Exploration of the Exciban gold-copper-tellurium vein system, Camarines Norte, Philippines

Abstract

In the Exciban area, gold occurrences found by small-scale miners in low-lying jungle-covered terrain were evaluated by standard exploration techniques. Surface sampling and mapping showed promising gold-tellurium base-metal anomalies, and suggestions of a substantial mineralized breccia body. A sulfide-quartz vein system in weakly metamorphosed to unaltered sheared Tertiary andesitic and arenaceous(?) rocks was evaluated by drill holes and rehabilitation of old underground workings along veins. The Exciban mineralization system occurs along and near faults, locally comprising the contact between andesites and volcanic siltstones. In places, nearly massive pyrite accompanies gold and quartz. Sphalerite and galena are rarely visible. The tellurium content of gold-rich samples ranges from a few to hundreds of parts per million. Free gold of high fineness occurs in nearby outcrops and placers. Detailed mineralogical and SEM studies show bismuth telluride minerals {tellurobismuthite (Bi 2Te 3), hedleyite (Bi 7Te 3) and tetradymite (Bi 2Te 3S)} to be closely associated with native gold. Gold tellurides and native tellurium were not found. The presence of daughter minerals in fluid inclusions suggests relatively high-salinity fluids of probable magmatic origin deposited the ores. Induced polarization (IP) and soil geochemistry generally proved useful in tracing concealed mineralization. Tellurium-bearing gold-quartz rocks vein systems in the environment described here (young volcanic rocks in island-arc settings) commonly are assumed to have formed under epithermal conditions. But at Exciban, analogies with known epithermal systems proved inadequate as exploration tools. It remains unclear whether this ore deposit formed under epithermal conditions.