A Platinum–copper Nugget Rich in Calcite: Evidence For Hydrothermal Pt₃Cu

An alluvial grain, 1.5 mm in length, occurs in the holotype material, a heavy-mineral concentrate, of the minerals kitagohaite, Pt₇Cu, and luberoite, Pt₅Se_4, 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.