Small-intrusion-hosted Ni-Cu-PGE sulfide deposits in northeastern Vietnam: Perspectives for regional mineral potential

Abstract

This paper discusses new data on the abundance and distribution of chalcophile elements (Ni, Cu, and PGE) in several small Permian–Triassic-aged sulfide-bearing mafic-ultramafic intrusions, namely the Dong Sang, Bo Ninh, Na Hoan and Ha Tri, belonging to the Cao Bang Complex in the Song Hien domain, Northeastern Vietnam. Mineral potential for Ni-Cu (-PGE) deposits in this domain is poorly understood in contrast to the Song Da domain in northwestern Vietnam, in which the well-known Ban Phuc Cu-Ni-PGE deposit is located. The disseminated sulfides from the Dong Sang, Bo Ninh and Na Hoan intrusions have very similar moderately fractionated chalcophile element abundance patterns, with positive Pd and negative Ru anomalies, and similar ratios of Pd/Ir (1–30) and (Pt+Pd)/(Ir+Ru+Os) (1–23), indicating a genetic link between these sulfides and the previously studied Suoi Cun intrusion, which also belongs to the Cao Bang complex and hosts Ni-Cu-PGE sulfides. The similar Cu/Pd ratios (3100–17100) of the sulfides from these intrusions are close to generally accepted ratios for the mantle range, and this indicates that sulfides were separated from a primary relatively PGE-undepleted magma as a result of a single sulfide segregation event. The sulfur isotopic data (δ34S) are in the range of −2.0 to −3.4‰ and indicate that the mantle is the dominant source of the sulfur, although the native values presumably allow for a small contribution from crustal S. In contrast, the disseminated and massive sulfides from the Ha Tri intrusion have high Pd/Ir (>100) and (Pt+Pd)/Ir (>140) ratios, indicating very high degrees of fractionation of a sulfide liquid, and it also has a wider range of δ34S values between +1.5 and −1.5‰ that overlap with that of the mantle. Mantle-like Cu/Pd ratios (3400–10600) indicate that no interaction has occurred between the sulfide liquid and a large volume of fresh magma in an open system. The results suggest two types of magmatic sulfide mineralization within mafic-ultramafic bodies of the Cao Bang Complex: (I) mineralization related to sulfide-bearing olivine-rich crystal mush and formed by multiple injections of magma into (and through?) the intrusion from the same source (as recorded in the Suoi Cun, Dong Sang, Bo Ninh, Na Hoan intrusions), and (II) mineralization hosted within closed system terminal chambers (as recorded in the Ha Tri intrusion).