Ancient Gold Mining at Ada Tepe, East Rhodopes, Bulgaria. Mineralogical Features of Au-Containing Fe-Oxides/Hydroxides from the Ada Tepe Gold Deposit. Their Significance in Clarifying the Ancient Gold Mining

Abstract

Ada Tepe is the oldest known open-pit mine on the Balkan Peninsula for mining gold from host rocks. The mine probably started operation around 1500 BC and continued to function until the end of the 12th to the middle of the 11th century BC. The sedimentary and hydrothermally altered host rocks in the upper zone of Ada Tepe are usually of a grey or yellow colour or, in some places, yellow-brown. The rock pieces in the waste dumps are usually darker, with a red or red-brown coloration, which is obviously not connected to geological processes. It is suggested that the difference in colour between the host rocks and the fragments in the waste dumps are due to the phase transformation of minerals caused by the ancient miners using fire to fracture the host rocks in order to extract the gold ore. The majority of the analysed rock samples have a polyphase composition of goethite, hematite, schwertmannite, Au-Ag alloys, iron ochres, quartz and adularia. The phase diagnostics were performed by ore microscopy and powder X-ray diffraction (PXRD), morphological peculiarities were determined by scanning electron microscopy (SEM) while the thermal behaviour was investigated by differential thermal analysis and thermo gravimetric (DTA-TG) method. This paper will present the analytical results, which led to the conclusion that the difference in the coloration is due to the processes of transformation of nano-sized Fe3+ (oxy)hydroxides and (oxy)hydroxysulfate minerals caused by anthropogenic thermal processing.