Geology, Mineralogy, and Formation Environment of the Disseminated Gold‐Silver Telluride Bulawan Deposit, Negros Occidental, Philippines

Abstract

: The Bulawan deposit is located in the porphyry copper belt of southwest Negros island, Philippines. Propylitic, K–feldspar, sericitic, and carbonate alteration types can be distinguished in the deposit. Propylite alteration occurs mainly in Cretaceous‐Eocene andesitic lavas and agglomerates while K–feldspar, sericite and carbonate alteration types occur mostly in the Middle Miocene dacite porphyry breccia pipes and stocks which were intruded into the andesites. K‐feldspar zones occur in the inner parts of the sericitized zone. Sericite alteration overprinted the propylitized and K‐feldspar alteration zones, at lower temperature than epidote and chlorite in the propylitized zone. Carbonate alteration is associated with the mineralization in the center of the breccia pipes and along faults. Mineralization consists of gold‐silver telluride ores that are hosted by the carbonate– and sericite‐altered dacite porphyry breccia pipes. The Bulawan ores occur mainly as disseminations, but unlike many epithermal gold deposits, lack classical epithermal colloform and crustiform quartz veins. The ore minerals are sphalerite, galena, chalcopyrite, pyrite and tetrahedite‐tennantite with minor amounts of electrum, calaverite, petzite, sylvanite, hessite, tellurobismuthite, coloradoite, altaite, and rucklidgeite. Electrum and telluride minerals are associated mostly with calcite and dolomite‐ankerite minerals. Fluid inclusions in quartz and calcite in clasts of propylitized andesite in the breccia pipes homogenize from about 300° to 400°C while fluid inclusions in quartz, calcite and sphalerite within the dacite porphyry breccia pipes homogenize between 300° to 310°C. The ores were formed around 300°C from hydrothermal solutions with salinity of about 6. 6 wt % NaCl equivalent. The presence of sylvanite and calaverite as intergrowths with each other, and the Ag content of calaverite are consistent with the above temperature estimate. Based on paragenesis, the Bulawan deposit formed in a pyrite‐stable environment, with pH between 3. 4 and 5. 5, fO2 between 10‐32 to 10‐30 atm, fS2 between 10‐9.8 to 10‐7.8 atm, fTe2 between 10‐8.9 to 10‐6.5 atm, and total sulfur content about 10‐2.8 molal. The dominant reduced sulfur species in the ore solutions may have been H2S(aq), and the likely aqueous tellurium species were H2Te(aq) and H2TeO3(aq). The ore minerals in the Bulawan deposit were probably formed by mixing of slightly saline and low salinity fluids.

Fluid Inclusion Microthermometry and Implications for the Mechanisms of Ore‐Grade Gold Mineralization in Epithermal System, Lalab Orebody, Sibutad, Zamboanga del Norte, Philippines

“…The T H are consistent with the temperature range of epithermal deposits, although relatively lower as compared, for example, with the Bulawan deposit (300–310°C), (Maglambayan et al , 1998), Antamok deposit (240–315°C), (Domingo & Sato, 1982), Acupan deposit (250–300°C), (Cooke et al , 1996) and Atlas Masbate deposit (200–300°C) (Aquino, 1989). The fluid inclusions measured are hosted in crystals, which represent different alteration events and mineralization stages (Table 1).…”

Oxygen and Carbon Isotope Study of Calcite and Dolomite in the Disseminated Au‐Ag Telluride Bulawan Deposit, Negros Island, Philippines

“…(Maglambayan et al, 1998) of fluid inclusions within the whole deposit suggest that modified, rather than pristine magmatic water was involved. The enrichment in 18 O in the Bulawan deposit maybe due to the reaction of 18 O-depleted fluids with limestone or other similar rocks with inherently elevated δ 18 O values.…”

“…A Middle Oligocene to Late Miocene belt of plutons occur landward from the trench (Burton, 1983). The Bulawan deposit is hosted by irregular, dacite porphyry breccia pipes and less brecciated dacite porphyry stocks which intruded Cretaceous-Paleogene basement complex (Basak Formation) consisting of lavas, moderately metamorphosed volcanic and sedimentary rocks (e.g., Maglambayan et al, 1998). The pipes and stocks appear to be younger than a batholithic, late Cretaceous to Oligocene quartz diorite pluton which cuts the basement rocks (Fig.…”

“…The common sulfides in their order of decreasing abundance are sphalerite, galena, chalcopyrite, pyrite and tennantite. Maglambayan et al (1998) reported the occurrence of the following telluride minerals: altaite (PbTe), calaverite (AuTe 2 ), hessite (Ag 2 Te), petzite (Ag 3 AuTe 2 ), sylvanite (AuAgTe 4 ), krennerite (Au 4 AgTe 10 ), tellurobismuthite (Bi 2 Te 3 ), coloradoite (HgTe) and ruck- O and C Isotope Study, Bulawan Deposit, Philippines vol. 51, no.…”

“…The fluid inclusions in calcite are classified into vapor-rich, two-phase inclusions and liquid-rich, two-phase inclusions. The following data are taken from Figure 13 of Maglambayan et al (1998).…”