Syenite mapping and prediction of geochemical Na versus K signatures: A novel remote sensing approach and implications for mineral resources

Syenites are highly valued for economic and strategic exploration programs worldwide, along with associated rocks (e.g., carbonatites), as a primary source for several industrial minerals and strategic elements, such as phosphate, potash, rare earth elements (REE), and Nb. Rapid identification and mapping of syenite outcrops and their major geochemical signatures (i.e. potassic and sodic) over vast areas are crucial for exploration programs aimed at identifying new economic deposits. For this, remote sensing provides an interesting way to delineate these alkaline igneous rocks and predicts their mineralogical and geochemical characteristics. In the literature, few remote sensing studies have been devoted to syenite outcrops, and an effective predictive tool for their mapping and mineralogical-geochemical classification remains to be done. Accordingly, we explore in the present study the potential of using ASTER thermal emissivity data for prospectivity mapping and predicting the main alkaline mineralogical and geochemical affinities of syenite rocks. This approach was applied to the Moroccan High-Atlas Mountains (area ∼ 42,000 km²), known for the presence of several alkaline intrusions hosting different alkali-syenites. Hence, a spectral syenite index [SyI = (B10/B11 * B10/B13 * B14/B13)] without the use of in-situ field data, was suggested in the first stage to quickly highlight potential areas, reducing the initial exploration zone to only ∼1000 km²/42000 km² that include all well-known syenites in the studied area. Subsequently, several in-situ field missions were conducted in the identified zones to sample and obtain precise GPS points of well-exposed syenitic outcrops. For discrimination of mineralogical and geochemical endmembers (Na vs. K) of syenites, two new spectral indices were proposed [SpI-A = (B14-B12)/(B14 + B12) for K syenite signature and SpI-B = (B10-B11)/(B10 + B11) for Na syenite signature]. These indices highlight the two major geochemical affinities in the High-Atlas Mountains: the potassic syenite character (K-feldspar- and nepheline-rich) in the Midelt-Tamazeght Cenozoic complex and the sodic syenite character (albite-rich) in the Imilchil Mesozoic massifs. The results have significant implications for early exploration programs, aiming to cost-effectively discover favorable areas of alkali syenites, potassic against sodic affinities, and optimize fieldwork as well as preliminarily mineralogical and geochemical characterization.