The origin of clays and clay minerals in the Paleocene Clayton (CF) and Porters Creek (PCF) Formations within the Mississippi embayment of central North America has been debated for more than 50 years. X-ray diffraction and petrographic analysis of samples of the CF and PCF from a mine in southeastern Missouri are used to evaluate contributions from Cretaceous-Paleogene (K-Pg) impact debris and the role of sediment diagenesis in the fine-grained sediment. Expandable clay minerals increase in abundance relative to illite and kaolinite above the K-Pg unconformity in the CF and PCF, and include dioctahedral smectite, vermiculite, and minor mixed-layered clay components, along with trioctahedral smectitic clays in the CF and lowermost PCF. Additional diagenetic phases include clinoptilolite (in the CF and lower PCF), pyrite, siderite, and opal CT (mainly in the PCF). The results of the petrographic analysis show no evidence for volcanic ash contributing directly to the sediment in the PCF. The detrital silicate minerals are mainly quartz, muscovite, biotite, and metamorphic minerals, consistent with an ancestral Appalachian Mountains source rather than volcanic ash or a Cretaceous western interior sediment source. The illite, kaolinite, dioctahedral smectite, and ordered illite-smectite mixed-layered clays are present in varying quantities in Cretaceous through Paleogene marine and nonmarine mudstones from the Mississippi embayment, and appear to be detrital in origin. Trioctahedral smectite and clinopti-lolite in the CF and lowermost PCF are argued to derive from alteration of glassy impact debris; clasts in the basal CF contain microtektites replaced by trioctahedral smectitic mixed-layered clay with randomly interstratified illite. The X-ray diffraction characteristics of the vermiculite in the PCF indicate a hydroxyl-interlayered aluminous variety that is argued to have a diagenetic origin, formed by clay mineral reactions under variably anoxic conditions in the PCF sediments during early diagenesis. Throughout the CF and PCF, pyrite precipitated under reducing conditions during diagenesis, locally along with siderite replacement of micritic carbonate. In addition, opal-CT precipitated as a result of silicate reactions and dissolution of diatoms under alkaline conditions and replaced matrix and calcitic microfossils throughout much of the upper PCF.

Abstract The Miocene metallogenic belt of central and northern Perú, extending for at least 900 km along the Western Cordillera and the adjacent high plateaus province, is defined by a large number of hydrothermal mineral deposits of different types that formed between about 6 and 20 Ma. The belt, centered east of the Mesozoic and early Paleogene Coastal batholith, is on mature continental crust that has undergone multiple episodes of compressive deformation from at least middle Paleozoic to latest Neogene time. Mineralization began before the early Miocene Quechua I compressive event and spanned later Quechua II tectonism. Mineral deposits are mostly hosted by shelf carbonates and other sedimentary rocks of Late Triassic,Jurassic, and Cretaceous age and by volcanic and intrusive rocks mainly of Neogene age. Base metal and precious metal mineralization was intimately associated in time and space with the eruption of calc-alkalic volcanic rocks of intermediate composition and the emplacement of mineralogically and chemically similar dikes and stocks. These igneous rocks are moderately potassic and the few available data suggest relatively nonradiogenic Sr, Nd, and Pb isotope compositions. Mineral deposits range from porphyry and associated proximal skarn deposits to polymetallic, precious metal, and mercury deposits formed at relatively lower temperatures. Porphyry deposits include the La Granja Cu porphyry, the Au-bearing Michiquillay Cu porphyry, the Mo-bearing Cu porphyritic rocks of Toro Mocho, Pashpap, and Páraq, the Mundo Nuevo-Tamboras-Compaccha Mo-W porphyry system, and the Cerro Corona, Minas Conga, Collpayoc, Laguna Chamis, Carhuacayán, and Puy-Puy Au-Cu porphyry deposits. Many of the classic base and precious metal deposits of central and northern Perú are within zoned polymetallic districts, some with one or more porphyry centers. Many districts have veins or replacement bodies containing enargite in their central parts, and a number are characterized by deposits of both vein and limestone replacement type. At a number of polymetallic districts, for example, Julcani, Yauricocha, Morococha, Casapalca, Huarón, Raura, Antamina, Pasto Bueno, Quiruvilca, Algamarca, and Hualgayoc, stocks containing high-salinity fluid inclusions are exposed, known from drill-hole data, or can be confidently inferred from fluid-inclusion or isotope data. Vein and limestone-replacement Pb-Zn ± Ag ± Cu deposits are common, and range from vertically persistent, high-temperature deposits, such as the veins of Casapalca, to largely stratabound deposits such as Cercapuquio and Azulcocha, that were formed at temperatures below 200°C. Although certain writers have interpreted some manto deposits to be diagenetic or syndiagenetic, field relations and lead isotope compositions argue strongly for an epigenetic origin. Vein systems or epithermal paragenetic stages in which silver is the economically most important metal, such as those of Milluachaqui, Millotingo, and Colqui, typically contain appreciable amounts of base metals and can best be considered a variant of the polymetallic vein group. The Huancavelica mercury deposit represents an extremely large geochemical anomaly, perhaps developed at the top of a polymetallic system. High-sulfidation-type Au-Ag deposits, such as Pierina and those of the Yanacocha district, are economically important. At Tantahuatay and Colquijirca, oxidized Au-bearing, vuggy silica rock occurs at higher elevations than surrounding, zoned, enargite-cored Cu-Pb-Zn-Ag veins and strata-bound replacement deposits. In contrast to the association of precious metals with enargite, tetrahedrite, and barite at Julcani and other reduced-type deposits, in moderate- to high-grade ores at Pierina and probably certain deposits in the Tantahuatay and Yanacocha districts, most of the gold is very late, following initial quartz-alunite-pyrite alteration, the destruction of alunite to form vuggy silica rock, and the subsequent deposition of pyrite and enargite accompanied by small amounts of gold. Gold and silver in economic quantities were then introduced by compositionally distinct, late fluids that oxidized pyrite and enargite, leached Cu, Zn, Se, Te, Tl, and other elements, and introduced Hg, Pb, Bi, Sb, and large amounts of barite. An analogous case for a distinct, compositionally different Au-Ag mineralizing pulse perhaps can be made for the sedimentary rock-hosted gold deposits of Purísima Concepción in Yauricocha. The ubiquitous presence of enargite, and the spatial and temporal association in several districts of pyrite + enargite, with modest gold content, and oxidized Au-rich ores, support the interpretation that bulk-mineable, volcanic-hosted gold deposits of a high-sulfidation type represent one of the many types of deposits related to the general class of porphyry-related, zoned polymetallic systems. The sandstone-hosted gold deposits of northern Perú also appear to be related to subjacent magmatic systems, although there are certain geological, mineralogical, and chemical differences from both volcanic-hosted, high-sulfidation and Purísima-type gold deposits. High W and Sn content of many of the sandstone-hosted ores of the Angasmarca district suggest that they are high-level manifestations of subjacent W-Mo ±Au systems such as are exposed at the nearby, more deeply seated Mundo Nuevo-Tamboras-Compaccha and Pasto Bueno districts. Several subsidiary belts are recognized within the Miocene metallogenic belt. A group of deposits in northern Perú, including the polymetallic deposits of the Quiruvilca district, the several Cu-Mo porphyry systems at Pashpap, and the Pierina high-sulfidation Au deposit, defines the 13 to 15 Ma or older Quiruvilca-Pierina subbelt in the western part of the metallogenic belt. The provisional Michiquillay-El Toro subbelt, including the Michiquillay Cu porphyry, the El Toro Au prospect, and probably the Au-Cu porphyry systems of the Minas Conga district, appears to have formed in northern Perú along the eastern margin of the metallogenic belt between about 18 and 20 Ma. A narrow, late Miocene subbelt that comprises a number of deposits dated at less than about 10 Ma, including Huachocolpa, Yauricocha, San Cristóbal, Morococha, Puy-Puy, Carhuacayán, Huarón, Raura, Huanzalá, Antamina, Pasto Bueno, and Angasmarca, extends from the Huachocolpa district at the southern end of the belt to the latitude of Santiago de Chuco in northern Perú. Deposits of the late Miocene subbelt postdate the 9 to 10 Ma Quechua II compressive pulse, and the initiation, location, and narrowness of the subbelt may have been related in some manner to this tectonic event. Intersections of successive, magmatic mineral axes with northeast-trending and other fault systems of probable crustal scale may have combined to influence the location of individual mineral deposits or clusters of deposits. Mineralization had ceased, and possibly was terminated, by the 5 to 7 Ma Quechua III compressive event. The emplacement of the 5.2 Ma late phase of the Cordillera Blanca batholith and the eruption of approximately coeval units of silicic ash-flow tuff and lava in northern and central Perú may reflect the subsequent relaxation of compressive stress, leading to the switching of axes of least and greatest principal stress indicated by 4 Ma north-south-trending dike systems in central Perú. Four important older districts within the Miocene metallogenic belt (Quicay, ca. 37.5 Ma; Uchucchacua, ca. 24.5 Ma), or bordering it on the east (Atacocha and Milpo, ca. 29-30 Ma), are related to older, and perhaps in part less intense, periods of magmatic activity. Although gold deposits may prove to be more important in northern than in central Perú, there is little indication that the concentrations of other metals vary markedly along or normal to the Miocene metallogenic belt. For example, porphyry molybdenum deposits are found in both the eastern and western parts of the belt. Moreover, particular types of deposits do not appear to be preferentially restricted to a given time period: several sandstone-hosted gold deposits in northern Perú have yielded ages ranging from less than 9 to greater than 18 Ma, and Au-bearing porphyry systems include examples of early, middle, and late Miocene age. Local geology and depth of erosion may be more important controls of deposit type. If future work shows that individual subbelts are as narrow and continuous as the present data suggest, areas within the narrow subbelts may prove to be the most prospective for mineral exploration.