Cone Negosa (Negoza)

Cone Negose is located immediately north of the fault system that defines the northern margin of the mid-Zambezi trough (rift). It forms a hill and is intruded into Precambrian metasedimentary rocks which are cut by numerous Karoo dolerite dykes. South of the east-west-trending fault that runs along the southern flank of Cone Negose are Karoo sediments that are cut by 14 satellite plugs lying up to 5 km south of the main body. Cone Negose is a circular, stock-like intrusion of trachytic and carbonate breccias or vent agglomerates of 4x3.5 km with a central 2 km diameter carbonatite. The breccias consist of fragments of highly altered trachytic and rhyolitic rocks with much limonite, hematite and secondary quartz in a carbonate-rich matrix. The satellite plugs to the south are of the same rock types. The carbonatite is of five main lithologies. The earliest is a grey carbonatite which occurs in the northwest; it contains pyrite, bastnasite, barite, pyrochlore and monazite. Partly overlying and partly intruding the grey carbonatite is a dolomitic variety, samples of which contained 7.8% bastnasite and 5% barite, which in turn is veined and brecciated by a dark red rock rich in hematite with 40% barite and up to 20% brookite containing as much as 15% Nb2O5. The central part of the hill comprises a heterogeneous carbonatite breccia which is banded and contains fragments of the earliest generations of carbonatite. The breccia contains fluorapatite, barite, monazite and pyrochlore and nodular masses containing concentrations of either haematite, goethite or phosphates. The iron-rich nodules include up to 5% pyrochlore and the phosphatic ones 50% apatite. The carbonatites are partly overlain by siliceous alteration products.