File:Magnesiocarbonatites (Iron Hill Carbonatite Comoplex, late Neoproterozoic, )570 Ma; Iron Hill, southeast of Powderhorn, southwestern Colorado, USA).jpg

English: Magnesiocarbonatite in the Precambrian of Colorado, USA.

Southwestern Colorado’s Iron Hill Carbonatite Complex (also known as the Powderhorn Carbonatite Complex) is a subvolcanic, alkaline intrusive complex consisting of a mix of unusual igneous rocks. The complex has a relatively high concentration of scarce elements - for example, niobium (Nb) and titanium (Ti). It was intruded more than 570 million years ago, during the late Neoproterozoic. Some Early Paleozoic dikes have cut through a portion of the northern part of the complex.

The principal reported lithologies in the the Iron Hill Carbonatite Complex include magnesiocarbonatite, pyroxenite, nepheline syenite, ijolite, and uncompahgrite.

The Iron Hill Complex outcrop seen here consists of weathered magnesiocarbonatites (formerly known as rauhaugites), which are dolomitic intrusive igneous rocks. Here, they also have calcite.

Magnesiocarbonatite is one of four types of carbonatite, which is a rare igneous rock composed principally of carbonate minerals. The most common is calciocarbonatite, with calcite (CaCO3) - these are essentially "igneous limestones". Other types include magnesiocarbonatite, with dolomite (CaMg(CO3)2), and ferrocarbonatite, which are highly ferruginous, or are dominated by siderite (FeCO3). The rarest type is natrocarbonatite, which only occurs at one locality on Earth - the Ol Doinyo Lengai Volcano in eastern Africa (see: www.flickr.com/photos/jsjgeology/albums/72157646442276517 ).

Locality: Iron Hill Northwest Outcrop - hillside exposures above/northeast of 27 Road, northwest of Eldorado Creek & northwest of Iron Hill, eastern side of Cebolla Creek Valley, southeast of town of Powderhorn, southern Gunnison County, southwestern Colorado, USA (vicinity of 38° 15’ 54.59” North latitude, 107° 04’ 39.44” West longitude)