Mineralogy of gold in the Elshitsa massive sulphide deposit, Sredna Gora zone, Bulgaria |
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| Authors: | K Bogdanov D Tsonev K Kuzmanov |
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| Institution: | (1) Department of Mineralogy, Petrology and Economic Geology, Faculty of Geology and Geography, Sofia University “St. Kl. Ohridski”, 15 Tsar Osvoboditel Bd., 1000 Sofia, Bulgaria, BG |
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| Abstract: | The Elshitsa volcanic hosted massive sulphide deposit occurs in the central part of the Srena Gora metallogenic zone in Bulgaria.
The gold-bearing massive sulphide mineralization is considered to be the product of an island arc volcano-plutonic process
and hydrothermal activity that took place during the Late Cretaceous. In addition to the major gold-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 and 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 tenor as a rule is less than 1 g/t. Native gold and electrum occur as blebs or intergranular particles in the sulphide
minerals. Gold in the early massive pyrite is of submicroscopic type (< 0,1 μm) and of colloidal ori-gin. Pyrite deformation
and recrystallization in the temperature range 250°–160 °C has led to Au and Ag migration to cracks and grain boundaries of
the sulphide minerals. As a result of these process the native gold and electrum grain size increases from submicroscopic
(< 0,1 μm) in the early colloform pyrite to microscopic (0,1–100 μm) and macroscopic (> 100 μm) in the late gold-sulphide
assemblages. The electrum fineness in 41 individually studied grains varies between 780 and 992‰ with a mean of 895‰. Native
silver was found in association with bornite. Cu, Te, Sb and Bi are the most common trace-elements in gold and electrum. The
Cu-Zn-Pb association is most important as a Au-Ag-carrier. A model for gold behaviour during sulphide deformation is proposed
involving coarsening of gold grain size from the earlier to the later sulphide mineral assemblages.
Received: 4 December 1995 / Accepted: 23 September 1996 |
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