Abstract

An alluvial grain, 1.5 mm in length, occurs in the holotype material, a heavy-mineral concentrate, of the minerals kitagohaite, Pt7Cu, and luberoite, Pt5Se4, from the Lubero region, North Kivu, Democratic Republic of the Congo. The grain is remarkable due to abundant inclusions of gangue within a metallic matrix. Here, the grain is characterized by electron-microprobe analysis as consisting of two Pt–Cu intermetallic compounds with calcite inclusions. The grain has a metallic core with Pt:Cu atomic ratios between 2.1 and 2.3. The core is enveloped by a Pt–Cu rim with Pt:Cu atomic ratios close to unity, corresponding to the mineral hongshiite (PtCu). The core composition is within the compositional field of synthetic Pt3Cu formed below about 600 °C in the Pt–Cu system. The calcite inclusions have MnO/FeO > 1, which is interpreted to reflect oxidizing conditions within the stability field of hematite. The low proportion of iron is compatible with the absence of iron in the Pt3Cu core and in the hongshiite rim. Hematite does occur as inclusions in other Pt–Cu nuggets of the heavy-mineral concentrate. The importance of the calcite-rich Pt–Cu nugget is that it indicates a calcite-lode setting, where the nugget is likely to have originated from a calcite-saturated hydrothermal brine.

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