Podiform Chromite Ore Bodies: a Genetic Model

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Abstract
The Carneal plug, Co. Antrim, Northern Ireland, is a Tertiary dolerite intrusion, 150 m in diameter, which incorporates blocks (up to several metres across) of the underlying Cretaceous Chalk and the flints originally contained in the Chalk. It is one of the rare natural occurrences showing reaction between basaltic magma and limestone under conditions of very high temperature and low pressure. The pressure was controlled by lithostatic load at about 0.2 kb; the upper temperature limit for the prograde reactions was close to the liquidus temperature of basaltic magma around 1100 °C and retrograde reactions with secondary hydrous fluids have produced a series of hydrated calc-silicates over the temperature range 700 °C to less than 100 °C. The rocks form complex assemblages with two complementary suites of rocks developed by the metamorphic and metasomatic reactions. An exomorphic suite, derived mainly from the chalk, consists of assemblages of larnite, spurrite, bredigite, merwinite, melilite (gehlenite), calcite, or spinel; and wollastonite, melilite, hibschite and quartz derived from flint. The endomorphic suite is derived by desilication reactions between the basic magma and the incorporated chalk and flint. The minerals are those of the dolerite, together with fayalitic olivine, sahlite, nepheline, aegirine, wollastonite, melilite (humboldtilite), merwinite, scawtite, and hydrogrossular. Retrograde processes provide a further range of scarce to rare mineral suites. The Carneal plug thus provides an ideal opportunity to investigate the application of strontium isotope-ratio measurements to crustal contamination of basic magmas, since the three rock end-members dolerite, chalk, and flint, are readily identifiable and relatively homogeneous, and the conditions of the formation of the resulting metamorphic and metasomatic rocks are well understood.

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