The Morphology and Composition of Native Gold from a Placer of the Dunitovaya River (Maimecha–Kotui Province, Polar Siberia)

Complex Au–Ir–Os placer deposits are associated with the Guli massif of ultramafic and alkaline rocks with carbonatites within the Maimecha–Kotui province in the north of the Siberian Platform. In contrast to Os and Ir minerals, which are genetically related to ultramafic rocks, the issue of the primary Au source is a matter of debate. We characterized for the first time the morphological and compositional peculiarities of Au minerals from Quaternary deposits of the Dunitovaya River in the southern part of the Guli massif. The native gold grains are subdivided into drop-shaped–round, lumpy, flattened–lumpy, and lamellar morphological types; the main morphometric parameters, the fineness of native gold, and the average statistical characteristics of chemical composition are determined. By inner structure, the studied native gold grains are subdivided on homogeneous, which mostly consist of homogeneous electrum, and heterogeneous, which contain (1) several minerals (e.g., electrum, tetraauricupride, and auricupride) or (2) electrum with a highly varying composition from Au-bearing silver to Ag-bearing gold. The first Cu isotopic composition of native gold of various morphological types of the Dunitovaya River is characterized by similar values of δ65Cu in a range of –0.59 to 0.11‰ (the average δ65Cu value = –0.30 ± 0.23‰, n = 5), indicating a primitive ore-matter source. Taking into account the geological setting of the riverbed and terrace deposits of the Dunitovaya River, insignificant transportation distance of placer gold (4–6 km), and similarity of Au minerals from the Dunitovaya River with those from calcite carbonatites, the rocks of the Maimecha–Kotui ijolite–carbonatite complex were the main source of the studied morphological gold types.