Gold occurrences in copper-magnetite-apatite deposit at Seruwila, Sri Lanka

This study presents petrology and evidence for possible gold occurrences in Seruwila copper–iron oxide–apatite (IOA) deposit, hosted in an ultramafic intrusion which is located at the boundary between the Highland and Vijayan complexes, within the intermediate-granitic basement in north-eastern Sri Lanka. The study is complemented with petrological observations and XRD and SEM analysis, respectively, to investigate the petrology/subsurface geology of the deposit and identify possible gold occurrences in the deposit. The ore-bearing rocks are mainly composed of magnetite and apatite in various proportions, hosted in an ultramafic intrusion with cumulate features within the granitic-intermediate basement. The secondary veins contain magnetite, chalcopyrite, pyrrhotite, and pyrite together with apatite and scapolite, tremolite, diopside, and minor actinolite and calcite, serpentinite, anhydrite, or gypsum. The clinopyroxene euhedral crystals show cumulus textures including grain triple junctions and large dihedral angles (∼120°), showing magmatic origin. Texturally two types of amphiboles are identified as coarse-grained (0.5–1 mm), pale green euhedral amphibole that is free of inclusions, and fine-grained (<0.1 mm) and brownish, occurring as anhedral inclusions in clinopyroxene. The deposit contains varying amounts of sulfides in which pyrite is the potential gold carrier in magmatic–hydrothermal processes. By the results of XRD analysis, it is evident that the presence of Au (111), Au (200), Au (220) and Au (311), although with low count values (50–500), probably due to the low concentration of gold. Therefore, particularly in the samples with veins or veinlets, gold was inferred to be present in pyrite/chalcopyrite as invisible structurally-bonded gold and/or gold nanoparticles. Hence, the results of this study, although at non-ore grade, veinlets of gold-bearing pyrite/chalcopyrite may serve as a promising target for gold occurrence, being a potential site of gold-mineralization in the context of the East-Gondwana.