Alteration of Platinum-Group and Base-Metal Mineral Assemblages in Ophiolite Chromitites from the Dobromirtsi Massif, Rhodope Mountains (Bulgaria)

Abstract

The Dobromirtsi Ultramafic Massif, located in the Rhodope Mountains (SE Bulgaria), is a portion of a Paleozoic sub-oceanic mantle affected by polyphase regional metamorphism. This massif contains several small, podiform chromitite bodies which underwent the same metamorphic evolution as their host peridotites. Like other ophiolite chromitites, those found in Dobromirtsi carry abundant platinum-group minerals (PGM) and basemetal minerals (BMM). The PGM consist mainly of Ru-, Os-, and Ir-based PGM (laurite RuS 2-erlichmanite OsS2, Os-Ru-Ir alloys, irarsite [IrAsS], Ru-rich pentlandite, and an unknown Ir-sulfide) but minor Rh-and Pd-based PGM (hollingworthite [RhAsS] and a series of unidentified stannides and sulfantimonides) are also present. In contrast, the BMM are dominated by pentlandite (Ni,Fe) 9S8, followed by heazlewoodite (Ni3S2), breithauptite (NiSb), maucherite (Ni11As8), godlevskite (Ni7S6), gersdorffite (NiAsS), millerite (NiS), undetermined minerals containing Ni, As and Sb, orcelite (Ni5-XAs2), awaruite (Ni3Fe) and chalcopyrite (CuFeS2). The detailed study of the textural relationships, morphology and composition of the PGM and BMM inclusions indicate the existence of two different PGM-BMM assemblages: (i) a primary or magmatic; and (ii) a secondary related with postmagmatic alteration. The PGM and BMM inclusions in unaltered zones of chromite crystals (mainly laurite-erlichmanite and pentlandite) are considered to be primary magmatic minerals formed under variable temperature (1200-1100°C) and sulfur fugacity (between -2 and -0.5 log fS 2). In contrast, PGM and BMM located along altered edges of chromite and serpentinised silicate matrix are considered to be secondary, formed from or re-equilibrated with altering fluids. Secondary PGM and BMM assemblages are considered as result of the combination of reducing and oxidising events related with regional metamorphism. Under low fO 2 states, fS2 also drops giving place to the formation of S-poor Ni-rich sulfides and secondary Ru-alloys by desulfurisation of primary S-containing minerals. In contrast, predominance of platinum-group elements and/or base-metal arsenides and sulfarsenides associated with the altered edges of chromite (chromite strongly enriched in Fe 2O3) is related with the fixing of remobilised PGE (mainly Ir, Rh and Pd) and base-metals (mainly Ni and Fe) when late oxidising fluids supplied As as well as Sb and Sn.

Petrological studies of neoproterozoic serpentinized ultramafics of the nubian shield: spinel compositions as evidence of the tectonic evolution of egyptian ophiolites

“…Very low Fe 3+ contents in the fresh chromian spinels indicate relatively low oxygen fugacity conditions at their primary source (Murck and Campbell 1986), while high Fe 3+ in the ferritchromite and Crmagnetite rims suggesting an oxidative state during metamorphism (Anzil et al 2012). Highly oxidising conditions favour the reaction of chromian spinel with serpentine to produce chlorite, ferritchromite and Crmagnetite (Mellini et al 2005;González-Jiménez et al 2009). Therefore, the development of ferritchromite rims around chromian spinel cores indicates their formation during prograde alteration and under oxidizing conditions (González-Jiménez et al 2009).…”

“…Highly oxidising conditions favour the reaction of chromian spinel with serpentine to produce chlorite, ferritchromite and Crmagnetite (Mellini et al 2005;González-Jiménez et al 2009). Therefore, the development of ferritchromite rims around chromian spinel cores indicates their formation during prograde alteration and under oxidizing conditions (González-Jiménez et al 2009). This alteration should have taken place during lower temperature amphibolite facies metamorphism (Suita and Streider 1996).…”

Composition and alteration of Cr-spinels from Milia and Pefki serpentinized mantle peridotites (Pindos Ophiolite Complex, Greece)

“…Farahat 2008;González-Jiménez et al 2009;Mukherjee et al 2010). Textural and compositional variations in the studied zoned Cr-spinel grains suggest that alteration to ferrian chromite took place after serpentinization.…”

“…Mellini et al 2005;Mikuš & Spišiak 2007;Farahat 2008;González-Jiménez et al 2009;Merlini et al 2009;Gervilla et al 2012;Singh & Singh 2013). According to Bach et al (2006) common low-T serpentinization, taking place during ocean-floor metamorphism cannot cause any changes in Cr-spinel composition.…”

“…More specifically, textural observations and mineral chemistry data indicate that hydrothermal alteration and metamorphism can significantly modify primary Cr-spinel composition (e.g. Bliss & MacLean 1975;Evans & Frost 1975;Wylie et al 1987;Kimball 1990;Suita & Streider 1996;Barnes 2000;Mellini et al 2005;Farahat 2008;Merlini et al 2009;Rollinson et al 2012;Sansone et al 2012 ', Spangenberg 1943;and cited in, among others, Evans & Frost 1975;González-Jiménez et al 2009;Mukherjee et al 2010;Gervilla et al 2012;Derbyshire et al 2013). Various investigations suggest that ferrian chromite forms during low-T hydrothermal alteration (e.g.…”