Abstract 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 Boyongan 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 a 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 surface based on 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 unsaturated 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 east, and exited along a high permeability zone to the west. The high relief (>550 m) on the elevation of the paleopotentiometric surface is consistent with an environment of high topographic relief, and the outflow zone to the west of the breccia complex probably reflects proximity to a steep 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 in supergene kaolinite from Boyongan suggest that local paleometeoric water involved in weathering had a δ 18 O 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 in an 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 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 Bayugo led to greater acid production during weathering and more 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.

Abstract Olympic Dam is a supergiant Fe oxide Cu-U-Au-Ag deposit that is also strongly enriched in a wide range of elements, including F, S, C, As, Ba, Bi, Cd, Co, Cr, Fe, In, Mo, Nb, Ni, P, Pb, Sb, Se, Sn, Sr, Te, V, W, Y, Zn, and rare earth elements (REE). The deposit contains more than 90 minerals. Mineralization was associated with intense, texturally destructive hematite and sericite alteration and brecciation of the primary host rock units, including Roxby Downs Granite, bedded clastic facies rocks, and mafic-ultramafic dikes. Based on comprehensive geological, geochemical, and mineralogical data sets collected during a deposit-scale resource delineation and sterilization drilling program (2003–2008), compiled with historical geological data and interpretations, we quantify geochemical and mineralogical associations and distribution patterns. The granite-derived elements (Al, Be, Ca, Hf, K, Li, Mg, Mn, Na, Rb, Si, Th, Ti, and Zr) are negatively correlated with Fe, whereas the hydrothermal elements (Ag, As, Au, Ba, Bi, Cd, Co, CO 2 , Cr, Cu, F, Fe, In, Mo, Nb, Ni, P, Pb, S, Sb, Se, Sn, Sr, Te, U, V, W, Y, Zn, and REE) are positively correlated; the ore and gangue minerals are also correlated with Fe abundance. There is a strong spatial association of Cu, U 3 O 8 , Au, and Ag. From the periphery inward and upward from depth toward the deposit center, the most significant zones are as follows: (1) reduced Fe oxide alteration (magnetite-apatite-siderite-chlorite-quartz) → oxidized Fe oxide alteration (hematite-sericite-fluorite) → hematite-quartz-barite alteration, (2) siderite → fluorite → barite, (3) sphalerite → galena → pyrite → chalcopyrite → bornite → chalcocite → nonsulfide, and (4) distal or paragenetically early (?) base metal-poor (Mo-W-Sn-As-Sb) → base metal-rich (Cu-Pb-Zn) minerals → sulfide-barren hematite-quartz-barite breccia in the deposit center. Spatially isolated remnants of advanced argillic alteration (sericite + quartz ± Al-OH) have been defined for the first time. Progressive Fe oxide addition to, and sericite replacement of the primary host rocks produced distinctive, albeit complex, hydrothermally altered and mineralized zones in the Olympic Dam deposit.