Cospatial Eocene and Miocene granitoids from the Jiru Cu deposit in Tibet: petrogenesis and implications for the formation of collisional and postcollisional porphyry Cu systems in continental collision zones

Yang, Zhiming, Hou, Zengqian, Chang, Zhaoshan, Li, Qiuyun, Liu, Yunfei, Qu, Huangchun, Sun, Maoyu, and Xu, Bo (2016) Cospatial Eocene and Miocene granitoids from the Jiru Cu deposit in Tibet: petrogenesis and implications for the formation of collisional and postcollisional porphyry Cu systems in continental collision zones. Lithos, 245. pp. 243-257.

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Abstract

Jiru is a poorly studied Cu deposit located in the west segment of the Gangdese porphyry Cu belt (GPCB), 200 km west of Lhasa. The deposit consists of both collisional- and postcollisional-stage porphyry-type Cu systems, which are genetically associated with the early Eocene granitoid batholith and the Miocene Jiru porphyry stock, respectively. In this study, we present zircon U–Pb LA-ICP-MS dates and Hf isotopes, whole rock geochemical and Pb isotope geochemical data for the main intrusions in the Jiru deposit. The early Eocene granitoid samples (~ 49 Ma) are characterized by magmatic arc geochemical features, slightly concave REE patterns and well-developed negative Eu anomalies. These geochemical characteristics suggest that the granitoid melts were generated by partial melting of a metasomatized mantle, and that the melt had undergone fractional crystallization of amphibole and plagioclase. In contrast, the Miocene porphyry intrusions (16.4–15.5 Ma) at Jiru are characterized by high K contents, adakitic affinities (e.g., high Sr/Y and La/Yb ratios), subduction signatures (e.g., enriched Cs, Rb, Ba and depleted Nb, Ta, Ti), positive zircon εHf(t) values (1–6) and variable 208Pb/204Pb ratios (38.5–39.0), similar to other post-collisional porphyry intrusions in the Gangdese belt. Based on the above features, we propose that the Miocene porphyry intrusions at Jiru were generated by partial melting of subduction-modified lower crust.

Well-developed negative Eu anomalies and low Sr/Y ratios (generally < 20) of the least fractionated samples of the Early Eocene granitoids indicate that water content of the primitive collision-related magma was < 4 wt.%, but increased to over 4 wt.% with fractional crystallization, as evidenced by very weak negative Eu anomalies and relatively high Sr/Y ratios (~ 40) for some samples with SiO2 contents of ~ 67 wt.%. Upper crustal differentiation, which would increase water content of residual magma, is thought to be a key step in the formation of the collision-stage Cu mineralization at Jiru. The presence of Eocene porphyry Cu–Mo mineralization indicates that sulfide precipitation at the base of the orogenic lower crust during the first-stage arc magmatism is not needed in the formation of the postcollisional porphyry Cu deposit at Jiru. This implies that the source of metal and S for postcollisional porphyry Cu deposits is more complex than originally considered.

Item ID: 39170
Item Type: Article (Research - C1)
ISSN: 1872-6143
Keywords: Jiru; Gangdese; Tibet; Porphyry Cu deposit; collisional setting; post-collisional setting
Funders: National Basic Research Program of China (NBRP), National Natural Science Foundation of China (NNSFC), Ministry of Land and Resources of China (MLRC)
Projects and Grants: NBRP No. 2011CB403104, NNSFC grant no. 41273051, NNSFC grant no. 41473041, MLRC 201011011, MLRC IGCP/SIDA-600
Date Deposited: 11 Jun 2015 01:48
FoR Codes: 04 EARTH SCIENCES > 0403 Geology > 040304 Igneous and Metamorphic Petrology @ 50%
04 EARTH SCIENCES > 0403 Geology > 040307 Ore Deposit Petrology @ 50%
SEO Codes: 84 MINERAL RESOURCES (excl. Energy Resources) > 8401 Mineral Exploration > 840102 Copper Ore Exploration @ 80%
84 MINERAL RESOURCES (excl. Energy Resources) > 8401 Mineral Exploration > 840199 Mineral Exploration not elsewhere classified @ 20%
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