Abstract:

 The Halasu copper deposit is recently discovered in Qinghe, Xinjiang, and proved to be a largescale copper prospect by the exploration in progress. It is located at the southeastern Altai and near the Ertix tectonic deformation zone. The region had undergone the Middle Paleozoic ocean plate subduction, the Later Paleozoic continental collision and the successive intraplate activation  processes. That which process was related to the copper ore mineralization has  caused much attention, and there are debates on the metallogenesis of the ore, such as,  porphyry copper type, hydrothermal copper veintype and volcanic related copper deposit.  The Middle Devonian basic volcanic rock (including picrites) and copperbearing altered porphyry including granodiorite porphyry, porphyritic granite, quartz monzonite porphyry and quartz diorite porphyry of different periods crop out mainly in Halasu copper ore district. The porphyrys SiO2 content is from 57.24% to 65.45%; the granodiorite porphyrys δ18OVSMOW  value is from 7.9‰ to 8.6‰, its εNd(t)  is from 7.3 to 8.5 (near the value of MORB), and (87Sr/86Sr)t is from 0.70383 to 0.70410 (near the original mantle value); these indicate a mantle or lower crust magma sources. The copperbearing alteration porphyry is whole mineralized (Cu 02%), the orebody (Cu over 0.3%) is lenticular and irregularly dendritic veinshaped and its occurrence is similar to that of  the porphyry, and over  95%  of  the orebodies are located within  the porphyry. The wallrock alteration  developed in a sequence of the zones of Kfeldsparbiotite, biotitechlorite, to propylite from the orebody to the porphyry and then to basic volcanic wallrock, and the later veinlets of  linear Kfeldsparization  overprinted on the earlier pervasive Ksilicate alteration. There is no secondary sulfide enrichment, and the primary copper ores are of two natural types : the veinletdisseminated and the veinoverprinted, the former is characterized by the typical metallic mineral association of pyritechalcopyritemolybdenite, and the latter is overprinted on the former by quartzpyritechalcopyrite veins. As compared with the former, the latter has the higher grade (Cu to 2.21% and Au to 0.83 g/t), and lower total amount of the trace and rare  earth elements with more gentle  REE pattern and more evident positive Eu anomaly and flatter spider diagram of the trace elements. The basic volcanic eruption, the mantlesource magma intrusion and the superposition of multiphase  metallization   are the key factors for the Halasu copper metallization. The porphyry copper mineralizations of early period  were overprinted by the later period of structuralhydrothermal metallization. The Sisotopic compositions of associated pyritechalcopyrite in the veinletdisseminated type of copper ores indicate the 420560 ℃ of porphyry copper mineralization temperature. The copper ore sulfide δ34SVCDT is mainly from -1‰ to -4‰, and the ore sulphur was from the mantle source porphyry intrusion (with minor mixing of  reduced sulphur  from the sulfates in surrounding strata ). The pyrite in the copper ores shows that 206Pb/204Pb is from 18.052 to 18.461, 207Pb/204Pb from 15.501 to 15.606 and 208Pb/204Pb from 37.813 to 39.335, and these are close to that of CuNi sulfides in Kalatongke ultrabasic intrusion. The metal was mainly from the mantle source porphyry intrusion. The parent hydrothermal solution (δ18OVSMOW=6.4‰10.2‰，δDVSMOW=-89‰-80‰) from which crystallized the copperbearing sulfide quartz vein in the veinoverprinted type of copper ores shows the O and Hisotopic compositions of magma source water. The isotopic chronology research of the porphyry intrusion, the copper ore and the alteration suggest that (1) the UPb concordant ages of (381.6±2.5) Ma and (371.8±96) Ma by zircons in copperbearing altered  porphritic granite and granodiorite porphyry, respectively, and the ReOs isochron age of (376.9±2.2) Ma by molybdenites in the veinletdisseminated copper ores are all the porphyry intrusion and porphyry copper metallization records of the oceanic platecontinent subduction period; that (2) the UPb concordant age of (265.6±3.7) Ma by zircon in copperbearing altered quartz monzonite porphyry and the ArAr plateau age of (269.2±3.2) Ma by Kfeldspar in the veinoverprinted type of copper ores are porphyry copper metallization and alteration records of the continentcontinent collision period; that (3) the UPb concordant age of (215.8±4.6) Ma by zircon in the copperbearing altered quartz diorite porphyry and ArAr plateau age of (198.2±2.3)(206.4±2.7) Ma by Kfeldspar in veinoverprinted type of copper ores are the copper mineralization and related alteration records of intraplate  activization period. The Halasu copper metallization is characterized by multiphase  tectonicmagmatichydrothermaloverprinting. This  study provides some new facts for the understanding of the superposition porphyry copper metallization in the tectonic domain of Central Asia.

Key words: ore deposits, geochemistry, UPb dating, ReOs dating, ArAr dating, porphyry