Prebatholithic clastic and minor carbonate and volcanic rocks are exposed along the 30th parallel of eastern Baja California Norte, Mexico. This formation has an apparent thickness of over 6,000 m and is subdivided into three informal members. The lower member (200 m) is comprised of quartz-rich, boulder-pebble conglomerate and sandstone, volcaniclastic sandstone, and minor limestone interpreted to have been deposited in a shallow-marine setting. The middle member (5,000 m) gradationally overlies the lower member, and consists of rhythmically thin-bedded, and laminated, black chert, fine-grained sandstone, and shale interpreted to represent a slope to basin-plain environment. The upper member (850 m) rests unconformably on the middle member and is composed of cobble conglomerate, sandstone, shale, and vesicular andesite. The sequence contains angular granules of black chert probably reworked from the middle member. The upper member is interpreted to have been deposited in a nonmarine environment. The age of the section is interpreted to be Aptian-Albian (possibly as young as Turonian) based on stratigraphic and intrusional relationships and nerineid gastropods from the lower member. The sequence is interpreted to have been deposited in a continental margin back-arc basin, contemporaneous with the Aptian-Albian Alisitos volcanic arc complex exposed 30 km to the west. The section was subsequently deformed into near-vertical, steeply plunging isoclinal folds, and metamorphosed to greenschist-facies grade.

Abstract Recent exploration of the historic Cerro San Pedro mining district located in the state of San Luis Potosí in central Mexico has resulted in the successful discovery of a large porphyry-related gold-silver deposit containing an estimated 251 million metric tons (Mt) averaging 0.49 g/t Au and 15.3 g/t Ag and representing a global resource of approximately 3.95 Moz gold and 123 Moz silver. Historic production for the district is estimated at 2.5 Moz of gold and 40 Moz of silver primarily from high grade veins, mantos, and breccias developed in the limestone hanging wall and footwall of a quartz dioritic porphyry intrusive, referred to here as the San Pedro porphyry. Between 1995 and 1997, Metallica Resources, Inc., successfully delineated a bulk mineable ore reserve currently estimated to contain a total of 64 Mt grading 0.62 g/t Au and 24.5 g/t Ag, representing 1.3 Moz of gold and 50 Moz of silver based on metal prices of US$300/oz gold and US$5.50/oz silver (or 2.0 Moz gold equiv). At the time of this writing the average waste/ore ratio for the proposed open pit is approximately 1.6:1. The San Pedro project is currently operated by Minera San Xavier S.A. de C.V. under a joint venture agreement between Metallica Resources, Inc., and Cambior Inc. The San Pedro district is located near the margin between the Eastern Sierra Madre and Central Mesa physiographic provinces. District stratigraphy consists of a well-defined sequence of Cretaceous limestones belonging to the Aptian age La Peña and Albian age Cuesta del Cura Formations. These units have been extensively deformed by a system of conjugate northeast and northwest-striking wrench faults and subordinate east-striking fault splays that has been superimposed upon a system of earlier north-northwest–striking folds and associated thrusts and reverse faults. The zone of intersection between these fault sets covers a roughly 1 × 2 km area within which the north-striking west-dipping San Pedro porphyry has been intruded. Emplacement of the San Pedro porphyry is inferred to have occurred during late Maastrichtian to early Paleocene time as indicated by a whole-rock K-Ar date of 64 ± 3.2 Ma for fresh porphyry. Brittle shearing of both the upper and lower porphyry-limestone contacts as well as faulting across the porphyry contacts indicates tectonic deformation continued well after intrusion and cooling of the San Pedro porphyry had been completed. In addition, nearly all structures in the deposit, including the limestone-porphyry contacts, act as primary ore controls and show little evidence of post-mineral displacement. These characteristics clearly indicate that gold-silver mineralization largely post-dates the principal deformation and intrusive events in the district. Limestone-hosted mineralization occurs as stratiform carbonate ± jasperoid replacement bodies, open space filling along zones of structural dilatancy and tectonic breccias by secondary calcite and quartz, discordant stockwork veining and recrystallization along vein selvages, and carbonaceous ± sulfide replacement within carbonaceous sections of the La Peña Formation. Economic limestone-hosted mineralization is associated with secondary calcite, iron and manganese oxides, jarosite, gypsum, anglesite, smithsonite, cerussite, barite, and occasional jasper and dolomite. Alteration of the San Pedro porphyry is characterized by five principal mineral assemblages that partially overlap one another and follow the principal structural ore controls in the deposit. In order of increasing alteration intensity, these assemblages include chlorite-illite (propylitic) alteration, illite-kaolinite (argillic) alteration, and illite-sericite/muscovite-quartz (sericitic), alunite-kaolinite-silica-jarosite (acid-sulfate) alteration, and montmorillonite-kaolinite-iron oxides (supergene argillic) alteration. The latter assemblage occurs in the uppermost near-surface levels of the deposit as an overprint upon the former four assemblages and is considered to be a product of super-gene weathering and oxidation processes. The propylitic, argillic, sericitic, and acid-sulfate assemblages are interpreted to be products of hypogene hydrothermal processes. Porphyry-hosted mineralization occurs in three distinct assemblages, defined on the basis of extent of oxidation of primary sulfides to secondary oxides, which include porphyry oxide, mixed oxide-sulfide, and porphyry sulfide. Approximately 70 percent of the mineable reserves occur as porphyry oxides hosted primarily within the argillic, sericitic, and acid sulfate alteration assemblages. Porphyry oxide ore is characterized by disseminated and stockwork goethite ± jarosite ± hematite in association with pervasive argillic ± silica alteration. Within this zone, gold and silver grades show a general increase with alteration intensity which in turn tends to be strongest along the principal ore-controlling structures in the deposit. Examples of the latter include the limestone-porphyry contact and the downdip extensions of faults that cut and offset the San Pedro porphyry. Deep exploration drilling through the central part of the deposit indicates that gold-bearing porphyry oxide mineralization extends several hundred meters below the main zone of currently economic gold-silver mineralization. Likewise, the mixed oxide-sulfide and sulfide zones below the main body of porphyry oxide ore are host to an appreciable volume of disseminated gold and silver mineralization which may represent a significant economic resource for future exploration and development. Geochemical sampling of surface exposures and drill holes at San Pedro indicates a relatively systematic trace element distribution pattern over the deposit. The patterns for anomalous gold and silver generally correspond well with the currently defined limits of economic gold-silver mineralization. Anomalous lead and zinc are likewise coincident with this zone, but tend to be strongest in the uppermost levels of the San Pedro porphyry and the limestone hanging wall. The distribution patterns for arsenic and antimony are characterized by semi-coincident anomalies over the northern portion of the deposit. Anomalous levels of copper occur in several isolated zones following a major northeast-trending fault system. Anomalous mercury is confined to a major fault intersection located in the southwestern part of the deposit. Fluid inclusion data for a limited set of samples suggests the presence of two distinct fluids related to mineralization; a highly saline fluid related to early sulfide mineralization, and a more dilute fluid related to overprinting sulfate and related oxide mineralization. Although the fluid inclusion data are limited in scope, they do suggest either an overall decrease in salinity following the deposition of primary sulfides from a single fluid, or an interaction between two fluids derived from different sources (magmatic and meteoric?) which may have led to the subsequent oxidation of the earlier sulfide assemblage. Additional work aimed at examining the relationships between the various alteration, mineralization, and oxidation assemblages is currently in progress to clarify these aspects of deposit metallogenesis.

Shallow-marine carbonate facies from volcanic-arc settings provide an important, but commonly overlooked, record of relative sea-level change, differential subsidence-uplift, paleoclimate trends, and other environmental changes. Carbonate strata are thin where volcanic eruptions are frequent and voluminous, unless shallow, bathy-metric highs persist for long periods of time and volcaniclastic sediment and erupted materials are trapped in adjacent depocenters. Carbonate platforms and reefs can attain significant thickness, however, if subsidence continues after volcanic activity ceases or the volcanic front migrates. The areal extent of shallow-marine carbonate sedimentation is likewise affected by differential tectonic subsidence, although carbonate platforms are most laterally extensive during transgressive to highstand conditions and when arc depocenters are filled with sediment. Tectonic controls on shallow-marine carbonate sedimentation in arc depocenters include (1) coseismic fault displacements and associated surface deformation; (2) long-wavelength tectonic subsidence related to dynamic mantle flow, flexure, lithospheric thinning, and thermal subsidence; and (3) large-scale plate deformation related to local conditions of subduction. Depositional controls on carbonate sedimentation in arc depocenters include (1) the frequency, volume, and style of volcanic eruptions; (2) accumulation rates for siliciclastic-volcaniclastic sediment; (3) the frequency, volume, and dispersal paths of erupted material; (4) (paleo)wind direction, which influences both carbonate facies development directly and indirectly by controlling the dispersal of volcanic ash and other pyroclastic sediment, which can bury carbonate-producing organisms; (5) the frequency and intensity of tsunami events; and (6) volcanically or seismically triggered mass-wasting events, which can erode or bury carbonate strata. Regarding platform morphologies in arc-related settings, (1) fringing reefs or barrier reef systems with lagoons may develop around volcanic edifices throughout the long-term evolution of volcanic arcs; (2) local reefs and mounds may build on intrabasinal, fault-bounded highs within underfilled forearc, intra-arc, and backarc basins; (3) isolated platforms with variable platform margin-to-basin transitions are common in “underfilled” and tectonically active depocenters; and (4) broad ramps and rimmed carbonate shelves are typically found in tectonically mature and sediment-filled depocenters.