The geochemistry of the carbonate-rich Espanola Formation (Huronian) with emphasis on the rare earth elements

The Espanola Formation, part of the Lower Proterozoic Huronian sequence, is unique in the Huronian succession in that it contains abundant carbonate material.Chemical analysis of bulk samples suggest that the dominant control on the distribution of Al, Si, Ti, K, P, Rb, Zr, Ba, Zn, Cr, Ni, Li, and Ga is the noncarbonate fraction of the rocks. This terrigenous component was derived from a source compositionally similar to that of other fine-grained Huronian formations. Sr is controlled by the substitution for Ca in calcite and dolomite. Na and Y are strongly affected by both the carbonate and noncarbonate fraction. Cu displays erratic behaviour. Microprobe analyses reveal high concentrations of Fe in calcite and dolomite, possibly related to substitution for Mg. Mn may have had a similar control. Whether this control is related to incorporation of Fe and Mn at the time of deposition, during diagenesis, or during metamorphism is uncertain.The distribution of the rare earth elements is similar to that of normal terrigenous clastic rocks of post-Archean age. However, high concentrations of REE in the soluble portion of carbonate-rich samples may suggest incorporation of REE into the carbonate structure. The lack of anomalous Ce behaviour implies that these rocks were deposited in water not equilibrated with manganese nodule formation.