Two pulses of mineralization and genesis of the Zhaxikang Sb-Pb-Zn-Ag deposit in southern Tibet: Constraints from Fe-Zn isotopes

Zhaxikang is one large Sb-Pb-Zn-Ag deposit located in the North Himalaya of southern Tibet. To date, the genesis of this deposit still remains controversial. Here, we present new pyrite Fe and sphalerite Zn isotopic data for the first three stages of mineralization, Fe-Zn isotopic data for Mn-Fe carbonate that formed during the first two stages of mineralization, and Zn isotopic data for the slate wall rocks of the Jurassic Ridang Formation to discuss the genesis of the Zhaxikang deposit. The overall delta Fe-56 and delta Zn-66 values range from -0.80 parts per thousand to 0.43 parts per thousand and from -0.03 parts per thousand to 0.38 parts per thousand, respectively. The delta Fe-56 values of Mn-Fe carbonates are lighter than those of associated pyrite in six mineral pairs, indicating that the iron carbonates are preferentially enriched in light Fe isotopes relative to pyrite. The sphalerite has lighter delta Zn-66 values than associated Mn-Fe carbonates in three mineral pairs. The delta Fe-56 values of pyrite that formed during the first three stages of mineralization gradually increase from stage 1 (-0.33 parts per thousand. to -0.09 parts per thousand) through stage 2 (-0.30 parts per thousand to 0.19 parts per thousand) to stage 3 (0.16 parts per thousand-0.43 parts per thousand). In comparison, the sphalerite that formed during these stages has delta Zn-66 values that gradually decrease from stage 1 (0.16 parts per thousand-0.35%parts per thousand) through stage 2 (0.09 parts per thousand-0.23 parts per thousand) to stage 3 (-0.03 parts per thousand to 0.22 parts per thousand). These data, in conjunction with the observations of hand specimens and thin sections, suggest that the deposit was overprinted by a second pulse of mineralization. This overprint would account for these Fe-Zn isotopic variations as well as the kinetic Rayleigh fractionation that occurred during mineralization. The temporally increasing delta Fe-56 and decreasing delta Zn-66 values recorded in the deposit are also coincident with an increase in alteration, again supporting the existence of two pulses of mineralization. The delta Fe-56 values of the first pulse of ore-forming fluid were calculated using theoretical equations, yielding values of -0.54 parts per thousand to -0.34 parts per thousand that overlap with those of submarine hydrothermal solutions (-1 parts per thousand to 0 parts per thousand). However, the delta Fe-56 values of the stage 3 pyrite are heavier than those of typical submarine hydrothermal solutions, which suggests that the second pulse of mineralization was probably derived from a magmatic hydrothermal fluid. In addition, the second pulse of ore-forming fluid has brought some Fe and taken away parts of Zn, which results the lighter delta Zn-66 values of sphalerite and heavier delta Fe-56 values of pyrite from the second pulse of mineralization. Overall, the Zhaxikang deposit records two pulses of mineralization, and the overprint by the second pulse of mineralization causes the lighter delta Zn-66 values and heavier delta Fe-56 values of modified samples. (C) 2017 Elsevier B.V. All rights reserved.