Tin mineralization in the giant Shuangjianzishan Ag-Pb-Zn deposit, Inner Mongolia

Abstract

The newly discovered Shuangjianzishan giant Ag-Pb-Zn deposit which located in the Southern segment of the Great Xing’an Range (SGXR), has controlled more than 18 000 t reserves of Ag, 1.9 Mt reserves of Zn, and 3.3 Mt reserves of Pb. Previous research has shown that Shuangjianzishan is a typical epithermal Ag-Pb-Zn deposit, however, mid- to high-temperature Cu-Sn (-W) mineralization are few studies. In this paper, a detailed mineralogical study reveals that a remarkable Cu-Sn (-W) mineralization is observed both in the upper tunnel and the deeper drill core samples. In the shallow ore-bodies, tin dominantly occurs as canfieldite in Pb-Zn-rich ores, minor as cassiterite-quartz veinlets. In the deep drill cores, tin occurs as cassiterite in the Cu-rich ores with the assemblage of quartz-arsenopyrite-chalcopyrite-stannite-(wolframite). Four primary paragenetic stages have been recognized. Stage I: cassiterite-quartz-arsenopyrite-chalcopyrite, including two sub-stage: cassiterite-arsenopyrite-pyrrhotite-quartz (stage I1) and sphalerite-chalcopyrite-galena-pyrite (stage I2); Stage II: galena-sphalerite-silver minerals; Stage III: Quartz-silver minerals, and stage IV: pyrite-carbonate.Electron probe micro-analysis (EPMA) show that the early mineralization (stage I) is characterized by Fe-rich sphalerite, Co-rich pyrite, Se-rich canfieldite, and Se-Bi-rich galena, while the later mineralization (stage II-IV) contains Fe-depleted sphalerite, As-rich pyrite, and Sb-bearing silver minerals. Laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis show that cassiterite has high contents of W, Fe, Sb, and Ti and depleted in Nb and Ta. Mineral associations and compositions implies a cassiterite-sulfide type tin mineralization in the Shuangjianzishan Ag-Pb-Zn deposit.A concealed granite porphyry intrusion was exposed in deep drill cores in the northwest part of the ore-field, with zircon U-Pb ages of (137.8 ± 1.1) Ma and (136.8 ± 1.0) Ma for fine- and coarse-grains granite, respectively. Bulk analysis shows that concealed granites are characterized by a highly fractionated, relatively reduced, and high K-calc-alkaline to alkaline, which belongs to A-type granites, and the remnant of Sn-bearing titanite suggests that it has the potential to provide the source of tin. Zircon U-Pb age (138–130 Ma) is well coupled with the previous data (ore-forming age, 136–129 Ma) of Shuangjianzishan, suggesting a close temporal and genetic relationship between tin mineralization and emplacement of the granite. This contribution implies that the Shuangjianzishan deposit may exist a middle- to high-temperature Cu-Sn (-W) mineralization center rather than a simple low-temperature hydrothermal system, and shows there is significant Sn-Cu mineralization potential in the peripheral and root zone of ore district.