Magmatic Fertility Indicators of the Chagai Arc: A Systematic Review of Published Whole-Rock Geochemistry and Implications for Porphyry Cu–Au Prospectivity

Mubashir Khan 

The porphyry copper–gold (Cu–Au) deposits of the world are the primary source for copper and are significant producers of gold and molybdenum, and the genesis is always linked to the geochemical characteristics of the arc magmas from which the deposits crystallised. The Chagai Arc of western Pakistan is a Neotethyan subduction-related magmatic belt with several significant porphyry prospects, namely the Reko Diq, the Saindak and the Koh-i-Sultan, which constitute one of the least-developed large porphyry Cu–Au provinces of the Tethyan metallogenic domain. The importance of the arc has not been assessed, however, as an integrated geochemical system within a fertility sensitive discrimination approach. Rock data sets already exist for each of the areas of the deposits, and these have been used and interpreted independently without a consistent criterion which makes it difficult to compare data from one area with another. Loucks, Henríquez and Fiorentini (2024, Economic Geology) proposed the whole-rock magmatic fertility framework, which is based on the ratios of Sr/Y, (La/Yb)N, Eu/Eu, V/Sc and Dy/Yb as proxies for fractionation depth and magmatic oxidation state, has not been applied to any arc segment in Pakistan. This study uses a set of validated, individually sourced published values (specifically Sr/Y and europium anomaly systematics by Richards et al. (2012) for ore-related Chagai and Iranian Tethyan intrusions, U–Pb zircon and molybdenite Re–Os geochronology by Wang et al. (2022) and Muhammad et al. (2024) for Saindak and Reko Diq, and compositional and petrogenetic data by Nicholson et al. (2010) for Koh-i-Sultan), not an independently re-compiled, sample-level multi-source database. The two rocks, Reko Diq and Saindak, have mean chondrite-normalised europium anomalies of Eu(N)/Eu = 0.88 and Sr/Y > 30, which are consistent with deep and hydrous differentiation that is preferred in the formation of porphyry Cu–Au-type ores under plagioclase-suppressed conditions (Richards et al., 2012). The evidence for the ore-forming event in the area of Saindak (~22.2 Ma) is much older than the main Reko Diq mineralising event (~11–12 Ma), suggesting that there was at least two distinct pulses of fertile magmas in this portion of the arc. Koh-i-Sultan (<2.5 Ma) is compositionally distinct (basaltic andesite to dacite, ordinary calc-alkaline trend) and has no published Sr/Y, Eu/Eu or V/Sc dataset available from this centre that could be found in this synthesis. This synthesis is the first application of the fertility discriminant scheme by Loucks et al. (2024) to a Pakistani arc system and it is concluded that the most direct path towards a fully quantitative, arc-scale map of the Chagai Belt fertility that could be used to guide porphyry exploration in the Chagai system and other similar Tethyan systems is through the re-extraction of samples from the cited datasets.