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БЪЛГАРСКО ГЕОЛОГИЧЕСКО ДРУЖЕСТВО, Национална конференция с международно участие „ГЕОНАУКИ 2015“BULGARIAN GEOLOGICAL SOCIETY, National Conference with international participation “GEOSCIENCES 2015”

Colloidal and physical transport textures of electrum in a sinusoidal-walled veinlet from the Khan Krum gold deposit, Eastern Rhodopes Mountains, BulgariaЕлектрумни текстури, указващи за колоиден транспорт на примера на синусовидна жилка от златното находище Хан Крум, Източни Родопи, БългарияIrina MarinovaИрина Маринова

Institute of Mineralogy and Crystallography, Bulgarian Academy of Sciences; E-mail: irimari@gmail.com

Key words: bonanza electrum, colloidal and physical transport textures, Khan Krum deposit, Rhodopes, Bulgaria.

Introduction

The term “colloidal and physical transport textures” was introduced by Saunders et al. (2011) for bonanza electrum ores from Western USA. Recently, Marinova (2014) documented a colloidal and physical trans-port texture of electrum from the Khan Krum deposit where the first-order textural feature was pronounced electrum enrichment only in the thicker veinlet por-tions, and made a conceptual model for formation of bonanza electrum based on this texture. During further textural investigations of the studied veinlet other col-loidal and physical transport textures of electrum of second and third orders were recognized in the thicker portions. These textures allow a refinement of the con-ceptual model already presented. In the present paper such textural features are illustrated and interpreted.

Material

Material is a polished section with an electrum-rich colloform micro-banding (Marinova, 2014). In the optical microscope the electrum-rich micro-banding consists from left to right of 4 micro-bands of nearly parallel walls and fifth band of variable width, resem-bling a sinusoidal shape, i.e. exhibiting an alteration of narrow and thick portions. The latter band is subject of the present work.

Optical observations and interpretation

The sinusoidal-walled veinlet and the other individual auriferous micro-bands are bounded from each side by a micro-crack of barren mineral filling. In transmitted light these micro-cracks exhibit mineral filling com-posed of comb quartz ± adularia whose crystals are much larger than the quartz-adularia matrix in the next

auriferous micro-bands. Large pores occur along the virtual centerline of the sinusoidal-walled micro-band (Fig. 1a). These central pores were empty (now some of them are filled partially with supergene goethite). Some pore walls are overgrown with micron-sized electrum aggregates (Fig. 1b). Across the thicker por-tions of the sinusoidal-walled micro-band the electrum impregnations and agglomerations are located in both areas between the large central pores and the micro-band margins thus forming two separate electrum-rich bands (Fig. 1a). At the left veinlet margin in addition, in places there are electrum aggregates situated per-pendicular to the veinlet length which have penetrated preceding micro-bands in a branch-like manner.

The bounding barren cracks were already examined by Marinova et al. (2014) in other colloform-banded hand specimens. Their origin was explained with a formation of cracks of syneresis. Large pores follow-ing the micro-band centerline are consistent with gas bubbles in two-phase (gas-liquid) fluid (Stanley et al., 1997), thus indicating phase separation of one-phase hydrothermal fluid, e.g. boiling. Focusing of electrum aggregates into two bands between the central large pores and the micro-band margins is a phenomenon observed in flows of colloidal solution and referred to as an effect of inertia-induced lateral migration of flowing particles. The position of these bands which appear equilibrium ones, depends on the combina-tion of forces acting on the flowing colloidal particles: shear-induced, wall-induced and rotation-induced (re-view in Zhou and Papautsky, 2013). Branch-like elec-trum aggregates which are laying perpendicular to the micro-band length appear likely a result of retention of electrum colloidal particles in micro-cracks already observed for gold nanoparticles trapped in natural mi-cro-cracks across quartz grains (Alonso et al., 2009). Trapping occurred depending on pH of the solutions

Citation: Marinova, I. 2015. Colloidal and physical transport textures of electrum in a sinusoidal-walled veinletfrom the Khan Krum gold deposit, Eastern Rhodopes Mountains, Bulgaria. - “Geosciences 2015” Nat. conf. with internat. participation, 10-11 Dec., Sofia, 25-26.

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liquid interface already described for colloidal parti-cles by Wan and Wilson (1994).

Conclusions1. The presented textures indicate flow of two-phase solution: a gas phase and a colloidal solution; 2. It is inferred that electrum colloidal particles flowing along saturated sinusoidal-walled micro-fissures experience (i) physical retention in relatively thick portions of the fissures, and in micro-cracks perpendicular to the for-mer, (ii) inertial focusing into equilibrium bands, and (iii) vapor-liquid interface capturing.

Acknowledgements: “Balkan Mineral and Mining” is thanked for the permission to take samples.

ReferencesAlonso, U., T. Missana, A. Patelli, D. Ceccato, N. Albarran,

M. Garcia-Gutierrez. 2009. Quantification of Au nanopar-ticles retention on a heterogeneous rock surface. – Colloids and Surfaces. A: Physicochem. and Eng. Aspects, 347, 230–238.

Marinova, I. 2014. A conceptual model for formation of bo-nanza electrum along steep veinlets in epithermal low-sulfidation gold deposits. Case study from the Khan Krum deposit, SE Bulgaria. – In: Proceedings XX Congress of the CBGA, Tirana, Albania, 204–207.

Marinova, I., V. Ganev, R. Titorenkova. 2014. Colloidal origin of colloform-banded textures in the Paleogene low-sulfi-dation Khan Krum gold deposit, SE Bulgaria. – Mineral. Deposita, 49, 49–74.

Saunders, J., P. Vikre, D. Unger, L. Beasley. 2011. Colloidal and physical transport textures exhibited by electrum and naumannite in bonanza epithermal veins from western USA, and their significance. – In: Steininger, R., W. Pennell(Eds.). Great Basin Evolution and Metallogeny. Geol. Soc. Nevada 2010 Symposium, 825–832.

Stanley, R., T. Ameel, R. Barron. 1997. Two-phase Flow in Microchannels. Technical report, Louisiana Tech. Univ., 348 p.

Wan, J., J. Wilson. 1994. Visualization of the role of the gas-water interface on the fate and transport of colloids in po-rous media. – Water Resources Res., 30, 11–23.

Zhou, J., I. Papautsky. 2013. Fundamentals of inertial focusing in micro-channels. – Lab Chip, 13, 1121–1132.

Fig. 1. a, photomicrographs (in reflected light) of a segment of the au-riferous veinlet represented from left to right by four micro-bands of almost parallel walls and fifth band of sinusoidal shape (reinforced with dotted line along both margins); b, stereomicrophotograph of a central pore with focus on the pore top overgrown with electrum (El) seen as light metallic speckles on the upper side of the pore; the pore bottom seen as black spot

used, surface roughness, defects, grain boundaries or small chemical heterogeneities. Overgrowing of cen-tral large pores with electrum aggregates is consistent with sorption of electrum colloidal particles at the gas-