Genesis of the sediment-hosted Haerdaban Zn-Pb deposit, Western Tianshan, NW China: Constraints from textural, compositional and sulfur isotope variations of sulfides

Abstract

The sediment-hosted Haerdaban Zn-Pb deposit is a newly discovered Zn-Pb deposit in the Sailimu terrane, Western Tianshan (NW China), containing ca. 0.64 Mt of ore at 6.19 wt% Zn and 1.09 wt% Pb. The mineralization consists mainly of veins, breccias and semi-massive sulfide ores within the dolomitic limestone and calcareous slate. Using LA-ICP-MS, this study investigates trace element and sulfur isotope composition of sulfide to constrain the genesis of the deposit. Four pyrite types were identified by variations in texture and composition. Porous pyrite (Py1) shows a significant enrichment in Mn, Ag, Sb, Tl, Ba, Pb, Cu, and Zn, whereas the pyrite overgrowth (Py2) and euhedral pyrite (Py3) are enriched in Co and As, but depleted in all other trace elements. This compositional trend is attributed to the release of trace elements from pyrite during recrystallization from a porous to massive texture. Pyrite (Py4) associated with sphalerite is inclusion-rich, but exhibits a depletion in most trace elements, except Ni and Sb. The underlying carbonaceous sediments are interpreted as a source of Ni, Sb and other metals for the ore-forming fluids. Sphalerite displays a wide variety of colors from dark brown to pale yellow, primarily related to variations in Fe contents. The overall Fe contents (0.25 to 6.12 wt%), Cd contents (524 to 1875 ppm) and Zn/Cd ratios (320 to 1258) in sphalerite are compatible with those of SEDEX Zn-Pb deposit. The contents of other elements in sphalerite are generally low. Calculated temperatures using trace element contents in sphalerite (GGIMFis geothermometer) range from 163 °C to 309 °C, indicating progressive cooling of ore-forming fluid. Pyrite, sphalerite and galena display a wide range of δ34S values from +3.9 to +18.3‰. The association of lower positive δ34S values (+3.9 to +8.8‰) and epithermal suite elements (Ni, As, Sb, and Tl) enrichment in porous pyrite indicate that sulfur in early sulfides was derived from thermochemical sulfate reduction (TSR) with a possible magmatic input. The dominant population of heavier δ34S values (+13.8 to +18.3‰) of massive sulfide support TSR as the main source of sulfur. The intermediate δ34S values (+7.0 to +13.0‰) of vein sulfide are likely a mixing of TSR-derived sulfur with light sulfur leached from diagenetic pyrite. The current data support the classification of Haerdaban as a metamorphosed SEDEX Zn-Pb deposit, where the main mineralization was the product of remobilization and upgrading of early exhalative ores during hydrothermal events.