Trends in metal enrichment in framboids from a metal-rich and a metal-poor estuary

Processes in which trace metals are incorporated into pyrite that forms under euxinic conditions in organic rich sediments are important for several reasons. Recent models proposed by Large et al. (2011) suggest that pyrite is an important source of gold in orogenic and Carlin style gold deposits. It is also possible that pyrite sequesters many potentially toxic trace metals and metalloids. Finally, as pyrite is a sink for trace metals it can be used as a tool for determining paleocean chemistry.
Traditional methods to determine trace metal content of syngenetic to diagenetic pyrite involve using partial extractions to release the trace metals contained within the sulphide phase. While this is a useful technique when pyrite is the only sulphide phase in the sample and there is only one phase of pyrite deposited, there are significant selectivity issues when these criteria are not met. In contaminated sites there are often several different sulphides and other phases that can dissolve under the same conditions as pyrite. Thus, overly high results often occur with sequential leach experiments in these environments. When studying rocks where there are several different generations of pyrite, a similar problem can occur. The reagent will dissolve every generation of pyrite and thus give a weighted average of the trace metal content of all the pyrite in the sample rather than the trace metal content of the pyrite of interest. To avoid these problems, we use LA-ICPMS analyses to analyze only the pyrite phase of interest, as selected by petrographic analysis.
We have analyzed pyrite framboids from the contaminated Derwent Estuary and the relatively uncontaminated Huon Estuary. The samples from the Huon Estuary resulted in very similar enrichment with depth (i.e. degree of trace metal pyritization) as noted previously by Huerta-Diaz and Morse. In the heavily metal enriched Derwent Estuary, however, the large concentration metals led to very different trace metal profiles than those observed in more pristine settings. Highly concentrated elements such as lead and zinc were incorporated into the pyrite crystal lattice in high abundance whereas, in uncontaminated estuaries, they generally reside as microinclusions in the pyrite. Similarly, other metals that are usually incorporated into the pyrite crystal lattice were not incorporated at the same level as the highest abundance of the element, but rather diffused to a deeper depth of sediment, presumably due to competition for adsorptive sites on the growing pyrite. An additional benefit to using LA-ICPMS is that it enables the analysis for gold, silver and tellurium, which, due to the low levels at which these elements occur in solution, cannot be analyzed for in sequential leach extractions. By comparing the depths in the sediment in which these elements are incorporated into pyrite relative to other elements, we were able to empirically add gold and silver to the relative rate of enrichment scheme proposed by Morse and Luther (1999) as follows: As = Mo > Cu = Fe > Co > Ni >> Mn =Au > Ag > Zn > Cr = Pb > Cd.