Genesis of the Saishitang skarn type copper deposit,West Qinling,Qinghai Province: Evidence from fluid inclusions and stable isotopes |
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| Affiliation: | 1. State Key Laboratory of Geological Processes and Mineral Resources, and School of Earth Science and Resources, China University of Geosciences, Beijing 100083, China;2. Gold Geological Institute of CAPF, Langfang 065000, China;3. James Cook University, Townsville 4811, Queensland, Australia;1. Institute of Geophysics, China Earthquake Administration, 100081, Beijing, China;2. Centre for Tectonics, Resources and Exploration, Department of Earth Sciences, School of Physical Sciences, University of Adelaide, SA 5005, Australia;3. School of Earth Sciences and Resources, China University of Geosciences, 100083 Beijing, China;1. Department of Geology, Payame Noor University, PO Box 19395-3697, Tehran, Iran;2. Department of Geology, Faculty of Science, Shahid Bahonar University, Kerman, Iran |
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| Abstract: | The Saishitang skarn type copper deposit, located in the southeast part of the Dulan–Ela Mountain Triassic volcanic–magmatic arc and forearc accretionary wedge, belongs to the Tongyugou–Saishitang tin–copper polymetallic ore field in West Qinling, Qinghai province. Based on the contact/crosscutting relationships, mineral associations and mineralization characteristics, hydrothermal fluid evolution can be divided into three stages: skarn (I), quartz sulfide (II) and polymetallic sulfide-bearing quartz–calcite vein (III). The quartz sulfide stage (II) can be further divided into a massive sulfide stage (II-1) and a layered sulfide stage (II-2). This paper presents detailed analysis of fluid inclusions, H–O, S and Pb isotope compositions of rock samples from each of the above three stages as well as analysis of fluid inclusions from quartz diorite. The homogenization temperature, salinity, density and pressure of fluid inclusions in quartz diorite and typical transparent minerals showed a tendency of gradual decline in these evolutionary stages. The ore-forming fluid can be classified as a Na+–Ca2 +–SO42 − Cl− system with a minor proportion of a Na+–Ca2 +–NO3− SO42 − system, which likely resulted from mixing of magmatic and formation water. The H–O isotope composition indicates that the proportion of formation water increased during the ore-forming process, and meteoric water was mixed in the late quartz–calcite vein stage. The δ34S (CDT) values (− 6.45–5.57‰) and Pb isotope compositions show that the ore-forming materials were mainly derived from magmatic fluid. Ore-forming fluid was boiling during the main ore-forming stage (II-1) due to pressure decrease. Consequently, the physical and chemical conditions (i.e., pH, Eh, fO2, fS2) changed, and metallic elements (including Cu) in the fluid could no longer exist in the form of complexes and precipitated from the fluid. According to the integrated analysis of ore features, mineral associations, alteration characteristics, ore-forming environment and fluid evolutionary process, it is concluded that the Saishitang deposit is a typical skarn deposit. |
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