The Qulong Giant Porphyry Copper Deposit In Tibet: Magmatism And Mineraliztion

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 thin dike(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 the intrusions suggest that the Miocene intrusions have similar origin and probably formed by fractionation of a deeper magma chamber. Emplacement of Miocene porphyries,controlled by Qulong anticline,is direct response of the rapid uplift/erosion of Gangdese arc hatholiths in the south Tibet.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(±K feldspar),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±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 7 million tons of contained copper are associated with slightly later quartz-biotite-anhydrite alteration.Barren assemblages are related to irregular quartz(-K feldspar±anhydrite) veins,which are truncated by the X porphyry.Cu sulfide-bearing assemblages are associated with discontinuous chalcopyrite(±biotite) and continuous quartz-anhydrite-chalcopyrite(±molybdenum) veins.Deposition of Cu-Mo with abundant anhydrite occurred during or between emplacements of closely related porphyries from high temperature magmatically-derived fluids,and was probably caused by hydrolysis of SO₂.