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Scientists review the metallogenesis and challenges of porphyry copper systems above subduction zone

Science China Press

As a typical magmatic hydrothermal metallogenic system, the porphyry copper (molybdate - gold) deposit is the most representative metallogenic type above the plate subduction zone, and has a very important economic value. Recently, the research progress of porphyry copper deposits has been reviewed in Science China: Earth Sciences, namely the latest progress and key scientific problems in the formation mechanism of porphyry copper deposits. The paper, titled "metallogenic mechanism and major challenges of the subduction porphyry copper ore system", was published in the recently published Science China: Earth Sciences (vol. 50, 2020). It was written by Professor Huayong Chen and Dr. Chao Wu from Guangzhou Institute of geochemistry, Chinese Academy of Sciences. The systematic literature investigation reveals that large PCDs are generally formed from initial arc magmas (from subduction-induced partial melting of the mantle wedge), which eventually ascend to the shallow crust (3-5 km) for mineralization after a series of complex evolution processes, and summarizes the current research on metallogenic mechanism, as well as existing scientific problems.

The PCDs consist of porphyry copper-molybdenum and copper-gold deposits, which are widely distributed in circle-Pacific and Central Asian metallogenic belt. PCDs are mostly spatially and genetically associated with shallow, intermediate to acid, porphyritic intrusions. PCDs usually have large reserves, shallow burial depths and are easily mined, which make them the key deposit type in the industry. As many world-class copper deposits are PCD-type, the physicochemical mechanism and geodynamic background of PCD formation have long been a hotspot of ore deposit research. This study summarizes the processes including (1) the dehydration or partial melting of subducting slab, which induces partial melting of the metasomatized mantle wedge; (2) the ascent of mantle-derived magma to the bottom of the lower crust, which subsequently undergoes crustal processes such as assimilation plus fractional crystallization (AFC) or melting, assimilation, storage and homogenization (MASH); (3) the magma chamber formation at the bottom of the lower, middle and upper crust; (4) the final emplacement and volatilization of porphyry stocks; and (5) the accumulation of ore-forming fluids and metal precipitation. Despite the many decades of research, many issues involving the PCD metallogenic mechanism still remain, such as (1) the tectonic control on the geochemical characteristics of ore-forming magma; (2) the reason for the different lifespans of the long-term magmatic arc evolution and geologically "instantaneous" mineralization processes; (3) the source of ore-forming materials; (4) the relative contributions of metal pre-enrichment to mineralization by the magma source and by magmatic evolution; and (5) the decoupling behaviors of Cu and Au during the pre-enrichment. These unsolved issues point out the direction for future PCD metallogenic research, and a resolution to them will extend our understanding of the metallogenesis at convergent plate boundaries, which will potentially benefit the industrial exploration for PCDs in Central Asian and Pacific Rim.

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This study was supported by the National Natural Science Foundation of China for Distinguished Young Scholars (Grant No. 41725009), the "135" Planned Project of Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (Grant No.135PY201606) and the Strategic Priority Program of the Chinese Academy of Sciences (Type B) (Grant No. XDB18030206).

See the article: Chen H, Wu C. 2020. Metallogenesis and major challenges of porphyry copper systems above subduction zones. Science China Earth Sciences, https://doi.org/10.1007/s11430-019-9595-8. http://engine.scichina.com/publisher/scp/journal/SSTe/doi/10.1360/SSTe-2019-0130?slug=abstract

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