Two international zircon standards and one laboratory zircon standard were systematically measured using a Thermo Finngan Neptune MC-ICP-MS system and a New Wave UP213nm laser ablation system. The obtained Hf-176/Hf-177 ratios are 0.282700 +/- 64 (2SD, N = 22) for TEMORA, 0.282008 +/- 25 (2SD, N = 26) for GJ1 and 0.282967 +/- 44 (2SD, N = 27) for FM02. The results are in good agreement with the previously reported data within errors. Comparative measurements of the FM02 zircon standard using different ablation diameter show that the results are consistent within errors. Zircons GJ1 and FM02 show narrower variations in Hf-176/Hf-177 ratios than those of zircons TEMORA and thus are ideal standards for the Hf isotope analysis. The origin and material source of zircons from four different areas are also discussed based on the analysis of the Hf isotope compositions of zircons.
Picritic lava flows near Lijiang in the late Permian Emeishan flood basalt province are associated with augite-phyric basalt, aphyric basalt, and basaltic pyroclastic units. The dominant phenocryst in the picritic flows is Mg-rich olivine (up to 91.6% forsterite component) with high CaO contents (to 0.42 wt %) and glass inclusions, indicating that the olivine crystallized from a melt. Associated chromite has a high Cr-number (73-75). The estimated MgO content of the primitive picritic liquids is about 22 wt %, and initial melt temperature may have been as high as 1630-1690 degrees C. The basaltic lavas appear to be related to the picritic ones principally by olivine and clinopyroxene fractionation. Age-corrected Nd-Sr-Pb isotope ratios of the picritic and basaltic lavas are indistinguishable and cover a relatively small range [e.g. epsilon(Nd)(t) = -1.3 to +4.0]. The higher epsilon(Nd)(t) lavas are isotopically similar to those of several modern oceanic hotspots, and have ocean-island-like patterns of alteration-resistant incompatible elements. Heavy rare earth element characteristics indicate an important role for garnet during melting and that the lavas were formed by fairly small degrees of partial melting. Rough correlations of isotope ratios with ratios of alteration-resistant highly incompatible elements (e.g. Nb/La) suggest modest amounts of contamination involving continental material or a relatively low-epsilon(Nd) component in the source. Overall, our results are consistent with other evidence suggesting some type of plume-head origin for the Emeishan province.
This paper, using gold deposits as example, attempts to setup a scientific linkage between ore geology and fluid inclusions, considering that in previous published works, observations and measurements of the fluid inclusions commonly were not well interpreted. In some cases, geological data did not agree with the results obtained from fluid inclusion studies. In this paper, we first review previous classifications of gold deposits, and then, subdivide gold deposits into five classes, based on the dominant ore-forming processes: 1) intrusion -related hypothermal systems, such as porphyry-systems, breccia-pipes, IOCG and skarns; 2) orogenic-or metamorphic hydrothermal type; 3) epithermal-type, i. e. reworking hydrothermal deposits hosted in continental-facies volcanic-subvolcanic rocks; 4) fine-grain disseminated type ( Carlin-type and/or Carlin-style), i. e. reworking hydrothermal deposits hosted sediments; and 5) hydrothermal metalliferous sediments related to submarine venting, such as VMS and SEDEX styles. In this work we select diagnostic geological and fluid-inclusion characteristics of these five classes of ore-systems, and clarify their key differences that can be used as genetic markers. Ore-fluids are classified into three end-members, namely reworking, metamorphic and magmatic fluids. Many ore-systems are known to form as a result of multiple fluids during multi-stage events; and their late-stage of mineralization always being caused by fluids with a high-proportion of reworking of the original ore systems or by renewed fluid flow. Therefore, the features of late-stage fluids, alteration and mineralization cannot be used to identify the origin and genetic type of an oresystem. Instead, we suggest that only the early-stage signatures can be employed to determine the origin and type of an ore-system. Reworking fluids are characterized by low-temperature ( < 300 degrees C), low-salinity and low-content of CO2, and sourced from meteoric and/or sea water; metamorphic fluids by moderate -temperature, low-salinity and high-content of CO2; and magmatic fluids by high-temperature, high-salinity and high-content of CO. Magmatic hydrothermal ore-systems contain multi-daughter-crystal-bearing and high-salinity, CO2-rich fluid inclusions; metamorphic ore-systems contain low-salinity, CO2-rich fluid inclusions; and the reworking hydrothermal ore-systems contain neither daughter-crystal-bearing nor CO2-rich/bearing fluid inclusions, but are populated by aqueous water-solution fluid inclusions. Finally, we discuss the tectonic settings of the ore-systems of the various classes. For examples, the orogenic-type formed during processes of crustal compression, orogenesis, metamorphism and uplift; submarine metalliferous sediments developed in the setting of rift basins; Paleozoic or earlier epithermal-type ore-systems can be preserved in accretionary orogens. It is suggested that the ore-systems and their fluid inclusions can be used as an ideal probe to trace geodynamic evolution of continents.
The Nanling region, located in the central part of South China, is the most important W-Sn metallogenic province in China and even over the world. Based on the latest highly precise radiometric dating of the ores and related granites and new progress in the study of geodynamic processes we propose that large-scale W-Sn mineralization occurred in the Nanling region in the Mid-Late Jurassic (165 to 150 Ma) and that its corresponding tectonic setting could be a back-are region of the continental margin. When the Paleo-Pacific plate was subducted beneath the Eurasian continent, a number of paralleling NE-trending extensional belts developed in a back-arc region where the mantle-crust interaction was strong. The intersection sites between these extensional belts and long-lived previous E-W-trending faults were the central areas of magmatism and mineralization, and the largest W-Sn-Mo-Bi-Be mineralization clusters in southern Hunan is located in the intersection site between the Shihang NE-trending fault belt and the E-W-trending deep fault in the middle ridge zone of the Nanling region. In the Mid-Late Cretaceous, because the Paleo-Pacific plate or Izalagi plate strike-shift to NNE-direction the Eurasian continent started to extension. In this geodynamic setting, the large-scale volcanic eruption mainly took place east of the Wuyi Mountains whereas range-basin tectonics appeared extensively in South China including the Nanling region, NNE-trending Cretaceous red bed-volcanic basins alternate with uplifts represented by granite, and some granite stocks emplaced at 130 similar to 90 Ma and their related tin mineralization can be found in some basins. For instance, the Yanbei and Taoxiba in Jinagxi province, Yinyan in Guangdong province and the Jiepailing in the Hunan province occurred in this period as porphyry or porphyry-skarn ores.