Pei Ni

SHRIMP U-Pb zircon dating for lamprophyre from Liaodong Peninsula： Constraints on the initial time of Mesozoic lithosphere thinning beneath eastern China

It is undebated fact that the lithospheric mantle beneath eastern China was considerably thinned during the Mesozoic time. However, it has no adequate evidence for the exact timing when the lithosphere thinning started. The Liaodong Peninsula is located in the eastern segment of the North China Craton and is one of the important domains to explore the event of lithosphere thinning. SHRIMP U-Pb zircon dating and geochemical study were carried out for the lamprophyre dike swarm that intruded into the magnesite ore-beds in the Dashiqiao Formation of Paleoproterozoic Liaohe Group at the Huaziyu magnesite ore district, Liaodong Peninsula. The results indicate that these lamprophyre dikes were intruded in late Jurassic (155±4 Ma) and show some geochemical characteristics of potassic magmas. It is now accepted that the lithosphere thinning took place in the late Mesozoic, and the peak thinning stage occurred in early Cretaceous (130–120 Ma). Considering the potassic mafic magmatism marking the onset of the lithospheric thinning, we therefore suggest that the studied late Jurassic potassic lamprophyre dike swarm could imply that the late Jurassic is the time that lithosphere thinning started.

Constraining ultrahigh-pressure (UHP) metamorphism and titanium ore formation from an infrared microthermometric study of fluid inclusions in rutile from Donghai UHP eclogites, eastern China

Eclogites in the Donghai area of the Sulu ultrahigh-pressure (UHP) metamorphic belt in eastern China contain both disseminated rutile and rutile deposits. The rutile occurs as small inclusions in garnet, omphacite, phengite, and quartz, as larger euhedral to subhedral grains in equilibrium with garnet, omphacite, phengite, and kyanite, and as discrete veinlets, aggregates, and megacrysts in cracks and along grain boundaries. To investigate the processes of rutile mineralization and its relationship to the evolution of the Sulu UHP metamorphic belt, we sampled the exposed Maobei deposit at Donghai and drill core between 100 and 2000 m in the nearby Chinese Continental Scientific Drilling Project (CCSD) main hole to carry out a detailed infrared microthermometric fluid inclusion study. Three types of fluid inclusions were identified: aqueous (Type I), CO 2 -H 2 O (Type II), and CH 4 (Type III). Type I and Type II inclusions are the most common. Primary and pseudosecondary Type I inclusions, which were trapped during recrystallization of the eclogite or early amphibolite facies retrograde metamorphism, have a narrow range of homogenization temperatures but a wide range of salinities. In contrast, secondary Type I inclusions, which formed entirely during retrograde amphibolite-facies metamorphism associated with exhumation of the UHP rocks, have higher and more varied homogenization temperatures but medium to low salinities. Type II inclusions formed contemporaneously with the Type I variety and are characterized by having small amounts of CO 2 and N 2 in addition to H 2 O. In addition, some of the rutile grains contain sparse inclusions composed of CH 4 (Type III). Our results combined with those of previous studies, suggest a great diversity of fluid composition in the peak metamorphic to early retrograde stages, implying very limited fluid-rock interaction during syn- to post-peak high-pressure (HP) and UHP metamorphism. This study shows that infrared microthermometric investigation of fluid inclusions can provide information not only on the composition of titanium ore-forming fluids but also on the relationship between mineralization and the evolution of UHP metamorphic belt.

Comparative study of ore-forming fluids of hydrothermal copper–gold deposits in the lower Yangtze River Valley, China

Fluid-inclusion and stable isotope studies were carried out on five types of Mesozoic (Yanshanian) hydrothermal copper–gold deposits in the lower Yangtze River Valley. Deposits include (1) copper in cryptoexplosive breccia pipes, (2) skarn copper, (3) porphyry copper, (4) high-temperature quartz vein-type copper and gold, and (5) medium–lower temperature fracture zone gold. This research has allowed a comparison between various types of ore-forming fluids. Melt-fluid inclusions in garnet from the matrix of the breccia pipe at the Shizishan copper deposit reveal the existence of a water-rich magma. In all deposit types, fluid temperatures and salinities were higher at early stages and generally decreased with time. Magmatic water is dominant in the high-temperature ore-forming fluids, whereas meteoric water was involved only in the medium–lower temperature Xiaomiaoshan gold deposit and in the post-mineralization stage of the Shaxi porphyry copper deposit. Fluid boiling played an important role in the mineralization of most deposits, particularly at Shizishan, where multi-stage boiling was associated with the formation of cryptoexplosive breccia, skarn, quartz-sulphide, and quartz-carbonate-sulphide stages. Boiling of an aqueous magmatic fluid system at high temperatures reflects the release of crystallization heat and increase of total volume of the magma–fluid system, and hence it can be referred to as active boiling. On the contrary, boiling of a fluid at lower temperatures is typically triggered by pressure release due to fracturing or dilation in the surrounding rocks, and is thus referred to as passive boiling. In general, passive boiling occurs more commonly at the higher levels of a hydrothermal mineral system and at later stages of the ore-forming process.

Spatiotemporal reconstruction of Late Mesozoic silicic large igneous province and related epithermal mineralization in South China: Insights from the Zhilingtou volcanic-intrusive complex

Silicic large igneous provinces (SLIPs) generally reflect large-scale melting of lower crustal materials and represent significant metal reservoirs. The South China Block-Coastal Region (SCB-CR) SLIP hosts several large epithermal deposits. To better understand these deposits, we document the spatio-temporal framework of the host SLIP across the SCB-CR. Using zircon U-Pb dating and geochemical and isotopic analysis, we identify four stages of emplacement. Stage 1 felsophyre (ca. 149 Ma) shows a chemical affinity to highly fractionated I-type granites. Stages 2 and 3 of low-Mg felsic volcanics (ca. 128 to 111 Ma) and stage 4 felsite (ca. 100 Ma) have higher eHf(t) and eNd(t) values than stage 1 felsophyre, suggesting a significant contribution of newly underplated juvenile crust to the magma sources. Stage 4 diabase (ca. 101 Ma) was likely produced by melting of subduction˗metasomatized asthenospheric mantle. Together with reliable published data, we build a new spatio-temporal framework of volcanics and infer that the majority of the SCB-CR SLIP was related to the gradual northwestward subduction of the Izanagi plate beneath South China in a continental arc setting during ca. 170 to 110 Ma, and minor contribution was from the eastward retreat of the subducting slab in a back-arc setting during ca. 110 to 90 Ma. We conclude that the large scale epithermal mineralization was generated by melting of the metal-rich, thin (30-40 km), newly underplated hydrous juvenile crust during the tectonic transition from arc to back-arc settings.

Hydrocarbon migration through salt: evidence from Kelasu tectonic zone of Kuqa foreland basin in China

Halite and other evaporates are rarely observed to degrade via brittle faulting because of their ductile properties. Hydrocarbons resulting from fractures and faults are rarely found in salt structures. However, large-scale hydrocarbons resulting from faulting are not well-documented. The Dabei Gasfield, which produces natural gases with few condensate oils, is found under Paleogene evaporates in the Kelasu tectonic zone of the Kuqa foreland basin. Above these evaporates, the Dawanqi Oilfield produces oils with few natural gases in the Kelasu tectonic zone of the Kuqa foreland basin. The biomarker distribution from oils and carbon isotope from natural gases between the Dabei Gasfield below evaporates and the Dawanqi Oilfield above evaporates are very similar, which indicates that they have same hydrocarbon source. The results provide direct evidences of the rare migration of hydrocarbon through salt due to fractures and faults to form oil reservoirs above the salt. This study indicates that hydrocarbons occasionally exhibit widespread migration through salt due to extensive brittle faulting.

Pyrrhotite Textures and Their Genetic Implications in the Hongtoushan Massive Sulphide Deposit, Liaoning Province, China

The Archaean massive sulphide Cu-Zn deposit at Hongtoushan has been metamorphosed to upper amphibolite facies. An investigation on ore textures shows that pyrrhotite in the deposit was originally produced by submarine exhalation, and subsequently metamorphosed, annealed and recrystallised. Deformation textures formed by progressive metamorphism of pyrrhotite were erased almost completely under peak conditions, and the presently-existing deformation textures and ore mylonites are largely resultant from diaphthoresis. Deformation and annealing of pyrrhotite could have been enhanced by fluids. Retrogressive fluids are characterized by high oxygen fugacity.

Spatial distribution and variation of ore body, alteration and ore-forming fluid of the giant Zijinshan epithermal Cu-Au deposit, SE China: implication for mineral exploration

The Zijinshan high-sulphidation Cu-Au deposit located in the west Fujian Province is the largest active gold mine in China with total production of gold over 300 tonnes to date. The high-sulphidation ore bodies are characterized by the upper supergene Au and lower hypogene Cu zones bounded roughly by the paleowater table near 600 m elevation. Au-bearing goethite and limonite are the main ore minerals in the supergene zone whereas the hypogene Cu mineralization is dominated by covellite and anilite (or digenite) with minor enargite. The expected porphyry mineralization that coupled with Zijinshan is so far not exposed even to a depth of 1500 m from surface. However, the Luoboling Cu-Mo deposit which occurs only 3 km NE to Zijinshan is identified as a typical porphyry deposit. Current debate centered on whether the Zijinshan high-sulphidation epithermal deposit and the adjacent Luoboling porphyry deposit are genetically linked or discrete hydrothermal systems. Recent deep exploration at Zijinshan revealed an extension trend of Cu ore bodies and quartz-alunite alteration zone toward SE below 0 m elevation. This contradicts previous considered NE trend above 0 m elevation. In this study, fluid inclusion mapping based on rigorous strategy and sample selection was carried out on 20 sample suites scattered over the whole high-sulphidation Cu-Au ore bodies from −450 to 800 m elevation. Aqueous fluid inclusions of unambiguous primary origin in syn-ore miarolitic quartz, coarse-grained alunite crystals and quartz overgrowth zones were measured. Microthermometry results show the average Th and salinity of each sample suite ranging from 205 to 304°C and 2.4 to 5.5 wt% NaCl equiv., respectively. Both fluid temperature and salinity exhibit negative correlation with increasing elevation, indicating convective cooling by mixing and circulating of meteoric water has involved. The good consistence of mineralization distribution pattern and fluid evolution trend implies that the southeastward extending ore bodies probably reflect the fluid pathway which is connected to the deeper porphyry mineralization and their magmatic source. Considering the temperature condition (370–400°C) required for brittle-plastic transition which commonly characterizes porphyry deposits, the potential porphyry mineralization coupled to Zijinshan should have occurred at greater depth under higher temperature. Combined with recent exploration progress and the fluid evolution pattern, we therefore proposed that the Luoboling porphyry Cu-Mo deposit and the Zijinshan high-sulphidation Cu-Au deposit are likely discrete hydrothermal systems, and future porphyry deposit exploration at the deep SE segment of Zijinshan is recommended.

Geology, mineral paragenesis and fluid inclusion studies of the Yueyang Ag-Au-Cu deposit, South China: implications for ore genesis and exploration

The Yueyang epithermal Ag-Au-Cu deposit is located in the Zijinshan ore district in western Fujian Province, South China. Orebodies are predominately hosted by the Jurassic Zijinshan granite batholith and controlled by a group of NW-trending faults. Roughly all minerals were deposited in a single extended period, and three stages can be further defined: (1) the early sphalerite stage; (2) the base metal-silver stage; and (3) the post-mineralization stage. Microthermometric measurements on quartz- and sphalerite-hosted, liquid-rich fluid inclusions indicate that mixing and dilution of the ore forming fluids took place from early to late stages. However, fluid boiling may locally occur and lead to silver participation, as suggested by the presence of fluid inclusion assemblage consisting of coexisting liquid-rich and vapour-rich inclusions, and also indicated by adularia in Ag-bearing veins and the breccia type ore of bonanza grade. Compared with typical low-sulphidation epithermal deposits, the higher Ag/Au ratios and fluid salinities, Fe-poor sphalerite and large proportion of sulphide minerals suggest that the Yueyang deposit is best classified as an intermediate-sulphidation epithermal deposit. Since intermediate-sulphidation epithermal deposits favour a magmatic affinity and usually exhibit spatial and temporal association with high-sulphidation and porphyry deposits, mineral exploration at the adjacent area of the Yueyang deposit is promising.

Mapping of fluid, alteration and soil geochemical anomaly as a guide to regional mineral exploration for the Dehua gold orefield of Fujian Province, southeast China

The Dehua gold ore-field, located in the central-eastern Fujian Province, SE China, is an important part of the renowned Dehua-Youxi-Yongtai gold ore production area. The ore-field contains several low-intermediate sulphidation epithermal gold deposits and numerous gold-bearing mineralized bodies. The extensive presence of intensely altered rocks, suitable structural environment, acidic intrusions and volcanic-subvolcanic rocks indicates an important potential for epithermal gold mineralization. In this work, fluid inclusion homogenization temperature and alteration mapping was carried out by fluid inclusion microthermometry in quartz and identification of alteration minerals closely related to the known mineralization. The result shows the trend of temperature contour curves as well as the shape of low argillic alteration zones implying that the area north of Qiucun might be profitable for future mineral exploration. The fluid and alteration mapping, in combination with soil geochemical mapping, defines the architecture of the epithermal-porphyry ore-forming system in the Dehua ore-field. Finally, the presence of intensely altered areas, high value zones of hydrothermal fluid flows and combined anomaly areas of multi-element association indicate that the region north of Qiucun has an important potential as a preferred target area for gold prospecting in the Dehua ore-field.

Constraints on the genesis of the Jiande polymetallic copper deposit in South China using fluid inclusion and O-H-Pb isotopes

The Jiande copper deposit is located in the Qin-Hang metallogenic belt, South China. The deposit is dominated by “stratiform-like” ores, which are hosted in the dolomite of the Upper Carboniferous Huanglong Formation. These ore bodies were previously proposed to be Carboniferous sedimentary exhalative (Sedex) style mineralization, but they also appear to be related to the Late Mesozoic granodiorite porphyry at Jiande. Three stages of mineralization can be observed. The prograde skarn minerals garnet and diopside were formed in the pre-ore stage. The “stratiform-like” ores, with minor quartz-polymetallic veins, were formed in the main mineralization stage. The post-ore stage is characterized by quartz-calcite ± pyrite veins. Fluid inclusions in quartz from the pre-ore skarn, the main stage of mineralization, and post-ore quartz–calcite ± pyrite veins were studied. Fluid inclusion petrography shows that two-phase liquid-rich (Type I), two-phase vapor-rich (Type II), and halite-bearing (Type III) fluid inclusions can be identified in the studied hydrothermal quartz samples. Primary Type II and Type III fluid inclusions only occur in quartz from the the main stage, whereas Type I fluid inclusions are present in all three stages of hydrothermal quartz.Type I fluid inclusions in pre-ore stage have homogenization temperatures of 290-368°C and salinities of 2.6-8.8 wt.% NaCl equiv. Type II and coexisting Type III fluid inclusions in the main stage share similar homogenization temperatures of 293 to 334 °C and 290 to 326 °C,but have two contrasting salinity ranges of 1.2 to 2.2 wt.% and 31.87 to 38.16 wt.% NaCl equiv, respectively. The coexistence of Type II and Type III fluid inclusions and their similar homogenization temperatures but contrasting salinities suggest that fluid boiling processes occurred. Type I fluid inclusions in the post-ore quartz–calcite veins have homogenization temperatures of 202-278 °C and salinities of 0.2-6.5 wt.% NaCl equiv. The hydrogen and oxygen isotopes (δD ranges from -78 ‰ to -61 ‰, δ18OH2O calculated from quartz are from 8.1 ‰ to 10.6 ‰) of fluid inclusions in quartz imply that ore fluids were principally derived from magmatic water. The lead isotopes of sulfide from the ores are close to the orogenic Pb evolution curve and are also similar to those of the Jiande Jurassic igneous rocks, suggesting that the metals were sourced mostly from the Jurassic igneous rocks. The skarn alteration, inferred boiling fluid inclusion assemblages, stable isotopic features of magmatic fluids, and metal source imply that the main stages of mineralization in the Jiande polymetallic copper deposit is of skarn-type, related to Jurassic granodiorites.