GEOLOGICAL SOCIETY OF THE PHILIPPINES

the geology of the boyOngan porphyry copper-gold deposit, surigao, Philippines

 

Patrick Waters, Geronimo Comia, Maximo Estaus, Rene Gonzales, Reynaldo Estacio, Jose Oliver Briones, Andrew Cabrera, Allan Ignacio and Oliver Briola

 

The Boyongan porphyry copper-gold system was discovered during systematic exploration and follow-up of regional geochemical anomalies by Anglo American Exploration (Philippines) Inc. Anglo embarked on a district-wide geological and geochemical campaign within the historic Surigao Gold District in January 1998. This was after a joint venture agreement was made with Philex Gold Inc.

The Boyongan porphyry mineralization exemplifies many features that typify Southwest Pacific porphyry deposits; however it is unique in some aspects. Just like other Philippine porphyries, it is hosted by a composite diorite porphyry stock containing at least seven distinct phases of dioritic intrusions. The stock unde,rwent pervasive K-silicate alteration and was affected by a weakly-developed intermediate argillic overprint and a localized overprinting advanced argillic lithocap that has been unroofed by erosion. Copper-gold mineralization associated with dense quartz stockwork development was introduced during the intrusion of the composite progenitor stock which consists of a series of coarse-grained diorite porphyries.

Deep supergene oxidation affected the Boyongan porphyry system during pre-Quaternary time. It was eventually concealed beneath a Quaternary lake by sediments and andesitic lavas that accumulated in it since 30,000 to 1,700 years b.p. The oxidized zone, up to 600 meters thick(possibly the deepest known supergene profile developed in porphyry copper deposits in the world), is characterized by an unusual abundance of cuprite (mostly chalcotrichite variety), along with azurite, malachite, chrysocolla, neotocite and native copper. Chalcopyrite and locally, bornite are the dominant hypogene sulfides.

 
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