Dikes of the McClure mountain — Iron mountain alkalic complex,Fremont county,Colorado, U.S.A.

The McClure Mountain — Iron Mountain alkalic complex, the largest of three Precambrian alkalic complexes in south-central Colorado, is an irregularly ovoid body about 8×6.5 kilometers in plan. It contains major petrologic units (in age sequence): 1) peridotite and ilmenite-magnetite lenses, 2) gabbro, 3) hornblende syenite, 4) ijolite and 5) nepheline syenite, whose internal arrangement is irregular. Dikes are numerous and varied both within the complex and in the surrounding Precambrian metasediments and meta-volcanics (Idaho Springs formation), granitoids and migmatites to a distance of 27 kilometers from the complex. Fenitization has only locally affected these wall rocks at their contacts with the complex. Dikes within the complex are chiefly light colored and of syenitic or nepheline syenitic composition. Texturally they range from aphanitic, through porphyritic with an aphanitic to fine-grained matrix, to uniformly medium-grained. Most of the intracomplex dikes (save the few carbonatites) are represented by compositionally equivalent major units within the complex. In contrast, the extracomplex dikes are chiefly lamprophyres and carbonatites, none of which has a compositionally similar unit within the complex. The carbonatite dikes are younger than the lamprophyres; where the two types co-occupy the same fracture, the lamprophyre has been carbonatized. The lamprophyres are represented by an extraordinary number of textural varieties, involving phenocrysts and phenocryst combinations of augite, barkevikite, olivine and plagioclase in varying matrix combinations of olivine, augite, barkevikite, biotite, magnetite, apatite, plagioclase and orthoclase. Many of the dikes can be classed as camptonites or olivine camptonites. The Goldie lamprophyres contain local marginal phases spectacularly marked by concentrically zoned ovoids. The Cabin dike is studded by megaphenocrysts of augite. The carbonatites are of three main types:

1. Relatively homogeneous calcitic or dolomitic carbonatites with few accessories.
2. Composite carbonate — hematitic potash feldspar bodies which, with diminution of carbonate, grade into feldspathic Th-RE veins.
3. Carbonatites with abundant barite and/or fluorite, and rarer species. The Goldie carbonatite also contains a complex suite of aluminofluorides.

Many carbonatites and Th-RE veins emit, when broken, a fetid gas consisting of a mixture of fluorinated hydrocarbons of the C₅ and C₆ types along with F₂, HF, and F₂O. The fluorine has been derived from structurally degraded radioactive fluorite; the hydrocarbon gases appear to represent primary inclusions. Within the dike halo are two small breccia pipes consisting of fenitized, locally derived Precambrian gneiss blocks with minor fragments of transported biotitized lamprophyre in a subordinate calcitic matrix also containing aegirine, crocidolite, potash feldspar and hematite. The abundance of the lamprophyres, the great width of the dike halo, the lack of ring structure within the complex, the restriction of carbonatite to dikes, and the subordinate fenitization effects all combine to suggest that the McClure Mountain — Iron Mountain complex is now exposed at a relatively deep niveau, possibly near the mesozone — catazone boundary.