川西甲基卡锂矿床伟晶岩结构分带及多期岩浆成矿作用

Textural zonation of pegmatites and multiple-phase magmatic mineralization in the Jiajika lithium deposit in western Sichuan

LCT(lithium-cesium-tantalum)伟晶岩的分带有两种形式,一种是单个伟晶岩脉的内部结构分带,这种伟晶岩脉通常体积巨大,内部结构显示出成分分带的特征,单一脉体中不同结构带中可能发育不同的稀有金属矿化带,例如新疆可可托海3号伟晶岩脉;另一种分带性表现为伟晶岩和成矿系统的区域分带,通常以高分异的过铝质花岗岩为中心,围绕花岗岩数以百计的伟晶岩脉显示出不同的伟晶岩类型和矿化类型,越向外围,伟晶岩的演化程度越高。区域分带内的伟晶岩单个脉体的规模不大、内部分带性差、含矿性各不相同,甲基卡锂矿就是这一类伟晶岩矿床的典型代表。对甲基卡矿区伟晶岩的野外观察发现,含Be的9号脉、含Li的133号脉和含Nb、Ta的528号脉具有相对较好的内部结构分带性,而含Li的104号脉和308号脉的内部分带性则相对较差,推测与脉体内部多期岩浆-热液活动有关。甲基卡矿区伟晶岩脉的相互穿插现象说明,矿区存在多期次的岩浆与成矿作用。科学钻探在JSD-2和JSD-3两个钻孔中均发现含锂辉石的伟晶岩脉和细晶岩脉侵入在不含矿的马颈子花岗岩中,佐证了多期岩浆与成矿事件的存在。伟晶岩中不同矿物U-Pb定年结果表明,全区伟晶岩的形成至少有两期,分别发生在213~206 Ma和199~191 Ma。依据钻孔JSD-1的研究表明,岩浆由深至浅逐步向高分异方向演化。从下向上,花岗岩与伟晶岩同步演化,花岗岩依次出现黑云母花岗岩、含白云母的黑云母花岗岩、二云母花岗岩、白云母花岗岩和钠长石花岗岩,伟晶岩依次出现微斜长石型伟晶岩、微斜长石-钠长石型伟晶岩、钠长石型伟晶岩和钠长石-锂辉石型伟晶岩。根据不同深度伟晶岩的矿化特点,矿床的垂向分带大致划分如下:0~105 m的Li-Be-Nb-Ta矿化带;0~860 m的Be-Nb-Ta矿化带;0~1730 m的第一期Nb-Ta矿化带;1730~3170 m的无矿伟晶岩带;3170~3211 m的第二期Nb-Ta矿化带。

LCT(lithium-cesium-tantalum)pegmatites exhibit two types of zonation:internal and regional.Internal zonation refers to the compositional zoning within a single,typically voluminous pegmatite vein.Different rare metal mineralizations can develop within distinct textural zones of a single vein,as exemplified by the Koktokay No.3 pegmatite in Xinjiang.Regional zonation,on the other hand,describes the distribution of pegmatite and ore-forming systems around a central,highly differentiated peraluminous granite body.Hundreds of pegmatite veins surrounding the granite show varying pegmatite types and mineralization types.Pegmatite evolution increases with increasing distance from the granite center.Consequently,single veins of such pegmatite in the regional zonation are smaller,exhibit less pronounced zonation,and possess different ore-bearing properties.The Jiajika lithium deposit is a typical representative example of this kind of pegmatite deposit.Field investigations at the Jiajika mining area show variations in internal zonation among pegmatite veins.Veins No.9(Be-bearing),No.133(Libearing),and No.528(Nb-Ta-bearing)display well-developed internal zonation patterns.In contrast,veins No.104 and No.308(both Li-bearing)exhibit less pronounced internal zonation,potentially due to multiple phases of magmatic-hydrothermal activity.The interpenetration of pegmatite veins in the Jiajika mining area suggests multiple phases of magmatic and metallogenic processes.Scientific drilling(JSD-2 and JSD-3 boreholes)confirmed this byencountering spodumene-bearing pegmatite and aplite veins intruding into the barren Majingzi granite.Mineral dating within the pegmatite indicates at least two stages of pegmatite formation:213~206 Ma and 199~191 Ma.Borehole JSD-1 data suggests a progressive evolution of magma from deep to shallow levels,with increasing differentiation.Granite and pegmatite compositions evolve synchronously from bottom to top.The granite sequence transitions from biotite granite to muscovite-bearing biotite granite,two-mica granite,muscovite granite,and finally albite granite.Similarly,the pegmatite sequence progresses from microcline type to microcline-albite type,albite type,and ultimately albite-spodumene type.According to the mineralization characteristics of the pegmatite at different depths,the vertical zonation of the Jiajika deposit can be roughly divided as follows:a Li-Be-Nb-Ta mineralization zone(0~105 m);a Be-Nb-Ta mineralization zone(0~860 m);a first phase Nb-Ta mineralization zone(0~1730 m);a barren pegmatite zone(1730~3170 m);and a second phase Nb-Ta mineralization zone(3170~3211)m.