Genesis and Multi-Episodic Alteration of Zircon-Bearing Chromitites from the Ayios Stefanos Mine, Othris Massif, Greece: Assessment of an Unconventional Hypothesis on the Origin of Zircon in Ophiolitic Chromitites

Argyrios Kapsiotis,Evangelos Tzamos,Aspasia Antonelou,Annie Ewing Rassios

doi:10.3390/min6040124

Argyrios Kapsiotis, Evangelos Tzamos + Show 2 more

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https://doi.org/10.3390/min6040124

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Abstract

Several small chromium (Cr) ore bodies are hosted within a unit of tectonically thinned dunite in the retired Ayios Stefanos mine of the western Othris ophiolite complex in Greece. Chromium ores consist of tectonically imprinted bodies of semi-massive to massive, podiform and lenticular chromitites composed of chromian spinel [Cr-spinel] with high Cr# [Cr/(Cr + Al) = 0.51–0.66] and Mg# [Mg/(Mg + Fe2+) = 0.58–0.76], low Fe3+# [Fe3+/(Fe3+ + Fe2+) ≤ 0.43] and low TiO2 (≤0.21 wt %) content. This composition is characteristic of Cr-spinels in equilibrium with melts of intermediate affinity between island-arc tholeiites (IATs) and mid-ocean ridge basalts (MORBs). Several Cr-spinel crystals in these ores exhibit imperfect zones made up of spinel hosting oriented lamellae of Mg-silicates (mostly chlorite) locally overgrown by porous domains along grain boundaries and fractures. From the Cr-spinel core to the lamellae-rich rim Cr#, Mg# and Fe3+# generally increase (0.68–0.87, 0.78–0.88 and 0.55–0.80, respectively), whereas from the core or the spinel zones with oriented lamellae to the porous domains Mg# and Fe3+# generally decrease (0.45–0.74 and ≤0.51, correspondingly). The lamellae-rich rims formed at oxidizing conditions, whereas the porous rims resulted from a later reducing event. Several tiny (≤30 μm), subhedral to anhedral and elongated Zr-bearing silicate mineral grains were discovered mainly along open and healed fractures cutting Cr-spinel. Most of the Zr-bearing silicate minerals (30 out of 35 grains) were found in a chromitite boulder vastly intruded by a complex network of gabbroic dykes. The dominant Zr-bearing silicate phase is by far zircon displaying a homogeneous internal texture in cathodoluminescence (CL) images. Raman spectroscopy data indicate that zircons have experienced structural damage due to self-irradiation. Their trace-element contents suggest derivation from a plagioclase-bearing, low-SiO2 intermediate to mafic source. Combined micro-textural and minerochemical data repeat the possibility of zircon derivation from limited volumes of high-T fluids emanating from the gabbroic intrusions. Once zircon is precipitated in cracks, it may be altered to Ca-rich Zr-bearing silicate phases (i.e., armstrongite, calciocatapleiite). Almost all zircons in these samples show evidence of gains in solvent compounds (CaO, Al2O3 and FeO) possibly due to re-equilibration with late deuteric fluids.

Highlights

Introduction and Rationale of the StudyDepleted sections of old sub-oceanic lithospheric mantle may contain substantial amounts of chromian spinel in the form of single but sometimes economically valuable deposits of Cr (e.g., [1])
It has been demonstrated that magmatic assimilation of a hydrated mantle protolith at Moho-level depths may dramatically affect the chemistry of a mid-ocean ridge basalt-type (MORB) melt causing increase of Cr concentration into economic ores [9]
Our results indicate that chromitites crystallized from melts that had an intermediate composition between that of high-Mg island-arc tholeiites (IATs: 11.4–16.4 wt % Al2 O3 ) [54] and mid-ocean ridge basalts

Summary

Introduction

Introduction and Rationale of the StudyDepleted sections of old sub-oceanic lithospheric mantle may contain substantial amounts of chromian spinel (hereafter Cr-spinel) in the form of single but sometimes economically valuable deposits of Cr (chrome ores) (e.g., [1]). It has been demonstrated that magmatic assimilation of a hydrated mantle protolith at Moho-level depths may dramatically affect the chemistry of a mid-ocean ridge basalt-type (MORB) melt causing increase of Cr concentration into economic ores [9]. Regardless of their origin, ophiolitic chromitites are widely considered as ‘miniature time capsules’ retaining priceless information on petrological issues of crucial importance such as: (i) the nature of the upper mantle [10]; (ii) mantle melting and subsequent melt extraction and interaction with peridotites [11] and lastly; (iii) recycling of the oceanic lithosphere [12]. The fundamental role of Cr-spinel and chromitites in resolving large scale petrological matters is reflected in their potential use to probe the differentiation processes in our planet’s interior up to the Transition Zone (410–660 km) [13]

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