Macro- to nanoscale mineral relationships in mining wastes of the As–Sb–Tl–Au Allchar mine, North Macedonia

In the mining waste dumps and tailings of the former As–Sb–Tl–Au Allchar deposit, North Macedonia, secondary oxides and oxy-salt minerals partially control the mobilization of arsenic (As), antimony (Sb) and thallium (Tl). Depending on the pH condition and the proportion of primary sulfide and sulfosalt minerals, we have observed two major scenarios for the retention of As, Sb and Tl through secondary minerals.

We have investigated three dozens of solid samples from the profiles and excavation holes at the three sites of former Alchar mine, analyzed them for their major and minor chemical elements, and characterized them for their mineralogical composition, with a special focus on Tl-secondary minerals at the nano- to centimeter scale.

At the Tl- and As-rich Crven Dol locality, As and Tl dissolved during weathering under circumneutral to slightly alkaline conditions are precipitated as micaceous crystals of poorly crystalline to amorphous thallium arsenates, forming porous aggregates up to 100 µm. These Tl arsenates are intergrown with dolomite and Ca-Fe-arsenates and appear as two different phases. In the first, more common phase Tl:As ratio range from ca. 2.1 to 4.1. In the second, Tl-richer phase, the Tl:As ratio varies from 5.1 to 8.4. In the waste dumps showing acidic pH-values common Tl precipitate is dorallcharite [TlFe³⁺₃(SO₄)₂(OH)₆]. Tl is also accumulated in Mn-oxides (up to 3.6 at.%), pharmacosiderite (up to 0.9 at.%), and jarosite-group minerals (up to 0.9 at.%).

The orpiment-rich tailings are mostly composed of orpiment, quartz, realgar and scorodite, followed by gypsum and kaolinite-group minerals. Realgar and orpiment are the major As-sources and Tl-sulfosalts lorándite, fangite, and raguinite are the primary Tl-sources. The most common Tl-bearing precipitate is dorallcharite mostly embedded in scorodite. Tl is also accumulated in Mn-oxides (up to 5 at.%) and thalliumpharmacosiderite, TlFe₄[(AsO₄)₃(OH)₄]·4H₂O.

In the deposit is Sb-rich central region, the primary Tl sources are sulfosalts such as fangite, lorándite, and pierrotite, while stibnite is the primary Sb source. Tl dissolved during weathering under circumneutral conditions is reprecipitated as avicennite, Tl₂O₃, and tiny, fibrous Tl-bearing Mn-oxides (up to 8.5% Tl). Furthermore, tiny spherulitic aggregates (up to 3 µm) of a Tl-Sb-oxide (a new mineral species) have been found intergrown with quartz, muscovite, and minor dolomite. TEM-based EBSD on Tl-Sb-oxide particles confirmed that the Tl-Sb-oxide is crystalline, and EDS-line and area scans confirmed a Tl:Sb ratio of 2.5, indicating that Tl enters the crystal structure of the new Tl-Sb oxides rather than being hosted in the nanophase.The oxidative weathering of Tl-bearing metal-sulfides generates both nano- and microcrystalline Tl-minerals.

Our future investigation focuses on the formation and dissolution of these phases and will offer a much deeper understanding of the mechanisms of mineral association formation.

Financial support of the Austrian Science Fund (FWF) [P 36828-N] is gratefully acknowledged.