Opaque oxide minerals of some diabase-granophyre associations in Pennsylvania

Abstract

Optical examination of the opaque oxide minerals in specimens of diabase, intermediate granophyric diabase, granophyre, and diabase pegmatite from Triassic intrusions of Pennsylvania revealed distinct changes through the diabase-granophyre differentiation sequence. Qualitative electron microprobe scans across complex mineral grains supplemented and confirmed the optical interpretations. The opaque oxide minerals consist almost entirely of ilmenite, magnetite, and ulvospinel, and they comprise between 1 and 10 percent of the rocks in the diabase-granophyre sequence. Magnetite occurs in two forms: the first contains exsolved ulvospinel and is intimately associated with ilmenite; the second is Ti-poor and it does not contain microscopically visible Ti-bearing phases. The second form occurs as rims on the first or as separate crystals in differentiated rocks. Ilmenite occurs as intergrowths, as skeletal growths, and as individual crystals. Accompanying the igneous differentiation sequence there is a gradation from titaniferous magnetite with ilmenite, to skeletal ilmenite retaining an original spinel outline. We suggest that during the stage of iron-enrichment in the igneous differentiation sequence an aqueous fluid developed within the crystallizing intermediate magma and was capable of dissolving iron from the titaniferous magnetite intergrowths, leaving skeletal crystals of ilmenite. Some or all of this iron was redeposited as a second generation of Ti-poor magnetite within the source rock. It is possible that iron-bearing solutions so derived could escape to higher levels where they would be a potential source of magnetite deposits of Cornwall type.