Zircon geochronological and geochemical study of the Baogaigou Tin deposits, southern Great Xing'an Range, Northeast China: Implications for the timing of mineralization and ore genesis

摘要

Tin deposits in China mainly occur in South China, while extensive Sn mineralization also occurred in the southern Great Xing'an Range (SGXR) of Northeast China. The Baogaigou deposit is a small typical greisen-type Sn deposit in the SGXR, yielding 1,136 t of Sn reserves with an average grade of 0.48% Sn. New whole-rock geochemical, zircon geochronological, whole-rock Sr-Nd isotope and zircon in situ Hf-O isotopic data are presented for granites from Baogaigou deposit. Two granite samples from the Baogaigou deposit yield identical secondary ion mass spectrometry zircon U-Pb ages of 145.6 +/- 0.8 Ma and laser ablation inductively coupled plasma mass spectrometry ages of 145.6 +/- 0.4 Ma, consistent with regional magma activity relating to Sn mineralization ages and prior to regional Sn mineralization ages (142-135 Ma) in the SGXR. The granites are strongly peraluminous, calc-alkaline granites enriched in light rare earth elements (LREEs), with fractionation of light over heavy REEs (LREE/HREE = 10.1-7.6) and strong negative Eu anomaly with Eu/Eu* values of 0.03-0.04. Primitive-mantle-normalized granitoid compositions display negative Ba, Nb, Ta, Pb, Sr, P, Eu and Ti anomalies, and positive Th, U and Nd anomalies. The granites have initial Sr-87/Sr-86 ratios, epsilon(Nd)(t) values, and T-DM(Nd) ages of 0.70532-0.70325, +2.5 to +2.1, and 783-743 Ma, respectively, with zircon epsilon(Hf)(t) values of 5.1-1.6, T-DM2 model ages of 1,086-880 Ma, and delta O-18 values of 5.70-4.91 parts per thousand. Geochemical and Sr-Nd-Hf-O isotopic data for the Baogaigou granites indicate they are A(2)-type peraluminous granites and were probably formed through partial melting of juvenile lower crust originating from a depleted mantle. The Baogaigou granite with Sn content (5.7-13.7 ppm; average = 9.6 ppm) contributed most of the ore-forming materials, while the highly fractionated and low magma fO(2) (Ce4+/Ce3+ and Eu/Eu* range from 1.0 to 49.3 and 0.03 to 0.04) nature of the Baogaigou granites would have facilitated development of Sn mineralization. Under extensional environment, the asthenosphere upwelling would result in large-scale transport of magma and heat from the mantle, and then extensive re-melting of the crust, generating magma and fluid to form a tectonic-magmatic-fluid metallogenic system.