CRYSTALLIZATION AND ALTERATION OF QUARTZ MONZONITE, IRON-SPRINGS MINING DISTRICT, UTAH - RELATION TO ASSOCIATED IRON DEPOSITS

The Iron Springs mining district of southwestern Utah contains the lar gest iron deposits in the western United States. These are related to emplacement, at minimum depths of 1 to 2.3 km, of three Miocene laccol iths of porphyritic quartz monzonite. When emplaced, the magma was nea tly half crystallized, on the verge of brittle behavior, and contained phenocrysts of plagioclase, amphibole, biotite, and clinopyroxene. Gr anophyric dikes in quartz monzonite formed from residual liquid, but m iarolitic cavities in these dikes are lined with alpha quartz, indicat ing that deposition continued from a volatile-rich phase below the sol idus. Pervasive high-temperature alteration led to: replacement of aug ite by diopside; oxidation of magnetite and ilmenite; replacement of a mphibole (completely) and biotite (partially) by aggregates of diopsid e, alkali feldspars, smectite-chlorite, magnetite, ilmenite, and calci te; albitization of groundmass alkali feldspar; ex change of Mg for Fe and of F for OH in biotite; and exchange of F and OH for Cl in apatit e. This alteration produced whole-rock enrichment in Fe3+, Na, and H2O , and losses in Fe2+, Ca, and K. Total Fe was effectively unchanged. A chilled, relatively impermeable margin, surrounding the pervasively a ltered interior of each laccolith, was mechanically bonded to wall roc k more strongly than to the magmatic interior, and deformed with the w all rock during subsequent intrusion and structural readjustment. In t his 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 periphe ral shell. In this zone, pervasively altered quartz monzonite was frac tured 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 se lvages, 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 str ong whole-rock depletion in light REE, total Fe, Mg, P, Rb, Y, Pa, and H2O-, but caused enrichment in Na. Magnetite-apatite veins commonly f ormed along the joints. Adjacent to zones of selvage joints, strata-bo und orebodies of magnetite, hematite, and apatite formed in limestone around the three laccoliths. Iron-bearing fluid was derived by subsoli dus alteration from quartz monzonite; there is no evidence supporting immiscible separation of iron-oxide-rich liquid from silicate magma. M ass balance calculations indicate that the Fe, P, Si, Al, and Mg in th e orebodies could have been entirely supplied by selvage alteration of 40 km(3) of quartz monzonite, in agreement with the exposed and confi dently inferred volume of altered intrusive rock. A fourth pluton, lac king magnetite veins, orebodies, and selvages, was emplaced closer to the surface and against sandstone and shale rather than limestone. Iro n liberated from hydrous mafic phenocrysts in this body was oxidized i n place, forming disseminated hematite.