Abstract
The Iron Springs mining district of southwestern Utah contains the largest iron deposits in the western United States. These are related to emplacement, at minimum depths of 1 to 2.3 km, of three Miocene laccoliths of porphyritic quartz monzonite. When emplaced, the magma was nearly half crystallized, on the verge of brittle behavior, and contained phenocrysts of plagioclase, amphibole, biotite, and clinopyroxene. Granophyric dikes in quartz monzonite formed from residual liquid, but miarolitic cavities in these dikes are lined with alpha quartz, indicating that deposition continued from a volatile-rich phase below the solidus. Pervasive high-temperature alteration led to: replacement of augite by diopside; oxidation of magnetite and ilmenite; replacement of amphibole (completely) and biotite (partially) by aggregates of diopside, alkali feldspars, smectite-chlorite, magnetite, ilmenite, and calcite; albitization of groundmass alkali feldspar; exchange of Mg for Fe and of F for OH in biotite; and exchange of F and OH for Cl in apatite. This alteration produced whole-rock enrichment in Fe (super 3+) , Na, and H 2 O, and losses in Fe (super 2+) , Ca, and K. Total Fe was effectively unchanged. A chilled, relatively impermeable margin, surrounding the pervasively altered interior of each laccolith, was mechanically bonded to wall rock more strongly than to the magmatic interior, and deformed with the wall rock during subsequent intrusion and structural readjustment. In this peripheral shell, clinopyroxene, amphibole, and biotite were less altered. Where the roof of each laccolith had a radius of curvature of less than 2 km, a heterogeneous jointed zone formed below the peripheral shell. In this zone, pervasively altered quartz monzonite was fractured by renewed intrusion or by structural readjustment of roof rocks. Along the resulting joints, quartz monzonite was altered to bleached zones (selvages) during a second stage of alteration. Within these selvages, complete decomposition of biotite that had survived pervasive alteration was accompanied by further growth of diopside, albitization of feldspars, and dissolution of some apatite. Unlike early pervasive alteration, selvage alteration was fracture controlled and caused strong whole-rock depletion in light REE, total Fe, Mg, P, Rb, Y, Ba, and H 2 O (super -) , but caused enrichment in Na. Magnetite-apatite veins commonly formed along the joint. Adjacent to zones of selvage joints, strata-bound orebodies of magnetite, hematite, and apatite formed in limestone around the three laccoliths. Iron-bearing fluid was derived by subsolidus alteration from quartz monzonite; there is no evidence supporting immiscible separation of iron-oxide-rich liquid from silicate magma. Mass balance calculations indicate that the Fe, P, S, Al, and Mg in the orebodies could have been entirely supplied by selvage alteration of 40 km 3 of quartz monzonite, in agreement with the exposed and confidently inferred volume of altered intrusive rock. A fourth pluton, lacking magnetite veins, orebodies, and selvages, was emplaced closer to the surface and against sandstone and share rather than limestone. Iron liberated from hydrous mafic phenocrysts in this body was oxidized in place, forming disseminated hematite.
