宁芜梅山铁矿床磁铁矿和磷灰石微区原位分析和成矿过程精细刻画

Meishan iron ore deposit is located in the northern section of Ningwu basin in the Middle-Lower Yangtze River polymetallic ore belt. It is a large “porphyrite iron ore deposit” related to continental volcanic rocks, with 338 million tons (Mt) of Fe metals and average grade of Fe 39.14%. There have always been controversial about genesis (magmatogenic vs. hydrothermal) and metallogenic mechanism. The previous work found that the pyroxene phenocryst of diorite porphyrite contained apatite microcrystals and magnetite bead inclusions, which may be the direct evidence of the liquid immiscibility of silicate magma to form iron-rich ore slurry or melt. In addition, magnetite and apatite are widely developed in each stage, which record the physicochemical conditions during hydrothermal activity. Therefore, magnetite and apatite provide an ideal proxies for discussing the genesis and mineralization enrichment mechanism of the Meishan iron deposit. This proposal will carry out on a detailed study using various in situ analysis techniques of laser ablation inductively coupled plasma mass spectrometry (LA- ICP -MS), laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS), and secondary ion mass spectrometry (SIMS) to obtained the high-precision sensitive elements of magnetite(e.g., Mg, Al, Ti, V, Cr, Mn, etc.) and apatite(e.g., F, Cl, OH, LREE, MREE, Sr, U, Th, etc.), and Sr-Nd-O isotope characteristics of apatite. Extracted the useful information based on the detailed study of the internal structure and complex formation process of magnetite and apatite, to meticulous depiction of the complex metallogenetic process, and to deepen the understanding of the genesis of porphyrite iron ore deposit.

梅山铁矿床是位于长江中下游成矿带宁芜盆地北段的玢岩铁矿床（3.38亿吨，Fe平均品位39.14%），其矿化富集机制及矿床成因类型（岩浆成因Vs.热液成因）一直存在争议。申请者前期工作发现该矿床辉石闪长玢岩中辉石斑晶含有磷灰石晶体及磁铁矿珠滴包体，可能是岩浆发生液态不混溶形成富铁矿浆或熔体的直接证据；而且各阶段磁铁矿-磷灰石广泛发育，记录着各阶段热液活动时物理化学环境的变化。因此，磁铁矿和磷灰石为研究该矿床矿化富集机制及矿床成因提供了理想研究对象。本项目拟运用多种原位分析技术（如：LA-ICP-MS、LA-MC-ICP-MS及SIMS分析等），获取磁铁矿和磷灰石中敏感元素（如：磁铁矿Mg、Al、Ti、V、Cr、Mn等及磷灰石F、Cl、OH、LREE、MREE、Sr、U、Th等），磷灰石Sr-Nd-O同位素特征，提取有用信息，对该矿床复杂的形成过程进行精细刻画，从而深化该类型铁矿床成因认识。

磁铁矿-磷灰石（Iron oxide-apatite, IOA）矿床又被为基鲁纳型铁矿床，其成因还存在激烈大争论。梅山铁矿床为长江中下游成矿带内宁芜盆地中规模最大的玢岩型铁矿床（IOA型矿床），磁铁矿与磷灰石发育，并贯穿整个岩浆与热液成矿过程，对其进行精细解剖，可为解决关键性科学问题“成矿是岩浆作用为主，还是热液成因为主”提供重要依据。.首先，在详细分析梅山铁矿床的矿床地质特征、矿物共生组合、成矿期次及矿石组构特征的基础上，结合BSE及CL阴极发光确定研究区发育至少2期4个世代磁铁矿及磷灰石。其次，磁铁矿及磷灰石主量元素地球化学结果表明，发育岩浆结晶分异作用的磁铁矿（Mt-1，TiO2=9.8~20.3 wt.%），热液成因的高钛磁铁矿（Mt-2，TiO2=0.5~1.8wt.%）、中钛磁铁矿（Mt-3，TiO2=0.1~0.5wt.%）及低钛磁铁矿（Mt-4，TiO2=0.1~0.5wt.%），均伴生有磷灰石，主要为氯-羟基-氟磷灰石（Cl-OH-F-apatite）。梅山铁矿床中磁铁矿及磷灰石普遍经历了结构再平衡，磁铁矿及共生磷灰石的主、微量特征元素及磷灰石中的卤素元素、SO3含量变化可有效反映其结晶温度（T）、氧逸度（fO2）及流体的氧化还原状态，揭示了从矿床深部高温低氧逸度岩浆→较低温度和较高的氧逸度岩浆-热液→中温高氧逸度热液作用形成条件连续转变特征。另外，梅山铁矿床中发育的高品位致密块状及“气孔构造”铁矿石是早期发育磷灰石-磁铁矿矿石中的磁铁矿遭受氧化物溶蚀重结晶作用而成，是热液充填作用的产物。最后，新鲜的辉石闪长玢岩中的斑晶单斜辉石及斜长石的矿物及化学不平衡结构，具岩浆混合作用过程特征。.综合梅山铁矿床成矿辉石闪长玢岩岩石学和矿石磁铁矿及磷灰石的矿物学研究工作，认为岩浆结晶分异期为磁铁矿成矿预富集期，并发生了岩浆混合作用，主成矿以热液交代作用为主。