PETROGRAPHIC CHARACTERIZATION OF MAGNETITE-APATITE DIKES AND ADJACENT ALTERATION HALOS, THREE PEAKS LACCOLITH, IRON SPRINGS MINING DISTRICT, UTAH

The Iron Springs mining district, near Cedar City, Utah, is one of the important iron mining districts in the United States. Much of the iron ore occurred as strata-bound bodies near the contact between the overlying limestone and the underlying quartz monzonites with the Fe-bearing fluid being derived by subsolidus hydrothermal alteration of the quartz monzonite (Barker, 1995). We report here on the mineralogy of several magnetite-apatite dikes that intrude granitoids of the Three Peaks laccolith and the alteration zone adjacent to these dikes. These dikes were not previously discussed as they crop out in the interior of the pluton where Fe-mineralization was not thought to occur. The dikes (1 to ~30 cm wide) have sharp contacts with the host rock. The host rock is altered for 10s to ~100 cm from the dike wall. The dikes are remarkable in that they exhibit euhedral apatite phenocrysts, mm to several cm in length that are oriented perpendicular to the dike walls. The textural characteristics of the dikes is strongly suggestive of an igneous origin similar to “Nelsonite” dikes commonly associated with Anorthosite-Mangerite-Charnockite-Granite complexes. However, the hydrothermal origin of the strata-bound iron ore bodies along the margins of the laccoliths is a likely process as well. To better understand the origin of these enigmatic magnetite-apatite dikes we are investigating the alteration zone associated with the dikes – can it be the source of the iron that was then concentrated into the dike or did intrusion of the dike produce the alteration halo? In addition, we hope that a comparison of the apatite in the dike with that of the apatite in the host granitoid may also constrain the origin of these magnetite-apatite dikes.