四川拉拉铜矿床赋矿层位变质沉积岩地球化学特征

拉拉铜矿区赋矿层位变质沉积岩的主量元素(ICV(av)=3.04,K2O/Al2O3(av)=0.22,CIA(av)=53.1)及Cs、Rb显著亏损特征,表明其物源区岩石只含有较少的粘土矿物和碱性长石,为中—低化学风化程度,且处于较不稳定的构造环境;多数样品的微量元素表现为显著富集高场强元素、亏损过渡族元素及Cr/Zr(av)=0.15、Th/Sc(av)=0.58,且具轻稀土富集、重稀土平坦((La/Yb)n(av)=25.7)的稀土元素配分模式,表明物源区以长英质组分为主;Sm、Nd同位素组成特征也显示其物源区以长英质组分为主,且沉积于大陆岛弧的弧后拉张环境。同时,结合矿区赋矿层位变钠质火山岩的构造判别结果,认为拉拉铜矿床也形成于该构造背景下。

Geochemical characteristics of the ore-bearing strata metasedimentary host rocks in the Lala copper deposit,Sichuan Province

Geochemical study of metasedimentary rocks in the Lala copper deposit was conducted to better constrain their provenance,chemical weathering condition and tectonic environment. These rocks have relatively low CIA values (44-65) and K2O/Al2O3 values (0.02-0.43) with high ICV values (1.44-5.11),suggesting that they underwent relatively weak chemical weathering and were derived from an immature source with minor clay minerals and alkali feldspar. Compared to PAAS,the samples mostly have relatively lower large ion lithophile element (LILE) contents (Cs,Rb),lower transitional element contents (V,Cr,Zn) and relatively higher high field strength elements (HFSE) contents (Y,U,Zr,Hf,Nb,Sc). The right-slope REE ((La/Yb)n(av)=25.7) pattern,low Cr/Zr (0.01-0.42) and Th/Sc (0.37-1.14) ratios,and Sm and Nd isotopic characteristics,indicate that these rocks were deposited in a continental back arc environment and derived from a felsic-dominated provenance. Furthermore,in combination with the results of tectonic setting discrimination of sodic metavolcanic rocks,we conclude that the Lala copper deposit may also have been formed in a continental back arc environment.