Genesis of the Cumbrian hematite deposits

Abstract

(Resume of an evening lecture given during the field meeting on the West Cumberland Hematite ore field, 15th – 17th July, 1983) West Cumbria and Furness have provided about 250 million tons of high-grade iron ore since mining began in earnest about the middle of last century. Iron contents in the range 50–60 per cent, with P2O5 less than 0.02 and S less than 0.06 have been normal as a result of the very simple mineralogy of the deposits, composed of massive and botryoidal hematite with dolomite and quartz as the only gangue minerals of quantitative importance. In a few places, a little chalcopyrite occurred as an early phase, while in others, small amounts of blue fluorite and of baryte were associated with specularite in late cavities. As J. D. Kendall (1881–82) first conclusively demonstrated, most orebodies were emplaced by metasomatism of limestone, in the course of which fossils were replaced molecule by molecule by hematite or quartz, and layers of chert nodules were preserved in situ in the orebodies, particularly those, like the example seen at Florence Mine, having the form of flats. The ores have been wrongly classified as of Bilbao type by United Nations investigators (United Nations 1970); this type is derived from siderite/ankerite protores by oxidation; careful determination of large numbers of carbonates has failed to show any evidence for such protores in West and South Cumbria. The hematite is unquestionably a primary mineral; in this respect the origin of the ores is radically different …