Hydrocarbons may also form zinc and lead deposits

Base metals such as zinc (Zn) and lead (Pb) are essential elements for our economy. They are mainly used for the galvanization of steel and large battery installations, respectively. These metals are often mined together from deposits hosted in ancient sedimentary rock formations. A new study is published in Scientific Reports by Saintilan (formerly at the University of Geneva and presently Swiss National Science Foundation Ambizione fellow at ETH Zürich) and colleagues and collaborators from the University of Geneva, the University of Lausanne, and the Geological Survey of Sweden (SGU). This work breaks with a widely accepted paradigm for the formation of an important type of sediment-hosted Zn-Pb deposits.

Multiple evidence suggests that most of those Pb-Zn deposits formed when metal-rich saline waters, which had previously leached metals from underlying rocks, flow into rock formations several kilometres below surface. There, they meet a hydrocarbon reservoir rich in reduced sulphur that cause precipitation of zinc and lead as sulphide minerals that we mine today. The new research challenges a key aspect of this model: it reports for the first time that in some ore deposits, significant amounts of the metals may have actually been transported by and sourced from hydrocarbons and not provided by the saline metal-bearing waters. So far, hydrocarbons had only been shown to hold the key role for providing reduced sulphur for mineralization.

Since the ban of leaded petrol in the 1990’s and the consequent decrease in environmental contamination of samples, the steady improvement of analytical techniques has permitted to analyse the isotopic composition of lead at an unprecedented level of precision. At the Laisvall Pb-Zb deposit in northern Sweden, which was the largest European lead mine in the course of the 20th century, the researchers noticed a peculiar chemical signal in the isotopes of lead in the sulphide minerals. Instead of being only sourced from underlying Precambrian granites and gneisses, as the "standard model" predicts, a significant fraction of the metals appears to have been sourced up to 60 % from a known hydrocarbon source rock rich in metals known as the Alum Shale. While hydrocarbons were being produced from the source rock, they scavenged metals from the Alum Shale and carried them to the site of mineralization. During alteration of the hydrocarbons, those metals alongside with a part derived from underlying basement rocks and carreid by saline waters were fixed as sulphide minerals in the Laisvall deposit.

This discovery opens a new way to look at mineral deposits in sedimentary basins and may lay a new path for exploration tools and strategies for sediment-hosted ore deposits containing industry critical metals such as nickel, cobalt, copper, zinc and lead.

See article “Petroleum as source and carrier of metals in sediment-hosted mineralization” by Saintilan et al., (2019) in Scientific Reports

June 13, 2019