The Characteristics Of Qulong Ore-bearing Porphyries And Their Relation To Cu(Mo) Mineralization In The Gangdese Belt, Tibet

Recent studies on Tibetan plateau show that there are a huge amount of potential resources in the Gangdese porphyry belt, especially Qulong porphyry copper deposit. In this his dissertation, the field geological work and modern analysis were finished on the Qulong ore-bearing porphyries. The mineralization chronology, petrology, element and isotopic geochemistry were studied on the porphyries. This dissertation also compare this work in Qulong ore-bearing porphyries with other ore-bearing porphyries in Gangdese belt, in order to probe into dynamical background and mineralization features of the ore belts in the condition of India-Eurasia collision. The main conclusions were drawn as follows:1. The Qulong ore-baring porphyries include beschtauite and biotite granodiorite porphyry. The biotite granodiorite porphyry is metaluminous, from calcalkaline to high-K calcalkaline; the beschtauite is from metaluminous to peraluminous, and high-K calcalkaline to shoshonitic.2. This dissertation got the deposit forming of Qulong copper (molybdenum) porphyry copper deposit by SHRIMP zircon U-Pb and Re-Os molybdenite methods. The SHRIMP age range from 16.35 Ma to 17.58 Ma, while the Re-Os molybdenite age is 16.85 Ma. The two groups of age sapan are very close. The age for rock-forming and ore-forming in Qulong porphyry Cu (Mo) ore deposit both lasts about 1 Ma. Further studies show that rock-forming and ore-forming processes are continuously in Qulong ore deposit.3. In the whole Gangdese porphyry ore deposit belt, the ore-forming ages are varied in differences places. Such as, 12.2-15.6 Ma in Chong jiang, 12.0-13.6 Ma in Lakange, 12.3-15.2 Ma in Jiama, 13.6-14.9 Ma in Nanmu, and 13.3-16.5 Ma in Bangpu. The ages for these ore-forming in Gangdese porphyry ore deposit belt range from 12 Ma to 17 Ma, showing eastward younger in the belt.4. The element geochemical features in Qulong ore-bearing porphyries show adakite magmatic affinity, similar to other ore deposits in Gangdese. But they are different to the subducted oceanic crust-origin classical adakitic rocks because of their high-K features. The rocks are enriched in LILE (e.g. Rb, Sr, U, Th) and LREE, and depleted in HFSE (e.g., Nb, Ta, Ti, Y), without significant Eu negative anomaly.5. The ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb values of the Gangdese ore-bearing porphyries range from 18.408 to 18.661 , from 15.567 to 15.733 and 38.561 to 39.153. This indicate magma originate from lower crust and are affected by neo-Tethys ocean crust.6. Comparison of Nd and Sr isotopic composition between Gangdese belt with Yulong ore deposit in eastern Tibet, show that ε_Nd values range from -6.18 to 1.81 in Gangdese, from -4.2 to -0.8 in Yulong. This indicate magma originate from lower crust or from neo-Tethys ocean crust. The ⁸⁷Sr/⁸⁶Sr values range from 0.704914 to 0.707920 in Gangdese, from 0.705817 to 0.708311 in Yulong, which show that ore-bearing porphyries originate from mixture of crust and mantle.7. Comparing the ores, alteration minerals and fluid inclusions of Gangdese ore deposits to isotope composition, trace elements, metal elements and other features, suggests that the ore-forming metals, ore fluid and heat source arte all originated from ore-bearing porphyry magmas.