Geodynamic significance and genesis of chromitites from the Islahiye ophiolite(Gaziantep,SE Anatolia)as constrained by platinum group element(PGE)compositions and mineral chemistry characteristics

Chromitites associated with intensely altered dunites and harzburgites from fourteen dif ferent localities in the Islahiye ophiolites(SE Anatolia)is reported here for the first time.These chromitites were observed as lenticular and banded bodies with disseminated and massive textures and containing magnesiochromite grains with the following composition:CrO=58.91–59.74 wt%,Al_(2)O_(3)=10.85–11.20 wt%,and TiO=0.09–0.13 wt%.The Mg#[Mg/(Mg+Fe)]values of magnesiochromite from the Islahiye ophiolite range between 0.52 and 0.60 and their Cr#[Cr/(Cr+Al)]values vary from 0.7802 to 0.7844.These contents vary with a constant pattern,coincident with the estimated parental liquids that have originated from the derivative of a single bulk of boninitic magma together with Al,Ti-poor,and Cr-rich initial contents.The chromitites are serpentinised in almost all parts of the study area,and harzburgite and dunite can be observed in different locations.Although the overall composition of platinum group elements(PGE)in most examined chromitites varies between 97 and 191 ppb,three chromitites from the Islahiye region present enrichments in overall PGE(up to 214 ppb).The mineralogical and geochemical features of chromitites from the Islahiye region exhibit a robust similarity to podiform chromitites in the mantle fragment of supra–subduction zone type ophiolitic bodies.The estimated parental magmas of the investigated chromites are consistent with the diff erentiation of arcrelated melts and do not suggest an oceanic spreading centre tectonic environment.The Islahiye chromites are enriched in IPGE(Ir,Os,Ru),with the occasional presence of Ru and Ir and higher Os contents in chromite.Furthermore,we did not find any platinum group minerals(PGM)associated with the serpentine silicate sample matrix,which would have stated a secondary enrichment in PGEs.All chromitites in the investigated region have high Cr and low Ti values,are defined as magnesiochromite and were crystallised from a characteristic boninitic magma.

Micromineralogy of ＂Black Shale＂ Disseminated- Sulphide Gold Ore Deposits of the Ayan-Yuryakh Anticlinorium （North-East of Russia）

The results of investigation of micromineralogy of disseminated-sulphide gold ore deposits of the Yana-Kolyma belt; the investigation was conducted via electron microscopy using a QEMSCAN （Quantitative Evaluation of Minerals by Scanning Electron Microscopy） hardware and software instrument equipped with a Quantax quantitative analysis system. The following micro-mineral phases have been detected： native gold and silver, selenides of Au and Ag （naumanite and fishesserite （？）, hessite, sulfoarsenides of Ni and Co （gersdorffite, cobaltite, rammelsbergite）, molibdenite and platinum group minerals. Our findings in disseminated sulfide ores selenides of Au and Ag, and molybdenite, together with the findings of platinum group minerals and Ni and Co, emphasize the ＂black shale＂ specifics character of these types of deposits.