The J-M platinum-palladium reef of the Stillwater Complex, Montana; I, Stratigraphy and petrology
The J-M platinum-palladium reef of the Stillwater Complex, Montana; I, Stratigraphy and petrology
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蒙大拿州斯蒂尔沃特综合体的 J-M 铂钯礁；
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发表时间：
1982
期刊：
影响因子：
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通讯作者：
T. Irvine
中科院分区：
文献类型：
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作者：
S. Todd;D. Keith;L. W. L. Roy;D. Schissel;E. L. Mann;T. Irvine
Johns-Manville公司对静水复合体的勘探导致发现和描绘了一个与Merensky礁相当的富含铂和钯的近似地层带。这一区域，在这里称为J-M礁，一般为1至3米厚，已追踪了40公里，基本上是侵入体露头区域内的最大可能长度。其特征是含有少量（0.5-1.0%）的黄铜矿、磁黄铁矿和镍黄铁矿，其中含有各种铂族矿物（主要是二水硬铝石、黄铜矿、铜铁矿、钛铁矿和铂铁合金）的微小颗粒。其中一段长5.5公里，2.1米处平均每短吨含铂和钯22.3克（0.65金衡盎司）。铂钯比值约为1：3.5。本文对静水杂岩进行了新的地层划分，将其划分为基底系列、超镁铁质系列和条带状系列。第一层包括基底苏长岩和基底古铜岩带，厚度共计50至150米;第二层为方辉橄榄岩带和上部古铜岩带，厚度共计800至1，200米。带状系列是一个4,500米长的富含斜长石的堆积层，J-M礁位于其基底上方400至450米处。它分为六个大旋回单元（I-VI，自下而上），其中主要带为：（I）norire，辉长岩;（II）norite，辉长岩;（III）斜长岩，troctolite-gabbro;（IV）斜长岩-troctolite，troctolite-gabbro;（V）斜长岩，troctolite-gabbro;和（VI）辉长岩，pigonite辉长岩。I和II两个薄的钙长岩-斜长岩带分别出现在I和II单元的顶部，钙长岩-斜长岩带I是J-M生物礁的寄主。钙长岩-斜长岩带I在杂岩的中西部最发育，厚约100 m，与10个相对连续的含橄榄岩段有关。这些岩段一般厚1 ~ 5 m，由橄榄岩、橄榄岩和橄榄苏长岩中的一种或多种组成。第1至4段交替出现各种不连续的辉长岩、苏长岩、辉石岩和斜长岩序列，这些序列统称为辉长岩亚带。第5 ~ 10段在斜长岩1、苏长岩和斜长岩2亚带中与斜长岩和苏长岩共生，它们都可以用一个旋回单元来识别。其中5个旋回单元的岩石类型序列为斜长岩-橄榄岩-橄榄岩;第6个旋回单元的岩石类型序列为斜长岩-橄榄长岩-苏长岩。与含橄榄石的第5段相关联的单元包含J-M礁，并延伸到复合体的两端;其他的已经不同程度地追踪了15至36 km的距离。矿物学数据和92个全岩分析，为troctolite斜长岩带1和相邻带。斜长石逐渐变得不那么钙向上通过这个带，和normatire钙长石/钠长石减少同情，但没有循环矿物学或化学趋势是明显的。镁铁质硅酸盐往往有较高的Fe/Mg内和以上的珊瑚礁比下-的差异，可能部分是主要的，但这在很大程度上是一个postcumulus效果有关的较大比例的被困（高Fe/Mg）堆间液积云（低Fe/Mg）镁铁质矿物在斜长岩1和2次带相比，辉长岩次带。镁铁质矿物中MnO和NiO也有类似的变化。S和Cu的全岩变化表明，条带状系列的母岩浆首先在J-M生物礁水平上达到连续饱和，并含有不混溶的硫化物液体，而从更高水平的其他硫化物带的成分来看，岩浆中的Pt和Pd含量似乎因生物礁的形成而有效地耗尽。积云硅酸盐矿物显示出两种主要的结晶顺序。在超镁铁质系列和巨旋回单位I和II中，较早的顺序是橄榄石、古铜岩、斜长石、普通辉石。第三至第六单元中明显的后一顺序是斜长石、橄榄石、普通辉石、古铜岩（或紫苏辉石）。钙长岩-斜长岩I带的复杂地层似乎代表了一个失败的过渡阶段。由于这两个目的是通过数千米厚的累积序列表现出来的，因此似乎涉及两种母体岩浆，第一种具有超镁铁质亲和力，第二种具有斜长岩亲和力，并且迹象表明J-M礁中的Pt和Pd来自第一种，而S来自第二种。通过混合这些液体形成珊瑚礁的机制将在本系列的第二篇论文中描述。
Exploration of the Stillwater Complex by the Johns-Manville Corporation has led to the discovery and delineation of an approximately stratigraphic zone rich in platinum and palladium comparable to the Merensky Reef. This zone, here called the J-M reef, is generally 1 to 3 m thick and has been traced for 40 km, essentially its maximum possible length within the outcrop area of the intrusion. It is distinguished by small concentrations (0.5-1.0%) of chalcopyrite, pyrrhotite, and pentlanditc in which are contained tiny grains of various platinum-group minerals (mainly moncheite, braggite, cooperite, kotulskite, and Pt-Fe alloy). One segment, 5.5 km long, averages 22.3 g (0.65 troy oz) Pt and Pd per short ton through 2.1 m. The ratio of Pt to Pd typically is about 1:3.5.A new stratigraphic classification of the Stillwater Complex is presented in which three long-standing, major divisions are renamed the basal, ultramafic, and banded series. The first comprises basal norite and basal bronzitite zones totaling 50 to 150 m in thickness; the second, harzburgite and upper bronzitite zones totaling 800 to 1,200 m. The banded series is a 4,500 m succession of plagioclase-rich cumulates with the J-M reef located 400 to 450 m above its base. It is divided into six megacyclic units (I-VI, from bottom to top) in which the principal zones are: (I) norire, gabbro; (II) norite, gabbro; (III) anorthosite, troctolite-gabbro; (IV) anorthosite-troctolite, troctolite-gabbro; (V) anorthosite, troctolite-gabbro; and (VI) gabbro, pigeonite gabbro. Two thin troctolite-anorthosite zones, I and II, occur at the top of units I and II, and troctolite-anorthosite zone I is host to the J-M reef.Where troctolite-anorthosite zone I is best developed in the west-central part of the complex, it is about 100 m thick and is associated with ten relatively continuous olivine-bearing members. These members are generally 1 to 5 m thick and are composed of one or more of peridotite, troctolite, and olivine norite. Members 1 to 4 alternate with varied, discontinuous successions of gabbro, norite, pyroxenite, and anorthosite that are collectively assigned to a gabbro subzone. Members 5 to 10 occur with anorthosite and norite in the anorthosite 1, norite, and anorthosite 2 subzones, and they can each be identified with a cyclic unit. The type rock sequence in five of the cyclic units is anorthosite-peridotite-troctolite; in the sixth, it is anorthosite-olivine norite-norite. The unit associated with olivine-bearing member 5 contains the J-M reef and extends to both ends of the complex; the others have variously been traced for distances of 15 to 36 km.Mineralogical data and 92 whole-rock analyses are presented for troctolite-anorthosite zone 1 and the adjoining zones. Plagioclase becomes gradually less calcic upward through this zone, and normatire anorthite/albite decreases sympathetically, but no cyclic mineralogic or chemical trends are evident. The mafic silicates tend to have higher Fe/Mg within and above the reef than below--a difference that may in part be primary but that is largely a postcumulus effect relating to the larger ratio of trapped (high Fe/Mg) intercumulus liquid to cumulus (low Fe/Mg) mafic minerals in the anorthosite 1 and 2 subzones as compared with the gabbro subzone. Similar variations are shown by MnO and NiO in the mafic minerals. The whole-rock variations of S and Cu indicate that the parental magma(s) of the banded series first reached continuous saturation with immiscible sulfide liquid at the level of the J-M reef, and from the compositions of other sulfide zones at higher levels it appears that the Pt and Pd contents of the magma(s) were effectively exhausted by the formation of the reef.Through the whole of the Stillwater Complex, two principal crystallization orders are indicated for the cumulus silicate minerals. The earlier order, defined in the ultramafic series and in megacyclic units I and II, is olivine, bronzite, plagioclase, augite. The later order, evident in units III through VI, is plagioclase, olivine, augite, bronzite (or hypersthene). The complicated stratigraphy of the troctolite-anorthosite zone I appears to represent an aborted transitional stage. Because both orders are manifest through cumulate successions thousands of meters thick, it would appear that two parental magmas were involved, the first with ultramafic affinities and the second with anorthositic affinities, and indications are that the Pt and Pd in the J-M reef came from the first, and the S from the second. A mechanism for forming the reef by mixing these liquids will be described in the second paper of this series.