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Colloidal origin of colloform-banded textures in the Paleogene low-sulfidation Khan Krum gold deposit, SE Bulgaria

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Abstract

We studied both colloform-banded macro- and micro-textures as well as the composition of electrum from the epithermal Khan Krum (or Ada Tepe) gold deposit, Bulgaria (5 Mt at 5.1 g/t Au + 2.7 g/t Ag) using optical and electron scanning microscopy, vibration spectroscopy, electron micro-probe analysis, and LA-ICP-MS. The individual textural bands differ in grain size, porosity, quartz-to-adularia ratio, and abundance of electrum, pyrite, and scattered dusty opaque micro-inclusions. The individual macro-bands have formed successively via intermittent fault dilation from episodic hydrothermal pulses, as a result of regional extension and most likely originated from colloidal solutions formed by vigorous boiling during rapid pressure drop and supersaturation with respect to amorphous silica in a closed or quasi-closed hydrothermal system. Electrum is highly concentrated in the finest quartz–adularia colloform micro-bands, which fill joints with widths up to 1–2 mm. We presume that the joints have filled in with more concentrated colloidal solutions than those in the macro-bands, most likely due to extreme boiling of fluids in open or quasi-open hydrothermal system. Electrum in the micro-bands forms transverse dendrite- and chain-like aggregates as well as oval clots along the banding, all of globular morphology. We explain these locations of electrum with reorientation of aggregated electrum globules during plastic deformation of a mixed electrum–silicate gel and in result from the consequent crystallization of silicates. At the same time, electrum is not present in pores and cracks of syneresis, which indicates that it had not been a soluble phase at the time of syneresis, rather colloidal particles. Electrum also forms dense sprinkles of globular morphology deposited in open space on the surfaces of some quartz–adularia bands, due to a condensation of gaseous phase, separated during the boiling of fluids. We found that the electrum-rich quartz–adularia micro-banding is enriched (in descending order) in Bi, Te, Cu, Fe, Pb, Au, As, U, Ba, Zn, Mg, Cr, Al, Tl, Na, K, and Th, and is slightly depleted in Si, Ti, Se, Ag, and Sb in comparison with the electrum-poor macro-bands. Ca displayed equal abundances in both macro- and micro-bands. The highest grades of electrum correspond to the highest abundances of Fe, As, Cu, Pb, Zn, Bi, and Te, which have deposited as sulfides and tellurides, thus revealing the voluminous electrum deposition in response to a significant decrease of sulfur and tellurium activities, and transport of gold and silver in the paleofluids in the form of sulfuric and telluric complexes. The epithermal Khan Krum deposit is Au dominant, Bi and Te rich, thus indicating its relationship to an igneous source.

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Acknowledgments

The National Science Fund of Bulgaria (project DO-02-82/2008) is acknowledged for funding a part of this study. The Bulgarian Ministry of Education, Youth, and Science is acknowledged for the English translation and editing of this article financed by the “Science and Business” scheme of the project BG051PO001-3.3-05/0001 through the “Human Resources Development” Operative Program. The authors thank their colleagues from “Balkan Mineral and Mining” Co, especially Sean Hasson, Danko Jelev, and Tanya Kazulova for technical assistance. Thanks are due to Luchezar Petrov (Institute of Mineralogy and Crystallography at Bulgarian Academy of Sciences) for the SEM study and EMPA, and to Lyubomir Dimitrov and Lyudmil Konstantinov (same Institute) for fruitful discussions. Thanks are due to James Saunders for providing the authors with new papers on bonanza epithermal ores and for useful recommendations. The authors thank Robert Moritz for the review and Bernd Lehmann and Robert Moritz for efforts to improve the manuscript.

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  1. Institute of Mineralogy and Crystallography, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Bl. 107, 1113, Sofia, Bulgaria

    Irina Marinova, Valentin Ganev & Rositsa Titorenkova

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  1. Irina Marinova
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Marinova, I., Ganev, V. & Titorenkova, R. Colloidal origin of colloform-banded textures in the Paleogene low-sulfidation Khan Krum gold deposit, SE Bulgaria. Miner Deposita 49, 49–74 (2014). https://doi.org/10.1007/s00126-013-0473-4

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