textural characteristics. The study of quartz vein fluid inclusions show that the inclusions can be divided into three categories: pure liquid or pure gas phase inclusions (type S), two‐phase gas and liquid inclusions (type W), and three‐phase inclusions rich in CO2 (type C). The results of laser Raman probe analysis show that the fluid inclusions are generally rich in CO2, thus forming a typical CO2–H2O–NaCl system. The fluid temperature results show that in stage I, only the type W fluid inclusions are identified by microthermometry, the homogenized temperatures (to liquid) are 210–343°C and mostly range from 260 to 280°C, the range of salinity is 1.57–10.98 wt.% NaCl, with a peak value of 8–11 wt.% NaCl, and the density is 0.65–1.02 g/cm3, with a peak value of 0.81–0.87 g/cm3; in stages II and III, the primary fluid inclusion assemblages include both type W and C inclusions, which show similar phase‐transition temperatures and are trapped between 190 and 300°C, and the peak values of salinities are 4–10 wt.% NaCl equivalent; and in stage IV, the total homogenization temperatures (Th‐TOT) into liquid range from 120 to 220°C, and the range of salinities is 4.2–7.9 wt.% NaCl. These values indicate that temperature, salinity, and density gradually decreased from the early to late mineralization stage. The ore‐forming fluid of the deposit was a thermal liquid system with medium–low temperature, medium–low salinity, and low density and was rich in reducing CO2. The H–O isotope shows that the ore‐forming fluids were dominated by metamorphic fluid with only minor amounts of meteoric water involved in the ore‐forming process. According to the S, Pb, and Sr isotopes analysis, it indicated that the source of ore‐forming materials is mainly crustal fluid. The measured Rb–Sr isochron age of pyrite is 123.4 ± 2.9 Ma, which indicates that the metallogenic age corresponded to the Early Cretaceous.

Origin and Evolution of Ore-Forming Fluid and Gold-Deposition Processes at the Sanshandao Gold Deposit, Jiaodong Peninsula, Eastern China

The Early Cretaceous Sanshandao gold deposit, the largest deposit in the Sanshandao-Cangshang goldfield, is located in the northwestern part of the Jiaodong peninsula. It is host to Mesozoic granitoids and is controlled by the north by northeast (NNE) to northeast (NE)-trending Sanshandao-Cangshang fault. Two gold mineralizations were identified in the deposit’s disseminated and stockwork veinlets and quartz–sulfide veins, which are typically enveloped by broad alteration selvages. Based on the cross-cutting relationships and mineralogical and textural characteristics, four stages have been identified for both styles of mineralization: Pyrite–quartz (stage 1), quartz–pyrite (stage 2), quartz–pyrite–base metal–sulfide (stage 3), and quartz–carbonate (stage 4), with gold mainly occurring in stages 2 and 3. Three types of fluid inclusion have been distinguished on the basis of fluid-inclusion assemblages in quartz and calcite from the four stages: Pure CO2 gas (type I), CO2–H2O inclusions (type II), and aqueous inclusions (type III). Early-stage (stage 1) quartz primary inclusions are only type II inclusions, with trapping at 280–400 °C and salinity at 0.35 wt %–10.4 wt % NaCl equivalent. The main mineralizing stages (stages 2 and 3) typically contain primary fluid-inclusion assemblages of all three types, which show similar phase transition temperatures and are trapped between 210 and 320 °C. The late stage (stage 4) quartz and calcite contain only type III aqueous inclusions with trapping temperatures of 150–230 °C. The δ34S values of the hydrothermal sulfides from the main stage range from 7.7‰ to 12.6‰ with an average of 10.15‰. The δ18O values of hydrothermal quartz mainly occur between 9.7‰ and 15.1‰ (mainly 10.7‰–12.5‰, average 12.4‰); calculated fluid δ18O values are from 0.97‰ to 10.79‰ with a median value of 5.5‰. The δDwater values calculated from hydrothermal sericite range from −67‰ to −48‰. Considering the fluid-inclusion compositions, δ18O and δD compositions of ore-forming fluids, and regional geological events, the most likely ultimate potential fluid and metal would have originated from dehydration and desulfidation of the subducting paleo-Pacific slab and the subsequent devolatilization of the enriched mantle wedge. Fluid immiscibility occurred during the main ore-forming stage due to pressure decrease from the early stage (165–200 MPa) to the main stage (90–175 MPa). Followed by the changing physical and chemical conditions, the metallic elements (including Au) in the fluid could no longer exist in the form of complexes and precipitated from the fluid. Water–rock sulfidation and pressure fluctuations, with associated fluid unmixing and other chemical changes, were the two main mechanisms of gold deposition.

Origin of the high-grade Early Jurassic Brucejack epithermal Au-Ag deposits, Sulphurets Mining Camp, northwestern British Columbia

The high-grade (8.1 million ounces of gold; 15.6 Mt grading 16.1 g/t Au; Pretium, 2016) Brucejack epithermal Au-Ag deposits are located in the Canadian Cordillera of northwestern British Columbia, and formed in association with extensive early Mesozoic island arc magmatism. Porphyry-type Cu-Au-Mo mineralization occurs nearby at the Kerr-Sulphurets-Mitchell (196–190 Ma; Febbo et al., 2015), Bridge Zone (191.7 ± 0.8 Ma, 191.5 ± 0.8 Ma), and West Zone (188.9 ± 0.9 Ma) prospects. Gold-silver vein-type mineralization at Brucejack is hosted by variably altered and deformed Early Jurassic porphyritic latite lava flows, volcaniclastic rocks, and volcanic-derived sandstones, siltstones, and conglomerates. This study focuses on the Valley of the Kings Zone at Brucejack, where host rocks have been dated at 188–184 Ma (U-Pb, zircon). Post-mineralization (but not post-alteration) basaltic and trachybasaltic dikes that cut the veins are geochemically similar to the host volcanic sequence and follow the same structural trends of the mineralized vein corridors. We suggest these dikes are broadly coeval with mineralization, and that ore formation at the Valley of the Kings formed during the overall Jurassic tectonomagmatic event.Gold-silver mineralization is hosted by quartz-carbonate veins that cut sericitized and pyritized volcaniclastic rocks. Six stages of veining are defined, with Au-Ag mineralization (electrum) focused in stages III–V: stage I–III veins consist of quartz with minor carbonate, chlorite, sericite, and sulfide minerals; stage IV quartz veins contain more abundant base metal sulfides (pyrite, sphalerite, and galena with minor sulfosalts); stage V veins are dominantly calcitic; late post-mineralization stage VI veins are quartz-calcite and contain sparse pyrite and chlorite, but no electrum. The veins are interpreted to be syn- to late-tectonic, with deformation decreasing from locally penetrative in the host-rocks prior to veining (resulting in local foliation of sericite and pressure shadows around pyrite), to dismemberment and shearing of early stage I–II quartz veins, brittle disruption of stage III–IV quartz and stage V carbonate veins, and minimal deformation of post-mineralization stage VI quartz-carbonate veins. A much younger set of extensional muscovite veins locally cuts the deposit with Late Cretaceous apparent ages, and appears to be associated with a weak thermal overprint that has reset K-Ar and 40Ar/39Ar ages in sericite throughout the district.The deformation experienced by the vein system, particularly in the later stages (III–V), was not uniform, and original undeformed vein textures and fluid inclusion assemblages are locally preserved. We report fluid inclusion microthermometric data from carefully selected primary fluid inclusion assemblages from quartz, calcite, and sphalerite from vein stages III–IV, many of which show evidence for boiling (coexisting liquid- and vapor-rich primary fluid inclusions). Liquid-rich fluid inclusions from stage III and V veins have moderate homogenization temperatures (∼170 °C and ∼160 °C respectively) and salinities of 2–8 wt% NaCl equiv., whereas inclusions from base-metal-sulfide-bearing stage IV veins show evidence of mixing with a cooler, more saline brine (∼140 °C, ∼10–15 wt% NaCl equiv.). Carbon dioxide was observed as clathrate during cooling in some fluid inclusions, suggesting that minor amounts of CO2 were present in the fluids.Calculated oxygen isotopic compositions of fluids in equilibrium with quartz and calcite from vein stages III–V range from δ18Ofluid = −10.7 to +1.8‰, whereas δ13CCO2 ranges from −9.5 to −4.5‰, and δ34Spyrite ranges from −1.7 to +0.6‰. Taken together, these data suggest a magmatic source for S and some C, carried by a fluid of evolved or diluted magmatic origin, which variably mixed with meteoric-derived groundwater or seawater containing carbon of sedimentary (organic) origin. Mixing is supported by large salinity ranges of vein stage IV fluid inclusions, where individual assemblages range from ∼1 to 15 wt% NaCl equiv.These fluid temperatures, salinities, and isotopic compositions are typical of epithermal deposits where distal magmatic fluids mix with local heated groundwaters. The low fluid temperatures but evidence of boiling suggest formation at shallow crustal depths. This is despite the evidence for penetrative deformation in the sericitized volcanosedimetary host rocks, which we attribute to rapid uplift immediately prior to or during the earliest stages of mineralization. The vein system is therefore interpreted to have formed during the later stages of a deformational event related to arc accretion.

Antimoniferous vein-type mineralization of the Berga Antiform, Eastern-Thuringia, Germany: A fluid inclusion study

The antimoniferous vein-mineralization in the Schleiz and Greiz area, eastern Thuringia, are bound to NE-SW striking Variscan fault zones along the margins of the Berga Antiform. The Sb-Pb-Zn ores form massive sulfide-sulfosalt-quartz ± carbonate veins, breccias or stockworks within Ordovician and Silurian metasedimentary units, Devonian metabasalts and metagranitoids. Mineralogical investigations from several Sb mines and hard rock quarries near Schleiz and Greiz, Germany, show that the Sb-Pb-Zn-sulfides are texturally in close association with quartz and carbonates. Stibnite is the major Sb-bearing mineral. Minerals of the Sb-Pb-Zn-sulfide assemblage replace an earlier Zn-Fe-As-sulfide-quartz assemblage (quartz I), named the ore-stage I mineralization. Stibnite and quartz II overgrow these early sulfides accounting for the ore-stage II. Lead-dominated Sb-sulfides (e.g., zinkenite, plagionite, semseyite) represent the ore-stage III phase followed by sphalerite II, galena and quartz III accounting for ore-stage IV. Fluid inclusion studies on ore and gangue minerals of the different ore stages give insights into the composition and temperature of the mineralizing fluid(s). Primary fluid inclusion assemblages in ore-stage II stibnite and co-genetic quartz II show variable vapor fractions within fluid inclusion assemblages. Heterogeneous trapping of CO2 at constant salinities of fluid inclusions of the same assemblage indicates the influx of a buoyant pure CO2 fluid into an aqueous fluid phase. The presented data demonstrates the first example of CO2-influx into an aqueous fluid identified by fluid inclusion analyses. During this ore-stage II a H2O-CO2-NaCl fluid, with eq. w(NaCl) of 1 to 2.7% at 220–226 °C prevails. In contrast, fluid inclusions in ore-stage III minerals (boulangerite) contain a homogeneously trapped H2O-NaCl-CaCl2 fluid, with eq. w(NaCl) up to 8.1% at temperatures ≥146 °C. Ore-stage IV minerals (sphalerite II, quartz III) contain a homogeneously trapped H2O-NaCl-CaCl2 fluid with eq. w(NaCl) of >20% and homogenization into the liquid phase at around 120 °C. Fluid inclusion data suggest P-T-conditions at the onset of mineralization at around 220 °C at 0.70 to 0.75 kbar (fluid pressure). The mechanisms of Sb-ore precipitation were induced by the influx of CO2, fluid cooling (quartz II – stibnite – boulangerite) and late stage mixing with a high salinity fluid resulting in the precipitation of sphalerite II, galena and quartz III. The unique opportunity to analyze fluid inclusions in many different, subsequently precipitating ore and gangue minerals enables the reconstruction of the temporal evolution of the Sb-veins. The distinct change of the ore-forming processes from the influx of CO2 into an aqueous fluid and cooling (ore-stage I, II) to mixing (ore-stage III, IV) as the dominant precipitation mechanisms are also accompanied by a drastic change in the mineralogy from Sb-sulfides to Pb-Zn-sulfides.Late-orogenic granitic intrusions served as a potential heat, metal and sulfur source. Variscan wrench and thrust tectonics and associated metamorphism account for the ore-stage I mineralization. Hydrothermal Sb-Pb-Zn assemblages (ore-stages II, III and IV) precipitated during the latest part of the Variscan orogeny most likely around the Carboniferous/Permian boundary. Although, fluid composition and formation temperatures of the Greiz and Schleiz mineralization are on the first view quite similar to those of other Variscan Sb mineralization, the observation of CO2-influx and fluid mixing with a high-salinity fluid is unique for European Sb-occurrences.

Late Pliocene High-Sulfidation Epithermal Gold Mineralization at the La Bodega and La Mascota Deposits, Northeastern Cordillera of Colombia

La Bodega and La Mascota are Plio-Pleistocene high- to intermediate-sulfidation epithermal gold deposits hosted in Proterozoic gneisses and Mesozoic granites of the Santander massif, eastern Cordillera of Colombia and are part of the California-Vetas mining district. Coeval volcanic rocks are not recognized in the district and volumetrically minor granodiorite porphyries dated at 10.1 Ma are the only igneous rocks younger than Jurassic known from the vicinity of the epithermal deposits. As of 2010, La Bodega and La Mascota contained an inferred resource of 3.47 Moz Au, 19.2 Moz Ag, and 84.4 Mlbs Cu at a 2 g/t Au cut off. Mineralization exhibits NE-trending, NW-dipping structural control associated with the right lateral strike-slip La Baja fault. Mineralization at La Bodega is composed of vein networks and tectonic-hydrothermal breccias while at La Mascota it is largely contained in tabular, fault-controlled tectonic-hydrothermal breccia bodies. Alteration and sulfide assemblages can be assigned to six distinct hydrothermal episodes which comprise early porphyry-style phases and a late epithermal-style phase with up to four distinct stages. Porphyry phases comprise stages 1 and 2. Stage 1 is characterized by propylitic alteration with epidote, chlorite, calcite, specularite veins, minor pyrite, and chalcopyrite. Molybdenite from veins collected 0.7 km southwest of La Mascota yielded an Re-Os age of 10.14 Ma which constrains the age of stage 1. Stage 2 is related to a separate, younger magmatic-hydrothermal system and characterized by phyllic alteration (sericite-illite, quartz, pyrite) associated with quartz-pyrite veins and its age is constrained by 40 Ar/ 39 Ar geochronology on hydrothermal muscovite (i.e., sericite) to ~3.4 Ma. The epithermal phase comprises stages 3 through 6 and is related to multiphase hydrothermal breccia development and quartz-alunite alteration. Stage 3 is defined by presence of copper sulfides such as bornite, covellite, and chalcopyrite, stage 4 by wolframite, stage 5 by enargite, and stage 6 by sphalerite. Pyrite, quartz, and alunite are common to all epithermal stages. Residual vuggy quartz is scarce. Gold-silver mineralization took place during stages 3 through 5 with some gold introduced during stage 2. Alunite 40 Ar/ 39 Ar ages constrain epithermal mineralization to between ~2.6 and ~1.6 Ma. Primary fluid inclusion assemblages representative of hydrothermal stages 4 and 5 contain liquid and vapor-rich inclusions and, together with quartz vein textures, suggest that boiling was an important ore-precipitating mechanism. Homogenization temperatures of these range from ~143° and 238°C and salinities from 0.5 to 5.6 wt % NaCl equiv. Pyrite δ 34 S ranges from −16.9 to −11.3‰ at La Mascota and −8.3 to −6.1‰ at La Bodega. The strongly negative sulfur isotope signature can be explained by fluid boiling or, alternatively, by biogenic sulfur source from the Proterozoic paragneisses. Based on δ 18 O and δ D data, alunite was precipitated from magmatic fluids and only stage 6 alunite shows clear evidence for mixing of the hydrothermal fluid with meteoric water.

Origin and evolution of ore fluid, and gold-deposition processes at the giant Taishang gold deposit, Jiaodong Peninsula, eastern China

The Early Cretaceous Taishang gold deposit is the largest deposit in the Linglong goldfield, Jiaodong Peninsula, with a pre-mining endowment of 326t of gold. It is hosted in the 165–150Ma Linglong biotite granite and controlled by the NEE- to NE-trending Potouqing Fault near the northern end of the regional Zhaoping Fault system. Hydrothermal alteration is well developed in the footwall of the Potouqing Fault and is characterized by a narrow zone of sericitization, followed by a wider zone of silicification, and an even wider zone of potassic alteration. The main orebodies (No. I orebody) comprise disseminated and stockwork-veinlet systems, although about 5% of the resources are in larger quartz veins (No. II orebody). Four stages have been identified for both styles of mineralization, on the basis of cross-cutting relationships and mineralogical and textural characteristics: pyrite–quartz–sericite (stage 1), quartz–pyrite (stage 2), quartz–pyrite–base-metal-sulfide (stage 3) and quartz–carbonate (stage 4). Gold was mainly deposited in stages 2 and 3, with minor amounts in stage 1.Petrographical, microthermometric, and laser Raman spectroscopic studies on fluid-inclusion assemblages in quartz and calcite from the four stages reveal three types of primary fluid inclusions: type 1 H2O-rich aqueous-carbonic, type 2 CO2-rich aqueous-carbonic, and rare type 3 carbonic inclusions. Stage 1 quartz primary inclusions are only type 1 inclusions, with an estimated composition of 88% H2O, 10% CO2, 4.5 wt.% NaCl equiv., and 0.5% CH4, with trapping at ~336°C and ~1.7kbars. The gold-rich stages 2 and 3 from both orebodies typically contain primary fluid-inclusion assemblages with both type 1 and 2 inclusions, which show similar phase-transition temperatures and were trapped between 246° and 294°C. The stage 4 quartz and calcite contain only primary type 1 inclusions, which are estimated to have a composition of 93% H2O, 6.0% CO2, 3.6 wt.% NaCl equiv., and trace amount of CH4, and were trapped at temperatures of >236°C.The δ34S values of hydrothermal pyrite from the four stages have a narrow range from 4.5‰ to 8.0‰, and are within the ranges for whole-rock sulfides from the Archean Jiaodong Group, and magmatic pyrite from Mesozoic granitoid and intermediate-basic dikes. The δ18O values of hydrothermal quartz range mainly from 10.9 to 12.5‰ and remain constant for all four stages; calculated fluid δ18O values are 1.3–10.0‰. The δDwater values calculated from hydrothermal sericite range from −60 to −45‰. Considering the fluid inclusion compositions, δ18O and δD compositions of ore-forming fluids, and regional geological events, the most likely ultimate potential fluid and metal reservoirs would be the Paleo-Pacific oceanic slab and its overlying sediments, which were thrust below the high-grade metamorphic rocks of the Jiaodong Peninsula.The initial ore-forming fluids were medium-high temperature, CO2-rich, and low salinity H2O–CO2–NaCl±CH4 homogeneous fluids. Over the duration of the hydrothermal system, the fluid remained fairly consistent in P–T–X, although becoming slightly more water-rich during final post-ore activity. Fluid immiscibility occurred during stages 2 and 3 ore deposition at pressures that fluctuated strongly from 1700 to 580bars during hydrofracturing. The (HS)2− ion was the most probable gold-transporting complex at Taishang. Wall-rock sulfidation and episodic pressure drops, with associated fluid unmixing and other chemical changes, were the two main mechanisms of ore deposition.

Structural Control, Hydrothermal Alteration Zonation, and Fluid Chemistry of the Concealed, High-Grade 4EE Iron Orebody at the Paraburdoo 4E Deposit, Hamersley Province, Western Australia

High-grade iron ore of the 4EE orebody of the 4E deposit (>200 Mt at 63.5 wt % Fe) occurs as a southerly dipping sheet within banded iron formation (BIF) of the Paleoproterozoic Dales Gorge and Joffre members of the Brockman Iron Formation. Structural reconstruction of the 4E deposit shows that reactivation of the 18E fault and development of the NW-striking, steeply SW dipping 4E and 4EE normal faults resulted in preservation of the 4EE orebody below the 4E deposit, and 400 m below the modern topographic surface. Three hypogene alteration zones between low-grade BIF and high-grade iron ore are observed: (1) distal magnetite-quartz-dolomite-stilpnomelane-hematite ± pyrite, (2) intermediate magnetite-dolomite-hematitechlorite-quartz- stilpnomelane, and (3) proximal hematite-dolomite-chlorite ± pyrite ± magnetite. Hydrothermal alteration is temporally and spatially constrained by NW-trending dolerite dikes that intruded the 4E and 4EE faults prior to hypogene alteration. Six vein types (V1-V6) are recognized at the 4E deposit. The veins both cut and parallel the primary BIF layers and were emplaced contemporaneously with the hydrothermal alteration zones that record the transformation of low-grade BIF to high-grade iron ore. Our integrated structural-hydrothermal alteration and fluid flow model proposes that during early stage 1a, hypogene fluid flow in the 4E orebody occurred during a period of continental extension and enhanced heat flow within sedimentary basins to the south of the Paraburdoo Range. Heated basinal brines were focused by the NW-striking, steeply SW dipping 4E and 4EE normal faults and reacted with BIF of the Dales Gorge and Joffre members. The warm to hot (160° -255°C), Ca-rich (26.6-31.9 equiv wt % CaCl2) basinal brine interacted with magnetite-chert layers, transforming them into magnetite-quartz-dolomite- stilpnomelane-hematite-pyrite BIF. The iron-rich brine (up to 2.8 wt % Fe) likely originated from evaporated seawater that had lost Mg and Na and gained Li and Ca through fluid-rock reactions with volcaniclastic rocks and carbonate successions within the Wittenoom Formation. The first incursion of deeply circulating, low-salinity (5.8-9.5 wt % NaCl equiv), heated (106°-201°C) modified meteoric water is recorded in late stage 1a minerals. This modified meteoric water had lost some of its Na through wall rock interaction with plagioclase, possibly by interaction with dolerite of the Weeli Wooli Formation that directly overlies the Joffre and Dales Gorge members. Stage 1b involved continuing reactions between the hydrothermal fluids and the magnetite-quartz-dolomite-stilpnomelane-hematite-pyrite BIF, and produced both the intermediate magnetite-dolomite-hematite-chlorite-pyrite and the proximal hematite-dolomite-magnetite-stilpnomelane alteration assemblages. Microplaty (10-80 μm), platy (100-250 μm), and anhedral hematite increasingly replace magnetite in the intermediate alteration zone, forming the proximal alteration zones that consist of microplaty, platy, anhedral hematite and magnetite. The intermediate and proximal alteration zones represent the mixing of a hot (250°-400°C), high-salinity, Ca-rich (30-40 wt % CaCl2 equiv), Sr-rich basinal brine with low-temperature and low-salinity (~5 wt % NaCl equiv) modified meteoric water that was heated (~100°-200°C) during its descent into the upper crust. Heterogeneous mixing of the two end-member fluids resulted in the trapping of primary fluid inclusion assemblages containing a wide range of trapping temperatures (up to 200°C) and salinities (up to 25 wt % NaCl equiv). Stage 1c of the hypogene hydrothermal fluid is characterized by low-temperature (<110°C), low-salinity (~5 wt % NaCl) meteoric water that interacted with the proximal hematite-dolomite-magnetite-stilpnomelane- altered BIF, leaving a porous, hematite-apatite high-grade ore. Supergene alteration affected the orebody since the Cretaceous and produced a hematite-goethite alteration assemblage, resulting in destruction of the hypogene alteration zones that are only preserved below the depth of modern weathering. Discovery of the concealed 4EE orebody of the 4E deposit demonstrates that structural geology plays a critical role in the exploration for high-grade iron orebodies. Structural reconstruction should be considered a critical exploration activity in structurally complex terranes where concealed orebodies may exist.

The influence of tectonics on petrological characteristics of anhydrite and anhydrite-halite intercalations in the Oldest Halite (Na1) (Zechstein, Upper Permian) of the Bądzów area (SW Poland)

The Oldest Halite (Na1) (Zechstein, Upper Permian) played a significant role in the development of geological structure of the Fore-Sudetic Monocline (SW Poland) because of its different rheological properties in comparison to the adjacent rocks. Intercalations of anhydrite and anhydrite-halite occurring within the Na1 show a lot of features of tectonically disturbed rocks. Macroscopic petrological observations revealed that anhydrite rocks represent four rock types with different structural and textural properties. On the microscopic scale, five types of crystals have been distinguished in all macroscopic types. They vary in size, shape and distribution of primary fluid inclusion assemblages. The anhydrite-halite rocks show two groups with slightly different petrological features that are apparently related to their tectonic deformation and recrystallisation processes. The main recorded directions of tectonic movements seem to be horizontal or sub-horizontal.

Genetic investigation and comparison of Kartaldağ and Madendağ epithermal gold deposits in Çanakkale, NW Turkey

Two epithermal gold deposits (Kartaldağ and Madendağ) located in NW Turkey have been characterized through the detailed examinations involving geologic, mineralogical, fluid inclusion, stable isotope, whole-rock geochemistry, and geochronology data.The Kartaldağ deposit (0.01–17.65ppm Au), hosted by Eocene dacite porphyry, is associated with four main alteration types with characteristic assemblage of: i) chlorite/smectite–illite±kaolinite, ii) quartz–kaolinite, iii) quartz–alunite–pyrophyllite, iv) quartz–pyrite, the last being characterized by three distinct quartz generations comprising massive/vuggy (early), fine–medium grained, vug-lining (early), and banded, colloform, comb (late) textures. Observed sulfide minerals are pyrite, covellite, and sphalerite. Oxygen and sulfur isotope analyses, performed on quartz (δ18O(quartz): 7.93 to 8.95‰ and calculated δ18O(H2O): −7.95 to 1.49‰) and pyrite (δ34S(pyrite): −4.8‰ and calculated δ34S(H2S): −6.08 to −7.20‰) separates, suggest a meteoric water source for water in the hydrothermal fluid, and an igneous source for the sulfur dissolved in ore-related fluids. Microthermometric analyses of primary fluid inclusion assemblages performed on quartz (late quartz generation) yield temperatures (Th) dominantly in the range of 245–285°C, and generally low salinity values at 0 to 1.7wt.% NaCl eq. Based on the quartz textures and the associated base metal concentrations, along with fluid inclusion petrography, the early vug-lining quartz is considered to have been associated with the mineralization possibly through a boiling and a late mixing process at >285°C.The Madendağ deposit (0.27–20.60ppm Au), hosted by Paleozoic mica schists, is associated with two main alteration types: sericite–illite–kaolinite, and quartz–pyrite dominated by two distinct quartz generations i) early colloform, comb and banded quartz and ii) late quartz, forming the cement in hydrothermal breccia. Whereas oxygen isotope analyses of quartz (δ18O(quartz): 9.55 to 18.19‰ and calculated δ18O(H2O): −2.97 to 5.54‰) suggest varying proportions of meteoric and magmatic sources for the ore bearing fluid, sulfur isotope ratios (δ34S(pyrite): −2.2‰ and calculated δ34S(H2S): (−3.63) to (−3.75) ‰) point to an essentially magmatic source for sulfur with or without contribution from sedimentary sources. Microthermometric analysis carried out on primary fluid inclusion populations of a brecciated sample (early quartz), give a temperature (Th) range of 235–255°C and 0.0 to 0.7wt.% NaCl eq. salinity. Based on the textural relationship, base metal and high gold contents, the ore precipitation stage is associated with late stage quartz formation via a possible boiling process.The presence of alunite, pyrophyllite and kaolinite, vuggy quartz and covellite suggest a high-sulfidation type of epithermal deposit for Kartaldağ. On the other hand, Madendağ is identified as an adularia-sericite type owing to the presence of significant sericite, neutral pH clays (mostly illite, chlorite/smectite, and kaolinite), low temperature quartz textures (e.g., colloform, comb, and banded quartz), and limited sulfide minerals.Given the geographical proximity of Kartaldağ and Madendağ deposits, the similar temperature and salinity ranges obtained from their fluid inclusions, and the similar ages of igneous rocks in both deposits (Kartaldağ: 40.80±0.36 to 42.19±0.45Ma, Madendağ: 43.34±0.85Ma) the mineralizing systems in both deposits are considered to be genetically related.

Fluid inclusion studies in datolite of low grade metamorphic origin from a Jurassic pillow basalt series in northeastern Hungary

Abstract The Jurassic pillow basalt blocks in the Szarvaskő Unit in the SW-Bükk Mountains are incorporated into an accretionary mélange of Cretaceous age which had been displaced from the Dinarides to NE-Hungary during the Alpine-Carpathian collision. The pillow basalt series is a part of an incomplete remnant of an ophiolite-like sequence from which ultramafic units and sheeted dikes are almost completely absent. At the studied Egerbakta quarry, closely packed pillow, pillow fragmented hyaloclastite breccia and peperitic facies of submarine basaltic volcanism were recognized. Peperitic facies was formed by local admixture of unconsolidated siliciclastic sediment into the basaltic lava. Basaltic pillows contains short and thin prehnite-chlorite-carbonate-quartz veins formed during the interaction of seawater with the cooling lava, as well as cross-cutting datolite-prehnite-quartzchlorite-albite bearing veins. Calcite in the short cooling cracks crystallised from upheated seawater at around 160°C temperature and 0.5–0.6 kbar pressure (water depth of about 5–6 km). Datolite precipitations in veins can be classified into at least three different textural-morphological types but all of them contain methane bearing primary fluid inclusion assemblages, with variable methane/water ratios, suggesting an inhomogeneous entrapment from a heterogeneous aqueous-carbonic fluid. End member primary fluid inclusions contain aqueous liquid with methane rich vapour phase and apparently methane only inclusions characterized by liquid phase on room temperature. A combination of results from fluid inclusion microthermometry, Raman spectroscopy (used not only for phase identification but also for determination of salinities) and temperatures calculated from chlorite compositions, show that the precipitation of datolite had taken place from low salinity (0.2–2 NaCl equiv. mass%) aqueous-methane fluids under increasing temperature and pressure (from 160 to 210°C, and from 0.6 to 1.1 kbar) conditions. The elevated pressures and temperatures in comparison to the submarine hydrothermal processes suggest that the datolite bearing mineral parageneses had been formed during the progression of Alpine very low grade metamorphic processes.

Nature of Hydrothermal Fluids at the Shale-Hosted Red Dog Zn-Pb-Ag Deposits, Brooks Range, Alaska

The Red Dog Zn-Pb-Ag district in the western Brooks Range, northern Alaska, contains numerous shale-hosted Zn-Pb sulfide and barite deposits in organic-rich siliceous mudstone and shale, chert, and carbonate rocks of the Carboniferous Kuna Formation. The giant Red Dog shale-hosted deposits consist of a cluster of four orebodies (Main, Qanaiyaq, Aqqaluk, and Paalaaq) that lie within distinct thrust panels that offset a single ore deposit during the Mesozoic Brookian orogeny. These Zn-Pb-Ag-barite orebodies contain one of the world’s largest reserves and resources of zinc. Fluid inclusions in samples of vein sphalerite, which accounts for about 20 percent of the ore in the Main deposit, and quartz that composes the bulk of the extensive silicification in the ore deposit, were studied by microthermometry, Raman spectrometry, and ion chromatography. The study of fluid inclusions in the vein sphalerite was limited by the intense postore deformation of the ore deposits. However, four primary aqueous fluid inclusion assemblages in vein sphalerite yield temperatures of homogenization of 115° to 120°C, 123° to 127°C, 110° to 120°C and 175° to 180°C. More abundant final-melting temperatures indicate that the fluid inclusions in sphalerite have salinities of about 14 to 19 wt percent NaCl equiv. The fluid inclusion electrolyte data show that the ore fluid responsible for the vein sphalerite derived its salinity from the evaporation of seawater. Considering the salinity of the fluid inclusions together with the electrolyte data, it is possible that the evaporative was initially about 30 wt percent saline fluid and that it mixed with a more dilute fluid somewhere along its flow path. The temperature, salinity, and electrolyte composition of vein sphalerite in the Red Dog deposits are remarkably similar to those characteristics in sphalerite veins near the Century zinc deposit, Australia. Together, these data compose the majority of information on the temperature and composition of sphalerite in deposits of this type. On the basis of data describing fluid inclusions in sphalerite and the geologic setting of the ore deposits, a brine model is suggested for the Red Dog deposits. In this model, brines were produced in evaporative environments in supratidal carbonate facies of the Lisburne Group less than 100 km from the Red Dog deposits. These reflux brines may have infiltrated the underlying rocks of Endicott Group or fractured metasedimentary basement rocks. In the absence of a local heat source at the Red Dog deposits, the temperature of the ore fluids (~100° to <200°C) requires that the fluids circulated at depths between ~ 2.4 and 7.4 km. In the Red Dog area, the metalliferous fluids ascended into the organic-rich rocks of the Kuna Formation, probably along zones of active extensional faults or breaches in the shale aquitards overlying the aquifers in the Endicott Group. Fluid inclusions were also studied in the abundant quartz that constitutes the majority of the silica rock in the ore deposits. This postore quartz extensively replaced barite and was traditionally thought to be part of the main ore event. Primary fluid inclusion assemblages contain two-phase aqueous inclusions, single-phase inclusions of dense methane, or both. Primary assemblages that contain single-phase, dense-methane inclusions together with two-phase aqueous inclusions yield consistent homogenization temperatures that provide unequivocal evidence for the coeval trapping of immiscible gas and aqueous fluids. The densities of the methane inclusions, together with the temperature of homogenization of coexisting aqueous fluid inclusions, show that these fluid inclusions were trapped between pressures of 800 and 3,400 bars and temperatures between 187° and 214°C. The pressures obtained provide unequivocal evidence that the quartz formed after ore deposition in the Carboniferous because such high fluid pressures could only have been produced from thrust loading during the Mesozoic Brookian orogeny. The observed large variation in pressure is best explained by transient fluid pressures from hydrostatic to lithostatic conditions during thrust loading. The 3,400 bars pressure corresponds with about 12 km of lithostatic burial, whereas the lower pressures (800 bars) correspond with about 8 km of hydrostatic pressure. Because of their low salinity (0–5 wt % NaCl equiv) the electrolyte compositions of the quartz fluid inclusions do not constrain their origin.

A coupled fluid-inclusion and stable isotope record of paleofluids in the Monterey Formation, California

Determining the history of fossil hydrologic systems that is recorded in vein minerals can be complicated by heterogeneity in the oxygen and carbon isotope compositions at millimeter and smaller scales. Several samples of diagenetic dolomite from two location in the Miocene Monterey Formation, California, have δ18O and δ13C values that vary over a few millimeters by ∼28‰ and 6.5‰, respectively (relative to the Pee Dee belemnite isotope standard). These variations reflect episodic flow and mixing of waters from different sources with distinct diagenetic histories. On the other hand, isotopic conpositions of dolomite in samples with gradual shifts in isotopic compositions and correlated δ18O and δ13C values may have been controlled by mass diffusion and thermal conduction between episodic flow events. Primary fluid-inclusion assemblages from samples collected at one location yielded homogenization temperatures of 87.8 ± 3.6 °C (2σ). These temperatures were combined with measured δ18O values very nearly collocated in each dolomite sample to show that δ18Owater values ranged between −0.8‰ and +2.5‰ (relative to standard mean ocean water). The samples from this Monterey Formation dolomite thus indicate their precipitation from water more enriched in 18O than late Miocene and younger seawater and must reflect modification of the δ18O values of the involved waters by mineral-dehydration reactions prior to dolomite precipitation. Samples from the other location contain no primary fluid- inclusion assemblages and few abrupt shifts in isotopic compositions, perhaps reflecting recrystallization. Small-scale variations in isotopic compositions of vein minerals could be interpreted as the result of a changing regional hydrologic pattern, but we think such an interpretation for the Monterey Formation samples would be incorrect. Instead, we infer that these variations provide important information about localized effects on the ancient hydrologic system and that these effects bear on the origins of the paleofluids responsible for the mineral precipitation.