钠化和赤铁矿化蚀变对Zr和Nb-Be-REE矿化的指示意义: 以内蒙古巴尔哲矿床为例

碱性花岗岩型稀有稀土金属矿床是Zr、Nb、Be及REE的重要来源, 其成矿经历了多种交代和蚀变过程, 但这些交代和蚀变与矿化的指示意义尚未明确。内蒙古巴尔哲碱性花岗岩型稀有稀土矿床拥有世界级的Zr、Nb、Be及REE储量, 发育强烈的钠质交代和赤铁矿化, 是探讨上述问题的理想对象。显微岩相学、全岩主微量和钠铁闪石主微量元素分析表明, 未蚀变界熔花岗岩贫Zr、Nb、Be和REE, 钠铁闪石具有较高的REE含量和较低的Na含量。钠质交代界熔花岗岩均发生了赤铁矿化蚀变, 主要发育雪球石英、钠锆石、兴安石、独居石、氟碳铈矿及Nb-Be-REE矿物假象。其主要特征为: 全岩贫F和富Zr、Nb、Be和REE金属元素; 钠铁闪石可见霓石化, 钠铁闪石具有较低的REE含量和较高的Na含量, 钠锆石原位分解再结晶为锆石-石英假象。仅赤铁矿化界熔花岗岩主要发育兴安石、独居石、氟碳铈矿及Nb-Be-REE矿物假象。其主要特征为: 全岩富REE、Be, 贫Zr、Nb金属元素; 钠铁闪石被赤铁矿、铌铁矿等含铁氧化物交代, 钠铁闪石具有较高的REE含量和较低的Na含量。界熔花岗岩钠质交代过程中, 钠铁闪石发生霓石化释放Ba、Zr、Nb和部分REE等元素, 钠锆石分解为锆石-石英假象释放约1%的CaO; 在赤铁矿化过程中, 钠铁闪石被赤铁矿、铌铁矿等矿物交代, 释放约2%的F, 流体中Ca和F的加入诱发成矿晚阶段LREE矿物的富集沉淀。钠铁闪石主微量元素特征和钠质交代界熔花岗岩中钠锆石的出现表明岩浆经历了贫Zr的单锆相到富Zr的多锆相的高分异演化。多锆相界熔花岗岩均发生钠质交代, 钠锆石几乎全部分解为锆石-石英假象, 表明钠质交代可指示界熔花岗岩的Zr矿化。赤铁矿化界熔花岗岩发育大量富Nb-Be-REE的热液矿物, 且Nb、Be及REE含量与赤铁矿化程度呈正相关, 表明岩浆晚期出溶的含矿流体交代界熔花岗岩, 发生赤铁矿化并沉淀金属元素。

Na-metasomatism and hematization implication of Zr and Nb-Be-REE mineralization: Case study of the Baerzhe deposit in Inner Mongolia

Alkaline silicate systems are significant sources of Zr, Nb, Be, and rare earth elements (REEs=lanthanides+Y), with intensive metasomatism and alteration. The relationship between alteration/metasomatism and mineralization, however, remain enigmatic. The Baerzhe alkaline granite-type rare metal deposit in Inner Mongolia represents a world-class mineralization of Zr, Nb, Be, and REE, which experienced Na-metasomatism and hematization, and thus provides an ideal case for addressing the aforementioned issues. Our microscopic observation, whole rock and arfvedsonite major-and trace-element analysis show that the unaltered transsolvus granite is poor in Zr, Nb, Be and REE. Arfvedsonite in unaltered transsolvus granite has high REE content and low Na content. The Na-metasomatized transsolvus granite has also experienced hematization. The rocks contained snowball quartz, elpidite, hingganite, monazite, bastnäsite and Nb-Be-REE mineral pseudomorphs. The arfvedsonite was replaced by aegirine and the elpidite was recrystallized to zircon-quartz pseudomorphs. The Na-metasomatized transsolvus granite is rich in Zr, Nb, Be and REE. Arfvedsonite in Na-metasomatized transsolvus granite has low REE content and high Na content. The hematized transsolvus granite contained hingganite, monazite, bastnäsite and Nb-Be-REE mineral pseudomorphs. The arfvedsonite was replace by Fe oxides such as hematite and niobite. The hematized transsolvus granite is rich in Be, REE and in poor Zr, Nb. Arfvedsonite in hematized transsolvus granite has high REE content and low Na content. In the process of Na-metasomatism of the transsolvus granite, the arfvedsonite was replaced by aegirine, releasing elements such as Ba, Zr, Nb and REE into the fluids, and the breakdown of elpidite releases about 1% CaO, in the process of hematization of the transsolvus granite, the arfvedsonite was replaced by Fe oxides such as hematite and niobite, releasing about 2% of F, and the addition of Ca and F in the fluids induced the enrichment and precipitation of LREE minerals. The composition of arfvedsonite and the occurrence of elpidite in the Na-metasomatized transsolvus granite indicate that the magma has experienced a high degree evolution from miaskitic phase to agpaitic phase. Na-metasomatism in the agpaitic transsolvus granite, induced the breakdown of elpidite to zircon-quartz pseudomorphs, suggesting that Zr mineralization is associated with Na-metasomatism. The Nb, Be and REE grades were positively associated with hematization, indicating that the ore-bearing fluid exsolved in the late-stage magma altered the transsolvus granite, resulting in hematization and Nb, Be and REE mineralization.