Abstract The Río Blanco-Los Bronces copper-molybdenum porphyry district in the late Miocene to early Pliocene magmatic arc of central Chile is currently being mined by state mining company CODELCO (Río Blanco) and Anglo American Sur (Los Bronces). Combined annual production in 2011 was nearly 450,000 metric tons (t) of copper plus by-product molybdenum. With Anglo American's recent high-grade discoveries in the district (3.7 billion tons (Bt) grading 0.7% Cu at San Enrique-Monolito, and 4.5 Bt grading 0.9% Cu at Los Sulfatos) adding more than 65 Mt of fine copper to the mineral inventory, the district now ranks as the world's largest by contained metal, with more than 200 Mt of copper. Volcanic and volcaniclastic rocks of the Abanico and Farellones Formations represent premineralization host rocks ranging in age between 22.7 ± 0.4 and 16.8 ± 0.3 Ma (U-Pb dating of zircons). The bulk of the copper-molybdenum porphyry endowment is related to evolution of the San Francisco batholith, a large (200 km 2 ) granodiorite-dominated complex with U-Pb zircon ages between 16.4 ± 0.2 to 8.4 ± 0.2 Ma. Three geologic domains are defined in the district on the basis of rock types, structural breaks, and age determinations: the Los Piches-Ortiga block in the west, the San Manuel-El Plomo block in the center, and the Río Blanco-Los Bronces-Los Sulfatos block in the east. These geologic domains are younger progressively to the east, with most of the known copper endowment on the easternmost (Río Blanco-Los Bronces-Los Sulfatos) block. Intrusive and hydrothermal activity in the Los Piches-Ortiga block spanned ∼ 2.5 m.y., from 14.8 ± 0.1 to 12.3 ± 0.1 Ma. Although these events apparently did not produce high-grade copper deposits, silver-bearing veins associated with a high sulfidation hydrothermal system are present in the block. To the east, in the San Manuel-El Plomo block, a series of magmatic-hydrothermal systems developed during a ∼ 3-m.y. period between 10.8 ± 0.1 and 7.7 ± 0.1 Ma. These events also apparently failed to generate high-grade copper systems. Magmatic-hydrothermal activity in the eastern Río Blanco-Los Bronces-Los Sulfatos block, hosting virtually all of the copper endowment recognized in the district, spanned a ∼ 4-m.y. period from 8.2 ± 0.5 to 4.31 ± 0.05 Ma. Copper endowment in the district is associated with vertically continuous breccia bodies and quartz-veinstockworked porphyries. Hydrothermal assemblages follow a characteristic vertical and lateral zonation pattern. Remnants of high sulfidation and/or advanced argillic assemblages (quartz-enargite-tennantite-galenasphalerite-gypsum-anhydrite with dumortierite-pyrophyllite-alunite) and peripheral sericite-illite reflect preservation of shallow levels, whereas K-silicate alteration assemblages (biotite-K-feldspar-albite) are present in association with chalcopyrite-bornite and chalcopyrite-pyrite at depth. Between the mineralized bodies, a distal assemblage of hydrothermal chlorite-epidote-specularite-pyrite predominates. At Río Blanco-Los Bronces, igneous and/or hydrothermal- and hydrothermal-cemented breccias developed in intimate association with porphyry phases. At shallow depths, the hydrothermal breccia cement generally comprises quartz-sericitetourmaline, and contains pyrite > chalcopyrite/molybdenite. At deeper levels the breccia cement is predominantly biotite-K-feldspar containing bornite-chalcopyrite-molybdenite. These progressively grade outward into chalcopyrite-pyrite-dominated zones and ultimately to pyrite-dominated zones. Within the Río Blanco-Los Bronces-Los Sulfatos block, the upper presence of the K-silicate assemblage varies from ∼ 3,000 m above sea level (a.s.l.) in the poorly telescoped northern area (Río Blanco), to ∼ 4,000 m a.s.l in the highly telescoped southern area (Los Sulfatos). These differences may reflect varying rates of synmineral structural exhumation, or varying depth of porphyry emplacement along the Río Blanco-Los Bronces-Los Sulfatos structural corridor. Key factors contributing to the copper productivity in the district are considered to reflect both far-field tectonic conditions, and district-scale structural controls. Following the last significant phase of volcanism documented in the district (∼ 16.8 Ma), a temporally discrete period of peak compression and rapid exhumation, between ∼ 6 to 3 Ma, affected the central Chilean Andes. This period of uplift relates to flat-slab subduction of sea floor containing the Juan Fernandez Ridge into the Chile Trench and overlaps part of the emplacement history of the Río Blanco-Los Bronces-Los Sulfatos block (8.2–4.31 Ma). The lack of contemporaneous volcanism and concomitant tectonic uplift are interpreted to reflect a state of increased horizontal crustal compression due to shear coupling of the downgoing slab. By suppressing volcanism, these conditions are considered to promote the retention of magma in the deep crustal environment, where higher pressures promote greater solubility of magmatic volatiles and higher temperatures may promote longer lived magma chambers by slowing fractionation processes. Under such conditions, the potential is enhanced for increased amounts of metals and volatiles by addition of fresh batches of magma to the deep magma chamber. At the district scale, closely spaced (2 km) structures that control the position of the porphyry and breccia bodies in the Río Blanco-Los Bronces-Los Sulfatos block appear to have focused long-lived, multistage magmatic-hydrothermal activity within a narrow structural corridor, contributing to the development of large, high-grade porphyry/breccia systems.

Stratigraphic and structural relations of syntectonic sedimentary sequences associated with Cordilleran metamorphic core complexes provide valuable information about the style and timing of extensional deformation related to tectonic denudation. Adjacent to the Catalina core complex, the San Pedro trough and other nearby depocenters contain multiple tilted half-grabens of conglomeratic mid-Tertiary strata partly buried beneath Neogene basin fill. Major episodes of local geologic history included mid-Proterozoic construction of continental crust, subsequent but intermittent platform sedimentation extending through Paleozoic time, mid-Mesozoic initiation of arc magmatism that persisted at intervals through mid-Tertiary time, complex Laramide orogenic deformation of latest Cretaceous to early Tertiary age, and Cenozoic extensional deformation involving both mid-Tertiary and basin-range phases of development. Precambrian basement includes lower Proterozoic Pinal Schist intruded by voluminous lower to middle Proterozoic granitic plutons. Pre-Laramide stratigraphic cover includes middle Proterozoic sedimentary strata and intercalated diabase sills, Paleozoic carbonate and clastic units, and Mesozoic volcaniclastic and clastic successions. Laramide assemblages include metaluminous plutons and andesitic to rhyolitic volcanic fields, synorogenic nonmarine sedimentary sequences, and large bodies of peraluminous two-mica granite. Laramide structural features include both premetamorphic and ductile synmetamorphic thrusts within the Catalina core complex, brittle thrusts of uncertain vergence and overall configuration outside the Catalina core complex, and folds of varied geometry related in part to local thrusts exposed nearby. Paleogene erosion had stripped Laramide volcanic cover from wide areas by mid-Tertiary time. Migratory Tertiary arc magmatism within the intermountain region gave rise to diachronous polymodal igneous suites, represented within and near the Catalina core complex by extensive volcanic fields and local granitic plutons of late Oligocene age. Across the whole Southwest Border region, analogous mid-Tertiary igneous activity was succeeded, following an intra-Miocene tectonomagmatic transition, by basaltic to bimodal suites erupted during subsequent block faulting. Tertiary intermountain taphrogeny included a mid-Tertiary phase marked by listric or rotational normal faulting associated with tectonic denudation of core complexes along detachment systems, and a later basin-range phase of widespread block faulting. Mid-Tertiary extension was apparently promoted by reduction of interplate shear, kinematic rollback of a subducted slab, lateral spreading of overthickened crust, advective softening of arc lithosphere, and possibly by counterflow of asthenosphere. Basinrange extension was evidently initiated by shear coupling of Pacific and American lithosphere. The Catalina core complex displays characteristic geologic features: mylonitic fabric overprinted near a detachment fault by brecciation and chloritic alteration, a brittle detachment surface abruptly separating rock masses derived from different crustal levels, cover strata broken into multiple tilted fault blocks forming a shingled imbricate array, and surrounding syntectonic sedimentary sequences containing intercalated megabreccia horizons. The domical uplift that controls the exposed extent of the Catalina core complex apparently reflects isostatic upwarp of the midcrust in response to tectonic denudation, coupled with rollover arching above listric faults cutting beneath the core complex and with later uplift of crustal masses along steep block faults. A belt of mylonitic gneiss 10 km wide and 100 km long lies along the southwest flank of the Catalina core complex adjacent to the downdip segment of the detachment fault. Updip segments of the detachment system, across which cumulative displacement is estimated as 20 to 30 km, curve over and around the exposed core complex to merge with an inferred headwall rupture along the trend of the San Pedro trough. The detachment system probably involved a gently dipping aseismic slip surface beneath surficial tilted fault blocks; alternatively, faults now dipping gently may have originated as steep structures that rotated to shallow dips as slip proceeded. Field relations around the Catalina core complex are seemingly more compatible with the former interpretation, but insights gained from the alternate interpretation are useful for structural analysis of rotated tilt-blocks above the master detachment surface. Kinematic considerations imply that displacements within the local detachment system were diachronous during its evolution, but mylonitic deformation and detachment faulting both occurred during late Oligocene and early Miocene time. Tilted homoclines of syntectonic mid-Tertiary strata and less deformed beds of younger basin fill are both composed dominantly of alluvial fan and braidplain facies that grade laterally to finer-grained fluvial and lacustrine facies. Subordinate deposits include landslide megabreccia, algal limestone, lacustrine diatomite, and playa gypsum. The most voluminous strata are crudely bedded conglomerate and conglomeratic sandstone deposited by braided depositional systems. Clast imbrication is the most widespread and reliable paleocurrent indicator. Mid-Tertiary aggradation in half-graben basins gave rise to sedimentary onlap and overstep of evolving tilt-blocks, intricate local facies relations, and marked stratigraphic contrasts between sequences deposited within partly isolated subbasins. Stratigraphic successions that are concordant within basin depocenters are commonly broken by unconformities and intervals of nondeposition on basin flanks and across crests of bounding tilt-blocks. Mid-Tertiary strata exposed as tilted homoclines along the flanks of the San Pedro trough and across broad uplands north of the Catalina core complex are assigned to the following formations, each of which includes informal local members and facies: (a) Mineta Formation, mid-Oligocene redbeds including both conglomeratic fluvial and finer-grained lacustrine deposits; (b) Galiuro Volcanics, including lavas and domes, air-fall and ash-flow tuffs, and intercalated volcaniclastic strata of late Oligocene to earliest Miocene age; (c) Cloudburst Formation, also of late Oligocene and earliest Miocene age but including a sedimentary upper member of conglomeratic strata as well as a volcanic lower member correlative with part of the Galiuro Volcanics; and (d) San Manuel Formation, composed of lower Miocene alluvial fan and braidplain deposits that display contrasting clast assemblages in different areas of exposure. Generally correlative Oligocene-Miocene strata exposed south of the Catalina core complex are assigned to the Pantano Formation, which contains similar lithologic components. Less-deformed Neogene strata of post-mid-Miocene basin fill are assigned to the Quiburis Formation along the San Pedro trough, but stratigraphic equivalents elsewhere lack adequate nomenclature. High benchlands mantled by paleosols mark the highest levels of Neogene aggradation. Successive stages of subsequent erosional dissection are recorded by multiple terrace levels incised into basin fill. Key exposures of syntectonic mid-Tertiary sedimentary sequences in several local subareas reveal typical structural and stratigraphic relationships. Multiple fault blocks expose pre-Tertiary bedrock overlain by tilted mid-Tertiary strata confined to intervening half-grabens. Bounding syndepositional faults dip southwest and associated homo-clines dip northeast. Fanning dips and buttress unconformities reflect progressive tilt and burial of eroding fault blocks. Dips of block-bounding faults are inversely proportional to the ages of the faults. Steeper dips for younger faults suggest either progressive erosion of successive listric faults or progressive rotation of successive planar faults. Uniformly moderate to steep dihedral angles between fault surfaces and offset homoclinal bedding imply that the faults dipped more steeply near the surface when syntectonic mid-Tertiary strata were subhorizontal. Although the inference of listric faulting best links apparent strands of the Catalina detachment system, the alternate interpretation of rotational normal faulting is compatible with local structural relationships including tilt of porphyry copper orebodies. Within the San Pedro trough, multiple homoclines of mid-Tertiary strata are exposed locally in tilt-blocks exhumed by Neogene erosion from beneath nearly flat-lying basin fill of the Quiburis Formation. Faults bounding the mid-Tertiary exposures include backtilted strands of the Catalina detachment system, somewhat younger listric or rotational normal faults, and steeper basin-range normal faults that display offsets both synthetic and antithetic to the flanks of the San Petro trough. In Cienega Gap, flanking the Tucson Basin, multiple tilt-blocks of the Pantano Formation form part of the upper plate of the Catalina detachment system. Initial construction of alluvial fans by generally westward paleoflow was followed by ponding of lacustrine environments along the foot of secondary breakaway scarps that also generated massive megabreccia deposits. In summary, syntectonic Oligocene to Miocene sedimentation succeeded a prominent pulse of polymodal mid-Tertiary volcanism and was coeval with mylonitic deformation and detachment faulting along the flank of the Catalina core complex. The headwall rupture for the detachment system migrated westward from an initial position along the range front of the Galiuro Mountains. After mid-Miocene time, accumulation and subsequent dissection of essentially undeformed basin fill was accompanied by basin-range block faulting. The most challenging structural issue is whether fault strands of the Catalina detachment system are interconnected or are disconnected rotational segments.

ABSTRACT This field trip integrates economic geology with structural geology and tectonics, as well as petrology, geochemistry, and regional geology, to examine a segment of the Laramide arc that includes part of the Laramide porphyry copper province of southwestern North America. The province arguably is the second-largest porphyry copper province in the world, hosting six of the world’s 25 largest porphyry deposits on the basis of contained copper metal. The Globe–Superior–Ray–San Manuel area includes about a dozen Laramide (Late Cretaceous to early Paleocene) porphyry copper deposits and the related granodioritic to granitic plutons. These plutons and their wall rocks were tectonically dismembered and variably easterly or westerly tilted (locally >90°) during Laramide contraction and subsequent mid-Cenozoic extension. The style of both shortening and extension here remains a subject of debate. Although this trip includes one brief mine visit and examination of drill core at the Resolution deposit, it will principally focus on: (1) different parts of various plutons and the associated alteration aureoles, including review of resultant mineralization, and the original sides, roots, and deep flanks of the hydrothermal systems; and (2) structure in the adjacent wall rocks and the implications for the style and timing of deformation in absolute and relative terms to hypogene ore formation. An increased understanding of the structural geology and the alteration-mineralization zonation of the dismembered hydrothermal aureoles allows an integrated view of the original geometry and size of the porphyry systems, the relationship between porphyry copper mineralization and crustal shortening, and possible origins of deep hydrothermal alteration.