Provenance and Genesis of Gem-Quality Rutile Revealed by Integrated Spectroscopic, Geochemical, and U-Pb Geochronological Signatures

Rutile is an oxide mineral widely distributed in igneous, metamorphic, and sedimentary rocks; it crystallizes in the tetragonal system. Trace-element abundances in rutile are influenced by the host-rock composition, redox conditions and crystallization history. Rutile exhibits high refractive index, strong dispersion, and adamantine luster. Its enrichment in high field strength elements (HFSEs) can be used to trace its formation environment. Owing to its inclusion-poor, compositionally uniform characteristics, rutile is particularly well-suited to in situ U-Pb geochronology. By integrating spectroscopic analysis, trace-element geochemistry, and U-Pb geochronology, this study systematically characterizes nine rutile samples from Madagascar, Pakistan, and Brazil, establishing a multi-dimensional scheme for origin discrimination. Spectroscopic analyses reveal that the infrared reflection band near 670 cm−1 varies systematically with provenance. It appears as a broad, strong band in Madagascar samples, becomes weaker and narrower in Brazilian samples, and is partially absent in Pakistani samples. The Eg Raman mode of Brazilian rutile is slightly left-shifted and exhibits lower intensity, indicating a distinct lattice strain state. Analyzed samples occupy well-separated compositional fields on Nb–V, V–Ta, Zr–Hf and Nb–Ta binary variation plots. Specifically, Pakistani samples are characterized by high Nb and Ta contents and relatively lower V contents than the Brazilian and Madagascar samples. Madagascar samples show pronounced W enrichment and very low Cr. Brazilian samples display elevated Cr, V, higher U contents and more radiogenic Pb isotope compositions. Zr–W systematics and Cr–Nb bivariate discrimination allow inference of geological genesis. The Madagascar rutile is of hydrothermal origin, whereas the Pakistani and Brazilian rutile are metamorphic, derived from felsic/pelitic and mafic protoliths, respectively. LA-ICP-MS U-Pb geochronology yields a lower-intercept age of 504 ± 13 Ma (MSWD = 1.1) for the Madagascar sample (MD-1). This concordant, low-common-Pb age corresponds to the Pan-African orogeny and suggests strong potential as an in-situ U-Pb dating reference material. The Brazilian and Pakistani samples yield lower-intercept ages of 486 ± 53 Ma and 36.8 ± 2.9 Ma, respectively. However, the larger data scatter precludes their use as reference materials.