Mineralogical assessment of orpiment-rich tailings from the abandoned Allchar mine, North Macedonia

The northern zone of the abandoned Sb–As–Tl–Au Allchar deposit in North Macedonia is characterized by high concentrations of thallium (Tl), iron (Fe) and arsenic (As), which are distributed among mining waste, technosols and soils (Đorđević et al., 2021; Vaňek et al., 2024). The high concentrations of Tl and As in carbonated, buffered environments have led to the formation of secondary minerals that are more widespread than is currently documented, and which can play an important role in reducing the dispersion of Tl and As from sources of contamination. Therefore, a detailed mineralogical assessment of Tl- and As-rich legacy tailings is important to understand the environmental risks and inform future strategies for the valorization or containment of these materials within a circular economy framework.

As part of the current study, we have begun investigating orpiment-rich tailings near Adit 25 in the northern part of the Allchar deposit. Tailings material was collected at four different areas of the tailings and bulk chemical, and mineralogical analyses were performed using powder X-ray diffraction (PXRD), Raman spectroscopy and inductively coupled plasma mass spectrometry (ICP-MS).

The preliminary results indicate the following: (i) Tl is present in amounts ranging from 210 to 3,950 ppm, with an average value of 2,035 ppm; (ii) As is present in amounts ranging from 6,750 to 210,000 ppm, with an average value of 124,550 ppm; (iii) Fe is present in amounts ranging from 6,500 to 50,500 ppm, with an average value of 34,700 ppm. Locally the samples show elevated concentrations of Sb (up to 1,150 ppm) and Hg (up to a maximum of 500 ppm). The main sources of As are orpiment (As₂S₃), locally realgar (As₄S₄) and scorodite (FeAsO₄·H₂O), followed by minor Fe-sulfate arsenates (e.g. bukovskyite, Fe³⁺₂(AsO₄)(SO₄)(OH)·9H₂O) and calcium arsenates. The main sources of Fe are pyrite (FeS₂), marcasite (FeS₂), goethite (Fe³⁺O(OH)), scorodite and amorphous Fe-oxides, as well as minor Fe-sulfate arsenates. The major sources of Tl are Tl-bearing sulfosalts, such as vrbaite (Hg₃Tl₄As₈Sb₂S₂₀) and simonite (TlHgAs₃S₆), which also serves as the sources of antimony (Sb) and mercury (Hg). Further dominant phases are gypsum, dolomite, quartz, calcite, muscovite and kaolinite, and barite in the heavy fraction (density >2.9 g/cm³).

While the present study is based on bulk chemical and mineralogical analyses, ongoing and planned micro- and nano-scale electron microscopy will extend these results by investigating metastable surface phases containing Tl and As, thereby improving the assessment of selective re-mining, containment, and valorization options for hazardous but resource-bearing mine waste within risk-aware circular economy frameworks.

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

Đorđević, T. et al. (2021): J. Appl. Geochem., 135, 105114–105130.

Vaňek, A. et al. (2024): Environ. Pollution, 357, 124413–124421.