4.1. Mineralogy of Slag Cu-(Fe)-Sulfides
Copper sulfides in investigated Uralian and Kazakhstan slags are presented by the mixture of covellite (CuS)–chalcocite (Cu2
S) (Table 2
). At the present time, this series includes eight IMA registered mineral species: covellite (66.5% Cu)–yarrowite (69% Cu)–spionkopite (73.4% Cu)–geerite (76% Cu)–anilite (77.6% Cu)–digenite (78.1% Cu)–djurleite (79.3% Cu)–chalcocite (79.9% Cu). However, it was not possible to diagnose any mineral in the mixture on the basis of chemical composition. Hereinafter, when referring to extreme mineral members, we assume the presence of intermediate species. On several archaeological sites, bornite and products of its alteration are identified (Cu5
, 63.3% Cu). A significant quantity of sulfides is presented by neogenic non-stoichiometric species with varying Fe contents. Sulfide droplets and grain sizes range from between several microns to inclusions measured in millimeters. Chalcopyrite (CuFeS2
34.6% Cu) and other copper sulfides are completely absent from the slags. An isolated grain of altered chalcopyrite is found in a slag from Taldysay.
According to morphological peculiarities, sulfide inclusions are divided into relict clast and melt droplets. Copper sulfide relicts formed isolated angular clasts and grains 3–4 mm up size (Figure 2
a). Relict clasts are found in slags from Kamenny Ambar, Konoplyanka, Katzbakh 6, Turganik, Tokskoe, Ivanovskoe, and Taldysay settlements. Melt inclusions are subdivided into primary-melt, which melted without major chemical and mineral composition transformation (Figure 2
b), and neogenic inclusions (Figure 2
c), which were separated from sulfide-silicate melt with new components absorption.
Covellite was observed to form relict fragments of several millimeters in size and melted grains or rare neogenic droplets (Figure 2
d). The color of covellite in fragments and smelted grains is bright blue up to dark-blue, while neogenic droplets are often light-blue color in reflected light. As copper sulfide, it prevails in slags from Trans-Urals settlements (Kamenny Ambar, Konoplyanka, Katzbakh 6, and Rodnikovoe). In relict and melted grains, covellite is often fractured, with copper oxide and carbonate and other supergenic copper mineral formations in the fractures. Neogenic aggregates formed rounded droplets with massive structures (Figure 2
Chalcocite prevails among the neogenic droplets and is widespread in the Cis-Urals, where it is strongly dominated on several settlements (Turganik, Ivanovskoe, Pokrovskoe, Rodnikovoe, Kuzminkovskoe 2). Chalcocite formed in neogenic droplets is yellow-grey colored, sometimes with a blue tinge in reflected light. Chalcocite formed small 1 µm inclusions and large 3–5 mm droplets. Rounded, crescent, and ring-shaped droplets are dominated and rarely amoeba-like aggregates. Ring-shaped and crescent shapes were often seen to frame metallic copper droplets (Figure 2
d). A wide range of covellite-chalcocite mixtures occurs in the Cis-Urals (Tokskoe, Bulanovskoe 2) and Kazakhstan (Sarlybay and Taldysay) settlements (Figure 2
The rarely-found bornite, which has a light pink color in reflected light (Figure 2
g), can serve as an indicator mineral. Bornite typically forms melt relic grains size up 1 mm. It was found only in Katzbakh 6, Ivanovskoe, Tokskoe, Turganik, and Taldysay settlements. Chalcopyrite occurs only in isolated cases. The only chalcopyrite grain was found out in samples from Taldysay (Figure 2
h). It was formed by round melted fragments which were secondary altered up to covellite. Additionally, many isolated droplets of metallic copper and bronzes were found in samples having sizes ranging from several microns up to 5 mm. However, the present paper does not deal with composition of metal droplets.
4.2. Trace Elements of Slag Cu-(Fe)-Sulfides
Copper sulfides vary widely in trace element contents. They contain a wide range of chalcophile, siderophile, and noble metal elements. In this paper, we determine the distribution of numerous elements, focusing on the marker elements that indicate ore source genesis, fluxes, and technological features of copper smelting.
Fe is a widespread minor element in copper sulfide. Moreover, in addition to copper and iron minerals (bornite and chalcopyrite) and their pyrogenic transformed species, it comprises a mixture in the covellite-chalcocite mineral group. Higher Fe contents are determined in Tran-Urals sulfides (Table 3
). Here, the average Fe contents reached 1–3 wt. % (up to 22 wt. % in some cases) in sulfides of samples in the Kamenny Ambar, Konoplyanka, and Katzbakh 6 settlements and slags from Kazakhstan (Sarlybay 3, Taldysay). In copper sulfides of Cis-Urals slags, Fe contents are lower, comprising 0.1–0.5 wt. % on average.
Arsenic (As) is an important element used to alloy copper in Eurasia [1
]. Arsenic in natural objects is confined to sulfide ores of different genesis located in ultramafic rocks. According to arsenic contents in sulfides (Figure 3
, Table 3
), four slag groups are divided. The first low-arsenic type with As contents less than 30 ppm is typical for the Alakul culture slags from Trans-Urals settlements (Kamenny Ambar, Konoplyanka, and Katzbakh 6) for which the ore source is not determined. Such contents are usual for several Cis-Urals settlements (Tokskoe, Rodnikovoe) for which the local raw material of cupriferous sandstones is supposed. The bulk of slag sulfides from Cis-Urals settlements is confined to the second group and contains arsenic up to 30–200 ppm. Sarlybay 3 slag sulfides can also be added for which the ore source is Sarlybay volcanogenic massive sulfide (VMS) deposit in basalts.
According to arsenic contents of 300–5000 ppm, the third sulfide group relates to the objects connected confined to ultramafic rocks. Due to ores application from these deposits, arsenic-copper alloys were obtained naturally. But As is not enough for arsenic bronzes production. These include the Early Yamna culture samples from Turganik settlements and Srubna slags from Pokrovskoe and Rodnikovoe settlements. The fourth group with high contents arsenic more than 0.5% is represented by special alloying addition arsenic minerals in slags. Copper arsenides often occur in a view of inclusions in copper droplets. These include samples from the Ustye and Taldysay.
Se replaces sulfur in minerals. In natural objects, Se is confined to the oxidized zone of VMS deposits in basalt-rhyolite and ultramafic complexes. Three main groups are subdivided according to their Se content. Low-selenium sulfides having less than 100 ppm of Se are typical for the Ustye and Katzbakh 6 sites, as well as for the Srubna culture slags from the Pokrovskoe, Turganik, and Ordynsky Ovrag sites (Figure 3
, Table 3
). An Se content of 100–1000 ppm is typical for glassy slags from cupriferous sandstones of Tokskoe, Ivanovskoe, Bulanovskoe 2, Kuzminkovskoe 2, Rodnikovoe, and Taldysay. Slags with high Se content (more than 1000 up to 7000 ppm) are found at the Kamenny Ambar, Konoplyanka, and Sarlybay 3 sites.
Sulphur can also be replaced by Sb in sulfides, where it can occur as microinclusions. Two main typological subdivisions can be distinguished according to Sb content: low-antimony (<10 ppm) and high-antimony with Sb contents up to 650 ppm. High Sb contents are typical for doped slags and reflect the input of alloying additions that is confirmed by correlation with As, Co, and Ni. These include samples from Ustye, Rodnikovoe, and Taldysay (Figure 3
, Table 3
Co and Ni are widespread in sulfides of slags and are subdivided into three groups. According to geochemical peculiarities these elements are both similar, e.g., in ultramafic rocks with high Co and Ni contents, and differ significantly, e.g., high Co and absent Ni in VMS deposits hosted in basalts. The majority of sulfides in slags from Cis-Urals settlements (Ordynsky Ovrag, glassy slags from Turganik, Tokskoe, Ivanovskoe, Bulanovskoe 2, and Rodnikovoe with possible sources from cupriferous sandstones, as well as Katzbakh 6) are related to low cobalt and low nickel group with less than 20 ppm content (Figure 3
, Table 3
). Sulfides from Kamenny Ambar, Konoplyanka, and Sarlybay are related to high-cobalt (more than 20 ppm) and low-nickel groups. Natural slags from Ustye, Rodnikovoe, and Taldysay, as well as those artificially-doped by arsenic, are related to high-cobalt and high-nickel sulfide groups; this also applies to Cr-rich spinel-containing olivine Turganik slags.
Despite ranging widely in terms of its contribution to the contents of sulfides, Ag can also serve as an indicator mineral. The studied slags are subdivided into two main types in terms of their Ag content. Low-silver sulfides having an Ag content of less than 30 ppm are typical for the Kamenny Ambar, Konoplyanka, and Sarlybay 3 sites. High-silver type slags with 50–500 ppm Ag content are typical for all Cis-Urals objects, as well as occurring at the Ustye, Katzbakh 6, and Taldysay settlements. In several samples, e.g., Kuzminkovskoe 2 and Rodnikovoe, the Ag content can exceed the second type to reach 0.4% (Figure 3
, Table 3
Ba can form neogenic sulfides and barite inclusions. A low Ba content (<50 ppm) is typical for sulfides of slags from the Cis-Urals and Kazakhstan (Figure 3
, Table 3
). Slags having a high Ba content ranging from 50 up to 1500 ppm are typical for Cis-Urals sites. Katzbakh 6 (Trans-Urals) and Rodnikovoe (Cis-Urals) settlements are characterized slags with a Ba content exceeding 0.5%, which probably indicates the addition of barite fluxes in charges.
The lowest values of Pb not exceeding 10 ppm are typical for Kamenny Ambar, Konoplyanka, and Sarlybay 3 slags, as well as the Cr-rich spinel containing olivine slags at the Turganik site. A slightly higher Pb (10–100 ppm) content is found in sulfides from Ustye, Katzbakh 6, glassy Turganik slags, Ivanovskoe, Bulanovskoe 2, Kuzminkovskoe 2, Pokrovskoe, and Rodnikovoe settlements. High Pb amounts (100–1000 ppm) are in slags from Ordynsky Ovrag, Tokskoe, and pyroxene slags of Rodnikovoe. Extremely high values (more than 0.1% Pb) are found in samples from the Taldysay settlement where numerous findings of galena and metallic Pb ingots were also discovered. A similar pattern is expressed in terms of Bi, although its contents are far smaller.
Zn is present in small amounts in slag sulfides ranging between 4 and 40 ppm. However, the contents are widely varied within the same object to reach 0.1–0.2% at Kamenny Ambar, Ordynsky Ovrag, Pokrovskoe, and Taldysay settlements. Due to the heterogeneity of content, Zn distribution cannot serve as an indicator of raw material sources.
Sn in sulfides of slags is significant, except for Taldysay samples. The majority of Sn contents in Cu-(Fe)-S ranges from a few tens of degree ppm up to the first ppm. Only a few values reach hundreds of ppm at Kamenny Ambar, Ustye, Sarlybay 3, and Taldysay settlements. At Taldysay, Sn rarely contributes as much as 0.45%.
Mo contents in slags are not suitable as indicators, since varying widely up to hundreds of ppm. The lowest values of a few ppm are typical for samples from Kazakhstan and olivine slags from Turganik. The highest concentrations contained in slag sulfides from the Tokskoe and Rodnikovoe settlements reached up to 600 ppm.
Although Cr generally appears in slag sulfides at values much smaller than the limit of detection, significant amounts are found in the Turganik olivine type. A few values within the a few tens of ppm are obtained in sulfides from Cis-Urals slags.
U is also undetected in any significant amounts or regularities. The elevated contents are in slags of the second type from the Rodnikovoe and Ivanovskoe settlements.
Other siderophile (Mn, V) and chalcophile (Ga, Ge, Cd, In, Au, and Hg) contents, as well as the majority of lithophile elements, are minor and, consequently, cannot serve as indicators.
Thus, several archaeological site groups differ in the composition of copper sulfides in slags. The first group is represented by sulfides with high contents of Co, Ni, Sb, and As, including slags from Ustye, Kuzminkovskoe 2, Taldysay, Turganik type 1, and Rodnikovoe type 2 (Table 3
). The second group is represented by high contents of Se and Co in copper sulfides, including slags from Kamenny Ambar, Konoplyanka, and Sarlybay 3 sites. The third group is represented by slag with high contents of Ag, Pb, and Ba, including Katzbach 6 and most of the Cis-Urals sites (Ordynsky Ovrag, Turganik, Tokskoe, Ivanovskoe, Bulanovskoe 2, Pokrovskoe, Rodnikovoe).