Abstract The southern Altiplano rift basin forms part of the Cretaceous rift system that extends from Peru to northwestern Argentina. Seismic and potential field data suggest that basin formation was controlled by a preexisting structural grain consisting of northwest-southeast and northeast-southwest tectonic lineaments within a Precambrian-Paleozoic basement. Variable extension was achieved through transfer fault zones that coincide with northwest-southeast trending lineaments. Rift subsidence can be divided into an early rift (Berriasian- Cenomanian) and a late rift episode (Turonian-Campanian) with a total accumulation of up to 1000 m of sandstones and shales. A period of tectonic quiescence followed that resulted in a regional sag basin characterized by thin-bedded, calcareous lacustrine deposits. This sequence includes the oil-prone shales and reservoir carbonates of the El Molino Formation, which has been correlated with the hydrocarbon-producing Yacoraite Formation in northwestern Argentina. At the beginning of the Tertiary, rift-related subsidence had ceased and was gradually replaced by subsidence resulting from Andean compression. An Eocene unconformity marks the subtle change to this new episode of basin formation. This was followed by a major Oligocene unconformity that characterizes the onset of increased subsidence rates related to the emplacement of large thrust sheets of the Eastern Cordillera. More than 4 km of synorogenic sediments accumulated in the adjacent Altiplano foredeep. Inversion of the Cretaceous rift structures took place during this compressional phase. In the southern Altiplano, this inversion resulted in hydrocarbon traps similar to those in the rift basins of northwestern Argentina.

Abstract Seabed faulting can have a significant impact on the routeing and behaviour of gravity currents depositing sand on deepwater basin floors. The Neogene El Cautivo Fault in the Tabernas-Sorbas Basin, SE Spain, is a rare example of a fault that demonstrably propagated through to the seabed during turbidite deposition, allowing the interplay between deepwater sedimentation and tectonics to be explored. The fault is associated with a wide (up to 350 m) gouge zone that varies significantly in thickness along its length, reflecting upward expansion towards the original seabed and progressive burial as fault activity ceased. Kinematic and stratigraphic evidence indicate that the fault was a dextral oblique strike-slip fault that accommodated an area of deeper ponded bathymetry (a ‘mini-basin’) and accelerated subsidence on its southern flank. Active faulting controlled the routeing of turbidity currents (revealed by changing provenance across the structure), rates of seabed deformation (resulting in differential subsidence and ‘growth’ of the stratigraphy), and the behaviour of the ponded currents (producing distinctive bipartite beds when deposition was in localized ponded depressions). The seabed expression of the fault varied from a forced fold, which warped the surface causing local wedging and onlap in the vicinity of the structure, and an unstable scarp that locally collapsed. The fault gouge fabrics and vein arrays are consistent with faulting of soft, water-rich sediment close to the seabed.

Abstract At least 14 porphyry copper-gold deposits and 19 epithermal gold deposits are known within 60 km of Cajamarca. The partly explored porphyry deposits vary in grade, Cu-Au-Mo proportions, and depth of erosion. Associated epithermal mineralization occurs at Perol, Peña de las Águilas, Kupfertal, Yanacocha Norte, Maqui Maqui, and Pampa Verde but not at Michiquillay, El Galeno, Chailhuagón, Cerro Corona, La Sorpresa, Colpayoc, and Chamis. These deposits are associated with Miocene magmatic activity, northwest-trending folds and thrusts, and northeast-trending faults. In the porphyry deposits, granular A quartz veins, associated with K-feldspar-biotite alteration and disseminated chalcopyrite-magnetite with bornite or pyrite, are typically present within and about multiple coeval porphyry intrusions. Banded quartz veins occur near the tops of some shallowly eroded systems, and late sericite-pyrite ± chalcopyrite is superimposed on most. Epithermal mineralization is mostly of high-sulfidation character, with pyrite-enargite-covellite typically underlying oxide Au zones leached of Cu. Epithermal Au-Cu is associated with multiple stages of brecciation and intense silicification, zoned outward and downward with decreasing SiO 2 and Au through quartz-pyrophyllite-diaspore-alunite-dickite to quartz-alunite and kaolinite. Structurally controlled, high-grade Au is apparently late and associated locally with intermediate-sulfidation assemblages, barite, and chalcedony. The transition between porphyry and epithermal environments is exposed at Perol and Huaylamachay, La Zanja, and especially Tantahuatay and Yanacocha. At Perol and Huaylamachay, porphyry gold-copper deposits are adjacent to generally contemporaneous volcanic vents altered to quartz-alunite with minor structures containing quartz-pyrophyllite-alunite-Au. At Perol, the dacitic vent is intruded by a later mineralized porphyry, whereas at Huaylamachay the vent breccia contains clasts with quartz-molybdenite veins and is cut by banded quartz veins, which we interpret as indicating a second, deeper porphyry Au system. At Tantahuatay, an andesitic dome complex is pervasively brecciated and altered to quartz-alunite-pyrophyllite-diaspore ± dickite, with extensive pyrite-enargite-covellite-(bornite) veins and disseminations beneath Aurich oxide mineralization. A gusano texture of soft, round patches of pyrophyllite-diaspore and/or alunite in a silicified matrix is widespread and associated with anomalous concentrations of Mo. Only one of several drill holes to 600-m depth encountered A quartz veins and minor porphyry intrusions. This hole provides evidence for prograde advance of quartz veining associated with one or more porphyry intrusions into the epithermal environment and subsequent retrograde collapse. At Yanacocha, the most abundant evidence of direct, albeit complex, spatial and temporal relationships between multiple centers of epithermal mineralization and porphyry intrusion and mineralization has been partially deciphered. At Kupfertal, the matrix of gusano alteration above the top of the porphyry becomes increasingly silicified and patchy downward, developing very contorted wormy quartz veins that overlap the top of A quartz veins. Intense pyritic quartz-pyrophyllite-diaspore-alunite and underlying sericite alteration is superimposed on K-feldspar-biotite alteration of the early stage. Fluid inclusions in quartz are vapor dominant, with downward-increasing proportions of high-salinity inclusions and amounts of minute relict chalcopyrite ± bornite grains “locked” in A vein quartz. A-veined and advanced argillic-altered xenoliths in pyroclastic rocks intruded by porphyries and hosting gold mineralization demonstrate multiple generations of porphyry and epithermal mineralization. Early Cu and Au of the porphyry event appear to have been remobilized and incorporated into the overlying epithermal system.