International Geologiical Congress - Oslo 2008

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MRD-20 Porphyry-type deposits

 

Geology of the qulong porphyry copper- molybdenum deposit, Tibet

 

Zhiming Yang, Institute of Gelogy, Chinese Academy of Geological Sciences (China)
Zengqian Hou, Institute of Gelogy, Chinese Academy of Geological Sciences (China)
Yucai Song, Institute of Gelogy, Chinese Academy of Geological Sciences (China)
Zhenqing Li, Institute of Mineral Resource, Chinese Academy of Geological Sciences (China)
 

 

Porphyry deposits usually form in arc settings, in association with subduction-related calc-alkaline magmas, although some deposits also occur in post-collisional extensional settings and are unrelated to subduction. These deposits remain poorly understood. Here we describe the igneous geology, alteration mineralogy, and mineralization history of Qulong, a newly-discovered porphyry Cu deposit in southern Tibet that belongs to the post-collisional class. The deposit is associated with Miocene monzogranite - granodiorite intrusions and is hosted partly in Jurassic andesitic-dacitic volcanics. A ∼19.5 Ma granodiorite pluton with diorite enclaves is the earliest Miocene intrusive unit. It was intruded by a regularly-shaped stock (P porphyry) and then irregularly-shaped thin apophyses (X Porphyry) of monzogranite about 17.7 m.y. ago. The main copper-molybdenum mineralization is associated with the P porphyry. A barren diorite porphyry, intruded the P and X porphyries, around 15.7 Ma. Petrologic trends of major and trace element compositions for the early granodiorite are distinct from the late P and X porphyries. Sr-Nd-Pb-Hf isotope data indicate that they have the same source and probably formed by fractionation of dioritic magma derived from a newly-formed mafic lower-crustal source. Miarolitic cavities and unidirectional solidification textures, the key evidence for volatile separation, have been recognized in the P and X porphyries, respectively. Early potassic alteration, characterized by quartz-K feldspar (+/- anhydrite), pervades the P porphyry and granodiorite. Laterally, this alteration grades into quartz-biotite-anhydrite (+/- anhydrite), which affects all Miocene intrusions except the latest dioritic porphyry. Wall rocks of granodiorite and Jurassic andesitic-dacitic volcanics within 1-1.5 kilometers of the porphyries are dominated by potassic alteration. An outer halo of propylitic alteration (epidote-chlorite+/- calcite) extends up to 2 km. Feldspar-destructive alteration (sericite-chlorite-pyrite +/-clay minerals) has overprinted most of the potassic and part of the propylitic alteration. The alteration is strongly pervasive in the interior of the porphyries and occurs as vein halos away from the porphyries. The earliest quartz-K feldspar alteration and veins are barren, whereas approximately 60 percent of the 10 million tons of contained copper are associated with slightly later quartz-biotite-anhydrite alteration. Barren assemblages are related to irregular quartz-K feldspar (+/- anhydrite) A-type veins, which are truncated by the X porphyry. Cu sulfide-bearing assemblages are associated with discontinuous chalcopyrite (+/- biotite) and quartz-anhydrite-chalcopyrite (+/- molybdenum) A-type veins. Deposition of Cu-Mo occurred during or between emplacement of closely related porphyries from high temperature magmatically-derived fluids.

 

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