Exploring Relationships Between the Chemical Evolution of the Atmosphere-Ocean System and the Secular Variation in BIF-hosted Gold Deposits

English Abstract:

As one of the most readily recognized and intriguing chemical sedimentary rock types, banded iron formations (BIFs) also serve as a useful and elegant archive of ocean chemistry. Their apparent secular distribution throughout geologic time has influenced models of evolving ocean chemistry for decades—that more or less stand today, with some important modifications. Recent intensive research concerning chemical changes in BIFs throughout geologic time has greatly expanded our knowledge of changing redox conditions as well as understanding controls on biological evolution. In particular, this work suggests that a dramatic change developed in the Orosirian period (2.05 to 1.8 Ga) resulting in a lower redox state of the atmosphere-ocean system. In contrast to the preceding Siderian and Rhyacian periods (2.5 to 2.05 Ga) that experienced high (modern-like?) oxygen levels, the Orosirian period records a deoxygenation event. There is a notable decrease in the abundance of some trace elements in BIFs, including U, Co, Zn, and Cr, after the end of the Lomagundi Excursion (ca. 2.06 Ga). The same trend is observed in organic matter-rich shales that show a shift in isotopic values and significantly lower trace element abundances. This time interval also coincides with a major peak in orogenic gold deposit occurrences (ca. 2.1 to 1.85 Ga) typically associated with BIFs. While this new model of evolving redox-state of the atmosphere-ocean system has been developing over the last few years, the implications of dynamic oxygen levels in the Proterozoic has not been fully explored. This talk will explore recent advances in our knowledge of the evolving redox state of the Precambrian atmosphere-ocean system, especially in the Orosirian period, and use this to consider ideas regarding controls on secular variations in mineral deposit occurrences.