Sulfide Minerals in the G and H Chromitite Zones of the Stillwater Complex, Montana GEOLOGICAL SURVEY PROFESSIONAL PAPER 694 Sulfide Minerals in the G and H Chromitite Zones of the Stillwater Complex, Montana By NORMAN J PAGE GEOLOGICAL SURVEY PROFESSIONAL PAPER 694 The relationship of the amount, relative abundance, and size of grains of selected sulfide minerals to the crystallization of a basaltic magma UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1971 UNITED STATES DEPARTMENT OF THE INTERIOR ROGERS C. B. MORTON, Secretary GEOLOGICAL SURVEY William T. Pecora, Director Library of CongresR catalog-card No. 70-610589 For sale by the Superintendent of Documents, U.S. Government Printin&' Otrice Washin&'ton, D.C. 20402 - Price 35 cents (paper cover) CONTENTS Page Abstract------------------------------------------------------------------------------------------------------------ 1 Introduction________________________________________________________________________________________________________ 1 Acknowledgments--------------------------------------------------------------------------------------------------- 4 Sulfide occurrences-------------------------------------------------------------------------------------------------- 4 Sulfide inclusions in cumulus minerals_____________________________________________________________________________ 4 Fabrtc______________________________________________________________________________________________________ 5 Phase assemblages__________________________________________________________________________________________ 5 Sulfide minerals associated with postcumulus materiaL------------------------------------------------------------ 5 Fabric------------------------------------------------------------------------------------------------------ 7 Phase assemblages__________________________________________________________________________________________ 7 Comparison of types of sulfide occurrence _____ ----------------------------------------------------------------------- 10 Variation of sulfide categories with stratigraphy---------------------------------------------------------------------- 12 Genetic implications------------------------------------------------------------------------------------------------- 15 Conclusions--------------------------------------------------------------------------------------------------------- 19 References cited---------------------------------------------------------------------------------------------------- 19 ILLUSTRATIONS Page FIGURE 1. Geologic index map and correlation diagram showing the chromitite zones in the Peridotite member of the Ultramafic zone ------------------------------------------------------------------------------------- 2 2. Geologic map of the Mountain View area showing sample localities ------------------------------------ 3 3. Sketch of polished sections showing shapes of mineral grains that form inclusions in olivine and chromite___ 6 4. Sketch of polished sections showing shapes of mineral grains associated with postcumulus minerals -------- 8 5. •.rriangular diagram showing volume percentages of pyrrhotite, pentlandite, and chalcopyrite in multiphase grains ---------------------------------------------------------------------------------------------- 9 6-9. Graphs showing: 6. Variation of volume percentage of sulfide minerals correlated with stratigraphic position ---------- 11 7. The size of sulfide grains and aggregates correlated with stratigraphic position -------------------- 13 8. Variation of abundance of phases correlated ·with stratigraphic position -------------------------- 14 9. Volume percentage of sulfide minerals versus Fe-P/ (Fe+2+Fe-P) ratio in chromite --------------- 18 TABLES Page TABLE 1. Abundance of mineral assemblages occurring as inclusions ------------------------------------------------- 5 2. Abundance of all mineral assemblages associated with postcumulus material ------------------------------- 9 3. Sulfide abundances in the G and H chromitite zones --~------------------------------------------------------ 10 III SULFIDE MINERALS IN THE G AND H CHROMITITE ZONES OF THE STILLWATER COMPLEX, MONTANA By NORMAN J pAGE ABSTRACT formal stratigraphic units-a lower Peridotite mem­ ber and an upper Bronzitite member. The Peridotite Pentlandite, pyrrhotite, chalcopyrite, niccolite, and platinum­ member contains about 15 cyclic units (Jackson, 1961, group minerals occur in two chromitite zones ( G and H) 1967, 1968) of which 12 or 13 contain chromitites. The of the Stillwater Complex. Detailed mineralogic studies of stratigraphic sections of both zones illustrate that amount, third stage began with the appearance of the first relative abundance, and size of sulfide minerals are related cumulus plagioclase. During this stage, the Banded directly to primary crystallization processes of a fractionating and Upper zones of the complex crystallized (Hess, basaltic magma. Four textural categories of sulfide grains and 1960). aggregates are present: inclusions in cumulus chromite and Sulfide minerals are concentrated in the Basal zone olivinP: grains associated with postcumulus phases: fracture fillings; and grains associated with alteration minerals. Aver­ (Howland, 1933), in the chromitite zones of the Peri­ aging the sulfide minerals in the two zones, 2.5 percent occurs dotite member of the Ultramafic zone (Page and Jack­ as inclusions, 88.4 percent occurs as grains associated with son, 1967), and in certain horizons of the Banded zone postcumulus phases, and 9.1 percent occurs as fracture fillings (Howland and others, 1936). Sulfide-rich pods and and in association with alteration minerals. Inclusions probably lenses, conformable with the igneous layering, occur represent both immiscible sulfide droplets and crystalline sul­ fide minerals trapped by crystallizing cumulus olivine and at the base of the G and H chromitite zones (E. D. chromite, whereas sulfide grains in postcumulus material rep­ Jackson, oral commun., 1968). resent interactions between sulfide droplets and the trapped The chromitite zones in the Stillwater Complex magma. Temperature, partial pressures of oxygen and sulfur, originated as direct crystallization products of a frac­ and growth and settling rates of the sulfide and silicate phases tionating sheet of basaltic magma (Jackson, 1961, were important factors in controlling the digtrihution of sulfide materials. 1963, 1967, 1969), and the sulfide minerals, which are intimately associated with these zones, probably are INTRODUCTION comagmatic with the complex. Page and Jackson (1967) suggested that the sulfide minerals were local­ Sulfide minerals have long been known to occur in ized either by a process of migration in fluids upward the Stillwater Complex. southwestern ~fontana (How­ in the crystal pile, the sulfide minerals being trapped land, 1933; Howland and others, 1936; Roby, 1949; owing to the relative impermeability of the fine­ Jackson and others. 1954; Peoples and others, 1954). grained chromitites, or by a mechanism of precipita­ The Stillwater Complex is a large tabular mass of tion of mattes from the individual batches of magma mafic and ultramafic rocks of Precambrian age (Kist­ that formed the cyclic units. This paper presents the ler and others, 1969) that crystallized in at least three available evidence concerning the relations of the sul­ distinct stages. The initial stage consisted of (1) the fide. silicate, and oxide phase assemblages within two intrusion of basaltic magma into pelitic rocks, quart­ chromitite zones. These data can be used as limiting zites, and iron-formation-now metamorphosed to the conditions on the mechanism of sulfide accumulation pyroxene-hornfels facies-and ( 2) the crystallization in the chromitites. of the Basal zone of the complex before the appear­ The descriptions of the sulfide mineralogy and pe­ ance of the first cumulus olivine (Hess. 1960; Jones trology are based on samples collected by E. D. Jack­ and others, 1960; Jackson, 1961). The be~inning of son from the G and H chromitite zones (figs. 1, 2) in the second stage, deposition of the Ultramafic zone, is the ~fountain View area of the Stillwater Complex. marked by the appearance of cumulus olivine.• Jack­ In this area the two chromitite zones are about 12 son (1961) has divided the second stage into two in- feet thick. 1 ~ 110°15' 10' 5' 110°00' 55' 50' 109°45' ~ d 0 2 3 4 5 MILES t"' ~ ~ tj ~ 45°30' .....~ z~ ~ ~ t"' 'X ~ ~ ~ ~ ~ 0 > 25' z..... ~ :I: 0 :I: ~ 0 ~ ~ WEST EAST 8 ~ Chrome Iron West Mountain Nye 8 Mountain Fork Initial View Lip Benbow Gish ~ N 0 u z G 0 0 ~ I NON (f). 20' H ozo } W<((l) 0 Sedimentary rocks ...J LIJ G <( ::E l'%j 0.. 8 - :I: ~ "'0 rtJ Metamorphic rocks ~ (f). =CIS 0 be, metasedimentary rocks 8 "'0 N gg, granitic gneiss ~ FEET "'0Q) '"'Q) z t"' 1000 = p. Principally norite, <( t"' CIS p. gabbro, and anorthosite 800 ~"' ~:::> 0:: ~ ,B al 600 Contact > member ~ ::E Q) <( 8 400 ;~,A c3 § u ~ !- N Bronzitite member LIJ ~ 200 ~ <:,) c::: 0.. 0 ~ <H Principally bronzitite Fault 0 CIS 0 ! s G chromitit~ ~ ~ ri) CIS zone ,±:! Thrust fault ~ 0 5 MILES '"' t"' :::> Peridotite member Sawteeth on upper plate 1.;1 Principally bronziti~. harzburgite, and chromititel ~ ~ ~ FIGUBE 1.-Geologic index map and correlation diagram showing the chromitite zones in the Peridotite member of the Ultramafic zone of the Still­ water Complex. After Page and Jackson (1967). ~ INTRODUCTION 3 EXPLANATION >- 0:: <( Landslide debris z 0:: w 1- <( :::l 0 Glacial deposits Basaltic