Implication of Apatite and Anhydrite for Formation of an Iron-Oxide-Apatite (IOA) Rare Earth Element Prospect, Benjamin River, Canada
Implication of Apatite and Anhydrite for Formation of an Iron-Oxide-Apatite (IOA) Rare Earth Element Prospect, Benjamin River, Canada
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磷灰石和硬石膏对氧化铁-磷灰石 (IOA) 稀土元素前景形成的影响，加拿大本杰明河
DOI：
10.1111/rge.12142
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发表时间：
2017
期刊：
影响因子：
1.4
通讯作者：
Kon Yoshiaki
中科院分区：
文献类型：
--
作者：
Hoshino Mihoko;Watanabe Yasushi;Kon Yoshiaki
位于加拿大新玩法北方的本杰明河磷灰石前景位于晚志留世的Dickie Brook深成杂岩中，该杂岩由以二长花岗岩、闪长岩和辉长岩为代表的侵入单元组成。本杰明河的IOA矿石主要由磷灰石、普通辉石和磁铁矿组成，在寄主火成岩中形成伟晶岩荚和透镜体，其中最大的长100 m，宽10-20 m，位于闪长岩和辉长岩单元中。在本次研究中，28 IOA矿石和岩石样品采集的闪长岩和辉长岩单位。矿物学观察表明，磷灰石-普通辉石-磁铁矿矿石中磷灰石、普通辉石和磁铁矿的含量各不相同，并伴有少量绿帘石族矿物（褐帘石、富稀土绿帘石和绿帘石）和微量钠长石、钛铁矿、钛磁铁矿、黄铁矿、方解石、方解石和石英。磷灰石和普通辉石颗粒含有小的硬石膏包裹体。这表明结晶磷灰石和普通辉石的岩浆具有高氧逸度。在背散射电子（BSE）图像中，矿石中的磷灰石颗粒具有两个不同外观的区域：（i）原生富稀土区;和（ii）多孔贫稀土区。多孔贫稀土区主要出现在磷灰石颗粒的边缘和/或内部，此外还存在完全由多孔贫稀土磷灰石组成的磷灰石颗粒。这种多孔贫REE磷灰石的特征在于低REE（<0.84重量%）、Si（<0.28重量%）和Cl（<0.17重量%）含量。绿帘石类矿物主要赋存于多孔贫稀土磷灰石和普通辉石之间的晶界。这表明，从原生富稀土磷灰石中浸出的稀土元素结晶为褐帘石和富稀土绿帘石。矿石中的磁铁矿常以细脉形式出现，切割磷灰石颗粒或以自形颗粒形式出现，替代部分普通辉石。这些结构表明磁铁矿在后期结晶。矿石中有黄铁矿脉，含大量石英和方解石脉。黄铁矿脉主要与普通辉石中的石英脉共生。这些结构表明黄铁矿脉是最新相。显微镜下，从Benjamin River前景采集的磷灰石-普通辉石-磁铁矿矿石、辉长岩-石英闪长岩和长石岩脉中含有脏纯钠长石（Ab 98 Or 2-Ab 100）。长石岩脉主要由脏纯钠长石组成。脏纯钠长石的出现表明原始斜长石的显著钠长石化（钠化蚀变）（Pilote等人，含SO 42 −岩浆结晶的原生富稀土磷灰石、普通辉石和硬石膏与含钠质流体中的Fe发生反应，导致Fe 2+氧化，S2−释放到含钠质流体中。被钠质流体蚀变的原生富稀土磷灰石、普通辉石和斜长石中的REE、Ca和Fe被释放到流体中。含钠流体中的Fe 3+以铁氧化物和绿帘石族矿物的形式沉淀在磷灰石-普通辉石-磁铁矿矿石中。Benjamin River IOA前景的成矿作用可分为4个阶段：（1）氧化岩浆阶段，主要结晶磷灰石、普通辉石和硬石膏;（2）钠交代阶段，伴随岩浆矿物蚀变;（3）氧化流体阶段，（4）还原流体阶段（黄铁矿成矿）。
The Benjamin River apatite prospect in northern New Brunswick, Canada, is hosted by the Late Silurian Dickie Brook plutonic complex, which is made up of intrusive units represented by monzogranite, diorite and gabbro. The IOA ores, composed mainly of apatite, augite, and magnetite at Benjamin River form pegmatitic pods and lenses in the host igneous rocks, the largest of which is 100 m long and 10–20 m wide in the diorite and gabbro units. In this study, 28 IOA ore and rock samples were collected from the diorite and gabbro units. Mineralogical observations show that the apatite–augite–magnetite ores are variable in the amounts of apatite, augite, and magnetite and are associated with minor amounts of epidote‐group minerals (allanite, REE‐rich epidote and epidte) and trace amounts of albite, titanite, ilmenite, titanomagnetite, pyrite, chlorite, calcite, and quartz. Apatite and augite grains contain small anhydrite inclusions. This suggests that the magma that crystallized apatite and augite had high oxygen fugacity. In back scattered electron (BSE) images, apatite grains in the ores have two zones of different appearance: (i) primary REE‐rich zone; and (ii) porous REE‐poor zone. The porous REE‐poor zones mainly appear in rims and/or inside of the apatite grains, in addition to the presence of apatite grains which totally consist of a porous REE‐poor apatite. This porous REE‐poor apatite is characterized by low REE (<0.84 wt%), Si (<0.28 wt%), and Cl (<0.17 wt%) contents. Epidote‐group minerals mainly occur in grain boundary between the porous REE‐poor apatite and augite. These indicate that REE leached from primary REE‐rich apatite crystallized as allanite and REE‐rich epidote. Magnetite in the ores often occurs as veinlets that cut apatite grains or as anhedral grains that replace a part of augite. These textures suggest that magnetite crystallized in the late stage. Pyrite veins occur in the ores, including a large amount of quartz and calcite veins. Pyrite veins mainly occur with quartz veins in augite. These textures indicate pyrite veins are the latest phase.Apatite–augite–magnetite ore, gabbro–quartz diorite and feldspar dike collected from the Benjamin River prospect contain dirty pure albite (Ab98Or2–Ab100) under the microscope. The feldspar dikes mainly consist of dirty pure albite. Occurrences of the dirty pure albite suggest remarkable albitization (sodic alteration) of original plagioclase (An25.3–An60in Pilote et al., 2012) associating with intrusion of monzogranite into gabbro and diorite.SO42−bearing magma crystallized primary REE‐rich apatite, augite and anhydrite reacted with Fe in the sodic fluids, which result in oxidation of Fe2+and release of S2−into the sodic fluids. REE, Ca and Fe from primary REE‐rich apatite, augite and plagioclase altered by the sodic fluids were released into the fluids. Then Fe3+in the sodic fluids precipitated as Fe oxides and epidote‐group minerals in apatite–augite–magnetite ores. Finally, residual S2−in sodic fluids crystallized as latest pyrite veins.In conclusion, mineralization in Benjamin River IOA prospect are divided into four stages: (1) oxidized magmatic stage that crystallized apatite, augite and anhydrite; (2) sodic metasomatic stage accompanying alteration of magmatic minerals; (3) oxidized fluid stage (magnetite–epidote group minerals mineralization); and (4) reduced fluid stage (pyrite mineralization).