Formation of the Bilyan-Tau gold ore occurrence (Khudolaz trough, Southern Urals)

Abstract

Introduction: lode gold deposits are of great economic importance. Within the Khudolaz trough in the Southern Urals, many small gold deposits and occurrences are distributed. They are the primary sources of placer gold deposits currently being developed. The genesis of these deposits has been poorly studied, and therefore the sources of ore-forming fluids are unclear.

Methodology: the mineral composition of ore-bearing veins was studied using a scanning electron microscope. To determine the sources of ore-forming fluids, the isotopic composition of sulfur in sulfides and arsenopyrite, the isotopic composition of oxygen in quartz, and the gas composition of fluid inclusions were determined. The PT parameters of the system were studied using microthermometry of fluid inclusions and mineral geothermometers.

Results and conclusions: the formation of the Bilyan-Tau gold ore occurrence is associated with the Khudolaz complex gabbro intrusion (325–329 Ma) into the tuffaceous rocks of the Biyagoda Formation (D₃f). Ore-bearing quartz veins were formed from magmatic fluid (separated from the gabbro intrusion) with the participation of host tuffaceous rocks along a system of NE-trending faults, typical of the entire Khudolaz trough. The temperature range of ore formation, according to the homogenization temperatures of fluid inclusions and thermometry of arsenopyrite and chlorite, was 200–450°C, the pressure range of 10–50 bar. Three stages of ore formation have been identified: 1) early polysulfide including arsenopyrite, 2) silicate-oxide, 3) late hydroxide. The deposition of gold in the ore is associated with the first highest temperature stage. The totality of data allows us to classify Bilyan-Tau as an orogenic-type gold deposit.