25th International Meeting on Organic Geochemistry IMOG 2011

P-041
Organic geochemistry of entrapped bitumen within
kerogen/mineral matrix of an Australian Late Paleoproterozoic
lead-zinc-silver deposit
Alexander Holman 1 , Kliti Grice 1 , Caroline Jaraula 1 , Arndt Schimmelmann 2
1 Western Australia Organic and Isotope Geochemistry Centre, Department of Chemistry, Curtin University,
Perth, Australia, 2 Department of Geological Sciences, Indiana University, Bloomington, United States of
America (corresponding author:alexander.holman@student.curtin.edu.au)
Hydrothermal fluid transport essential in forming
the Late Paleoproterozoic McArthur River or "Here's
Your Chance" Pb/Zn/Ag ores in the Northern Territory
of Australia is also reflected in low δD and δ 13 C
values of n-alkanes and a decrease of polyaromatic
hydrocarbon content with diminishing hydrothermal
alteration. Estimates of maturity based on organic
molecular proxies calculated from abundances of
freely extractable hydrocarbons (Bitumen I) in this
ore, however, may not be reliable due to recent
hydrothermal alteration and transport processes.
Maturity proxies are calculated with abundances of
isomers of methylphenanthrene, phenanthrene,
steroids and triterpenoids. There is strong evidence
for differing thermal stabilities of these compounds
with increasing clay content, such that a more pristine
record of maturity is preserved in hydrocarbons
occluded in the kerogen/mineral matrix (Bitumen II)
(Nabbefeld et al., 2010). To test this hypothesis, we
have investigated the biomarker distributions and
stable carbon and hydrogen isotope ratios of Bitumen
II and compared these with those reported in Bitumen
I (Williford et. al. 2011). Bitumen II has been obtained
after initial extraction of Bitumen I by Williford et al.
(2011) through extensive demineralistion with HCl
and HF/H3BO3 (as shown by Nabbefeld et al. 2010
and references therein) to isolate organic components
entrapped within the kerogen/mineral matrix. The
differing mineral structures within the matrix have
been investigated with microscopy techniques.
References
Nabbefeld B, Grice K, Schimmelmann A, Summons
R, Troitzsch U, Twitchett RJ (2010). A comparison
of thermal maturity parameters between freely
extracted (Bitumen I) and a second extract
(Bitumen II) from within the kerogen matrix of
Permian and Triassic sedimentary rocks. Org.
Geochem. 41, 78-87.
Williford KH, Grice K, Logan G, Chen J, Huston D
(2011). The molecular and isotopic effects of
hydrothermal alteration of organic matter in the
Paleoproterozoic McArthur River Pb/Zn/Ag ore
deposit. Earth Planet. Sci. Lett. 301, 382-392
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