Gold and Base-Metal Sulfide Mineralogy and Paragenesis of the Lalab Orebody, Sibutad, Zamboanga del Norte, Philippines: Clues to the Fluid Composition and Formation of Gold-Rich Zones

Abstract

The Sibutad gold deposit is one of the precious and base-metal deposits in the northeastern portion of the Zamboanga Peninsula, Mindanao island, the Philippines. The host rocks for gold-bearing quartz veins and disseminated to vein-associated base-metal sulfides are the andesite flows of the lower member Malindang Volcanics. In the Lalab orebody, these andesite flows vary from aphanitic to porpyhritic. Gold and base-metal sulfides, such as sphalerite ± galena, chalcopyrite, and bornite, occur in the variably altered andesite and hydrothermal breccia bodies. Clast type, alteration overprint, and cross-cutting relationships indicate the occurrence of three stages of hydrothermal activities, namely pre-breccia wallrock alteration, post-breccia wallrock alteration, and gold mineralization. Pre-breccia wallrock alteration occurred during Stage I. Post-breccia altered wallrocks and milky quartz veins and veinlets are characterized by marginal grade gold mineralization, with assays from 0.5 to 1.0 g/ton Au, and ore grade, with >1.0 g/ton Au, rocks, respectively. Post-breccia wallrock alteration refers to Stage II and formation of ore-grade-bearing milky quartz veins and veinlets are grouped in Stage III. Stage I is characterized by the assemblage of pyrite-sphalerite ± galena-chalcopyrite-bornitehematite. Stage II is represented by pyrite-chalcopyrite-bornite ± sphalerite ± gold-hematite. Stage III shows pyrite-bornite-chalcopyrite-gold-hematite assemblage. Sphalerite content decreases from Stage I through Stage II until Stage III. Bornite content increases and gold forms from Stage I and Stage II to Stage III. The occurrence of ore-grade gold zones in the central (100 m to 200 m above sea level) and deeper (50 m below sea level to 100 m above sea level) portions of the Lalab orebody are evidenced by the distribution of milky quartz veins. Gold-rich zones occur in Stage III milky quartz veins and veinlets in specific horizons within Stage II altered wallrocks. The Stage III milky quartz veins and veinlets were the most favorable for the precipitation of gold. The dominant metal ligand in the mineralizing solution apparently changed from chloride in Stage I to bi-sulfide during stages II and III. The prolific gold-bearing Stage III hydrothermal solution, with bi-sulfide as the dominant gold carrier, has precipitated its gold in favorable depositional settings as dictated by increased permeability, pervasive silicification, and depth-temperature condition.