Trace elements in sedimentary pyrite through time: application to understanding temporal changes in ocean oxygenation and ore deposit cycles in marine basins

This research outlines a new and innovative method to determine variations in the trace metal content of the oceans through time, by LA-ICPMS analysis of sedimentary pyrite in black shales from early Archean to present day. The oceans are a significant source of metals that can inform us about economic accumulations in sedimentary basins; in particular of gold, iron, zinc, copper and uranium. Preliminary research shows that certain metals (e.g. Ni, Co, Cu and Au) have generally decreased in the oceans through time whereas other metals (e.g., Mo, Se and Zn) have generally increased. These changes appear to be controlled by many factors including; 1) residence times of the metals in the ocean, 2) gradual oxygenation of the Earth’s atmosphere/oceans, 3) the composition of exposed crustal rocks 4) seafloor hydrothermal activity, and 5) cycles of ocean anoxia. This preliminary work also shows links between temporal curves of trace metals in the ocean and cycles of major metal accumulations of banded iron formations, sedex Zn-Pb-Ag, stratiform Cu and sediment-hosted gold (including Carlin-type) deposits. The advantages of measuring trace elements in sedimentary pyrite, rather than whole-rock shales, to develop ocean proxies are; 1) pyrite concentrates most of the elements of interest well above the whole rock value, 2) pyrite can be analysed by LA-ICPMS to very low detection levels, giving both maximum and minimum values, 3) pyrite is one mineral, rather than a mix of minerals, and thus does not require normalisation by another element, and 4) petrography and etching of pyrite, prior to analysis, provides information on metamorphic and hydrothermal effects.