Trace Element Geochemistry and Metasomatic Origin of Alluvial Sapphires From the Orosmayo Region, Jujuy Province, Northwest Argentina

Abstract

Abstract This study primarily sought to investigate the trace element composition of alluvial sapphires from the Orosmayo region, Jujuy Province, northwest Argentina. In situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis ( n = 69) showed that the sapphires exhibit a predominant metasomatic trace element signature, including low Fe/Mg (4–117) and Ga/Mg (0.3–10.4) ratios. However, the sapphires also display magmatic associations, including elevated Ga (up to 239 ppm) and Si (up to 3890 ppm), which indicates magmatic-metasomatic inputs. Enrichments in LREEs (Y, La, Ce, and Nd), Zn, and Pb, likely contained within remnant formational fluids, and elevated Be, Nb, Ta, Sn, Th, and U contents, likely contained within mineral inclusions, suggest that the sapphires were influenced by highly evolved felsic melts resembling those of granitic composition. The analyses were complemented by mineral inclusion identification using visual microscopy, and confirmed by energy dispersive spectroscopy (EDS). Identifiable co-genetic mineral inclusions include Al and Ti-rich magnetite, pyrochlore, albite, columbite-(Fe), rutile, phlogopite, trikalsilite, pyrite, baryte, monazite-(La), xenotime-(Y), and zircon. Secondary inclusions included hematite and Mg-hercynite which formed as reaction rims surrounding three of the sapphires. The inclusion mineralogy supports the notion of a mixed genetic scenario, with a distinct bimodal nature highlighting interactions between Si-poor and evolved Si-rich melts. Morphological and textural analysis using scanning electron microscopy (SEM) revealed dominant corrosive features on the sapphires. These corrosive features, coupled with the presence of spinel reaction rims, suggests that the sapphires were transported to the surface as xenocrystal fragments that were in chemical dis-equilibrium with their transport medium. Detailed consideration of their geochemical composition in relation to the known geology of the surrounding region was used to narrow down potential sources. Until other evidence surfaces, a working hypothesis for sapphire genesis involves a three-stage process, including: (1) their crystallization via metasomatic fluid exchange between an evolved crustal felsic system, such as crustal pegmatitic/granitic fluids, and intruding coeval mantle-derived carbonatized lamprophyre during the Lower Cretaceous (∼150–100 Ma) as part of alkaline magmatism within NW Argentina; (2) their incorporation as xenocrysts within the effusive volcanic products of the Neogene Granadas Volcanic Complex (∼9.8 Ma); and (3) subsequent host-rock erosion to form the present-day alluvial placer deposits of Orosmayo. The large trace element variation within individual sapphire grains identified in this work also highlights the importance of detailed in situ analysis across corundum grains in order to better understand the complex changing physiochemical conditions during their growth.