The
Figueroa Sulfide is located in the
San Rafael Mountains and is part of the
Franciscan Complex, which formed during subduction of the
Farallon Plate. This geological formation includes altered basalts,
serpentinites, cherts, and tectonically sheared mudstones. The
Figueroa Sulfide consists of a 1.5-meter-thick sulfide lens with a silicified core, flanked by more friable sulfides, and is structurally overlain by altered volcanics and cherts. The deposit shows evidence of tectonic brecciation and oxidation before silica deposition. Though small and not viable for large-scale mining, its mineralization and vent structures suggest similarities to modern
hydrothermal vent systems. Biostratigraphic analysis of the
Figueroa Sulfide, based on radiolarian fossils extracted from chert samples, indicates a late Pliensbachian to early Toarcian age. The presence of key radiolarian taxa, such as
Parahsuum ovale and
Bagotum sp., supports this dating. Taphonomically, fossils found in the deposit, including brachiopods, gastropods, and vestimentiferan tubes, are preserved as pyrite molds, replicating fine details of the original shells and tubes. These fossils suggest an ancient hydrothermal vent community, where organisms like tube worms and brachiopods likely relied on chemosynthetic bacteria for nutrition, mirroring modern vent ecosystems. The Figueroa sulfide fossil assemblage differs from modern vent communities in species diversity, composition, and trophic structure. Unlike modern vents, the
Figueroa Sulfide has only three species, lacks common taxa such as
Vesicomyidae and
Bathymodiolus bivalves, arthropods, and gastropods, and has no predators. Possible explanations for these differences include taphonomic bias, where only high-temperature vent species were fossilized, and a genuinely low-diversity Jurassic vent ecosystem. Some "missing" taxa, such as
Vesicomyidae, likely evolved later, supported by their fossil record starting in the early
Cretaceous and molecular divergence estimates. == Biota ==