Sheng-Rong Li

Stable isotopes and noble gases in the Xishimen gold deposit, central North China Craton: metallogeny associated with lithospheric thinning and crust–mantle interaction

Quartz-vein type gold mineralization at Xishimen is a recently discovered gold deposit in the central North China Craton. More than 50 auriferous quartz veins occur in this region within a NNW–SSE-trending fault zone 4600 m in length and 3–10 m wide. Wall rocks are mainly Precambrian tonalite–trondhjemite–granodiorite (TTG) gneisses and associated supracrustals, modified by K-feldspathization and pyrite-phyllic hydrothermal alteration. Based on detailed field and petrographic studies, we identify five episodes of mineralization: pyrite-phyllic stage (I), coarse-grained pyrite-milky white quartz stage (II), fine-grained smoky grey quartz-pyrite stage (III), fine-grained smoky grey quartz-polymetallic sulphide stage (IV), and quartz-carbonate stage (V). We present results of δ34S analysis of sulphide minerals from the different stages which show tightly clustered values in the range of –1.0‰ to 2.1‰, close to those of mantle and meteorite sulphur. Lead isotopic ratios of pyrite from the early to main stages also show restricted ranges with 206Pb/204Pb of 16.289–17.286, 207Pb/204Pb of 15.217–15.453, 208Pb/204Pb of 37.012–38.232, implying lower crustal input. 3He/4He and 40Ar/36Ar ratios of fluid trapped in pyrite are 0.68 Ra to 1.20 Ra (where Ra is the 3He/4He ratio of air = 1.4 × 10−6) and 540.9–1065, respectively. 3He and 4Ar concentrations vary from 10.05 to 18.5 (10−7 cm3STP/g) and 6.15 to 17.4 (10−7cm3STP/g), respectively, with calculated mantle helium ranging from 8.47% to 14.96% (average 11.01%). δ18OQ and δ18DQ values of quartz range from 8.0‰ to 13.2‰ and –101.9‰ to –70.5‰, respectively, with calculated δ18OW values of the mineralizing fluid ranging from 1.11‰ to 5.72‰, suggesting the mixing of magmatic aqueous fluid with meteoric water during gold precipitation. We correlate the mixed crust–mantle signature of the ore-forming sources to magmatism and metallogeny associated with Mesozoic inhomogeneous lithosphere thinning in the central North China Craton.

Stable isotope geochemistry and Re–Os ages of the Yinan gold deposit, Shandong Province, northeastern China

The Yinan gold deposit in the Luxi area of Shandong Province in northeastern China is a skarn-type ore deposit. In this article, we present results from sulphur, lead, carbon–oxygen, and helium–argon isotope chemistry to characterize the ore genesis and source features. We also present rhenium–osmium ages from molybdenite to evaluate the timing of ore formation. The δ34S values of pyrite from the ore deposit range from 0.7‰ to 5.60‰ with a mean at 2.70‰, close to mantle and meteorite sulphur. Among Pb isotopes, 206Pb/204Pb values range from 18.375 to 18.436, 207Pb/204Pb values from 15.694 to 15.8, and 208Pb/204Pb values from 38.747 to 39.067. The δ13C values of calcite associated with the ores range from −0.2‰ to −0.5‰ and their δ18O values show variation from 9.4‰ to 12.6‰, suggesting a mixed fluid source. The 3He/4He and 40Ar/36Ar ratios of fluids trapped in pyrite are in the range of 0.27–1.11 Ra and 439.4–826, respectively, with calculated proportion of the mantle-derived He ranging from 3.25% to 14.03% and atmosphere argon ranging from 35.8% to 67.3%. The data suggest that the ore-forming fluids were derived from the crust and were mixed with a distinct contribution of mantle helium. The Re and Os values vary from 32 × 10−6 to 93.02 × 10−6 and from 0.01 × 10−9 to 0.34 × 10−9, respectively. The model ages of molybdenite range from 126.96 ± 1.82 Ma to 129.49 ± 2.04 Ma, with a weighted mean age of 128.08 ± 0.75 Ma and isochron age of 130.3 ± 3 Ma. These ages are close to the age of the associated quartz diorite porphyrite pluton, suggesting a close relationship between Cretaceous magmatism and metallogeny in NE China. A comparison of the Yinan gold deposit in the Luxi area with those of the Jiaodong area shows that the contrast in metallogenic features between the two are linked with the tectonic and geodynamic history.

Crystal phases of calcium carbonate within otoliths of Cyprinus carpio . L. from Miyun Reservoir and Baiyangdian Lake, China

Soil and un-husked maize samples were collected from 29 different locations belonging to three distinct ecological zones (Swat, Hazara and Peshawar) of Khyber Pakhtunkhwa, Pakistan. The samples were evaluated for the incidence of aflatoxigenic strains of Aspergillus flavus. The soil samples collected from Peshawar (100%) and Hazara (66%), and grain samples collected from Swat (64%) and Peshawar (55%) were severely infected with aflatoxigenic strains of A. flavus. The strains isolated from maize kernels of Manyar, Jalala (Swat zone), Palosi and Takkar (Peshawar zone) produced the highest amount of aflatoxin B1 (324 to 514 µg g -1 ) and B2 (23 to 486 µg g -1 ). Similarly, the strains isolated from soils of Bannu (Peshawar) and Huripur (Hazara) were prominent in B1 (662 to 1323 µg g -1 ) and B2 (145 to 826 µg g -1 ). Microbial analysis of the surface sterilized grains showed that the strains isolated from Jalala (Swat) and Palosi (Peshawar) samples were high in B1 (62 to 79 µg g -1 ) and B2 (21 to 36 µg g -1 ). It was concluded, therefore, that A. flavus from soil might contaminate maize crop, but not directly from the same field. The most probable contamination occurred through air borne spores. It is thus recommended that protection from air borne spore should be devised.