The Platreef Unit is a deceptively complex sequence of layered cumulates located in the northern limb of the 2.055 Ga-old Bushveld Complex. The unit includes the Platreef, a thick, richly mineralized stratabound PGE orebody which differs markedly from the comparatively thin, predominantly stratiform Merensky Reef found in the Upper Critical Zone of the eastern and western limbs. The Platreef Unit is, however, interpreted as a localized facies of the Upper Critical Zone, despite layering being neither as systematic nor as clearly defined as in the equivalent stratigraphy found in the other limbs. The Platreef Unit in the Akanani project area includes well-defined layers of feldspathic harzburgite and norite, in addition to the ubiquitous feldspathic orthopyroxenite-melanorite that characterizes other sections. The paucity of floor-rock xenoliths is an additional feature. The relatively well-developed nature of the layering and paucity of xenoliths in the Platreef Unit at Akanani is explained by separation of the unit from the floor of the intrusion by a thick succession of ultramafics assigned to the Lower Critical Zone. We identify three lithological subgroups in the Platreef Unit at Akanani. They do not define an upward-younging stratigraphy. The primary stratigraphy, or PU1 subunit, is dominated by multiple layers of feldspathic orthopyroxenite, melanorite, and norite. This subunit built up from incremental addition of relatively small magma pulses. Repeated magma replenishment induced concomitant partial melting of earlier-formed layers. The PU1 subunit includes thin chromite stringers that contain Cr-spinels with unusual, amoeboidal textures consistent with several stages of growth and re-equilibration. The feldspathic harzburgite of the younger PU2 subunit was emplaced non-sequentially into the already complexly-layered PU1 subunit as a series of sinuous lenses or syn-intrusive sills. One of the PU2 sills contains the richest and most consistent of the mineralized sections at Akanani, i.e., the Main Mineralized Reef (MMR). The irregularly-developed pegmatoidal lithologies of the PU3 subunit are ascribed to recrystallization of earlier-formed cumulates (PU1 and PU2).Whole rock isotopic data for a section of the Platreef Unit, together with the overlying Lower Main Zone and underlying Lower Critical Zone, mostly from drill-hole ZF-1, demonstrate a complex pattern in both Sr₈₇/Sr₈₆ initial ratios and Ԑ_Nd values. These patterns are consistent with multiple lineages of parental magmas. The Lower Main Zone and the majority of the Platreef Unit are characterized by anomalously high Sr initial ratios (with a large degree of scatter) and low Ԑ_Nd values (relatively tightly constrained). Harzburgite layers from the Lower Critical Zone have a low Sr initial ratio and a relatively high Ԑ_Nd value. The new isotopic data suggest these sequences crystallized from multiple magma batches, broadly constrained within the U-type (ultramafic) and A-type (tholeiitic) lineages, derived from mantle sources and/or staging chambers which experienced varying degrees of crustal contamination. The MMR crystallized from a specific pulse of the U-type magma lineage characterized by a high Sr₈₇/Sr₈₆ initial ratio (average of 0.71113) and a markedly low Ԑ_Nd value (average of -11.35). The olivine-saturated magmas associated with the MMR were derived from a localized mantle source and yet underwent an unusually high degree of crustal contamination. Some of layered PGE orebodies in the Bushveld Complex, including the Platreef and Merensky Reef, were emplaced as syn-magmatic sills which crystallized from anomalously PGE-rich parental magmas with an unique isotopic fingerprint.

中文翻译：

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Bushveld复杂区北肢Akanani的Platreef单元非连续注入富含PGE的超镁铁基岩：来自Sr和Nd同位素系统学的证据

Platreef单元是位于2.055 Ga的Bushveld复合体北肢的一个层状堆积体的看似复杂的序列。该单元包括Platreef，这是一种厚的，矿化程度高的地层结合的PGE矿体，与在东西四肢的上临界区发现的相对较薄的，主要为层状的梅伦斯基礁明显不同。然而，尽管分层不像在其他肢体中发现的等效地层那样既没有系统性也没有明确定义，但Platreef单元被解释为上临界区的局部相。阿卡纳尼项目区的Platreef单元，除了长满其他地区特征的长石质正辉石-黑铁矿外，还包括定义明确的长石玄武岩和红土层。地层岩石异质岩的匮乏是一个附加功能。在Akanani的Platreef单元中，异岩的分层和稀疏性相对发达，这是由于该单元与侵入层底部之间的分离是由一系列分配给下临界区的超镁铁质岩层造成的。我们在Akanani的Platreef单元中确定了三个岩性亚组。它们没有定义向上的地层。主要的地层学或PU1亚基由多层长石的正辉石，黑铁矿和红土组成。该子单元由相对较小的岩浆脉冲的增量相加而建立。反复的岩浆补充导致较早形成的层同时发生部分熔融。PU1子单元包含稀铬铁矿纵梁，其中包含带有异常，变形虫的纹理与生长和重新平衡的几个阶段一致。较年轻的PU2亚基的长石性哈茨石以一系列弯曲的晶状体或同侵入的基石非顺序地置于已经复杂分层的PU1亚基中。PU2基石之一包含Akanani中最丰富，最一致的矿化剖面，即主要矿化礁（MMR）。PU3亚单元的不规则发育的钉状岩性归因于较早形成的堆积物（PU1和PU2）的再结晶.Platreef单元的一部分的整个岩石同位素数据，以及上覆的下主区和下下临界区，大部分来自ZF-1钻孔，在两个 较年轻的PU2亚基的长石性Harzburgite作为一系列弯曲的晶状体或同侵入的基石非顺序地置于已经复杂分层的PU1亚基中。PU2基石之一包含Akanani中最丰富，最一致的矿化剖面，即主要矿化礁（MMR）。PU3亚单元的不规则发育的钉状岩性归因于较早形成的堆积物（PU1和PU2）的再结晶.Platreef单元的一部分的整个岩石同位素数据，以及上覆的下主区和下下临界区，大部分来自ZF-1钻孔，在两个 较年轻的PU2亚基的长石性Harzburgite作为一系列弯曲的晶状体或同侵入的基石非顺序地置于已经复杂分层的PU1亚基中。PU2基石之一包含Akanani中最丰富，最一致的矿化剖面，即主要矿化礁（MMR）。PU3亚单元的不规则发育的钉状岩性归因于较早形成的堆积物（PU1和PU2）的再结晶.Platreef单元的一部分的整个岩石同位素数据，以及上覆的下主区和下下临界区，主要来自ZF-1钻孔，在两个 PU2基石之一包含Akanani中最丰富，最一致的矿化剖面，即主要矿化礁（MMR）。PU3亚单元的不规则发育的钉状岩性归因于较早形成的堆积物（PU1和PU2）的再结晶.Platreef单元的一部分的整个岩石同位素数据，以及上覆的下主区和下下临界区，主要来自ZF-1钻孔，在两个 PU2基石之一包含Akanani中最丰富，最一致的矿化剖面，即主要矿化礁（MMR）。PU3亚单元的不规则发育的钉状岩性归因于较早形成的堆积物（PU1和PU2）的再结晶.Platreef单元的一部分的整个岩石同位素数据，以及上覆的下主区和下下临界区，大部分来自ZF-1钻孔，在两个₈₇ / Sr ₈₆初始比率和_Nd值。这些模式与父母的岩浆的多个谱系一致。下部主区和大部分Platreef单位的由异常高的Sr初始比（具有大的程度的散射）和低Ԑ特征_的Nd的值（相对严格的限制）。从下临界区方辉层具有低的Sr初始比和相对高的Ԑ_钕值。新的同位素数据表明，这些序列是从多个岩浆批次中结晶出来的，广泛地限制在U型（超音波）和A型（可塑性）谱系中，这些谱系来自地幔源和/或经历地壳污染程度的分期室。该MMR从U型岩浆谱系特征在于高的Sr的特定脉冲结晶₈₇ / SR ₈₆初始比（平均的0.71113）和显着地低Ԑ_钕值（平均值为-11.35）。与MMR相关的橄榄石饱和岩浆来自局部地幔源，但经历了异常高的地壳污染。布什维尔德综合体中的一些分层的PGE矿体，包括Platreef和Merensky礁，被放置为同岩浆基岩，从异常富集PGE的具有独特同位素指纹的父母岩浆中结晶出来。