黑云母和锆石化学组分对岩浆结晶条件的约束:以滇西北衙超大型金矿床为例

Constraints of chemical compositions of biotite and zircon on crystallization conditions of magma: An example from the Beiya giant Au deposit,SW China

中酸性岩浆含矿性差异一直是矿床学的研究热点。滇西北衙超大型金多金属矿床(探明金资源量超过370t)内发育成矿二长花岗斑岩体和非成矿黑云母二长花岗斑岩体、煌斑岩体,是研究岩体含矿性差异、富碱岩浆结晶时物理化学条件及其成矿效应的良好选区。本文在详细的岩相学观察基础上,对成矿的二长花岗斑岩、未成矿的黑云母二长花岗斑岩和煌斑岩中黑云母和锆石开展矿物化学分析,厘定了北衙富碱岩浆的结晶条件。北衙成矿二长花岗斑岩体的锆石结晶温度(843℃)稍高于黑云母二长花岗斑岩体的锆石结晶温度(807℃),鉴于金在熔体中溶解度随温度升高而增大,表明在岩浆演化初期二长花岗斑岩体具有更高的金溶解度。同时,利用锆石微量元素组分估算的二长花岗斑岩体的lg(f_(O_2))(-10. 67)高于黑云母二长花岗斑岩体(-15. 00),表明二长花岗斑岩体具有更高的氧逸度。在岩浆演化过程中高氧逸度会抑制金以硫化物形式沉淀,从而增强了二长花岗斑岩体的成矿潜力。除此之外,二长花岗斑岩具有最低的黑云母结晶温度(二长花岗斑岩、黑云母二长花岗斑岩、煌斑岩依次对应644℃、723℃、766℃)和最浅的侵位深度(1. 46～1. 74km、4. 03～5. 02km、2. 53～2. 72km)。高压条件下母岩浆中出溶的流体几乎没有能量形成裂隙,而且也很难发生对金属富集有重要影响的流体不混溶作用。二长花岗斑岩体侵位深度与矽卡岩中石榴石发育的含石盐子晶的三相包裹体的捕获深度(～2km)近似,进一步暗示二长花岗斑岩体侵位后发生流体沸腾作用。因此,岩浆氧逸度和侵位深度的差异可能是黑云母二长花岗斑岩体和煌斑岩体未成矿的原因。

The reasons for the difference of mineralization of intermediate-felsic magmas have always been controversial.The Beiya Au deposit is the biggest Cenozoic Au deposit in the Jinshajiang-Ailaoshan porphyry metallogenic belt,with Au reserve more than 370 t.This deposit develops metallogenic monzogranite porphyry,non-metallogenic biotite monzogranite porphyry and lamprophyre,and thus provides a good example to study the constraint of magmatic physicochemical conditions for mineralization.Based on detailed petrographic observation,the mineral chemistry of biotites and zircons from the Beiya monzogranite porphyry,biotite monzogranite porphyry and lamprophyre are presented in this paper.The crystallization temperatures of monzogranite porphyry(averaged at 843℃)estimated by zircons is lower than that of biotite monzogranite porphyry(averaged at 807℃),indicating the monzogranitic porphyry has a higher solubility of Au at the beginning of the magma evolution,because the solubility of Au in the melt increases with temperature.Moreover,the lg(fO2) of the monzogranite porphyry(-11.05^-7.98) is higher than that of the biotite monzogranitic porphyry(-18.55^-15.59),suggesting the monzogranite porphyry has more potential for mineralization.Moreover,the emplaced depth of monzogranite porphyry(1.46~1.74 km) is lower than that of biotite monzogranite porphyry(4.03~5.02 km) and lamprophyre(2.53~2.72 km).Under high emplacement pressure,the magmatic fluid dissolved from the parental magma has no energy to form crack,and it’s difficult to occur the immiscibility of the fluid that has important influence on metal enrichment.In addition,the emplacement depth of the monzogranite porphyry is similar to the capture depth(~2 km) of the three-phase inclusions containing garnet,suggesting fluid boiling after the emplacement of the monzogranite porphyry.Therefore,the difference in magmatic oxygen fugacity and emplacement depth may be the reasons why the biotite monzogranite porphyry and lamprophyre are not mineralized.