The hydropyrolysis (HyPy) release of hydrocarbon products from a high maturity kerogen associated with an orogenic Au deposit and their relationship to the mineral matrix

Abstract

An innovative experimental approach has been applied to a metamorphosed (> 550 °C) kerogen associated with orogenic Au mineralisation. The novel application of catalytic Hydropyrolysis (HyPy) to mineralised sediments associated with Au-mineralisation successfully liberated GC–MS-detectable hydrocarbons interpreted as indigenous to the high maturity organic matter (OM), and previously proven unamenable to more traditional methods of organic geochemical analyses. These organic compounds were likely sequestered within condensed polyaromatic structures of the high maturity kerogen that exhibited an intimate relationship to the mineral matrix as documented using high resolution in-situ microscopy. The major pyrolysis products were the polycyclic aromatic hydrocarbons (PAHs) pyrene (detected in particularly high abundance), coronene and hydrogenated analogues of these 4- and 7-ringed systems. To help distinguish the importance of specific organic–mineral relationships, each sample was subjected to a sequential demineralisation process using three increasingly stronger acids (i.e. 3 M HCl, 12 M HCl, HF). The kerogen isolated following each demineralisation step was subject to HyPy and then GC–MS analysis. Pyrene and hydrogenated analogues were again the major HyPy products of all demineralised kerogens (similar to the parent kerogen result). Other 2–7 ringed hydrocarbons and a homologous series of n-alkanes also were detected. Subtle differences in the distributions of compounds released and a pronounced increase in the abundance of most HyPy products at each demineralisation step (TOC also showed a general increase) were observed. These observations could be attributed to the increasing release of mineral-bound hydrocarbons, indicating the strong organic affinity for a wide variety of mineral types. The intimate association of the kerogen and minerals was clearly evident in the SEM–EDS photomicrographs of several freshly fractured surfaces that showed OM along mineral boundaries as inclusions, and as continuous infill in mineral grains. Most of the OM is intricately embedded in the clay structures. These mineral–kerogen relationships are retained even when the samples were finely powdered. Laser Raman analysis identified highly ordered and aromatic graphitic like carbonaceous material (CM-1) associated with the sedimentary matrix, although a distinct type of less ordered CM (CM-2) was also identified in proximity to sulphide veins — reflecting a separate organic origin of preservation pathway. The sequestration and protection of these hydrocarbons from thermal destruction likely result from an intimate structural relationship between the minerals and OM. This pioneering analytical approach represents a major breakthrough to the organic characterisation of sediments metamorphosed by high temperature mineralisation processes.