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Stillwater Complex, Montana— Structure, mineralogy, and petrology of the basal zone with emphasis on the occurrence of sulfides

1979; United States Government Publishing Office; Linguagem: Inglês

10.3133/pp1038

2330-7102

Norman J Page,

Geochemistry and Geologic Mapping

The Basal zone of the Stillwater Complex is divided into two members, a Basal norite and an overlying Basal bronzite cumulate.The Basal norite is composed of alternating lensoid masses or layers of rock and is extremely variable both laterally and vertically in mineralogy, texture, grain size, and crystallization sequences, and the Basal bronzite cumulate is composed of layers of fine-to coarse-grained granular orthopyroxene cumulates and exhibits relatively regular properties laterally.The contact between the Basal zone and the metasedimentary rocks it intrudes does not follow a single stratigraphic horizon but cuts across lithologic and structural units in the country rocks.The overall shape of the Basal zone is sheetlike with an extremely large length-to-thickness ratio, but in detail it consists of a series of basinlike shapes linked by thinner units of rock.The Basal zone is composed of a variety of rocks, including: Cumulates, principally orthopyroxene cumulates but with minor amounts of one-and two-phase cumulates dominated by olivine, orthopyroxene, clinopyroxene, inverted pigeonite, plagioclase, or chromite; noncumulate igneous rocks with ophitic, subophitic, and ragged textures; contaminated and mixed rocks; rocks derived by local partial melting; sulfide-bearing and sulfide rocks; and complex breccias of the above types.The rocks and their constituents are characterized by fluctuating and repeating patterns of chemical and physical properties that, from bottom to top, include: (a) Lithologic sequences of ophitic, subophitic, ragged, and cumulus-textured rocks;(b) cumulate sequences of orthopyroxene-ortho-pyroxene+plagioclase, olivine+orthopyroxene-orthopyroxene, and plagioclase-plagioclase+ orthopyroxene; (c) sequences of fine-to mediumto coarse-grained cumulate rocks; (d) variation in the volume of cumulus phases present; (e) variation of orthopyroxene compositions; (f) variation of plagioclase compositions; and (g) variation of the distribution of sulfide minerals.The model for the formation of the Basal zone proposes that immediately after or during initial emplacement of magma, several characteristics of the basal contact zone developed.Silicate magma and a sulfide-oxide liquid were injected into the metasedimentary rocks as tongues, dikes, and lenses, accounting for locally complicated geologic relations.Some early silicate magma was emplaced in rocks that were cool enough to allow it to crystallize as ophitic, subophitic, and ragged-textured noncumulates.Subsequent injections of magma cooled more slowly and developed textures characteristic of cumulate rocks.The massive sulfide lenses crystallized as temperatures decreased.Noncumulate igneous rocks crystallized by relatively rapid cooling or quenching of magma within the main chamber near the contact with metasedimentary rocks.Large blocks and inclusions of metasedimentary rocks locally having rinds of noncumulate rocks were rafted upward in the magma chamber.Locally, cumulate crystalization began, was interrupted either by new influxes of magma or by overturning in convection cells, and was reestablished.This process was probably the major mechanism operating during the development of the Basal norite member.The approach of the magma to thermal and chemical equilibrium was also perturbed by influxes of new magma or convection.The same cycle of crystallization and perturbation affected the remaining magma, probably producing the chemical and mineralogical repetitions and inhomogeneities exhibited by the norite member of the Basal zone.Perhaps, by the time the upper part of the Basal norite member was crystallizing or at least by the time the Basal bronzite cumulate member was accumulating, there were no new influxes of magma, and the crystallization pattern was established.'"5 CO c/2 ..CO