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The Himalayan Yulong Porphyry Copper Belt: Product of Large-Scale Strike-Slip Faulting in Eastern Tibet

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Hou, Z and Ma, H and Zaw, K and Zhang, Y and Mingjie, W and Zeng, W and Guitang, P and Renli, T (2003) The Himalayan Yulong Porphyry Copper Belt: Product of Large-Scale Strike-Slip Faulting in Eastern Tibet. Economic Geology, 98 (1). pp. 125-145. ISSN 0361-0128

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Abstract

The Yulong porphyry copper belt is the most significant porphyry copper belt in Tibet and is located in the
Qiangtang terrane of the Himalayan-Tibetan orogen. The terrane is a collage of continental blocks joined by
ophiolitic sutures and volcano-plutonic arc complexes. The Yulong belt is approximately 300 km long and 15 to
30 km wide, contains one giant, two large, and two medium- to small-sized porphyry copper deposits, and more
than 20 mineralized porphyry bodies. The Yulong belt is located in the Changdu continental block that comprises
Proterozoic to early Paleozoic crystalline folded basement and middle to late Paleozoic platform facies
carbonate and clastic sedimentary rocks similar to the Yangtze continent. The porphyry belt is closely associated
with Tertiary potassic volcanic rocks and alkali-rich intrusions in the area and controlled by north-south-north-northwest, large-scale, strike-slip faults, which are perpendicular to the collision zone between the
Indian and Asian continents. Isotopic age determinations of the ore-bearing porphyries indicate that the magmatism
occurred over at least three stages, peaking around 52, 41, and 33 Ma, respectively. The timing of middle
and late shallow-level emplacement of these magmas is consistent with the ages of associated potassic volcanism
and alkali-rich magmatism in the area. Although the porphyry deposits in the Yulong belt were
developed in the intracontinental convergent environment, their mineralization styles and features are comparable
to porphyry copper deposits in arc environments.
Compared to ore-bearing calc-alkaline porphyries in island arcs or continental margin arcs, the porphyritic
intrusions in the Yulong belt are characterized by high K2O contents and enrichment in Rb and Ba, suggesting
a shoshonitic magmatic affinity. Strong negative anomalies for Nb, Ta, P, and Ti and positive anomalies for Rb,
Ba, Th, and LREE, normalized by chondrite, are characteristic of arc magmas. These intrusions yield a narrow 143Nd/144Nd range varying from 0.51243 to 0.51253 and 87Sr/86Sr values from 0.7065 to 0.7077, which are transitional between type II enriched mantle and mid-ocean ridge basalt (MORB) values and closer to the former
in terms of epsilon Nd- epsilon Sr. This suggests that the porphyritic magmas were derived either from a hydrous-enriched mantle metasomatized by components such as H2O, K, Rb, Ba, Th, and LREE or by melt derived from the
subducted oceanic slab of the Paleozoic Jinshajiang oceanic plate. The hypothesis is supported by Pb isotope
data for the intrusions.
Large-scale strike-slip faults in eastern Tibet, which accommodated the compressive strains produced by the
Asian-Indian continent collision, also localized the porphyry Cu mineralization. North to north-northeast-directed convergence and collision produced a dextral strike-slip fault system around 60 to 70 Ma. Northeast-directed wedging of the Indian continent and subsequent collision with the Yangtze continent during the Paleocene-
Eocene produced conjugate strike-slip fault zones. The transition from a dextral strike-slip fault system
to conjugate strike-slip zones resulted in stress relaxation and formation of strike-slip pull-apart basins. Crustalscale strike-slip faulting may have caused upwelling and partial melting of the hydrous-enriched mantle by decompression and facilitated the rise of a large volume of volatile-enriched porphyry magma that had ponded
near the base of the lithosphere during this period.

Item Type: Article
Keywords: Porphyry copper, Strike-slip fault, Yulong, Himalayan, Tibet, China
Journal or Publication Title: Economic Geology
Page Range: pp. 125-145
ISSN: 0361-0128
Identification Number - DOI: 10.2113/98.1.125
Date Deposited: 06 Sep 2007
Last Modified: 18 Nov 2014 03:22
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