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Geological and chronological constraints on the long-lived Eocene Yulong porphyry Cu-Mo deposit, eastern Tibet, China: implications for lifespan of magmatic-hydrothermal processes forming giant and supergiant porphyry Cu deposits

Chang, J.; Li, J.-W.; Selby, D.; Liu, J.-C.; Deng, X.-D.

Geological and chronological constraints on the long-lived Eocene Yulong porphyry Cu-Mo deposit, eastern Tibet, China: implications for lifespan of magmatic-hydrothermal processes forming giant and supergiant porphyry Cu deposits Thumbnail


Authors

J. Chang

J.-W. Li

J.-C. Liu

X.-D. Deng



Abstract

The Yulong porphyry Cu-Mo deposit, the third largest porphyry Cu deposit in China, contains proven reserves of > 6.5 million metric tons (Mt) Cu and 0.4 Mt Mo. Previous radiometric dating studies have provided numerous ages for this deposit, but the timing and duration of the process governing the deposition of Cu and Mo remains not well constrained. In this paper, we first document multiple stages of mineralization and hydrothermal alteration associated with distinct magmatic pulses at Yulong by field and textural relationships, and then present high-precision molybdenite Re-Os ages of 14 quartz-molybdenite ± chalcopyrite veins representing these stages to precisely constrain the timing and duration of Cu-Mo mineralization. The ore-hosting Yulong composite stock consists of three successive porphyry intrusions: (1) monzonitic granite porphyry (MGP), (2) K-feldspar granite porphyry (KGP), and (3) quartz albite porphyry (QAP). The vein formation, Cu-Mo mineralization, and ore-related alteration are grouped into early, transitional, and late stages with respect to the intrusive history. The first two porphyry intrusions are followed by cyclical sequences of veining that are mainly associated with potassic alteration and have formed (1) ME vein/USTT, (2) EBE/T veins, (3) A1E/T veins, (4) A2E/BT veins, and (5) A3E/T veins. A2E/BT and A3E/T veins of the early and transitional stages are dominated by quartz and chalcopyrite ± pyrite, respectively, and represent the main Cu-Mo mineralization events. More than 80% of Cu and Mo at Yulong were deposited in the early stage with the remainder being formed in the transitional stage. The late-stage pyrite-quartz veins (DL), which are characterized by sericitic alteration halos, postdate the intrusion of QAP dikes and have no economic significance. Molybdenite Re-Os ages of A2E and BT veins indicate that sulfide deposition at Yulong was episodic over a prolonged history lasting over 5.13 ± 0.23 m.y. (1σ). However, the bulk Cu-Mo ores formed in a shorter time interval of 1.36 ± 0.24 m.y. (1σ) with most Cu precipitated in a more restricted timespan of 0.82 ± 0.24 m.y. (1σ) in the early stage. These results, combined with geochronologic data from porphyry copper deposits elsewhere, confirm that multiple magmatic-hydrothermal pulses with a lifespan of tens to hundreds of thousands of years are sufficient to form a giant porphyry copper deposit. Factors such as metal concentration, volume, and focusing efficiency of ore-forming fluids could have played important roles in producing a giant porphyry Cu deposit regardless of a short- or long-lived magmatic-hydrothermal system.

Citation

Chang, J., Li, J., Selby, D., Liu, J., & Deng, X. (2017). Geological and chronological constraints on the long-lived Eocene Yulong porphyry Cu-Mo deposit, eastern Tibet, China: implications for lifespan of magmatic-hydrothermal processes forming giant and supergiant porphyry Cu deposits. Economic geology and the bulletin of the Society of Economic Geologists, 112(7), 1719-1746. https://doi.org/10.5382/econgeo.2017.4527

Journal Article Type Article
Acceptance Date Jun 23, 2017
Online Publication Date Nov 1, 2017
Publication Date Nov 1, 2017
Deposit Date Jul 5, 2017
Publicly Available Date Mar 29, 2024
Journal Economic Geology and the Bulletin of the Society of Economic Geologists
Print ISSN 0361-0128
Electronic ISSN 1554-0774
Publisher Society of Economic Geologists
Peer Reviewed Peer Reviewed
Volume 112
Issue 7
Pages 1719-1746
DOI https://doi.org/10.5382/econgeo.2017.4527

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