安徽沙坪沟斑岩钼矿床赋矿岩体钾质交代作用及其微量元素和Sr-Nd同位素响应

摘要

This study focuses on the potassic metasomatism within quartz syenite and granite porphyry which constitute the main host rocks of the Shapinggou Mo deposit.Through a comparative study of petrography,geochemistry and isotopic characteristics of igneous rocks with different alteration intensities,the authors investigated specific mineral assemblage within different alteration types,elemental migration and Sr-Nd isotopic variation during the hydrothermal alteration and the genesis,physicochemical condition difference and the genetic link to mineralization,and then revealed the features and origin of ore-forming hydrothermal fluid.The results indicate that the ore-bearing quartz syenite and granite porphyry were derived from similar magma sources as shown by their uniform geochemical features,and they are characterized by potassic metasomatism of different intensities.The K-altered igneous rocks exhibit high K2O and Rb and low Na2O,CaO,Sr and Ba.The quartz syenite and granite porphyry with different intensities of K-metasomatism show varying ratio of Rb/Sr and (87Sr/86Sr)0,but data from granite porphyry reveal more discrete and even abnormally low (87Sr/8rSr)0 values.This indicates that the Rb-Sr isotopic system was sensitive to the hydrothermal fluid,while the Sm-Nd system was insensitive.These phenomena were observed in some Mo deposits with great reserves and drastic hydrothermal alteration in the east Qinling-Dabie Mo metallogenic belt,suggesting the consistent hydrothermal alteration features and similar hydrothermal fluid origins and evolving processes.This was the uppermost Mo mineralization stage between the late period of K-feldspathization to beresitization,implying that the declining of pH value in the fluid system triggered the Mo precipitation from fluid.Based on a comparison with porphyry Cu and Cu-Mo deposits,the authors hold that these porphyry deposits have all experienced alkali-metasomatism,but there existed obvious difference in the alteration type,intensity of Rb-Sr fractionation of hydrothermal fluid and alteration of the primary trace elements as well as the isotopic system of igneous rocks,implying their respective exclusive rock-and ore-forming materials,fluid origins and tectonic settings.%以沙坪沟钼矿主要的赋矿岩石——石英正长岩和花岗斑岩为对象,通过对比不同蚀变强度岩石的岩相学、岩石地球化学和同位素特征,研究该矿床的钾质交代作用-矿化特征,探讨不同热液蚀变的元素组合、蚀变过程中的元素迁移和Sr-Nd同位素的变化及其成因、不同蚀变的物理化学条件差异及其与矿化的关系,进而揭示蚀变-成矿热液流体的特征和起源.研究表明,石英正长岩和花岗斑岩的地球化学特征总体相似,显示其属同源岩浆演化产物,二者均受到钾质蚀变,但蚀变强度相差较大.钾质蚀变岩石的化学成分表现为高K2O、Rb和低Na2O、CaO、Sr、Ba,不同蚀变强度的岩石Rb/Sr和Sr同位素组成差别较大,花岗斑岩样品数据更显离散,甚至出现异常低的锶同位素初始值,表明热液蚀变强烈改造了Rb-Sr同位素体系,而Sm-Nd体系基本保持稳定.这一现象在东秦岭-大别钼矿带中典型的斑岩钼矿床也有出现,显示该成矿带具有相似的蚀变类型、热液起源和演化特征.而且钾长石化后期至黄铁云(绢)英岩化阶段也是最主要的钼成矿期,表明这期间流体系统pH值的降低致使Mo元素从流体中沉淀成矿.对比斑岩铜、铜-钼矿床和钼矿床的蚀变特征及其过程中元素和同位素的变化可以发现,这3种矿床均发育碱质交代作用,但蚀变强度、热液的Rb-Sr分异程度及其对原岩的改造程度存在较大差异,这暗示了各自特有的成岩、成矿物质和流体来源及大地构造背景.