吉林四方甸子钼矿床成岩成矿时代及岩石地球化学特征

摘要

The Sifangdianzi Mo deposit in Jilin Province is a recently discovered quartz-vein type molybdenum deposit.The orebodies are mainly hosted in monzogranite.Its ore-forming processes can be divided into three stages: barren quartz-vein stage, quartz-molybdenite-pyrite stage and quartz-carbonatate stage.By the LA ICP MS U Pb dating of zircons,we obtained the age of 170.0 ±1.0 Ma for granodiorite,and the age of 179.0 ±1.0 Ma for monzogranite.Based on the Re Os dating of molybdenite from quartz vein type ore,the model ages obtained are from 175.0±2.6 to 176.5±2.6 Ma averaged at 176.1± 1.1 Ma,and having an isochronic age of 176.6 ±4.2 Ma.It is indicated that the Sifangdianzi Mo deposit was formed in the early Jurassic Period.SiO2 values of the granitic rocks change from 66.17% to 76.60%,Na2 O and K2 O are in the range of 3.15% to 5.40% and 2.42% to 5.42%respectively,their Rittmann indexes vary from 2.17 to 2.93;Their REE patterns demonstrate right-oblique linear REE patterns,the trace elements show relative enrichment of the large ion lithophile elements,and depletion of the high field strength elements.The geochemical characteristics of the granitic rocks imply they belong to I-type granite.The geotectonic mechanisms responsible for the diagenesis and mineralization were probably related to subduction of the paleo-Pacific Plate to the Eurasian continent.%吉林四方甸子钼矿床是一新发现的石英脉型钼矿床,矿床产于二长花岗岩中。矿床成矿包括无矿石英脉阶段、石英辉钼矿黄铁矿阶段和石英碳酸盐化阶段。通过对矿区内花岗质岩石进行 LA ICP MS 同位素测年,获得花岗闪长岩成岩年龄为(170.0±1.0)Ma,二长花岗岩成岩年龄为(179.0±1.0) Ma。对含矿石英脉中辉钼矿 Re Os 同位素测年分析,获得辉钼矿模式年龄值为(175.0±2.6)~(176.5±2.6)Ma,加权平均值为(176.1±1.1)Ma,等时线年龄为(176.6±4.2)Ma,表明四方甸子钼矿床形成于早侏罗世晚期。岩体 w(SiO2)为66.17%~76.60%,w(Na2 O)和 w(K2 O)分别为3.15%~5.40%和2.42%~5.42%,里特曼指数σ为2.17~2.93。稀土元素分布形式呈右倾型,轻、重稀土元素分馏明显。微量元素总体显示相对富集 Rb、Ba、Th、U 等大离子亲石元素(LILE),亏损 Nb、Ta、Ti、P 等高场强元素(HFSE)。综合分析表明,岩石为弱过铝质 I 型花岗岩。成岩成矿动力学背景为燕山早期古太平洋板块向欧亚大陆俯冲作用的大陆边缘环境。