Coupled and uncoupled to?S-34 behavior of gold and silver in pyrite and marcasite from the low-sulfidation Sarnak gold deposit, SE Bulgaria

In this study, the contents of Au and Ag in five pyrite +/- marcasite samples from the Sarnak gold deposit, the respective delta 34S, and the sample distances to the contact between metamorphic basement and overlying sedimentary cover were considered. We found that two samples from lower horizons (levels of 69 and 30 m below the contact) display relatively low Au and Ag contents, negative values of delta 34SV-CDT (-3.14 and -6.42%o) and pronounced oscillatory zoning, resulting from oscillating contents of As. Three samples from higher horizons (levels of 8, 7 and 0 m below the contact) have higher Au and Ag contents, positive or slightly negative values of delta 34SV-CDT (+0.5, +1.73 and -1.85%o) and poorly expressed or absent arsenian zoning. Two of them contain microscopic electrum and adularia. Based on these contrasting features we assume different mechanisms of precipitation: intense fluid-rock interaction for samples from lower horizons under steady conditions and boiling of fluid for samples from higher horizons under fluctuating conditions.