The early Tertiary Sanerlin uranium deposit is located near the southwestern margin of the Chaling- Yongxing pull-apart basin defined by the Chaling-Yongxing and Chenxian-Linwu sinistral strike-slip faults in southern China. The uranium ores are hosted in 15 breccia-veinbodies, which are separately located in the cores of three secondary anticlines of the Upper Permian Dangchong Formation. Individual breccia-vein bodies are composed of fragments of silicified shale and sandstone from the Dangchong Formation, and quartz veinlets as cements. These fragments, together with quartz veins, form a mosaic texture. Hydrothermal pitchblende is the only commercial uranium mineral, mainly occurring as disseminated grains within quartz veins or coating fragments. Other metallic minerals include molybdenite, pyrite, chalcopyrite, galena, sphalerite, and red microcrystalline hematite. Fluid inclusions in quartz veins have homogenization temperatures ranging from 150 to 280 C, and calculated salinity values between 5.6 and 13.4wt% NaCl equivalent. Stable isotope analyses show that the mineralizing fluid was characterized by d18O values of-2.2 to +2.6 & and dDH2O values of-134 to-110 &. These analytical data demonstrate that hydrothermal fluids were mainly derived from formation waters (brines) of the Chaling-Yongxing basin. Fluid overpressuring was caused by an abnormal geothermal gradient and impermeable shales inthe deposit area. The geometry, texture, and structure of the breccia-veinsystem, along with the fluid pressure estimates, suggest that hydraulic fracturing generated the mineralized breccia-vein system. Pitchblende and associated minerals were deposited when gaseous phases were released abruptly from the ore fluids due to the hydraulic fracturing.

One hundred and twenty-five mineral grains from 45 visually pure K-bearing Mn oxide (hollandite group) samples collected from weathering profiles in the Mt Tabor region of central Queensland, Australia, were analysed by the 40Ar/39Ar laser probe technique. These K-Mn oxides precipitated mainly through a process of cavity filling (direct precipitation from weathering solution), with botryoidal texture formed by micrometric mineral bands. Well-defined and reproducible plateau ages have been obtained for most samples, ranging from 27.2

Manganese oxides in association with paleoweathering may provide significant insights into the multiple factors affecting the formation and evolution of weathering profiles, such as temperature, precipitation, and biodiversity. Laser probe step-heating analysis of supergene hollandite and cryptomelane samples collected from central Queensland, Australia, yield well-defined plateaus and consistent isochron ages, confirming the feasibility dating very-fined supergene manganese oxides by 40Ar/39Ar technique. Two distinct structural sites hosting Ar isotopes can be identified in light of their degassing behaviors obtained by incremental heating analyses. The first site, releasing its gas fraction at the laser power 0.2 0.4W, yields primarily 40Aratm, 38Aratm, and 36Aratm (atmospheric Ar isotopes). The second sites yield predominantly 40Ar* (radiogenic 40Ar), 39ArK, and 38ArK (nucleogenic components), at 0.5 1.0W. There is no significant Ar gas released at the laser power higher than 1.0W, indicating the breakdown of the tunnel sites hosting the radiogenic and nucleogenic components. The excellent match between the degassing behaviors of 40Ar*, 39ArK, and 38ArK suggests that these isotopes occupy the same crystallographic sites and that 39ArK loss from the tunnel site by recoil during neutron irradiation and/or bake-out procedure preceding isotopic analysis does not occur. Present investigation supports that neither the overwhelming atmospheric 40Ar nor the very-fined nature of the supergene manganese oxides poses problems in extracting meaningful weathering geochronological information by analyzing supergene manganese oxides minerals.

Two alkaline syenite plutons, the Tieshan and Yangfang plutons, have recently been recognized within NE-trending fault zones in eastern Cathaysia, South China. The rocks are very enriched in K2O (6.28-9.39wt.%), rare earth elements (REE; particularly light REE) and large ion lithophile elements, but are relatively low in high field strength elements. Isotopically, they are characterized by high initial 87Sr/86Sr (0.7093 to 0.7123) and low eNd(t) values (5.64 to 10.63). The geochemical data suggest that the alkaline syenites most likely formed via fractional crystallization of enriched mantle-derived magmas. Sensitive High-Resolution Ion Microprobe zircon U-Pb dating indicates that these two intrusions have Late Permian (254F4Ma) and Early Triassic (242F4Ma) crystallization ages, respectively. Our data suggest that a tectonic regime dominated by transtension probably existed from at least the latest Permian into the Triassic and was responsible for the formation of the Tieshan and Yangfang alkaline syenites. When combined with previous paleomagnetic, structural, and sedimentology data, we suggest that the transtension along the NE-trending strike-slip fault zones was related to oblique subduction of the Pacific plate underneath South China.

The Jiaodong gold province is the largest gold repository in China. Both mineralization and granitoid hosts are spatially related to the crustal-scale Tan-Lu strike-slip fault system, which developed along the Mesozoic continental margin in eastern China. A series of 40Ar/39Ar laser incremental heating analyses of hydrothermal sericite/muscovite from three major gold deposits (Jiaojia, Xincheng, and Wangershan) and igneous biotite from the granodiorite hosts were performed to establish a possible temporal link between gold mineralization, magmatism, and movement along the Tan-Lu fault zone. Magmatic biotite crystals yield well-defined and concordant plateau ages between 124.5 0.4Ma and 124.± 0.4Ma (2σ), whereas sericite and muscovite samples (a total of 30 single separates) give reproducible plateau ages ranging from 121.± 0.4Ma to 119.2±0.2Ma (2σ). An integration of our 40Ar/39Ar results with age data from other major gold deposits in Jiaodong demonstrates that widespread gold mineralization occurred contemporaneously during a 2-3-m.yr. period. Most gold deposits show intimate spatial associations with abundant mafic to intermediate dikes. The mafic dikes have K-Ar ages of 123.5-119.6Ma, in excellent agreement with those of the gold deposits. These newly obtained 40Ar/39Ar ages, in combination with other independent geological and geochronological data on granodioritic intrusions (130-126Ma), volcanic rocks (1243.6-114.7Ma), and deformed rocks within strike-slip faults (132-120Ma) in Jiaodong or adjacent areas, also support the idea that gold mineralization postdated the granodioritic magmatism but was contemporaneous with mafic magmatism and volcanism, all controlled by the transtensional motion along the Tan-Lu fault in the Early Cretaceous.