Geology of the Ungay Malobago Volcanogenic Massive Sulfide Cu-Zn-Au-Ag
Deposit, Rapu Rapu Island, Albay, Philippines

Dr. Bruce Rohrlach, Nolito Nuñez and Roderick Watt

LaFayette Mining

Abstract

The Ungay Malobago volcanogenic massive sulfide (VMS) Cu-Zn-Au-Ag deposit lies at the eastern end of Rapu Rapu Island in the province of Albay, Philippines. The island hosts two known VMS deposits (Ungay Malobago and Hixbar) and two stratabound zones of alteration that lie north and west of these deposits. The Ungay Malobago deposit has a global resource of 7,140,000 tonnes at 2.54 g/t Au, 1.23% Cu, 2.05% Zn and 27.37 g/t Ag. Rapu-Rapu Minerals Incorporated and its foreign partner Lafayette Philippines Incorporated have commenced construction of mine infrastructure at the Ungay Malobago deposit following approval of all regulatory commitments, and acquisition of finance. The project has a capital investment of US$42 million, and with a projected revenue in excess of US$300 million, the project will make a significant contribution to the export income of the Philippines. It will be the first major deposit to be developed since the inception of the Philippine Mining Act of 1995.

Rapu Rapu Island lies at the southern end of the northwest-trending Eastern Regional Metamorphic Complex (Gervasio, 1966). This belt of metamorphic rocks forms the basement to the eastern part of southeast Luzon and is overlain by younger arc-related volcanic rocks of Neogene age along the Bicol peninsula. The eastern half of Rapu Rapu island comprises metamorphosed mafic and felsic volcanics and intercalated quartzo-feldspathic sediments of Cretaceous to Paleogene age. The metamorphic grade of the host rocks to the Ungay Malobago deposit is upper greenschist to lower amphibolite facies. The mafic volcanic sequence is dominated by massive and pillowed basaltic flows and sills that are intercalated with mafic epiclastic volcanics. These submarine basalts cover ~80% of the eastern part of the island. Sub-ordinate dacitic volcanics are spatially and temporally associated with massive sulfide mineralization on Rapu Rapu island. Thick accumulations of dacitic volcanics in the Ungay Malobago and Hixbar ore environments represent the locus of discrete volcanic centers. The dacite flows are inferred to have erupted as flow-dome complexes onto the palaeo-seafloor, and established subsurface hydrothermal convection cells that precipitated massive sulfide mineralization in topographic lows around the margins of the volcanic piles.

The Ungay Malobago deposit is associated with a single massive sulfide lens that occurs along a complexly folded contact between hanging-wall mafic volcanics and sericite-quartz-pyrite-altered metadacites in the stratigraphic foot-wall. The sulfide lens has been folded and attenuated in an east-west direction by a deformation event that accompanied regional metamorphism during the Miocene. The host sequence has been overturned by recumbent folding so that the stratigraphic foot-wall sequence is the geometric hanging-wall. The massive sulfide horizon has a shallow-west-plunging, pipe-like geometry and is folded along axes that plunge at ~10 degrees to the west. The ore crops out at surface as a gold-enriched gossan at the eastern end of a west-plunging synform. A thick saprolite profile occurs over the deposit. The sericite-quartz-pyrite alteration of the stratigraphic footwall volcanics is transitional outward to quartz-chlorite-sericite alteration. Discordant bodies of quartz-pyrite alteration that transect the felsic volcanic units are interpreted to define a silicified fault conduit that channeled ascending hydrothermal fluids from below the sulfide lens. The massive sulfide bodies comprise 50-98% sulfide dominated by pyrite. The principal base-metal sulfides are chalcopyrite, bornite and sphalerite, with minor galena, tetrahedrite-tennantite, telluride and native gold. Base metal sulfides occur as grains and laminar grain clusters that are disseminated in the interstitial matrix to massive and semi-massive pyrite. The pyrite and base-metal sulfides occur within a silica- and barite-rich matrix. The Hixbar Au deposit is a smaller, Au-dominated VMS system that lies ~2.5 km WNW of the Ungay Malobago deposit. Mineralisation in the Hixbar region comprises Au-bearing pyritic ores within thin sulfide lenses that lie along the stratigraphic contact between metamorphosed mafic volcanics (chlorite-actinolite schists) and altered dacitic felsic volcanics (quartz-sericite-pyrite schists). Stockpiles of oxidized Au-bearing rock at Hixbar remain as a viable source of ore for the future process plant at Ungay Malobago. Additional stratabound zones of hydrothemal alteration on Rapu Rapu Island occur along coarse, permeable, mafic epiclastic horizons that are altered to an epidote-silica-pyrite-chlorite ± magnetite assemblage with Pb, Zn, Ba and Mn enrichment.