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 δ 56 Fe values of Mn-Fe carbonate and δ 66 Zn 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 δ 66 Zn 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 δ 56 Fe i and δ 66 Zn i values (initial Fe-Zn isotopic compositions) of the ore-forming system are in the range of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M1"><mml:mo>-</mml:mo><mml:mn fontstyle="italic">0.5</mml:mn><mml:mtext>‰​</mml:mtext><mml:mi>​</mml:mi><mml:mtext> ~</mml:mtext></mml:math>−<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M2"><mml:mn fontstyle="italic">1</mml:mn><mml:mtext>‰</mml:mtext></mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M3"><mml:mo>-</mml:mo><mml:mn mathvariant="normal">0</mml:mn><mml:mo>.</mml:mo><mml:mn mathvariant="normal">28</mml:mn><mml:mtext>‰</mml:mtext><mml:mi mathvariant="normal">​</mml:mi><mml:mtext> ~</mml:mtext><mml:mn mathvariant="normal">0</mml:mn><mml:mtext>‰</mml:mtext></mml:math>, respectively. To conclude, the EPMA data, Fe-Zn isotopic characteristics, and fractionation models support the SEDEX model for the first pulse of mineralization.