西藏甲玛铜多金属矿床矽卡岩形成及矿化过程中的元素地球化学行为

Geochemical Behavior of Elements During Formation and Mineralization Processes of Skarn in the Jiama Copper Polymetallic Deposit of Tibet

西藏甲玛铜多金属矿床是冈底斯成矿带中东段近年来取得重大找矿突破的超大型斑岩-矽卡岩型矿床。矽卡岩型矿体作为该矿床最主要的矿体类型,前人对其在矿体模型、矽卡岩分带、矽卡岩矿物学等方面做了大量的研究工作;但对于不同类型矽卡岩的形成及矿化过程中元素的地球化学行为缺乏系统的研究工作。文章一方面对弱蚀变大理岩、石榴子石为主无矿矽卡岩(石榴子石含量大于80%)、硅灰石为主无矿矽卡岩(硅灰石含量大于80%)、石榴子石为主含矿矽卡岩和硅灰石为主含矿矽卡岩分门别类进行了研究,认为矽卡岩的形成过程综合了大理岩与花岗岩类的成分特征,而矽卡岩的矿化过程是岩浆热液的叠加过程,促进了元素的进一步富集与贫化;另一方面对矽卡岩形成与矿化过程中元素的迁移情况进行了定量讨论,矽卡岩形成过程中主量元素Na2O含量变化不大,CaO含量有少量减少,微量元素中Tl、Ba含量基本保持不变,Sr含量减少约10%,其它组分均为增加,增加量:TiO2>P2O5>TFe>MnO>Al2O3>K2O>SiO2>MgO;Be>Bi>Th>Ga>U>V>Cu>Li>Au>Sc>Cr>Co>Zn>Ni>Pb;In>W>Sn>Ta>Hf>Nb>Mo>Sb>Y>∑REE>Zr>As>Cd>Cs>Rb;而矽卡岩矿化过程中,Na2O表现出活动性,Cd与Bi都为增加较多的组分,增加量超过40%,K2O、In增加量也超过20%;相对减少的组分是P2O5、MnO、MgO、∑REE,减少量大于10%。除此之外,TFe、Sn、Rb、Sr、Sb、V表现出少量增加,增加量小于10%;TiO2、Ge、Y、CaO有少量减少,减少量小于5%;Al2O3、Ga、Cr、SiO2在矽卡岩矿化阶段基本保持不变。

The Jiama copper-polymetallic deposit is a super-large-scale porphyry-skarn-type deposit located at the eastern Gangdise metallogenic belt, in which a major prospecting breakthrough has been made recently. The skarn body is the main ore body type of the deposit. Previous workers sors have done a lot of researches,including models of ore bodies,zonation and mineralogy of skarn and other aspects. However,on the formation of different types of skarn and the geochemical behavior during mineralization processes,there is no systematic research yet. This article firstly analyzes the weakly altered marble,garnet dominant barren skarn( content of garnet more than 80%),wollastonite dominant barren skarn( content of wollastonite over 80%),garnet dominant ore-bearing skarn and wollastonite dominant ore-bearing skarn separately. The results show that the formation process of skarn may have the characteristics of incorporating the constituents of marble and granites,while the mineralization process of skarn may be the superposition process of magmatic fluid,promoting the further enrichment and depletion of the elements. Then,element migration in the formation and mineralization processes of skarn have been quantitatively discussed. It is suggested that in the formation process of skarn,the content of the main element Na2O changed little,CaO with a small reduction; the trace elements Tl and Ba remained basically unchanged; Sr decreased by approximately 10%; other components increased with increasing amounts in the order: TiO2> P2O5> TFe > MnO > Al2O3> K2O > SiO2> MgO; Be > Bi > Th > Ga > U > V > Cu > Li > Au > Sc > Cr > Co > Zn > Ni > Pb; In > W > Sn > Ta > Hf > Nb > Mo > Sb > Y > ΣREE > Zr > As > Cd > Cs > Rb. In the mineralization process of skarn,Na2O showed activity,Cd and Bi were added many other components,increasing amount of which is more than 40%,the increasing amount of K2O more than 20%; relatively reduced components are P2O5,MnO,MgO,ΣREE,decreasing by more than 10%. Besides,TFe,Sn,Rb,Sr,Sb and V increased by a small quantity,less than 10%; TiO2,Ge,Y and CaO presented a slight decrease,less than 5%; Al2O3,Ga,Cr and SiO2essentially unchanged in the skarn mineralization stage.