GENESIS OF VEIN-STOCKWORK CRYPTOCRYSTALLINE MAGNESITE FROM THE DINARIDE OPHIOLITES

Abstract

Vein and stock-work cryptocrystalline magnesite deposits, known also as Kraubath type, Gelmagnesite, and Khalilovo type, are widespread in the Tethyan ophiolites from the Alps to the Zagros mountains, including the Dinarides, but also in other ophiolite suites like California, etc. Thirteen samples from the magnesite deposits of the Dinaric and Vardar zone ophiolite belts were analyzed for C- and O-isotopes, major and trace elements, and REE. Genesis of cryptocrystalline magnesite has received two controversial interpretations, involving circulation of Mg-bearing ore fluids, “per ascendum” or “per descendum” mechanisms. Stable isotope data on C- and O-isotopes in magnesite deposits show significant positive correlation along the light-heavy isotope ratio from vein to stock-work and sedimentary type ore bodies. The thirteen new samples confirmed the same regularity, as a rule for cryptocrystalline magnesite. It should be stressed that there are some exceptions, marked by different isotope patterns, represented by deposits located close to the products of the Tertiary volcanic activity. This paper brings arguments both in favor and against the two genetic models, supported by the new geochemical data. The authors favour, however, the per descendum model, stating generation beneath the lateritic weathering crusts, which covered a wide area of the obducted ophiolites, subjected to warm, humid tropical climate in the Early Cretaceous. The laterite crusts acted as a chromatographic column separating immobile elements from mobilized magnesia in solution, which percolating down through a fracture system, deposited magnesite as a result of increased pH. The self refining of ore forming fluids produced almost monomineralic magnesite veins, with or without selvages. The fractionation of light C- and O-isotopes in magnesite needs isotopic light CO2, commonly interpreted as a derivate of deep seated decarboxylation of organic rich sediments. However, this behaviour of light C- and O-isotopes on a regional scale can be convincingly explained also by closed/semiclosed fractionation between atmospheric CO2 and magnesite in the descending fluid, controlled by the Rayleigh equation.