摘要:
木落寨矿床是冕宁—德昌稀土元素(REE)矿带内的主要稀土矿床, 位于矿带北段, 雅砻江断裂以西.该稀土矿床的形成与正长岩-碳酸岩杂岩体密切相关.与冕宁—德昌REE矿带中其它矿床例如牦牛坪、大陆槽矿床不同的是, 牦牛坪和大陆槽矿床的围岩主要是石英闪长岩, 而木落寨 REE 矿床中围岩较复杂,主要有大理岩、绿片岩、碱性花岗岩等.已有研究表明石英闪长岩并不是牦牛坪和大陆槽矿床稀土元素的主要来源, 对木落寨矿床来说, 围岩对成矿的作用还需进一步研究.采用ICP-MS与XRF, 对大理岩和绿片岩这两种主要的围岩进行主微量元素分析, 全岩稀土配分曲线显示轻稀土亏损(大理岩3×10–6~20×10–6, 绿片岩62×10–6~74×10–6), 重稀土平坦, 总稀土含量低(大理岩4×10–6~21×10–6, 绿片岩86×10–6~97×10–6), 与本次研究的木落寨正长岩中稀土含量(592×10–6~2026×10–6)和以往研究的冕宁—德昌成矿带其他三个矿床的碳酸岩-正长岩杂岩体中稀土含量(碳酸岩2470×10–6~40807×10–6, 正长岩630×10–6~3233×10–6)相比, 大理岩和绿片岩中稀土元素含量过低, 显示在稀土物质来源上, 这两种围岩可能对成矿贡献不大, 根据剖面展示, 矿体大部分出现在正长岩裂隙中, 极少部分出现在正长岩和大理岩接触面中, 大理岩可能提供碳酸根离子, 促进交代和成矿作用的进行.木落寨矿石类型主要是细脉浸染型, 少量为条带型.通过手标本、镜下和 BSE 图像观察, 脉石矿物主要有萤石+方解石+重晶石+石膏+黄铁矿+方铅矿+石英+金云母等, 矿石矿物为氟碳铈矿, 氟碳铈矿叠加在已形成的脉石矿物之上, 显示矿床主要形成于热液阶段最晚期.
Abstract:
The Muluozhai deposit is a main REE deposit in the Mianning–Dechang REE belt, which is located in the north of the belt and the west of Yalong River fault. The formation of this REE deposit was closely related to the carbonate-syenite complexes. Unlike other REE deposits in the Mianning–Dechang REE belt, such as the Maoniuping deposit and the Dalucao deposit, whose main wall rocks are quartz diorite, the wall rocks in the Muluozhai deposit are more complex, and the main types are marble, green schist, alkali granite and some other rocks. It has been shown that quartz diorite was not the main REE source of Maoniuping and Dalucao deposit, and the effect of these wall rocks on mineralization of the Muluozhai deposit needs further studies. ICP-MS and XRF were used to analyze the major and trace elements of the main wall rocks—marble and green schist, the REE patterns of these wall rocks exhibit a declined LREE (3×10–6~20×10–6for marble, 62×10–6~ 74×10–6for green schist), flat HREE, low total REE content (4×10–6~21×10–6for marble, 86×10–6~97×10–6for green schist). Compared with the REE content (592×10–6~2 026×10–6) of syenite of Muluozhai and total REE in carbonate-syenite complexes of other three deposits in the Mianning–Dechang metallogenic belt (carbonate 2 470×10–6~40 807×10–6, syenite 630×10–6~3 233×10–6) studied in the past, the REE values of marble and green schist are too low, as shown in the source of REE materials, and hence these two types of wall rocks might not have contributed much to ore formation. According to the profile display, most of the orebodies appear in the syenite fractures, with a few occurring in the contact surface between syenite and marble, and marble probably provided CO2–3 and promoted metasomatism and mineralization. The orebodies of the Muluozhai deposit are mainly of veinlet dissemination type, followed by stripe type. Hand specimens as well as microscopic and BSE image observations show that gangue minerals are mainly fluorite + calcite + barite + gypsum + pyrite + galena + quartz + phlogopite, and ore mineral is bastnaesite. Bastnaesite overlies these gangue minerals, indicating that the deposit was mainly formed at the latest stage of the hydrothermal period.
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