MINERALOGY OF GOLD IN THE ELSHITSA MASSIVE SULFIDE DEPOSIT, SREDNA GORA ZONE, BULGARIA

The Elshitsa volcanic hosted massive sulphide deposit occurs in the ce ntral part of the Srena Gora metallogenic zone in Bulgaria. The gold-b earing massive sulphide mineralization is considered to be the product of an island are volcano-plutonic process and hydrothermal activity t hat took place during the Late Cretaceous. In addition to the major go ld-hosted opaque minerals such as pyrite, chalcopyrite, sphalerite and galena there are minor phases of tennantite, goldfieldite, Se-bearing aikinite, native silver and bornite in the massive sulphide lenses an d stringer zones. Most of the sulphide minerals are Se-bearing. All of the six mineral assemblages that were deposited during the pyrite and copper-pyrite stages of mineralization are gold-bearing. The gold ten or as a rule is less than 1 g/t. Native gold and electrum occur as ble bs or intergranular particles in the sulphide minerals. Gold in the ea rly massive pyrite is of submicroscopic type (< 0,1 mu m) and of collo idal ori-gin. Pyrite deformation and recrystallization in the temperat ure range 250 degrees-160 degrees C has led to Au and Ag migration to cracks and grain boundaries of the sulphide minerals. As a result of t hese process the native gold and electrum grain size increases from su bmicroscopic (< 0,1 mu m) in the early colloform pyrite to microscopic (0,1 - 100 mu m) and macroscopic (> 100 mu m) in the late gold-sulphi de assemblages. The electrum fineness in 41 individually studied grain s varies between 780 and 992 parts per thousand with a mean of 895 par ts per thousand. Native silver was found in association with bornite. Cu, Te, Sb and Bi are the most common trace-elements in gold and elect rum. The Cu-Zn-Pb association is most important as a Au-Ag-carrier. A model for gold behaviour during sulphide deformation is proposed invol ving coarsening of gold grain size from the earlier to the later sulph ide mineral assemblages.