A structural approach to the genesis of chrome ores within the Vourinos ophiolite (Greece): Significance of ductile and brittle deformation processes in the formation of economic ore bodies in oceanic upper mantle peridotites
Graphical abstract
Introduction
Ophiolites are suites of temporally and spatially associated ultramafic, mafic, and felsic rocks that represent on-land remnants of ancient oceanic crust and upper mantle (Dilek and Furnes, 2011). They are the best archives of the evolutionary history of ancient oceanic lithosphere from its rift–drift and seafloor spreading stages to subduction initiation and recycling into the mantle (Dilek and Eddy, 1992, Dilek et al., 2008, Dilek and Yang, 2018). Recent studies of ophiolites and their results have shown that the mineralogy and geochemical compositions of crustal and mantle sequences in most ophiolites do not represent a simple melt–residua relationship, as presumed in the original 1972 Penrose definition, because refertilization of mantle peridotites and their interactions with off-axis melts near mid-ocean ridges and with slab–derived fluids and melts resulted in the modification of their whole-rock chemistry and mineralogical compositions (Dilek and Furnes, 2014, and references therein). Chromitite deposits are a significant component of an ophiolite litho-stratigraphy, occurring in the upper mantle sequence, within the Moho transition zone, and even in the lowermost fossil oceanic crust. These chromitite occurrences are highly important not just because of their economic value, but because of their significance in better understanding the mode and nature of those fluid/melt and rock interactions in the oceanic mantle and the geochemical differentiation processes taking place in the deeper mantle (Dilek and Yang, 2018). Chromites in the peridotites of most suprasubduction zone (SSZ) – originated ophiolites contain inclusions of diamonds, ultra high-pressure minerals and native elements, indicating very high pressure–temperature and ultra–reducing conditions of the mantle transition zone (MTZ) for the integration of these highly unusual earth material into chromites. This means that petrogenesis of ophiolitic chromites might have started at significantly deeper levels in the mantle than shallow partial melting depths beneath oceanic spreading centers, as the conventional models for oceanic lithosphere formation predict (Dilek and Yang, 2018, and references therein). Thus, chromites and chromitite deposits in ophiolites are extremely valuable repositories of material and information to explore the mantle dynamics, chemical fluxes within the oceanic mantle, and the two–way mass transfer between different layers of the earth (González-Jiménez et al., 2014).
The formation of chromitite deposits in ophiolites has been investigated extensively, particularly in regard to their geochemistry and mineralogy in relation to the geochemistry and melt evolution of their host peridotites, and their platinum-group element (PGE) abundances and patterns. There are different types of chromitites based on their geochemical features, and models for the formation of these geochemically and isotopically different chromitites vary significantly (see González-Jiménez et al., 2014, and the references therein for an overview). However, structural studies of chromites and chromitites in ophiolites have been limited and less definitive than geochemical approaches, even though chromitites are spatially and temporally associated with some of the most deformed ultramafic rocks and high–strain zones, where melt migration and transport played a significant role in the mantle dynamics.
In this paper, we present a comprehensive synthesis of fieldwork and analytical research conducted over 40 years of exploration (much of it previously unpublished) in the chromitite deposits of the Jurassic Vourinos ophiolite in northern Greece. Our field observations and interpretations are based on macroscopic-, mesosocopic-, and microscopic-scale analyses of a wide–spectrum of ductile and brittle deformation structures displayed by chromitites and their host peridotites in this ophiolite. We use these observations and interpretations to discuss the physical conditions and structural processes of the formation of chromitites in the upper mantle peridotites and within the Moho transition zone, without getting into the topic of geochemical processes involved in their formation. We first document characteristic features of chromitite bodies and their host peridotites in Vourinos and fabric–texture types of Cr-spinel in ore hosts, and then discuss processes of mechanical enrichment of chrome ores through deformation. Next, we evaluate the types and scales of different processes of re-equilibration of chromite in deforming host peridotites, and we discuss the changes in the chromite mineral chemistry along–across ductile–brittle deformation zones. In the last section of the paper, we discuss deformation as a critical factor in generating economic–scale chrome deposits in the Vourinos ophiolite. This paper, thus, presents a unique case study with a structural approach to the genesis of chrome ores, and highlights the significance of ductile–brittle deformation processes in the formation of economic ore bodies in the oceanic upper mantle. The combination of our structural observations and data with a comprehensive mineral chemistry database (see the Supplementary file) should provide the global scientific community of ore deposit studies with a rich resource of three-dimensional images of chromitite deposits with associated structural fabrics and microscopic-scale chemical profiles.
Section snippets
Why the Vourinos ophiolite for an In-depth study of Chromitites?
The Vourinos ophiolite of northern Greece (Fig. 1) figured prominently in the documentation of ophiolites as on-land analogues of ancient oceanic lithosphere during the emergence of plate tectonic theory (Moores, 1969, Moores, 2003). Even earlier, Vourinos was well-known as a host to economic-scale chrome ore deposits (Zachos, 1954). Explorationist studies in the late 1970′s to 1990′s produced detailed field maps (1:10,000 scale) of all chromitite occurrences, and maps (down to 1:1000 and 1:500
Materials and methods
More than 640 peridotite and chromite ore samples were collected during more than four-decades of systematic fieldwork by the authors in the Vourinos ophiolite. The majority of these samples were systematic collections within ore-bearing dunite and through exploratory drill cores. Structural analysis of deformation fabrics was done by conventional microtectonic mapping at various scales and by evaluation of oriented rock samples: field analyses for estimating temperatures and pressures of
Chrome ore setting
The Vourinos ophiolite has been interpreted as a supra-subduction zone (SSZ) ophiolite (Dilek and Furnes, 2009, Dilek and Furnes, 2011, Mosier et al., 2012, Furnes et al., 2020), as are the majority of ophiolite complexes around the world (Metcalf and Shervais, 2008, Dilek and Thy, 2009, Dilek and Furnes, 2014). Many authors have concluded that SSZ oceanic lithosphere, particularly ultra-depleted harzburgite in a forerarc mantle, is a preferential host for ophiolitic chromite deposits due to
Deformation of Cr-Spinel
The deformation of Cr-spinel is not well studied. Strain patterns within the chromite lattice demonstrate that dislocation occurs under conditions of high temperature ductile deformation with annealing of chromite by grain boundary migration (mantle flow, Christiansen, 1986). Many dunites within the ultramafic rocks of Vourinos have a mosaic fabric characteristic of annealed olivine (such fabrics are commonly retained within serpentinised dunite). Small Cr-spinel grains in mosaic–fabric dunite
Processes of Re-equilibration
Following the general description of deformation and rheological conditions, we now consider how these conditions could affect the composition of Cr-spinel. The temperature conditions of re-equilibration entirely overlap with those of ductile deformation conditions (Rassios and Kostopoulos, 1990, Grieco et al., 2018).
Pristine magmatic chromite compositions evolve via subsolidus re-equilibration during adiabatic cooling within a lithospheric slab “drifting” away from a spreading center.
Origin of chromitites: A structural perspective
The parental magmas of chromitites need not initially contain original high chrome concentrations (Boudier and Al-Rajhi, 2014). Boninitic magmas contain ~ 200–1800 ppm Cr (e.g. Hickey and Frey, 1982). MOR averages overlap the lower end of this range with 81–406 ppm (Gale et al., 2013). Chrome ore-bearing layered igneous intrusions contain levels of Cr in parental magmas (estimated about 400 ppm in continental tholeiites) similarly overlapping the boninitic group. Some layered igneous intrusions
Conclusions and new investigative directions
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Chromitites crop out within the entirety of the 8 – km – thick section of the Vourinos ultramafic section in sub- and supra Moho dunite hosts. The various textural types of these chromitites occur independently of their structural positions, indicating that regional stress conditions do not contribute to an ore-positive zone at some constant depth or location beneath the ridge crest. Pressure changes accompanying magmatic intrusion could contribute to a “rapid” response of chromite
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Early parts of the study were financed by a National Science Foundation Grant (1975–1980), by Greek Public Work research grants, and a Brite-Eurham grant (1986–1994). E Tzamos would like to thank Greek National Scholarships Foundation (IKY) for its financial support during his doctoral and postdoctoral research. Y Dilek acknowledges financial support from Miami University and for his fieldwork in the Vourinos ophiolite. We thank many colleagues who have discussed the geology of the Vourinos
References (98)
- et al.
Podiform chromitites do form beneath mid-ocean ridges
Lithos
(2015) The elastic thickness of the lithosphere in the Pacific Ocean
Earth Planet. Sci. Lett.
(1987)Deformation of chromite: SEM investigations
Tectonophysics
(1986)- et al.
Fingerprints of metamorphism in chromite: New insights from minor and trace elements
Chem. Geol.
(2014) - et al.
Structure and geochemistry of Tethyan ophiolites and their petrogenesis in subduction rollback systems
Lithos
(2009) - et al.
Island arc tholeiite to boninitic melt evolution of the Cretaceous Kizildag (Turkey) ophiolite: Model for multi-stage early arc-forearc magmatism in Tethyan subduction factories
Lithos
(2009) - et al.
Chromitites in ophiolites: How, where, when, why? Part I. A review and new ideas on the origin and significance of platinum-group minerals
Lithos
(2014) - et al.
Processes of primary and re-equilibration mineralization affecting chromite ore geochemistry within the Vourinos ultramafic sequence, Vourinos Ophiolite (West Macedonia, Greece)
Ore Geol. Rev.
(2018) - et al.
Refractory chromitites recovered from the Eretria mine, East Othris massif (Greece): Implications for metallogeny and deformation of chromitites within the lithospheric mantle portion of a forearc-type ophiolite
Chem. Erde
(2019) Compositional signatures of SSZ-type peridotites from the northern Vourinos ultra-depleted upper mantle suite NW Greece
Chemie der Erde-Geochem.
(2014)
Compositional fingerprints of chromian spinel from the refractory chrome ores of Metalleion, Othris (Greece): Implications for metallogeny and deformation of chromitites within a “hot” oceanic fault zone
J. Geochem. Explor.
The structure of and origin of nodular chromite from the Troodos ophiolite, Cyprus, revealed using high resolution X-ray computed tomography and electron backscatter diffraction
Lithos
Rotational deformation in the Jurassic Mesohellenic ophiolites, Greece, and its tectonic significance
Lithos
The Vourinos ophiolite complex, Greece: the tectonite suite
Tectonophysics
Major and minor element geochemistry of chromite from the Xerolivado-Skoumtsa mine, Southern Vourinos: Implications for chrome ore exploration
J. Geochem. Explor.
Metallogeny of the Chrome Ores of the Xerolivado-Skoumtsa Mine, Vourinos Ophiolite, Greece: Implications on the genesis of IPGE-bearing high-Cr chromitites within a heterogeneously depleted mantle section
Ore Geol. Rev.
Ophiolite-type diamond: A new occurrence of diamond on the Earth
Geophys. Res. Abstracts
Possibly diamond-bearing mantle peridotites and podiform chromitites in the Luobusa and Donqiao ophiolites
Tibet. Can. J. Earth Sci.
High pressure experimental calibration of the olivine-orthopyroxene-spinel oxygen geobarometer: Implications for the oxidation state of the upper mantle
Contrib. Miner. Petrol.
Chromite in Komatiites, II. Modification during greenschist to mid-amphibolite facies metamorphism
J. Petrol.
Structural control on chromitite deposits in ophiolites: the Oman case
Geological Society, London, Special Publications
Rheology of the lower crust and upper mantle: Evidence from rock mechanics, geodesy, and field observations
Ann. Rev. Earth Planet. Sci.
Formation of olivine pseud-crescumulates by syntectonic axial planar growth during mantle deformation
Geol. Mag.
Ophiolites, Ancient Oceanic Lithosphere?
The Troodos (Cyprus) and Kizildag (S. Turkey) ophiolites as structural models for slow-spreading ridge segments
J. Geol.
Ophiolite genesis and global tectonics: Geochemical and tectonic fingerprinting of ancient oceanic lithosphere
Geol. Soc. Am. Bull.
Origins of ophiolites
Elements
Ophiolites, diamonds, and ultrahigh-pressure minerals: New discoveries and concepts on upper mantle petrogenesis
Lithosphere
Tethyan ophiolites and Tethyan seaways
J. Geol. Soc. London
Tectonic evolution of the Troodos ophiolite within the Tethyan framework
Tectonics
Ore Deposit Models 7. Magmatic Segregation Deposits of Chromite Geoscience Canada
J. Geol. Assoc. Canada
Equilibria involving Al-Cr spinel: Mg-Fe exchange with olivine experiments, thermodynamic analysis, and consequences for geothermometry
Am. J. Sci.
Spinel-olivine geothermometry in peridotites from ultramafic complexes
Contrib. Miner. Petrol.
Geochemical characterization of ophiolites in the Alpine-Himalayan Orogenic Belt: Magmatically and tectonically diverse evolution of the Mesozoic Neotethyan oceanic crust
Earth Sci. Rev.
The mean composition of ocean ridge basalts
Geochem. Geophys. Geoyst.
Formation of ferrian chromite in podiform chromitites from the Golyamo Kamenyane serpentinite, Eastern Rhodopes, SE Bulgaria: a two-stage process
Contrib. Miner. Petrol.
Structure and tectonics of subophioliticmelanges in the western Hellenides (Greece): Implications for ophiolite emplacement tectonics
Int. Geol. Rev.
Spinel composition, PGE geochemistry and mineralogy of the chromitites from the Vourinos ophiolite complex, northwestern Greece
Can. Mineral.
Drilling for “blind“ podiform chrome orebodies at Voidolakkos in the Vourinos ophiolite complex Greece
Econ. Geol.
Multiple intrusive events documented from the Vourinos ophiolite complex, northern Greece
Am. J. Sci.
Geochemical characteristics of boninite series volcanics; implications for their source
Geochim. Cosmochim. Acta
Chromian spinel as a petrogenetic indicator: Part 1 Theory
Can. J. Earth Sci.
Chromian spinel as a petrogenetic indicator: Part 2. Petrologic applications
Can. J. Earth Sci.
The Vourinos ophiolite, Greece: Cyclic units of lineated cumulates overlying harzburgite tectonite
Geol. Soc. Am. Bull.
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