Mineralization Characteristics Of Yanshanian Granitic Small Intrusions And Batholiths In South Hunan

South Hunan region has experienced a long-term development of multi-cycle struture,and there occurred a massive magmatism during the time period from Middle to Late Jurassic,when formed a large number of granitic intrusions with varying sizes,including batholiths and small intrusions,as well as a series of polymetallic deposits.Despite a huge amount of research has been done on the mineralization characteristics of granitic intrusions in this study area,there are rare research which focuses on the different mineralization between granitic batholiths and small intrusions,which could help identify ore bearing intrusions effectively and would be favorable for further prospecting in South Hunan Province.This paper takes 5 batholiths(Jiufeng,Dayishan,Dadongshan,Jiuyishan and Qitianling)and 5 small intrusions(Xianghualing,Wangxianling,Yaogangxian,Huangshaping and Shangbao)in South Hunan as research objects,on the basis of previous research done by other reserachers,and several months' field work and experimental analysis,I have completed the comparison between batholiths and small intrusions on petrology,mineralogy,geochemistry and mineralization characteristics,and summarized several conclusions below:1.Lithology of batholiths are mainly biotite granites and two-mica granites,with a small amount of amphiboles in the early granites(e.g.Qitianling or Shaziling)and possibly little muscovite(e.g.Pangxiemu)in the late granitic rocks,there also might be some lepidolite(e.g.Dayishan).Accessory minerals are various,including zircon,apatite,fluorite,sphere,magnetite etc.On the other hand,granites of small intrusions have different minerals,such as lithium mica or iron lithium mica.And there are a wide range of accessory minerals,including different rare metal or rare earth minerals.2.Compared with ore-bearing batholith,small intrusions are rich in silicon,manganese,alkali and heavy rare earth elements,while have low content of aluminum,magnesium and titanium,this means that small intrusions have experienced higher degrees of fraction crystallization.3.Ore-bearing intrusions are mainly composite rock bodies with petrogenic time between 150170Ma(apart from supplemental body of Dayishan batholith,which were emplaced during during late Yanshaninan period);small intrusions were emplaced during the similar period,but the time difference between two emplacing stages sometimes are longer,for example,early stage of Wangxiangling intrusion were emplaced at Indo-Chinese epoch,which was connected with tin mineralization,while latest stage of the intrusion was formed during Yanshanian period without any mineralization.4.Most ore-bearing batholiths were related with tungsten-tin deposits while mineralization of small intrusions was more complex,including tungsten-tin-lead-zinc,as well as rare metal deposits.5.There are several discrimination criteria of ore-bearing Yanshanian intrusions in South Hunan Province:the first one is denudation degree of intrusions.By studying the elevation of the top of the intrusion or concealed rock bodies is able to help distinguish possible orebody concealed space and denudation degree of orebodies.The second one is petrological and mineralogical features.The appearance of amphibole indicates that granites is not in favour of mineralization,while the emergence of iron lepidolite shows that these granites are beneficial to the formation of tin deposits.In most tungsten-tin deposits,original biotite has experienced different degrees of alteration with the formation of fluorite;the accessory minerals of granites which are related with tungsten-tin deposits are mainly garnet,zircon-garnet or zircon-monazite;while the accessory minerals of granites that are related to lead-zinc normally are zircon-apatite,zircon-sphene,magnetite-apatite etc.The third one is geochemical features.Tungsten-tin intrusions usually have higher content of SiO2(around 75%),and the percentage of K2O+Na2O is normally 5?9%,while the figure for MgO and TiO2 are lower(?0.4%,0.3%respectively)and the content of CaO is usually 0.3?1.0%.In addition,the heavy rare earth element content of late stage of intrusion,which has higher degrees of fraction crystallization,is higher than that of early stage of the same intrusion.The forth one is isotope chronology.Most ore-bearing intrusions were emplaced during the time period from Middle to Late Jurassic.And these intrusions normally are composite rock bodies with banded tungsten-tin-lead-zinc mineralization in or around some parts of the rock mass;intrusions emplaced at the late stage are more beneficial to the formation of deposits since these granites normally have experienced higher degrees of fraction crystallization.6.Detailed comparisons between Qitianling and Xianghualing intrusions shows that the lithology of Qitianling batholith are mainly(amphibole)biotite monzonite granites,while rocks of Xianghualing intrusion are usually lepidolite granites,muscovite granites and iron lepidolite granites;Xianghualing granites have higher content of silicon,alkali,fluorine,and heavy rare earth elements.In addition,it would be beneficial to focus on comprehensive evaluation of rare metals on eastern part of Qitianling batholith,especially Changchengling area,and tin mineralization is possible to be found on the western part of Qitianling,especially in someplace where small intrusion emplaced.