滇西北衙金矿床富水岩浆对成矿的制约

成矿岩浆富水(4%)是形成斑岩型矿床的关键。滇西北衙金矿床是金沙江-哀牢山新生代富碱斑岩成矿带中规模最大的金多金属矿床,目前探明金资源储量超过320吨,伴生的铅锌、银、铜、铁、硫也达到大-中型规模。本文以北衙成矿的二长花岗斑岩中角闪石斑晶和锆石为研究对象,开展岩相学和地球化学研究,厘定成矿岩浆的特征,并探讨富水岩浆对成矿的制约。研究表明,北衙二长花岗斑岩角闪石斑晶发育、轻稀土富集而重稀土亏损、具有高Sr(300×10~(-6))、低Y(10×10~(-6))和高(La/Yb)N(6.19~26.8)的地球化学特征,暗示在岩浆结晶早期有角闪石斑晶的晶出。角闪石晶出的条件是岩浆中水含量大于4.5%,因此北衙金矿床成矿岩浆演化早期角闪石的晶出显示成矿岩浆相对富水。角闪石矿物组分计算表明岩浆熔体中水含量在3.8%~4.1%之间,进一步证实北衙金矿床成矿岩浆相对富水。北衙金矿床这种具有高水含量的岩浆在源区岩石部分熔融的过程中,水的加入可以降低其熔点,促进源区含Cu、Au硫化物重熔,或萃取岩石中Cu、Au成矿元素,或聚集岩浆中分散分布的金属元素,形成富金属、富水的岩浆。含矿岩浆就位后,其通过压力的降低(岩浆房脆性破裂或岩浆向上侵位)来降低水的溶解度,或通过岩浆持续的结晶(在等压条件下晶出无水矿物)逐渐提高岩浆水含量,促使熔体中水含量大于水在熔体中溶解度,从而导致岩浆水达到饱和状态,流体开始出溶。总体而言,北衙金矿床富水岩浆不仅能促进北衙矿床成矿流体的形成与演化,也对成矿流体中Cu、Au等成矿元素的富集具有重要的作用。

The Beiya gold deposit:Constraint from water-rich magmas to mineralization

The Beiya gold deposit is the biggest Cenozoic gold deposit in the Jinshajiang-Ailaoshan porphyry metallogenic belt, with gold reserve more than 320t. The study of the petrography and geochemical features of the Beiya monzogranite porphyry presented in this paper shows that the Beiya monzogranite porphyry is characterized by amphibole phenocrysts, with high Sr (>300×10^-6), low Y (<10×10^-6) and high (La/Yb)_N (6.19~26.8) values, which indicates that amphibole crystalizes in early magmatic differentiation. The presence of amphibole during the early stages of crystallization indicates that the Beiya ore-related magma were very hydrous melts. At the same time, the contents of amphibole suggest that the water contents of the Beiya melts are varying between 3.8% and 4.1%, which further confirmed Beiya ore-related magma is relatively water-rich. During the process of partial melting in the source of the Beiya gold deposit, the addition of H₂O can reduce the melting point of magma, promote remelting of the source containing copper, gold sulfide, or extraction of copper, gold ore-forming elements from rock, or gather the scattered distribution of metal elements in magma, forming metal-rich magma. After the emplacement of the Beiya ore-related magma, through the pressure decrease to reduce the water solubility, or by magma crystallization increasing the water contents of the Beiya melts, prompted the magmatic water earlier reached saturation and dissolved fluid. Then providing main ore-forming fluids and metal source for the porphyry mineralization and generating (or improving) the metallogenic potential of the Beiya water-rich magma. In general, the water-rich magma in the Beiya gold deposit not only plays an important role during the process of ore-fluid formation, but also the components of Cu, Au and other metal of the ore-forming fluid.