Canadian Mineralogist Vol. 12, pp. 475-498 (1974) GREEIUAJjTE,STILPI\IOMELANE, MINNESOTAITE CROCIDOLITEAND CARBONATES IN I\ VERY LOW.GRADE METAMORPHIC PRECA,MBRIANIRON.FORMATION CORNELISKLEIN, JR. Department ol Geology, Indiana University, Bloomington, Indiana 47401 AssrRAcr of possible solution of earlier phases and deposition of later phases were not found. The Sokoman Formation in the Howells River area, ai the western edge of the Labrador Trough, INrnouucrrou is part of a relatively undisturbed sedimentary se- quence which dips between 5' and l2o east-nortl- 16s mineral assemblagcin this study are east. The finely banded silicate-carbonate members part of the silicate-richhorizons of the Protero- of this Aon-formation were studied by detailed pe- trographic and electron microprobe techniques. Most of the assemblages show complex textures in which early diagenetio features are overprinted by later ones which are the result of very low-grade meta- morphio reactions. Of the three Fe-silicates, greena- lite, stilpnomelane, and minnesotaite, greenalite ap pears to be of very early origin (sedimentary to diagenetic). Stilpnomelane appears to be of diage- netic origin, and minnesotaite probably began its formation in diagenetic stages as well, but is gen- erally concluded to be of very low-grade metamor- phic origin. Greenalite which occurs as oolites, gtrarules, and later cemeot, is cryptocrystalline to microcrystalline in texture, and of homogeneous chemical composition. Stilpnomelane is well-crystal- lized, at times medium-grained. Minnesotaite, in well-crystallized rosettes and sprays, replaces earlier greenalite and stilpnomelane as well as much of the associated carbonates. Carbonate compositions are represented by members of the dolomite-anke- rite series, siderite, and calcite. Ankerite and side- rite are the most common carbonate species. Man- ganoan dolomite occurs in well-crystallized oolites ffi:lINDEX MAP SHOWING tvhich show relict banding as outlined by discon- LocArloNoF LABRAD.RrRouGH tinuous concentric bands of siderite rhombohedra. Calcite occurs sporadically as cement around silicate granules and in granul:tr ocqurences of calcite-side- I t" rite pairs. Magnetite occurs in coarsely recrystallized O ulo Miles bands, oolites, granules and iregular masses. Quartz # or chert occur throughout all of the assemblages. Crocodolite is found only in higbly deformed, cre- nulated crocodolite-mapetite-stilpnomelane-quartz assemblages in exposures near Schefferville, P.Q., and northwest thereof, where bulk compositions are Na-rich and structural deformation is pronounced. The primary Fe-silicate materials were probably hydrous silicate gels of greenalite-type and stilpno- melane-type compositions. Greenalite and stilpno- melane appear to be the recrystallization products thereof. Minnesotaite is a reaction product of earlier silicate and carbonate-chert assemblages. All Frc. 1. Location of the Howells River area, New- carbonates and magnetite appear to be of primary foundland and Schefferville, P.Q., with respect origin but are nolq coarsely recrystallized. Although to the iron-formation horizons in the Labrador recrystallization and reaction t€xtures are present Trough (after Gross 1968). Location of biotite in all assemblages,replacement textures indicative isograd after Fahrig (1967). 475 476 THE CANADIAN I\{INERALOGIST zoic iron-formation sequence at the western Schefferville mining areas. ODe sample of Ruth edge of the Labrador geosyncline. Most of the Slate from the Schefferville mining area and samples were obtained from the Sokoman For- one crocidolite-rich assemblage from Walthier mation in the Howells River area which is 1o- Lake, 23 miles northwest of Scheffervillb, were cated approximately twelve miles west of Schef- also studied. ferville, P.Q. (Fig. 1). Several additional samples The Sokoman Formation in the Howells River were obtained from the Sokoman Formation in area consists of conformable sedimentary units tho iron mining district of Schefferville, p.e., which are relatively undistur ed and strike ap- and 23 miles northwest thereof, in the Walthier proximately 35'NW and dip about 5 to 12" Lake area. east-northeast. Faults and other evidence of The iron-formation sequencein the Howells structural deformation are extremely rare. The River area was selectedfor this study of silicate. Sokoman Formation in the Schefferville area, carbonate assemblages.b.ecause the Sokoman however, has undergone complex structural Formation in this area is unleached, and be- changes and locally strong leaching and super- cause it probably is the least metamorphooed gene enrichment. and least altered iron.formation in the Labrador The first detailed study of the geologic setting, Trough. Although the mineral assemblagesdis- mineralogy and petrology of the Sokoman For- play some very low-gtade metamorphic fea- mation in the Howells River area was made by tutes, all preserve diagenetic and possibly pri- Perrault (1955). mary textures and chemistry. ANALYTTCALMBrnoos Gnorocrc SBrtrNc rN THE Howslrs RrvBrr. petrographic AREA Detailed studies of mineral as- semblagesand their textural relations were made The lowermost member of the proterozoic prior to electron probe microanalysis. Specific Kaniapiskau Supergroup which unconformably assemblageswere photographed at a magni- overlies the Ashuanipi Basement complex of fication similar to that of the highest power of Archean ageois the Wishart Formation, a basal the light optical system of the electron probe guartzite with an average thickness of approxi- (64OX). Chemical analysesfor nine oxide com- mately 58 feet (Fink 1972) in the Howells River ponents in the silicates, carbonates and oxides area. This is overlain by the Ruth Slate, a thin- were performed on polished thin sections using ly laminated slate consisting of carbonate, chert a three-spectrometerEtec Autoprobe. Operating and greenalite and sporadic pyrite and graphite. conditions were 15kv accelerating voltage and The Ruth Slate averages L7 feet in thickness. 0.02 microampere sample current, standardized The Sokoman Formation, with an averagethick- on quartz, using beam current integration. The ness of 373 feet, overlies the Ruth Slate. The electron beam size was adjusted to a relatively most silicate-rich members of the Sokoman For- large size of about 2 to 8 microns in diameter mation in the Howells River area are referred to because of the possibility of breakdown of the as the "Lower Iron Formation" (LIF) and the hydrous silicates and the carbonates with a "Lean Chert" (LC) by Iron Ore Company of higher electron loading per unit area. A com- Canada geologists (Fink 1972). The Lower Iron bination of well-analyzed, homogeneous syn- Formation member directly overlies the Ruth thetic silicate glasses and naturally occurring Slate, whereas the Lean Chert member forms crystalline silicates was used as standard mate- tle uppermost member of the iron-formation rials. The Etec Autoprobe spectrometers ale sequence.The Lower Iron Formation member controlled by an onJine PDP-11/05 (1.2K me- averages 27 feet in thickness and the Lean mory) computer which also performs the data Chert member averages 83 feet in thickness. handling for each analysis point. The computer The majority of the assemblagesof this study program for partial instrument control and 'oLlF" data were obtained from the and "lC" mem- handling was developed by Dr. Larry W. Finger bers of the Sokoman Formation in the Howells of the Geophysical Laboratory, Washington, River region. Samples were obtained from dia- D.C. (Finger & Hadidiacos L972; Finger 1973). mond drill cores 1032D and 1039D of the Iron Data reduction procedures were those of Bence Ore Company of Canada (Fink L97I, L972) & Albee (1965) with correction factors from and from field occurrences. A few additional Albee & Ray (1970). samples of silicate-rich members of the Soko- Complete chemical analyses of bulF samples man Formation, the silicate-carbonateiron-for- of the silicate-rich iron-formation were made mation (SCF) member and the "lower red by a combination of gravimetric, flame photo- cherty" (LRC) members were obtained in the rnetric and colorimetric techniques. LOW.GRADE METAMORPHIC PRECAMBRIAN IRON-FORMATION 477 BuLr CnsrursrRy oF SrLrcerE-RtcH 1 to 5. Analysis 7, which was made on a minne- InoN-FonvrerroN sotaite-rich band of the silicate-carbonate iron- formation member (SCIF) in the Schefferville Complete chemical analyses of eight bulk mining area, is almost equivalent to that of pure samplesof silicate-rich members of the Sokoman minnesotaite (see Table 4 for comparison). Formation and Ruth Slate are given in Table 1. Analysis 8, made on a highly oxidized part.of Analyses 1 to 5 represent samples from the un- the Ruth Slate, consists of hematite-limonite- leached and unaltered iron-formation in the chert and lesser, brown oxidized minnesotaite. Howells River area. In each of these, FeO ex- ceedsFezOa, reflecting the presenceof abundant siderite and Fe-silicates (which contain large MINERAL OnnrvrrstnY amounts of FeO), and lesser amounts of magne- Many of the minerals in this only very slightly tite. Oxide percentages in these five analyses are metamorphosed iron-formation are very fine- simil4l to those given by James (1966) for tle grained and finely intergrown with each other. silicate-carbonate iron-formation samples from Although such assemblagesare difficult to study the Mesabi district, Mimesota and the Gogebic by methods which require mineral separation district, Michigan. Analysis