The Fe-Zn Isotopic Characteristics and Fractionation Models: Implications for the Genesis of the Zhaxikang Sb-Pb-Zn-Ag Deposit in Southern Tibet

Abstract

The genesis of the Zhaxikang Sb-Pb-Zn-Ag deposit remains controversial. Three different geological environments have been proposed to model mineralization: a hot spring, a magmatic-hydrothermal fluid, and a sedimentary exhalative (SEDEX) overprinted by a hot spring. Here, we present the electron probe microanalysis (EPMA) and Fe-Zn isotopic data (microsampled) of four samples from the first pulse of mineralization that show annular textures to constrain ore genesis. The Zn/Cd ratios from the EPMA data of sphalerite range from 296 to 399 and overlap the range of exhalative systems. The δ56Fe values of Mn-Fe carbonate and δ66Zn values of sphalerite gradually decrease from early to late stages in three samples. A combination of the EPMA and isotopic data shows the Fe-Zn contents also have different correlations with δ66Zn values in sphalerite from these samples. Rayleigh distillation models this isotope and concentration data with the cause of fractionation related to vapour-liquid partitioning and mineral precipitation. In order to verify this Rayleigh distillation model, we combine our Fe-Zn isotopic data with those from previous studies to establish 12 Fe-Zn isotopic fractionation models. These fractionation models indicate the δ56Fei and δ66Zni values (initial Fe-Zn isotopic compositions) of the ore-forming system are in the range of -0.5‰​​ ~−1‰ and -0.28‰​ ~0‰, respectively. To conclude, the EPMA data, Fe-Zn isotopic characteristics, and fractionation models support the SEDEX model for the first pulse of mineralization.

The Sr–He–Ar isotopic and elemental evidence constraints on the ore genesis of the Zhaxikang Sb–Pb–Zn–Ag deposit in southern Tibet

“…The ores in Stage 2 exhibit banded (Figure d), massive (Figure e,f), globular (Figure e), and concentric annular (Figure f) textures. Concurrently, previous studies of Zhaxikang deposits also found the disseminated, brecciated, fine‐grained layered and colloform, net‐veined, and Dal Matianite ore textures (Wang et al, ; Wang, Zheng, Mathur, & Wu, ), as well as syndepositional structure and even ancient hydrothermal vent (Zheng et al, ) in the first episode of mineralization. The above characteristics of the ores from the first episode of mineralization in Zhaxikang deposit are similar to those of the ores from Red Dog SEDEX‐type ore district in Alaska, which reveals typical submarine hydrothermal sedimentation (metasomatism) origin.…”

“…Detailed regional settings of NH have been reported in the previous literature (Wang et al, ; Wang, Zheng, Mathur, & Wu, ; Sun et al, ; Zhou et al, ), here is only a brief description. The strata cropping out in NH mainly include the Precambrian Laguigangri Group and a series of Upper Triassic, Jurassic, Lower Cretaceous, and Quaternary sediments (Figure b).…”

“…The Zhaxikang deposit is located ~48 km west from Longzi County Town within NHMB (Figure b; Wang, Zheng, Mathur, & Wu, ). This deposit contains Zn + Pb 2.066 Mt at average grades of 6.38%, Sb 0.235 Mt at an average grade of 1.14%, Ag 2660.6 t at an average grade of 101.64 g/t, Au 6.5 t at an average grade of 2.9 g/t (independent gold ore body), associated Au 3.9 t and Ga 361 t, and 20 Mt Mn–Fe carbonates ores at an average grade of 42% for Fe + Mn (the latest exploration results of Huayu Mining Company).…”

“…(a) Geological map of the Zhaxikang Sb–Pb–Zn–Ag deposit (modified from Zheng et al, ; Wang, Zheng, Mathur, & Wu, ); (b) cross section along the Exploration Line 8 (Wang et al, ) [Colour figure can be viewed at wileyonlinelibrary.com]…”

“…Wang and Zhu () suggested that the non‐traditional isotopic research of ore‐forming elements (e.g., Fe, Cu, Zn, Cd, and Mo) can more directly constrain the ore genesis and restore the ore‐forming process than traditional isotopes (e.g., C, H, O, and S). Accordingly, Wang et al () and Wang, Zheng, Mathur, and Wu () studied the Fe–Zn isotopes of Zhaxikang deposit. According to the combined research of Fe–Zn isotopic fractionation models and geological evidence, the δ 56 Fe and δ 66 Zn values of the first episode of ore‐forming fluid are supposed to range from −0.5‰ to −1‰ and −0.28‰ to 0‰, respectively.…”