Petrology of the Low-Grade Rocks of the Gunflint Iron-Formation, Ontario-Minnesota PAP?k^N I Department of Earth and Space Sciences, State University of New York, Stony Brook, New York 11794 ABSTRACT tent with textural and compositional data tinuation of the iron formation. Marsden supporting a primary origin for the iron and others (1968) used the term "Animikie The relatively unmetamorphosed middle silicates. Quartz, recrystallized carbonate Iron Formation" for the correlated seg- Precambrian Gunflint Iron-Formation of cements, microcrystalline siderite, hematite, ments of the Cuyuna, Mesabi, and Gunflint Ontario has undergone considerable post- and possibly magnetite are also considered Ranges of Minnesota and Ontario. depositional recrystallizarion and locally in- primary phases. Key words: mineralogy, The iron formation is structurally simple tense replacement. Although these tend to sedimentary petrology, crystal chemistry, and uncomplicated. It is nearly flat lying obscure primary textural-mineralogical re- sheet silicates. with an average southeast dip of 5°. Local lations, textural elements similar to those of folding and brecciation, often accompanied limestone can be identified and their INTRODUCTION by gravity faults, are, however, present. mineralogy defined. Two fundamentally This type of deformation was attributed by different kinds of iron formation are recog- This report deals with the mineralogy Goodwin (1956) to penecontemporaneous nized: (1) cherty iron formation, which and petrography of the relatively un- volcanic disturbances. consists of granules, ooliths, and interstitial metamorphosed Gunflint Iron-Formation The Gunflint Iron-Formation and the cements; and (2) banded or slaty iron for- of Ontario. Emphasis is placed on defining overlying Rove Formation (with which it mation, which is composed of matrices the textural relations and chemistry of forms a gradational contact) comprise the (fine-grained internally structureless silicate- and carbonate-bearing assemb- middle Precambrian Animikie Group. The laminae). Cherty iron formation corre- lages. A succeeding paper will deal with as- Rove Formation consists of interbedded ar- sponds broadly to the thick-bedded taco- pects of the contact metamorphosed por- gillite and graywacke as much as 1,000 m nite and algal chert facies of Goodwin; slaty tion of the iron range that is largely thick (Morey, 1967). The average thickness iron formation encompasses the thin- confined to northern Minnesota. of the Gunflint Iron-Formation in Ontario banded chert-carbonate and tuffaceous The Gunflint Iron-Formation of Ontario is about 120 m (Goodwin, 1956). In Min- shale facies. contains an abundance and variety of iron nesota the Animikie is divided into three Greenalite associated with cherty quartz silicate and carbonate minerals whose formations, including a basal quartzite unit and minor minnesotaite are dominant min- chemistry within iron formations is virtu- that forms the lowermost member of the eral constituents of granules; stilpnomelane ally unknown. We relate compositional var- iron formation in Ontario. Unconformities and hematite are less common. Recrystal- iation in some of the individual mineral are present at the base and top of the lized calcite, ankerite, and siderite occur phases to specific textural occurrences and Animikie strata in Ontario and Minnesota. locally as cements and as replacement min- stratigraphic position; these variations ap- On a regional scale, basic intrusions of erals. The most common cement is quartz. pear to be retained in the contact aureole. late Precambrian (middle Keweenawan) Iron silicate and siderite matrices are major Preservation of relict textures and the ap- age are intimately associated with the constituents of slaty iron formation, which parently isochemical nature of the Gunflint and Rove Formations. In Ontario also contains considerable amounts of sec- metamorphism (except for loss of H20 and these comprise the Logan intrusive rocks, ondary calcite and ankerite. Stilpnomelane COz) should enable correlation between which consist of numerous sill-like diabase and chamosite are locally abundant in slaty metamorphic mineral assemblages and sheets (Moorhouse, 1960). These are found rocks as apparent pseudomorphs after their sedimentary precursors as well as throughout the Animikie rocks, but indi- shards. quantification of some metamorphic reac- vidual sills rarely exceed 30 m in thickness. Microprobe analyses of greenalite reveal tions. In short, the Gunflint Iron-Formation little compositional variation; stilp- offers a rare opportunity to study the pro- Previous Work nomelane from slaty iron formation is ex- gressive transformation of an undeformed tremely heterogeneous. Both siderite and sedimentary rock to the pyroxene-hornfels Detailed geologic investigation of the ankerite exhibit considerable substitution facies. Gunflint Iron-Formation began with the of Fe by Mg (and Mn locally) whereas cal- studies of Tanton (1923, 1931) and Gill cite is almost pure CaC03. Geologic Setting (1924, 1927). Gill (1927) described some of Comparison of the greenalite, min- the Gunflint textures but not their detailed nesotaite, and stilpnomelane crystal struc- The main portion of the iron formation mineralogy. The iron formation was di- tures reveals many similarities. The crystal extends about 170 km northeast from the vided by Goodwin (1956) into six major chemistry of magnesium and nickel International Boundary at Gunflint Lake sedimentary facies representing four mem- analogues (serpentine, talc, and garnierite) (Fig. 1). In Minnesota the iron range forms bers. He drew attention to evidence of has been used to predict structural details of a narrow belt 20 km long and is truncated widespread volcanism associated with the iron silicate minerals. These are consis- to the southwest by the Duluth Complex. iron-formation deposition and suggested a !f Present address: Manned Spacecraft Center, TN6, Erosional remnants northeast of Thunder volcanic source for the iron and silica. The Houston, Texas 77058 Bay at Schreiber, Ontario, suggest a con- cyclic nature of the deposition is shown by Geological Society of America Bulletin, v. 86, p. 1169-1190, 14 figs., September 1975, Doc. no. 50901. 1169 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/9/1169/3418213/i0016-7606-86-9-1169.pdf by guest on 28 September 2021 1170 FLORAN AND PAPIKE IV^M Duluth Complex ") p-p-n , „ . f Keweenawan Group (U. Precambrian) I.:'.•! Sibley Series J I I Rove Fm •) S Animikie Group (M. Precambrian) Guntlint Fm J Undivided (L. Precambrian) Upper Limestone Member 100 -\ Upper Guntlint Member Lower Guntlint Member Basal Conglomerate Member 1 Lower* and Upper algal chert facies 4 Lower* and Upper* banded chert-carbonate facies 2 Lower and Upper tuffaceous shale facies 5 Upper limestone member 3 Lower west* and east taconite; Upper* taconite facies »abundantly sampled Figure 1. Generalized geologic map and stratigraphic section of Gunflint Iron-Formation (after Morey, 1973, modified from Goodwin, 1956), the repetition of facies in the Lower tablished the time of sedimentation. the sediments is shown by abundant fea- Gunflint and Upper Gunflint members (Fig. Nevertheless, these studies (Hurley and tures of shallow water (above wave base) 1). The upper tuffaceous shale unit others, 1962; Faure and Kovach, 1969; origin. In addition, at least a small part of (argillite-tuff horizon of Moorhouse, 1960) Misra and Faure, 1970) and indirect evi- the iron formation appears to have been forms a marker bed traceable throughout dence (Hanson and Malhotra, 1971) sug- deposited subaerially (Goodwin, 1956; much of the iron formation. The strati- gest the Animikie sediments were deposited Walter, 1972). graphic units defined by Goodwin (1956) in slightly less than 2,000 m.y. B.P. Ontario can be readily correlated with the Many workers have drawn attention to Analytical Procedures fourfold division used on the Mesabi the abundant sedimentation and penecon- Range (Lower Cherty, Lower Slaty, Upper temporaneous deformation features that The facies that were extensively sampled Cherty, Upper Slaty). This division was ex- occur within the Gunflint Iron-Formation are indicated in Figure 1. A large range of tended to the Gunflint Iron-Formation in (Broderick, 1920; Gill, 1927; Goodwin, rock types was collected, but detailed study Minnesota by Broderick (1920); he sug- 1956; Moorhouse, 1960; Mengel, 1963, was directed to the silicate- and (to a lesser gested that the two ranges are probably 1965; Barghoorn and Tyler, 1965; extent) carbonate-bearing rocks. Labora- continuous beneath the Duluth Complex. LaBerge, 1967a, 1967b; Gross, 1972; Wal- tory work integrated petrographic, x-ray Goodwin (1960) and Moorhouse (1960) ter, 1972). Most of these structures are diffraction, and electron microprobe published outcrop maps (scale 1:31,680) of common to clastic limestone, shale, and analytical techniques. X-ray diffractograms most of the iron formation in Ontario; sandstone and serve as indicators of the were obtained on a Picker diffractometer Morey and Papike (see Sims and others, physiochemical environment of deposition; using monochromatic CuKa radiation. 1969) presented a preliminary geologic others are unique to chemical precipitates These supplemented optical identification map of the range west of Gunflint Lake in and suggest the former existence of gels. of major and minor mineral constituents. Minnesota. The variety and abundance of these struc- Chemical