Chromite compositional variability and associated PGE enrichments in chromitites from the Gomati and Nea Roda ophiolite, Chalkidiki, Northern Greece

Abstract

The Gomati and Nea Roda ophiolites are located into the Serbo-Macedonian massif of the Chalkidiki peninsula (Northern Greece). The present work focuses on the variability of platinum-group elements (PGEs), geochemistry, spinel mineral chemistry, and platinum-group minerals (PGMs) — base metal minerals (BMMs) assemblage in chromitites of three Gomati localities (St. George, Tripes, and Limonadika) and Nea Roda. The studied chromitites show variable textures and are heavily altered. Primary silicates are almost completely replaced by chlorite, and chromite rims are altered into ferrian chromite. The variability of spinel mineral chemistry in terms of Cr# [Cr/(Cr + Al)] and Mg# [Mg/(Mg + Fe2+)], and the PGE contents, argues for a genesis in a supra-subduction setting (SSZ), at different stratigraphic positions in the ophiolite section. Chromitites from Tripes have the lowest Cr# (0.5–0.6) and the highest PGE contents (3516 ppb), similar to some chromitites formed in small magma chambers in the cumulate sections above the Moho. The high PGE contents of Tripes chromitites are due to an IPGEs-enriched melt derived from critical melting of mantle peridotites. Limonadika and St. George show the highest Cr# (0.77–0.96 and 0.74–0.87, respectively) and variable PGE contents (175 ppb and 383 ppb on average respectively), compatible with a genesis from boninitic magmas in the mantle section. Nea Roda chromitites have intermediate to high Cr# (0.66–0.75) and low PGE contents (135 ppb on average) and show similarities to other intermediate chromitites formed from evolving magma sources at subduction initiation. BMMs detected in both ophiolites are primary (pentlandite) and secondary (mainly millerite and heazlewoodite) sulfides. All the detected PGMs are primary, crystallized from the melt, and entrapped into chromite, and they are mainly laurites. In the studied chromitites, the absence of alloys indicates that the circulating fluids during chloritization were at high fS2 and fO2, and did not remobilize the PGEs. The same fluids are probably responsible for the low-T crystallization of an uncommon suite of arsenides and antimonides at St. George.

Post-magmatic processes recorded in bimodal chromitites of the East Chalkidiki meta-ultramafic bodies, Gomati and Nea Roda, Northern Greece

“…The first scenario, although plausible in principle, is supported by only one piece of evidence, which is the presence of a single secondary Pd-Sb PGM within the G1 chromitite. The analyzed primary PGM from the Gomati chromitites (subhedral laurite included in chromite) contain no appreciable PPGE and this agrees with findings by Bussolesi et al (2022b). Irarsite was encountered in all chromitites.…”

“…Indeed, the studied chromitites display a gradual enrichment in IPGE and ΣPGE from the G1 towards NR and G2 occurrences. The average ΣPGE contents demonstrate a decrease from G2-245.0 to NR-154.6 and to G1-98.8 (collective data: this study;Economou-Eliopoulos, 1996;Bussolesi et al, 2022b). This trend has been previously described by Uysal et al (2016) referring to chromitites produced by a successively fractionating melt with deepercrystallized Cr-rich chromitites being more enriched when compared to the more shallow "intermediate" counterparts.…”

“…Taking into consideration the secondary and modified primary PGM, as well as other base metals that were encountered (Ni-Sb) as also reported by Bussolesi et al (2020Bussolesi et al ( , 2022b, it is evident that there is an enrichment in As + Sb during a post-magmatic event. Bussolesi et al (2020Bussolesi et al ( , 2022b assigned this metasomatism to the 53-55 Ma Ierissos pluton intrusion or porphyry copper related mineralization. However, no alteration effects related to the metallogenesis of Kassandra district have been reported for the Ierissos pluton as well as along the 12 km Gomati Fault (Siron et al, 2018 and references therein).…”

“…PPGE/ IPGE ratios are consistently low for the G2 and NR Cr-rich chromitites with an average of 0.24 unlike the G1 chromitite with average ratios of 0.93. Previous studies refer to ratios of 0.07 and 0.23 for Cr-and Al-rich chromitites of Gomati, respectively (Economou-Eliopoulos, 1996;Bussolesi et al, 2022b). Yet, the similar PGE patterns of the compositionally different chromitites studied may be interpreted as products of continuous melt production from a common source (Rollinson, 2005).…”

“…FIGURE 10 (A) Chondrite normalized PGE patterns of the Gomati and Nea Roda chromitites, CC1 Values after Naldrett and Duke (1980) (B) Pd/Ir vs. Pt/Pt* = Pt N /(Rh N × Pd N ) 1/2 plot of the studied chromitites (afterGaruti et al, 1997). Purple field after Economou-Eliopoulos (1996) and references therein; gray field afterTarkian et al (1991) and González-Jiménez et al(2015); dotted line field after González-Jiménez et al(2015); orange field after Economou-Eliopoulos (1996) andSideridis et al (2021); G1 and G2 chromitite from previous studies are afterEconomou-Eliopoulos (1996) andBussolesi et al (2022b).…”