Evolution Of Ore-forming Fluids Research Articles

Zn–S isotopic fractionation effect during the evolution process of ore-forming fluids: A case study of the ultra-large Huize rich Ge-bearing Pb–Zn deposit

The fluid evolution process is a key research subject in the field of ore deposit geology. An accurate inversion of the evolution history of ore-forming fluids (OFFs) can play a crucial role in the investigation of mineralisation processes and metallogenesis. In addition, analysing the combination of Zn–S isotopes eliminates the uncertainty and unilaterality of monoisotopic tracing and provides a more accurate interpretation of the evolution of OFFs during the mineralisation process. To trace the migration direction and evolution process of OFFs, based on a collected and tested batch of Zn–S isotope compositions, the characteristics of Zn–S isotopic fractionation and the type of metallogenesis were comprehensively analysed to determine the basis for building a metallogenic model of the Ultra-large scale Huize rich Ge-bearing Pb–Zn Deposit, composed of Kuangshanchang deposit and Qilinchang deposit. Previous studies have shown that OFFs migrate upward from deep sections, whereas stratum brine and atmospheric precipitation containing reduced S migrate downward from shallow sections. Rayleigh fractionation of Zn–S isotopes occurred during the migration and evolution of OFFs, resulting in the accumulation of the heavier isotopes of Zn and S present in the fluids. For example, from shallow to deep sections, δ34S values were 5–9‰ (2400–2200 m), 8.5–17.2‰ (1900–1400 m), and 13.6–15.2‰ (1274 m), the δ66Zn values of ore minerals in Qilinchang were 0.205–0.307‰ (1832–1840 m) and 0.203–0.247‰ (1261 m), while the δ66Zn values of ore minerals in Kuangshanchang were 0.290–0.424‰ (1752 m), 0.169–0.3157‰ (1463 m), and 0.033–0.123‰ (1274 m), respectively. Consequently, the isotopic composition of sulfides exhibited an obvious spatiotemporal zoning. Heavier Zn isotopes and lighter S isotopes gradually accumulated in the sulfides at increasingly shallow depths; temporally, the sulfides gradually enriched in heavier isotopes between the early and late stages of mineralisation.

Evolution of Ore-Forming Fluids and Gold Deposition of the Sanakham Lode Gold Deposit, SW Laos: Constrains from Fluid Inclusions Study

The Sanakham gold deposit is a newly discovered gold deposit in the Luang Prabang (Laos)–Loei (Thailand) metallogenic belt. It consists of a series of auriferous quartz-sulfide veins, which is distinguished from the regional known porphyry-related skarn and epithermal gold deposits. There are four mineralization stages identified in Sanakham, with native gold grains mainly occurring in stages II and III. Evolution of ore-forming fluids and gold deposition mechanisms in Sanakham are discussed based on fluid inclusion petrography, microthermometry, and Laser Raman spectroscopy. The original ore-forming fluids belong to a medium-high temperature (>345 °C) CH4-rich CH4–CO2–NaCl–H2O system. In stages II and III, the ore fluids evolve into a NaCl–H2O–CO2 ± CH4 system characterized by medium temperature (~300 °C), medium salinity (~10 wt% NaCl eq.), and CO2-rich (~10% mol). They might finally evolve into a NaCl–H2O system with temperature decreasing and salinity increasing in stage IV. Two fluid immiscibility processes occurred in stages II and III, which created high-CH4 & low-CO2 and low-CH4 & high-CO2 end-members, and CO2-poor and CO2-rich endmembers, respectively. Gold-deposition events are suggested to be associated with the fluid immiscibility processes, with P–T conditions and depth of 236–65 MPa, 337–272 °C, and 8.7–6.5 km, respectively.

Texture, in-situ geochemical, and S isotopic analyses of pyrite and arsenopyrite from the Longshan Sb-Au deposit, southern China: Implications for the genesis of intrusion-related Sb-Au deposit

Longshan is an important Sb-Au ore deposit (3.7 Mt @4.5 wt% Sb and 4.6 g/t Au) in the Xiangzhong metallogenic province (XZMP), southern China. The Sb-Au mineralization is characterized by vein ores comprising quartz, stibnite, native gold, auriferous pyrite, and arsenopyrite within the Neoproterozoic Jiangkou Formation slate. The paragenesis of the Longshan deposit can be divided into three stages, i.e., (1) stage 1 (pre-ore) composed of euhedral coarse to nodular sedimentary pyrite and deformed irregular quartz veins, (2) stage 2 marked by NE-striking and NW-dipping quartz, stibnite, pyrite, arsenopyrite, and minor native gold veins, and (3) stage 3 characterized by NW-striking and NE-dipping quartz, stibnite, pyrite, arsenopyrite, scheelite, and minor native gold veins. Seven subtypes of pyrite and four subtypes of arsenopyrite from the three stages are recognized based on mineral assemblages, textures, and geochemistry. Visible gold shows the complexity with three distinct categories, including Au1 from stage 2, Au2 and Au3 from stage 3. In this study, we integrated laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) analyses to document textural, compositional, and S isotopic variation among texturally complex pyrite and arsenopyrite assemblages, and constrain the genesis of the Longshan Sb-Au deposit.Py1a and Py1b of sedimentary origin from stage 1 are enriched in δ34S (from 13.3 to 16.9‰, average 15.3‰, n = 8) and have low Au concentrations (from 0.43 to 4.10 ppm, n = 37). Two subtypes of pyrite (Py2a–Py2b) and arsenopyrite (Apy1a–Apy1b) from stage 2 are characterized by oscillatory zoning textures in backscattered electron (BSE) images and are obvious by their relatively high content of invisible Au (up to 351 and 1357 ppm for pyrite and arsenopyrite, respectively) and restricted range of δ34S values (from 2.1 to 4.7‰, average 3.7‰, n = 19). Three subtypes of pyrite (Py3a–Py3c) and two subtypes of arsenopyrite (Apy2a–Apy2b) from stage 3 have relatively low Au concentrations (up to 95.1 and 64.9 ppm for pyrite and arsenopyrite, respectively), and show a narrow variation in δ34S values (from –2.5 to 3.7‰, average 0.8‰, n = 22). These data suggest that magmatic-hydrothermal ore-forming fluids contribute to most of the Au and S budget in the Longshan Sb-Au deposit.The major differences between mineralization in terms of texture, trace elements, and S isotopic compositions of pyrite and arsenopyrite can reflect contrasting mechanisms of Au precipitation and provide implications for the genesis of this intrusion-related Sb-Au deposit. The new data underpin the importance of performing complementary in-situ mineralogical analyses to elucidate the source and evolution of ore-forming fluids and enable correct interpretation of the hydrothermal Sb-Au system architecture.

Textural and chemical evolution of magnetite from the Paleozoic Shuanglong Fe-Cu deposit: Implications for tracing ore-forming fluids

Abstract The Aqishan-Yamansu belt in Eastern Tianshan (NW China) hosts several important Fe and Fe-Cu deposits, the origin of which is the subject of considerable debate. The coexistence of various types of ore-forming fluids makes it difficult to distinguish the genesis of the Fe-Cu deposits. We present detailed textural and compositional data on magnetite from the Paleozoic Shuanglong Fe-Cu deposit to constrain the formation of iron oxides and the evolution of the ore-forming fluids and thus define the genesis of the Fe-Cu ores. Based on the mineral assemblages and crosscutting relationships of veins, two mineralization stages were established, including the early Fe mineralization and late Cu mineralization stage. Three types of magnetite, i.e., platy (MA), massive (MB), and granular (MC) magnetite occur in the Fe mineralization. Backscattered electron (BSE) images identified display oscillatory zoning in an early hematite and transformational mushketovite phase (MA-I), characterized by abundant porosity and inclusions, as well as two later generations, including an early dark (MA-II, MB-I, and MC-I) and later light magnetite (MA-III, MB-II, and MC-II). The MA-I has extremely high W contents and mostly displays as micro- and invisible scheelite inclusions, which were probably caused by the W expulsion during mushketovitization. The texture and composition of magnetite suggest that the later light magnetite formed via dissolution and reprecipitation of the precursor dark magnetite, and the temperature and oxygen fugacity of fluids decreased over time. Our study also shows the MB-II magnetite and coexisting chlorite display synchronous oscillatory zoning, with the calculated temperature from 444 to 212 °C. Such variations could indicate the incursion of external low-temperature fluids with high salinity, which can dissolve the primary dark magnetite. This study provides a good example of using magnetite to trace the complex evolution and multiple sources of ore-forming fluids.

Oxygen isotope evidence for input of magmatic fluids and precipitation of Au-Ag-tellurides in an otherwise ordinary adularia-sericite epithermal system in NE China

Abstract Tellurium-rich (Te) adularia-sericite epithermal Au-Ag deposits are an important current and future source of precious and critical metals. However, the source and evolution of ore-forming fluids in these deposits are masked by traditional bulk analysis of quartz oxygen isotope ratios that homogenize fine-scale textures and growth zones. To advance understanding of the source of Te and precious metals, herein, we use petrographic and cathodoluminescence (CL) images of such textures and growth zones to guide high spatial resolution secondary ion mass spectroscopy (SIMS) oxygen isotope analyses (10 µm spot) and spatially correlated fluid inclusion microthermometric measurements on successive quartz bands in contemporary Te-rich and Te-poor adularia-sericite (-quartz) epithermal Au-Ag vein deposits in northeastern China. The results show that large positive oxygen isotope shifts from –7.1 to +7.7‰ in quartz rims are followed by precipitation of Au-Ag telluride minerals in the Te-rich deposit, whereas small oxygen isotope shifts of only 4‰ (–2.2 to +1.6‰) were detected in quartz associated with Au-Ag minerals in the Te-poor deposits. Moreover, fluid-inclusion homogenization temperatures are higher in comb quartz rims (avg. 266.4 to 277.5 °C) followed by Au-Ag telluride minerals than in previous stages (~250 °C) in the Te-rich deposit. The Te-poor deposit has a consistent temperature (~245 °C) in quartz that pre- and postdates Au-Ag minerals. Together, the coupled increase in oxygen isotope ratios and homogenization temperatures followed by precipitation of Au-Ag tellurides strongly supports that inputs of magmatic fluid containing Au, Ag, and Te into barren meteoric water-dominated flow systems are critical to the formation of Te-rich adularia-sericite epithermal Au-Ag deposits. In contrast, Te-poor adularia-sericite epithermal Au-Ag deposits show little or no oxygen isotope or fluid-inclusion evidence for inputs of magmatic fluid.

Complementary Textural, Trace Element, and Isotopic Analyses of Sulfides Constrain Ore-Forming Processes for the Slate-Hosted Yuhengtang Au Deposit, South China

Abstract Yuhengtang is a representative slate-hosted Au deposit in the Jiangnan orogenic belt, South China, with a reserve of ~55 t Au and an average grade of ~3.9 g/t. Gold mineralization is characterized by veinlet and disseminated ores comprising native gold, auriferous pyrite, and arsenopyrite. Paragenesis of the Yuhengtang deposit can be divided into three stages. Pre-ore stage 1 is composed of bedding-parallel layers of pyrite in slate of the Neoproterozoic Banxi Group. Main ore stage 2 represents the Au mineralization stage, and two distinct types of mineralization can be distinguished: visible Au-arsenopyrite-pyrite in quartz veinlets and auriferous arsenopyrite-pyrite disseminated within altered slate. Post-ore stage 3 consists of quartz-pyrite-calcite-ankerite veins. In this study, we integrate electron microprobe, laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and high-resolution ion microprobe (SHRIMP) analyses to document textural, isotopic, and compositional variation among texturally complex pyrite and arsenopyrite assemblages in veinlet and disseminated ores. Additionally, LA-ICP-MS sulfur isotope mapping of pyrite highlights the covariation behavior between trace elements and sulfur isotopes at the grain scale, thus allowing the factors controlling sulfur isotope fractionation in hydrothermal Au deposits to be constrained. Pyrite, of sedimentary origin (stage 1), hosts negligible Au (<1.6 ppm) but is enriched in δ34S (15.6–25.8‰). Pyrite and arsenopyrite from stage 2 veinlet mineralization both display porous and dissolution-reprecipitation textures, have low Au concentrations (<4 and <78 ppm, respectively), and show a large variation in δ34S (–2.7 to 14.7‰ and –10.3 to 12.1‰, respectively). Pyrite and arsenopyrite from disseminated mineralization are, in contrast, characterized by oscillatory zoning textures and homogeneous appearance in backscattered electron (BSE) images, respectively, and are obvious by their relatively high contents of invisible Au (up to 90 and 263 ppm, respectively) and restricted range of δ34S values (0–5.3‰). These data suggest that magmatic-hydrothermal fluids contribute most of the Au and S budget in the Yuhengtang Au deposit. The major differences between veinlet and disseminated mineralization in terms of texture, trace element concentrations, and δ34S signatures of pyrite and arsenopyrite reflect contrasting mechanisms of Au precipitation and an evolution of physicochemical parameters of the ore-forming processes, particularly fO2 and the intensity of fluid-rock interaction. Pyrite from stage 3 appears homogeneous in BSE images yet displays a wide variation in δ34S values (1.2–31.4‰), further highlighting the controlling role played by physicochemical condition (i.e., pressure) on the δ34S signature of sulfides. Results of the coupled LA-ICP-MS sulfur and trace element mapping reveal that some zoned pyrite grains from stage 2 formed via overgrowth of Au-rich, light δ34S (2.4‰) hydrothermal rims onto Au-poor, heavy δ34S (18.1–18.5‰) sedimentary cores. All results support that multiple depositional mechanisms within a dynamic mineral system were responsible for Au concentration and define the specific textural, compositional, and sulfur isotope signatures of sulfides in coexisting vein/veinlet and disseminated mineralization. The new data highlight the ore-forming processes-based interpretation for ore genesis and underpin the importance of performing complementary in situ mineralogical analyses to elucidate the source and evolution of ore-forming fluids and enable correct interpretation of the architecture of the hydrothermal Au system.

Ore-Forming Fluid Evolution of Shallow Polyhalite Deposits in the Kunteyi Playa in the North Qaidam Basin

Polyhalite occurrence in the Kunteyi Playa in the Qaidam Basin has been known for many years. However, the genetic mechanism of this deposit remains unclear. In this study, a typical section in the playa depocenter is selected to study the polyhalite mineralogy combined with the homogenization temperature and composition of halite fluid inclusions in shallow evaporitic strata. The results show that 1) the main evaporite minerals in the strata are halite and polyhalite; no common gypsum is found; 2) analyses of homogenization temperatures of halite fluid inclusions indicate that a higher temperature is needed for polyhalite generation compared with other saline minerals; and 3) the fluid inclusion chemical analysis shows that they are sulfate-type minerals with a shortage of Ca. Thus, it can be concluded that the formation of polyhalite is not related to gypsum replacement, and deep oilfield brines may provide a Ca source and a higher temperature for polyhalite formation, where the mixing and interaction occurred between K- and Mg-enriched sulfate brines and deep Ca-enriched brines under the control of climate and tectonics in the study area. While most polyhalite was generated natively, some formed during secondary generation, which was potentially related to replacement with carnallites or sylvites.

Genesis of the Longkou–Tudui gold deposit, Jiaodong Peninsula, eastern China: Constraints from zircon U-Pb dating, fluid inclusion studies and C–H–O–S stable isotopes

The Jiaodong Peninsula in eastern China hosts numerous large and giant gold deposits. Recently, a set of gold deposits distributed along the Guocheng Fault have been identified as a new gold mineralization type, of which the Longkou–Tudui gold deposit is spatially associated with the Muniushan monzogranite whereas its genesis is poorly constrained. A systematic study including zircon U-Pb dating, fluid inclusions, C–H–O–S stable isotopes was carried out in order to determine the genesis of the Longkou–Tudui gold deposit. Zircon dating results indicate that the Muniushan monzogranite and dioritic porphyrite were emplaced at 2089–2066 Ma (with a metamorphism probably occurring at 1860 ± 27 Ma, MSWD = 0.98) and 115.6 ± 1.4 Ma, respectively. Combined with cross-cutting relationships of the Muniushan monzogranite, dioritic porphyrite, and gold orebodies, it is inferred that the gold mineralization occurred in the early Cretaceous. Fluid inclusion study demonstrates a H2O–NaCl ± CO2 system, with a progressive decrease in average values of homogenization temperature and salinity from pre-ore to post-ore stages but a slight increase in bulk density. The ore-forming fluid is characterized by reductive, acidic, high to middle temperature, low salinity, and low density as revealed by a combined study on fluid inclusion and stable isotopes. H–O–C–S isotopic compositions indicate that the ore-forming materials have multiple sources including an inherited S source from the Jingshan Group, a mantle-derived C source, and a mixed H–O source induced by the addition of meteoric water. Compared with other Jiaodong gold deposits, the Longkou–Tudui deposit does not differ in mineralization age, nature and evolution of ore-forming fluid, and source of ore-forming materials except for orebody occurrence and heavier δ34S values inherited from the Jingshan Group, and this deposit was a byproduct of oceanic subduction of the Paleo Pacific Ocean.

Genesis of the Erdaohe skarn Pb-Zn-Ag deposit in the Great Hinggan Range, NE China: Evidence from geology, fluid inclusions, and H–O–S isotope systematics

The large Erdaohe Pb-Zn-Ag deposit is an important and representative skarn deposit in the middle segment of the Great Hinggan Range. The Pb-Zn ore bodies are mainly hosted in and among different lithological layers of volcaniclastic and carbonate rocks. The Si–Ca interface with abrupt changes in rock chemical properties and the bedding detachment faults played important roles in controlling mineralization. The mineralization of the deposit can be divided into three stages: (1) pre-ore skarn stage (I); (2) syn-ore quartz–sulfide stage (II); and (3) post-ore quartz–carbonate stage (III). Here, we present a combined study of geology, fluid inclusions (FIs), laser Raman analysis and H–O–S isotopes to constrain the origin and evolution of ore-forming fluids and the source of ore-forming materials. Four types of FIs are identified in the Erdaohe deposit, including liquid-rich (L-type), vapor-rich (V-type), CO2 (C-type) and daughter mineral-bearing (S-type) FIs. The L-type, V-type, C-type and S-type FIs appear simultaneously in stages I and II, and their homogenization temperatures (Th) are similar, indicating that boiling of the ore-forming fluids occurred. From stages I to III, Th varies from 338 to 475 °C, 194 to 383 °C and 157 to 323 °C, with salinities ranging from 3.87 to 50.72 wt% NaCl equiv., 0.35 to 39.05 wt% NaCl equiv. and 0.18 to 6.88 wt% NaCl equiv., respectively. The main ore-forming fluid was a H2O–NaCl ± CO2 system. From stages I to III, the variations in δ18Ofluid range from −2.3 to 7.2‰, −8.8 to 1.0‰, and −13.6 to −3.8‰; the ranges of δD are −141 to −208‰, −133 to −161‰, and −126 to −142‰, respectively. H–O isotope studies reveal that the initial ore-forming fluid was mainly derived from magma and underwent intense degassing, followed by mixing with meteoric water. The δ34S values of sulfide minerals vary in a small range, are concentrated from 5.2 to 10.0‰, and are characteristic of deep-sourced magmatic sulfur. The high δ34S values in the Erdaohe deposit are probably the result of magmatic sulfur mixing with sulfur from the surrounding Duobaoshan Formation. The alteration style related to mineralization, combined with FIs, laser Raman analysis, and H–O–S isotopes, suggests that the Erdaohe Pb-Zn deposit is a typical skarn deposit associated with magmatic activity. The boiling of ore-forming fluids and mixing with meteoric water played crucial roles in ore precipitation.

Constraints on fluid evolution and growth processes of black smoker chimneys by pyrite geochemistry: A case study of the Tongguan hydrothermal field, South Mid-Atlantic Ridge

Up to now, the Tongguan hydrothermal field is the southernmost-distributed modern submarine massive sulfide discovered on the South Mid-Atlantic Ridge. A well-preserved black smoker chimney which contains three distinct concentric zones was collected from the active vent field. The external outer wall of the chimney contains mainly pyrite with minor sphalerite, chalcopyrite and galena. The sulfides within the internal middle layer consist of predominant pyrite, with lesser amounts of marcasite. The axial conduit is dominated by chalcopyrite, with minor pyrite. To clarify the ore-forming fluid evolution during the black smoker chimney growth, we performed detailed petrographic and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICPMS) studies on pyrite grains from the chimney. Seven types of pyrite: framboidal, relict, colloform, dendritic, massive, euhedral and lined pyrites, are identified from exterior to interior of the chimney.As shown by the results of LA-ICPMS, geochemical compositions of pyrite vary across the chimney. Generally, low-temperature elements (Mg, V, Mn, Pb, Zn, Ag and Tl) are enriched in pyrite in the outer wall of the chimney and decrease toward the interior zones. Elements indicative of high temperature conditions, such as Se and Te, show opposite trends and are enriched in pyrite within the inner zones of the chimney. Framboidal, relict, colloform and dendritic pyrites precipitate from low to mid temperature fluids (<250 °C), while massive and euhedral pyrites are products of ore-forming fluids characterized by mid to high temperature (250 to > 335 °C). Relict and lined pyrites have the same chemical signatures, suggesting that they precipitated under similar physicochemical conditions. The observed patterns are resulted from the changes in temperature and fluid chemistry caused by interactions of between seawater and hydrothermal fluids with each other in the chimney.Cobalt concentrations in pyrite from the modern black smoker chimneys located on the mid-ocean ridge apparently vary systematically in response to salinity change of vent fluids. The low Co contents (average 0.32 ppm) in pyrite suggest that the hydrothermal fluids vented from the black smoker chimney are Cl-depleted compared to seawater, most likely caused by phase separation. The systematic variations in geochemistry and morphology of the pyrite across the chimney reflect multiple stages of mineralization. Different degrees of fluid-seawater mixing and phase separation are the main controlling factors during the chimney formation.

Iron and sulfur isotopic compositions of carbonatite-related REE deposits in the Mianning–Dechang REE belt, China: Implications for fluid evolution

Carbonatite-related rare earth element (REE) deposits are the most significant source of REEs worldwide. The processes of REE precipitation, enrichment, and mineralization remain controversial. The Cenozoic Mianning–Dechang (MD) REE belt, located in Sichuan Province, southwestern China, comprises one giant (Maoniuping), one large (Dalucao), and two small–medium (Muluozhai and Lizhuang) deposits. These deposits provide a continuous record of fluid evolution, and thus are ideal for investigating the processes of REE mineralization in carbonatite-related REE deposits. Given that sulfate (i.e., barite and celestite) and sulfide (i.e., pyrite and galena) minerals crystallized and precipitated in the pegmatitic to hydrothermal stages, respectively, the REE minerals formed later than the sulfate minerals. However, the formation sequence of the sulfide minerals and bastnäsite is unclear, although both pyrite and bastnäsite formed in the late hydrothermal stage. We used S isotope data for sulfate and sulfide minerals and Fe isotope data for pyrite to investigate the composition and evolution of ore-forming fluids during the magmatic–hydrothermal stages. The sulfate minerals have positive δ34SCDT values (+3.2‰ to +8.3‰), and the sulfide minerals have negative δ34SCDT values (−13.5‰ to − 5.6‰) in the four REE deposits. In the Maoniuping deposit, δ34SCDT values for barite from the pegmatitic stage (+4.7‰ to +5.7‰) are higher than for barite from the hydrothermal stage (+4.1‰ to +4.5‰), which indicate that hydrothermal activity led to relative enrichment in isotopically light S. The δ34SCDT values for barite (+3.2‰ to +5.5‰) are lower than for celestite (+6.2‰ to +7.2‰) from the pegmatitic stage in the Dalucao deposit. The δ34SCDT values for galena (−13.5‰) are also lower than for pyrite (−13.5‰ to −7.2‰) from the hydrothermal stage in the Guangtoushan section. In general, δ34SCDT values change from positive to negative values (+8.3‰ to −16.4‰) as the fluids evolved from the pegmatitic to hydrothermal stages, which can be attributed to a decrease in oxygen fugacity (fO2) and addition of sediment containing isotopically light S. Iron isotopic compositions of pyrite from the hydrothermal stage show significant variations (δ56FeIRMM-014 = −0.03‰ to +0.65‰ for the Maoniuping deposit; −0.14‰ to 0.00‰ for the Dalucao deposit; +0.05‰ to +0.35‰ for the Lizhuang deposit), and are higher than those for the carbonatites (δ56Fe IRMM-014 = −0.47‰ to −0.17‰). These data indicate there are two sources of Fe in the MD REE belt, which are the carbonatite–nordmarkite magma and 56Fe-rich sediment. Paleozoic–Mesozoic volcanic–sedimentary and Mesozoic clastic and carbonate rocks are exposed in the MD REE belt. In general, the S–Fe isotope data, along with geological and petrographic observations, indicate that the REE minerals formed later than the sulfate minerals, and the S–Fe were derived from both carbonatite magma and sediment containing isotopically light S and heavy Fe.

Textural and geochemical characteristics of garnet from the Luoyang Fe skarn deposit, eastern China: implications for ore-forming fluid evolution and mineralization conditions

AbstractThe late Palaeozoic Yong’an–Meizhou depression belt is an important iron (Fe) and polymetallic metallogenic belt in southern China. It has undergone a transformation from Tethys to the circum-Pacific tectonic domain. The Luoyang deposit is one of the typical Fe skarn deposits in the Yong’an–Meizhou depression belt of eastern China. Garnet is a characteristic mineral in the deposit. Two generations of garnets are detected in the deposit based on their textural characteristics and trace-element contents, and are represented by Fe-enriched andradite. The first generation of garnets (Grt1) have two types of garnets (Grt1-A and Grt1-B). Type A garnets of the first generation (Grt1-A) (Adr80-88) replaced by massive diopside-magnetite assemblage exhibit distinct oscillatory zonings and display patterns of enriched light rare earth elements (LREE) to weak heavy rare earth elements (HREE), with weak negative to positive Eu anomalies, and highest U, ΣREE and Sn contents. Type B garnets of the first generation (Grt1-B) are irregular zones (Adr94-96) coexisting with magnetite, in which Grt1-A is generally dissolved, and have obviously LREE-enriched and HREE-depleted patterns, with weak negative to positive Eu anomalies, and moderate U, ΣREE and Zn contents. Garnets of the second generation (Grt2) (Adr96-99) that replaced massive magnetite together with sphalerite show unzoned patterns, with a flat REE pattern and pronounced negative Eu anomalies as well as contents of lowest U and ΣREE, and highest W. The substitution of REEs in garnets occurs as [X2+]VIII –1[REE3+]VIII +1[Si4+]IV –1[Z3+]IV +1in an Al-enriched environment. Luoyang hydrothermal fluids shifted from reducing conditions with relatively high-U and -ΣREE characteristics to oxidizing conditions with relatively low-U and -ΣREE characteristics. The reduced siderophile elements and increased fO2 in fluid during Grt1-B formation caused magnetite mineralization and reduced Zn contents during Grt2 formation, causing the deposition of sphalerite. All garnets formed from magmatic fluid and were controlled by infiltrative metasomatism in an opened system.

Ore-Forming Fluid Evolution in the Giant Gejiu Sn–Cu Polymetallic Ore Field, SW China: Evidence From Fluid Inclusions

The giant Gejiu Sn–Cu polymetallic ore deposit is one of the largest Sn producers in the world, and is related in time and space to highly evolved S type granitic intrusion. The mineralization processes can be divided into four stages: (I) skarnization; (II) greisenization; (III) cassiterite–sulfid; and (IV) cassiterite–tourmaline–quartz. Five types of fluid inclusions were recognized using optical petrography, microthermometry, and Raman spectroscopy. The results of microthermometry revealed the evolution of the ore-forming fluid, from a high temperature with low–to–high salinity to a low temperature with low–to–intermediate salinity. Stage I, skarn Sn–Cu ores were formed by bimetasomatism between the granitic intrusion and the surrounding rock under near–critical conditions with the help of ore-forming fluid. Stage II, the fluid was separated into the coexisting liquid and vapor phases in equilibrium condition, and a large amount of cassiterite–scheelite–beryl–lithium muscovite minerals were formed during greisenization. Stage III, mixing, boiling and immiscibility of different types of fluid solutions took place with a decline in temperature and pressure as well as a change in the Eh–pH, which caused amounts of cassiterites and sulfides to precipitate. Stage IV, stockwork ores characterized by cassiterite–tourmaline–quartz minerals were formed associated with the low temperature and low salinity hydrothermal liqiud activity. The laser Raman spectra identified CH4 in all ore-forming stages, indicating that the ore deposits might have been formed in a relatively reduced environment. CO2 appeared in all stages in addition to Stage I, and might have been formed due to both immiscibility of fluid solutions with dropping pressure as well as temperature and mixing of different types of fluid solutions. In conclusion, the bimetasomatism, mixing, and immiscibility of fluid solutions should have been responsible for the formation of giant Sn–Cu polymetallic deposits.

Characteristics and evolution of ore-forming fluids in the Diyanqinamu Mo deposit, Inner Mongolia: Evidence from LA-ICP-MS analysis of individual fluid inclusion

The Diyanqinamu Mo deposit in Dong Ujimqin Banner of Inner Mongolia is a large deposit. In this study, hydrothermal activity is divided into five stages: quartz - potassium feldspar - fluorite, quartz - muscovite, epidote - magnetite, chlorite - carbonate – pyritization, and argillization. The results show that early exsolved fluid has the characteristic of high temperature (up to 470°C), high oxygen fugacity, and Fe- and sulfide-rich in daughter minerals. LA-ICP-MS analysis of individual fluid inclusion exhibit early fluid is rich in W, Mo, Pb, Zn, Fe, Mn, As, Ag, Sn, and rare earth elements of La, Ce. The ore-forming fluids are NaCl-H2O system and contain gases CO2 and a small amount of H2 and N2. In general, the ore-forming fluids evolved from high temperature (340 °C - 470°C) with high salinity (up to 63.9 wt%NaCl equiv) to low temperature (189 °C - 202°C) with low salinity (as low as 0.18 wt% NaCl equiv). The ore-bearing fluid comes from the exsolution of the volatile-rich magma, and hydrothermal fluid boiling result from the drop of the pressure may have led to Mo precipitation. Based on the alteration, mineralization, and fluid characteristics, the Diyanqinamu Mo deposit is a porphyry deposit related to magmatic activity in Yanshanian.

Fluid inclusion and C-O isotopic constrains on the origin and evolution of ore-forming fluids of the Badaguan Cu-Mo deposit, Inner Mongolia

The Badaguan porphyry Cu–Mo deposit is located in the Derbugan metallogenic belt. Aqueous (W-type), CO2-rich (WC-type), vapor-rich (V-type), and daughter mineral–bearing inclusions (S-type) were recognized in the hydrothermal quartz. Fluid inclusions in the early stage show high homogenization temperatures (247–374 ℃) and high pressures (66–191 MPa), with salinities of 0.95–12.15 wt% NaCl equiv. The presence of anhydrite in veins and phenocrysts suggests high oxygen fugacity conditions. The veins in the Mo- and Cu mineralization stages were formed by immiscible fluids at temperatures of 244–366 ℃ and 183–342 ℃, and pressures of 39–137 MPa and 30–98 MPa, respectively. They yielded salinities of 1.69–10.52 and 0.35–6.85 wt% NaCl equiv, respectively. Homogenization temperatures of fluid inclusions in late stage range from 194 to 269 ℃ with a salinity of 0.6–5.88 wt% NaCl equiv. It is concluded that the ore-forming fluids in the early stage were magmatic in origin, then gradually diluted and cooled by meteoric water. During this process, fluid immiscibility and CO2 release induced by pressure drop and oxygen fugacity decrease resulted in Mo mineralization, while temperature drop was the main trigger for the precipitation of chalcopyrite. Laser-Raman analyses suggest that the fluid inclusions of the Badaguan porphyry Cu-Mo deposit in Inner Mongolia are rich in CO2. Minor CH4 occurs in the Mo- and Cu-mineralization stage, which may be generated during water–rock interaction. The hybrid carbon isotope compositions of the fluid inclusions, mainly representing CO2, show a very depleted δ13CPDB (−21‰ to −28.3‰), which can be explained by the joint influence of ~ 0.5% subducted oceanic sedimentary contamination in the mantle and carbon isotope fractionation induced by 35–80% CO2 degassing. Petrographic observation, previous studies on the geochemical and isotopic characteristics of ore-causative rocks rule out the possibility of contamination of ascending magma with reducing wall rocks. The large variations in δ18Owater (−6.6‰ to +2.4‰) for quartz and calcite suggest the participation of meteoric water during water–rock interaction.

Lithium isotopic systematics of ore-forming fluid in the orogenic gold deposits, Jiaodong Peninsula (East China): Implications for ore-forming mechanism

There are rare studies on the Li isotope systematics of ore-forming fluids in the mesothermal deposits, especially in the orogenic gold deposits. We carried out systematic Li isotopic analyses on whole-rock granite samples, and quartz and pyrite were separated from both the altered-rock type and quartz-vein type gold deposits from the Canzhuang gold deposit of the Jiaodong Peninsula (East China) with purposes of shedding lights on the ore-forming fluid evolution and ore-forming mechanism. Altered granite and unaltered granite yield δ7Li values of 12.6‰ and 1.9‰, respectively, indicating that fluid-related alteration can change the Li isotopic composition of granite significantly and ore-forming fluid is enriched in heavy Li isotope. The δ7Li values of quartz and pyrites from the altered-rock type gold deposits range from 5.5‰ to 21.1‰ and from 0.5‰ to 16.8‰, respectively. The δ7Li values and Li contents of quartz are positively correlated in the altered-rock type gold deposit, with the two figures in the first mineralization stage (5.5–13.7‰ and 0.3–1.1 ppm) being significantly lower than that in the second stage (17.9–20.8‰ and 1.0–10.4 ppm) and the third stage (17.3–21.0‰ and 1.1–4.2 ppm). This is consistent with the lithium isotopic compositions of pyrites of the altered-rock type gold deposits, with δ7Li values of pyrites in the first stage (0.5–2.8‰) being significantly lower than that in the second stage (2.3–16.8‰) and the third stage (5.8–6.2‰). Collectively, these features demonstrate that the Li isotopic compositions of the altered-rock type gold deposits were controlled mainly by interaction between ore-forming fluid and host granites, and that the ore-forming fluids in three stages belong to different pulses of fluid injection and evolve separately. On the contrary, the quartz and pyrite from the quartz-vein type gold deposits yield δ7Li values of 9.4‰ ~33.6‰ and 0.5‰ ~24‰, respectively, which are different from that of the altered-rock type gold deposits. In addition, the lithium isotopes of quartz show no correlation with the lithium contents of quartz, and the δ7Li values of quartz and pyrites decrease gradually from the first to the third metallogenic stage, with δ7Li ranging from 33.6‰ to 9.4‰ for quartz, and from 24‰ to 0.5‰ for pyrite. Based on the fractionation calculation, we suggest that Li isotope variation of the quartz-vein type gold deposits was ascribed to the Rayleigh fractionation in a close metallogenic system. This work demonstrates that the orogenic gold deposits display a significant Li isotopic fractionation and the lithium isotopes of quartz and pyrite are potential in providing insights into the evolution processes of mesothermal hydrothermal systems.

Tungsten mineralisation in the Zhazigou W–(Mo) deposit, Henan Province, China: Constraints from scheelite geochemistry and molybdenite geochronology

Skarn-type scheelite deposits are one of the most strategically and economically significant resources in China. The Zhazigou deposit, which contains 27.27 Mt of ore reserves at an average grade of 0.64% WO3 and 0.02% Mo, is a large W–Mo deposit located within the East Qinling Orogenic Belt between the North China and Yangtze cratons. This study presents new scheelite mineralogical and molybdenite geochronological data for the deposit, and uses these data to explore the evolution of ore-forming fluids associated with the deposit and the processes that formed the mineralisation. The deposit is a redox-intermediate W–(Mo) skarn deposit, and records four stages of mineralisation, namely, skarn (I), quartz–scheelite (II), quartz–sulphide (III), and carbonate (IV) stages. The W mineralisation mainly developed during the quartz–scheelite stage (II), whereas the Mo mineralisation generally developed during the quartz–sulphide stage (III); trace molybdenite that coexists with scheelite formed during the quartz–scheelite stage (II). Re–Os geochronological analysis of molybdenite intergrown with scheelite yields a weighted mean age of 146.3 ± 0.9 Ma that is consistent with a 145.4 ± 2.6 Ma Re–Os isochron age and a 147 Ma zircon U–Pb age for the Yuku porphyritic granite, which is close to the Zhazigou deposit. The substitution of the rare earth elements (REEs) into scheelite is controlled predominantly by the reaction 3Ca2+ = 2REE3+ + □Ca, which indicates that the REE characteristics of the scheelite in the Zhazigou deposit were inherited from the mineralising fluids. The REE and Sr–Nd isotopic compositions of the scheelite within the deposit are similar to those of the Yuku porphyritic granite, indicating that the ore-forming fluid was mainly magmatic in origin. Scheelite samples from the skarn and quartz–scheelite stages (I–II) have high Mo contents (up to 38930 ppm), whereas notably decreasing Mo contents (727–2629 ppm) characterise the quartz–sulphide and carbonate stages (III–IV). The changing redox state had a key role in affecting Mo abundances in the Zhazigou scheelite. A relatively oxidising fluid in the skarn (I) and quartz–scheelite (II) stages transformed into a reducing fluid in the quartz–sulphide (III) and carbonate stages (IV). Changes in the mineral assemblages from ferric- (e.g., andradite) to ferrous- (e.g., pyrrhotite) dominated also record these redox changes. The scheelite was precipitated as a result of fluid–rock interaction that released Ca2+ into the mineralising fluids. Oxygen fugacity exerted an important control on molybdenite precipitation, and a decrease in oxygen fugacity might have promoted the formation of scheelite, as well as the change from high- to low-Mo scheelite.

Source and Evolution of the Ore-Forming Fluids of Carlin-Type Gold Deposit in the Youjiang Basin, South China: Evidences from Solute Data of Fluid Inclusion Extracts

The source and evolution of ore-forming fluids is important to understand the genesis of Carlin-type gold deposit. Constraints on the source and evolution of ore fluid components by the conventional geochemical methods have long been a challenge due to the very fine-grained nature and complex textures of hydrothermal minerals in these deposits. In this study, we present the crush-leach analyzed solute data of fluid inclusion extracts within quartz, calcite, realgar, and fluorite from the Shuiyindong, Nibao, and Yata gold deposits in the Youjiang Basin, providing new insights into the source and evolution of ore-forming fluids. The results show that the high molar Cl/Br ratios up to 2 508 in fluid inclusion extracts are indicative of a contribution of magmatic hydrothermal fluids. Fluids mixing between basinal and magmatic-hydrothermal fluids are evident on the plots of Cl/Br versus Na/K ratios, showing that ore-stage milky quartz near the magmatic-hydrothermal fluids reflects magma origin of the ore-forming fluids, whereas late ore-stage drusy quartz and realgar near the defined basinal fluids suggest the later input of basinal fluids in late-ore stage. Although the predominately host rocks in Shuiyindong, Nibao and Yata gold deposit are bioclastic limestone, sedimentary tuff, and calcareous siltstone, respectively, the solute data of fluid inclusion extracts records they underwent the similar fluid-rocks reactions between the Na-rich magmatic hydrothermal fluids and the Ca- and Mg-rich host rocks. This study highlights the solute data of fluid inclusion extracts obtained by crush-leach analyses have the potential to fingerprint the source and evolution of ore-forming fluids of the Carlin-type gold deposit.

Multistage hydrothermal quartz veins record the ore-forming fluid evolution in the Meiling Cu–Zn (Au) deposit, NW China

The Meiling Cu–Zn (Au) deposit, located in the center of the southeastern Kalatag district in the eastern Tianshan, NW China, is characterized by multistage ore-bearing quartz veins. The fluid inclusion, textures, and trace element compositions of the quartz associated with three main mineralization stages—Stage I: quartz (Qtz1) + pyrite ± chalcopyrite ± sphalerite veins; Stage II: quartz (Qtz2) + pyrite + chalcopyrite + sphalerite veins; and Stage III: quartz (Qtz3) + pyrite + chalcopyrite + covellite veins—are investigated to reveal the ore-forming fluid evolution. Stockwork and breccia Qtz1 (245 °C to 348 °C) with impure surfaces have high concentrations of the trace elements Li, Ga, Ge, Sb, and Sn, indicating that Qtz1 crystallized during a fluid boiling event under relatively hydrofracturing conditions. The fluid composition, temperature, and fluid flow rate largely influenced the trace elements in Qtz2-1 (wide temperature range of 111 °C to 279 °C) and Qtz2-2 (121 °C to 218 °C), which are characterized by smoky gray and milky quartz veins, respectively. Decreasing temperature and pressure and enhanced fluid-rock reactions may have been the factors causing the precipitation of chalcopyrite and sphalerite in Stage II. Qtz3 (110 °C to 202 °C) + chalcopyrite + covellite veins in Stage III mostly show comb-like and porous textures, and Qtz3 has relatively low Li, Al, Ti, Ga, Ge, and B contents and Li/Al ratios, indicating that Qtz3 crystallized under open and oxidized conditions. The locally higher Li, Al, and Ti contents in the rims than in the cores of Qtz2-2 and Qtz3 can be attributed to rapid growth rates. The Ti contents and fluid inclusion of quartz indicate a decreasing trend of temperature and salinity during the evolution of ore-forming fluid from the early to late stages. Furthermore, the low Ti contents (<10 ppm) of quartz in the main mineralization stages generally reveal that the mineralization temperatures were less than 350 °C, as evidenced by the fluid inclusion microthermometry data (150 °C to 230 °C). The quartz data indicate that the Meiling deposit probably shows an affinity with epithermal deposits. Nevertheless, the medium- to high- temperature and salinity fluid in Stage I and high-Ti and high-Al field of epithermal deposits, as well as the complex “C”-shaped trend in the Ti vs. Al discrimination diagram, indicate that the Meiling deposit underwent potentially magmatic-hydrothermal fluid evolution and metallogenesis.

Fluid inclusion and stable isotope constraints on the source and evolution of ore-forming fluids in the Bailongshan pegmatitic Li-Rb deposit, Xinjiang, western China

The newly discovered Bailongshan pegmatitic lithium–rubidium (Li–Rb) deposit in West Kunlun–Karakorum (western China) is a world-class Li deposit. In this paper, we present detailed field observations and fluid inclusion (FI) data for four major mineral zones at Bailongshan; i.e., the fine albite (FAZ), blocky feldspar (BFZ), quartz–muscovite (QMZ), and spodumene–quartz (SQZ) zones. FIs are abundant in quartz and spodumene, and include four types: (1) two-phase L-type, (2) two-phase V-type, (3) three-phase S-type, and (4) three-phase (CO2–H2O–NaCl) or two-phase (CO2-rich) C-type. Quartz contains all four FI types, whereas spodumene contains mainly C-type FIs. Microthermometric measurements show that the L-type FIs in the FAZ, BFZ, and SQZ homogenised at 283–338, 156–224, and 223–421 °C, respectively, with corresponding salinities of 15.4–26.5, 4.4–10.4, and 10.5–30.6 wt% NaCl equivalent. The S-type FIs in the FAZ and SQZ homogenised at 183–285 and 214–298 °C, respectively. The C-type FIs in spodumene and quartz homogenised at 256–321 and 234–286 °C, with corresponding salinities of 4.4–18.1 and 4.4–12.6 wt% NaCl equivalent, respectively. The ore-forming fluids were of medium–low-temperature and medium–low-salinity, and no systematic fluid compositional variations were identified in each zone. The average CO2 densities and trapping pressures were estimated at 0.72–0.91 g/cm3 and 3.00–3.75 kb, respectively, corresponding to mineralisation depths of 9–11 km. Supercritical fluids likely contributed to the concentration of ore-forming elements in the fluids, and fluid boiling may have led to CO2 degassing and Li ore deposition. H–O isotope data show that quartz grains from all four zones have δ18OV-SMOW values of 8.6‰–10.2‰ and δDV-SMOW values of −38‰ to 107‰. The ore-forming fluids were likely sourced from pegmatitic magmas.