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THE FORMATION OF EPITHERMAL DEPOSITS IN THE

MANKAYAN MINERAL DISTRICT: CONDITIONS OF MINERALIZATION

IN AN ANCIENT GEOTHERMAL SYSTEM?

Rene Juna R. Claveria
Coffey Philippines Inc., Ortigas Center, Philippines,

 

 

 

 

ABSTRACT



The Mankayan Mineral District is one of the well-known Cu-Au districts in northern Luzon, Philippines where the Far Southeast porphyry copper and the Lepanto copper-gold and the Victoria gold epithermal deposits are located.

The Lepanto copper-gold deposit is an epithermal high sulfidation vein-replacement type of mineralization. The alteration haloes around the veins are relatively wide. Pervasive alteration zones that are commonly observed are silicification, advance argillic and propylitic zones going outward from the veins. The principal ore minerals are enargite and luzonite, with significant presence of tennantite (-tetrahedrite), base metal sulfides, electrum and tellurides. Paragenetic studies show consistent stages of sulfide evolution, and these are the enargite-luzonite-pyrite stage, followed by tenantite (-tetrahderite)-chalcopyrite-sphalerite stage and a late gold-telluride stage. The Lepanto copper-gold deposit formed in the early stages of mineralization characterized by high sulfur activity (enargite-luzonite-pyrite assemblage). The subsequent cooling of the mineralized solution and their dilution by mixing with groundwater decreased the sulfur activity and formed the tennantite (—tetrahedrite)- chalcopyrite-sphalerite-pyrite assemblage. Later, fluctuations in the SO2 content of the solutions caused the deposition of electrum and tellurides.

The Victoria gold deposit is an epithermal gold-base metal rich quartz vein type of mineralization. In an outward sequence, silicic alteration occurs very near the base metal sulfide-quartz veins, followed by sericitic—argillic alteration to propylitic zones. Gold is associated with quartz and base metals. The predominant base metals present are sphalerite, galena and chalcopyrite. Preliminary paragenetic studies show three stages of mineralization, the early quartz (-gold) stage, a carbonate stage and a late gypsum stage.

Based on the textural evidences, the Victoria gold mineralization evolved from early stages of silica super saturation leading to quartz deposition. This is followed by the deposition of sulfides (+gold) and then alternating with carbonates (+quartz). During the stages of continuous cooling (and CO2 loss), silica occurs either in the form of quartz pseudomorphs after carbonate blades or as open space comb crystals. Gypsum and anhydrite are mainly occurring as overprints cutting the carbonate-silica stages. Their presence would manifest deep incursions of cool supergene meteoric waters.

The Lepanto copper-gold and the Victoria gold deposits could have some genetic associations with each other. The hydrothermal fluids, responsible for the alteration and ore deposition in Lepanto have magmatic attributes and such contributions may have come from the FSE copper-gold porphyry system. The observed cross-cutting and overprinting of enargite-bearing veins (Lepanto) by the quartz-gold-base metal veins (Victoria) suggest that the Victoria gold hydrothermal fluids were modified from earlier, more magmatic components. These magmatic-derived early fluids were probably neutralized by circulating meteoric waters during the waning stages of hydrothermal activity in the district.
 

 

 

 

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