New mineralogical data on platinum-group minerals from the Río Santiago alluvial placer, Esmeraldasprovince, Ecuador

Abstract

Mineralogical studies on platinum-group minerals found in placer deposits from the Río Santiago (Ecuador) are scarce. In this investigation, one sample collected from the Río Santiago alluvial placer was studied via a multi-disciplinary approach, including optical microscopy, scanning electron microscopy, electron microprobe, and Raman spectroscopy. Whole-rock geochemistry data of the sample confirms elevated Au and platinum-group elements contents and the chondrite-normalized pattern reveals pronounced positive Ir and Pt nomalies. Free grains of platinum-group minerals were separated via hydroseparation techniques and identified as: i) Pt-Fe alloy (Pt3Fe), ii) tulameenite (Pt2FeCu) and iii) hongshiite (PtCu). The most abundant platinum-group mineral is Pt-Fe alloy (85%) that occasionally hosts cuprorhodsite (CuRh2S4) inclusions. Although the primary source remains unknown, the geochemical and mineralogical data suggests that the source of platinum-group minerals in the Río Santiago alluvial placer is a mafic-ultramafic Ural-Alaska type complex. Possible primary sources are the mafic and ultramafic rocks found in the mafic basement of the coastal region and the Western Cordillera (Piñón, San Juan and Pallatanga units), which derive from the Late Cretaceous Caribbean Colombia Oceanic Plateau (CCOP).