From economic geology to ocean chemistry: the wide range of information that can be obtained from a humble pyrite grain

Pyrite incorporates a portion of several trace elements dissolved in the fluids from which it formed. Thus, trace element analysis of pyrite grains can give us information of these past fluids with applications as varied as understanding changes in ocean chemistry to identifying ore deposit type. We have analysed over 1400 pyrite grains from sedimentary rocks through geologic time using laser ablation ICPMS. Periods in geologic time when several trace elements are enriched in pyrite match with times where whole rock analyses of redox sensitive trace elements also show enrichments. This suggests that pyrite chemistry, like these traditional whole rock studies, can be used to understand changes in ocean chemistry and the atmosphere through Earth history. In this presentation we evaluate the efficacy of this by investigating nano-scale zoning of pyrite framboids from euxinically formed pyrite, the most likely pyrite to retain the trace element signature of the water column from which it formed.

Pyrite also incorporates trace elements from hydrothermal fluids. Because pyrite forms in many different deposits and those deposits have different fluid compositions pyrite chemistry should be a viable way to identify ore deposit type. This is important as near surface deposits are increasingly exploited and new, large deposits will be found at deeper and deeper depths. If pyrite can be utilized to identify ore deposit type it will allow for application of geological models early in an exploration program enhancing efficiency of the drilling. In this study we a Random Forest data learning algorithm to identify ore deposit signatures based on the trace element composition of the pyrite associated with them. We will also present a combination of LA-ICPMS, synchrotron XRF mapping and XANES analysis to identify redox fluctuations in the formation of a gold deposit.

Bio
 

In 2007 Daniel Gregory graduated with an honours degree in geology from the University of British Columbia.  After which he worked as a field geologist exploring for a wide variety of different mineral deposit types, including stratiform Ni-Mo mineralization, in the Yukon Territory, Canada. In 2010 he started his PhD in economic geology and geochemistry at the University of Tasmania.  During the course of his dissertation, he investigated three different questions regarding pyrite chemistry: how are trace elements incorporated into pyrite, can pyrite trace element chemistry be used to infer the chemistry of ancient oceans, and can sedimentary pyrite be a major source of gold in Archean lode gold deposits?

After his PhD research Daniel worked as a post-doctoral researcher at CODES.  In this work he continued to investigate pyrite chemistry and how it could be used to interpret paleo-ocean chemistry but also developed new research interests. These included using in situ analyses of different minerals (magnetite, pyrite, chlorite, titanite, hematite) coupled with machine learning techniques to both identify ore deposit style and to vector towards economic mineralization. In 2015 he started a post-doctoral position to work with Tim Lyons at UCR in the department of Earth Sciences. During his post doc he investigated what factors affect the trace element content of sedimentary pyrite and how trace elements are held in pyrite using nano-scale analytical techniques. In August 2018 he joined the Earth Sciences department at the University of Toronto as an Economic Geologist and received tenure July 2024. Here he continues to research trace element content of pyrite, utilizing a number of techniques including atom probe tomography, transmission electron microscopy, XANES, XAFS, and LA-ICPMS to better understand how trace elements are incorporated into pyrite.

For the Zoom link please email Xueyao Cheng  > xc272@stanford.edu