The Ni-Cu Mineralization And Geodynamic Setting Of Neopaleozoic Mafic Intrusions In Kalatongke Area,Xinjiang,NW China

The Kalatongke Cu-Ni magmatic sulfide deposit occurs along the Irtysh fault zone in the northern margin of East Junggar structural belt,part of the Central Asian Orogenic Belt?CAOB?.At present,there are 13 mafic intrusions in the Kalatongke area and four of them contain significant sulfide mineralization.The total reserves of copper and nickel in the Kalatongke deposit is the first in xinjiang and the fourth in China.With the development of the metallogenic theory and the progress of exploration,there are still some scientific questions to be discussed,such as the forming relationships among the different intrusions,cause of sulfide saturation and sulfide enrichment process.In addition,the tectonic setting of the these mafic intrusions and magma sulfide deposits is highly debated and some researchers believe that the Tarim flood basalts and the Kalatongke intrusions are related to the same mantle plume.Therefore,discussing these scientific question is of great significance for the metallogenic theory of Cu-Ni magmatic sulfide deposit in CAOB and the regional mineral exploration.On the basis of previous studies,this paper has carried out an integrated study that is composed for three parts.In the first part,we use zircon SHRIMP U-Pb data,chemical compositions in silicate minerals,apatite Sr-Nd isotopic data,zircon Hf-O isotopic data and sulfide S isotopic data,combined with whole rock major and trace elements and Sr-Nd isotopic data to evaluate the triggers of sulfide saturation and enrichment of metal sulfide to better understand the origin of the Kalatongke deposit.In the second part,we suggest a geodynamics background model for Kalatongke mafic intrusions according to the regional geological data and geochemical characteristics.In the third part,we evaluate the mineralizing potential in deep part of Kalatongke deposit preliminarily.The most important ore bodies are hosted in the mafic units of the Y1,Y2,Y3 and G21mafic intrusions.In Y1 and Y2 intrusions,important sulfide mineralization occurs in the olivine norite and norite units,including massive and heavily-disseminated ores.In Y3 and Y9intrusions,sulfide mineralization occurs in the norite and gabbro units respectively,which are mainly disseminated ores.Recently,sulfide mineralization has also been found by drilling in a Y5 and G21 mafic intrusions,and some disseminated ores is present in the leucogabbro unit of the G21 intrusion.The chronological data of mafic intrusions in the Kalatongke area show that the mafic intrusions are not coeval whereas can be divided into two stages,Late Carboniferous and the Early Permian.The zircon SHRIMP U-Pb ages for the diorite and norite in the Y3 intrusion are²90 Ma and²83 Ma,respectively and the leucogabbro in the G21 intrusion is²81 Ma,which are also identical to the zircon U-Pb ages for the sulfide-bearing norite and gabbro units of the Y1 and Y9 intrusions.The zircon U-Pb ages for diorite and gabbro in the Y5 intrusion are³20 Ma,obviously earlier than the other intrusions.The olivine Fo contents,The average Mg^#of pyroxenes and the plagioclase An contents indicate that the parental magmas for the mafic intrusions are fractionated,and the Late Carboniferous is more fractionated than those for the Early Permian;The trace element patterns for the mafic rocks in all intrusions are similar,showing light REE enrichments relative to heavy REE and pronounced negative Nb anomalies.In addition to pronounced negative Nb-Ta anomalies,the gabbros and diorites of the Y5 intrusion are also characterized by mod-erate negative Zr-Hf anomalies.The different intruions have the different Sr-Nd-Hf-O data,and the Late Carboniferous intrusion has lower(⁸⁷Sr/⁸⁶Sr)_i and?¹⁸O_zircon and higher?Hf_?t?values.The Nd isotope compositions of apatite and whole rock samples from the Early Permian mafic intrusions in the Kalatongke area are similar to those of depleted mantle.Whole-rock and apatite Sr-Nd isotope data,zircon Hf-O isotope data from this study indicate that the parental magmas of Kalatongke intrusions all experienced up to 15 wt%crustal contamination.Our new data and the previous data together show that most of the?³⁴S values?-1.5^-3.1‰?for the Permian magmatic sulfide deposits in the Kalatongke district are lower than the MORB values,indicating addition of¹0 wt.%crustal sulfur to the parental magmas of the deposit.According to the modeling results about fractional crystallization and crustal contamination,we suggest that contamination siliceous crust and assimilation crustal surfur can increase the silicate-sulfide cotectic ratio and play a key role in triggering sulfide saturation in the parental magmas of Kalatongke deposit.The modeling results show that the variations of PGE tenors in the Kalatongke magmatic sulfide deposit can be explained by a common parental magma with 0.02 ppb Ir and 0.6 ppb Pt initially and variable R factors from 200 to 1600.The high R factor contribute to the enrichment of Ni and Cu.The initial Ir-Pd contents in the parental magmas for the Kalatongke deposit estimated by us are one order of magnitude lower than the values in the PGE-undepleted picrites associated with continental flood basalts.The reason of the PGE depletions in the parental magmas are due to previous sulfide segregation during magma storage or ascent before their emplacement in the Kalatongke area.The characteristics of the sulfide mineralization in the Kalatongke Y2 and Y3 intrusions can be explained by accumulation of immiscible sulfide droplets by gravitational settling,but the accumulation of sulfide in Y1 and Y9 intrusions are related with flow differentiation.New and existing geochronological and geochemical data do not support a mantle plume model for the protracted Paleozoic mafic intrusions in the Kalatongke area.An alternative explanation is that these mafic intrusions are the products of mafic magmatism in a convergent tectonic zone.The Y5 mafic intrusion is interpreted to be subduction-related,with the primary magma produced by flux melting at the base of the mantle wedge.This maga have similar characteristics with island arc calcic alkaline basaltic magma,such as significance of negative Nb-Ta-Zr-Hf anomalies.The younger mafic intrusions in the Kalatongke area are interpreted to have formed in the second stage,with the primary magma generated by decompress melting of the upwelling asthenosphere due to delamination of the overlying lithosphere.In this model,the arc geochemical signatures in the post-subduction mafic intrusions in the Kalatongke area are attributed to a source mantle that was metasomatized previously by slab-derived,Nb-Ta depleted fluids associated with the previous subduction events.Compared with Early Permian mafic intruions in Kalatongke area,the parental magma of Carboniferous intrusion is highly fractionated liquids.The simulated calculation of Sr-Nd-Hf isotopic data indicated significantly lower degrees of crustal contamination in the magma so that it may be not reach the sulfide saturation.For these reasons and more,we propose that the Y5 mafic intrusion may not have metallogenetic potentiality;Geological,geophysical data,and mineralization suggest that the Early Permian intrusions still have exploration potential at depth.In addition,the Ni exploration in the region should focus on the olivine-bearing intrusions that were emplaced in the Early Permian,in which the parental magma of this intrusions have experienced crustal contamination and added crustal sulfur.