Origin and significance of aromatic hydrocarbons in giant iron ore deposits of the late Archean Hamersley Basin, Western Australia

Late Archean to earliest Paleoproterozoic shales associated with two giant iron ore deposits in the Hamersley Province, Western Australia, contain traces of solvent extractable saturated and aromatic hydrocarbons. The host rocks belong to the 2.5 billion years (Ga) old Mt McRae Shale and Brockman Iron Formation, Hamersley Group, and were collected in mines near Tom Price and Newman (Mt Whaleback). The saturated hydrocarbons in the rock extracts have the composition of highly mature gas condensates. The aromatic fraction predominantly consists of unsubstituted two and three ring hydrocarbons that are demonstrably indigenous and syngenetic based on their unusual pyrolytic composition and the presence of hydrocarbons with similar attributes covalently bound to the kerogen. The bitumen composition is interpreted as recording the interaction of organic matter with hydrothermal fluids or oxidizing solutions overprinted by regional low-grade metamorphism. The organic matter could potentially have recorded flow direction, redox potential and temperature of fluids that formed the iron deposits. Thus, a new organic geochemical approach is suggested for determining hypogene and supergene controls on iron mineralization in the Hamersley Province and related ore deposits. Crown Copyright # 2003 Published by Elsevier Ltd. All rights reserved.