Introduction

Iron sulphides have attracted the attention of geologists owing to their frequent occurrence in modern sediments, sedimentary rocks and ore deposits. They are of importance in studies of environmental pollution and in economic contexts: e.g. as ‘sinks’ for toxic heavy metals (Dekkers & Schoonen, 1994), as raw materials for sulphuric acid production (Lin, 1997) and as indicators of the presence of gold-bearing ores (Menyah & O'Reilly, 1991). Additionally, however, iron sulphides are attracting increasing attention within the contexts of environmental magnetism and palaeomagnetism. One reason for this interest is the increasing frequency with which the presence of authigenic magnetic iron sulphides is being reported, particularly in lake and brackish water sediments. These occurrences highlight the need for careful evaluation of sedimentary magnetic records in terms of palaeoenvironmental inferences (see Chapter 7). Magnetic iron sulphides have also been shown to play an important role in palaeomagnetic investigations of Quaternary (and older) sediments. For example, in sediments bereft of any detrital magnetic minerals, ‘syn’-depositional formation of authigenic greigite or pyrrhotite can provide a means of reliable remanence acquisition and hence contribute a robust and useful palaeomagnetic record. Conversely, as discussed below, post-depositional formation of such minerals may result in unreliable and ambiguous palaeomagnetic directions.

The formation of iron sulphides in sedimentary environments frequently involves geochemical processes that are mediated by micro-organisms, in particular bacteria responsible for the decomposition of organic matter. Sulphur isotope studies of Archean and Proterozoic rocks have revealed that sulphate-reducing bacteria have played an important role in the state of the global sulphur and oxygen cycles since an early stage in Earth's geological history (Ohmoto et al., 1993).