Ruizhong Hu

Mantle, crustal and atmospheric noble gases in ailaoshan gold deposits, Yunnan Province, China

Abstract Helium and argon isotopic and elemental compositions of fluids released by crushing pyrite grains are reported from the Ailaoshan Gold Province, China. The province is a series of gold deposits hosted on a 200-km segment of a major normal fault formed during Eocene extension. The deposits have been well characterised using conventional geochemical and microthermometric techniques, the results of which are consistent with a predominantly high temperature “magmatic” fluid present in the ore minerals. In contrast, analyses of noble gases from three gold deposits demonstrate the palaeofluids were a mixture between a mantle-derived, magmatic fluid and two different low temperature fluids similar to modern groundwaters. The specific isotopic and abundance characteristics of helium and argon allow unequivocal identification of both a mantle-derived component and a surface-derived component to these fluids. The magmatic fluid dominates deposits from the northern end of the fault. The second groundwater is present only in deposits in the south of the province. By assuming fluid inclusion homogenisation temperatures in coexisting quartz are representative of fluid temperature, 3He/heat ratios can be estimated. The 3He/heat ratios vary systematically along the fault, from ≈ 30 × 10−12 cm3 STP J−1 in the northern deposits to

Multiple Mesozoic mineralization events in South China—an introduction to the thematic issue

Mesozoic mineral deposits in South China include world-class deposits of W, Sn and Sb and those that provide the major sources of Ta, Cu, Hg, As, Tl, Pb, Zn, Au and Ag for the entire country. These deposits can be classified into polymetallic hydrothermal systems closely related to felsic intrusive rocks (Sn–W –Mo granites, Cu porphyries, polymetallic and Fe skarns, and polymetallic vein deposits) and low-temperature hydrothermal systems with no direct connection to igneous activities (MVT deposits, epithermal Au and Sb deposits). Recent studies have shown that they formed in the Triassic (Indosinian), Jurassic–Cretaceous (Early Yanshanian), and Cretaceous (Late Yanshanian) stages. Indosinian deposits include major MVT (Pb–Zn–Ag) deposits and granite-related W–Sn deposits. Early Yanshanian deposits are low-temperature Sb–Au and high-temperature W–Sn and Cu porphyry types. Many Late Yanshanian deposits are low-temperature Au–As–Sb–Hg and U deposits, and also include high-temperature W–Sn polymetallic deposits. The formation of these deposits is linked with a specific tectonothermal evolution and igneous activities. This special issue brings together some of the latest information in eight papers that deal with the origins and tectonic environments of mineral deposits formed in these stages. We anticipate that this issue will stimulate more interests in these ore deposits in South China.

Helium and Argon isotope systematics in fluid inclusions of Machangqing copper deposit in west Yunnan province, China

Abstract This paper reports the helium and argon isotope compositions of fluid inclusions hosted in pyrites from Machangqing alkali–porphyry copper deposit in west Yunnan province, China. There is an excellent positive correlation between 3He/36Ar and 40Ar/36Ar, and between 40Ar*/4He and 3He/4He in the inclusion-trapped fluids. This indicates that the ore-forming fluid can be considered as a mixture of two endmember components, a modified mantle fluid and crustal fluid. From the noble gas results and combined noble gas and stable isotope behavior, the authors conclude that the former is a high-temperature magmatic fluid differentiated from Machangqing alkali–porphyry and enriched in volatiles such as sulfur and carbon. The crustal fluid is likely a low-temperature meteoric fluid enriched in crustal radiogenic helium but with atmospheric argon isotopic composition. The sulfur and carbon concentrations in the low temperature fluid are probably too low to produce a distinct C or S isotope signature: the mixing between high and low temperature fluids can only be traced using noble gases. In addition, the positive relationship between 3He/4He and δ34S reflects the evolutionary physicochemical conditions of the mixed fluid.

Samarium–neodymium isotope systematics of hydrothermal calcites from the Xikuangshan antimony deposit (Hunan, China): the potential of calcite as a geochronometer

Abstract The Xikuangshan antimony mine located at Central Hunan, China, is the largest antimony deposit in the world. Hydrothermal calcites from the Xikuangshan Mine display relatively large variations in REE abundance and Sm/Nd ratios and are characterized by MREE- and HREE-enriched and LREE-depleted patterns. Sm–Nd isotopic compositions of syn-sulfide calcites fall on well-defined lines in the isochron plot. Calcites from early- and late-stage mineralization form two distinct isochrons corresponding to 155.5±1.1 and 124.1±3.7 Ma, respectively. The isochron ages suggest that Sb mineralization took place during the Late Jurassic–Early Cretaceous Period in the Xikuangshan district, consistent with previous geological observations and with independent isotope data. Samarium–neodymium dating of calcites shows potential for determining ages of hydrothermal mineralization.

Petrogenesis of the Pt–Pd mineralized Jinbaoshan ultramafic intrusion in the Permian Emeishan Large Igneous Province, SW China

The Jinbaoshan ultramafic intrusion is a sheet-like body with a thick wehrlite unit in the center and thin pyroxenite units at the margins. PGE are enriched in several disseminated sulfide zones in the intrusion. Olivine from the intrusion has low Fo and depleted Ni contents compared to olivine from coeval Emeishan picrites. Whole rock major and trace element concentrations suggest that the Jinbaoshan wehrlites originally contained <30% trapped liquid. The total amount of sulfide in the rocks exceeds that which could have been dissolved in the trapped liquid. The Jinbaoshan wehrlites are interpreted to represent residual assemblages formed by dissolution of plagioclase by passing magma. No clear evidence of crustal contamination is indicated by S, Nd and Os isotopes. We envision that sulfide saturation occurred at depth due to olivine and chromite crystallization. Immiscible sulfide droplets were transported to the Jinbaoshan conduit where they accumulated and reacted with magma successively passing through the conduit to achieve high PGE concentrations.

Cenozoic high Sr/Y volcanic rocks in the Qiangtang terrane, northern Tibet: geochemical and isotopic evidence for the origin of delaminated lower continental melts

Geochemical and Sr–Nd–Pb isotopic data are presented for volcanic rocks from Zougouyouchaco (30.5 Ma) and Dogai Coring (39.7 Ma) of the southern and middle Qiangtang block in northern Tibet. The volcanic rocks are high-K calc-alkaline trachyandesites and dacites, with SiO 2 contents ranging from 58.5 to 67.1 wt % The rocks are enriched in light REE (LREE) and contain high Sr (649 to 986 ppm) and relatively low Yb (0.8 to 1.2 ppm) and Y (9.5 to 16.6 ppm) contents, resulting in high La/Yb (29–58) and Sr/Y (43–92) ratios, as well as relatively high MgO contents and Mg no., similar to the compositions of adakites formed by slab melting in subduction zones. However, the adakitic rocks in the Qiangtang block are characterized by relatively low e Nd (t) values (−3.8 to −5.0) and highly radiogenic Sr (( 87 Sr/ 86 Sr) i = 0.706–0.708), which are inconsistent with an origin by slab melting. The geochemistry and tectonics indicate that the adakitic volcanic rocks were most likely derived from partial melting of delaminated lower continental crust. As the pristine adakitic melts rose, they interacted with the surrounding mantle peridotite, elevating their MgO values and Mg numbers.

Continental hydrothermal sedimentary siliceous rock and genesis of superlarge germanium (Ge) deposit hosted in coal: A study from the Lincang Ge deposit, Yunnan, China

There are abundant hydrothermal sedimentary structures and plant fragment fossils in the siliceous rocks from the Lincang Ge deposit. The major element compositions of these siliceous rocks are characterized by high content SiO2, low TiO2 and Al2O3 concentrations, and low Al/(Al+Fe+Mn) ratios (0.010 on average). The siliceous rocks are distinctly enriched in Ge, Sb, As, W, and secondly enriched in Cs, U, Mo and Tl. Their total REE content are generally less than 1μg/g, LREE relatively concentrated, and the values of Eu anomaly and Ce anomaly vary from 0.452 to 5.141 and 0.997 to 1.174, respectively. Their NAS-normalized REE patterns are plain or left-inclined. The Oxygen isotope compositions of these siliceous rocks are similar to those of the hydrothermal siliceous sinter. The above characteristics, as well as the geological setting of the deposit, indicate these siliceous rocks formed in continental hydrothermal environment. As the interlayer or cliff of the Ge-rich coal seams, siliceous rocks tightly contacted with ore-body, and the contents of Ge in siliceous rocks vary from 5.6 to 360 μg/g (78 μg/g on average). The Ge content increased in coal which close to the siliceous rocks. With the increase of Ge content, the typical trace element ratios (i.e., Ge/Ga, Nb/Ta and U/Th) and REE patterns of Ge-rich coal are more close to those of the siliceous rocks. The Ge concentrated in coal seams of the Lincang Ge deposit might be transported by the hydrothermal water, which demonstrated by the siliceous rocks, during the coal-forming processes.

Helium and argon isotopic geochemistry of Jinding superlarge Pb-Zn deposit

The study results of He and Ar isotopes from fluid inclusions in pyrites formed during mineralization stage of Jinding superlarge Pb-Zn deposit in west Yunnan, China are reported. The data show that the40Ar/36Ar and3He/4He ratios of fluid inclusions are respectively in the range of 301. 7–385. 7 and 0. 03–0.06Ra, suggesting the oreforming fluid is a kind of air saturated meteoric groundwater. On the basis of research on coupled relationships among He, Ar, S and Pb isotopes, the evolution history of ore-forming fluid of the deposit can be summarized as (i) air saturated meteogenic groundwater infiltrated down and was heated→ (ii) leached S, C and radiogenic He, Ar from the basinal strata → (iii) leached Pb and Zn from mantle-derived igneous rocks located in the bottom of the basin→ (iv) ore-forming fluid ascended and formed the deposit. Due to this process, the isotope signatures of crustal radiogenic He, atmospheric Ar (with partial radiogenic40Ar), crustal S and mantle-derived Pb remained in the ore-forming fluid.

Elemental and Sr–Nd–Pb isotopic geochemistry of Mesozoic mafic intrusions in southern Fujian Province, SE China: implications for lithospheric mantle evolution

Cretaceous mafic dykes in Fujian province, SE China provide an opportunity to examine the nature of their mantle source and the secular evolution of the Mesozoic lithospheric mantle beneath SE China. The mafic rocks have SiO 2 ranging from 47.42 to 55.40 wt %, Al 2 O 3 from 14.0 wt % to 20.4 wt %, CaO from 4.09 to 11.7 wt % and total alkaline (K 2 O+Na 2 O) from 2.15 wt % to 6.59 wt %. Two types are recognized based on their REE and primitive mantle-normalized trace element patterns. Type-A is the dominant Mesozoic mafic rock type in SE China and is characterized by enrichment of light rare earth elements (LREE) ((La/Yb)n = 2.85–19.0) and arc-like trace element geochemistry. Type-P has relatively flat REE patterns ((La/Yb)n = 1.68–3.43) and primitive mantle-like trace element patterns except for enrichment of Rb, Ba and Pb. Type-A samples show EMII signatures on the Sr-Nd isotopic diagram, whereas type-P rocks have high initial 143 Nd/ 144 Nd ratios (0.5126–0.5128) relative to the type-A rocks ( 143 Nd/ 144 Nd = 0.5124–0.5127). The type-A rocks have 207 Pb/ 204 Pb ranging from 15.47 to 15.67 and 206 Pb/ 204 Pb from 18.26 to 18.52. All the type-A rocks show a negative correlation between 143 Nd/ 144 Nd and 206 Pb/ 204 Pb ratios and a positive relationship between 87 Sr/ 86 Sr and 206 Pb/ 204 Pb ratios, indicating mixing of a depleted mantle source and an EMII component. Geochemical modelling shows that the parental magmas were formed by 5–15 % partial melting of a spinel lherzolite, and contaminated by less than 1 % melt derived from subducted sediment. The type-P magmas were derived from a mantle source unmodified by subduction components. The wide distribution of type-A dykes in SE China suggests that subduction-modified lithospheric mantle was extensive beneath the Cathaysia Block. Geochemical differences between Mesozoic and Cenozoic mafic rocks indicate that lithospheric thinning beneath SE China occurred in two episodes: firstly by heterogeneous modification by subducted components in early Mesozoic times, and later by chemical–mechanical erosion related to convective upwelling of the asthenosphere during Cenozoic times.

Review of the Link between the Hongge Layered Intrusion and Emeishan Flood Basalts, Southwest China

Generation of the Late Permian Emeishan large igneous province has recently been attributed to the ascent of a mantle plume head. The Hongge layered intrusion, hosting a giant Fe-Ti-V deposit and Ni-Cu-PGE mineralization, is contemporaneous and chemically correlated with high-Ti type Emeishan flood basalts in the Pan-Xi area, southwestern China. Basalts within the lower part of the lava sequence (HT3) were strongly contaminated by material mainly from the middle to upper crust, whereas overlying basalts (HT2) were contaminated by a gabbroic layer near the crust-mantle boundary. In contrast, the magma parental to the Hongge intrusion assimilated appreciable amounts of a plagioclase-rich lower-crustal end-member. The Hongge intrusion may have acted as an open-system conduit through which the lavas erupted. Equilibration of Fe-Ti oxides and immiscible sulfide liquids with successive batches of magma produced the giant Fe-Ti-V deposit, and a gradual increase in the Ni, Cu, and PGE contents of later differentiates.