期刊
ORE GEOLOGY REVIEWS
卷 134, 期 -, 页码 -出版社
ELSEVIER
DOI: 10.1016/j.oregeorev.2021.104166
关键词
Ophiolitic chromitites; Unusual (exotic) mineral assemblages; Trace elements; Platinum group minerals (PGM); Subduction-initiation; Fore-arc basalts (FAB)
资金
- Fondo Europeo de Desarrollo Regional (FEDER) Funds
- Spanish Projects [CGL2015-65824, RTI2018099157AI00, PID2019105625RBC21, A.RNM.186.UGR18]
- FPUPhD grant by the Ministerio de Educacion of the Spanish Government
- Ramon y Cajal Fellowship [RYC-2015-17596]
- Mexican research program CONACYTCiencia Basica [A1-S-14574]
- Mexican research program UNAM-PAPIIT grant [IA-101419]
Research on the chromitite orebodies in the Bou Azzer area of Morocco reveals the presence of igneous relicts and suggests formation from different pulses of parental melts. Platinum group elements mineralization is found in the chromitites, along with unusual mineral phases, indicating formation in a subduction-initiation geodynamic setting.
The Neoproterozoic Bou Azzer ophiolite in the Moroccan Anti-Atlas Panafrican belt hosts numerous chromitite orebodies within the peridotite section of the oceanic mantle. The chromitites are strongly affected by serpentinization and metamorphism, although they still preserve igneous relicts amenable for petrogenetic interpretation. The major, minor and trace element composition of unaltered chromite cores reveal two compositional groups: intermediate-Cr (Cr# = 0.60 - 0.74) and high-Cr (Cr# = 0.79 - 0.84) and estimates of parental melt compositions suggest crystallization from pulses of fore-arc basalts (FAB) and boninitic melts, respectively, that infiltrated the oceanic supra-subduction zone (SSZ) mantle. A platinum group elements (PGE) mineralization dominated by Ir-Ru-Os is recognized in the chromitites, which has its mineralogical expression in abundant inclusions of Os-Ir alloys and coexisting magmatic laurite (RuS2) and their products of metamorphic alteration. Unusual mineral phases in chromite, not previously reported in this ophiolite, include super-reduced and/or nominally ultra-high pressure minerals moissanite (SiC), native Cu and silicates (oriented clinopyroxene lamellae), but exotic zircon and diaspore have also been identified. We interpret that clinopyroxene lamellae have a magmatic origin, whereas super-reduced phases originated during serpentinization processes and diaspore is linked to late circulation of low-silica fluids related to rodingitization. Zircon grains, on the other hand, with apatite and serpentine inclusions, could either have formed after the interaction of chromitite with mantlederived melts or could represent subducted detrital sediments later incorporated into the chromitites. We offer a comparison of the Bou Azzer chromitites with other Precambrian ophiolitic chromitites worldwide, which are rather scarce in the geological record. The studied chromitites are very similar to the Neoproterozoic chromitites reported in the Arabian-Nubian shield, which are also related to the Panafrican orogeny. Thus, we conclude that the Bou Azzer chromitites formed in a subduction-initiation geodynamic setting with two-stages of evolution, with formation of FAB-derived intermediate-Cr chromitites in the early stage and formation of boninite-derived high-Cr chromitites in the late stage.
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