"Invisible" gold in synthetic and natural arsenopyrite crystals (Vorontsovka deposit, Northern Urals)
In many types of hydrothermal ore deposits Au occurs in “invisible” state in most common minerals of the Fe-As-S system. It is supposed that the state of the “invisible” Au may be either non-structural (nano-sized inclusions of metal and its compounds) or chemically bound (isomorphous solid solution). Here we report results of investigation of the state and the concentration range of “invisible” Au in synthetic and natural arsenopyrites FeAsS (Vorontsovka deposit, North Urals, type Carlin). Conditions that favor the formation of Au-bearing arsenopyrite were identified. The synthesis experiments were carried out in Au-saturated system by means of salt flux method with a stationary temperature gradient. The temperature at the cold end of the ampole was 400-500 °С. The chemical composition of arsenopyrite was determined by electron probe microanalysis. The composition of the synthesized arsenopyrite varied within [at.%]: Fe from 32.6 to 34.4, As from 30.0 to 36.5, S from 29.4 to 36.0. The Au content in arsenopyrite varied from the detection limit (< 45 ppm) to 3 wt.%. A strong negative correlation between the concentrations of Au and Fe was observed in the synthesized arsenopyrite grains. The slope of the correlation lines corresponds to the formation of the Au-bearing solid solution where Au isomorphically substitutes for Fe at the parameters of the synthesis experiments. In addition, there is a weaker positive correlation between Au and As: higher Au concentrations are characteristic of arsenic-rich compositions (As/S [at.%] > 1) and those close to stoichiometric arsenopyrite, whereas in sulfur-rich arsenopyrite the concentration of Au is lower and does not exceed 0.25 wt.%. The positive Au-As correlation appears not only on a local level within a single crystal of synthetic and natural arsenopyrite, but is valid on the Vorontsovka deposit scale: As-rich arsenopyrite formed at lower temperature and sulfur fugacity (t = 250-370 °C, log f S2 = -12 ÷ -17) contains more Au than the As-poor early arsenopyrite (t = 270-400 °C, log f S2 = -7 ÷ -9). Comparison of these results with the literature data shows that the positive correlation between the concentrations of Au and As in arsenopyrite and the negative correlation between the concentrations of Au and Fe are the common features of ores of the Carlin-type deposits. We suggest that, in contrast to negative correlation Au-Fe, the positive correlation Au-As cannot be explained in terms of crystal chemistry, but can result from the effect of external factors among which are the difference in composition of ore-forming hydrothermal fluids and the sulfur fugacity.