Die Entstehungsprozesse der Sulfidminerale und Goldspeziation in diesen Mineralen
Final Report Abstract
Pyrite crystal growth: The rationale of this part of the investigation was to i) determine the CSDs of hydrothermal pyrite crystals from the Nízke Tatry Mountains (Slovakia) and ii) estimate nucleation and growth processes for each sample according to the mathematical models developed by Eberl et al. (1998). The distribution type for all measured CSDs was lognormal or pseudo-lognormal. The simulations with the GALOPER program attest that the hydrothermal pyrite grew in an open system. The statistical tests of the simulations showed that for some samples, the surface-controlled growth is the most likely growth mechanism whereas the others can be described by the supply-controlled mechanism. These results show that the dominant growth processes can be diverse within one ore-forming system, or even within a small volume, like a hand-sized specimen of the ore. The electron microprobe investigation of the pyrite crystals showed two different types of arsenic zonation in the crystals. There are i) pyrite crystals with an arsenic-rich core and an arsenic-depleted rim and ii) crystals with an arsenic-rich rim and an irregular (porous) arsenicdepleted core. Especially the crystals with the irregular cores raise the question of possible dissolution and re-nucleation processes and their impact on the simulation. Based on repeated measurements and simulations, it is at least possible to rule out that dissolution and renucleation affected the simulation significantly. Although it is not possible to exclude slow surface- or supply-controlled growth, it is likely that the growth of the pyrite crystals and so the formation of the whole ore body must have occurred on a relatively short geological time scale. These preliminary results pose more questions related to the emplacement, circulation, and supersaturation of hydrothermal fluids and to the formation of large ore bodies in a short time. With a number of assumptions, we calculate the growth time of the pyrite crystals of 60-2000 years, depending on their size. However, this estimate bears a very large uncertainty and could be revised in future studies. Arsenic in pyrite – similarities and differences: The similarities and differences of arsenic-rich pyrite were the object of this part of the investigation. For that purpose, samples with different arsenic concentrations and distributions were investigated. The pyrite crystals from Zigana were found to be almost arsenic-free. With a maximum-concentration of 0.05 wt.%, the pyrite from Zigana could be a candidate for nano-particular arsenic. The pyrite from Dve Vody shows arsenic concentrations up to 5.81 wt.% and internal zones splitting the pyrite into arsenic-rich and arsenic-poor pyrite. The microanalytical investigation shows that there is a negative correlation between arsenic and sulfur which hints at an arsenic-for-sulfur substitution. In the pyrite samples from Dve Vody, it is also possible to see arsenopyrite crystallization in the arsenic-richest domains within the pyrite. This supports the theory that there is a maximum arsenic incorporation capacity in pyrite which, if exceeded, will be responsible for a arsenopyrite decomposition. The pyrite samples from the Hishikari gold deposit show arsenic contents up to 11 wt.%. The observations at a microscale show that there is no arsenopyrite. It is possible to speculate that the gold content of the Hishikari pyrite is responsible for the greater arsenic acceptance. The Eger samples show arsenic amounts up to 7 wt.%. The analyses on pyrite from the Eger Graben show that the arsenic-rich pyrite can be split into two groups: I) arsenic-rich pyrite with relatively stoichiometric Fe contents and a negative correlation towards sulfur and II) arsenic-rich pyrite with a negative correlation towards Fe and sulfur. The measured Fe deficit in the group II was a basis for speculation if there is a possible As3+ substitution for Fe on the cation-site. The results of the synchrotron based X-ray absorption spectroscopy reveal that there are no oxidized arsenic species in the pyrite samples from the Eger Graben. However, the Eger samples and the Hishikari samples will be part of future investigations. Especially the measured Fe deficits will be subjects of a re-examination with the electron microprobe, transmission electron microscope, and X-ray absorption spectroscopy.
Publications
- (2014): Mechanisms of crystal growth and zoning of hydrothermal pyrite from Sb-Au Deposits in Nizke Tatry, Slovakia. The Canadian Mineralogist, 52, 555-568
Kiefer, S. & Majzlan, J.
(See online at https://doi.org/10.3749/canmin.52.3.555)
