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

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• 作者：Li Zhang^a ; Yi Zheng^a ; YanJing Chen^b ; ^{yjchen@pku.edu.cn}
• 关键词：Tiemurt Pb&ndash ; Zn&ndash ; Cu deposit ; Fluid inclusion ; CO2-rich fluid ; Orogenic lode deposit ; Altay orogenic belt
• 刊名：Journal of Asian Earth Sciences
• 出版年：2012
• 出版时间：30 April, 2012
• 年：2012
• 卷：49
• 期：Complete
• 页码：69-79
• 全文大小：2409 K

文摘

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 ¡ãC 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 ¡ãC, 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 ¡ãC, 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 ?3 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.