Ore geology and fluid inclusion geochemistry of the Tiemurt Pb–Zn–Cu deposit, Altay, Xinjiang, China: A case study of orogenic-type Pb–Zn systems

The Tiemurt Pb–Zn–Cu deposit is hosted in a Devonian volcanic-sedimentary basin of the Altay orogenic belt, and is thus interpreted to have formed by sea-floor hydrothermal exhalation in previous studies. Our investigation discovered that the deposit is not stratiform or stratabound, but structure-controlled instead. The hydrothermal ore-forming process can be divided into the early, middle and late stage, represented by pyrite-quartz, polymetallic sulfide-quartz and carbonate-quartz veinlets, respectively. The early-stage veins and contained minerals are structurally deformed and brecciated, suggesting a compressional or transpressional tectonic regime. The middle-stage veinlets intrude and infill the fissures of the early-stage assemblages, and show no deformation, suggesting a tensional shear setting. The late-stage veinlets mostly infill open-space fissures that crosscut veins and replacements formed in the earlier stages.Four types of fluid inclusions (FIs), including aqueous (type W), carbonic-aqueous (type C), pure carbonic (type PC) and solid-bearing (type S), are identified at the Tiemurt deposit. The early-stage minerals contain the C- and W-type primary FIs that are totally homogenized at temperatures of 330–390 °C with low salinities of 0.8–11.9 wt.% NaCl eqv.; whilst the late-stage quartz or calcite contains only the W-type FIs with homogenization temperatures of 118–205 °C, and salinities of 1.4–3.4 wt.% NaCl eqv. This indicates that the ore fluid system evolved from CO2-rich, probably metamorphic to CO2-poor, meteoric fluids; and that a significant CO2-escape must have occurred. All the four types of FIs can be only observed in the middle-stage minerals, and even in a microscopic domain of a crystal, representing an association trapped from a boiling fluid system. These FIs homogenize at temperatures ranging from 270 to 330 °C, with two salinity clusters of 1.9–14.5 and 37.4–42.4 wt.% NaCl eqv., respectively. This implies that metal precipitation resulted from fluid boiling, CO2-escape and transient oversaturation. The estimated trapping pressures of FIs range from 130 to 380 MPa, suggesting an alternating lithostatic–hydrostatic fluid-system, controlled by a fault-valve at the depth of ∼13 km. Therefore, the Tiemurt Pb–Zn–Cu deposit is likely an example of orogenic Pb–Zn + Cu systems formed in collision orogeny, rather than a pre-collision VMS or Sedex system developed in Devonian; and a new metallogenic model is proposed to interpret the formation of the Tiemurt Pb–Zn–Cu deposit.