Chemical composition of pyrite from the Utanakh gold occurrence in diaphtorites of the Verkhne-Timptonkiy area, Aldan-Stanovoy shield (based on EPMA and LA-ICP-MS data)

In the Verkhne-Timptonkiy ore district, despite the presence of large placer deposits and more than a century-long mining history, no comparable primary gold deposits have been identified, and information on their possible types remains highly limited. The aim of this study was to investigate the mineralogical characteristics and chemical composition of pyrite from the Utanakh occurrence in order to reconstruct the conditions of ore formation and to classify the type of gold mineralization. The composition of pyrite from the Utanakh gold occurrence in the Verkhne-Timptonkiy district was examined using electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The Utanakh occurrence is hosted in diaphtorites (biotite–muscovite and chlorite schists) of the Kholodnikan greenstone belt.Mineralization is represented by carbonate–quartz veins and lenses 0.11.5 m thick, and by adjacent metasomatites containing disseminated sulfides. Two generations of pyrite were distinguished. Pyrite-1 hosted in epidote–quartz metasomatites is close to stoichiometric composition and forms fine cubic aggregates up to 200 µm in size. Pyrite-1 is enriched in Ni, Cu, As, Pb, Te, and Zn, contains moderate amounts of Co and Se, and exhibits well-developed sectoral zoning: the cores are enriched in As and Ni, whereas the rims are enriched in Cu, Co, and Pb. Pyrite-2 occurs as anhedral grains 0.8–7.0 mm in size, with chalcopyrite and bornite crystallizing in structural defects in association with native gold. Pyrite-2 from carbonate–quartz veins differs from Py-1 by anomalously high concentrations of Co and Se, low contents of Ni, Cu, and As, and elevated Co/Ni ratios. Gold concentrations in both pyrite types are extremely low. The obtained results indicate a transition from early As–Ni-rich metamorphic fluid sources to late Co–Cu-rich magmatic-derived fluids. The established geochemical indicators are typical of orogenic gold mineralization controlled by diaphtorite zones within granulite and amphibolite complexes. The study demonstrates that pyrite chemistry is an effective tool for classifying poorly investigated gold occurrences.