Geological characteristics and ore-forming conditions of the Tasikmadu porphyry Cu–Au prospect in Trenggalek, East Java, Indonesia

Abstract

Tasikmadu is a newly discovered porphyry Cu–Au prospect in the eastern Sunda arc, Indonesia. This study aimed to elucidate salient diagnostic characteristics and ore-forming conditions of the prospect. Fieldwork and various laboratory analyses for a suite of representative samples were performed for mineralogy, bulk-geochemistry, mineral chemistry and ore fluid characterization. The study area is composed of three diorite porphyries, that is, fine-grained, medium-grained and coarse-grained diorite porphyry, respectively. The intrusions are calc-alkaline with a high Sr/Y value, which is similar to many ore-bearing intrusions in the eastern Sunda arc. Ore mineralization occurs in quartz veins and veinlet stockwork, centred in the potassic zone, and dominated by chalcopyrite and bornite occurring in A and B veins, which cut earlier barren (EB) and M veins. The mineralization core has an average grade of 0.63 wt% Cu and 0.25 ppm Au, respectively. Outwardly, the potassic zone changes to the propylitic zone, which still bears copper in the quartz and pyrite veinlets, although the grade is low. Fluid inclusion microthermometry revealed that the A and B veins in the potassic zone formed at 464 and 390°C by hypersaline boiling fluids, respectively. The temperature temporally and spatially decreased, that is, in the propylitic zone, the quartz veinlets formed at 260–400°C. Hypogene mineralization that formed the A veins occurred at 1.5 km below the palaeosurface, indicating a relatively shallow depth as a porphyry deposit. Nevertheless, the δ³⁴S_CDT values of sulphides range from −2.0 to −0.1‰, inferring a magmatic origin. The Tasikmadu prospect shares some similarities compared with other porphyry deposits worldwide, but it also reveals unique characteristics that differ from others, for example, potassic-altered rocks are only typified by secondary biotite without/rare secondary K-feldspar reflecting the lack of magma contamination by continental crustal components. In addition, current surface geological features and shallow depth erosion level of the prospect may imply that the potential of Cu–Au mineralization underneath is still open to depth.