Geochemistry and Geochronology of Magmatic Rocks in Southwestern and Northeastern Cambodia: Implications for the Tectonic Evolution and Metallogenesis of the Loei Fold Belt

ABSTRACT The Phnom Sro Ngam and Halo Prospects are potentially important targets for exploring porphyry copper‐molybdenum, epithermal and skarn‐related gold deposits in Cambodia. These areas are in the southwest and northeast of Cambodia, extending from the highly mineralized Loei Fold Belt in Thailand and Laos. However, the metallogenic characteristics and tectonic significance of magmatic rocks within this fold belt in Cambodia remain ambiguous. In this paper, we present new geochemical data and zircon U–Pb ages for the magmatic rocks from the Halo and the Phnom Sro Ngam prospects in northern and southwestern Cambodia. Geochemically, the magmatic rocks from both areas in Cambodia were formed and emplaced in a subduction‐related tectonic environment under a volcanic arc setting. All magmatic rocks plot into I‐type and S‐type granite on the (Na 2 O + K 2 O) versus (Zr + Nb + Ce + Y) diagram. Trace element spider diagrams normalized to primitive mantle display a strong enrichment in large‐ion lithophile elements such as Rb, Ba and K and depletion in some high‐field‐strength elements such as Nb and Ti, suggesting that the magma was generated in a subduction‐related tectonic setting. The REE patterns show slight enrichments in light REEs relative to heavy REEs and weak negative Eu anomalies. A distinct negative Eu anomaly indicates possible fractionation of Eu to plagioclase. Considering La/Yb as a crustal thickness proxy in intermediate to felsic calc‐alkaline rocks, the arc crustal thickness in Phnom Sro Ngam was probably calc‐alkaline granitoid. Zircon U–Pb dating yielded 229 to 282 Ma corresponding to Late Permian to Triassic and they are similar to magmatic activity in the Loei Fold Belt, which occurred during the subduction of the Paleo‐Tethys Oceanic crust towards the east beneath the Indochina terrane. The trace element geochemistry of the zircon grains strongly indicates great potential in porphyry copper mineralization in the prospects.