Abstract Ponding conditions of basalts erupted from fissure zones at São Miguel Island (Azores) were investigated through microthermometry of fluid inclusions, hosted in olivines and clinopyroxenes, and whole-rock and mineral chemistry. The Região dos Picos and Achada das Furnas fissure zones formed between central volcanoes and erupted geochemically similar magmas for the last 30 kyr. Hydrocarbonic fluid inclusions that survived to diffusion and re-equilibration recorded a maximum density value of 1000 kg m −3 (29.3 km) at both fissure zones, at the intersection with the feeding systems of older central volcanoes. The maximum density decreases to 875 kg m −3 (23.5 km) towards the central segment of Região dos Picos. Further trapping of low-density fluids is occasional and limited to a few samples. While the deepest event is interpreted to mark the vertical variation of the Moho at each location, all other events are only representative of short-time ponding with fluid re-equilibration. The deepening of the Moho towards the central volcanoes might be the effect of long-lasting underplating. However, the periodic stretching of the lithosphere, in response to the differential stress field acting at central volcanoes, would have been responsible for the thinning of the crust at the central segment of the fissure zone. Supplementary material: Whole-rock geochemistry with international standard used, and representative analyses of selected mineral phases from the two segments of the Região dos Picos fissure zone is available at www.geolsoc.org.uk/SUP18818

Analysis of regional joint orientations, vein mineral paragenesis, and fluid inclusion microthermometry provides valuable insights into the deformation history of the Central Appalachians. These data indicate that deformation occurred as a continuum from hinterland to foreland, and they reflect a progressive rotation in shortening direction. However, separate deformation milestones punctuate the history. Prior to North Mountain thrust emplacement, the rocks in the Valley and Ridge Province were fractured under a NNW-directed shortening. The emplacement of the North Mountain thrust sheet occurred under a more NW-directed shortening and resulted in rapid loading of the Paleozoic section in the footwall by 7–10 km of rock that extended 20–30 km toward the hinterland from the North Mountain thrust ramp. The loading of the Paleozoic section resulted in overpressuring of Cambrian Waynesboro décolle-ment and the development of the Valley and Ridge duplex, and the formation of the Adams Run–Cacapon Mountain anticlinorium in the northern part of the study area. Growth of this anticlinorium shed more sediments toward the foreland, thereby driving addition horse formation. Continued NW- and then WNW-directed shortening resulted in the formation of the Wills Mountain duplex under the load of the North Mountain thrust sheet and sediments derived from the Adams Run–Cacapon Mountain anticlinorium. The growth of the Wills Mountain duplex resulted in duplication of the Cambrian–Ordovician carbonate section with coeval syntectonic erosion, shedding 3–4 km of sediments into the plateau, and triggering the formation of several thrusts in the Cambrian–Ordovician carbonate section. A final E-W–directed shortening event resulted in extensive regional mesoscale fracturing.

Fluid inclusion microthermometric data from five temporally controlled vein sets in the southeastern Piedmont province record changes in fluid composition and deformation conditions during regional exhumation and cooling related to postorogenic Mesozoic rifting. In general, compositions of postmetamorphic fluids are remarkably consistent across the southeastern Piedmont, indicating regional fracture connectivity during fluid-trapping events between the end of the Alleghanian collisional orogeny and the opening of the Atlantic Ocean. Late Paleozoic P 1 and P 2 quartz veins associated with post-Alleghanian stress relaxation contain metamorphic CO 2 -H 2 O-NaCl fluids that were trapped under fluctuating lithostatic (125–240 MPa) to hydrostatic pressures (20–100 MPa) at burial depths of up to 9.3 km. Maximum depths are similar to emplacement depths of post-kinematic plutons, suggesting a period of rapid isobaric cooling. Fluids trapped in veins in the Modoc fault zone give similar trapping pressures, but significantly higher temperatures, suggesting fluid migration from depth along the zone. Triassic TR 1 quartz veins related to early failed rifting record regional decompression as fracturing above the brittle-to-ductile transition allowed regional pore-fluid pressure to drop to hydrostatic levels. During this time, extensive fluid mobility occurred due to opening of regional fractures associated with initiation of rifting and tapping of deep, hot, saline fluid reservoirs. Calcite in TR 2 veins, which are related to the development of Triassic basins, and later TR 3 veins related to initial Jurassic dike emplacement host common H 2 O inclusions. These inclusions are evidence for complete opening of the hydrologic system and for convective circulation of meteoric water, which resulted in dilution of “in situ” fluids, and ultimately to a pure-H 2 O system. These fluids continued to be trapped in vein minerals through the Early Cretaceous.

Epithermal ore deposits have traditionally been the most economically important in México, with renowned world-class deposits like those in the Pachuca–Real del Monte, Guanajuato, Fresnillo, Taxco, Tayoltita, and Zacatecas districts. Whereas in certain areas (like the Great Basin in Nevada) intermediate and low sulfidation deposits have been found to be mutually exclusive in time and space; in the case of epi thermal deposits in México, the intermediate and low sulfidation types do not appear to be mutually exclusive and, to the contrary, they coexist in the same regions, formed during the same time spans, and even occur together within a single deposit. These deposits are all Tertiary in age, ranging from middle Eocene to early Miocene, with the possible sole exception of a Paleocene deposit. Their space and time distribution follows the evolution of the continental arc volcanism of the Sierra Madre Occidental and Sierra Madre del Sur. The vast majority of epithermal deposits in México belong to the intermediate (IS) or low (LS) sulfidation types; only a few high sulfidation (HS) deposits have been described in the NW part of the country (e.g., El Sauzal, Mulatos, Santo Niño, La Caridad Antigua, all of them in Sonora and Chihuahua). Because most epithermal deposits in México exhibit composite characteristics of both IS and LS mineralization styles (as well as scarce characteristics of HS), they cannot be simply characterized as IS (polymetallic deposits associated with the most saline brines) or LS deposits (mainly Ag and Au deposits associated with lower salinity brines). Thus, in this paper we propose to use an empirical classification for IS + LS deposits (that is, alkaline/neutral epithermal deposits) into three types of mineralization; namely, A, B, and C. Type A (or IS type) comprises those deposits that generally formed at greater depths from highly saline but unsaturated brines and contain exclusively from top to bottom IS styles of mineralization with a consistent poly-metallic character. Type B (or LS-IS type) comprises those deposits that exhibit dominant LS characteristics but have polymetallic IS roots (Zn-Pb-Cu); this is the most widespread type of epithermal mineralization in México. Types A and B generally exhibit mineralogic and/or fluid inclusion evidence for boiling. Type C (or LS type) comprises those deposits that exhibit only LS styles of mineralization, formed generally by shallow boiling of low salinity fluids, and have relatively high precious metal and low base metal contents. In this paper, we also review other known or attributable aspects of Mexican epithermal deposits, including ore and gangue mineralogy and their evolution in time and space, structure, geothermometry, stable iso topic composition of mineralizing fluids and other components of the deposits, chemistry and sources for mineralizing fluids, and the plausible mechanisms for the mobilization of deep fluid reservoirs and for mineral deposition in the epithermal environment.

The Late Archean Witwatersrand Basin of South Africa represents the world's foremost gold deposit. The basin is also the erosional remnant of the world's oldest (2.02 Ga) and largest known impact structure, the Vredefort impact structure. The debate about the origin of the Witwatersrand gold resource—as a placer deposit, through influx of gold-bearing hydrothermal solutions into the basin or mobilization of basinal fluids, or in the form of a so-called modified placer deposit—has been ongoing for about a century. Currently, most active Witwatersrand workers favor either a hydrothermal process or the modified placer theory, although the nature and timing of hydrothermal events affecting the basin remains a matter of debate. Here, results of fluid inclusion petrographic and quadrupole mass spectrometric analyses on Ventersdorp Contact Reef conglomerate and footwall quartzite samples, as well as Vredefort impact-related pseudotachylitic breccias from the bedding-parallel fault zones at the top and bottom of the Ventersdorp Contact Reef, are reported. These analyses were obtained in an attempt to derive new information on the types of fluids that may have been involved in gold mineralization during the history of this important ore deposit. Samples from Tau Lekoa Mine, previously known as the Vaal Reefs No. 10 Shaft Mine in the Klerksdorp goldfield, and samples from the western part of Elandskraal Mine, previously known as the Elandsrand Gold Mine in the Carletonville gold-field, were analyzed. Three populations of predominantly secondary fluid inclusions are recognized. The earliest one comprises small, wispy-textured inclusions. The second involves a moderately saline fluid and the third one a CO 2 -rich fluid with variable CO 2 /H 2 O ratios. Microthermometry indicates fluid entrapment temperatures from 220 to 370 °C at 2 kbar. Tau Lekoa fluid inclusions are enriched in CO 2 compared to the CH 4 -enriched inclusions in Elandskraal samples. These differences are explained by comparatively more reducing conditions at the time of fluid entrapment at Elandskraal. Detailed textural studies indicate that fluid entrapment occurred at a late stage in the evolution of the Witwatersrand Basin. This is corroborated by previous argon chronological information as well as detailed mineralogical analysis of impact-related pseudotachylitic breccias, which also suggest that fluid migration occurred at the time of the Vredefort impact event. Thermodynamic calculations based on information gained from entrapped remnants of mineralizing fluids indicate that the solubility of gold in these fluids is, at 1–10 parts per billion, an order of magnitude lower than previously thought. These results cast doubt on the hydrothermal model for authigenic gold mineralization in the Witwatersrand Basin and favor a modified placer model.