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The volcanosedimentary rocks that host the Au-rich porphyry Cu deposits of the Cadia Valley preserve the products of episodic volcanism that erupted into a large sedimentary basin. Volcanogenic sedimentation, including the Forest Reefs Volcanics, overwhelmed the fine-grained sedimentary component that characterized much of the Weemalla Formation. The Forest Reefs Volcanics evolved as a relatively low relief, multiple-vent submarine volcanic complex. The vents comprised mafic to intermediate lava flows, cryptodomes, and subvolcanic intrusions (dikes and sills). Stacked lava sequences, including hyaloclastites, massive lavas, and their reworked equivalents, are up to 1 km thick, forming significant intrabasinal topography. Explosive volcanism occurred during the late stages of Forest Reefs Volcanics deposition. These air-fall deposits, combined with coexisting shallow-water faunal assemblages, imply that volcanism became locally emergent. Continuity of sedimentation between underlying deep marine basin deposits of the Weemalla Formation and Forest Reefs Volcanics, coupled with the predominance of sheet-like, laterally continuous debris flow and other coarse-grained sedimentary deposits, implies that volcanism and related sedimentation persisted in an active sedimentary basin marginal to an oceanic island arc. Deposition of the Forest Reefs Volcanics spanned the Late Ordovician to Early Silurian. Monzonite fragments (identical to the ore-related intrusions) are abundant in sedimentary breccias found at the top of the preserved volcanic stratigraphy. This finding, combined with available absolute ages of crosscutting intrusions and associated hydrothermal alteration and mineralization, suggests that some volcanosedimentary units were deposited synchronously with or immediately after the last known porphyry-related hydrothermal event in the Cadia Valley. The Au-rich porphyry deposits were therefore emplaced into an evolving sedimentary basin with episodic intrabasinal magmatism. Permeable horizons and volcanic lithoiacies can preferentially host alteration and mineralization that can extend over several kilometers in lateral extent. This finding suggests that hydrological models of fluid flow in porphyry systems need to take basin architecture into account. [ABSTRACT FROM AUTHOR]

Ore deposits spatially and temporally related to high-K calc-alkaline intrusions or domes are widespread in the central Peruvian Cordillera Occidental. Geochronology and petrochemistry of intrusive rocks associated with mineralization reveal that most ore deposits (1) were emplaced in the late Middle to early late Miocene and (2) are related to calcalkaline igneous rocks of granodioritic composition with Sm/Yb 13, Sr/Y1 40, and Y/Th ! 1.5. The largest deposits are associated with intrusions with Sm/Yb 1 4. Strontium isotopic compositions of the igneous rocks vary from 87Sr/86Sr = 0.7038 to 0.7089, but the 87Sr/86Sr ratio values exhibit only a poorly defined overall trend toward increased crustal contamination through time. Lead isotopic ratios generally correlate with the Sr isotopes and range from 206Pb = 18.64 to 18.91, from 207Pb/204Pb = 15.58 to 15.66, and from 208Pb/204Pb = 38.60 to 39.06. The overall Sr and Pb isotopic compositions of the igneous rocks indicate a generally moderate crustal assimilation of ancient radiogenic rocks. The Pb isotopic composition of sulfides from 14 polymetallic ore deposits in the study area lie on parallel arrays between the Pb isotopic compositions of spatially related igneous rocks and higher 208Pb/204Pb and 207Pb/204Pb sources such as Paleozoic and Mesozoic siliciclastic sedimentary rocks or the Carboniferous and older crystalline basement. The trace element and radiogenic isotope signature of igneous rocks related to mineralization indicates that these rocks have undergone limited midcrustal and shallow crustal assimilation and reflect melt equilibration in the lower crust at a crustal thickness of more than 40-45 km under conditions where hornblende and garnet were stable in the basaltic lower crustal residuum. Mineralization was favored by a change from normal to flat subduction, resulting in crustal thickening as well as broadening and subsequent cessation of the magmatic arc. However, some igneous rocks apparently unrelated to mineralization have high Sm/Yb and Sr/Y ratios as well. Petrographic and Pb isotopic evidence indicates that for some of these rocks, these trace element ratios may be better explained by shallow crustal hornblende fractionation or assimilation of pelitic material. [ABSTRACT FROM AUTHOR]

The Pliocene El Dorado low-sulfidation epithermal Au-Ag vein system, located within an 84-km² district in north-central E1 Salvador, is hosted by volcanic rocks formed along the Caribbean plate margin in response to the subduction of the Cocos plate during the Tertiary. The principal volcanic host rocks to epithermal veins, and effective basement in the district, consist of a >400-m-thick sequence of basaltic to andesitic lava flows and volcanogenic sedimentary rocks. Locally, porphyritic basaltic to andesitic domes and dikes intruded this sequence during the late Miocene (10.7 ± 1.9 Ma). The mineralized volcanic basement rocks, outcropping mainly in north and central El Dorado, represent different parts of a complex volcanic sequence composed of multiple superimposed volcanic centers. Vein formation in the district occurred in the Pliocene between 4.7 ± 0.2 Ma and 4.06 ± 0.29 Ma, based on K-Ar and 40Ar-39Ar plateau ages on vein adularia. Vein formation overlapped with Pliocene felsic volcanism (3.94 ± 0.29 Ma to 3:36 ± 0.49 Ma), based on field relationships and 40Ar-39Ar geochronology. Pliocene felsic volcanic rock sequences crop out widely in southern El Dorado at lower elevations relative to central and northern El Dorado. The felsic rocks have sinters intercalated with pyroclastic and sedimentary rocks at the base of the sequence that define the paleosurface of the epithermal vein system, at least in this part of the district. The depth of erosion in central and northern El Dorado, the primary Au-Ag resource, is below the paleosurface, as only the low sulfidation-style epithermal veins are present. Field relationships and geochronology suggest that the onset of extensive felsic volcanism marks the waning stage of hydrothermal activity associated with vein formation and mineralization. From north to south, the El Dorado district represents an oblique cross section through the volcanic and epithermal vein system formed during east-west extension. Northern and central El Dorado, dominated by mineralized mafic to intermediate volcanic basement rocks, represent the deeper parts of the volcanic and hydrothermal system. Southern El Dorado is the shallow level of the volcanic field and epithermal vein system, preserving the surface and shallow subsurface environments, including the sinters mad contemporaneous to postmineral felsic volcanic rocks. Although we cannot preclude that the distribution of volcanic rocks represents lateral facies changes in a Pliocene felsic dome field or that the variations in depth within the low-sulfidation epithermal system represent simply lateral flow driven by topography in the geothermal system, the preponderance of evidence suggests that the oblique section is principally due to postmineral deformation associated with formation of large Pliocene and Pleistocene Rio Titihuappa basin lying to the immediate south. [ABSTRACT FROM AUTHOR]