The Metallogeny Of The Miocene Gangdese Porphyry Copper Belt In Tibetan Collisional Orogen

The Gangdese porphyry copper belt is located in the central-east segment of the Gangdese granitoid batholiths, southern margin of the Lhasa terrane in the Tibetan collisional orogen, paralleling with the Yarlung-Zangbo suture. The Miocene mineralized porphyry bodies, intruded the Gangdese granitoid batholiths and surrounding the Triassic-Cretaceous sedimentary sequence as multiple hypabyssal intrusions, were locally controlled by either near NS-striking normal faults or the intersection of thrust zone with normal fault. Across the batholiths, these intrusive appear as beaded clusters trending within the normal faulting systems or graben basins.The ¹⁸⁷Re- ¹⁸⁷Os isochron ages yielded by molybdenite samples from the deposits in the belt vary from14 Ma to 17 Ma, suggesting that Cu (-Pb-Zn) mineralization took place in the middle-late stage of the lifetime of the Miocene magmatic Cu systems during post-collisional crustal extension period.The dominant porphyries are granitic, and monzogranitic, with subordinate quartz monzoniti1, quartz synitic, granitic, and granodiorite. The porphyries characterized by high-SiO₂, Al₂O₃, K₂O, and Mg# and low Y, and HREE contens with high Sr/Y are shoshonitic and high-K ealc-alkaline. The rocks are enriched in LILE(e.g. K, Rb, Th, U, Sr, Pb) and LREE, and depleted in HFSE(e.g. Nb, Ta, Ti, Yb), showing adakite magmatic affinity. The characteristics of the rare elements compositions and the signatures of Sr, Nd, and Pb isotope of the rocks inferred that the mineralized porphyry related to the partial melting of the subducted oceanic slab remaining in the mantle and of the lower crust under Gangdese and to the interaction between slab-derived melt and overlying mantle.The Gangdese porphyry copper belt consists of two mineralize subzones, viz. porphyry type copper mineralize zone in the south Gangdese belt and skarn type copper polymetal mineralize zone in the north Gangdese belt, which form the metallogenic system of Gangdese porphyry copper deposit. Two styles of mineralization can be recognized in the porphyry deposits. One style is the Cu or Cu-Mo mineralization in the interior of porphyry intrusions with veinlet-disseminated and fine veinlet pyrite + chalcopyrte+molybdenite+bornite assemblages, the Cu orebodies mostly occurred in K-silicate zone whereas Mo orebodies in quartz-sericite zone. The second mineralization predominantly produced stratiform or lenticular Pb-Zn-Cu polymetallic orebodies, mainly associated with the skarn bodies along the contact between the porphyries and surrounding carbonate rocks.Multiple hydrothermal alterations progressing from early central K-silicate styles to late sericitic, silicification and argillic types were extensively developed around mineralizing intrusions. Three zones can be distinguished according to the altered mineral assemblages. The concentric alteration zones from the inner outward are successively the K-silicate zone, the quartz-sericite zone, and the propylitic zone. The orebodies mostly occur in the K-silicate alteration zones. The argillic alteration controlled by structure locally occurs as patches, overprinted the other alteration zone.The feature of the inclusions in quartz phenocrystal, vein quartz and anhydrite and the compositions of the ore-fluid in the deposits indicate that the fluid is the type of Na⁺-K⁺-SO₄^2--Cl^--H₂O-CO₂. The salinities of the fluid vary remarkable from 1.91 to 66.75 wt%NaCl equivalent. The homogenization temperatures of the fluid inclusions are various, ranging from 191℃to 550℃, with mostly between 300℃and 550℃. The range of the homogenization pressures is from 64.33×10⁵Pa to 236.42×10⁵Pa. The fugacity of oxygen and that of sulfur are high, varying from -20.763～-38.078 (lgfo₂) and -2.21～-7.62 (lgf_s2, respectively. The compositions of hydrogen, oxygen isotope of the fluid and the signatures of sulfur, lead isotopes of the rocks and the sulfide from the deposits indicate that the ore fluid and ore-forming matter are directly from the magmas developed closely in high fugacity of oxygen. The mineralization mainly took place in the postmagmatic stage in high temperatures. The alterations and the mineralization resulted from the unilateral evolution of the magmatic fluid. The secondary boiling of the magmas in high temperature and the mixing of different fluids are two important mineralizing ways in the copper deposits in the Gangdese belt. The size of the single copper deposit in Gangdese is restricted by the limited scale of the oceanic slab remaining in the mantle under the Gangdese.The comparison of the age date yielded by the phenocrystal plagioclases and by the molybdenites from the mineralized porphyry intrusions suggests that the ore fluid developed in a short session with time limit of active no more than 2 Ma, and the time span of mineralizing is even more shorter, 1 Ma or so, implying the speciality of rapid mineralizing of the copper deposits in Gangdese.Based on the mineralization characteristics of the Gangdese copper deposits and the contrast of the copper deposits occurred in different settings, the tectonic model of the mineralized magmas and the metallogenic model of the Gangdese Miocene porphyry copper deposits in the Tibetan collisional orogen have been proposed in this paper.