Gold mining in Western Borneo can be traced back over a century in the Dutch era. Many of these gold deposits are hosted in auriferous quartz veins, yet their geology and alteration/mineralization features have never been well documented. In this study, we present the first detailed account on gold mineralization in Western Borneo, using the Monterado goldfield as an example. Our results reveal that the auriferous quartz veining occurred in both intrusions (Mensibau granodiorite and Sintang Suite diorite porphyry) and volcanic-sedimentary sequences (Raya Volcanics and Hamisan Formation), and wallrock alterations are well developed. Distributions of the veins are controlled largely by N-S-trending (tensional veins, e.g., Nek Rimban and Muisan) and NW-SE (en-echelon tensional gash veins, e.g., Mertua, Lipan, and Sekar) structures. Vein textures include mainly massive, breccia, comb, drusy, and stockwork. Advanced Mineral Identification and Characterization System mineral mapping on the ore veins and mineralized wallrocks show the presence of electrum, acanthite, pyrite, chalcopyrite, bornite, sphalerite, galena, and supergene products such as malachite, covellite, cuprite, chalcocite and rosasite. Polymetallic sulfide mineralization is spatially associated with adularia within auriferous quartz veins and silicic alteration. Ore chemistry on 285 vein ore samples yielded on average 1.46 g/t Au and 6.58 g/t Ag. Fluid inclusion microthermometry indicates that the homogenization temperature ranges from 273 to 362? (avg. 338?), with salinities of 1.1-8.7 wt% NaCl eq. (avg. 4.4 wt% NaCl eq.). Accordingly, the Monterado Au-Ag quartz vein-type mineralization is best classified as intermediate-sulfidation epithermal style.
