Κοιτασματογένεση πλατινοειδών ορυκτών και χρωμιτών συνδεόμενων με την πετρογενετική εξέλιξη των οφιολιθικών συμπλεγμάτων Βούρινου και Πίνδου
Authors: Καψιώτης Αργύριος 
Published: January 2014

In the present thesis the processes of the genesis of the chomitite occurrences of the Vounnos and Pindos ophiolite complexes combined with their content in platinum group elements are studied Special attention is given to the petrogenetic evolution of the mantle units of both ophiolitesequences in an attempt to understand better the origin and the formation of the ophiolites of the NW Greek area In geotectonic terms the ophiolite complex of Vounnos belongs to the Ypopelagonic isopic zone while the Pindos ophiolites are tectonically thrusted onto the Eocenic flysch of the Pindos zone Both complexes are considered as remnants of a certain ocean which today are found tectonically thrusted over the west margin of the Pelagonic micro plate Beyond the fact that both complexes show characteristics of intense tectonic deformation they do preserve all the types of the petrologie sequences that compose a complete ophiolite complex All the formations that have been studied in the present thesis are coming from the mantle units of both complexes The main part of the Vounnos ophiolite complex is composed of mantle tectonites while the mantle formations in thePindos ophiolite complex compose the so called Dramala unit In the Vounnos complex the samplingcovered the most important ore hosting areas of its northern and southern part while in the Pindoscomplex several samples were taken from the central southern parts of the Dramala unit and fromexotic blocks of mantle origin of the Avdella melange unit The field investigation in both complexes has led to the following conlusions a) The northern part of the Vounnos ophiolite complex shows clearly more characteristics of plastic deformation compared to its southern part in which characteristics of brittle to semi plastic deformation prevail A similar but not so well expressed discrimination can be made as far as concerns the structure of the areas of Miha and Pefki in the Pindos complex b) The statistical analysis of several motional indicators shows a NE thrust direction for the northern sector of the Vounnos complex and the area of Miha and a SE direction of motion for southern Vounnos and Pefki c) The chromitites are hosted in dunites and in the majority of the chromitite occurrences the accompanying rock type is thingrained dunite (serpentinised or not) In contrast to coarse grained dunite the thin-grained variety of dunite is not foliated d) The chromitites of northern Vounnos and Miha are usually of podiform type discordantly to sub-concordantly developed to their neighbouring thick-grained dunitic to harzburgitic pendotites The chromite ores of southern Vounnos and Pefki are stratiform having a concordant to sub concordant structure e) Chromitites of both complexes show a great variety of chromite textures although in each deposit a certain type of texture predominates In northern Vounnos and Miha the dominant textural type is massive and leopard chromitite while in southern Vounnos and Pefki the most common texture in chromitite is schlieren f) In southern Vounnos and Pefki are commonly observed interlayered bodies of harzburgite and dunite which are indicative of a stratified structure of the residual mantle superimposed on these areas The number of the pyroxenite dykes (rodingitised or not) is higher in these areas compared to those in northern Vounnos and Miha In their vast majority these dykes cut chromitite bodies which indicates that they are not genetically related to the ores except for one case of a chromitite that is hosted in a pyroxenite dyke in the northern part of the area of Pefki g) Always in the vicinity of the chromitites the lithology follows the sequence ±Clinopyroxene bearing harzburgite normal harzburgite transitional harzburgite coarse grained dunite thin-grained dunite chromitite A different lithological transition is quite unusual h) The common entrapment of harzburgite blocks in dunites indicates that the latter comes from the former although replacements of foliated harzburgites from thin grained dunite that show no mark of foliation supports the fact that some rocks may not be simply of residual origin I) The presence of clinopyroxene bearing harzburgites is more common in the Pindos ophiolite complex while in the Vounnos ophiolite complex bodies of thin-grained dunite in the form of dykes layers or lenses are more commonly observed The pétrographie investigation of the mantle pendotites has shown that the clinopyroxenebeanng harzburgites and the normal harzburgites are characterized by protogranular texture However the transition from normal to transitional harzburgites is marked by the appearance and the dominance of the porphyroclastic texure Porphyroclastic to mylonitic is the texture of the coarsegrained dunites while their thin grained variety shows (equi )granular texture The most important pétrographie feature of all the investigated rock types except for the thin grained dunites is the replacement of the orthopyroxene porphyroclasts by olivine and generally their enrichment in olivine Sparsely neo-formation of pyroxene has been observed and especially replacement of orthopyroxene by clinopyroxene and of olivine by orthopyroxene The changes in the morphology of accessory Cr spinels with the type of the pendotite is another significant feature of the investigated rocks The most fertile rocks preserve anhedral brown Cr spinels while the most depleted euhedral dark Cr rich spinels In very few cases anhedral pyroxene spinel pairs which are interpreted as break down product of a garnet precursor have been observed Serpentinisation effects in various extents are common as well In the Pindos ophiolite complex was noticed the occurrence of serpentinites that show clear features of re crystallisation like hour glass and interpenetrating texture high contents of magnetite and chlorite In the assemblage of the chromitites participates olivine and rarely other mineral phases as pyroxenes The serpentinisation effect leads to the transformation of the silicate matrix of the chromitites into serpentine tremolite and chlorite In both chromitites and pendotites primary hydrous minerals were not found except for one case of Mg rich horneblende in a harzburgite from the Pindos massif The inclusions that were found in the chromite crystals of the chromitites are both primary and secondary in origin Except for olivine pyroxenes serpentine chlorite actinolite tremolite rare base metal sulfide (BMS) and alloy crystals in the Vounnos chromitites edenitic amphibole was also found as inclusion In the Pindos chromitites edenitic amphibole phlogopite and hydrogamet also take part as inclusions The presence of hydrous minerals in the form of inclusions in the chromitites of both complexes indicates the hydrous nature of the magma that lead to their formation Additionally the absence of phlogopite in the Vounnos chromite ore deposits is indicative of a high ratio of fluid/rock The majority of the inclusions was found in chromitites of massive and leopard texture The general absence of BMS as inclusions in the mantle phases and their restncted occurrence in interstitial places in the pendotites of both complexes indicates a) Depletion of the pendotites in sulfides because they have been affected by high degrees of melting b) later processes of impregnation of the pendotites by melts that were efficient at crystallising interstitially grains of Fe Ν ι sulfides As for the chromitites only these of Korydallos do clearly present syncrystallisation phenomenae of chromite with sulfides The sulfides occur both as inclusions in chromite and in interstitial places as well which means that the magma could generate immiscible sulfide droplets even at the time of chromite crystallisation However the common assemblge Awaruite pentlandite miliente that was described in the mantle pendotites of Vounnos and Pindos can not be formed as product of the fractionation process of a sulfide melt that is exsolved from a silicate magma This assemblage represents the final product of two combined processes a) The local reduction of primary sulfide grains and b) the neo formation of part of them via hydrothermal fluids The accessory Cr-beanng spinels of the ultrabasic rocks from Vounnos and Pindos are classified as magnesiochromites as do the chromite crystals of the ores The same phases of the pendotites and a part of those in the chromitite from the area of Korydallos are classified as spinels due to their more ΑΙ-rich composition The magnesiochromites of the clinopyroxene bearing and normal harzburgites have Cr# smaller than 0 60 and are usually depleted in Ti02 The magnesiochromites of the other formations are richer in Ti02 and have Cr# greater than 0 60 The spinels of the Korydallos pendotites have Cr# that ranges between 0 36-0 42 The magnesiochromites of the Vounnos chromitites have Cr# that range from 0 58 up to 0 83 while those from the Pindos ores have Cr# between 0 74 0 84 The spinels of the chromitite from Korydallos have Cr# that range from 0 43 to 0 69 The compositions of the accessory magnesiochromites from the clinopyroxene bearing and the majority of the normal harzburgites reflect the residual origin of these pendotite types which have been formed in a mid-ocean ridge (MOR) The magnesiochromites of higher Cr# indicate that the pendotites that host them constituted part of a mantle wedge above a supra subduction zone (SSZ) These magnesiochromites are similar in compositional terms to spinels in equilibrium with boninite melts The compositions of the mantle spinels from the area of Korydallos are similar to those from Ν MOR basalts Mantle pyroxenes of both complexes show compositional characteristics representative ofthose from SSZ and especially fore arc pendotites Additionally mantle clinopyroxenes presentanomalous compositional core-nm zoning The Vounnos mantle clinopyroxenes show an increase inSi02 Ti02 and CaO contents from core to rim while mantle clinopyroxenes from Pindos show anincrease in Ti02 and CaO contents towards their rims The fact that mantle clinopyroxenes of bothcomplexes exhibit high Mg# (>0 94) (higher than that of the other mantle phases) is indicative of an episode of cryptic metasomatism by which the pendotites have been affected Another important compositional characteristic of the pyroxenes is their depleted nature in REE+Y The depletion is extreme for the LREE-MREE which is supportive of their residual rather than of their magmatic origin The olivine crystals of the pendotites of both complexes exhibit high Mg# which is characteristic of olivines from pendotites of SSZ origin Their high NiO content does not follow thenormal compositional trend of mantle olivines a feature that supports the addition of olivine in therocks by metasomatic processes The olivine crystals that are hosted in the chromitites of sourthernVounnos chromitites tend to be poorer in NiO and richer in MnO compared to those of the chromitites of the northern part of the complex The plots of the compositions of chromite crystals of all the mantle formations in the projections Cr#Mg^hr Fooi indicate that the magnesiochromites of the chromitites and of some pendotites of the Vounnos massif do not follow the expected compositional trend of mantle spinelolivine pairs In particular their plots follow the fractionation line of boninites which means that they have been crystallised from a bonmite melt that had the opportunity to react with the hosting depleted pendotites because of a lack of thermo chemical equilibrium For the Pindos massif the same plot shows that some magnesiochromite olivine pairs correspond to unexpectedly high degrees of partial melting The positive correlations of Cr# with DR# the relatively high Ti02 contents and the high Fe+3# of magnesiochromites of both complexes do not agree with their residual origin but indicate that metasomatic processes have also played role in their formation The morphological change from vermicular and anhedral to euhedral magnesiochromites with the hthological transition from chnopyroxene bearing harzburgites and normal harzburgites to dunites indicate significant differences in the process of their genesis The high Cr# Ti02 Fe+3# and DR# of the dunitic magnesiochromites means that they are crystallisation products of a boninitic melt The magnesiochromite crystals of the transitional harzburgites from the Vounnos complex that have high Cr# and DR# are of magmatic origin as well In contrast the magnesiochromites of the same pendotite type from the Pindos ophiohtes are not well shaped (DR# <0 4) and have a residual origin The smaller dimensions of the Pindos chromitites compared to those from Vounnos reflect lower melt/rock ratios and more restricted lateral modifications of the neighbouring pendotites caused by the impregnating meltAlteration phenomena have affected the magnesiochromite spinel crystals of severalpendotites and chromitites of both complexes The accessory magnesiochromites of the intenselyserpentmised pendotites have been marginally altered to femt chromite showing increasing Cr#Fe+3# Fe+2# Fe+3/Fe+2 and decreasing Mg# towards their rims The magnesiochromite crystals of the chromitites from the areas of Voidolakkos and Xerolivado in the Vounnos massif commonlyexhibit marginal de colounsation zones From core to rim are characterised by increase of Cr# andrestricted increase of Fe+3# and Fe+3/Fe+2 The spinel grains of the chromitite from the area ofKorydailos in the Pindos massif are marginally altered to fernt chromite showing the followingmodifications from core to rim Increase in Cr# Fe+3# Fe+2# Fe+3/Fe+2 and reduction of Mg# The fact that the magnesiochromite crystals of this ore body do not present such alteration effects may be due to their less vulnerable structure to secondary processes (magnesiochromite normal & spinelinverse molecular structure) In all cases of altered magnesiochromite spinel grains the neighbouring silicate phase is Cr-bearing chlorite that is serpentine which has been transformed to chlorite due to metasomatic addition of Af 3 and Cr+3 to it with the progress of the alteration process The assemblage Femt-chromite+antigonte+tremolite+chlonte+chrysotile in the chromite ores which is representative of lower amphibolite facies metamorphism is in accordance with the assemblage Tremolite-talc-antigonte recognised in the pendotites The absence of fernt-chromite rims in the magnesiochromites of the chromitites from Voidolakkos and Xerolivado shows that these ore bodies have been altered at lower conditions (greenschist facies) The geothermometnc study of olivine spinel pairs of the chromitites from both complexes revealed Τ s between 650-850 C which do not respond to the magmatic Τ s but represent Γ s of re equilibration recrystalhsation However the chromitites of the southern Vounnos massif are characterized by lower Τ s of re-equilibration than those of the northern part Moreover the geothermometnc results show that the Τ s that are taken from the marginal compositions of the orthopyroxenes are systematically lower (767-1006 C) than those from the compositions of their cores (821 1126 °C) This difference may also represent a feature of metasomatic processes on the pendotites The compositions of the orthopyroxene cores respond to Ρ s of pendotite formation between 30-40 kbar The range of AlogffO^FMQ values is wide enough for the pendotites of both massifs Actually it covers the whole range of AlogffO^FMQ values between abyssal and SSZ type pendotites However higher nlogifO^FMo values are taken from the chromitites ( 2 to +1 ) which is in accordance with the high f02 conditions that prevail in an SZZ environment The pendotites of both ophiolite sequences are highly depleted This is indicated by their low concentrations in low melting point elements (Al 2 03 CaO Ti02 V Se REE) and their enrichment in compatible elements (Cr Ni Mg) The low concentrations in REE and the different normalized REEpatterns of the pendotites reflect their formation after several episodes of melt extraction The enrichment of certain pendotites in LREE combined with the restricted presence of newly formed phases in them indicate their affection by cryptic metasomatism The characteristic U shaped normalized REE patterns of some harzburgites reveal impregnation processes of these rocks by boninitic melts under open system conditions The different levels of pendotite enrichment in LREE especially in the Vounnos complex reflect variations in the degree of the metasomatism The melt/rock ratio was progressively reduced from the reaction front (transitional harzburgites) to the more fertile ultrabasic rocks (normal harzburgites and chnopyroxene bearing harzburgites) The process of percolative fractional crystallisation is responsible for the lower levels of enrichment of the more fertile pendotites in LREE Thus the so called REE paradox which is when the LREE content of the pendotites and their degree of depletion are not positively correlated is quite common in Vounnos and Pindos mantle pendotites The small degree of impregnation of the pendotites of both complexes with melts is also reflected in the fact that pendotite samples with anomalous REE contents have been rarely found However the small melt/rock ratios are able to perturb the composition of the pendotites in major elements Infact the mantle rocks of both massifs are enriched in Ti02 and less in CaO and Al 2 03 in comparison with their experimentally expected concentrations in depleted pendotites The Τι Sc AI2O3 versus Ni plots reflect a possible affection of the pendotites by two different melts during two distinct episodes of mantle metasomatism REE modeling shows that the pendotites of the Vounnos and Pindos ophiolites have been formed by several episodes of partial melting of a relatively fertile source The completely depleted nature of the pyroxene porphyroclasts in LREE-MREE indicates a possible origin of the pendotites from a garnet-bearing mantle protolith The low concentrations of REE in mantle pyroxenes combined with the fact that these pyroxenes are contained in rocks relatively enriched in LREE supports the presence of interstitially trapped melts in the pendotites The ultra depleted nature of the pendotites is in accordance with their SSZ origin The rich in Cr chromitites of both complexes have total PGE concentrations which are typical of ophiolitic chromitites (<400ppb) and negatively slopped normalised PGE patterns The lower chromitite horizon from the area of Korydallos has lower PGE concentrations (294 1 ppb) compared to the highe one which exhibit extreme PGE abundance (28830 ppb) The former has relatively flat normalised PGE patterns while the latter positively slopped PGE patterns The relative enrichment of the rich in Cr chromitites in IPGE is due to the mechanical entrapment of rich in IPGE phases in them The high content of the Al rich chromitite horizon of Korydallos in PPGE (>13 ppm) is attributed to the exsolution of immiscible sulfide droplets from the magma contemporaneously with the crystallisation of spinel The sulfide droplets scavenged the highly chalcophile PGE from the magma The decrease of the PGE content of the southern Vounnos chromitites with the Cr# of their magnesiochromite grains indicates crystal fractionation processes The high Cr# of the magnesiochromites of the chromite ores from the Vounnos massif and the areas of Milia and Pefki from the Pindos massif in combination with their high contents in IPGE are indicative of their formation from boninitic melts The Al rich character of the chromitite of Korydallos and it s high content in PPGE and BMS are supportive of their formation from a tholeitic magma These two different types of melts can be produced in a SSZ environment In particular the boninites can be formed straight under the arc and in the fore arc area of a SSZ while the MOR basalts can be produced in the opening back arc basin (BAB basalts) The PGM study was based on both the in situ and m-concentrates investigation The poor nature of the studied chromitites in PGE made the application of the recovery methods (hydroseparation and superpanners) necessary for obtaining representative results about the real PGM assemblage of each chromitite body The Vounnos chromitites host PGM assemblages typical of ophiolitic chromitites These are characterised by the predominance of members of the launte erhchmanite series and alloys of Os Ir composition The launte crystals of the southern Vounnos chromitites represent a wider range of OsRu substitution and are more Os enriched compared to those from the northern Vounnos chromitites This is indicative of the fact that the former have been formed under conditions of higher fS2 and/or lower Τ Phenomena of transformation of bi-sulfides into Ru-Os Ir Fe Ni bearing oxides are common due to hydrothermal alteration and/or weathering effects Some of these oxides exhibit internally strange and unknown until now in such minerals uncommon rosette textures Rare composite crystals consisting of erhchmanite Os Ir alloys and sulfasrenides which have been recovered from the chromitites of Voidolakkos and Xerolivado areas are indicative of the rapid rise of the fS2 locally in the magma during the formation of these chromite ore deposits The PGM assemblage of the chromitites from the area of Miha is quite similar to that of theVounnos chromitites The PGM assemblage of the area of Pefki is different with bi-sulfidessulfarsenides bi arsenides and PPGM The restricted number of Os Ir alloys in the Pefki schromitites and the high amount of sulfarsenides and bi-arsenides indicate conditions of higher fS2fAs and/or lower Τ than those prevailing during the formation of the chromitites of the area of MiliaDe arsenication and de sulphurisation phenomenae have also been observed but are restrictedSome Os Ir alloy crystals from the chromitites of Milia represent anomalously enriched compositionsin Ru+PPGE and relatively flat primitive mantle mormalised PGE patterns These anomalouscompositions are indicative of their residual instead of their magmatic origin which means that thePGM may exist as independent grains even in the residual pendotites of that certain areaThe less differentiated chromitite horizon of the Korydallos chromite ore contain inclusions oflaunte erhchmanite and sperrylite a feature that indicates it s genesis under conditions of very highfS2 fAs and/or low Τ Their serpentinised chlontised matrix hosts several Pt Fe alloys and Rh-ncharsenides of secondary origin From the upper stratigraphie level of the chromitite Pt Pd and Auphases have been recovered Pt phases occur as exsolutions from altered grains of BMS while thePd and Au phases occur as epitaxic growths onto the altered BMS crystals as well as independentgrains Hydrothermally altered zoned grains of Pd rich tetraauncupnde and PdCu4 have also beenfound The process of the hydrothermal alteration of chromitites was responsible for the depletiontransport and re-deposition of Pt Pd Au Cu and Fe The combined results of the present thesis are supportive of the higher stratigraphie position of the southern part of the Vounnos ophiolite complex compared to the northern one as well as of a stratigraphie evolution from Milia to Pefki as far as the Pindos ophiolite complex is concerned The chromitite of the area of Korydallos in the Pindos massif has similar characteristics to the chromitites from the upper parts of the mantle section or those from the lower crustal section of the ophiohtes That is why the area of Korydallos represents the higher-most mantle levels in the whole mantle unit of the Pindos ophiolite complex Finally taking into account all the available geological mineral chemistry and penological data it can be said that the mantle units of both complexes have been initially formed in a mid ocean ridge and then have been furtherly affected by the processes that take place in the mantle wedge above a SSZ Thus the transition from Vounnos to Milia and Pefki and then to Korydallos is obviously equivalent to the passage from a fore arc basin to the arc and finally to a small back arc basin.