Ultra-Deep Oxidation and Exotic Copper Formation at the Late Pliocene Boyongan and Bayugo Porphyry Copper-Gold Deposits, Surigao, Philippines: Geology, Mineralog, Paleoaltimetry, and their Implications for Geologic, Physiographic, and Tectonic Controls

The Boyongan and Bayugo porphyry copper-gold deposits are Part of an emerging belt of intrusion-centered gold-rich deposits in the Surigao district of northeast Mindanao, Philippines. Exhumation and weathering of these Late Pliocene-age deposits has led to the development of the world's deepest known porphyry oxidation profile at Boyongan (600 m), and yet only a modest (30-70 m) oxidation profile at adjacent Bayugo. Debris flows, volcanic rocks, and fluviolacustrine sediments accumulating in the actively extending Mainit graben subsequently covered the deposits and preserved the supergene profiles. At Boyongaii and Bayugo, there is a vertical transition from shallower supergene copper oxide minerals (malachite + azurite + cuprite) to deeper sulfide-stable assemblages (chalcocite +/- hypogene sulfides). This transition provides it time-integrated proxy for the position of the water table at the base of the saturated zone during supergene, oxidation, Contours of the elevation of the paleopotentiometric sill-face based oil this mineralogical transition show that the thickest portions of the unsaturated zone coincided with a silt-sand matrix diatreme breccia complex at Boyongan. Within the breccia complex, the thickness of the misaturated zone approached 600 m whereas outside the breccia complex (e.g., at Bayugo), the thickness averaged 50 m. Contours of the paleopotentiometric surface suggest that during weathering, groundwater flowed into the breccia complex from the north, south, and cast, and exited along it high permeability zone to the west. The high relief, (>550 m) on the elevation of the, paleopotentiometric surface is consistent with all environment of, high topographic relief, and the Outflow Zone to the west of, the breccia complex probably reflects proximity, to a steel) scarp intersecting the western breccia complex margin. Stable isotope paleoaltimetry has enabled estimation of the elevation of the land surface, which further constrains the physiographic setting during supergene oxidation Isotopic measurements of Oxygen ill supergene kaolinite from Boyongan suggest that local paleometeoric water involved ill weathering had a delta O-18 composition of approximately-5.7 per mil. At the latitude of the southern Philippines, this value corresponds to Pleistocene rain water condensing at elevations between 750 and 1,050 m above contemporary sea level, providing a maximum estimate for the surface elevation during weathering of the porphyry systems. Physiographic reconstructions suggest that the deep oxidation profile at Boyongan formed ill all environment of high topographic relief immediately east of a prominent (>550 m) escarpment. The high permeability contrast between the breccia Complex and the surrounding wall rocks, coupled with the proximity of the breccia complex to the escarpment, led to a depressed groundwater table and a vertically ex(extensive unsaturated zone in the immediate vicinity of Boyongan. This thick vadose zone and the low hypogene pyrite/copper sulfide ratios (0.6) at Boyongan promoted in situ oxidation of copper sulfides with only modest (<200 m) supergene remobilization of copper. In contrast, higher hypogene pyrite/chalcopyrite ratios (2.3) at Bayngo led to greater acid production during weathering and wore complete leaching of copper above the base, of oxidation. This process promoted significant (600 m) lateral dispersion of copper down the paleohydraulic gradient into the diatreme breccia comple, ultimately leading to the formation of an exotic copper deposit.