四川拉拉IOCG矿床方解石REE与C，O同位素地球化学特征及意义

摘要

对拉拉铁氧化物−铜−金(IOCG)矿床中与铜矿化密切相关的变质期方解石(Ⅰ)和热液期方解石(Ⅱ)的稀土元素(REE)及C，O同位素地球化学特征进行研究，以探讨方解石成因及沉淀影响因素、成矿流体来源及性质。研究结果表明：方解石(Ⅰ)具有极低的REE总质量分数(wREE平均75.98×10−6)，方解石(Ⅱ)的REE总质量分数相对较高(平均313.85×10−6)；方解石(Ⅰ)的δ(13CPDB)为−3.84‰~0‰，方解石(Ⅱ)的δ(13CPDB)为−3.98‰~−1.40‰，二者大小接近且相对变化较小；方解石(Ⅰ)的δ(18OSMOW)为9.45‰~12.2‰，方解石(Ⅱ)的δ(18OSMOW)为8.1‰~11.6‰，后者相对低于前者。分析认为：方解石(Ⅰ)不是变质成矿期的主要REE载体，其成矿流体中REE含量很低；方解石(Ⅱ)为热液期REE的主要载体，其成矿流体中REE含量较高。两期方解石均为热液成因，但二者成矿流体不同源；水岩反应及降温作用耦合影响本矿床两期方解石的沉淀。方解石(Ⅰ)的REE配分曲线为平坦型，既有正Eu异常也有负Eu异常，无Ce异常，配分模式受流体中REE络合物稳定性控制，其成矿流体为大量LREE沉淀后的残余高温变质成矿流体，继承了海相喷发的火山围岩部分特征；方解石(Ⅱ)的REE配分曲线为左倾型，显著正Eu异常，无Ce异常，配分模式受方解石晶体化学因素控制，其成矿流体为大气降水与围岩反应形成的低温高氧逸度热卤水。方解石(Ⅰ)的成矿流体中的C由深源C和海水C共同提供；方解石(Ⅱ)的成矿流体中的C来自地幔柱成因的基性辉长岩浆分异释放的CO2，O同位素受大气降水影响发生明显负漂移。%Calcite (Ⅰ) from metamorphic mineralization stage (MMS) and calcite (Ⅱ) from hydrothermal mineralization stage (HMS) have close relationship with copper mineralization in the Lala IOCG deposit. Factors controlling the genesis of calcite, and the features and source of ore-forming fluids were discussed through systematic investigation of REE and C-O isotope geochemistry of calcites. The results show that the average åwREE ,δ(13CPDB) andδ(18OSMOW) of calcite (Ⅰ) are 75.98×10−6, −3.84‰−0‰ and 9.45‰−12.2‰, respectively, while the calcite (Ⅱ) are 313.85×10−6,−3.98‰~−1.40‰and 8.1‰~11.6‰, respectively. The results show that calcite (Ⅰ) is not the main carrier of REE in the MMS, and its ore-forming fluid has very low åwREE . Calcite (Ⅱ) is the dominate REE carrier of HMS, and its ore-forming fluid has higher åwREE than calcite (Ⅰ). Both calcite (Ⅰ) and calcite (Ⅱ) are hydrothermal origin,water-rock interaction and cooling effect the precipitation of calcite, but their ore-forming fluids have different sources. REE distribution pattern of calcite (Ⅰ) is flat type. There are both positive and negative Eu anomalies, and no Ce anomaly, which is controlled by the stability of REE complexes in the fluid. The ore-forming fluid of calcite(Ⅰ) is a kind of high temperature residual fluid after the precipitation of a large number of LREE, and has inherited characteristics of the surrounding marine eruption volcanic rock. REE distribution pattern of calcite (Ⅱ) is leftist type, with significant positive Eu anomaly and no Ce anomaly, which is controlled by crystal chemical factors. The ore-forming fluid of calcite (Ⅱ) is a kind of hot brine originated from the reaction between meteoric water and surrounding rocks, which has a feature of low temperature and high oxygen fugacity. C of calcite (Ⅰ) is derived from mantle fluids and marine carbonates, while C of calcite (Ⅱ) is sourced from CO2 which is released by gabbro magma. O isotope obviously has negative drift being affected by meteoric water.