斜长岩体中Fe-Ti-P矿床的特征与成因

岩体型斜长岩为由90%以上斜长石组成的岩浆岩,具变压结晶的特点,仅形成于元古宙(2.1～0.9Ga),常赋存有Fe-Ti-P矿床。Fe-Ti-P矿体既呈整合层状也呈透镜状和席状等不规则形式产出;矿石类型有块状和侵染状,前者矿石矿物含量>70%,后者矿石矿物含量为20%～70%;矿物组成上,不同矿床稍有差别:部分矿床的Fe-Ti氧化物以钛磁铁矿为主、钛铁矿次之,而其他矿床则以赤钛铁矿为主、磁铁矿次之。一些矿床磷灰石含量较高,出现仅由Fe-Ti氧化物和磷灰石组成的铁钛磷灰岩。研究表明,Fe-Ti-P矿床由富Fe、Ti的岩浆演化形成,其母岩浆是在深部岩浆房中大量结晶斜长石后的残余岩浆。部分学者认为不同矿石经正常的结晶分异作用并堆晶形成,但该机制很难解释呈不规则状产出的矿石;其他学者则认为不混熔作用对矿石的富集(尤其是脉状、席状的铁钛磷灰岩)有重要作用,但该机制缺乏岩相学和地球化学方面的证据。河北大庙Fe-Ti-P矿体呈透镜状、席状等不连续地分布于斜长岩中,矿体不发育明显岩浆分层,但仍出现不同矿石的相带。依据详细的岩相学、矿体中矿物含量和成分的变化规律以及全岩地球化学特征,我们判断大庙矿床中不同矿石为堆晶矿物和晶隙流体的混合产物,它们由铁闪长质岩浆经结晶分异和堆晶作用形成,与不混熔作用关系不大。矿体不规则状产出的特点可能与岩浆动力分异作用有关,并伴随有小范围的亚固相迁移。

Characteristics and origin of Fe-Ti-P oxide deposits associated with Proterozoic massif-type anorthosite

Massif-type anorthosite is referred to igneous rocks consisting of over 90% plagioclase. The massif-type anorthosite bodies formed in the Proterozoic ranging from 2.1 Ga to 0.9 Ga, and commonly host abundant Fe-Ti-P oxide ores. Fe-Ti-P ore bodies occurred as conformable layers or irregular shapes as lenses, sheets or veins. The ores usually are of massive and disseminated types;the former type contains more than 70% ore minerals, and the latter one contains 20% to 70%. Ore minerals from some deposits are mainly titanomagnetite with minor ilmenite, whereas the others are mainly hemo-ilmenite with minor magnetite. Some of ores contain abundant amounts of apatite, which forms nelsonite containing only Fe-Ti oxides and apatite. These Fe-Ti-P ores have been commonly interpreted as forming from a Fe-Ti-rich magma which usually represents residual magma after extensive crystallization of plagioclase in a deep chamber. However, it is controversial on the ore-formation mechanisms. They were assumed to have crystallized from immiscible Fe-Ti-P liquids separated from silicate magma or represent cumulate rocks that directly crystallized from silicate magmas. The Damiao Fe-Ti-P oxide deposit is the unique deposit hosted in massif-type anorthosite in China. Its ore bodies commonly occur as irregular lenses, veins or pods with sharp contact with anorthosite, and consist of different zones of Fe-Ti-P ores upwards instead of obvious igneous layering. Based on detailed petrography, mineral proportions, mineral compositions and whole-rock geochemistry, we conclude that various ores at Damiao are mixtures of cumulus mineral and trapped liquids, and that they formed from a ferrodioritic magma by fractional crystallization and accumulation. The discordant occurrence of various ores may be related to the dynamic crystallization process and/or subsolidus re-mobilization.