Metallogenic Regularity And Metallogenic Prediction For Ga’erqiong-galale Copper-gold Concentrated Area,Tibet

The Ga’erqiong-Galale copper-gold ore-concentrated area is located in the western part of the Bangong-Nujiang suture zone north of the Lhasa Terran, which is the typical skarn(porphyry) ore-concentrated area. This paper conducted a systematic study on the geological characteristics, metallogenic evolution and tectonic setting in the Ga’erqiong deposit and Galale deposit using techniques of detailed geological record and microscopic identification, isotopic geochronology, isotopic geochemistry and lithogeochemistry, and established exploration model and metallogenic model and summarized the metallogenic regularity of the deposit in the ore-concentrated area. The research results have important guiding significance for the regional prospecting work.The orebodies of the Ga’erqiong deposit are composed of the porphyry molybdenum(copper) orebody, skarn copper-gold-molybdenum orebody and iron oxide-copper-gold type orebody, which are formed in the quartz diorite porphyry, contact skarn belt and fault fracture zone respectively.While the orebody of the Galale deposit is composed of the single skarn copper-gold orebody. The major skarn minerals in the Ga’erqiong district are garnet, diopside, wollastonite, tremolite, epidote, chlorit, which has similar characteristics as most typical calcareous skarn. The major skarn minerals in the Galale district include olivine, serpentine, diopside and phlogopite, while the minor skarn minerals are composed of pleonaste, tremolite, garnet, epidote, brucite and muscovite. The mineral assemblage of the skarn is the typical magnesian skarn. The ore-forming elements(Cu, Au, Ag) are mainly in the form of an independent mineral, the gold is mainly occurring in the simple form of fissure gold, intercrystalline gold, inclusion gold, film gold and isomorphous.The systemic diagenetic and metallogenic chronology study showed that there had six stages of magmatic activities, which include the Early Cretaceous volcano-magmatic activities, the Late Cretaceous volcano-magmatic activities and the Late Cretaceous magmatic activities. According to the results, the crystallization age of quartz diorite(ore-forming mother rock) in the Ga’erqiong deposit is 87.1 ± 0.4 Ma, which is later than the age of granodiorite(ore-forming mother rock) in the Galale deposit(age = 88.1 ± 1.0 Ma),they both main shows positive εHf(t) values(3.5~10.5, 5.84~9.20, respectively), suggesting that the magmatic source of the main intrusions in the ore-concentrated area has the characteristics of mantle source region. The Re-Os isochron age of molybdenite in the Ga’erqiong district is 86.9 ± 0.5 Ma, which is later than the mineralization age of the Galale district(88.6 ± 0.6 Ma). The main intrusive rocks in the ore-concentrated area have similar lithogeochemical characteristics, for they both show the relative enrichment in large-ion lithophile elements(Rb, Sr, K, Th etc.), and relatively depleted in high field strength elements(Nb, Ta, Zr, Hf, Yb, Ti etc.), and show the characteristics of magmatic arc.The Ga’erqiong deposit and the Galale deposit are the products of the same structure-magmtism-mineralization event.Isotope geochemistry and fluid inclusions geochemistry data showed that the fluid of the mineralization stage was high temperature-high salinity fluid, which mainly came from the magma, and also mixed with precipitation in the late mineralization stage. The processes of fluids boiling after decompressed was an important mechanism of metal mineral precipitation. The ore-forming minerals of the deposits are mainly derived from the mantle, with a few upper crustal materials contamination.Comprehensive aforementioned studies and regional tectonic evolution, we identified the porphyry-skarn deposit which were formed in the stage of the Bnangong Co-Nujiang tethys southern subduction-collision tectonic evolution and the porphyry+skarn+ iron oxide-copper-gold “trinity ” type orebodies, which is pointed out the direction for regional prospecting. This paper established the Late Yanshan metallogenic sub-series which was related to the mantle magmatism, proposed new idea that the mineralization in the ore-concentrated area was related to the continent-continent collision and the metallogenic magma was derived from the partial melts of the newborn lower crust. Based on the mineralization and exploration model, combined geochemistry, alteration mapping and index minerals identification, this paper had made a metallogenic prediction for the ore-concentrated area and provided the new ideas for the district regional prospecting.