Deciphering the Tanzanian Ruby–Zoisite Enigma: A Confluence of Geochemistry, Microtextures, and Mineralogy

The Longido region (Tanzania) hosts a distinct corundum–zoisite paragenesis, renowned for its ornamental value and geological significance as a tracer of Pan-African tectonothermal events. Through integrated analyses—including electron probe microanalysis (EMPA), LA-ICP-MS, XRF mapping, and SEM-EDS on five representative samples—we posit that the genetic model for the zoisite–corundum is that pargasite and early-stage corundum were the protolith, which experienced zoisitization prior to hydrothermal fluid influx. This fluid event induced the replacement of zoisite and mechanical compression by newly crystallized corundum. Key findings include textural–chemical concordance: rubies exhibit Al2O3 >98 wt.% with Si anomalies (>5000 ppm) in transitional zones, indicative of fluid-mediated replacement of precursor zoisite. Combined mineralogy, this study explored the debated genesis of Longido red corundum–zoisite assemblages, and a rough model was obtained.