Elucidating the Role of Hydrocarbons in Cinnabar (HgS) Ore Formation: A Model for Hg Mineralization in the Terlingua Mining District, Big Bend National Park, SW Texas

Abstract The genetic relationship between organic-rich source rocks and Hg deposits remains the subject of debate. This paper evaluates the role of organic-rich source rocks in cinnabar ore formation in the Terlingua mining district, Texas, which was deposited at relatively shallow depths in Cretaceous sedimentary rocks spatially related to intrusive alkali igneous rocks. The mineralization formed at ~45 bar and ~200°C. The aqueous ore-forming fluid had a pH of ~5 to 7 and was H2S saturated. Cinnabar was deposited as a result of H2S oxidation through mixing and cooling with local meteoric water. Both Hg0(org) and Hg0(aq) species were likely important in cinnabar ore formation. However, recent studies on the solubility of Hg0 in hydrocarbons show that at cinnabar saturation, Hg0 is more than an order of magnitude more soluble in hydrocarbons (Hg0(org) = 163 mg/kg) than Hg0 in water (Hg0(aq) = 10.8 mg/kg). Despite their proximity in some deposits, conditions of ore formation of the rare Hg oxychloride and sulfate minerals are not compatible with conditions under which most cinnabar ores formed, requiring fO2 conditions orders of magnitude more oxidizing, a relatively high chloride ion activity (>10–1), and alkaline conditions, with pH > 10. Mass dependent fractionation versus mass independent fractionation of Hg isotope data from Hg-bearing minerals in Terlingua support a genetic link to the source of Hg being the organic-rich marls and tuffaceous black shales of the Lower Eagle Ford Formation. This source rock is chronostratigraphically equivalent to the Lower Cretaceous oceanic anoxic event (OAE-2), which defines the Cenomanian-Turonian boundary. OAE-2 represents the culmination of a global anoxic ocean event at ~94.1 Ma, believed to be a consequence of volcanic activity associated with a large igneous province. Mercury was sequestered by the organic-rich source rocks of the Lower Eagle Ford Formation and associated coeval ash and tuffs. Tabular igneous bodies that intruded and uplifted the local stratigraphy formed the Terlingua monocline and provided a source of heat for hydrothermal activity and maturation of organic matter. The subsequent generation and migration of liquid hydrocarbons and formation brines from the Lower Eagle Ford Formation transported Hg0(org) updip toward the hinge line of the Terlingua monocline where mixing with oxygenated meteoric water and subsequent oxidation of H2S produced the cinnabar-rich ores.