A comprehensive contrast of ore-forming geological background and ore-forming fluid features, especially fluid ore-forming processes, has been performed between the Tianmashan and the Datuanshan ore deposits in Tongling, Anhui Province. The major reasons for the formation of the stratabound skarn Au-S ore deposit in Tianmashan and the stratabound skarn Cu ore deposit in Datuanshan are analyzed in accordance with this contrast. The magmatic pluton in Tianmashan is rich in Au and poor in Cu, but that in Datuanshan is rich in Cu and Au. The wallrock strata in Tianmashan contain Au-bearing pyrite layers with some organic substance but those in Datuanshan contain no such layers. Moreover, the ore-forming fluids in Tianmashan are dominantly magmatic ones at the oxide and sulfide stages, but those with high content of Cu in Datuanshan are mainly groundwater fluids. In addition, differences in compositional evolution and physicochemical condition variation of the ore-forming fluids result in gradual dispersion of Cu or Au in the late stage of the fluid ore-forming process. This is also an important metallogenic factor for the stratabound skarn Au-S ore deposit in Tianmashan and stratabound skarn Cu ore deposit in Datuanshan.
Mafic xenoliths from Paleozoic Fuxian kimberlites (Liaoning Province) mainly in- clude garnet granulite, with minor pyroxene amphibolite, meta-gabbro and pyroxene syenite. The SiO2 contents of the xenoliths are from 47.3 wt% to 49.9 wt%. The granulites are mainly coarse-medium grained and reasonably with well-developed granoblastic (mostly polygonal) texture, contain the assemblage of garnet + plagioclase + pyroxene + K-feldspar ±phlogopite. The mineral assemblage of the amphibolites is plagioclase + pyroxene + amphibole ±K-feldspar, with the equilibrium temperature and pressure conditions of 744—821℃ and 0.76—0.88 GPa. The granulites are regarded as derivation from the lower crust depth (more than 29 km), below the amphibolites. Garnet granulites compositionally correspond to sub-alkalic basalt, and have wide ranges of Ni abundance (133—840 ppm), and Nb/Y (0.12—1.85), Nb/U (3.51—53.86) and Ta/U (0.38—2.48). The amphibolite and the syenite correspond to alkalic basalt. The Fuxian ma- fic xenoliths are regarded as the metamorphic product of the underplated magmas (including fractional crystallization or not) experienced the contamination with the pre-existed crustal com- ponents, and partly effected by kimberlitic magmas. The concordant ages of zircons for the gab- bro (2610—2580 Ma) and the near-concordant upper intercept ages of zircons for the garnet granulite and pyroxene amphibolite (2578—2538 Ma) indicate that they are currently known as the oldest deep-seated xenoliths from the lower crust of the North China Craton. These ages recorded the formation of the united Eastern Block of the North China Craton, That is, Neoar- chean (2.6—2.5 Ga) is an important continental crustal growth period of the North China Craton. The lower intercept age of the garnet granulite (1853 Ma) recorded an important tectonic thermal event in Paleoproterozoic. This event was probably related to collision of the East and West block, and resulted in the final assembly of the North China Craton around at 1.8 Ga.
ZHENG Jianping1,YU Chunmei1,LU Fengxiang1 & LI Huimin2 1.Faculty of Earth Sciences,China University of Geosciences,Wuhan 430074,China
A simple and precise method for direct determination of molar ratio of 206Pb/ 204Pb, 207Pb/ 204Pb and 208Pb/ 204Pb by ICP-MS was developed. The data acquirement parameters of MS for Quadrupole setting time(QST), Dwell time(DT), Integrated time and number of sweeps were investigated in detail. The relative standard deviation(RSD) of different Pb isotope ratios were less than 0.32%[n=5, ρ(Pb)=20 ng/mL]. The mass discrimination and dead time of detector was calibrated by using NIST981 solution. The mass bias factor(α) of 206Pb/ 204Pb, 207Pb/ 204Pb and 208Pb/ 204Pb were 0 002 5, 0 002 9 and 0 002 0, respectively. The dead time of detector was 7 ns. The proposed method has been applied to the analysis of geological standard reference materials containing AGV-1, BCR-2 and BHVO-2 .The relative error between this work and the recommend values or reported values was better than 0.73% and the relative standard deviation was better than 0.93%(n=5).
The early Paleozoic alkali dyke swarms in the Ziyang-Langao areas of south Qinling are characterized by bimodal volcanic association. They show the patterns of enriched LREE with high fractionation of light and heavy REE and enrichment of incompatible elements in the primitive man-tle-normalized spider-diagrams with peaks of Ba and Nb and valleys of Pb and Sr. The ratios of Ta/Hf are larger than 0.3 and those of Th/Ta less than 1.6, indicating that they were derived from enriched mantle sources closely related to plume. The occurrence of alkali dyke swarm demon-strates an extensional event of crust resulting from deep plume activity in the Qinling orogeny dur-ing early Paleozoic. The rifting extension could not continue due to the short activity of plume. Consequently, no new oceanic basin was formed.
