Lower Palaeozoic iron oolites and volcanism from a Baltoscandian perspective

The Ordovician iron oolites in Baltoscandia are generally geographically extensive but lean, [<30% ooids sensu (Bhattacharyya, D.P., 1989. Concentrated and lean oolites: examples from Nubia Formation at Aswan, Egypt, and significance of the oolite types in ironstone genesis. In Young, T.P., Taylor, W.E.G. (eds.) Phaneorozoic Ironstones. Geological Society Special Publication 46, 93–103)] and have undergone minimal reworking and diagenetic alteration. The ooids were formed in a normal shallow-marine environment around a nucleus of a calcite skeletal grain. The oolites were formed during the period of the highest global iron oolite production ever. Most of the major oolites from this period have been altered in various ways, which has obscured their origin. The presence of volcanic matter in the Baltoscandian and some other European oolites points to a volcanic origin, a conclusion further supported by the REE distributions in iron ooids, which are close to those in associated volcanic tuffs. The global 87Sr/86Sr curve for Phanerozoic seawater in general, and for the Palaeozoic in particular, shows that negative excursions in the Sr isotope ratio often coincide with periods of high global iron oolite productivity. This temporal coincidence points to a possible causal link. Iron ooids, volcanic ashes and bentonite beds occur in the Ordovician of Baltoscandia during a negative excursion in the 87Sr/86Sr curve. The variation in the 87Sr/86Sr ratio in seawater mirrors the balance between the two major sources of Sr, mid-ocean ridge volcanism and continental weathering, and indicate that volcanism could be the general cause of most Palaeozoic iron oolites. Two Cambrian iron oolites occur in Baltoscandia, and at least one of them is suggested to be of volcanic origin.