Geological, Geochemical Characteristcs And Prediction Of Qingmingshan Cu-ni Sulfide Deposit In Horthern Guangxi, China

Sipu group is the oldest Proterozoic stratum which exposed in Sibao region in north Guangxi, China, have large quantity of mafic volcanic rocks, mafic intrusive rocks, intermediate-acid intrusive rocks,is the important component of Cu-Ni-PGE and Sn, Cu, Pn, Zn, Sb, W, U, Au metallogenic belts in northern Guangxi province, and also one of the great potential polymetallic prospective zones in Southern China. This paper is based on the detailed research on regional geological background and its evolution, the comprehensive geological, geophysical, geochemical, survey and mining data of deposits(mineral points) in the research area. According to the guidance of regional metallogeny and volcanic mineralization theory, and to the main line of structure-magmatism-metallogenesis, it's the first time for us to do in-depth analysis and system research on typical deposit-Qingmingshan copper-nickel sulfide deposit, representative mafic intrusive rocks, mafic volcanic rocks, and Sn-Cu-Pb-Zn etc. deposits compatibly. We achieve the following main results and understanding:(1) Sibao region located in southeast margin of the Yangtze platform in tectonic setting, the early Mesoproterozoic, the happening tectonic movement, which ancient Huana oceanic crust nose down below the Yangtze platform, induced the forming of the basin behind island-arc, received a large number of hemipelagic sea-deep sea clastic deposition (Sibao Group); magmatic diapir and intruding which partial melting of subducted oceanic crust formed large-scale polycyclic mafic volcanic rocks; in the period of late Mesoproterozoic to early Neoproterozoic, accompanied by the further evolution of rift basins and volcanic-sedimentary thickening, mafic magmatism mainly changed as magmatic intrusion; in the same time, the differentiation and evolution of mafic magma, crustal contamination and partial melting formed a granodiorite; the late Neoproterozoic, with the plate convergence movement, the Movement of arc basin rift over, folds and deep magmatic orogenic thermal erosion, melting of continental crust formed a large number of massive biotite granite. (2) Sipu group are hemipelagic sea-deep sea clastic deposition with characteristics of turbidite, which consist of sandstone, arkose lithic sandstone, mudstone, and mafic volcanic rocks, which the feature of the composition of petrochemistry is rich in pro-iron elements, shch as Mn, Cu, Ni, Co, Cr, V, Pb, Zn. Danzhou group are transition facies-marine facies deposition, which consist of conglomerate, sandstone lithic sandstone and carbonatite. Which is enrichment in Zn, Ba and other acidic elements. There are widely distribution of Cu-Ni-PGE series of deposits, Sn, Cu, Pn, Zn, Sb, W, U, Au series of deposits in Sipu groups.(3) Mafic Intrusive rocks and volcanic rocks interdepended and associated with each other in spatial and temporal distribution, many characteristic of which include chemical composition, the feature and pattern of trace elements and rare earth elements, and the tectonic discrimination result are same, such as which is poor in TiO2 and K2O, rich in MgO, belong to tholeiite series; Trace Elements richen in large ionic radius elements Rb, Cr, Ba and Th, poor in Sr, depleted in high field strength elements Ta, Nb, P and Ti. Light rare earth slightly enriched, HREE loss, weak or no dEu aberrant, lean right in REE distribution pattern; Th/Nb ratio both are much higher than MORB and OIB rocks, Th/Nb ratio, and also higher than Th/Nb ratio of Yangtze block in the region; Nb/Ta and Nb/U ratio all much lower than normal ridge basalt and chondrite values; have the feature of higher 87Sr/86Sr ratio(> 0.7033), lower 143Nd/144Nd (<0.5130) and negative eNd(t)(<0), what mentioned above indicate that magam originated from partly melted mantle(EMI) and mixed by crust materials in continental margin near island arc, belong to the same source magma and formed in dissimilar time.(4) Diorite granites and biotite granites both are enriched on aluminum in the composition of petrochemistry, and trace elements enrichen in K, Rb and Th, loss of Ti, LREE enrichment, dEu significant losses, lean right in REE distribution pattern; Diorite granite is a mixed-source-type granite, and biotite granite is a remelting-type granite.(5) Sipu group formed in early-middle Proterozoic, about 1190-1800 Ma; Danzhou group formed in the 822-890 Ma; Proterozoic volcanic rocks formed in the mid-late Proterozoic, about 1090-1734Ma; mafic intrusive rocks formed in roughly late, about 982-1088 Ma, diorite granite formation age is about 820-1064 Ma, biotite granite formed in the 820-898 Ma. Qingmingshan Cu-Ni deposit formation age is about 1087±56Ma.(6) Qingmingshan's mafic intrusive rocks contained copper-nickel sulfide occurred in the form of dike, with integrated magmatic differentiation, from bottom to top of the mafic intrusive rocks lithology may divided into pyroxenite-gabbro-diabase-diorite etc.; The chemical feature of mafic-ultramafic intrusive rocks?(Si02) is 42.68?45.84%,?(Al2O3) is 6.62?10.58%, co(MgO) is 17.87?25.12%,?(TiO2) is 0.33?0.52%,?(K20) is 0.05?1.19%. Trace elements rich in large ionic radius elements Rb, Depleted in high field strength elements Ta and Nb; REE rare earth elements feature SREE content is 40.95?63.7×10-6, SLREE/SHREE is 1.82?1.93, LaN/YbN is 2.92?3.2, dEu is 0.56?1.13; light rare earth is relatively slightly enriched, HREE loss, weak or no dEu aberrant, what mentioned above indicate that Qingmingshan's mafic intrusive rocks have arc-magma geochemical characteristics.(7) Qingmingshan copper-nickel sulfide deposits distributed the bottom of mafic-ultramafic intrusive rocks, controlled by pyroxenite lithofacies. Ore body is east-west direction, bedding output, tendency south, dip angle is 28°?52°, thickness is 20?250 m, length is 1.0?3.5 Km. From the surface to down, ore-bearing rock currently controlled by drills etc. gradually become larger in aspects of thickness, and corresponding to become rich on grade of copper-nickel ore. Qingmingshan copper-nickel sulfide deposit is divided into north and south mining area. In south mining area Qingmingshan copper-nickel sulfide orebody extend more than 1500m, thickness is 0.81?3.78m, Ni grade 0.31%?0.49%, Cu grade 0.14%?0.27%, Co 0.008%?0.016%; In north mining area Macaoshan copper-nickel orebody's surface is mostly covered by quaternary landform. Ore controlled extend more than 500 m, thickness is 0.8?3.5m, Ni grade 0.20%?0.30%, Cu grade 0.10%?0.30%. There are four mainly types of ores:disseminated ore, taxiticstructure ore, massive ore and ore vein; most of the metal minerals are pyrrhotite, pentlandite, chalcopyrite, and so on. In Copper-nickel ore d34S is-0.5%o?+3.60%o, in mafic rock d34S is 6.3%o-+24.9%o. Compareing and analyzing with the geological and geochemical on Duluth Complex Cu-Ni deposits, show that Qingmingshan copper-nickel sulfide deposit is similar to Duluth Complex on mentioned above all, have many dfferent characters contrasting with Jingchuan Cu-Ni deposits; all these are arosed by different about tectonic settint.(8) The formation of Cu-Ni sulfide deposits on mafic intrusive rocks in Qingmingshan area are controlled by magmatic immiscibility, then crystallization differentiation, the central factor begeting magmatic immiscibility and saturation of S are magmatic mixed by sulfur-rich volcanic-sedimentary rocks, then rapid decline in magmatic temperature and fractional crystallization.(9) There are two series of deposits in Qingmingshan area, Cu-Ni-PGE series formed in stage of the nosing down of ocean crust and rift valley forming of deposition-basin, formed in period between Mesoproterozoic Era and Newproterozoic Era, fomed by liquid immiscibility and differentiation from melting mafic intrusive rocks, distributed in mafic-ultra mafic rocks bottom. Sn, Cu, Pb, Zn, Sb, W, U, Au mineralization series formed in stage of deposition-basin folding, orogeny and granitizing, formed in middle-late phase of Newproterozoic Era, related to location, evolution and hydrothermal circulation of biotite granite in genesis, distributed in the peaks and fault near granite. Analyse on the cause of ore-forming of the two metallogenic series, show that abundant component of metallogenic series in Sipu group stratum is basic, the evolution on plate tectonics and tectonics is main dynamic cause, the mafic-ultramafic rock formation, segregation, differentiation and positioning, evolution and hydrothermal circulation of biotite granite is the key. On this basis, established the Qingmingshan's metallogenic model of copper-nickel deposits in genesis.(10) The major controlled ore-forming factors are:the tectonic setting-sea basin behind island arc, secondary structures-fold near Sipu fault (Qingmingshan double folds), lithofacies of ultramafic intrusive rocks, lithologic interface between intrusive rocks and country rocks, metamorphosed volcanic-sedimentary rocks in Sipu group. The direct and effective clues summarized for prespecting are secondary structures-fold near regional deeper and bigger fault, the bette-differentiation ultramafic rock, pyroxenite lithofacies, mineralized outcrops, integrated geophysical and geochemical anomalies.(11) Comprehensive geological, geophysical, geochemical and geological data deposits, indicated that there is better and bigger potential for prospecting on big-type copper-nickel sulfide deposits, middle-small type granite-related tin-polymetallic deposits. The major direction of prospecting are:to explore the the large-scale massive copper-nickel sulfide ore bodies distributed in bedrock and channel of mafic-ultramafic intrusive rocks. All analysis mentioned above suggest 3 prospecting target in Qingmingshan area:No.?geophysical anomaly forecast district near southeastern Qingmingsahn mining area, No.?geophysical anomaly forecast district near eastern Macaoshan mining area, and No.?geophysical anomaly forecast district near western Qingmingsahn mining area.