Abstract The carbonate Vegadeo Fm (Lower-Middle Cambrian, NW Spain) hosts abundant Zn-Pb mineralization. Minor stratiform ore showings occur in mineralogically and geochemically undisturbed limestones of the Lower Member. They are interpreted as related to diagenetic processes, with sulfur derived from the abiogenic reduction of sulfates interbedded in the carbonates and lead leached from the nearby detrital rocks. The major mineralization, stratabound deposits in the Upper Vegadeo Fm and discordant replacements (Rubiales-type), occurs in relationship with pervasive epigenetic silicification and minor hydrothermal breccias. They are interpreted as of Late Variscan age. Here, the mineralization seems to be formed by reaction of the hosting carbonates with low-saline (<7 wt% NaCl eq.) water-rich fluids that circulated along major lithologic contacts or extensional faults. Isotopic composition of the hydrothermal fluids falls within the field of low-grade metamorphic or basinal waters (δ 18 0=-1.0 to +6.3%; δD=-43 to -31%0). The ore-forming process occurred at temperatures between 150 and 250°C and at low fluid pressures (<500 b). Lead isotopes suggest that most of the base metals were derived from the hydrothermal remobilization of the diagenetic ores in a “lead-frozen” system. Some of the sulfur appears to have had the same source, but there was also a significant input from biogenically-reduced sulfur from the enclosing shales. These Variscan ores have many of the descriptive features of carbonate-hosted deposits, but the fluid compositions, P-T conditions of formation, and tectonic setting were fundamentally different. They can be interpreted as equivalent to MVT deposits, but formed in the internal zones of orogenic belts.

Abstract The Llanos Basin of Colombia is located along the eastern flank of the Andes in northern South America. The basin covers an area of approximately 194,000 km 2 and it is drained toward the east and northeast by the Arauca, Meta, Casanare, and Vichada Rivers and their tributaries that join the Orinoco River. Petroleum exploration activity in this basin began with the drilling of the first well in 1944. Since then, two oil giants (Cano Limon and Castilla), three major oil fields (Rubiales, Apiay, and the Tame Complex), and more than 50 minor oilfields have been discovered, representing 1500 MBOE of cumulative production and estimated remaining reserves of more than 3050 MBOE. These hydrocarbon volumes make the Llanos Basin very prospective for future giant- to medium-size field discoveries, for which a complete understanding of the basin’s geologic evolution is necessary. Key factors in the petroleum potential of the Llanos Basin include the deposition of Cretaceous marine source rocks that were deeply buried and matured and multiple phases of orogeny that generated numerous structural and stratigraphic traps and pathways for oil migration. The main aim of this study is to provide an overview of the tectonostratigraphic evolution of the northern and central parts of the Llanos Basin, based on previously published information and our own seismic interpretations and flexural modeling of the foreland basin.