Genesis of the Taishanmiao Au Deposit in the Ningshan-Zhenan Ore Field,South Qinling Orogen,China:Evidence from Geochemical,Geochronological and S-Pb-H-O Isotopical Study

The Taishanmiao Au deposit is in the western part of the Ningshan-Zhenan ore field,in the South Qinling orogen.Based on geological and geochemical features,we propose that the Taishanmiao Au deposit is a magmatichydrothermal type of deposit.All samples have high SiO_(2),K_(2)O+Na_(2)O contents and differentiation index values,low CaO,MgO,P_(2)O_(5),and TiO_(2)contents,are enriched in high field-strength elements,and depleted in large ion lithophile element.The stable isotopeδ^(34)S values of pyrite vary from 6.8%-7.8%,and the H-O isotopic compositions of quartz from quartzpyrite veins indicate the ore-forming fluid is a mixture of a small amount of magmatic-hydrothermal solution and groundwater.Lead isotope ratios of pyrite and silicalite can show that the ore-forming materials were derived from a mixed source containing mantle and crustal materials.At the same time,the LA-ICP-MS U-Pb dating of monzogranite is 198.4±4.2 Ma.Combined with the regional geological background,the intracontinental extension in the late collisional orogeny and large-scale lithospheric thinning associated with mantle uplift may lead to large-scale mineralisation in the region.

Discovery of late Paleozoic fossils in tectonic blocks of the Dabao Formation in the South Qinling Orogenic Belt and their significance

The Dabao Formation in the South Qinling Orogenic Belt was previously regarded as Ordovician in age and consists of clastic matrix and blocks of siltstone,limestone,chert,and volcanic rocks.However,some Middle Devonian corals,conodonts,and other fossil fragments within the limestone blocks were discovered in recent field investigations,indicating that the Dabao Formation was formed during late Paleozoic.Combined with other regional geological data,the Dabao Formation in the Southern Qinling Orogenic Belt is considered to be a late Paleozoic or early Mesozoic accretionary complex.

Mesozoic uplift of the Dabashan and Micangshan-Hannan Dome in the South Qinling orogenic belt

The topographic evolution of continental orogens is important for understanding continental orogenic processes,geodynamic mechanisms,and climatic and environmental changes.The Qinling Orogen is a major orogenic belt in China,and its uplift history can provide insights into the tectonic configuration and geodynamics of China and East Asia.Previous studies have shown that the Dabashan and Micangshan-Hannan Dome(MHD)in the South Qinling orogenic belt were uplifted during the Mesozoic.However,the magnitude of the uplift remains unclear.In this study,using sedimentary records in the northern Sichuan Basin and lithospheric flexural modeling,we estimated the magnitude of Mesozoic uplift of the Dabashan and MHD,along with the effective elastic thickness(Te)of the Sichuan Basin.The Dabashan and MHD were uplifted by approximately 1220 and 880 m during the Middle Jurassic and Early Cretaceous,respectively.Therefore,we propose that the present-day elevation of the Dabashan and MHD is primarily the result of Mesozoic uplift.The differences in the duration and amount of uplift between different tectonic units indicate that the uplift processes and driving mechanisms in the South Qinling orogenic belt were different in the Mesozoic and Cenozoic.Mesozoic uplift was the result of convergence of the North China and South China blocks advanced from east to west,whereas Cenozoic uplift was driven by ongoing indentation of the Indian Plate into Eurasia from southwest to northeast.The lithospheric strength of the northern Sichuan Basin was weakened from the Middle Jurassic to Early Cretaceous,and Tedecreased from 73 to 57 km.This may have been caused by the flexure-related bending stresses in the lithosphere that developed due to the large topographic loading.

Rock Assemblages and Formation Ages of the Baishuijiang Group in the Southwest Qinling Orogenic Belt, Northwest China

The Baishuijiang Group, located in the southwest Qinling orogenic belt, is divided into three belts according to the characteristic of the matrix and rock blocks based on the large scale geological mapping. The north belt and south belt are composed of abyssal mudstone and siltstone, and limestone, chert and basic and ultrabasic rock blocks. The middle belt consists of a few limestone blocks and turbidites, which were formed in the trench environment. At present, in the Baishuijiang Group, many fossils were found in matrix and rock blocks, the fossils contain the Cambrian small shell fossils(Xiao, 1992;Tao et al., 1992), Silurian chitinozoas, scolecodonts and spores, and Ordovician graptolites, and middle Devonian Coral and conodonts in limestone and chert blocks(Wang et al., 2011a), and Permian radiolarians in the matrix(Wang et al., 2007). The volcanic rock blocks have undergone different degree of metamorphism. Their geochemical characteristics indicate that the rocks are similar to oceanic island arc and seamount(Wang et al., 2009), and SHRIMP U-Pb dating yielded ages from Neoproterozoic to early Paleozoic(Yan et al., 2007;Wang et al., 2009, 2011b). Therefore, comprehensive analysis of regional data, the Baishuijiang group is an accretionary complex which was consisted of matrix and blocks, and was finally formed during Permian-Triassic.