滇西保山核桃坪铅锌矿V_1矿体成矿流体演化的历史分析
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摘要
保山核桃坪铅锌矿床位于保山—镇康铅锌铜多金属成矿带核桃坪铅锌多金属矿集区内,赋存于上寒武统核桃坪组大理岩化灰岩中,是受构造控制的矽卡岩型矿床。近年来许多地质工作者对该矿集区进行了研究,取得了许多认识。然而,对矿床围岩蚀变特征、流体包裹体地球化学特征、流体演化历史等方面的研究比较缺乏,造成对该区矿床成因及作用过程的认识分歧较大,也无法有效地指导矿区及外围地质找矿工作的开展。本文依托国家自然基金项目《滇西核桃坪铅锌矿区断裂带成矿流体的显微构造解析》,对核桃坪铅锌矿床成矿流体演化做了深入的研究,主要取得以下认识和成果:
     (1)系统的流体包裹体岩相学、显微测温及包裹体成分分析表明,本区流体包裹体多为负晶形、椭圆形及不规则形状,以纯液相与富液相为主;成矿阶段包裹体发生“沸腾”,造成各相包裹体共生;从早期矽卡岩阶段至晚期碳酸盐阶段,均一温度、盐度及密度总体上呈现逐渐下降的趋势。包裹体均一压力的计算显示,其变化范围较大,认为其原因是两种流体端员的混合与硫化物的沉淀所致,而闪锌矿在温度190~220℃、压力7~16Mpa时发生过大量沉淀。根据石英中包裹体压力值计算,成矿作用发生在0.25~1.7km的浅成环境。
     (2)群体包裹体成分分析表明,流体液相成分阳离子以Ca2+、Na+为主,兼有少量的Mg2+,阴离子以Cl-、F-为主,Cu、Pb、Zn成矿元素在流体中以氯络合物形式搬运。而气相成份以H2O、CO2为主,及少量CH4、C2H6、N2等还原性气体。矿化早阶段还原系数(R*)为1.14-1.47,平均为1.28;晚阶段为1.66,明显高于早阶段,表明流体演化晚阶段比早阶段更为还原,更利于硫化物的沉淀。
     (3)根据流体性质,将流体分为高温低盐度与中低温中高盐度两个端员,提出流体的沸腾与矿质的沉淀为两种端员流体的混合所致。
     (4)流体包裹体同位素分析证明,成矿流体主要来自深部幔源流体分异水,并在后期成矿作用过程中有地层建造水或大气降水的加入。
     (5)核桃坪铅锌多金属矿床是燕山晚—喜马拉雅早期(64.5±0.7Ma-35.3±0.7Ma)持续的印-亚大陆挤压碰撞和澜沧江脆—韧性剪切带推覆、走滑构造体制下,与深部幔源成矿流体贯入有关的构造热液脉型铅锌多金属矿床,其成矿作用受控于深部过程的驱动和控制。
Hetaoping skarn-type lead-zinc deposit in western Yunnan province which is hosted in marbleization limestone of the upper Cambrian Hetaoping Formation is located in Hetaoping lead-zinc muti-metal deposit cluster area of Baoshan-Zhenkang lead-zinc-copper polymetallic belt. The deposit is controlled by tectonic. In the past few years, this particular deposit has been receiving increasing attention from researchers, and many results were obtained. Because of lacking of wall-rock alteration characteristics, geochemistry of fluid inclusions characteristics and the history of the fluid evolution, there are many diffents understandings ore genesis and mineralizations. Aim to solve this problem, it is obtained many results through researching for the above aspects in this paper. The results are list as follow:
     (1)Petrographic observations,temperature results and analysis of fluid inclusion component indicate that the main types of fluid inclusions are NaCl-H20 solution and aqueous inclusions in seven mineralization stages and the shapes of fluid inclusions of Hetaoping are mainly of ellipse and irregular shape. Due to sudden falling of pressure(decompression) and joining of meteoric water, boiling happened in the ore-forming fluid. Generally, homogenization temperature, salinity, density and homogenization pressure of the fluid are stably decreased from early skarn stage to carbonation late stage. Zinc precipitation mainly occur in190~220℃and 7-16Mpa. The mineralization occurr under the epithermal conditions of 0.25~1.75km. So it is epithermal deposit.
     (2)Inclusion groups analysising show that the ore-forming fluid are rich in Ca2+, Na+, K+, F-,Cl-, SO42- and CO2,H2S,CH4,C2H6. The deoxidize coefficient (R*)in early mineralization stage is 1.14~1.47, and in the late is 1.28.Indicate that the environment of ore-forming fluid in early stage is more deoxidize than late stage.
     (3)Based on the study, fluid end members are divided into high temperature-low salinity and low temperature-high salinity,and mixing of fluids cause the boiling and sulfide precipitation.
     (4)Isotope of fluid Inclusions analysis indicate that ore-forming elements were carried by tectonic-fluids from magma or mantle to orogenic belt.
引文
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