Layering and Precious Metals Mineralization in the Rincón del Tigre complex, Eastern Bolivia

M.D.Prendercast

摘要

Geologic investigations supporting recent,technically successful, platinum exploration of the Rincón delTigre Complex-a ca. 4.6-km-thick layered sill-have providedsignificant new understandings of the layering development, andprecious metals mineralization of one of South America's largestmafic-ultramafic intrusions. Three macrocyclic units arerecognized-from the base up Basal unit, Unit 1, and Unit 2-eachmost likely the product of a major replenishment with maficmagma of one compositional type. The ultramafic rocks of allthree macrocyclic units largely comprise repeated minor cyclicunits of poikilitic harzburgite-> granular harzburgite (-> olivinebronzirite) each a few meters thick, with occasionaldevelopment of bronzitite at the top. Mafic rocks, absent fromUnit 1 and present only locally in the Basal unit, form a well-developed norite ->gabbro->magnefite gabbro sequence in theupper half of Unit 2.Associated with the base of the magnetite gabbro is the PreciousMetals zone, a persistent zone of low-grade (subeconomic)sulfide and precious metals mineralization, 80 to 185 m thick.The Precious Metals zone comprises an upper Cu sulfide-richportion, its base (the sulfide phase boundary) about 23 to 70 mabove the magnetite phase boundary, and a lower preciousmetals-rich portion, the lower part straddling the magnetitephase boundary. The individual precious metals are concentratedin separate, sulfide-poor subzones, each up to many tens ofmeters thick, in the stratigraphic order (Rh ->)Pt -> Pd -> Au,with peak offsets at the same scale. Modal layering of silicatesand magnetite is well developed in the mafic rocks as is modaland cryptic (precious metals) layering of the sulfides in thePrecious Metals zone, all at three different superimposed scalesfrom about 3 m to several tens of meters. A possible interpretiveframework for the cryptic layering (and perhaps for much of theother layering, including the meter-scale layering of theultramafic rocks) is provided by the likely compositionalstratification of the resident magma initiated by the fluiddynamics of the replenishment and mixing processes and, in themagnetite gabbro, by repeated convective overturns caused bythe onset of magnetite precipitation.The characteristics of the Precious Metals zone fit anorthomagmatic model. The composition, stratigraphic relations,and modal correlation (with magnetite) of the sulfides all implythat S saturation was induced by loss of FeO to magnetiteprecipitation. This took place after prolonged closed-systemfractionation (following the last major magma injection at thebase of Unit 2) during which crystallization of olivine, pyroxene,plagioclase, and (finally) magnetite precipitation led to overall S,Cu, and precious metals enrichment, and Ni and (finally) Fedepletion of the magma. The development of the mineralizationand its complex layering was controlled by sulfide segregation,Rayleigh fractionation, the different but generally very highpartition coefficients (D_(silicate/sulfide)) for each preciousmetal, and repeated convective overtures at three different superimposed scales.The decoupled metals distribution is analogous and similar inorigin to that of the Main Sulfide zone of the Great Dyke(Zimbabwe) and of the Main Sulfide layer (offset portion) of theMunni Munni Complex (Western Australia), but thestratigraphic order of metal enrichment peaks is different (Pt->Pd-> Au vs. Pd -> Pt ->Au). This suggests a reversal in therelative magnitudes of the partition coefficients D_(pt) andD_(Pd) that was possibly brought about by changes in thetemperature and 0, 5, and Fe activities of the magmas betweenthe middle and late stages of crystallization. Constancy of Dvalues during fractionation of mafic magma should not be assumed.The Precious Metals zone is one of several Skaergaard-typestrata-bound precious metals zones associated with magnetitegabbros in the upper levels of many layered intrusions, and theorigin of the Skaergaard mineralized zone itself-incl

机译：地质调查支持RincóndelTigre Complex-a ca的近期技术上成功的铂金勘探。厚达4.6公里的分层基岩对南美洲最大的镁铁-超镁铁矿侵入之一的分层发展和贵金属矿化提供了重要的新认识。识别出三个大环单元-从基本向上的基础单元，单元1和单元2，最有可能是具有一种成分类型的黑手岩浆的主要补充的乘积。所有三个大环单元的超镁铁质岩石主要由重复的次要循环单元组成，即钾长石->颗粒状长石（->橄榄石青铜矿），每层厚几米，偶尔在顶部发育青铜矿。 1号单元中缺少的镁铁质岩石仅在基底单元中局部存在，在2号单元的上半部形成了发育良好的Norite-> gabbro->菱镁矿辉长岩序列.PreciousMetals带是磁铁矿辉长岩的底部。低品位（亚经济）硫化物和贵金属矿化的持久带，厚度为80至185 m。贵金属区包括一个富集铜的上部硫化物，其基底（硫化物相界）约23至70磁铁矿相界，以及下部富含贵金属的部分，下部跨越磁铁矿相边界。各个贵金属以地层顺序（Rh-> Pt-> Pd-> Au）集中在单独的，贫硫化物的分区中，每个分区的厚度可达数十米，峰值偏移量相同。在镁铁质岩石中，硅酸盐和磁铁矿的模态层发育良好，在贵金属带中硫化物的模态和隐性（贵金属）层也是如此，这三个层次的叠加比例都在大约3 m至几十米之间。隐伏层的可能解释框架（也许还有其他许多层，包括超塑性岩石层的米级分层）是由补充和混合过程的流体动力学以及磁铁矿引起的常驻岩浆的可能成分分层提供的。由于磁铁矿沉淀的开始而引起的反复对流翻转，从而形成了辉长岩。贵金属带的特征符合反磁模型。硫化物的组成，地层关系和模态相关性（与磁铁矿）都暗示着S饱和是由于FeO损失到磁铁矿沉淀中而引起的。这发生在长时间的封闭系统裂解之后（在单元2的底部进行了最后一次大岩浆注入之后），在此期间橄榄石，辉石，斜长石和（最终）磁铁矿的结晶导致了S，Cu和贵金属的整体富集，并且镍和（最后）岩浆的吞没。矿化的发展及其复杂的分层受硫化物的偏析，瑞利分馏，每种贵金属的不同但通常很高的分配系数（D_（硅酸盐/硫化物））以及在三个不同叠加比例下的对流序曲的控制。与大堤（津巴布韦）的主要硫化物带和Munni Munni复杂物（西澳大利亚州）的主要硫化物层（偏移部分）相似，但金属富集峰的地层顺序不同（Pt- > Pd-> Au与Pd-> Pt-> Au）。这表明分配系数D_（pt）和D_（Pd）的相对大小发生了逆转，这可能是由于温度的变化以及结晶中期和后期之间岩浆的0、5和Fe活性引起的。在镁铁质岩浆分馏过程中Dvalues的常数是不恒定的。贵金属带是许多分层侵入体上层中与磁石辉石相关的几个Skaergaard型地层结合贵金属带之一，Skaergaard矿化带本身的起源-含

Layering and Precious Metals Mineralization in the Rincón del Tigre complex, Eastern Bolivia

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