埃达克岩:斑岩铜矿的一种可能的重要含矿母岩——以西藏和智利斑岩铜矿为例

Adakite, A Possible Host Rock for Porphyry Copper Deposits: Case Studies of Porphyry Copper Belts in Tibetan Plateau and in Northern Chile

作者通过对 3个重要的斑岩铜矿带的综合研究和对比分析发现 ,最具成矿潜力的含矿斑岩不是典型的岛弧岩浆岩 ,而是一种高SiO2 〔w(SiO2 ) >5 6 %〕、高Al2 O3〔w(Al2 O3) >15 %〕、富Sr(多数wSr>40 0× 10 -6)、低Y(多数wY<16× 10 -6)的岩石 ,具有埃达克岩地球化学特征 ,显示埃达克岩岩浆亲合性。含矿的长英质岩浆并非来自地幔楔形区或壳幔过渡带 ,而是来自俯冲的洋壳板片的直接熔融。该俯冲板片熔融前通常变质为含水的榴辉岩。在安第斯弧造山带 ,大洋板块低缓、快速、斜向俯冲 ,诱发洋壳板片直接熔融 ,形成埃达克质熔体 ,后者通过分凝和封闭性演化 ,形成安第斯中新世_上新世巨型斑岩铜矿系统 ;在青藏高原碰撞造山带 ,俯冲并堆积于地幔岩石圈的古老洋壳物质的变质和拆沉 ,诱发榴辉岩部分熔融 ,产生埃达克质熔体 ,并与幔源熔体混合 ,形成西藏冈底斯和玉龙斑岩铜矿系统。

Porphyry-type copper deposits are the most important type of copper deposits. According to the traditional genetic model, porphyry copper deposits were formed in island or continent-marginal arc environments, the magmas of ore-bearing porphyries were derived from partial melting of the mantle wedge or the crust-mantle transitional zone and, through crystal fractionation and /or crustal contamination, these magmas evolved into calc-alkaline felsic rocks typical of island arc. However, based on a comprehensive study and comparative analysis of three important porphyry copper belts, the authors hold that, instead of being a typical arc magmatic rock, the most potential ore-bearing porphyry is a kind of rock rich in SiO 2(>56%), Al 2O 3(>15%), Sr(mostly>400×10 -6), and poor in Y(mostly <16×10 -6). It has geochemical characteristics of adakite and shows magmatic affinity of this rock. Rather than being derived from the mantle wedge or the crust-mantle transition zone, such a kind of ore-bearing felsic magma must have originated from direct melting of subducted oceanic crust slabs. The subducted slabs was metamorphosed to water-bearing eclogite before being melted. In the Andean arc orogenic belt oceanic plates were subducted rapidly at a low degree and in an oblique direction. Direct melting produced adakitic magmas which then experienced differentiation and closed evolution and finally formed Andean Miocene-Pliocene giant porphyry copper system. In Tibetan collision-orogenic belts, old oceanic crustal rocks were subducted and accumulated in mantle lithosphere, where metamophism and detachment caused their partial melting under the condition of eclogite facies and resulted in the generation of adakitic magmas. Such magmas were mixed with mantle melts and finally produced the Gangdese and the Yulong porphyry copper systems.