PETROGRAPHY AND GEOCHEMISTRY OF EPIDOSITES FROM THE MIRDITA OPHIOLITE, ALBANIA

Hydrothermally altered rocks from the sheeted dike and overlying volcanic sequence of the Mirdita ophiolite record evidence of subseafloor seawater-rock interaction. Samples are variably altered to greenschist facies and lower mineral assemblages including chlorite, epidote, actinolite, and smectite. Epidote and quartz are commonly developed in greenschist facies metabasalts and plagiogranites (tonalite, trondhjemite, and quartz diorite). In extreme cases, pervasive alteration has produced rocks consisting almost entirely of epidote and quartz called epidosites. Epidosites are characterized by the complete replacement of primary igneous textures by granoblastic assemblages of epidote + quartz ± chlorite ± actinolite ± ilmenite ± magnetite ± sulfides. Epidosites metasomatically replace basalt and plagiogranite protoliths and represent extremely pervasive alteration resulting from high-temperature fluid-rock interaction at high water-rock ratios. Epidosites are well documented in other ophiolites including the Troodos ophiolite in Cyprus and Semail ophiolite in Oman. Within sheeted dike complexes, epidosites commonly form as either patches in single dikes or dike outcrops or compose large zones as much as tens of meters wide, parallel to the axis of spreading. Plagiogranite-hosted epidosites more commonly occur as irregular patches, centimeters to meters wide. Preliminary temperatures calculated from oxygen isotope analyses of quartz-epidote mineral pairs from the Mirdita ophiolite indicate the epidosites formed at temperatures between approximately 200º to 400º C, which is consistent with studies from other ophiolites. Field and petrological data suggest that epidosites form at the base of ore-forming hydrothermal systems and may be useful in exploration for massive sulfide deposits.