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49EARTH SCIENCES
GOLD-BEARING ARSENIAN PYRITE AND ARSENOPYRITE FROM VORONTSOVKA CARLIN-
STYLE GOLD DEPOSIT IN THE NORTH URALS
Tyukova E.E.
candidate of geological and mineralogical sciences Scientific Geoinformation Center, Russian Academy of Sciences
Moscow, Noviy Arbat,11
Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry (IGEM), Russian Academy of Sciences Moscow, Staromonetniy per.35 Vikentyev I.V.
Doctor of Geological and Mineralogical Sciences
Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry (IGEM), Russian Academy of Sciences Moscow, Staromonetniy per.35 Kovalchuk E.V.
Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry (IGEM), Russian Academy of Sciences Moscow, Staromonetniy per.35 Borisovsky S.E.
Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry (IGEM), Russian Academy of Sciences Moscow, Staromonetniy per.35 Tagirov B.R.
Doctor of Geological and Mineralogical Sciences
Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry (IGEM), Russian Academy of Sciences Moscow, Staromonetniy per.35
Abstract
Invisible gold in hydrothermal ores is frequently scattered in the most abundant minerals of the Fe-As-S system. It is assumed that invisible gold does not incorporate into the mineral structure (nanoscale inclusions of the metal or its compounds) or is chemically fixed (isomorphous substitution). The aim of this study is to determine the concentration range of invisible gold, species of its occurrence in arsenian pyrite and arsenopyrite in the natural samples from the Vorontsovka Carlin-type deposit in the Northern Urals. According to EMPA, LA-ICP-MS analyses, the number of trace elements Sb, Co, Ni, Ag, Cu in arsenian pyrite and arsenopyrite does not exceed 0.1 wt% in total. Specially selected survey conditions were used to determine the Au impurity content. Under these conditions, the detection limit for Au is 0.0045 wt%. The As content in pyrite varies from 0.28 to 4.316 wt%, Au - from 0.2 ppm to 0.074 wt%. The Au content in arsenopyrite reaches 1.23 wt%. Correlation dependences of Au and As contents in pyrite, Au and the main components in arsenopyrite suggest the occurrence of Au in the structure of the minerals.
Keywords: «invisible» gold, arsenian pyrite, gold containing arsenopyrite, the North Urals, EPMA.
1 Introduction
Gold in hydrothermal ores is frequently associated with As-bearing minerals: arsenopyrite FeAsS, arsenian pyrite FeS2, and less frequent lollingite FeAs2. Arsenopyrite and arsenian pyrite are the major ore minerals at some hydrothermal deposits, including large and superlarge, where gold is the main or byproduct. These are Carlin-type gold deposits [1, 2]; epithermal [3], mesothermal, or orogenic gold deposits; and Cu-Au skarn and massive sulphide deposits [4, 5]. In the ores of these deposits, sulphides, in particular, arsenopyrite and arsenian pyrite, contain invisible gold, which could not be identified by optical or scanning electron microscopy. Invisible gold is scattered in sulphides as an isomorphous admixture, nanoscale Au0 inclusions or Au-semimetal (As, Sb, Bi, Te)
compounds [6]. Variable composition ((As/S and Fe/(As+S)) within crystals, contrast growth zoning, and structure imperfection of arsenopyrite facilitate Au scattering in this mineral. Invisible gold is refractory and is not recovered by conventional or environmentally friendly processing methods; the majority of such gold passes into waste. Therefore, determination of the conditions favoring the formation of refractory ores and refinement of invisible gold species in sulphides are necessary to improve deep recovery technologies for gold from refractory ores.
2 Geological setting
The Vorontsovka gold deposit (59°39'5'' N, 60°12'56'' E) is situated in the Krasnoturinsk District, Sverdlovsk oblast, in the Auerbakh volcanoplutonic
belt. It has been operated as an open pit since 1999 by ZAO Zoloto Severnogo Urala; more than 55 t of gold and 50 t of silver have been molded into bars. The
Turya-Aurbakh ore district hosting this deposit corresponds to the eponymous volcanotectonic depression with walls gently dipping (15°-30°) toward the cen ter of the structure (Pic. 1).
volcanic rocks of Tura formation(Upper Silurian-Lower Devonian):
Pic. 1. Schematic geological map of the Turya-Auerbakh ore district. Compiled by V. Murzin using Bobrov (1991) and materials of Rindzyunskaya et al (1995). Skarn deposits (after Podlessky 1979): 1 - Auerbakh, 2 -Novo-Peschanka, 3 - North- and South-Peschanka, 4 - West-Peschanka, 5 - Poludenskoe, 6 - Vorontsovka Fe and North-Vorontsovka Fe, 7 - South-Vorontsovka Fe, 8 - Garevskoe, 9 - Vladykinskoe, 10 - Kakva, 11 - 34th quarter, 12 - Troitsk-Mikhaylov, 13 - Bogoslovsk andBashmakov, 14 - Vadimo-Aleksandrovka, 15 -Nikitinskoe, 16 - Uspenskoe, 17 - Frolov, 18 - Suvorovskoe, 19 - Vasilyev, 20 - Sukhodoyskoe, 21 -
Psarevskoe.
It is filled with sedimentary, volcanosedimentary, and volcanic rocks of the Emsian to Lower Eifelian Krasnoturinsk Formation. The Auerbakh group of iron skarn deposits and Vorontsovka gold deposit located in the southern part of the district are related to the Early to Middle Devonian Auerbakh gabbro-diorite-gran-odiorite pluton [7]. Initially, the Vorontsovka deposit contained 101 t Au @ 7 g/t Au, and120 t Ag @ 8 g/t Ag; including 30 t Au in regolith [8]. The deposit is mined by Polymetal Company from 1999 in two open pits to a depth of 200 m and 80 m. Orebodies correspond to the fine-disseminated low-sulphide (3-5 vol%, rarely up to 30 vol%) zones; gold-quartz veinlets occur rarely. The main gold endowment associates with the following two ore types: the gold-pyrite-arsenopy-rite ore in the argillic altered tuff-clastic rocks and the gold-pyrite-realgar ore in the silicified limestone breccia with relict carbonate-volcanoclastic cement.
3 Mineralogy
There are four groups of mineral assemblages in the orebodies of the Vorontsovka deposit. [9] The later
assemblages often overprint the early ones. Group 1 comprises pyrite ± sphalerite ± chalcopyrite ± pyrrhotite and is located in the limestone breccia and in the lowest part of the overlying tuff-clastic rocks. (Pic. 2) Group 2 comprises fine-disseminated gold-pyrite-arsenopyrite in the tuff-clastic rocks. Such sulphide impregnation is locally redistributed with segregation into pyrite veinlets. Pyrite forms euhedral crystals and spherical aggregates (from 0.01 to 0.n mm). Arsenopyrite commonly occurs in the tuff-clastic rocks if they have evidences of hydrothermal alteration and primary pyrite recrystallisation. The As/S atomic ratio is close to 1. Group 3 includes garnet-magnetite prograde skarn; epidote retrograde skarn; brecciated carbonate rock with cocarde texture; polysulphide-carbonate-quartz and massive chalcopyrite veins. In the skarn, overprinting sulphide association of pyrite ± pyrrhotite ± chalcopyrite ± sphalerite ± galena is common, i.e. sulphides are epi-skarn. Later, along with silicification, arsenopyrite ± pyrrhotite ± sphalerite ± fahlore + Pb-Sb-sulphosalts + native gold were formed. The As/S atomic ratio in
arsenopyrite is close to 1. Group 4. The Carlin-style gold-pyrite-realgar assemblage is superimposed over different breccias. The sequence of the mineral deposition is as follows: pyrite + arsenopyrite ^ (silicification) ^ Pb-Sb-sulphosalts + sphalerite + chalcopyrite ^ (argillisation) ^ native arsenic + S-lollingite + native gold ^ thallium minerals + stibnite + realgar + orpiment + native gold. The As/S atomic
For the study, samples from the 3rd and 4th groups of associations were used. Formation conditions of assemblages:
Group 3 - T, = 400-270°C; P = 0.2-0.6 kbar; log f S2 = -7 to -9;
Group 4 - T, = 370-250°C; P = 0.15-0.2 kbar; log f S2 = -12 to -17.
ratio in ;
yrite is greater than 1.
Pic. 2. Ores from the Vorontsovka deposit: (a) carbonate breccia with laminated semi-massive sulphide cement, Vr131-3; (b) layered tuff-clastic rock with disseminated of sulphides (pyrite, chalcopyrite, sphalerite, marcasite) altered and recrystallised along veinlet, Vr131-10; (c) brecciated limestone of cocarde structure with zones of carbonaceous matter and sulphide impregnation (pyrite, arsenopyrite, magnetite, pyrrhotite, native gold), Vr11— 6; (d) skarnoid with epidote and pyrite zones in carbonate rock, Vr9-7; (e) carbonate breccia, containing disseminated realgar with impregnation of gold, sulphides and thallium minerals, Vr128-1; (f argillic altered tuff-clastic rocks, rich in thin impregnation of acicular arsenopyrite (Apy) which is intersected and cemented by
native arsenic (As), associated with native gold, Vr1-2.
4 Analytical methods
The chemical composition of arsenopyrite and pyrite were measured using EPMA (JXA-8200). The major constituents (As, Fe, S) were determined at an accelerating voltage of 20 kV, current intensity on a Faraday cylinder of 20 nA, and beam diameter of 1 ^m. The acquisition time for As (La, TAP), Fe (Ka, LIF), and S (Ka, PETH) was 10 s at the peak and 5 s on both sides. Operating conditions for determination of gold trace concentrations are Ma line, 20 kV, 300 nA, peak time 200 c, background time 100 c, beam diameter 2-3
mkm. These operating conditions and correctly selected background points made it possible to decrease Au detection limit down to 78 and 45 ppm for lines La and Ma, respectively.
4.1 Gold in Arsenopyrite
The data obtained for arsenopyrite crystals indicate an inverse correlation between Au and S and a direct correlation between Au and As (Pic. 3). The relationship between Au and Fe within individual crystal zones is weaker.
Pic. 3. Distribution of gold content along the profiles in arsenopyrite: a,b - from skarn (Vr134-15); c-e - high-arsenic arsenopyrite of the gold-pyrite-realgar assemblage (Vr1-5). Arsenopyrite with profile C-D is a relic one of the skarn assemblage. Detection limit of gold 45ppm, dotted line.
4.2 Gold in arsenian pyrite arsenic zones in grains with oscillatory zoning con-
Grains of pyrite from the epidote skarn have spotty sistent with the symmetry elements of pyrite crystals
and oscillatory zoning. The maximum contents of As (Pic. 4-5). (4.316 wt.%) and Au (0.079 wt.%) are typical for high-
O.OOl-,-,-,-2-,-,-,-,
0 10 20 30 40 50 60 70
Profile points
Pic. 4. Gold-bearing pyrite of zonal structure (Vr10-17, BSE images) and distribution of arsenic (a) and gold (b) along profile A-B; B - scan profiles: -. The dotted line indicates the detection limit for gold content (45 ppm). The correlation coefficient between the elements is 0.91.
0 10 20 30 40 50 60
Profile points
Pic. 5. Distribution of As and Au in zonal pyrite Vr10-17. A and B section and profile through a pyrite crystal; C-distribution of as and Au contentsin pyrite obtained on the JXA-8200 (JEOL) device using a special precision technique for simultaneous determination of Fe, S, As and Au. The detection limit (3 a) for As is 0.01 wt%, for
Au-0.0045 wt%.
5 Discussion
The data obtained for arsenopyrite crystals indicate an inverse correlation between Au and S and Au-Fe and a direct correlation between Au and As (Pic.5).
Pic. 6. Dependence of the Au content on the main elements in the studied arsenopyrite samples. Blue dots-arsenopyrite from skarn (Vrl34-15); dots: red-relic arsenopyrite (Vrl-5); green-highly arsenic arsenopyrite of
the gold-pyrite-realgar assemblage (Vrl-5).
For a gold content up to ~0.6 wt%, there is a strong correlation between Au and As: the Au concentration increases in arsenopyrite enriched in As. This relationship becomes weaker for a gold content above 0.6 wt%. Meanwhile, a strong inverse correlation between Au and Fe is observed over the entire compositional range of the Au-bearing arsenopyrite studied here, which again proves the formation of the solid solutionin which Au substitutes for Fe in the structure of arsenopyrite. Since similar dependences of the Au content on the arsenopyrite composition were identified at other Carlin-
type deposits [1], it can be assumed that they are contained at deposits of this type in general.
The data obtained for arsenian pyrite indicate the presence of two groups of impurity contents in pyrite (Pic. 7): one-has low as impurity contents and close to zero level of Au concentrations. While the second-begins near 1.5 wt% of the as impurity and with increasing its concentration linearly increases the Au impurity to 0.074 wt%. In the analysis of pyrite probably two aspects should be considered: the As impurity in pyrite and the Au impurity in pyrite and their relationship.
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
Pic. 7. Distribution of As and Au contents in pyrite, Vr10-17. The dotted line is the limit of Au detection.
So the maximum solubility of As in pyrite according to experimental data by L. Clark [10] at 600°C is 0.53 wt%. and in natural pyrites, the As content reaches 9.3 wt% [1] and some researchers determined 14.1 wt% As in pyrite [11]. And in most cases, researchers note, that the sum of S and As contents in pyrite is preserved by 66.7 at% and its low concentrations are interpreted (up to 1.2 wt% As on average) as a solid solution with local clustering of As atoms. However, at higher concentrations of As in pyrite (6-9 wt%), researchers [2, 12] detect arsenopyrite domains in HRTEM images.
Gold impurity in pyrite in which the arsenic content does not exceed 0.2 wt% is no more than 700 pbb or is completely absent [13, 14]. Significant gold content appears when arsenic content in pyrite is at least 1.5 wt% [11, 14, 15]. In many cases, there is a linear positive dependence of Au and As contents. But at the same time, there are three maximum arsenic contents:
0-1. 5; 1.5-5 and above 5 wt% [11]. The entry of As admixture into pyrite, and with it Au, can be explained by a change in the structural form of the entry of Au Into As-pyrite: at relatively low concentrations of As (0-5 wt%) gold enters the solid solution S22-^ AsS3-, at high concentrations of As, gold is included in the pyrite and marcasite-structural (arsenopyrite) domains, which are identified in [16].
6 Conclusions
Thus in our studied sample of arsenopyrite and As-pyrite, it is also possible to assume the occurrence of Au in its structure.
Funding: project no. 20-05-00849 Sources of hydrothermal fluids and mechanisms of concentration of heavy metals in oceanic and paleoceanic complexes.
Conflicts of Interest: The authors declare no conflict of interest.
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