宁芜玢岩铁矿磷灰石的稀土元素特征

Rare Earth Elements in Apatite from Porphyrite Iron Deposits of Ningwu Area

文章分析了宁芜玢岩铁矿 4种产状磷灰石的稀土元素组成 ,并与Kiruna型铁矿和斜长岩、苏长岩及钛铁霞辉岩中磷灰石的稀土元素组成进行了对比。结果表明产地和母岩不同的矿床中 ,它们的磷灰石稀土元素分布型式一致 ,以轻稀土富集和Eu负异常明显为特征 ,属陆相岩浆成因。前 3种产状磷灰石的ΣREE变化于 30 31.48×10 -6～ 12 0 80× 10 -6,第 4种产状磷灰石的ΣREE仅为 195 8× 10 -6,反映岩浆演化到热液的晚期阶段成矿溶液稀土元素含量较低。尽管辉长闪长玢岩与磷灰石的稀土元素分布型式一致 ,但辉长闪长玢岩无Eu异常或有弱Eu正异常 ,代表它们的地幔源区低氧逸度的还原环境 ,或反映氧逸度较高情况下的分离结晶作用。不混溶作用形成的矿浆在冷凝过程中 ,Eu2 + 优先被透辉石捕获 。

There are four modes of occurrence for apatites in porphyrite iron deposits: (1) magnetite-apatite-actinolite bodies in iron ores and country rocks, (2) apatite-magnetite veins hosted in iron ores, (3) apatite-magnetite veins or nodules hosted in gabbro-diorite porphyrite, and (4) apatite-quarte-pyrite veins hosted in country rocks. This paper investigates rare earth elements in apatites of four modes of occurrence in porphyrite iron deposits. The REE patterns of apatites in porphyrite iron deposits are in general similar to those of apatites in anorthosite, norite and jacupirangite. The apatites are enriched in light REE, with pronounced negative Eu anomaly. The similarity of REE distribution in apatites from various deposits indicates the existence of a common process for the formation of various ore types, i.e., immiscibility. Early magmatic or magmatic-hydrothermal transitional apatite contains (3?031.48～12?080)×10 -6 REE. Later hydrothermal apatite contains 1?958×10 -6 REE, suggesting that later hydrothermal ore-forming solution contains lower REE. Although gabbro-diorite porphyrite and apatite exhibit similar REE patterns, gabbro-diorite porphyrite does not show europium depletion, possibly caused either by reduction environment of mantle source or by fractionation and crystallization (immiscibility) under the high fugacity condition. Negative Eu anomaly of later apatites resulted probably from the acquisition of Eu 2+ by earlier diopsite during the cooling of ore magma.