An Early Cretaceous gold mineralization event in the Triassic West Qinling orogen revealed from U-Pb titanite dating of the Ma'anqiao gold deposit

The West Qinling orogen in central China, formed from continental collision between the North China and Yangtze cratons in the Late Triassic, hosts numerous gold deposits with a total Au endowment of about 2000 t. Most deposits were emplaced at ca. 250-195 Ma and are genetically associated with the Triassic orogenesis. Here in situ U-Pb titanite dating with laser ablation inductively coupled plasma mass spectrometry indicates the Ma'anqiao gold deposit in the northern portion of this orogen has a distinctive age and under a contrasting tectonic regime. This structurally controlled gold deposit is hosted in Late Ordovician to Early Silurian sub-greenschist facies metasedimentary rocks. The gold mineralization is hosted in quartz-pyritepyrrhotite veins and pyrite-pyrrhotite disseminations in hydrothermally altered rocks, which are crosscut by K-feldspar-calcitechlorite +/- pyrite veins. Titanite, present both in the disseminated sulfide ores and later veins, was used for in situ U-Pb dating. Titanite from three disseminated sulfide ore samples with Th and U averaging 27.46 and 39.31 ppm (1 ppm=1 mu g g(-1)), respectively, yielded lower-intercept ages of 121.1 +/- 3.1 to 120.7 +/- 3.5 Ma (2 sigma) in the Tera-Wasserburg diagram. Titanite from three later vein samples with much lower Th and U concentrations averaging 2.74 and 16.21 ppm, respectively, yielded overlapping ages of 120.8 +/- 3.2 to 120.3 +/- 5.8 Ma (2 sigma). These new titanite U-Pb ages tightly constrain the formation of the Ma'anqiao gold deposit at ca. 121-120 Ma and, when combined with independent geological data, indicate it is not related to the Triassic Qinling orogeny. Rather, its formation is attributed to lithospheric thinning and destruction of the North China craton during the Late Jurassic to Early Cretaceous which has generated numerous gold deposits along the southern margin of this craton. This catastrophic event caused extensive magmatism in large areas of the North Qinling terrane and northern edge of the West Qinling orogen immediately to the south of the North China craton. The heat flux and elevated geothermal gradients associated with this magmatism could have induced prograde metamorphism of the Paleozoic sedimentary infrastructure in and beneath these areas with the derived fluids ascending along pre-existing crustal-scale regional structures to form the Ma'anqiao gold deposit. We suggest that areas in the North Qinling terrane that have been affected by the Late Mesozoic magmatism are potential targets for future exploration of the decratonization-related gold deposits. Additionally, this study highlights the use of titanite U-Pb dating as a robust geochronometer for metasedimentary rock-hosted gold deposits in Phanerozoic orogens, which has previously not been utilized.

Automated summary

The West Qinling orogen in central China, formed from continental collision between the North China and Yangtze cratons in the Late Triassic, hosts numerous gold deposits. In this study, in situ U-Pb titanite dating was used to determine the age of the Ma'anqiao gold deposit in the northern portion of the orogen. The results show that the deposit was formed at ca. 121-120 Ma, indicating that it is not related to the Triassic orogeny but rather to the thinning and destruction of the North China craton during the Late Jurassic to Early Cretaceous. The study also highlights the potential for future exploration of similar gold deposits in areas affected by Late Mesozoic magmatism.