Alteration and lithogeochemistry in the Masara gold District, Eastern Mindanao, Philippines, as tools for exploration targeting

Abstract

The intermediate to low sulfidation epithermal gold deposit in Masara, Compostela Valley, Eastern Mindanao, Philippines, is associated with a diorite porphyry from the Late Miocene Lamingag Intrusive Complex. Detailed mineralogical investigation of the host rocks in the deposit reveals five major alteration zones and at least two mineralizing events. A potassic alteration zone characterized by stockwork magnetite veins, secondary biotite and magnetite and an early-stage chlorite-sericite alteration composed of quartz+chlorite+illite+sericite+pyrite±biotite±magnetite±calcite assemblages are interpreted to be linked with an earlier porphyry copper system in the area. These were overprinted by late-stage chlorite-sericite alteration consisting of quartz+chlorite+illite+sericite+pyrite±adularia±magnetite±epidote and sericite assemblages which generally contain quartz+illite+sericite+pyrite. These alteration overprints are inferred to be related to a younger low to intermediate-sulfidation epithermal gold mineralization which involved near-neutral fluids. Advanced argillic alteration comprising quartz+kaolinite+magnetite+dickite±illite±calcite was also observed in the area. Isocon analysis shows that alteration zones associated with epithermal mineralization are generally characterized by enrichment in SiO2 and K2O and depletion of CaO and Na2O as well asadditions of Au, Cu, Zn and Pb. Pearce Element Ratio (PER) analysis indicates high degrees of sericitization in the altered diorite porphyry, tuff and quartz diorite units. At least three major alteration centers are defined based on the spatial distribution of the Alteration Index of surface samples calculated from the PER analysis. The results of this study support the use of whole-rock geochemistry asa complement to traditional techniques in exploration geochemistry.