Metallogenic Process And Exploration Of Magmatic-association Metallic Sufide Deposits Of Qilian Mountain, Northwest China

Qilian mountains is the most important metallogenic zone of Early Palaeozoic marine volcanic-association massive sufide (VHMS) copper-multimetallic deposits in China, and Longshou mountains also zone of Proterozoic magmatic nickel-copper sulfide (platinum group metal) deposits. In this paper, the both metallogenic zone, in the eyes of the metallogenic development, are identified as a wide-sense Qilian mountains with close association to be researched, but also the Proterozoic and Early Palaeozoic metallizations are took as the important study targets. The metallogenic area of Proterozoic magmatic Ni-Cu sulfide (PGM) deposits includes all continental blocks and micro-continental blocks with the pro-Changcheng Period (early period of Middle Proterozoic) basements, and the metallizations of VHMS copper-multimetallic deposits only found in the Early Palaeozoic, locally the end period of Neo-Proterozoic, marine volcanic area of the original Qilian mountains.Jinchuan world-class supper large type mineral deposit is a typical representative for the metallizations of Proterozoic magmatic Ni-Cu sulfide (PGM) deposits in the wide-sense Qilian mountains, of which rock-mineral Sm-Nd isochron dating is 1508 ±31Ma, and Sm-Nd isochron dating 1511 ± 67Ma, of Zangbutai non-mineralize metamorphic-ultramafic rocks with komatiite features located on the western side of Jinchuan intrusion. In addition, the continental flood basalt (CFB) had been recognized in Zhulongguan group of the Jingtieshan micro-continental block in the western part of North Qilian, that their ages confirmed in Proterozoic era although the dada of different isotopic age determinations had a wide period, and in the Hualong micro-continental block of South Qilian Lashuixia small type Ni-Cu sulfide (PGM) deposits which nearly whole mineralizations in the intrusion and other neighboring mafic-ultramafic intrusions also formed undoubtedly in Proterozoic era in spite of no certain age data in present. The paper suggests that these Proterozoic CFB, komatiite, intruded mafic-ultramafic rock bodies and magmatic Ni-Cu sulfide (PGM) mineral bodies location on the basements of different continental blocks and micro-continental blocks were resulted from large igneous provinces(LIP), which created by large scale magmatism to respond to cracking of Columbia super-continent, successively extension of the Qilian paleocontinent about 1.5Ga. As a result of process between rising mantle plume origin in D layer of core-mantle boundary and base of lithospheric mantle to generate parting melt that yielded a large scale magmas source from much more materials, the Qilian ancient LIP was finally formed by the rising eruption rocks (CFB and komatiite/picrite), the eruption canals (basic dyke swarms are exemplified by large volumes of close distributions of gabbro sills location on Longshou mountains micro-continentalblock) and the intruded layer intrusions and the magmatic Ni-Cu sulfide (PGE) ore bodies(zonal mafic-ultramafic rock body groups and local high-mineralize rock bodies).Comparison with most world-known LIPs location on stable craton, Qilian's LIP is different, because after the LIP formation Qilian palaeo-continent blocks had been disintegrated during Palaeozoic era and become a orogenic belt expanding in the western part of China. So, to restore Qilian's palaeo-LIP is relatively difficult and more detailed researchs will be required to do. In fact, more and more evidences show that most large scale magmatic Cu-Ni sulfide (PGE) deposits in world are understood as a result of processing of mantle plume which is divided into both the mantle plume origin in the core-mantle boundary and the one regeneration in the asthenosphere mentale, the former may formation large scale nickel and copper mineral deposits and the latter very difficult generation to assemble large volume of nickel and copper metals because of formation in post-orogenic phase.Geological and Geochemical characteristics of Jinchuan mineral deposit show that chemical compositions of the whole rocks belong to lherzolite, mineralizes of the whole rock body high into sixty percent and Lashuixia more high to ninety percent, and such high assembling of metallic sulfides is no as a result of magmatic liquation in suit and then should produce certainly from injection of metallic sulfide-bearing magma and/or metallic sulfide liquid coming from immiscible phase in bigger magma chamber in depth. Jinchuan rock body, e Nd(t) =-1.9 ~ -4.3, ( La/Yb ) N=5.39 ~ 79.15, averagel5.04, (87Sr/86Sr)I=0.702547~0.711761, e Sr (t) >0, even high into +128.8, explains that mineralize magma was subjected to the contamination of crust or indicated as the information of the origin of lower mantle, and Zangbutai non-mineralize ultramafic rocks, e Nd(t) =+2.6 — +2.9, show that its sulfur was unsaturated owing to no joining in crustal substances. According to remarkable difference between the western segment,2REE 3.39—23.57X 10"6, average 14.12X10"6, (La/Yb) N 5.39—14.01, average 7.50, PGE enrichment, and the eastern segment of Jinchuan rock body, SREE 15.25—55.87 X 10"6, average 30.64X 10"6, (La/Yb) N 7.02—79.15, average 23.52, the quantities of PGE lower, it is realized that there were several intrusion and injection of magmas at least two end of magmas. Remarkable depleted nickle and cobalt in early olivine and chromite accumulations indicate that immiscible of sulfide liquation occur in early stage, that is taking place liquation in the deep-seated chamber. These characteristics of examples illustrate that in the Qilian ancient continental blocks owing to the processing of Proterozoic mantle plume the large igneous provinces are produced, and then the ore-bearing intrusions are formed at the high-level of sulfide-bearing magma chambers, from which basic melt of partial melting form and raise to the deep-seat of crust because of interaction between the heads of mantle plume and decompression of the lithosphere at depth form, and sulfur occur oversaturation and immiscible sulfide liquation occur in magmas when crustal substances contamination. The deep-seat of chambers not only have several ones but also continuous feeder magmas from different compositions, and the magmas intrusion and injection are also several stages from the deep-seat of magma chambers into the high-level of sulfide-bearing magma chambers. These realizations supply the determined symbols to look for prospecting targets for Jinchuan type mineral deposit, and suggest that the Lajishan region in the Hualong micro-continental block of South Qilian should be an important prospecting target.In Qilian mountains the Palaeozoic VHMS deposits can be divided into both continental crust environment including island arc and oceanic crust environment including oceanic basin and regeneration oceanic basin in back-arc basin types. The former is representative of Baiyinchang ore deposit, which mainly show that there are large volume of intermediate-acid volcanic rocks, as a whole enriched lead from continental crust in mineralize elements, and in Qilian mountains the prospecting objective of Palaeozoic marine volcanic-association massive sulfide deposits is main this type; the latter because of thrust and subduction. of Qilian ancient oceanic basin to conserve ore deposits is difficult even ifformation ores, only preservation of ore deposits formed in oceanic crust environment of back-arc basin may been done. At present found Shijuligou is representative of the latter, which it is characterized in large volume of basic volcanic rocks as the part of ophiolite, deleted lead and enriched copper, locally zinc high contents, in mineralize elements. Owing to prospecting potential of ophiolite-association ancient VHMS deposits in the world is not distinct, which only in one fourth ophiolites mineralize were found, in China reported affirmatively ophiolite-association VHMS deposits only in North Qilian mountains, Shijuligou typical representative, the research and exploration of Shijuligou ore deposit is very important. Being based on the researchs of metallogenic setting of North Qilian's VHMS deposits, this paper demonstrates in sedimentology that North Qilian's Continental rift occurs at latter stage of Neo-Proterozoic era due to Roidian supper Continent break up, and suggests that Ordovician subduction and convergent fashion of North Qilian oceanic basin to the north keep a structural transomation, which the middle-east part was a continental margin of trench-arc-basin system to relate to ocean-ocean collision, but the west part transform to an active continental margin to relate to ocean-continent collision. So, the more the oceanic crust subductes to the west, the more subduction and convergent location is to the continent, and Shijuligou region location on the middle-west part appears a splitiing of island arc-oneself which then develops from island rift to back-arc basin with more island matters in its volcanic rocks. Nevertheless, owing to the mineralization is decided by magma fluid of post-volcanic action, the magma fluid of oceanic crust of back-arc basin applies mineralize metallic assemblage of characteristic oceanic crust type in hydrothermal mineralization.In Shijuligou mineral district, the basalt , TiO2 contents 1.30'— 1.21 uB%, jaspilite of close leaning against copper ore body 8 30Si -0.1%c~-0.9%e, approaches oceanic basalt; § 34S value of pyrite and chalcopyrite changes from +4.95%o to +8.88%o, and explains sulfur mixing of sea-water; quartz in ore, 8 180=+ll.l%o~+18.6%o, approaches magma fluid of boundary of convergent plate (+2.7-+8%o) . Above characteristics explain that magma fluid is main supply of ore-forming elements in hydrothermal mineralization system of primary fracture in control of the volcanic apparatus.In this paper, as a setting of exploration of solid metallic mineral resources in the whole Qilian mountains, a exploration recognition of three steps had been suggested, that is opening up new region, screening target with the measure of area of geophysical and geochemical prospecting and deep exploration in important area to look for targets of large or/and supper-large mineral deposits. It is specially emphasized that to open up new region of exploration must depend on the fundamental research, and confirming and application of important indicators had been profoundly researched to look for in Qilian mountains the exploration targets of the magmatic Cu-Ni-PGE sulfide deposits in terms of the project practice on the screening target of the Hualong zone of South Qilian. In addition, suitability of exploration thought and method assemblage in ophiolite-association VHMS deposits had been probed to be based on the exploration of Shijuligou ore deposit in Qilian mountains.