Hydrocarbons in silica: PVTX properties of fluids and the genesis of diamond quartz from Caravia-Berbes Fluorite district (Asturias, Spain)

Abstract The fluid history of “diamond” quartz crystals from Caravia-Berbes Fluorite district is intimately related with episodes of maturation and migration of hydrocarbons within the Asturian basin, probably linked to the structural activation of a Mesozoic rift system. These “diamond” quartz record the migration of hydrothermal silica-rich brines and associated hydrocarbons that recorded a temperature increase (outlined by fluid inclusion studies) from ∼90 to ∼140 °C. This gradient is also recorded by a change in the organic matter species trapped in quartz, from bitumen located in the inner quartz growth zones to liquid hydrocarbons trapped within the outer quartz overgrowths. Secondary ion mass spectrometer (SIMS) oxygen isotope microanalyses allowed to differentiate two δ 18 O H2O values corresponding to the oil-dominated zone (core) and the water-dominated zone (outer growth zone). The heavier δ 18 O H2O values in the water-dominated zone do not only record a higher temperature of formation, but also an increase in the δ 18 O H2O of the parental fluid. Isotopic compositions of diamond quartz suggest a heterogeneous source of surficial fluids and formation-waters expelled from claystones. Within the basin history, the diamond quartz precipitation took place during early Jurassic to Eocene subsidence periods. Pressure Temperature and composition (PTX) and δ 18 O H2O characteristics point out a probable relationship with Albian diagenetic events reported in Cantabrian and north Pyrenean basins.