Iron Hill (Powderhorn)

The pear-shaped Iron Hill complex covers some 31 km2 and is emplaced in Precambrian metamorphic and igneous rocks, principally granites in the vicinity of the complex, which are fenitized. Drilling seems to indicate that the complex tapers downwards (Temple and Grogan, 1965, p.684). The northwestern part of the complex is overlain by Oligocene rhyolitic ash-flow tuffs, while much of the western contact is concealed between Recent deposits along Cebolla Creek. The principal rocks, from oldest to youngest, are pyroxenite, uncompahgrite, ijolite, nepheline syenite and carbonatite. Heterogeneous pyroxenite underlies more than half of the exposed part of the complex and layering occurs locally, pyroxene-rich layers alternating with biotite or vermiculite-rich layers; textures are very variable. Pyroxene, principally augite, but sometimes diopside, aegirine-augite or aegirine, varies from 20-90% but generally constitutes 55-70% by volume. Biotite ranges from 0-52% but is generally about 10%, while the ubiquitous apatite varies from 5-10% but may reach 48%. Melanite is widespread and a number of different amphiboles occur, often rimming pyroxene. Magnetite-ilmenite ranges from 10-15% and other significant minerals include perovskite, sphene, calcite and dolomite, and relatively rare orthoclase and microcline. Uncompahgrite forms irregular bodies and apophyses in the pyroxenite, and is composed of melilite, of approximately equal proportions of Na melilite and akermanite (Nash, 1972, p. 1368) and its alteration products, about 15% diopside and a little magnetite, apatite, phlogopite-biotite, melanite, calcite and perovskite. Ijolite cuts pyroxenite and uncompahgrite in the southeast of the complex and consists of nepheline, aegirine-augite, melanite and accessories; locally the ijolite grades to urtite and alternating pyroxene- and nepheline-rich layers occur. Nepheline syenites are confined to the eastern end of the complex where they cut the country rock granite, fenite and pyroxenite, and fenitization appears to be more extensive in their vicinity (Olson and Hedlund, 1981, p. 15). The nepheline syenites are trachytic textured and of orthoclase microperthite laths, largely altered nepheline, aegirine-augite, melanite and accessories. Carbonatite is the youngest rock type within the complex and forms a plug of 4 km2 and numerous dykes within and outside the complex. Contacts of the plug are poorly exposed, but a mixed zone 100-300 m wide can be seen on the southern and eastern margins against pyroxenite and uncompahgrite in which biotite and sodic amphibole are developed. The carbonatite is variably fine grained to pegmatitic, foliated and sheared and may contain partially replaced xenoliths. It consists of dolomite and minor fluorapatite, phlogopite, biotite, baryte, calcite, magnetite, pyrite, pyrochlore, bastnaesite, synchisite etc. Several hundred carbonatite dykes have been mapped within and around the complex (Olson and Hedlund, 1981, p. 19), the majority 1-1.5 m in thickness. In order of abundance they contain dolomite, calcite, siderite, biotite, phlogopite and a range of accessories including fluorite, synchisite and parisite. The fenite aureole is best developed along the northeastern contact and involves the development of aegirine-augite and sodic amphibole; some fenites may have been rheomorphosed and Armbrustmacher (1983) suggests that nephelinization also took place. As well as carbonatite dykes the country rocks are cut by numerous 'thorite veins' (Olson and Hedlund, 1981, p. 22 and Fig. 20) up to 5 m thick consisting principally of K-feldspar and quartz with a range of accessory minerals including thorite; wall rocks are partly replaced by reddish orthoclase. The classic account of the petrology and mineralogy of the complex is that of Larsen (1942), but other detailed descriptions are given by Temple and Grogan (1965), Nash (1972) and most recently by Olson and Hedlund (1981). Analyses of apatites, including REE data, from a range of rock types are given by Nash (1972).