The Mineralogical Characteristics And Metallogenic Process Of Middle-low Grade Metamorphic Rutile Deposits In The Eastern Qinling Orogenic Belt

Titanium is one of the critical metals and is called the"third metal"charactered by its unique function.Metamorphic rocks-type rutile deposits are widely distributed in high-grade metamorphic rocks ranging from eclogite to granulite facies in the world.Recently rutile deposits are also discovered in middle-low grade metamorphic rocks.This article chose the East Qinling orogenic belt Fangcheng Baishugang-Wujianfang rutile deposit and the Xixia Bamiao-Qingshan rutile deposit as the research objects.Based on field investigations and petrographic studies,we used in-situ LA-ICP-MS U-Pb datings of rutile and zircon,and electron probe analysis of rutile and gangue minerals to better understand the characteristics of middle-low grade metamorphism and mineralization.The main results and implications are as follows:(1)The Fangcheng Baishugang-Wujianfang rutile deposit occurs in the North Qinling Kuanping Group,and the Xixia Bamiao-Qingshan rutile deposit occurs in the South Qinling Liuling Group.The degree of metamorphism of the ore-hosting strata is amphibolite facies.Rutile is generally granular or long columnar,with high contents of Ti O₂(97%?99%),and its color is generally orange or yellow-brown.It mainly occurs in the interstitial spaces of biotite,hornblende quartz and wrapped in the crystals of the above minerals.The results of electron probe analysis showed that the biotite,hornblende and titanomagnetite in the two deposits all had high Ti O₂ contents,indicating that the Ti originated from the above-mentioned minerals.(2)The trace element analysis results of the rutile minerals in the two deposits show that the contents of Nb(>800 ppm)is higher than the Cr,indicating that rutile was formed in metapelitic rocks rather than metamafic rocks.Zr-in-rutile thermometer of the two deposits ranged from580°C to 630°C,indicating that these deposits formed in amphibolite facies with the estimated depth is 25 km.It is different from other rutile deposits formed by high-grade metamorphism in the Sulu-Dabie area.(3)Previous studies suggested that the rutile deposits in the East Qinling orogenic belt were formed in the Early-Middle Devonian.In this study,by using in-situ LA-ICP-MS rutile U-Pb dating,the ages of the Fangcheng Baishugang-Wujianfang and the Xixia Bamiao-Qingshan rutile deposits are 386±16 Ma and 260.8±7.6 Ma,respectively.As a result,this study suggests that a new perspective of multi-stage metamorphism and mineralization related to rutile in this area.(4)On the basis of geochronology of the deposits and the tectonic evolutions,the two rutile deposits related to middle-low grade metamorphic rocks,are the response of regional strata to tectonic movement.In the Early-Middle Devonian,the Shangdan Ocean was gradually closed due to northward subduction,subsequent to arc-continent collision occurs.Under the huge stress effects,the Kuanping Group were metamorphosed,and the biotite and other Ti-rich minerals released a large amount of Titanium,resulting in the formation of Fangcheng Baishugang-Wujianfang rutile deposit.After the Shangdan Ocean was completely closed,the South Qinling terrane began to subduct below the North Qinling terrane subsequent to a continent-continent collision.In the Late Permian,the continuous collisions of the South-North Qinling terranes caused the Liuling Group to deform and metamorphose strongly in the foreland basin.Silicate minerals dominated by biotite released a large amount of Titanium,which contributed to form the Xixia Bamiao-Qingshan rutile deposit.