USING COMPOUND SPECIFIC HYDROGEN AND CARBON ISOTOPES TO UNDERSTAND THE NATURE OF HYDROTHERMAL ALTERATION OF SEDIMENTARY ORGANIC MATTER IN THE HYC PB/ZN/AG DEPOSIT

Although the ~1639 Ma Barney Creek Formation (BCF) in the Northern Territory, Australia exhibits the best known preservation of sedimentary organic matter on Earth older than one billion years, its HYC (or "Here’s Your Chance") Pyritic Shale Member contains a major sedimentary exhalative Pb/Zn/Ag ore deposit, the result of the interaction of hot, metal-rich brines with BCF sediments and organic matter. This unit offers an extraordinary opportunity to study the effects of hydrothermal alteration on otherwise well preserved and extremely ancient organic matter and to develop a refined depositional model for this deposit that will aid future mineral exploration efforts.

Five samples were taken along the apparent flow path of the HYC ore fluid and a thus along a gradient of hydrothermal alteration. The average δD of four n-alkanes recovered from these samples (n-C₁₈-n-C₂₁) is -75‰, compared to a previously reported average of -130‰ for the same compounds found in a bitumen extracted from an immature, unmineralized core of the BCF [1], suggesting that significant hydrogen exchange has occurred in the HYC member. It has been previously shown that n-alkanes undergo hydrogen exchange under high thermal maturity [2], and the decreasing δD of the n-alkanes with distance from the source of the HYC ore fluid implies a decreasing degree of hydrothermal hydrogen exchange.

The δ¹³C values of HYC n-alkanes range from -26 to -30‰, showing no clear relationship between δ¹³C and distance. Polycyclic aromatic hydrocarbons (PAHs) in these samples are significantly depleted in ¹³C relative to saturated hydrocarbons. PAHs also show a general depletion in ¹³C with distance from the fluid source and with increasing molecular weight and aromaticity, supporting earlier findings that the HYC ore fluid contained a significant quantity of hydrothermal PAHs [3].

These results support other recent work on the HYC which has estimated ore fluid temperatures to be above 300°C [3, 4].

[1] A.L. Sessions, S.P. Sylva, R.E. Summons, J.M. Hayes, 2004. Geochim Cosmochim Ac 68: 1545-1559.

[2] D. Dawson, K. Grice, R. Alexander, 2005. Org Geochem 36: 95-104.

[3] J.H. Chen, M.R. Walter, G.A. Logan, M.C. Hinman, R.E. Summons, 2003. Earth Planet Sc Lett 210: 467-479.

[4] D.T.A. Symons, 2007. Econ Geol 102: 1295-1310.