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MINNESOTA GEOLOGICAL SURVEY D.L. SOUTHWICK, Director

HYDROTHERMAL SYSTEMS IN -RICH -FORMATION OFTHE CUYUNA NORTH RANGE, : GEOCHEMICAL AND MINERALOGICAL STUDY OF THE GLORIA DRILL CORE

Frank Melcher, G.B. Morey, Peter L. McSwiggen,

Jane M. Cleland, and S.E. Brink

Report of Investigations 46 ISSN 0076-9177

UNIVERSITY OF MINNESOTA

Saint Paul - 1996

HYDROTHERMAL SYSTEMS IN MANGANESE­ RICH IRON-FORMATION OF THE CUYUNA NORTH RANGE, MINNESOTA: GEOCHEMICAL AND MINERALOGICAL STUDY OF THE GLORIA DRILL CORE Copyright 1996 by the Regents of the University of Minnesota

The University of Minnesota is committed to the policy that all persons shall have equal access to its programs, facilities, and employment without regard to race, color, creed, religion, national origin, sex, age, marital status, disability, public assistance status, veteran status, or sexual orientation. CONTENTS f

Abstract ...... 1 Introduction ...... 2 Regional geologic setting ...... 2 General stratigraphy ...... 3 Mahnomen Formation ...... 3 Trommald Formation ...... 3 Zone A-Thin-bedded silicate- iron-formation ...... 4 Zone B-Thin-bedded iron-formation ...... 6 Zone C-Thin-bedded manganese-oxide iron-formation ...... 7 Zone D-Thick-bedded manganese-rich oxide iron-formation ...... 7 Zone E-Thick-bedded oxide iron-formation ...... 8 Rabbit Lake Formation ...... 9 Magnetic susceptibility ...... 9 Geochemistry...... 10 Major element attributes ...... 20 Minor and trace element attributes ...... 22 Rare earth element attributes ...... 22 ...... 23 Silicates ...... 23 ...... 23 Chlorite ...... 26 -like phases of uncertain affinity ...... 26 ...... 27 ...... 27 Alkali feldspar ...... 27 ...... 27 Clay-like ...... 27 ...... 27 Iron ...... , ...... 28 ...... 28 ...... 28 ...... 29 and martite ...... 29 Phosphates ...... , ...... ,...... 29 Sulfates ...... 30 Manganese oxides and oxyhydroxides ...... 30 ...... 32 Discussion ...... 34 Evidence for a hydrothermal system ...... 35 Rare earth element signatures ...... 35 Discrimination diagrams ...... 36 Correlation coefficient analysis ...... 38 Mass-balance calculations ...... 40

111 Paleogeographic implications ...... 43 Conclusions ...... 44 Acknowledgments ...... 45 References cited ...... 45

ILLUSTRATIONS

Figure 1. Generalized geologic map of the Cuyuna range showing the location of the Gloria drill site and other localities mentioned in the text ...... 2 2. Generalized lithologic column of strata intersected in the Gloria core summarizing various nomenclatural schemes used on the North range ...... 4 3. Stratigraphic summary of chemical data compiled on Tables 1-3 ...... 19 4. Rare earth element enrichment diagrams ...... 23 5. Silica vs. alumina diagram of Crerar and others showing the dominantly attributes of these constituents ... 37 6. vs. diagram of Nicholson for manganese-rich chemical sediments ...... 37 7. vs. thorium diagram ...... 38 8. Isocon diagram of -bearing silicate-carbonate iron-formation vs. sulfide-free silicate-carbonate iron- formation ...... 41 9. Isocon diagram of thin-bedded oxide iron-formation vs. silicate-carbonate iron-formation ...... 41 10. Isocon diagram of thin-bedded Mn-bearing oxide iron-formation vs. silicate-carbonate iron-formation .... .41 11. Isocon diagram of thick-bedded Mn-bearing oxide iron-formation vs. silicate-carbonate iron-formation ...... 41

TABLES

Table 1. Whole rock chemical data from the Gloria core ...... 11 2. Whole-rock and selected minor element data of core samples selected for leach tests ...... 15 3. Chemical data from the Gloria core ...... 16 4. Mean and range of zone averages from the Gloria core ...... 21 5. Rare earth element characteristics of zones in the Gloria core compared to average Biwabik Iron Formation ...... 24 6. Chemical parameters ...... 25 6a. Mean and range of stilpnomelane in the Gloria core ...... 25 6b. Mean and range of Fe-rich and chlorite in the Gloria core ...... 25 7. Minerals detected by XRD in manganese- and iron-rich from the Gloria core ...... 31 8. Chemical parameters of arsenopyrite in the Glorial core ...... 33 9. Spearman rank correlation coefficients where N=35 ...... 39 10. Summary of mass balance calculations for different zones of the Gloria core compared to unaltered Zone A average ...... 42

APPENDIX

Appendix 1. Lithologic and selected chemical attributes of the Gloria core ...... 49

iv HYDROTHERMAL SYSTEMS IN MANGANESE-RICH IRON-FORMATION OF THE CUYUNA NORTH RANGE, MINNESOTA: GEOCHEMICAL AND MINERALOGICAL STUDY OF THE GLORIA DRILL CORE

By

Frank Melcher!, G.B. Morey, Peter L. McSwiggen, Jane M. Cleland, and S.E. Brink2

ABSTRACT

The iron-rich Trommald Formation of Early Proterozoic age on the Cuyuna North range in east-central Minnesota is the largest resource of manganese in the . To better elucidate the complex history of the manganese oxides and to investigate their compatibility with in situ leaching techniques, the U.S. Bureau of Mines drilled a core near the Gloria mine (sec. 28, T. 47 N., R. 29 W.) to an inclined depth of about 1200 feet. It intersects a complete section of Trommald Formation 553 feet thick, as well as short intervals of the overlying Rabbit Lake and underlying Mahnomen Formations. At the Gloria site the lowermost part of the Trommald Formation consists of chlorite-bearing hematite iron­ formation, and contains features indicative of syndepositional reworking-possibly under shallow-water conditions­ that include granule-rich layers and pebble-size conglomerate. Much of the overlying thin-bedded facies consists of carbonate-silicate iron-formation broken in places by beds of breccia. In the breccia, sulfides--especially ­ fill interstitial voids or form discordant composite veins along with , manganese oxides, carbonates, and stilpnomelane. Pyrite contains included pyrrhotite, , and arsenopyrite, and is replaced by magnetite, in turn replaced by martite. The silicate-carbonate iron-formation contains little manganese «2 wt. percent MnO), but the breccias are marked by elevated values of Mn, as well as Cu, As, Sb, S, Sr, Y, Ca, and P. Within the thin-bedded facies, a transitional interval of about 15 feet separates silicate-carbonate iron-formation below from oxide iron-formation above. Isocon analyses show that oxidized strata can be derived from unoxidized strata by a variety of decomposition reactions, all essentially removing Mg, K, Ca, Na, P, and resulting in a mass loss of 40 to 50 percent. Manganese oxides appear in oxidized strata as discordant veins and as concordant, massive layers and lenses, where they are admixed with goethite and hematite. Massive layers are partly enveloped by brecciated oxides of several kinds, have maximum MnO values of 12 wt. percent, and are enriched in Ba, Sr, Ag, andU. Manganese values increase markedly just above the contact between the thin-bedded facies and the overlying thick-bedded facies. -rich layers just above the contact have brecciated or "gnarled " textures and are conformably interlayered with intervals of decomposed oxide iron-formation containing abundant secondary . The manganese oxide-rich layers are intercalated with thick beds of admixed chert and hematite; they have a relict micronodular texture and discordant textural features, involving manganese minerals such as , , and . Whole rock assays show that they can contain as much as 50 wt. percent MnO. They also have elevated Ca, Mg (attributed to secondary carbonates), and P values and are enriched in Ba (as much as 1.8 percent), Pb, Sr, Ag, As, and the LREE. The uppermost part of the thick-bedded facies consists of oxide iron-formation where primary hematite is abundant but where manganese oxides are lacking. The overlying Rabbit Lake Formation is an epiclastic unit that contains thick beds of carbonaceous , thin layers of tuffaceous material, and intercalated beds of sulfide (pyrite) and oxide (hematite) iron-formation. The sulfide iron-formation (35.7 wt. percent Fe203, 26 percent S) contains elevated values of Au (31 ppb), As, Cu, Co, Ni, Pb, Sr, V, Mo, and Se (230 ppm). Many of the textural and mineralogical features observed in the Gloria core are consistent with hydrothermal fumerolic processes that started in late Mahnomen time and persisted into early Rabbit Lake time.

1 Current address: Department of Geosciences, University, 8700 Leoben, . 2U .S. Bureau of Mines, 5629 Minnehaha A venue S., Minneapolis, MN 55417. INTRODUCTION range and contiguous areas to the south and east. consists of an allochthonous fold-and-thrust belt that contains The Trommald Formation on the Cuyuna North range several structural discontinuities which probably are zones is unique in that it contains the largest reported manganese of thrusting. The discontinuities bound discrete structural values of any Early Proterozoic iron-formation in the panels or enclaves that possess small-scale structural world (Kimberley, 1989b). In addition, it is the largest features consistent with large-scale north-northwest­ resource of manganese in the United States (Laznicka, verging nappes (Holst, 1984; Southwick and Morey, 1992). In order to determine the potential of using in 1991). The belt of strongly deformed rocks flanks a situ mining techniques to recover manganese from the tectonic foredeep which extends to the Mesabi iron range Trommald Formation, the U.S. Bureau of Mines (USBM). in northern Minnesota where rocks of the well-known in cooperation with the Minnesota Geological Survey are exposed. Because the basin is filled (MGS). University of Minnesota. undertook a research with strata that can be assigned to the Animikie GrouP. program that in part involved drilling a deep hole near Southwick and others (1988) have referred to the foredeep the now abandoned Gloria mine in the northern part of as the Animikie basin. a usage somewhat at odds with the Cuyuna North range. Technical aspects of this drilling that in the earlier literature (Trendall. 1968; Morey. 1983). program. as well as preliminary data on the feasibility of The structural studies of Southwick and others (1988) in situ leaching have already been published (Dahl and also showed that iron-formation was deposited at different others. 1992; Dahl, 1992; Saini-Eidukat and others. 1993). times in different parts of the Penokean orogen (Fig. 1). Several other recently published reports (e.g., Morey and others. 1992; Cleland and others, 1993; McSwiggen and others. 1994a, b) have emphasized the importance of hydrothermal processes in the origin of the manganese deposits. The emphasis of this report is on the geologic factors that control the distribution of manganese in the North range. What follows is a brief description of the structural and stratigraphic attributes of the iron-rich host rocks. together with a summary of their principal sedimentological attributes. A description also is given of the extent. location. and origin of the manganese­ bearing materials that occur within the iron-formation. Although preliminary in nature. this report provides some of the geologic data necessary for the ultimate economic utilization of the manganese deposits.

REGIONAL GEOLOGIC SETTING

Our view of the stratotectonic evolution of the 94° Cuyuna range has changed considerably over the past 10 r- High-angle fault [:":':':'lMahnOmen Fm years (Southwick and others. 1988). Previous studies "rr"" Thrust fault Mille Lacs Group viewed the Cuyuna range as part of a single miogeoclinal r- Contact to eugeoclinal complex that was more deformed to the ~ south and less deformed to the north (Morey and Van Schmus, 1988). The tectonic framework is now Figure 1. Generalized geologic map of the Cuyuna range envisioned as the product of a collisional event-the (modified from Southwick and others, 1988) showing the Penokean orogeny of Goldich and others (1961)­ location of the Gloria drill site and other localities between 1900 and 1760 Ma. with most activity in the mentioned in the text. interval between 1870 and 1850 Ma (Morey and Van Thus earlier stratigraphic interpretations founded on the Schmus. 1988). We now think that the zone of deformed hypothesis of a single iron-rich interval are no longer rocks produced by the Penokean orogeny and earlier Early valid. This change in stratigraphic thinking is of Proterozoic tectonic events. which includes the South importance in understanding the origin of the

2 manganiferous iron ores. We can no longer extrapolate Trommald Formation sedimentological and mineralogical attributes from one part of the range to another. Therefore the geology of A complete section of the iron-rich Trommald each district must be evaluated separately, because each Formation, some 553 feet thick, was intersected at the can have its own unique stratigraphic and tectonic history, Gloria site. The contact between underlying terrigenous even though the deposits themselves may share many and overlying chemically precipitated iron-formation is similar attributes. transitional over several feet, and it is arbitrarily placed at the top of a conformable quartz-pebble conglomerate, GENERAL STRATIGRAPHY about 5 cm thick. In that conglomerate, matrix-supported, egg-shaped pebbles are aligned parallel to bedding and USBM drill site Gloria Number 1 is located near grade upward both in size and abundance. The maximum Ironton, Minnesota (NW1I4SEl/4SE1I4, sec. 28, T. 47 N., pebble-size clast is about 6 cm. The matrix consists of R. 29 W.), Crow Wing County. It was drilled between intergrown hematite, limonite, and green chlorite. Beds October 15, 1991 and November 6, 1991 at an inclination of conglomerate are intercalated with laminae of dark­ of 55° and an azimuth of N.300W. to a hole depth of green chlorite-rich schist and light-green quartz-rich 1200 feet. According to Dahl (1992), below 85 feet of schist. glacial deposits, 117 feet of Rabbit Lake Formation, 965 The Trommald Formation was described in detail by feet of Trommald Formation, and 33 feet of Mahnomen Schmidt (1963) who divided it into a thin-bedded member Formation were cored. The drilled strata have a nearly and a thick-bedded member; throughout much of the vertical dip and form the northern limb of a synclinal North range, the former stratigraphically overlies the structure having an east-northeast-trending axis (Fig. 1; latter. Using mainly bedding criteria, USBM geologists e.g., Morey and Morey, 1986). When corrected for subdivided the formation at the Gloria site into a thick­ apparent dip, the Trommald Formation is approximately bedded facies 135 feet thick, a mixed thick- and thin­ 553 feet thick, a value that corresponds well with a bedded facies 127 feet thick, and a thin-bedded facies thickness of 565 feet at the Merritt site a mile to the 291 feet thick. They also recognized an oxidation southwest (McSwiggen and others, 1995). boundary at a depth of 922-943 feet that divided the The Gloria core intersects three lithostratigraphic core into an upper oxidized interval some 430 feet thick, entities corresponding to the Mahnomen, Trommald, and and a lower unoxidized interval 130 feet thick (Dahl, Rabbit Lake Formations of Schmidt (1963). They 1992). Broadly speaking, the upper oxidized interval compose the North range group of Southwick and others can be classified as an oxide iron-formation, whereas the (1988). Descriptions of each formation follow with lower un oxidized interval can be classified as a carbonate­ emphasis on manganese- and iron-rich strata of the silicate iron-formation. This stratigraphic arrangement Trommald Formation. is similar to that recognized in nearby drill cores. For example, McSwiggen and others (1995) divided iron­ Mahnomen Formation formation at the Merritt site a mile to the southwest (Fig. 1) into an upper interval of thick-bedded oxide iron­ The lowest strata of the Mahnomen Formation consist formation, 173 feet thick, and a lower interval of dominantly of medium- to light-gray, massive to vaguely carbonate-silicate iron-formation 392 feet thick. They bedded, fine-grained sericitic quartzite and interlayered further divided the carbonate- and silicate-rich strata into phyllite. The latter contains porphyroblasts of biotite an upper laminated interval and a lower thin-bedded approximately 0.25-1 mm in diameter. A thin bed, about interval. (Fig. 2). These relationships are consistent with 2 cm thick, of massive, dark-green to black, chlorite schist the work of Schmidt (1963) who observed that bedding occurs at 1173 feet. It is marked by abundant millimeter­ attributes can vary considerably over relatively short long needles of tourmaline oriented in two directions, as strike distances. well as by scattered grains of rutile and some sulfides. In this report we have expanded on the subdivisions Above that the Mahnomen is chlorite rich and very fine envisioned by Dahl (1992) and have recognized five grained. Small clasts of hematite first appear at about zones, or more properly stratigraphic intercepts, using 1190 feet and become more abundant and larger in size an expanded array of attributes, including in addition to toward the contact with the overlying Trommald bedding characteristics, (1) oxidation state (unoxidized Formation. The uppermost part of the Mahnomen vs. oxidized), which is closely related to mineralogy contains discordant veinlets of hematite and quartz. (strata dominated by silicate-carbonate ± magnetite vs.

3 Clastic intervals disappear at about 1166 feet and are replaced by' granule-bearing iron-formation. Most granules are composed of a green silicate partly replaced by an . Thin laminae composed almost entirely of an oxide-stained green silicate first appear at 1165.5 feet, whereas chert-rich laminae first appear at 1163 feet. Hematite-rich layers disappear at 1160 feet, which marks the top of the A.l interval. Interval A.2, between 1160 and 1115 feet, consists predominantly of thin-bedded silicate-carbonate-magnetite iron-formation and at least four prominent intervals of conglomerate and breccia. Individual laminae are typically dark green to black, consist predominantly of silicates and carbonates, and are intercalated with light­ greenish-gray, chert-rich laminae. Synsedimentary deformation structures are common, ranging from irregularly folded laminae to complex slump structures marked by overturned folds and synsedimentary faults. Several kinds of conglomerate have been recognized, which range in thickness from less than 1 to more than 5 feet. The lowest of these at 1155.5 feet contains centimeter-size clasts of a brownish-green carbonate-rich rock of uncertain affinity set in a matrix of quartz, 1200 silicates, magnetite, and sulfIdes. At 1154.5 feet some of the clasts, which are as much as 3 cm in diameter, Figure 2. Generalized lithologic column of strata intersected in resemble the quartzitic rocks in the Mahnomen Formation. the Gloria core summarizing various nomenclatural Many are rimmed by selvages of intergrown schemes used on the North range. stilpnomelane and iron-rich carbonates. A conglomerate in the interval between 1141 and 1137 feet also contains strata dominated by oxides); (2) geochemistry (mainly clasts of quartzite and chert cut by veins of stilpnomelane. manganese content); and (3) magnetic susceptibility (high The uppermost part of this conglomeratic interval also vs. low), which is closely related to the extent that martite contains clasts of magnetite-rich iron-formation that replaces magnetite. Magnetic susceptibilities were resemble an overlying layer of strongly deformed, measured using a hand-held susceptibility meter, whereas magnetite-rich iron-formation. relative abundances of magnetite vs. martite were Beds of breccia contain angular clasts of obvious observed petrographically using thin sections. intraformational origin. Sulfides are ubiquitous mainly Stratigraphic features are summarized in Appendix in discordant stilpnomelane-rimmed vugs, together with 1, together with magnetic susceptibility and manganese admixed carbonate, quartz, and stilpnomelane. Sulfides assay values. A more general summary is given in Figure of this generation also rim many clasts but rarely penetrate 2, which broadly delineates the five stratigraphic them. intercepts-A through E-that have been recognized. Straight, zoned, carbonate-rich extension veins, which are typically arranged en echelon, cut clasts and matrix Zone A-Thin-bedded silicate-carbonate iron­ in both conglomerate and breccia. The veins are sulfIde­ formation bearing where they transect iron-formation, but are barren where they transect clasts. Zone A, between 1167 and 912 feet, consists Interval A.3, between 1115 and 1080 feet, also predominantly of unoxidized, thin-bedded, silicate­ consists of thin-bedded silicate-carbonate-magnetite iron­ carbonate oxide iron-formation. Although the lowermost formation, but lacks appreciable chert. This rather interval, A.I, between 1167 and 1160 feet, contains monotonous interval is characterized by wavy laminae appreciable terrigenous material, it is assigned to the and abundant synsedimentary deformation structures. A Trommald Formation because of appreciable hematite.

4 particularly pronounced example occurs at 1087 feet very dark green to black, laminated silicate-carbonate where a laminated chert layer is deformed into a doubly­ magnetite iron-formation. Layers with granular textures plunging fold. Magnetite-rich iron-formation directly are abundant, as are layers and lenses of chert as much above that layer is folded and broken by rip-up structures. as 20 mm thick. The upper part of this interval is marked The ripped-up clasts in turn are fractured, and the by intercalated layers of monomineralic chert and fractures are filled with sulfides. Granular fabrics are magnetite. Much of the chert is relatively pure and is common above 1095 feet where conglomeratic intervals either porcellanitic or has a recrystallized granular texture. and thin chert-rich laminae are sparingly present. Sulfides Some of the thicker beds are zoned with fine-grained occur sparingly as disseminated grains in the chert. At interiors and recrystallized rims; the rims contain 1083 feet, dark granules of uncertain mineralogy are disseminated sulfides. In many parts of the A6 interval, embedded in a matrix consisting of quartz and sulfides beds of chert pass upward into granule-rich beds and in equal amounts. This layer is truncated by a sulfide­ into layers of chert-pebble conglomerate. The chert beds bearing vein that lacks quartz. Carbonate-rich, straight are associated with a variety of synsedimentary veins are common and the interval 1082-1080 feet deformation structures involving various combinations contains numerous irregular mobilizates of quartz, iron of folding and faulting. Several kinds of veins cut the carbonates, and some sulfides. deformed layers. These include straight veins filled with Interval A4, between 1080 and 1049 feet, resembles quartz and carbonate. Late brittle fractures are filled interval A2. It contains layers of conglomerate and with stilpnomelane and sulfides, whereas discordant riedel breccia, as well as beds of chert, which are as much as shear structures in chert are filled with sulfides. 40 mm thick. Conglomeratic layers 1.5 to 3 feet thick Breccia zones also are abundant in interval A6. are common and generally contain clasts of Most are thin and marked by sulfide-filled vugs.One intraformational origin, mostly chert. However one layer breccia, at 947 feet, contains angular clasts of chert and contains quartz-sericite clasts of Mahnomen affinity. The silicate-carbonate iron-formation set in a matrix of clasts in the thicker beds generally coarsen upward to a carbonate, hematite, and limonite. The clasts were maximum of about 5 cm. Many of the smaller clasts are deformed before the enclosing sediments were compacted. rimmed by chert and carbonate. Sulfides, together with The uppermost 7 feet of interval A6 is somewhat stilpnomelane, are present in some of the chert pebbles, oxidized, as shown by the first appearance at 950.3 feet but these minerals rarely occur in the matrix. Matrix of red colors that parallel bedding in green-laminated material consists mostly of chert admixed with silicate­ silicate-carbonate iron-formation. Additionally, much of rich intraclasts and structures that resemble oncolites. the magnetite is partially to completely altered to The latter have single- quartz cores and rims of hematite. stilpnomelane and carbonates. Interval A7, between 943 and 912 feet, is a transition Massive sulfides occur throughout AA, both as layers between thin-bedded, silicate-carbonate magnetite iron­ as much as 4 cm thick, and as discordant and concordant formation below and thin-bedded oxide iron-formation veins. Most of the layered occurrences are associated above. Below a depth of 935 feet hematite replaces with laminated and brecciated intervals. magnetite as in the upper part of the A6 interval, but Interval A5, between 1029 and 1019 feet, generally above that level, textural evidence implies that hematite lacks chert. The lower half consists of alternating layers is a primary phase. rich in chlorite, hematite, stilpnomelane, and martite Breccia or rubble zones are fairly common. intercalated with layers as much as 3 cm thick of martite Interestingly, the lowest of these occurs at a depth of after magnetite. The upper half is marked by centimeter­ 928 feet just within the oxidation boundary recognized thick beds that either have a granular texture or are by Dahl (1992). Chert units within rubble zones above internally laminated. Slightly martitized magnetite occurs 928 feet contain small amounts of idiomorphic, slightly as discrete lenses or pods. The upper 5 feet also contains oxidized sulfides, which imply that oxidation there may vugs, partly to completely filled with sulfides. Discordant be a secondary event. en echelon veins filled with coarse quartz and iron We also recognized several rock types not directly carbonates are fairly abundant. Those in the lower few related to the precipitation of iron-formation in Zone A feet also contain stilpnomelane. These include beds of chlorite schist and tourmaline­ Interval A6, between 1019 and 943 feet, resembles bearing sericite schist in the interval between 1049 and intervals A3 and A4 in consisting for the most part of 1029 feet (units X.l and X.2 in Appendix 1) and a

5 pronounced quartz-carbonate vein in the interval between texture, are spotted with hematite, and contain 997 and 995 feet. idiom orphic grains of martite as much as 2 mm in size. Light-green, generally structureless chlorite schist Hematite iron-formation also contains thin laminae of between 1049 and 1045 feet most likely represents martite oriented parallel to a foliation. Some parts of carbonized tuff of mafic to intermediate composition. The the interval are altered and leached to argillized, gray to schist is spotted with small rhomb-shaped grains of light-reddish-gray iron-formation. carbonate that probably replace feldspar porphyroblasts. Interval B .3, between 854 and 830 feet is similar to Some rhombs are associated with small grains of biotite B.2 but differs by having thick intersects of shale or tuff. or tourmaline. Very thin hematite laminae define a weak As in B.2, hematite iron-formation contains laminae and foliation, especiaIly toward the top of the unit, and the streaks of martite, I to 2 mm thick, and chert beds have uppermost foot or so is marked by a diffuse reddish zone a granular texture. Some beds contain disseminated composed of hematite or oxidized iron carbonates. martite that passes transitionally into disseminated The lowest part of the tourmaline-bearing sericite hematite near contacts. schist is marked by red spots of altered titanite. However The interval is strongly brecciated and has a variety much of this schist is a gray to green, inhomogeneous, of replacement textures. Rubble zones marked by angular relatively coarse-grained sericite schist that carries in clasts of chert and massive hematite are as much as 2 addition to the tourmaline and titanite, elongate clasts of feet thick. Shaly or tuffaceous material is strongly sericite, carbonate, and hematite. The tourmaline grains argiIIized and partly replaced by hematite. Chert is and the oxides are aligned paraIlel to a weak foliation oxidized and leached; vugs are filled with manganese which defines a large angle with presumed master bedding and iron oxides and chert. Massive hematite is altered planes that separate iron-formation and chlorite schist. to reddish-brown goethite. The uppermost foot of Zone X consists of a laminated, Interval B.4, between 830 and 820 feet, also consists tourmaline-poor, chlorite-sericite schist marked by a dominantly of thin-bedded hematite iron-formation, but vuggy structure and a leached appearance. Many vugs is distinguished from underlying units by layers or lenses are filled with an iron carbonate. of yellowish-green chert and in oval Zone A is cut by abundant veins. In the interval structures as much as 1.5 mm in diameter. Thin-bedded between 997 and 995 feet, the largest vein cuts banding hematite iron-formation is martite-rich and partly replaced of the iron-formation at an angle of approximately 70°. by goethite. Beds of granular chert are also partly Interior parts of this vein contain coarse grains (2-5 mm) replaced by goethite and . The entire interval of brown carbonate and white quartz surrounded by is cut by thin, irregular veins filled with quartz, Mn radiaIly grown aggregates of stilpnomelane. Interestingly, carbonates, Fe oxides, apatite, and traces of barite. Wall where this vein intersects chert, the carbonate phases all rocks near the veins are porous, are dominated by but disappear, and stilpnomelane in long needles oriented hematite, and carry martite stringers that more or less perpendicular to vein walls becomes abundant. parallel the veins. Interval B.5, between 820 and 757 feet, consists of Zone B-Thin-bedded oxide iron-formation thin-bedded goethite-rich iron-formation that has a pronounced yeIlow-brown color. Individual beds 1 to 5 Zone B, between 912 and 757 feet, consists mm thick contain various proportions of fine-grained predominantly of thin-bedded hematite-goethite iron­ chert and goethite. The interval also contains beds of formation. The zone has been divided into six intervals. granular chert and conformable stringers of martite after Interval B.l, between 912 and 883 feet, contains magnetite. The goethite is clearly a replacement . appreciable silicate-carbonate iron-formation. However It occurs as needle-like oriented more or less it also contains many thin layers of massive hematite as perpendicular to bedding. Manganese oxides are sparsely well as scattered beds of chert that contain discrete grains present as small oval-shaped patches in beds of chert of martite after magnetite. Some of the beds of chert are and as thin films on bedding and surfaces. The repiaced by secondary hematite. entire interval has a porous and possibly leached Interval B.2, between 883 and 854 feet, is appearance; porosity measurements by Dahl and others distinguished from B.l by the presence of thick intersects (1992) indicate a drastic increase in porosity compared of finely laminated hematite iron-formation intercalated to thin-bedded iron-formation in the lower part of the with layers of massive hematite or chert. Many of the formation. chert layers are as much as 5 cm thick, have a granular

6 Interval B.5 contains two short intervals of thin­ halos (mostly <5 mm) around the veins, at places bedded, hematite-martite iron-formation, both penetrating bedding planes for several millimeters. Zoned characterized in Appendix 1 as Unit B.6. They occur veins, 1-2 cm wide, occur at several places. They have between 820 and 810 feet and 800 to 794 feet. Both are manganese oxide-rich cores and goethite-hematite-rich dominated by hematite and martite. The martite is rims. However apart from such local signs of remobilized particularly abundant in closely spaced laminae as much manganese, the interval between Mn 1 and Mn2 is as 5 mm thick. Thin-bedded hematite iron-formation generally poor in manganese. appears decomposed and is typically friable. Porosity The interval between Mnl and Mn2 also contains 9 values are high and susceptibilities vary considerably over feet of rubble marked by a red or red-brown, friable, short distances. hematite- and goethite-bearing breccia. The breccia has Massive quartz veins typical of the upper part of the a variety of complicated textures including and iron-formation-and especially Zones C and D-first open cleft (shrinkage?) structures filled with manganese appear at 817 feet. They are typically associated with oxides. hematite or goethite rubble zones and with transitional The second of the stratiform, iron- and manganese­ intervals between hematite and goethite. rich intervals, Mn2, is 5 feet thick and consists predominantly of dark-green to black manganese-oxide­ Zone C-Thin-bedded manganese-oxide iron­ bearing goethite ore. It is underlain by medium-thick formation beds of hematite and overlain by thick beds of hematite above. The latter has a pronounced porosity and bleached Zone C, between 757 and 568 feet, is generally appearance. Subordinate amounts of occur similar to Zone B, but is distinguished from it by lenses in small granules embedded in a goethite-rich oxide layer and layers of manganese oxides and by two stratiform at 644-637 feet. This layer is extensively brecciated and intervals marked by layers that almost entirely consist of veined by quartz, especially at bounding contacts. admixed iron and manganese oxides. They are termed A variety of replacement textures involving goethite Mnl and Mn2 (Appendix 1). These stratiform intervals after hematite are found in Zone C, generally above 634 are typically associated with brecciated intervals of feet. Masses of goethite discordantly replace thin-bedded hematite-rich iron-formation cut by numerous veins of iron-formation and related manganese oxides and are quartz. veined in turn by a second generation of manganese Mn l, the lower of the stratiform intervals, is a oxides. Thus the distinction between "late" replacement composite unit consisting of 3.6 feet of admixed goethite textures is difficult to impossible to recognize in many and manganese oxides, overlain by 13 feet of thin-bedded small-scale situations. The replacement of hematite by goethite iron-formation that contains variably thick lenses goethite above 634 feet seems to mark a second oxidation and pods of manganese oxide and thin brecciated layers front, possibly related to surficial processes. marked by fragments of goethite and manganese oxides. The uppermost beds of Zone C or conversely the Goethite iron-formation is in turn overlain by 7 feet of lowermost beds of Zone D include a thin layer of fine­ alternating beds of manganese-rich and iron-rich oxides. grained volcanoclastic or terrigenous material, almost The ore is very hard and has a metallic luster because of completely decomposed to green and gray clay minerals. appreciable manganite. On a microscopic scale the ore exhibits a very irregular crenulation, in part due to the Zone D-Thick-bedded manganese-rich oxide iron­ replacement of magnetite by martite. Manganite-rich formation layers are disrupted by veins filled with coarser grained manganite. The overall appearance is that of a strongly Zone D, between 568 and 258 feet, is typically thick deformed and altered, but primary oxide precipitate. bedded. Beds of granular chert as much as 20 cm thick Mn 1 is overlain by interlayered hematite- and are interlayered with meter-thick intervals of hematite goethite-rich, thin-bedded iron-formation interlayered iron-formation. The latter may be either structureless or with thin and commonly broken beds of chert. Granular finely laminated. Thin-bedded goethite-rich iron­ ooids and synsedimentary slump structures are common. formation, which dominates Zones Band C, and Manganese oxides occur in 3- to 4-mm clots and as thin pronounced layers of manganese oxides, which dominate discordant veins that follow joints that transect the Zone C, are lacking. The manganese-bearing intervals bedding at a large angle. Manganese oxides form narrow in Zone D are typically stratiform breccias, which have a variety of replacement textures.

7 The least destructive replacement textures in Zone laminated iron-formation; the latter contains scattered D occur as discrete, globular pods and elongated lenses beds of granular chert. The interval also contains a of goethite and manganese oxides that generaIly foIlow variety of breccia zones where gnarled ore is common sedimentary bedding planes. More extreme textures and where crystaIline manganite replaces iron-formation involve the nearly total replacement of iron-formation along bedding planes. However, manganese oxides occur by masses both of earthy, ocherous yeIlow-brown and of predominantly as earthy materials in the breccia zones. hard, brown goethite. The goethite in turn is replaced by The breccia zones also contain appreciable secondary irregular "blotchy" masses of manganese oxides. Extreme carbonates, including rhodochrosite that is partly replaced replacement results in the development of "gnarled ore" by manganese oxides. which Schmidt (1963) describes as bodies of more or Unlike the underlying strata, interval DA, between less ore that enclose masses of smaIl ore fragments 383 and 303 feet, contains thick beds of chert that are separated by many voids apparently formed by shrinkage. intercalated with beds of martite iron-formation, Schmidt further distinguishes between "columnar" gnarled structureless oxides, and breccia zones of variable and "ellipsoidal" gnarled ore. Columnar ore consists of thickness. The chert has a yellow, pink, or brown color parallel columns of goethite, whereas eIlipsoidal ore and a granular texture; many beds are replaced by iron resembles closely spaced cross sections of cabbage (0.5 and manganese oxides and by pink and white carbonates. - 5.0 cm in diameter), where the leaves are plates of Martite iron-formation contains nodules several goethite (Dahl and others, 1992). Although gnarled ore millimeters in diameter of manganomelane. Oxide-rich is clearly a post-deformative phenomenon, its origin is intervals include beds of hematite or admixed goethite unknown. and manganese oxides. They are cut by composite Zone D is subdivided into five intervals, based on veinlets of quartz and carbonates. Breccia zones contain the presence or absence of chert and the nature of the either goethite and quartz or manganese oxides, goethite, replacement phenomena. Interval D.I, between 568 and quartz, and carbonate. Mixed manganese oxide-goethite 536 feet, lacks massive chert. It is characterized by thick­ assemblages have gnarled, mottled, or massive textures. bedded hematite iron-formation interlayered with chert, Interval D.5, between 303 and 258 feet, also contains and by structureless lenses of admixed manganese oxides, very thick beds of granular chert, much of which has a goethite, and hematite. The oxide lenses are broadly porous appearance. Relict granules of uncertain stratiform, but clearly replace original iron-formation on composition are replaced by quartz, carbonate, and both the micro and macro scales. acicular silicates. Fibrous manganese oxides intergrown Interval D.2, between 536 and 438 feet, also lacks with acicular goethite in turn are replaced by a second appreciable chert, and much of it consists of thick beds generation of manganese oxides that form massive of laminated hematite iron-formation. Laminations are aggregates. defined by changes in the grain size and the composition Manganese oxide-rich intervals include breccias filled of chert-rich, hematite-rich, and manganite-rich laminae. with masses of soft, yellow-brown and hard, brown Much of the microlayering is disrupted by synsedimentary goethite. Some breccia also contains relict clasts of slump features. argillized, thin-bedded hematite iron-formation that are Manganite and hematite are chiefly secondary phases interlayered with bedding-conformable layers of that replace an earlier phase, possibly or manganese oxides. Other oxide-rich breccias are marked . Veins of coarse manganite and pyrolusite by gnarled textures that contain massive heterogeneous are relatively abundant. Manganese oxides also occur domains of limonite and several manganese oxides within brecciated, cataclastic zones generally less than a including psilomelane, pyrolusite, and fine-grained, meter thick. The manganese and iron oxides typically possibly amorphous phases. Coarse 'manganite occurs as are segregated into ellipsoidal "nodules" which overprint veins, as well as in vugs. The vugs also contain quartz a weak layering. Quartz occurs in aggregates between or chert. nodules and in veins also filled with or acicular stilpnomelane. Zone E-Thick-bedded oxide iron-formation Interval D.3, between 438 and 383 feet, also lacks appreciable chert, but contains abundant manganese. The Zone E, between 258 and 202 feet, consists of interval consists of thick-bedded hematite iron-formation intercalated beds of white to gray granular chert and intercalated with strongly oxidized, light-colored intersects of more or less monomineralic hematite. Individual beds are typically structureless and range in

8 thickness from a few millimeters to more than 15 cm; summarized in Appendix 1 show that the iron-formation most average 5 cm in thickness. There is no simple can be subdivided into three broad stratigraphic intercepts paragenetic relationship between the chert-rich and the characterized by: (1) large values generally greater than hematite-rich layers. Beds of hematite fill depressions 1000 cgs and maximum values around 70,000 cgs; (2) in the upper surfaces of several chert layers. Beds of intermediate values in the range 500-1000 cgs; and (3) hematite also contain rounded or elliptical clasts of chert small values generally less than 500 cgs. which are deformed and partly dissolved. In contrast, Especially large values of the first intercept generally beds of granular chert contain clasts and irregular laminae correspond to unoxidized, thin-bedded iron-formation in of hematite. Zone A. Five distinct maxima recognized in Zone A Laminae of brown to green phyllite mark the correspond to stratigraphic intervals exceptionally rich uppermost part of Zone E and become more prominent in magnetite. The second stratigraphic intercept, toward the contact with the overlying Rabbit Lake generally corresponds to oxidized, thin-bedded iron­ Formation. They probably represent tuffaceous material formation at the top of Zone A and all of Zones Band added to the depositional basin toward the end of the C. In detail, the lower part of Zone B has very small Trommald time. susceptibility values, whereas the remainder of that zone Many of the replacement textures that characterize and all of Zone C have mixed intermediate values. Many Zone D are lacking in Zone E. Both hematite and chert of the intermediate values toward the lower end of the are altered to goethite, and replaced by it in a few places. spectrum correspond to hematite-rich intervals that Manganese oxides are only sparingly present as small contain martite after magnetite. In contrast, values toward pods or as thin films along fractures in altered intervals. the upper end of the spectrum correspond to goethite­ A weakly cemented hematite-rich rubble zone at 243 to rich intervals that also contain martite and magnetite. 240 feet also contains sparse amounts of manganese Small susceptibility values of the third stratigraphic oxides. intercept correspond to the lower part of Zone B, to much of Zones D and E, and to the lowermost few feet of Rabbit Lake Formation Zone A, to tourmaline-bearing chlorite schist, and to sulfide iron-formation in the Rabbit Lake Formation. The Rabbit Lake Formation, intersected between There is no single, simple explanation for the depths of 202 and 85 feet, is a heterogeneous interval observed susceptibility values. It is possible that they consisting of thinly laminated phyllite, pyritic argillite, reflect the extent to which magnetite is preserved. sulfide iron-formation, oolitic hematite-chert iron­ However, such a correlation was not found in polished formation, and layers of laminated tuffaceous material. sections of the core. It is also possible that the values Sulfides are abundant between 193 and 186 feet, where reflect martite that retained some of the magnetic pyrite-rich laminae occur in packages 0.5 to 6 cm thick. character of the magnetite as it was oxidized. Banerjee Pyrite occurs as finely disseminated grains with relict (1991) has shown that under equilibrium conditions and framboidal textures; as recrystallized and rotated at temperatures between 400° and 600°C, magnetite centimeter-size nodules along with arsenopyrite and oxidizes to hematite or alpha-Fe203. However, at lower arsenian pyrite; and as large idiomorphic crystals temperatures (<300° -400°C), under nonequilibrium associated with quartz (chert?)-rich layers and veins. conditions, and especially in the presence of hydrothermal Phyllite consists of microcrystalline quartz and muscovite. solutions, magnetite oxidizes to a cation-deficient , The latter occurs in two generations-older fine-grained , or gamma-Fe203. Although maghemite is sericite, and younger mica flakes elongated in the foliation an important carrier of natural remanent magnetization direction. Units of porous oolitic (probably ) (NRM), it is a metastable phase, which upon heating hematite-chert or granular hematite-chert iron-formation inverts rapidly to hematite. Furthermore polished sections are intercalated with graded clastic intervals of terrigenous show that hematite rather than maghemite is the stable origin and with beds of fine-grained, green laminated oxidation product in Zones Band C. Thus maghemite tuff of the "pietra verde" type. cannot be responsible for the observed susceptibility values. As a third possibility, Stacey and Banerjee (1974, MAGNETIC SUSCEPTIBILITY p. 40) note that goethite, although an antiferromagnetic mineral, may acquire a thermal remanent magnetization Magnetic susceptibility was measured on more than (TRM) when cooled through the Neel point (120°C). 600 samples using a hand-held susceptibility meter. Data

9 Thus it is possible that the large susceptibility values in show that three broad intervals can be distinguished Zones Band C are related to the presence of goethite between 202 and 790 feet. These are: that formed at some temperature above the Neel point, (1) The interval between 202 and 258 feet, or rather than to the presence of relict magnetite. essentially Zone E. It is iron rich and manganese poor In contrast to the situation in Zones Band C, polished with very small MnlFe ratios «<0.01). The interval sections show that samples of Zone D having large consists of admixed hematite and chert as indicated by a susceptibility values carry incompletely oxidized strong negative correlation between SiOz and Fe(T)(R=- magnetite. Goethite in Zone D is not associated with 0.73). Somewhat elevated alumina values imply that the large susceptibility values. Instead it occurs with composition is influenced by extrabasinal material secondary manganese oxides in obvious crosscutting possibly of volcanogenic or terrigenous origin. relationships, and most likely formed during a late-stage (2) The interval between 258 and 580 feet, or Zone mobilization event. Thus, the goethite in Zones Band C D and part of Zone C, is rich in both iron and manganese appears to be an early, higher-temperature phase, whereas (as much as 27 wt. percent MnO) with variable MnlFe that in Zone D appears to be a late, lower-temperature ratios (0.56-0.85). Three popUlations of manganese values phase. It also appears that susceptibility is a useful guide can be distinguished: (1) a manganese-poor «2 wt. in delineating the two phases. percent MnO) population marked by widely scattered SiOz (15-85 percent) and Fe203 (50-70 percent) values; GEOCHEMISTRY (2) a manganiferous (2-7 wt. percent MnO) population marked by scattered values of Si02 (10-70 percent) and The results of two different geochemical surveys are Fe203 (20-75 percent); and (3) a highly manganiferous summarized here. These are the major element analyses population with 7-27 percent MnO and lower values for of 108 5-foot intersects by the U.S. Bureau of Mines Fe203 (20-50 percent) and Si02 (20-60 percent). according to methods described by Dahl and others Manganese and silica show a negative correlation in the (1992). Their analyses extend over the manganese­ third population. A few samples from it have elevated bearing parts of the core from 202 to 790 feet (Table 1). potassium values (as much as 2.57 wt. percent K20), Additionally, 22 especially manganese-rich samples that implying the possible presence of cryptomelane; were selected for leach tests were analyzed by the Bureau potassium also vaguely correlates with aluminum, for their major element constituents, as well as Pb and , and , implying the presence of As (Table 2). In order to complete the geochemical manganomelane group minerals which also may characterization of the Gloria core below 790 feet, and incorporate aluminum. A good correlation also exists to obtain trace element and rare earth element data from between barium and potassium in manganese-rich leach­ well-defined domains, 39 visually homogeneous samples test samples that have low KzO values. This implies the were selected and analyzed as part of this report. The possible presence of , romanechite, or data, techniques employed, and detection limits are psilomelane. The interval between 280 and 520 feet summarized in Table 3. also contains considerable magnesium and . Although the data summarized in Tables 1-3 were Calcium values (as much as 5.6 wt. percent CaO) are obtained using considerably different protocols, plots of weII above background between 288 and 515 feet. CaO concentration vs. depth (Fig. 3) show consistent results and MnO show a positive correlation implying the that do not differ significantly for most elements. presence of a manganiferous calcite, the carbonate most Exceptions include sodium which was not detected by likely present as matrix material in manganese-rich the analytical methods used by the USBM. Additionally breccia. Magnesium values (as much as 5 wt. percent we could not confirm some of the high phosphate values MgO) are well above background between 303 and 504 reported by the Bureau in some of the manganese-rich feet. Magnesium does not correlate with CaO or Fe203, parts of the core. but does correlate slightly with MnO and Ab03 and As noted previously, the USBM divided the Gloria distinctly with LOI (400°C). These correlations imply core into three intersects based on bedding the presence of either some Mg-Mn carbonate and/or characteristics-thick-bedded, mixed thick- and thin­ some Mg-bearing aluminosilicate such as stilpnomelane. bedded, and thin-bedded. Starting at the intersect The transition between intervals 1 and 2, which boundaries, one-quarter portions of 5-foot intervals were corresponds to the transition between Zones E and D, is crushed and a 100-gram split was analyzed. These data marked by an abrupt upward decrease in MnO values, a

10 Table 1. Whole rock chemical data from the Gloria core (data from Dahl and others, 1992) Interval 202-203 203-208 208-213 213-218 218-223 223-228 228-233 233-238 238-243 243-248 248-253 253-258 258-263 263-268 depth (m) 61.9 63.4 64.9 66.4 68.0 69.5 71.0 72.5 74.1 75.6 77.1 78.6 80.2 81.7 Si02 87.91 60.11 59.25 51.55 40.00 29.52 51.12 65.03 35.29 25.24 61.82 59.89 72.73 65.67 Ti02 0.08 0.18 0.18 0.15 0.13 0.12 0.07 0.02 0.12 0.17 0.03 0.02 0.02 0.05 AI203 4.16 6.23 5.86 4.72 4.34 3.78 2.08 1.53 3.97 6.04 1.47 0.60 0.49 1.51 Fe203(T) 4.86 30.16 30.73 39.31 50.18 58.75 37.74 26.88 51.46 57.47 28.30 31.74 18.58 23.59 FeO 0.30 0.30 0.30 0.30 0.30 <0.01 0.45 0.30 <0.01 <0.01 <0.01 0.30 <0.01 <0.01 MgO 0.13 0.02 <0.02 0.05 0.05 0.03 0.08 0.07 0.13 0.05 0.07 0.03 0.02 0.05 MnO 0.01 0.03 0.01 0.04 0.09 0.17 0.15 0.14 0.40 0.59 1.07 0.14 0.53 0.39 CaO 0.08 0.08 0.08 0.08 0.18 0.17 0.08 0.08 0.11 0.17 0.15 0.08 0.66 0.21 K20 0.31 <0.06 <0.06 <0.06 0.07 0.07 <0.06 <0.06 <0.06 <0.06 <0.06 <0.06 <0.06 <0.06 Na20 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 P20 5 0.06 0.19 0.17 0.15 0.19 0.27 0.21 0.11 0.17 0.22 0.16 0.08 0.64 0.23 LOIlOOO 1.80 2.90 2.80 2.40 2.70 2.90 4.30 4.80 4.50 5.00 3.10 3.60 2.40 3.90 L0I400 0.60 0.68 0.61 0.62 0.87 1.30 2.60 2.30 2.80 2.50 1.80 2.90 1.80 3.00 Total 99.42 99.96 99.17 98.52 97.94 95.78 95.90 98.71 96.22 95.01 96.24 96.25 96.12 95.65 - - Table 1. (continued) Interval 268-273 273-278 278-283 283-288 288-293 293-298 298-303 303-308 308-313 313-318 318-323 323-328 328-333 333-338 depth (m) 83.2 84.7 86.3 87.8 89.3 90.8 92.4 93.9 95.4 96.9 98.5 100.0 101.5 103.0 Si02 54.12 59.25 26.52 58.61 35.08 34.87 50.91 33.15 20.11 27.59 25.24 27.59 46.42 23.53 Ti02 0.02 0.02 0.05 0.07 0.05 0.05 0.03 0.07 0.05 0.10 0.18 0.10 0.07 0.08 A1203 0.49 1.08 1.45 1.89 1.17 1.25 0.66 1.13 1.04 2.27 4.72 1.83 0.93 1.25 Fe203(T) 20.44 23.30 35.60 25.02 39.17 41.03 29.88 40.31 33.45 37.60 45.60 47.60 39.60 39.31 FeO <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 MgO 0.03 0.08 0.12 0.05 0.13 0.13 0.12 0.22 0.20 0.27 0.83 1.13 1.18 0.71 MnO 14.33 8.00 21.30 4.91 12.65 7.49 4.91 11.62 23.37 10.46 4.52 11.75 3.87 14.85 CaO 0.45 0.08 0.08 1.96 0.64 2.94 2.24 0.25 4.19 3.91 3.22 2.38 2.52 3.08 K20 0.08 <0.06 0.25 0.10 0.23 0.10 0.11 0.10 0.42 0.12 <0.06 0.06 <0.06 0.08 Na20 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 P20 5 0.48 0.07 0.08 1.42 0.14 0.89 0.12 0.07 0.09 0.08 0.08 0.08 0.07 0.08 LOllOOO 5.30 5.10 9.30 5.10 7.50 7.70 6.20 7.70 12.30 11.30 11.50 10.90 6.80 11.20 LOI400 2.90 3.30 5.70 3.40 5.00 4.80 3.60 5.10 4.50 5.60 6.60 6.20 3.50 5.70 Total 95.74 97.04 94.76 99.11 96.76 96.44 95.17 94.62 95.22 93.70 95.95 103.42 101.51 94.17 Table 1. (continued) Interval 338-343 343-348 348-353 353-358 358-363 363-368 368-373 403-408 408-413 413-418 418-423 423-428 428-433 433-438 depth (m) 104.6 106.1 107.6 109.1 IlO.6 112.2 113.7 124.4 125.9 127.4 128.9 130.5 132.0 133.5 Si02 29.30 28.66 62.03 31.66 43.64 30.80 53.90 30.37 28.02 26.74 37.65 31.44 19.68 31.87 Ti02 0.08 0.08 0.07 0.08 0.05 0.07 0.03 0.08 0.08 0.10 0.32 0.05 0.00 0.07 AI203 1.68 2.08 2.46 2.83 1.59 2.08 1.38 2.08 1.89 1.89 2.27 1.36 0.94 1.68 Fe203(T) 40.74 37.88 25.45 38.03 30.88 38.03 23.73 34.31 31.74 36.88 33.59 43.46 34.74 40.17 FeO <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 MgO 1.61 0.98 0.86 0.65 0.70 0.66 0.95 1.66 0.81 0.85 1.46 0.70 0.17 0.88 MnO 12.26 14.98 6.97 11.62 9.30 10.72 8.26 17.43 21.95 19.24 12.39 10.97 26.08 12.26 CaO 1.40 0.94 1.38 1.68 3.50 3.91 3.50 0.67 0.63 0.45 0.69 0.31 1.82 0.08 K20 0.13 <0.06 <0.06 <0.06 <0.06 <0.06 <0.06 0.28 0.70 0.31 0.13 0.11 0.06 0.17 Na20 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 P20S 0.07 0.09 0.08 0.09 0.08 0.09 0.07 0.08 0.11 0.10 0.10 0.10 0.11 0.11 LOIIOOO 10.60 10.50 7.50 10.00 9.80 11.70 8.80 10.30 9.40 9.00 8.80 7.60 11.50 7.90 L0400 6.20 6.00 4.20 5.90 4.70 5.90 3.80 5.60 4.70 5.50 4.90 4.10 4.40 4.60 Total 97.89 96.25 106.86 96.70 99.57 98.12 100.68 97.26 95.33 95.55 97.39 96.10 95.09 95.19

...... tv Table 1. (continued) Interval 438-439 439-444 444-449 449-454 454-459 459-464 464-469 469-474 474-479 479-484 484-489 489-494 494-499 499-504 depth (m) 133.8 135.3 136.9 138.4 139.9 141.4 143.0 144.5 146.0 147.5 149.1 150.6 152.1 153.6 Si02 70.80 45.56 28.88 55.83 35.29 31.44 48.34 37.65 32.09 36.15 35.72 33.58 36.36 49.20 Ti02 0.10 0.13 0.07 0.07 0.08 0.20 0.10 0.12 0.12 0.10 0.12 0.13 0.08 0.10 Al203 2.08 1.70 1.34 1.45 1.89 2.64 2.27 2.27 2.46 2.08 2.46 3.02 2.27 2.27 Fe203(T) 15.58 24.16 46.32 24.73 26.73 34.02 27.16 26.73 33.16 28.59 28.59 28.73 35.74 31.02 FeO <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 MgO 3.15 0.75 0.50 1.43 0.91 0.91 1.23 1.03 1.49 1.11 0.86 1.03 0.48 0.48 MnO 2.71 15.88 10.72 4.78 19.24 16.91 9.17 16.40 14.98 16.78 17.43 17.17 11.75 7.49 CaO <0.08 0.14 0.89 1.40 0.27 1.17 0.32 1.68 0.75 0.77 0.87 1.54 0.88 1.40 K20 <0.06 0.34 0.19 <0.06 1.12 1.01 0.14 0.16 0.22 0.08 0.06 <0.06 <0.06 <0.06 Na20 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 P20S 0.05 0.08 0.99 0.80 0.15 0.15 0.12 0.12 0.11 0.09 0.14 0.10 0.10 0.08 LOIlOOO 6.10 8.00 7.20 6.50 9.20 10.00 7.20 10.10 10.70 10.00 9.70 10.50 8.50 6.70 LOI400 4.10 4.10 4.30 3.90 4.80 5.10 4.10 5.40 6.50 5.10 5.00 5.20 4.90 3.30 Total 100.71 96.74 97.09 97.04 94.88 98.47 96.05 96.24 96.08 95.76 95.94 95.87 96.22 98.79 Table 1. (continued) Interval 504-509 509-514 514-519 519-524 524-529 529-534 534-539 539-544 544-549 549-554 554-559 559-564 564-569 569-574 depth (m) 155.1 156.7 158.2 159.7 161.2 162.8 164.3 165.8 167.3 168.9 170.4 171.9 173.4 175.0 Si02 38.50 36.36 44.92 44.92 47.06 27.81 44.92 62.03 27.81 51.34 55.61 40.64 53.48 54.76 Ti02 0.12 0.10 0.12 0.08 0.08 0.10 0.12 0.17 0.18 0.20 0.17 0.23 0.90 0.95 Al203 2.83 2.83 2.83 2.27 2.46 2.27 3.02 4.16 4.72 4.53 4.34 6.42 9.26 8.69 Fe203(T) 33.45 38.03 30.31 33.88 39.31 35.31 33.02 23.44 50.60 25.87 26.73 33.02 28.02 28.88 FeO <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.13 MgO 0.27 0.18 0.08 0.08 0.12 0.10 0.12 0.05 0.07 0.05 0.02 0.17 0.15 0.13 MnO 11.36 9.68 10.72 10.72 2.97 20.01 9.17 4.26 5.03 12.65 7.36 8.26 1.81 2.19 CaO 2.38 1.40 0.21 0.11 0.70 0.10 0.13 0.11 0.10 0.11 <0.08 0.15 0.08 0.00 K20 <0.06 <0.06 <0.06 0.08 <0.06 0.33 0.14 <0.06 <0.06 <0.06 <0.06 0.23 0.10 0.18 Na20 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 0.08 <0.07 P20 5 0.10 0.12 0.09 0.08 0.14 0.09 0.10 0.11 0.14 0.08 0.10 0.14 0.10 0.09 LOIIOOO 9.20 9.00 6.80 6.30 5.10 8.10 7.30 5.10 7.10 5.60 5.00 7.90 5.90 5.40 L0400 4.20 5.20 3.90 3.60 3.70 4.60 4.70 3.00 4.80 2.30 2.40 4.40 2.90 1.80 Total 98.27 97.76 96.13 98.52 98.00 94.21 98.03 99.49 95.82 100.50 99.48 97.17 99.79 101.28

Table 1. (continued) -w Interval 574-579 579-584 584-589 589-594 594-599 599-604 604-609 609-614 614-619 619-624 654-659 659-660 660-665 665-670 depth (m) 176.5 178.0 179.5 181.1 182.6 184.1 185.6 187.2 188.7 190.2 200.9 201.2 202.7 204.2 Si02 46.84 25.03 23.96 34.65 40.21 26.74 34.22 33.58 41.07 16.68 37.01 16.04 30.37 44.71 Ti02 0.17 0.22 0.17 0.13 0.07 0.02 0.13 0.07 0.13 0.08 0.18 0.05 0.02 0.02 A1203 3.02 4.34 3.40 3.21 2.27 1.04 2.46 1.66 2.83 2.27 3.97 1.62 0.66 0.66 Fe203(T) 39.60 59.61 65.19 56.18 50.75 62.47 56.75 58.18 50.03 71.48 45.03 72.05 61.61 45.17 FeO <0.13 <0.13 <0.13 <0.13 <0.13 <0.13 <0.13 <0.13 <0.13 <0.13 <0.13 <0.13 <0.13 <0.13 MgO 0.15 0.22 0.23 0.15 0.15 0.15 0.12 0.10 0.10 0.15 0.05 0.07 0.03 0.02 MnO 3.87 1.42 1.23 0.76 0.58 0.48 0.32 0.48 0.23 0.76 5.03 0.52 0.23 0.15 CaO 0.10 0.10 0.11 0.08 <0.08 0.14 <0.08 0.39 0.53 0.13 <0.08 0.08 <0.08 0.11 K20 0.18 <0.06 <0.06 <0.06 <0.06 <0.06 <0.06 <0.06 <0.06 <0.06 <0.06 <0.06 <0.06 <0.06 Na20 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 P205 0.12 0.14 0.13 0.11 0.12 0.12 0.09 0.18 0.20 0.19 0.08 0.30 0.11 0.16 LOIlOOO 6.10 8.40 8.10 7.00 7.00 6.90 7.10 4.80 5.60 7.40 5.40 7.40 5.40 4.10 L0I400 3.60 5.30 5.40 4.40 4.40 4.80 4.80 2.90 3.40 4.70 3.10 6.40 4.80 3.20 Total 100.15 99.53 102.57 102.34 101.29 98.11 101.34 99.50 100.79 99.20 96.89 98.19 98.59 95.16 Table 1. (continued) Interval 670-675 675-680 680-685 685-690 690-695 695-700 700-705 705-710 710-715 715-720 720-725 725-730 depth (m) 205.7 207.3 208.8 210.3 211.8 213.4 214.9 216.4 217.9 219.5 221.0 222.5 Si02 51.76 45.13 50.27 43.21 37.22 31.44 32.94 2S.66 45.99 41.28 57.54 22.03 Ti02 0.03 0.10 0.10 0.Q7 0.13 0.47 0.12 0.10 0.12 0.15 0.12 O. [0 AI203 0.70 2.64 2.S3 1.21 3.59 5.29 2.46 2.0S 2.S3 2.64 3.40 2.46 Fe203(T) 39.03 44.60 39.SS 50.46 50.46 50.S9 54.75 56.32 43.74 50.46 35.02 64.47 FeO <0.13 <0.13 <0.13 <0.13 <0.13 <0.13 <0.13 <0.13 <0.13 <0.13 <0.13 <0.13 MgO 0.08 0.Q7 0.05 0.03 0.10 0.12 O.OS 0.13 0.10 0.03 0.03 0.10 MnO 0.13 0.12 0.13 0.26 1.02 0.65 0.56 0.66 0.34 0.54 0.27 l.OS CaO 0.22 0.08 O.OS 0.13 0.34 0.24 0.15 0.41 0.15

>-' +"-

Table l. (continued) Interval 730-735 735-740 740-745 745-750 750-755 755-760 760-765 765-770 770-775 775-780 7S0-785 785-790 depth (m) 224.0 225.6 227.1 22S.6 230.1 231.7 233.2 234.7 236.2 237.7 239.3 240.S Si02 S.9S 11.55 IS.40 24.39 IS.IS 25.67 37.22 32.51 41.71 29.95 27.S1 22.25 Ti02 O.OS 0.13 O.lS 0.10 0.10 0.Q7 0.07 0.07 0.10 O.OS 0.07 0.10 AI203 2.64 4.16 4.91 4.72 2.0S 1.32 1.0S 1.15 1.72 1.02 1.23 1.3S Fe203(T) 72.33 6S.04 66.76 57.75 71.33 64.47 56.S9 5S.IS 52.32 63.33 65.19 71.19 FeO <0.13 <0.13 <0.13 <0.13 <0.13 <0.13 <0.13 <0.13 0.30 0.44 0.44 0.59 MgO O.OS 0.12 0.1 0 0.12 0.23 0.13 0.07 0.15 O.OS O.OS O.Q7 O.OS MnO 4.39 4.7S 0.S4 5.29 0.75 1.42 0.21 1.55 0.26 0.26 0.15 0.12 CaO 0.22 0.43 O.OS 0.24 O.ll 0.14 0.10 0.S5 0.15 0.24

Table 2. (continued) -Vl Sample 6B 7T 7B 8T 8B 9T 9B lOT lOB lIT lIB Depth (ft) 388 391 391 428 428 471 471 505 505 536.6 536.6 Depth (m) 118.3 119.2 119.2 130.5 130.5 143.6 143.6 153.9 153.9 163.6 163.6 Si02 8.13 27.38 28.02 16.47 5.99 2l.82 18.82 30.37 34.22 42.78 32.09 A1203 l.89 2.46 l.61 l.30 2.27 1.89 3.97 3.59 4.34 1.89 3.78 Fe203 47.60 41.60 38.17 25.45 26.02 34.59 33.59 44.03 38.17 3l.45 30.02 FeO <0.12 <0.12 <0.12 <0.12 <0.12 <0.12 <0.12 <0.12 <0.12 <0.12 <0.12 MgO 0.08 2.16 0.36 <0.02 <0.02 0.76 <0.02 0.12 <0.02 <0.02 MnO 24.79 16.40 19.11 43.64 48.93 26.85 25.05 7.75 12.01 15.49 21.95 CaO 0.73 0.52 0.29 <0.08 <0.08 0.39 0.39 1.96 0.62 <0.08 <0.08 K20 0.25 0.16 0.27 0.16 0.19 0.11 2.53 <0.06 <0.06 Na20 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 <0.07 P20 S 0.23 0.21 0.21 0.16 0.16 0.11 0.11 0.25 0.32 BaO 0.31 0.26 0.31 0.33 0.41 0.19 0.16 0.26 0.25 1.12 1.12 LOI 15.3 11.9 9.0 7.9 11.4 9.3 11.2 9.6 9.1 7.0 9.0 Total 99.31 103.02 97.34 95.51 95.47 96.02 95.82 97.88 99.20 99.83 98.05 As ppm 126 62 80 112 62 59 95 27 17 120 82 Pb ppm 84 2.2 64 186 66 13 11 3 3.8 74 56 Li ppm <25 <25 F ppm <50 <50 Cl ppm 35 45 Table 3. Chemical data from the Gloria core [Major elements in wt percent oxides; minor elements in ppm; Au and Ir in ppb] Sample G 193 G 215 G 237 G 287 G 299 G 416 G 451 G 481 G 527 G 642 G 735 G 750 G 769 Zone FEE D.5 D.5 D3 D2 D2 D2 C C-Mn 1 C-Mn 1 B.5 Sulf-IF Hm-IF Chert Mn-Fe ore "Chert" Mn-Fe ore Oxide-IF Fe-Mn ore Chert Fe ore Fe-Mn ore Fe-Mn ore Oxide-IF SiO, 33.24 20.09 95.80 11.47 43.10 20.38 33.80 35.65 82.99 10.61 2.78 2.97 24.89 TiO, 0.22 0.26 0.03 0.05 0.06 0.12 0.09 0.05 0.05 0.08 0.09 0.09 0.05 Aha, 4.64 6.45 0.57 1.35 1.56 2.44 2.09 1.47 1.40 1.91 2.12 2.02 1.18 Fe,03(T) 35.76 67.82 1.35 33.68 38.56 35.08 27.64 33.54 4.27 73.94 66.80 64.37 59.62 MnO 0.06 0.03 0.02 42.76 2.60 28.70 26.46 16.78 0.06 1.01 11.62 14.52 3.27 MgO 0.58 0.35 0.28 <0.01 038 1.24 2.03 1.29 0.99 0.53 0.09 0.09 0.22 CaO 0.62 0.41 0.25 0.28 3.88 0.74 0.33 1.19 5.38 0.78 0.43 5.04 2.23 Na,O 0.48 0.19 0.18 0.15 0.32 0.02 0.13 0.10 0.09 0.09 0.14 0.17 0.11 K,O 1.36 0.03 0.01 0.45 0.32 0.30 0.01 0.27 0.02 0.01 0.05 0.14 0.04 P,O, 0.48 0.22 0.02 0.22 0.16 0.44 0.11 0.19 1.93 0.36 0.54 3.61 036 LOI 20.14 2.95 0.45 9.17 7.84 9.98 6.89 8.67 2.57 9.08 14.93 6.67 7.84 Total 97.58 98.80 98.96 99.58 98.78 99.44 99.58 99.20 99.75 98.40 99.59 99.69 99.81

Be (2) <2 <2 <2 4 3 6 <2 4 <2 4 <2 <2 <2 B (4) S (3) 265100 Sc (4) 7.9 7.9 0.2 1.3 2.7 29 1.6 0.4 2.4 2.1 22 1.3 V (2) 724 412 2 2fj 31 40 15 35 9 34 28 40 22 Cr (4) 58 61 11 9 13 13 16 16 14 17 16 11 20 Co (4) 9) 3 1 24 9 24 10 28 1 13 22 19 18 Ni (2) 150 15 6 17 12 18 5 2fj 2 25 21 14 13 Cu (2) 173 8 4 53 34 12 10 44 4 8 21 12 10 Zn (2) 30 25 2 30 18 55 9 37 2 36 36 38 24 Ga (3) <2 As (4) 230 31 2 80 41 79 18 32 <2 70 59 64 120 Se (4) 230 <3 <3 <3 <3 <3 <3 <3 <3 <3 <3 <3 <3 Br (4) <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 Rb (4) 20 <10 <10 <10 <10 <10 Rb (3) 18 Sr (I) 1096 113 3 220 76 434 139 419 71 9) 204 472 54 Y (I) 8 8 <1 15 6 38 16 12 10 12 23 115 17 Zr (I) 124 44 7 39 18 22 59 21 25 46 53 37 32 Nb (3) 3 Mo (4) 57 10 <5 23 7 15 6 12 5 <5 <5 9 <5 Ag (2) 1.1 <0.4 <0.4 23 0.5 2.0 2.9 0.9 <0.4 <0.4 2.0 22 <0.4 Cd (2) <0.5 4.8 <0.5 <0.5 <0.5 0.5 <0.5 2.0 <0.5 4.6 <0.5 8.0 3.8 Sn (3) <5 Sb (4) 2.9 02 <0.2 03 0.5 0.4 0.4 02 <0.2 0.3 0.3 0.7 0.8 Cs (4) <0.5 <0.5 <0.5 <0.5 <0.5 0.6 <0.5 <0.5 <0.5 <0.5 <0.5 1.0 0.5 Ba (I) 446 87 46 18409 1122 5236 145 3549 72 186 231 226 61 Hf (4) 1.6 0.9 <0.5 <0.5 <0.5 l.l 0.6 0.6 <0.5 <0.5 <0.5 <0.5 <0.5 Ta (4) <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1

La (4) 7.1 16.9 0.4 14.3 7.7 30.5 43.9 10.3 9.3 5.1 14.9 31.2 12.3 Ce (4) 13 31 <3 20 11 39 81 30 21 12 T7 56 24 Nd (4) 6 12 <5 9 5 19 36 9 9 <5 8 33 9 Sm (4) l.l 1.8 <0.1 1.8 0.8 3.9 4.7 1.7 0.9 0.7 1.6 5.5 1.4 Eu (4) 0.4 0.6 <0.1 0.8 0.3 1.7 1.5 0.7 0.4 0.2 0.6 2.0 0.5 Gd (4) Tb (4) <0.5 <0.5 <0.5 <0.5 <0.5 0.8 0.5 <0.5 <0.5 <0.5 <0.5 12 <0.5 Yb (4) 1.0 0.7 <0.1 1.0 0.5 22 12 0.9 0.4 1.0 1.0 2.9 0.9 Lu 4 0.20 0.12 <0.05 0.15 0.09 0.25 0.17 0.14 <0.05 0.16 0.17 0.46 0.16 1, Fusion-ICP; 2, total digestion; 3, XRF pellets; 4, INAA

16 Table 3. (continued) Sample __G~80~5 __ ~G~8~1~5 ___G~82~8~.5 ___G~83~6 __ ~G~8~5~3 __~G~8~55 ____ G~88~1 __~G~8793~ __G~92~4~~G~9~3~5 __~G~9~50~ __G~96~6 __ ~G~9~7~2 __ Zone B.5 B.6 B.5 B.4 B.4 B.3 B.3 B.2 A7 A7 A6 A6 A6 Oxide-IF Oxide-IF Oxide-IF Oxide-IF Oxide-IF Oxide-IF Oxide-IF Oxide-IF SCO-IF SCO-IF SCM-IF SCM-IF SCM-IF SiO, 41.71 20.75 50.49 53.84 24.51 35.19 10.09 38.32 48.15 36.44 38.03 45.11 57.01 TiO, 0.06 0.05 0.04 0.05 0.07 0.07 0.08 0.31 0.19 0.10 0.18 0.09 0.04 AWl 1.39 1.11 1.43 1.45 1.45 2.56 2.31 7.30 5.76 3.11 5.97 2.79 1.90 Fe,Ol(T) 53.15 73.42 42.86 35.48 55.15 52.37 82.16 47.83 28.26 48.24 37.69 42.46 22.81 MnO 0.08 0.34 0.21 0.09 2.74 0.18 0.18 0.04 1.01 0.93 0.90 0.30 0.78 MgO 0.14 0.06 0.15 0.22 0.29 0.37 0.09 0.36 2.70 2.23 2.69 1.36 1.77 CaO 0.31 0.26 0.19 0.35 1.05 0.45 0.81 0.62 3.52 3.79 2.67 1.99 4.09 Na,O 0.13 0.09 0.09 0.17 0.17 0.12 0.17 0.15 0.20 0.23 0.47 0.55 0.46 K,O 0.03 0.05 0.01 0.06 0.05 0.31 0.02 0.02 0.96 0.73 0.43 0.53 0.43 P,o, 0.23 0.17 0.13 0.16 1.16 0.19 0.60 0.38 0.09 1.49 0.56 0.51 0.09 WI 2.29 1.74 4.19 7.79 12.97 8.20 2.34 4.35 8.37 1.22 10.09 4.09 8.88 Total 99.52 98.04 99.79 99.66 99.61 lOOm 98.85 99.68 99.21 98.51 99.68 99.78 98.26

Be (2) <2 4 3 8 7 2 3 5 2 3 3 4 B (4) S (3) <50 <50 <50 <50 390 Sc (4) 1.3 1.2 1.2 2.2 2.8 2 2 4.9 4.8 2.1 4.9 2J 1.3 V (2) 25 29 24 3J 24 32 44 43 35 21 31 21 13 Cr (4) 7J 16 17 Al 22 Al 26 43 39 18 42 21 15 Co (4) 12 10 13 10 38 10 9 5 12 8 13 8 4 Ni (2) 9 15 16 13 Al 8 14 10 Al 10 19 8 7 Cu (2) 8 8 71 17 13 7 12 13 6 25 3 5 13 Zn (2) 8 9 33 15 45 3 5 6 25 12 3J 14 6 Ga (3) <2 <2 <2 <2 <2 As (4) 14 71 21 25 32 81 100 82 21 15 <2 12 4 Se (4) <3 <3 <3 <3 <3 <3 <3 <3 3 <3 <3 <3 <3 Br (4) 1 1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 Rb (4) <10 <10 <10 <10 <10 18 <10 Rb (3) 44 18 10 11 9 Sr (1) 11 ff) 13 17 50 44 64 49 53 61 33 25 21 Y (1) Al 19 13 19 13 Al 14 21 6 37 16 8 8 Zr (1) 26 21 14 21 23 37 56 101 47 31 55 (fJ 17 Nb (3) 4 <2 3 <2 4 Mo (4) <5 5 5 <5 5 <5 <5 6 <5 <5 <5 <5 <5 Ag (2) <0.4 <0.4 0.4 <0.4 0.7 <0.4 <0.4 <0.4 0.4 0.4 <0.4 <0.4 <0.4 Cd (2) <0.5 3.1 1.2 <0.5 <0.5 <0.5 2.0 <0.5 1.6 0.6 3.6 2.9 <0.5 Sn (3) 7 <5 <5 <5 <5 Sb (4) 0.4 0.8 0.5 0.5 0.6 0.5 0.9 0.4 <0.2 <0.2 OJ 0.4 Cs (4) <0.5 0.7 0.9 2.4 1.4 1.7 <0.5 <0.5 7.1 2J 1.7 1.2 0.9 Ba (1) 37 78 53 47 46 63 ff) 127 114 64 130 58 40 Hf (4) <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 0.6 1.6 1.3 <0.5 1.2 0.7 <0.5 Ta (4) <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 W (4) <3 <3 <3 <3 <3 <3 <3 <3 <3 <3 <3 <3 <3 Ir (4) <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 Au (4) <5 <5 <5 <5 <5 <5 5 <5 <5 <5 <5 <5 <5 Hg (4) <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 Pb (2) <5 6 <5 <5 15 5 <5 6 <5 <5 <5 <5 <5 Pb (3) <5 <5 6 7 <5 Bi (2) <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 Th (4) 1.0 0.6 <0.5 0.9 0.9 1.4 1.1 4.2 3.4 1.7 3.9 1.8 1.0 U (4) <0.5 0.6 1.4 1.0 5.0 0.8 1.1 0.8 0.7 1.1 0.7 <0.5 <0.5

La (4) 10.2 7.1 7.9 12.6 9.6 15.0 9.8 19.4 3.1 22.8 18.2 6.5 2.5 Ce (4) Al 17 17 7J 23 37 21 52 8 41 41 14 8 Nd (4) 6 5 7 7 7 12 8 17 <5 15 15 7 5 Sm (4) 1.2 l.l 1.2 1.5 1.3 1.4 1.5 3.0 0.6 2.6 2.4 1.1 0.5 Eu (4) 0.4 0.4 0.5 0.5 0.5 0.4 0.5 0.7 0.3 0.9 0.7 OJ 0.2 Gd (4) Tb (4) <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 0.5 0.6 <0.5 <0.5 Yb (4) 0.9 0.9 0.7 1.2 1.0 1.0 0.8 1.2 0.5 1.6 1.2 0.6 0.6 Lu (4) 0.12 0.12 0.11 0.17 0.18 0.15 0.11 0.17 0.11 0.23 0.16 0.09 0.10 1, Fusion-ICP; 2, total digestion; 3, XRF pellets; 4, INAA 17 Table 3. (continued) Sample _G~9~94..:....-_G::::....:1~0076::..:..2==---..;;G::..1:..::0:;:2~9_..;;G~17:0~3.:::..6_..::::G71~04~8~_G::::....:1.:::06.:::..1~....;G:::....:.1.::::08:..:7_....:G::....:..171O::.:;3~....:G::....:...l1~2:.:::9_..::G::....:...;11:,::4.!.1_..::G::..1;...:1~5:::..5 _,;;;G:..;1:....:1..::::65:::...... ~G::....:...11:....:7.::.3_ Zone A6 A6 AS X.2 X.1 A4 A3 A3 A2 A2 A2 Al Mahn Fm SCM-IF SCM-IF SCM-IF Tourm ChI seh SCM-IF SCM-IF SCM-IF SCM-IF SCM-IF SCM-IF SCO-IF Tourm SiO, 33.74 34.85 16.82 53.18 33.95 31.86 30.13 37.70 22.93 2l.l4 19.72 37.92 52.92 TiO, 0.08 0.08 0.13 1.74 1.47 0.14 0.04 0.19 0.09 0.09 om 0.29 0.65 AI,OJ 2.64 2.52 4.06 14.86 11.44 4.31 1.46 6.19 2.44 2.74 2.23 8.81 13.80 Fe:,O,(T) 43.42 41.68 51.84 14.28 27.71 42.97 42.53 41.38 43.41 46.99 50.40 31.54 23.56 MnO 1.27 1.16 0.39 O. I 4 0.36 1.34 0.87 0.24 0.57 0.63 1.23 0.22 0.12 MgO 2.12 2.19 3.88 6.81 7.49 2.70 1.94 2.71 1.91 2.35 3.17 3.41 3.81 CaO 2.45 4.29 7.26 0.52 6.29 2.17 10.76 2.11 10.98 6.55 3.64 7.04 0.19 Na,O 0.66 0.58 0.20 0.35 0.12 0.76 0.47 0.82 0.64 0.54 0.45 0.15 0.25 K,O 0.64 0.58 1.22 1.31 0.01 0.82 0.13 0.97 0.55 0.56 0.61 0.77 0.09 P20, 1.39 1.00 1.97 0.26 0.31 0.92 1.41 1.21 4.04 4.33 0.50 3.64 0.06 LOr 11.24 10.12 11.81 4.76 10.84 11.58 10.20 6.07 12.18 13.93 17.55 5.82 4.04 Total 99.65 99.05 99.58 98.21 99.99 99.57 99.94 99.59 99.74 99.85 99.57 99.61 99.49

Be (2) 3 2 <2 <2 4 2 <2 3 <2 3 <2 4 <2 B (4) 10 1352 14 17 2052 S (3) 125 <50 <50 <50 115 1480 <50 6600 200 2365 310 Se (4) 1.9 2.1 3.9 35 7] 35 0.8 5.2 2.1 2 2 6.7 II V (2) 23 18 25 219 196 26 14 31 21 19 18 57 43 Cr (4) 22 18 7] 38 32 31 12 49 13 20 16 ff) 100 Co (4) 12 9 26 34 39 IS 7 IS IS 5 4 12 7] Ni (2) 8 4 12 44 35 10 10 25 18 14 9 22 54 Cu (2) 7 2 15 24 6 6 29 6 54 IS 45 36 62 Zn (2) 9 16 14 101 ff) 22 13 28 6 17 10 32 101 Ga (3) <2 <2 <2 17 <2 <2 <2 <2 <2 <2 8 As (4) 71 22 56 2 14 21 40 10 26 130 28 54 6 Se (4) <3 <3 <3 <3 <3 <3 <3 <3 <3 <3 <3 <3 <3 Br (4) <1 <1 I

La (4) 11.3 13.6 20.5 21.6 14.7 13.5 9.6 14.1 21.4 16.4 5.6 25.9 25.5 Ce (4) 24 25 41 47 34 30 17 35 ?f) 24 12 47 62 Nd (4) 10 12 16 23 Xl 10 7 12 18 14 6 21 23 Sm (4) 1.6 1.7 2.9 45 3.4 1.7 1.3 2.1 3.4 2J 0.8 3.7 3.6 Eu (4) 05 0.6 1.2 1.4 0.9 05 0.4 0.6 1.0 0.9 OJ 1.3 1.1 Gd (4) 1.5 3.7 1.5 2.2 3.8 Tb (4) <0.5 <0.5 0.7 <0.5 05 <0.5 <0.5 <0.5 05 05 <0.5 <0.5 <0.5 Yb (4) 0.9 1.0 1.4 1.9 1.8 0.9 0.7 0.9 1.1 0.9 0.7 1.7 1.4 Lu (4) 0.14 0.14 0.18 0.33 0.28 0.14 0.13 0.13 0.20 0.13 0.10 0.25 0.22 I, Fusion-rep; 2, total digestion; 3. XRF pellets; 4, INAA 18

100

200

300 ISO .g ~ ..." ..."'" 400 ..." ~ ~ SOO c ~ 0

5600 100 E'" .c g 0 is. u.. ~ .,~ .., 12 .1l 700 <.> ] :s 800

SO 900

1000

1100

0 1200 1 I 0.1 ppm ppm ppm

Figur e 3. Continued. more continuous upward decrease in CaO values and low in manganese, averaging 0.79 wt. percent MnO, a relatively high P205 values (as much as 1.4 wt. percent value close to the Biwabik average of Morey (1992; 0.88 P20 5). There are two other intercepts having anomalous wt. percent MnO) but considerably less than values P20 5 values. In all three, positive correlations between observed at many other localities on the North range P20 5 and CaO imply the presence of apatite. Correlations (Lepp, 1968). Average MnO values for thin-bedded rocks also exist between P20S and MnO and between P20 S and at the Merritt, Arko, and Hillcrest mines range from 4.3 Ah0 3' (Hillcrest) to 6.5 (Arko) wt. percent. It is only the (3) The interval between 580 and 790 feet, or Zone manganese values in the thin-bedded rocks of Zones B C and part of B, is very iron rich (60-80 percent F~03) and C that approach these reported values. and manganese poor; MnlFe ratios are low «0.10). Low Silicate-carbonate-oxide iron-formation at Gloria Mg, Ca, and LOI (400°C) values imply a lack of contains appreciable potassium (0.65 wt. percent K20 carbonates, whereas high Al and Ti imply the presence compared to 0.44 wt. percent at Merritt, 0.26 wt. percent of clastic material at 570 feet. at Arko, and 0.19 wt. percent for the Biwabik), illustrating the dominance of stilpnomelane over other Fe silicates; Major Element Attributes Morey (1992) among others has suggested that elevated potassium and alumina contents are indicative of a Data in Table 4 show that unoxidized, thin-bedded terrigenous or volcanogenic component. silicate-carbonate-oxide iron-formation in Zone A is a A very striking feature of the thin-bedded rocks of typical Lake Superior-type iron-formation composed of the Gloria core is the high content (as much approximately equal amounts of Si and Fe, followed in as 4.33 wt. percent P20 5). As in the data of Dahl and abundance by CO2 (LOI in this case), and with Al20 3, others (1992), we found a strong positive correlation MgO and CaO values in the 1 to 5 percent range. It is between calcium and phosphorus, which implies that the

20 Table 4. Mean and range of zone averages from the Gloria core (Major elements in wt. percent; Au in ppb; Ce/Ce* is the Ce anomaly normalized to the NASC standard; X Mg = 100*Mg/(Mg+Fe+Mn); n = number of analyses. Average for the Biwabik Iron Foramtion alter Morey (1992). Zone X = tourmaline-bearing rocks in Gloria) Zone A: unox. IF B: leached thin-b. C: ox. Mn-rich thin-b. D: Mn-rich thick-b. E: thick-b. hm-chert F: Sulf IF Biwabik X Depth (It) 1165-913 913-769 760-569 564-258 258-202 193 n 15 9 36 (3) 83 (6) 14 (2) 1 67 3

Si02 34.10 16.8-57.0 33.30 10.0-54.0 31.80 2.8-57 35.40 2.1-72.7 53.00 20-96 33.24 43.61 46.70

Ti02 0.12 0.04-0.29 0.09 0.04-0.31 0.16 0.02-0.95 0.07 <0.32 0.11 0.02-0.26 0.22 0.12 1.29

AI 203 3.79 1.46-8.81 2.24 1.1-7.3 3.00 0.66-9.26 2.20 <6.42 3.70 0.57-6.45 4.64 1.24 13.40 F~03(l) 41.04 22.8-51.8 55.80 35-82 54.80 28-74 33.50 5.1-54.6 36.90 1.3-67.8 35.76 38.2 21.80 MnO 0.79 0.22-1.34 0.79 0.04-3.27 1.92 0.12-14.5 15.10 0.4-48.9 0.21 0.01-1.07 0.06 0.88 0.21 MgO 2.48 1.36-3.88 0.21 <0.37 0.12 <0.53 0.64 <4.97 0.09 0.01-0.35 0.58 3.04 6.04 CaO 4.89 1.99-10.9 0.70 <2.23 0.32 <5.04 1.14 <5.59 0.14 <0.41 0.62 2.71 2.30 Na20 0.48 0.15-0.82 0.13 <0.17 0.13 <0.17 0.13 <0.32 0.48 0.07 0.24

K20 0.66 0.13-1.22 0.06 <0.31 0.07 <0.18 0.21 <2.53 0.07 <0.31 1.36 0.19 0.47

P20S 1.54 0.09-4.33 0.37 0.1-1.16 0.25 <3.61 0.18 <1.42 0.16 <0.27 0.48 0.09 0.21 LOI 9.54 1.2-17.6 5.7 1.2-12.9 6.53 3.4-14.9 9.18 2.4-15.3 3.16 <5.0 20.14 6.5 Be 2.8 <5 3.7 <8 3 <4 3.5 <6 <2 <2 <2 2.8 2.7 S 796 <50-6600 <50 265000 500 <50 Sc 3 0.8-6.7 2.1 1.2-4.9 2.2 2.1-2.4 1.7 0.4-2.9 7.9 7.9 4 24 V 25 13-57 30 22-44 34 28-40 26 9.0-40 412 724 50.4 153 Q 27 12.0-69 23 16-43 15 11.0-17 13.5 9.0-16 61 58 16 57 Co 11 <26 14 <38 18 13-22 16 <28 3 99 13 33 Ni 13 4.0-25 13 8.0-20 20 14-25 13 <26 15 150 9 44 Cu 18 2.0-54 18 7.0-71 14 8.0-21 26 <53 8 173 8.4 31 Zn 17 6.0-32 16 3.0-45 37 36-38 25 <55 25 30 43 90 As 34 2-130 61 14-120 64 59-70 68 17-160 31 230 15 7 Rb 17 6.0-44 <10 <10 <10 <10 <10 <10 <10 18 Sf 61 21-145 41 11.0-69 258 99-472 226 71-434 113 1096 22 87 Y 21 6.0-39 17 13-21 50 12-115 16 6.0-38 8 8 5 22 ?J 42 15-93 37 14-101 45 37-53 31 18-59 44 124 20.6 135 Nb 3 2.0-5.0 3 Mo <5 <6 <5 <6 <5 <9 11 <5-23 10 57 2 <5 Ag <0.4 <0.6 0.4 <0.7 1.5 0.4-2.0 1.5 0.4-2.9 <0.4 1.1 <0.5 <0.4 Cd 2.2 0.5-5.8 1.4 0.6-1.6 4.4 0.5-8.0 0.7 <2.0 4.8 <0.5 <2 0.7 Sn <5 <7 <5 <5 50 Sb 0.49 0.2-1.3 0.6 0.4-0.9 0.43 0.3-0.7 0.3 0.2-0.5 <0.2 2.9 9 <0.2 Cs 2.6 2.3-9.4 1 0.5-2.4 <0.5 <1 <0.5 <0.5 <0.5 <0.5 0.6 Ba 70 15-130 65 37-127 214 186-231 5176 145-19200 67 446 27 376 HI 0.7 0.5-1.4 0.6 0.5-1.6 <0.5 <0.5 0.6 <1.1 0.9 1.6 3.4 Ta <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 W <3 <4 <3 <3 <3 <3 <3 <5 <3 <3 <10 <3

Au (ppb) <5 <5 <5 <5 <5 <5 <5 <5 <5 31 2 <5 Hg <1 <1 <1 <1 <1 <1 <1 <1 <1 4 0.21 <1 Pb <5 <8 6 <15 <5 <5 49 2-212 <5 56 44 20 Bi <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 <5 Th 2.2 0.7-5.1 1.3 0.5-4.2 1.2 1.0-1.3 0.93 0.5-2.0 3.4 3.2 0.84 6.1 U 0.75 0.5-1.3 1.4 0.5-5.0 8.4 3.8-14 5.8 2.0-10 3.1 5.6 0.55 1.2 IREE 56 13-101 50 32-94 68 19-132 72 26-169 63 29 24.59 97 Ce/Ce* 0.94 0.7-1.17 1.11 0.9-1.25 0.88 0.88 0.6-1.3 0.89 0.87 0.96 0.98 Mn/Fe 0.022 <0.04 0.015 <0.06 0.038 <0.25 0.600 0.02-7.3 0.007 0.002 0.022 0.010 X MJl 5 2.7-8.5 0.35 0.18 2.1 1.4 1.4 6 20

21 latter is contained in apatite. The large phosphorous However, other elements such as Zn, V, and Pb are values contrast markedly with those observed in thin­ markedly depleted. Compared to thin-bedded rocks of bedded rocks at the Merritt, Arko, and Hillcrest mines, the Merritt, Arko, and Hillcrest sites (McSwiggen and which contained maximum values of 1.13, 1.18, and 0.82, others, 1995), Zone A rocks contain equal amounts of respectively (McSwiggen and others, 1995). Such Sc, V, Cr, Ni, Cu, Sb, Pb, and U. They are depleted in variability appears to be an attribute of the North range Co, Zn, Ba, and Be, and enriched in As, Sr, Y, Zr, Cd, in general. For example, an unoxidized thin-bedded and Th. Base metal and precious metal contents are argillaceous iron-carbonate rock from the Kennedy mine very low (Au <5 ppb), a feature generally observed in contained 4.02 wt. percent P20 S (Schmidt, 1963), but Lake Superior-type iron-formations. this was the only analysis published by him that exceeded Trace element values from rocks in Zone B generally

I wt. percent P20 S. Similarly 67 analyses from drill are similar to those in Zone A, but As and V are enriched. cores stored and logged by the Minnesota Department of Trace elements in Zone C are enriched in Ba, Zn, Sr, V, Natural Resources, Minerals Division, Hibbing contain Ag, Cd, and U. Zone D is enriched in Cu, Ag, Pb, and

no more than 0.97 wt. percent P20 S and averages only U and depleted in average amounts of V, Co, Ni, Zn, 0.16 wt. percent. and Sb compared to thick-bedded rocks at the Merritt Oxidized thin-bedded iron-formation from Zone B site. Both sites are characterized by similar large Ba, Sr, is characterized by high and variable Fe values-at As, and Mo values. In contrast, Zone E is enriched in constant Si, Ti, AI, Mn-compared to unoxidized iron­ V, Cr, and Sr, a feature observed in only the uppermost formation from Zone A. Iron values as large as 82 wt. layers of the thick-bedded facies at the Merritt site percent Fe203 were obtained from intervals of nearly pure (McSwiggen and others, 1995). hematite. In these samples, Mg, Ca, Na, K, P, and LOI One analysis of sulfide-rich iron-formation in the are obviously depleted constituents. MnO values average Rabbit Lake Formation produced exceptionally large base 0.79 wt. percent and Mn/Fe ratios average less than 0.06. metal, in addition to elevated As, Sb, Mo, Se, V, Sr, and The major element composition of oxidized thin-bedded Au values (31 ppb). iron-formation in Zone C is similar to that in Zone B. However, the latter is characterized by MnO values that Rare Earth Element Attributes on average are more than double (1.92 wt. percent) those of Zone B. MnlFe ratios reach 0.25, a value similar to Iron-formation in the Gloria core has REE attributes that reported for thin-bedded un oxidized iron-formation similar to those of other iron-formations in general at the Merritt, Arko, and Hillcrest mines (McSwiggen (Danielson and others, 1992) and those in the Biwabik and others, 1995). Iron Formation in particular (Morey and Boerboom, Oxidized thick-bedded iron-formation in Zone D is 1992). However, average REE (La+Ce+Nd+Sm+Eu+Tb+ generally similar in composition to thick-bedded strata Yb+Lu) totals of 50 to 70 ppm are twice those observed

at the Merritt site. Average Si02 values (35.4 wt. percent in the Biwabik Iron Formation. Samples normalized to at Gloria, 36.7 wt. percent at Merritt), Fe203(T) (33.5 chondri tic values generally show a negative slope from wt. percent at Gloria, 36.5 wt. percent at Merritt), and LREE to HREE (LalYbcH =3-25) with no or only weakly MnO (15.1 wt. percent at Gloria, 16.6 wt. percent at positive Eu anomalies; negative Ce anomalies are present Merritt) are remarkably similar: MnlFe ratios average in several of the manganese-rich samples. Where 0.6 at Gloria and 0.7 at Merritt. Potassium and sodium normalized to, the North American Shale Composite values however are significantly larger at the Merritt Standard (NASC), Gloria samples have distinctly positive locality, basically because of the anomolous presence of Eu anomalies. Normalized LalYbNASC is low (0.5-2.4), hyalophane (K) and aegIrIne (Na) (McSwiggen and indicating flat, shale-like patterns (Fig. 4). others, 1994a, b). REE characteristics of the different zones are summarized in Table 5 where they are compared with Minor and Trace Element Attributes Biwabik averages. It is evident that Zones A and B have generally similar REE patterns. Sulfide-enriched Some trace elements including As, Sr, V, Zr, Cd, strata from Zone A, and oxidized thin-bedded iron­ Ba, and Th are significantly enriched in the unoxidized formation of Zone B both have slightly negative Ce thin-bedded iron-formation (Table 4) compared to average anomalies. Manganese-bearing and manganese-rich iron­ values from the Biwabik Iron Formation (Morey, 1992). formations from Zones C and D have enriched REE totals

22 5 bedded facies where the dominant silicate assemblage Zone 0: Mn ores was controlled either by the bulk-rock composition or by o 1 the metamorphic grade. Manganiferous siderite «C/) coexisting with and ferroan kutnahorite z dominates in the lower, thin-bedded member at the Merritt ~ Q o site, whereas carbonates of the calcite-rhodochrosite a: 0.1 (kutnahorite) group dominate in the upper thick-bedded ~Zone C: Mn ores Biwabik" average member. Aegirine (McSwiggen and others, 1994a), , and hyalophane (McSwiggen and others, 1994b) were found in micronodular structures only in 0.01 -t---r---r---r-'-'-'--.---'--'-'--'-'--~ thick-bedded rocks. 5 As part of this study, 75 polished thin sections from unoxidized thin-bedded all parts of the Gloria core were evaluated. Brief o 1 summaries of the major mineral groups are given below. «C/) ~ ,0': ,',:',:.:'.: •...... Silicates oQ a: 0.1 Silicates are abundant in un oxidized thin-bedded iron­ ~oxidized thin-bedded Biwabik average formation and include stilpnomelane, chlorite, and biotite­ iron-formation " like phases of uncertain affinity. Alkali feldspar occurs along the selvages of crosscutting sulfide-mineralized veinlets. Muscovite and tourmaline are present in schist LaCe NdSmEu Tb YbLu of the Mahnomen Formation, and in schist intercalated Figure 4. Rare earth element enrichment diagrams. A, with unoxidized iron-formation. Oxidized iron-formation manganese-rich intervals in Zones C and D. B, oxidized carries low amounts of biotite-like phases, some and unoxidized thin-bedded iron-formation in Zones A and minnesotaite, and clay minerals. B. Also shown are average data for the Biwabik Iron Formation (Morey, 1992). All data are standardized Stilpnomelane against the North American shale composite (NASC). Stilpnomelane is the dominant in unoxidized thin-bedded iron-formation. It occurs as both green (ferro-) and brown (ferri-) varieties, commonly as and distinctly negative Ce anomalies. However, samples aggregates formed by interlocking acicular crystals from Zone D have distinctly higher La/Yb and SmlYb several m to several millimeters in length. Although ratios and are enriched in the LREE. Thick-bedded different generations can be distinguished in individual hematite iron-formation in Zone D also is enriched in samples, stilpnomelane lacks much compositional the LREE. In contrast, hematite iron-formation from diversity. There are only minor variations in X Mg Zone E has considerably lower REE totals. Sulfide iron­ [100Mg/(Mg+Fe2++ Mn)] and X Mn [100Mn/(Mg+Fe2++ formation from the Rabbit Lake Formation has even lower Mn)] with depth (Table 6); such differences can be REE totals (30 ppm) and is marked by a negative Ce attributed to small differences in bulk rock composition. anomaly, and a very flat, shale-like pattern. X Mg in stilpnomelane is higher than the respective whole rock value and in one sample ranges from 4.3 to 10.8 MINERALOGY with averages around 8.0 to 10.2 (Table 6). XMn is The mineralogy of the Trommald Formation has been considerably lower than the whole rock value and ranges described by Grout and Wolff (1955), Schmidt (1963), from 0.35 to 1.19; MnO contents in stilpnomelane are and Blake (1965). More recently McSwiggen and others generally lower than 0.5 wt. percent. Although MgO (1994a, 1994b, 1995) have reexamined selected materials contents are homogeneous (2.03 ± 0.25 wt. percent), a from the North range using electron microprobe few grains that coexist with pyrite + magnetite + siderite techniques. They showed that minnesotaite, chamosite, both in sulfide-rich iron-formation (G 1059) and in and stilpnomelane were abundant phases in the thin- sulfide-bearing vugs (G 1155) have MgO contents as low as 0.9 wt. percent.

23 Table 5. Rare earth element characteristics of zones in the Gloria core compared to average Biwabik Iron Formation. Zone A A-sulfides B C D E F Tourm Biwabik IREE 62 54 50 68 72 63 29 173 25 Range 29-84 17-101 32-94 19-132 26-269 76-334 1.0-84

CHONDRITE-NORMALIZED Ce/Ce* 1.00 0.92 1.09 0.86 0.88 0.89 0.87 0.98 0.96 Range 0.9-1.2 0.7-1.2 0.9-1.3 0.8-0.9 0.6-1.4 0.9-1.1 0.8-1.5 EulEu* 1.25 1.23 1.11 1.31 1.39 0.94 0.98 1.00 1.08 Range 1.0-1.5 1.0-1.4 0.8-1.4 1.1-1.6 0.7-1.3 0.7-3.1 La/Sm 4.44 4.41 4.88 4.67 5.36 5.91 4.06 3.6 Range 3.7-5.0 3.1-5.2 4.1-6.7 3.5-5.9 3.8-6.5 2.7-4.5 LalYb 9.4 9.17 8.1 6.9 12.9 16.3 4.8 11.7 8.38 Range 7.3-10.0 2.8-13 5.3-10 3.4-10 7.7-24 5.5-16 5.0-25 Sm/Yb 2.13 2.06 1.68 1.5 2.37 2.76 1.18 3.2 2.29 Range 1.8-2.5 0.9-3.3 1.3-2.7 0.7-2.0 1.7-4.2 2.0-4.6 1.0-6.4 Eu/Sm 0.86 0.90 0.89 0.91 1.08 0.88 0.97 0.77 0.96 Range 0.7-1.1 0.8-1.1 0.6-1.1 0.7-1.0 0.8-1.2 0.5-1.0 0.5-2.1

NASC-NORMALIZED EulEu* 2.23 2.20 1.99 2.34 2.49 1.63 2.3 1.56 1.64 Range 1.8-2.6 1.8-2.5 1.5-2.4 2.3-2.4 2.0-2.6 1.2-1.9 1.1-4.9 LalYb 1.39 1.35 1.19 1.02 1.91 2.41 0.78 1.73 1.24 Range 1.1-1.6 0.4-1.9 0.8-1.6 0.5-1.5 1.1-3.6 0.8-2.5 0.7-3.7 A-sulfides, sulfide-bearing zone A iron-formation; Tourm, tourmaline-beru;ing rocks of the Cuyuna Range Ce/Ce* =3Ce/(2La+Nd); Eu/Eu*=Eu/[(Sm+Tb)/2]

The "A-position" in the stilpnomelane structure wt. percent, LaBerge, 1966b). When calculated to 22 (Eggleton, 1972; Floran and Papike, 1975) may contain +water, combined Na+K values in stilpnomelane either Ca, Na, or K, but is typically dominated by K. typically should be in the 0.3 to 0.6 range and generally In the Gloria core Ca was for the most part not detected; should not exceed 1. Seven out of 39 analyses from Gloria Na contents as large as 6.26 percent were observed. exceed that value. Although the Si/Al and Fe/Mg ratios The K/(Na+K+Ca) ratios range from 0.34 to 0.83 and in these grains are typical of stilpnomelane, the large are typically smaller than those in mica minerals. amounts of sodium and potassium probably cannot be Potassium contents average 1.6 to 3.1 wt. percent K20, accommodated in the stilpnomelane structure. Therefore but values as large as 5 wt. percent were detected in it is assumed that much of the stilpnomelane that was one sample (G 1155). That value clearly exceeds the analyzed is very finely interlayered with either a biotite­ compositional limit of 3.4 wt. percent established in the like phase, or a low-AI biotite precursor phase. literature (Eggleton and Chappell, 1978). Stilpnomelane It is assumed that the alkali content in stilpnomelane from the various iron-formations worldwide contains as is a function of its availability in the bulk rock because, much as 2.9 wt. percent K20 (FIoran and Papike, 1975; in the absence of mica minerals, the stilpnomelane Gole, 1980, Klein and Gole, 1981; Haase, 1982). A concentrates nearly all the alkali elements. In contrast Ca compilation of high-quality analyses from various iron­ is strongly partitioned into carbonates. In the Gloria core, formations compiled by FIoran and Papike (1975) shows coarse (30 m long) stilpnomelane in alteration assemblages, high K20 in the Brockman Iron Formation (as much as especially inside carbonate-quartz-sulfide vugs, tends to 5.63 wt. percent: LaBerge, 1966a; TrendaII and have K contents larger than those in matrix stilpnomelane. Blockley, 1970; Ayres, 1972) and in South Africa (7.2 This probably reflects potassium metasomatic processes

24 Table 6a. Chemical Earameters (mean and range) of stilEnomelane in the Gloria core. Sam Ie G1155 GI106 G1059 G996 G984 G972 ALL Number of analyses 8 6 5 8 8 4 39 100 XMg 9.6 (8.7-10.7) 10.1 (9.3-10.6) 8.0 (4.3-10.3) 9.7 (9.2-10.2) 9.5 (9.2-10.0) 10.2 (9.7-10.8) 9.5±1.18 100XMn 0.43 (0.38-0.51) 0.63 (0.46-1.19) 0.49 (0.37-0.67) 0.66 (0.59-0.74) 0.72 (0.54-0.87) 0.64 (0.56-0.80) 0.59 ± 0.17 K/(Na+Ca+K) 0.69 (0.34-0.83) 0.68 (0.50-0.78) 0.73 (0.64-0.82) 0.54 (0.39-0.65) 0.67 (0.57-0.76) 0.64 (0.62-0.66) 0.65 ± 0.11

wt% K20 3.06 (0.73-5.08) 1.80 (1.19-3.55) 1.69 (0.91-2.30) 1.61 (0.99-3.10) 2.34 (1.46-3.22) 2.49 (1.79-3.34) 2.19 ± l.15 wt% MnO 0.16 (0.13-0.19) 0.24 (0.17-0.47) 0.19 (0.14-0.26) 0.26 (0.23-0.29) 0.27 (0.20-0.33) 0.25 (0.21-0.31) 0.23 ± 0.07

100 XMg = 100*Mg/(Mg+Fe+Mn); 100 XMn= 100*Mn/(Mg+Fe+Mn)

tv VI Table 6b. Chemical Earameters (mean and range) of Fe-rich mica and chlorite in the Gloria core. Fe-rich mica minerals Fe-rich chlorite (Si per 28 ox) Sample G 1029 G930 GI173 G1165 GI048 Gl029 G930 Number of analyses 8 5 7 2 5 6 5 100XMg 19.7 (8.0-28.0) 26.1 (23-28.5) 22.9 (22.2-23.8) 15.9 (15.5-16.2) 30.4 (30.1-30.9) 12.5 (9.4-13.8) 14.1 (13.4-14.8) 100 XMn 0.18 (0.0-0.75) 0.10 (0.0-0.21) 0.32 (0.21-0.45) 0.21 (0.17-0.25) 0.20 (0.12-0.27) 0.19 (0.14-0.23) 0.27 (0.22-0.29) K/(Na+Ca+K) 0.85 (0.60-0.94) 0.95 (0.93-0.98) 5.19 (5.09-5.32) 5.44 (5.43-5.45) 5.55 (5.51-5.58) 5.58 (5.43-5.72) 5.65 (5.37-5.98)

wt% K20 4.09 (1.73-6.56) 4.88 (2.76-6.34) 0.00-0.12 0.06-0.13 0.00-0.02 0.01-0.24 0.02-0.78 wt% MnO 0.05 (0.0-0.022_0.02 (0.0-0.04) 0.10-0.20 0.08-0.12 0.08-0.13 0.06-0.11 0.10-0.14

100 XMg = 100*Mg/(Mg+Fe+Mn); 100 XMn= 100*Mn/(Mg+Fe+Mn) in the vicinity of veins and vugs, a feature consistent are as large as 6.6 wt. percent and average 4.1 (G 1029) with selvages of alkali feldspar and biotite around and 4.9 wt. percent (G 930). The analyses are marked mineralized veins. by variable silica values (G 1029: 39.6 ± 8.3 wt. percent; G 930: 48.9 ± 5.6 wt. percent) and generally low totals Chlorite «90 wt. percent), implying that the phases contain appreciable contents of OH, H20, or ferric iron. Why Minerals of the occur in the Gloria the microprobe analyses are so inconsistent with respect core: (1) in the Mahnomen Formation, (2) within the to Si, Fe, and alkalis is not yet clear. The biotite-like lowermost 6 feet of the iron-formation, (3) in interlayered phase could be finely interlayered with quartz, or it could volcanogenic schist, (4) in un oxidized thin-bedded iron­ be unstable under the electron beam and simply evaporate. formation adjacent to (3), and (5) in the transition zone Regardless, sample G 1029 calculated to 22 between Zones A and B. Typically chlorite is oxygens+water yields a structural formula consistent with interlaminated with magnetite, siderite, and dolomite­ alkali-bearing di- or tri-octahedral sheet silicates. A . Chlorite also occurs in two samples interlayered general formula can be given as with a K-rich AI-bearing sheet silicate similar to biotite. Chlorite apparently does not coexist with stilpnomelane. K>I(Fe3.oMgo.gAlu)s(Alo.3.1.oSi7.o-7.7)g020(OHk

Chlorite compositions are generally Fe-rich with XMg If the analyses in G 1029 also are normalized to 14 values ranging from 10 to 30. Silica contents vary from cations, calculated ferric iron values would range from 5.0 to 6.0 cations per 28 oxygens; compositions plot into 1. 7 to 16.7 wt. percent, with ratios of Fe3+/Fe2+= 0.1 to the Fe-rich thuringite (Si<5.6; ripidolite and daphnite) 1.0. The assumption of stoichiometric OH contents adds and chamosite (Si>5.6) fields of the classification of Hey approximately 4.5 wt. percent H20 to the analyses, (1954). MnO contents are typically less than 0.2 wt. resulting in relatively acceptable totals of 95 wt. percent percent. The Mg ratio in chlorite correlates well with for the best analyses. the Mg number of the enclosing whole rock, whereas The phase in G 930 contains excess silica and if Mn contents in chlorite do not increase with Mn contents calculated to 22 +water yields a structural formula in the whole rock. such as K>I(Fe2.3Mgo.gAI1.4)4.5Sig.102o(OHk If all of the The chlorite geothermometer of Cathelineau (1988), iron is calculated as ferric iron, Si comes to 7.6 to 7.8 which is based on the Si content in the tetrahedral site, per 22 oxygens and total cations would add to about 13. indicates formation temperatures of between 264 and The resulting formula then would be: 407°C. Interestingly when averaged by depth, the temperatures consistently increase with depth: 316°C (930 feet), 327°C (1029 feet), 333°C (1048 feet), 351°C (1165 feet), and 390°C (1173 feet). The analyses very much resemble those of , an Fe-rich dioctahedral mica with tetrahedral AI( or 3 Biotite-like phases of uncertain affinity Fe +»0.2 per formula unit, and octahedral R3+>1.2 pfu (Odom, 1984). Glauconite occurs mainly in Mica-like minerals that are considerably different unmetamorphosed marine sedimentary rocks, but that from either stilpnomelane or chlorite were found in two paragenesis should not preclude the possibility that a samples of chlorite-bearing iron-formation (G 1029 and glauconite-type mineral is present at the Gloria site. The G 930). They occur as slightly crenulated and deformed data may be explained in several ways. First, the aggregates that in some cases have a radial texture. They glauconite-like mineral may in fact be finely interlayered have a good cleavage, are green and pleochroic, and in stilpnomelane and biotite. Second, although places resemble phengitic muscovite. Microprobe stilpnomelane and biotite are not isostructural entities, analyses show them to be a biotite-like phase that differs the mineral may be a phase with an intermediate from stilpnomelane in having larger Mg, AI, Ca, and K compOSItIOn. Third, the phase may be a poorly contents, and smaller Mn, Na, and Fe contents. AI 20 3 crystallized, K-deficient and AI-poor "proto"-biotite (or contents are relatively constant, and are, on the average, "proto-annite"). Our data are insufficient to resolve this 1-2 wt. percent higher than those in stilpnomelane (G problem at this time. 1029: 8.03 ± 1.17 wt. percent. G 930: 7.00 ± 0.86 wt. A biotite-like phase containing elevated K20 (6.58- percent). CaO is detectable in all grains in the 0.1 to 0.6 6.78 percent), Ti02 (0.81-0.92 percent), and AI 20 3 (8.78- wt. percent range, whereas Na is lacking. K20 values 9.13 percent) compared to stilpnomelane is also described

26 from the low-metamorphic grade Marra Mamba Iron of the North range (Cleland and others, 1993 and in Formation, (Klein and Gole, 1981). The Dales press). The Mg/(Mg+Fe) ranges from 0.32 to 0.61, and Gorge Member of the Hamersley Group, Australia, NaI(Na+Ca) from 0.85 to 0.94; MnO is less than 0.13 contains Fe-rich mica with 35-39 wt. percent Si02, 1.8- wt. percent, and Ti02 is as high as 1.6 wt. percent. 5.5 wt. percent A120 3, 30-41 wt. percent FeO, 7.7-12.2 Tourmaline thus is a member of the schorl-dravite solid wt. percent MgO, and 7.8-8.8 wt. percent K20 (Miyano, solution series. 1982). Floran and Papike (1978) present an analysis of fine-grained interstitial brown stilpnomelane plus biotite Alkali feldspar in grunerite-magnetite granules of the Gunflint Iron Formation (subzone 3b), which indicated 10 percent Alkali feldspar occurs as a selvage along discordant quartz-rich veins. In sample G 1106, it forms euhedral Al20 3 and 3.57 percent K20. Miyano (1987) assumes that ferri-annite easily replaces stilpnomelane by the to subhedral crystals some 20 to 40 m in size. These addition of K at low temperatures (100o-150°C), a process crystals coexist with stilpnomelane, siderite, and quartz. that has been observed in the zone of the Dales The vein also contains the assemblage quartz-barite­ Gorge Member, Brockman Iron Formation (Australia). anhydrite. Microprobe analyses show that the feldspar A second AI-rich silicate characterized by 36-44 wt. is pure K-spar; no Na or Ca were detected and Fe and percent Si02, 5.6-16.2 wt. percent A120 3, 6-12 wt. percent Mn are present in only trace amounts. A calculated FeO, 3-4 wt. percent MgO, variable amounts of MnO formula, however, indicates a considerable potassium (0.9-2.5 (?) wt. percent), and 1.3 to 3.4 wt. percent K20 deficiency (15 percent), which implies that some of the has been found filling cracks in rhodochrosite; it occurs alkali feldspar may contain barium. as a replacement phase in oxidized iron-formation (G 853). Analytical totals are low (72-88 wt. percent), but Minnesotaite calculation to 22 oxygens+water results in a mica-like Minnesotaite is rare in the Trommald Formation at structural formula such as: the Gloria site. However, it has been found intergrown with goethite and quartz in oxidized thick-bedded iron­ formation (G 298). The iron-rich silicate has a calculated As such, the phase is similar to glauconite-like minerals structural formula that contains 13.7 cations per 22 in G 1029, but has higher Mg and Mn values. oxygens and thus approximates the structural formula of . However, the analysis is at best only approximate, Muscovite because it totals only to 79 wt. percent. No elements Phengitic muscovite is common in the uppermost other than Si, Fe, and small amounts of Mn, AI, and Mg parts of the Mahnomen Formation where assemblages were detected. consisting of muscovite-rutile-zircon (±tourmaline) are interlayered with assemblages consisting of tourmaline­ Clay-like minerals chlorite-quartz-rutile. The muscovite there has an average Soft Mg-rich silicates interlayered with quartz are formula of enclosed within a manganite matrix in sample G 451. Analyses total only to 70 wt. percent and consist of Si, AI, and appreciable Mg, with small amounts of Fe, Mn, This indicates that the A-position is only half filled (K20 Ca, and K. Calculation to 22 oxygens+water reveals 14 = 5.7-6.3 percent). The Si content of 3.15 per formula cations, implying the phase is probably a . unit indicates minimum pressures of formation around High Mg contents are typical of the vermiculites (34 2.5 kbar (Massonne and Schreyer, 1987) calculated for percent Si02, 15.4 percent A120 3, 8 percent Fe203, 22.6 400°C. MgO percent and 20 percent H20; Deer and others, 1992), which typically form by breakdown of biotite through hydrothermal or weathering processes. Tourmaline

In the massive tourmalinite detected in the uppermost Carbonates Mahnomen rocks of the Gloria core, tourmaline Carbonates in thin-bedded, unoxidized iron-formation compositions resemble those analyzed from other parts at the Gloria site generally lack manganese and plot in

27 the compositional of either siderite or the and relatively pure rhodochrosite. Similarly, in many of assemblages dolomite-ferroan dolomite-ankerite. In a the vein assemblages, textural evidence suggests that chlorite-rich sample from near the base of the iron­ rhodochrosite and apatite replace an older carbonate of formation (G 1165), elongated lath-shaped grains of unknown composition, but whose shape and idiomorphic dolomite (CaI54Mgs36Sid3Rdc7) coexist with chlorite and crystal form are still preserved. they collectively define a foliated fabric. That dolomite Lastly almost pure calcite (Cals7Mgs2Sidjl) occurs is overgrown by euhedral grains of complexly zoned as a late replacement mineral in thick-bedded oxidized dolomite of a second generation (core: iron-formation (G 298). Cal49Mgs30SidjsRdc3; rim: Cal5jMgs37SidjjRdcd. Elsewhere, deformed and corroded grains and thin Iron Oxides laminae of pure dolomite (CaI54Mgs43Sid2Rdcj) occur in chlorite schist (G 1048) together with chlorite, altered A variety of iron oxides occur in the Trommald ilmenite (pseudorutile), and quartz. All are overgrown Formation. These include hematite, magnetite, ilmenite, by post-kinematic blasts of ankerite and a variety of their alteration products. Ilmenite and (CaI50Mgs2jSid27Rdc2), which in turn is rimmed by narrow rutile and their alteration products occur in unoxidized seams of quartz. Clearly carbonates were precipitated iron-formation. In contrast, oxidized iron-formation throughout the pre-metamorphic to post-metamorphic contains goethite, martite, some lepidocrocite, and minor history of the iron-formation. hematite. The centers of sulfide-rich vugs and veinlets contain a siderite with 2 to 6 wt. percent MnO. Such vugs are Magnetite rimmed typically by an ankeritic dolomite. Mn contents Magnetite is especially abundant in thin-bedded iron­ are higher in siderite within mineralized vugs than in the formation where much of it is variably martitized. In matrix. general two generations of magnetite can be distinguished Typically only Fe-rich minerals of the dolomite­ using textural criteria (see sulfide discussion). ankerite series coexist with siderite in thin-bedded iron­ Nonetheless, random microprobe analyses show that formation. However in many samples, dolomite and various samples differ very little in having low trace siderite do not appear to be coexisting phases, one phase element contents; Ti02 is less than 0.03 wt. percent, MnO or the other being restricted to vein assemblages. is less than 0.05 wt. percent, and A1 20 3 is less than 0.03 Nonetheless it is possible to apply the dolomite/ankerite wt. percent. The samples consistently contain between + /siderite geothermometer (McSwiggen, 1993) 1.2 and 2.3 wt. percent Si02• The Si02 may be attributed to three samples. In pairs of coexisting Mn-poor to a theoretical Fe2Si04 molecule. It probably reflects a (XSid.Mn<0.07) siderite and ankerite; in matrix material as reaction involving primary sedimentary Fe minerals well as in crosscutting veins, KD values range from 0.2 (oxides or silicates), quartz, and carbon to produce to 0.3, yielding temperatures from 277° to 380°C. XSid.Mg magnetite + quartz + CO2, in these pairs ranges from 0.06 to 0.1. Carbonates in alteration zones around sulfide-bearing veins yield Hematite temperatures of about 280°C (G 972), which suggest that equilibration was obtained. However, analyses from vein­ Primary hematite is present in the basal Fe-chlorite matrix pairs do not provide reasonable results, implying schist, in interlayered chlorite-tourmaline schist, and in that there was no such equilibration between these two the transitional interval between Zones A and B. In ages of material. each it occurs as small ovoid masses that are aligned Manganese-rich carbonates (Calg.2oMgs6.7Sidj.jsRdcn- parallel to the foliation and interlayered with acicular 79) occur both in apatite-carbonate veins (G 853) and in silicate and chert (G 913). Euhedral, flaky hematite also pods intergrown with goethite or hematite. In both, the occurs in chlorite-"mica"-magnetite-carbonate iron­ carbonates postdate oxidation processes in the iron­ formation (G 1029) just above tourmaline-bearing schist. formation. Paragenetic relationships can be complex. In In oxidized thin-bedded iron-formation (Zones Band some pods large crystals of an older manganese-rich C), hematite is a secondary phase. In thick-bedded iron­ carbonate with relict Fe-rich cores and Ca-rich formation (Zones D and E), it is a primary phase. rhodochrosite rims are partly replaced by fine-grained

28 llmenite Goethite and martite

Small amounts of ilmenite were detected in several Goethite occurs in a variety of textural habits. In samples. Typically it occurs as anhedral to subhedral thin-bedded oxidized iron-formation it is a major grains 10 to 50 m in length that are associated with constituent that has an acicular habit similar in shape magnetite, stilpnomelane, quartz, and siderite (G 996). and size to stilpnomelane; that goethite may have replaced The anhedral to subhedral grains in turn contain irregular stilpnomelane. Rhombohedral grains of goethite replace inclusions and blebs «10 m across) of a manganoan similarly shaped grains of primary carbonate minerals. ilmenite that contains as much as 10 wt. percent MnO, Goethite also occurs as rims surrounding disseminated corresponding to 23 mole percent of the pyrophanite euhedra of martite after magnetite, and as continuous component. In contrast, enclosing ilmenite contains only layers of massive material having a radial texture. These 2.1 wt. percent MnO or 5 mole percent of the pyrophanite layers in places contain pseudomorphs after silicates and component. martite and typically are brecciated. Ilmenite also occurs in a variety of other textural Microprobe analyses show that martite basically settings. In volcanogenic chlorite schist it occurs retains the chemical signature of magnetite with abundantly as elongated crystals that parallel the foliation. detectable Si, but low values of AI, Mn, Ti, and Mg. Single grains as much as 50 m in length are also present Goethite contains, in addition to iron, 1.3-5.8 wt. percent within postkinematic ankerite blasts. Lastly ilmenite is Si02, 1.5-3.4 wt. percent A1 20 3, as much as 0.23 wt. associated in places with euhedral grains of monazite 1- percent MgO, as much as 0.6 wt. percent MnO, <0.06 3 m in size. Regardless of the textural setting, all are wt. percent CaO, <0.1 wt. percent Na20, and <0.05 wt. deficient in FeO and contain elevated MnO (as much as percent K20. Some of the goethite pseudomorphs have 3.8 wt. percent), Si02, A1 20 3, and MgO contents. The measurable phosphorus and fluorine contents. Ti/(Ti+Fe) ratios range from 0.56 to 0.61; altered ilmenite of this composition is defined as "hydroilmenite" by Frost Phosphates and others (I983). Rydroilmenite analyses can be calculated to a pseudorutile formula, assuming Ti = 3.0 Apatite commonly occurs in thin-bedded unoxidized (Mucke and Bhadra Chaudhuri, 1991). This results in a iron-formation within massive pyrite-rich lenses where generalized formula of Fe2+o.o.osFe+31.86.1.93 Ti30 9. Low it is associated with stilpnomelane, magnetite, dolomite, totals indicate the presence of considerable amounts of siderite, arsenopyrite, and quartz. Sparse apatite also water and OR. occurs associated with magnetite in laminae intercalated In chlorite schist near the base of the iron-formation in chlorite-rich intervals in Zone A (G 1029). Apatite (G 1165), plates of ilmenite as much as 20 m in length also is a minor constituent in quartz-carbonate-silicate­ are interwoven with hematite. The ilmenite has been barite-anhydrite veinlets (G 1106). In all cases, the apatite oxidized to pseudorutile with a formula Fe2.,Ti309. is an Fe-bearing fluorapatite with 1.2 to 1.7 wt. percent Almost all of the iron is in the ferric state and Ti/(Fe+ Ti) FeO, 0.03 to 0.08 wt. percent CI, and 1.5 to 2.1 wt. ratios average around 0.59. Pseudorutile is considered percent F. Formula calculations to 26 (0, OR, F, CI) to be a low-temperature alteration product of ilmenite. range from 5.7 to 6.0 cations of P, 10.2 to 10.8 cations As such it has an intermediate composition between of Ca+Fe+Mg, and 0.9 to 1.1 cations of F + Cl. X-ray ilmenite and leucoxene (rutile + hematite). It is diffraction analysis of a manganese-bearing interval in commonly observed in ground-water alteration zones, but Zone C (Ml, G 750) also revealed the presence of some pseudorutile occurs in rocks of low to medium significant apatite, in addition to goethite, hematite, and metamorphic grade. It seems to be fairly common in manganite. Precambrian Fe- and Mn-rich sedimentary rocks (Mucke Some intervals of thick-bedded iron-formation and Bhadra Chaudhuri, 1991; Melcher, 1995) and was contain as much as 1.4 wt. percent P20 S' One of these observed in a Mesozoic greenschist-grade metamorphic intervals (G 298) contains appreciable apatite as a matrix paleoweathering layer in the European Alps (Melcher, mineral along with calcite and quartz in martite-hematite­ 1991). goethite-manganomelane-bearing granular iron-formation. The apatite is fluorapatite with trace amounts of FeO

29 «0.1 wt. percent), MgO «0.05 wt. percent), and Cl of (Ca l.3FeO-2.1 Mn 1-3)2.6-4SAI6(PO 4h 1-3.6( °H) 10' H20 and «0.04 wt. percent), but 2.2-4.7 wt. percent F. thus imply a slight deficiency in P04, which possibly A Ca-rich, and Mn- and Fe-bearing phosphate of evaporated away under the electron beam. The X position uncertain affinity was found along with rhodochrosite, contains excess cations, implying that Fe and Mn are not quartz, and barite in a crosscutting vein in thin-bedded part of the structure. A Sr content of approximately 0.7 oxidized iron-formation (G 853). Analyses of the cations for 21 oxygens (6-7 wt. percent SrO) would be phosphate total sum to 88-91 wt. percent and include 1.4 needed to produce an intermediate crandallite-goyazite to 1.8 wt. percent MnO, 0.01 to 4.1 wt. percent FeO, composition. <0.16 wt. percent A1 20 3, <0.04 wt. percent MgO, 48 to The phase also could be a hydrated Mn-bearing 52 wt. percent CaO, and 34 to 37 wt. percent P20 S' aluminophosphate such as found in the series; Halogen contents are less than 0.1 wt. percent for CI and these minerals have an ideal structural formula of less than 0.3 wt. percent for F. Formula calculations, (Fe,Mn)AI(P04)(OH)2·H20 with 3-31 wt. percent MnO, assuming the structural formula for apatite, total 5.7 to 16-23 wt. percent A120 3, 1-31 wt. percent FeO, 30-31 6.0 cations for P and 10.9 to 11.7 cations for combined wt. percent P20 S, and 15-17 wt. percent H20. Thus, Ca+Fe+Mn+Mg+Al. Thus it is possible that this phase minerals of the series contain more P and combined is not apatite but rather either a carbonate apatite or a Fe+Mn than found in sample G 735. hydrated phosphate. A second AI-rich phosphate of uncertain identity Sulfates occurs as very small (1 to 3 m), octahedral or pseudocubic crystals scattered within a nodular aggregate of manganite Barite is the principal sulfate from the Trommald and Mn-Fe oxyhydroxide (G 735). This phase occurs Formation. It typically occurs in veinlets where it is within both manganese-bearing phases, but not within intergrown with quartz. The barite contains euhedral or goethite. The phosphate crystals are zoned with bright subhedral inclusions of a calcium sulfate, most likely rounded cores and darker rims that impart a euhedral anhydrite. The inclusions are about 10 to 20 m in shape. Analyses suggest the following compositional diameter and are associated with apatite, quartz, pyrite, variability: 6.2-19.3 wt. percent MnO; 0.8-7.4 wt. percent and stilpnomelane. Barite also occurs as small inclusions FeO; 23.8-30.3 wt. percent Ah03; 5.6-7.6 wt. percent in quartz in rhodochrosite-apatite-quartz veins that cut oxidized thin-bedded iron-formation. In transmitted light CaO; and 19-24 wt. percent P20 S' Minor elements include <0.08 wt. percent MgO, 0.03-0.04 wt. percent the barite appears as slightly green, round, fluid inclusion­ K20, <0.04 wt. percent BaO, 0.007-0.016 wt. percent like droplets as much as 20 m in size in thin-bedded CoO, and 0.03-0.7 wt. percent PbO. Sr contents, although strata. not measured precisely, may be in the 5-percent range, Thick-bedded, manganese-rich iron-formation probably similar to Ca. The identity of this mineral is generally contains appreciable barium, but much of it is uncertain. Schmidt (1963) mentions a Sr-bearing mineral incorporated within Mn phases. In one gnarled of the plumbogummite group in the 525-foot level of the ore sample, however (G 288), perfectly idiomorphic plates Armour 1 mine together with specularite. The of barite as much as 1 mm in length are present in vugs, plumbogummite group (better termed the crandallite where they are replaced by romanechite. group) has the principal formula: Manganese oxides and oxyhydroxides

The textural and chemical attributes of manganese with X=Pb (plumbogummite), Ba (gorceixite), Sr oxides and in iron-formation of the Gloria (goyazite), or Ca (crandallite). Minerals of this group core are complex. A variety of phases can be contain 10-20 wt. percent H20, 15-34 wt. percent P20 S, distinguished on the basis of color, reflectivity, and and 20-40 wt. percent A120 3; goyazite has 17-22 wt. textural attributes. In reflected light most of the phases percent SrO, crandallite has 7-15 wt. percent CaO, and are extremely fine grained or amorphous. The reflectivity deltaite has 22-27 wt. percent CaO. However, no of manganomelane minerals varies considerably with minerals of the plumbogummite group are known to composition; e.g., K-rich cryptomelane has higher contain appreciable manganese and iron. Nonetheless reflectivity values than does Ba- or Mg-rich romanechite. calculations of the available analyses to 21 (0, OH, F, Microprobe analyses can distinguish between minerals Cl) as in plumbogummite result in a structural formula of the manganomelane group (AI_2BgOI6·nH20, with A =

30 K, Ba, Na, Pb; B = Mn4+, Mn2+, Fe3+, AI, Co, etc.), and 3 wt. percent BaO, 0.3-1.3 wt. percent K20, and with "pure" tetravalent or trivalent oxides. A further BaO/K20 ratios 2.4 to 5.1). Ba-rich romanechite (BaO distinction between tetravalent oxides (pyrolusite or ~­ 12-16 wt. percent) and manganite are later phases. Pods Mn02; Mn4+I.xMn3+x02.xOHx; or y­ of romanechite contain domains that differ in their Ba Mn02) is not easily possible. Phases that total to less and K contents, and locally Ba-rich and Ba-free oxides than 95 wt. percent are assigned to the trivalent are interlayered on a small scale. Mn minerals in G 288 manganese oxyhydroxides such as manganite (y-MnOOH; contain only trace amounts of Co «0.04 wt. percent) or the polymorphs feitknechtite, ~-MnOOH, and , and Pb «0.02 wt. percent). a-MnOOH). A mineral that can be easily identified in Manganese minerals are typically concentrated in both reflected light microscopy and in microprobe pods or nodules in cherty iron-formation (G 298) having analyses is Iithiophorite (simplified (AI,Li)OH2'Mn02)' abundant martite, goethite, quartz, calcite, and apatite. This strongly pleochroic and anisotropic mineral contains Two textural types can be distinguished-coarse-grained high alumina values. tabular, and fine-grained acicular. They are X-ray powder diffraction analyses of massive Mn­ compositionally similar and consist of intermediate and Fe-rich intervals were carried out at the Department cryptomelane-romanechite minerals with BaO values of of Geosciences, North Dakota State U ni versity. 3.7 to 6.3 wt. percent and K20 values of 3.5 to 6.0 wt. Manganese and iron oxides are variably crystalline. percent. Manganomelane minerals contain detectable Cryptomelane, manganite, pyrolusite, and lithiophorite values of Co and Pb (as much as 0.2 wt. percent). (or woodruffite?) were detected as manganese minerals. Alumina contents are in the 2 to 4 wt. percent range, and A list of phases is given in Table 7. iron may be as much as 10 wt. percent. Preliminary microprobe data show that the Mn phases Laminated Mn-rich iron-formation (G 451) carries within thick-bedded iron-formation are either abundant cubes of a pseudomorphosed Fe- and Mn-rich "manganite," lithiophorite, or manganomelane group phase. Lamellar replacement similar to trellis-type minerals, with romanechite (Ba) and cryptomelane (K) lamellae is commonly observed. Microprobe analyses prevailing. Water-free minerals like pyrolusite or vary between 1 and 37 wt. percent MnO and 37 to 86 hollandite are excluded because of consistently low totals. wt. percent Fe203; precursor minerals probably were It is assumed that all measured phases contain OH and jacobsite, Mn-Fe spinel, or hausmannite, which have been H20. oxidized to hematite and manganite. Manganite, "Gnarled ore" (G 288) consists dominantly of containing small amounts of AI, Fe, and Mg, forms a interlayered colloform-botryoidal goethite, barite, and matrix around replaced Fe-Mn oxide in oxide-rich layers. "psilomelane" (mixed romanechite-cryptomelane with 1- All Mn minerals carry less than 0.15 wt. percent Pb, and less than 0.1 wt. percent Co.

Table 7. Minerals detected by XRD in manganese- and iron-rich ores from the Gloria core Sample G 390 G435 G 451 G463 G 521 G 579a* G 579b" G642 G750 Quartz x x x x x x x Calcite x Apatite x Goethite x x x x x x x Hematite x x x x x x Manganite x x x Pyrolusite Mn02 x x Cryptomelane-type x x x x minerals Lithiophorite or woodruffite x *porous or "vuggy" portion "massive portion

31 Conspicuously banded Mn-Fe ore interlayered with anisotropic pyrite. The rims contain included calcium­ chert (G 574) consists of acicular goethite, masses and rich carbonates, stilpnomelane, and apatite. Anisotropic veins of manganomelane, and grains of lithiophorite. pyrite is in tum rimmed by intergrown arsenopyrite. A Romanechite (11-16 wt. percent BaO, <3 wt. percent few lenses have outermost rims of magnetite intergrown K20) and intermediate romanechite-cryptomelane (BaOI with pyrite and xenomorphic platelets of chalcopyrite. K20 from 1 to 3) are interlaminated on a small scale. Chalcopyrite, pyrrhotite, and trace amounts of Crosscutting veins generally consist of B a-rich also may be included phases in coarse-grained pyrite romanechite. Lithiophorite occurs as anhedral grains 20 cores. Arsenopyrite and slightly martitized magnetite, to 50 microns in size and contains 51-53 wt. percent the latter typically containing included sulfides, also occur Mn02, 24-25 wt. percent A120 3, 1.0-3.7 wt. percent MgO, in the matrix outside the sulfide lenses. and less than 1 wt. percent Si02, Fe203, CaO, and K20. Sulfides in zoned vugs and veinlets are associated Elevated Co contents are characteristic (0.2-0.6 wt. with intervals that have undergone intense brecciation. percent Co). They have a variety of textures and paragenetic Millimeter-size pods of Mn hydroxides in sample G relationships. Vugs typically have cores of intergrown 735 contain cores of relatively large, tabular crystals as quartz and iron-rich (Sid84Mgs IORdc4Ca12) carbonate. much as 50 m in length of a Mn hydroxide with almost They are replaced in part by goethite having included Mn02 stoichiometry, probably manganite [MnOOH]. The hematite or ilmenite. Some vugs are rimmed by quartz, cores are surrounded by thick rims of very fine grained, needle-shaped grains of stilpnomelane, and relics of heterogeneous material that contains 31 to 49 wt. percent pyrite, marcasite(?), and chalcopyrite, in tum overgrown Mn02, 32 to 48 wt. percent Fe203, and lesser amounts of and replaced by hematite or martite. Other vugs are Si, AI, Mg, K, and Ca. Totals of 83 to 86 wt. percent lined by intergrowths of pyrite and arsenopyrite. and calculated formulae do not match the compositions Pyrrhotite, chalcopyrite, probably , and a white, of Mn4+ minerals. Therefore the rims are probably an strongly anisotropic mineral similar to are included amorphous mixture of Fe and Mn hydroxides, or hydrated in the pyrite. Mn3+ oxide, e.g., . It is noteworthy that the Mn Coarse magnetite occurs in many vugs where it is minerals in G 735 are essentially free of Ba «0.03 wt. rimmed by martite. Much of the magnetite contains percent), but carry consistent Co values (0.02-0.07 wt. included chalcopyrite (common), pyrite and marcasite percent) and, rarely, Pb «0.07 wt. percent). (less common), and a yellowish to gray mineral that is either pyrrhotite or bornite (rare). The included sulfides Sulfides have a skeletal texture suggestive of rapid quenching and also are rimmed by martite. Groundmass material The lower part of the Gloria core is atypically between vugs also contains magnetite, but this phase lacks enriched in sulfides, especially between 920 and 1115 included material and is not martitized. Apparently the feet. They occur in at least five different structural and brecciated intervals contain two generations of magnetite tectonic settings. These are: (1) more or less concordant distinguished by size, presence or absence of included layers and lenses (G 1059); (2) discordant, irregularly sulfide, and degree of martitization. Some of the martite shaped and complexly zoned vugs and vein lets associated is rimmed by fresh magnetite, which in tum is rimmed with quartz, carbonate, and stilpnomelane (G 1150, G by a second generation of martite suggesting several 1103, G 1072, G 984, and G 972.7); (3) euhedra sparingly oxidizing events. distributed or concentrated along contacts between chert Still other vugs contain cores of goethite rimmed by and iron-rich strata (G 984, G 9728, G 957, and G 935); pyrite or marcasite and by chalcopyrite intergrown with (4) euhedra disseminated within chert and quartz-sericite stilpnomelane. Idiomorphic pyrite contains included clasts in conglomerate or breccia (G 1062); and (5) pyrrhotite and chalcopyrite and is overgrown by acicular discordant, straight, en echelon veins associated with arsenopyrite and fine-grained "framboidal-like" pyrite or quartz and chert. Regardless of setting, the sulfides have marcasite, which in tum contains included stilpnomelane. a complex paragenesis with oxides, and to a lesser extent, Vein lets associated with the second type of sulfides with silicates and apatite. have similar mineral assemblages. Goethite occurs as Sulfides of the first type-concordant layers and large acicular aggregates, which locally contain narrow lenses-typically are zoned with cores of massive plates of hematite and various combinations of isotropic pyrite and rims of finer grained, porous, slightly chalcopyrite, pyrite, and marcasite. The chalcopyrite

32 occurs as subhedral crystals in the inner part of the veins, Table 8. Chemical parameters of arsenopyrite as blebs, and within cracks in pyrite. Pyrite also contains in the Gloria core small euhedral crystals of galena and pyrrhotite. [number of analyses =13; measured in atom percent] However, most of the sulfides consist of a complexly mean std.dev. range intergrown, slightly pleochroic, and highly anisotropic S 32.47 0.8 31.49-33.83 mineral that most likely is marcasite. In a few places As 34.06 0.72 32.93-34.88 sulfide-bearing carbonate veinlets fill radial fractures Fe 32.82 0.62 31.63-33.54 developed in the outer rims of mineral-filled vugs. Cu 0.023 0.047 0.00-0.14 Abrupt termination of the fractures and their contained Ni 0.01 0.022 0.00-0.08 minerals at the outer edge of the vugs implies that they Co 0.62 0.53 0.07-2.03 formed as contraction features, such as shrinkage cracks. Sulfides of the third type typically occur as disseminated grains or irregularly concentrated aggregates along contacts between layers of chert and hematite plus chert. They have a relatively simple paragenesis 1.01 ASo.99.1.05 SO.94.1.01. In general the iron content of most consisting mostly of monomineraIic pyrite or natural arsenopyrite is typically close to 33.5 atom arsenopyrite. In one aggregate, pyrite, marcasite, percent; lower values in some of the Cuyuna samples chalcopyrite, and arsenopyrite are intergrown with coarse most likely reflect the diadochous exchange between stilpnomelane. .iron and . Most natural arsenopyrite is rich in Sulfides of the fourth type, which occur within (Klemm, 1965), and examples associated with conglomeratic clasts of chert and quartz-sericite, include pyrite typically contain less than 33.3 atom percent As marcasite, pyrite, and chalcopyrite. The two latter (Kretschmar and Scott, 1976). That value is within one sulfides also occur as included grains in partly martitized standard deviation of the invariant composition at 33.0 magnetite. atom percent. Some samples of arsenopyrite (Table 8) Sulfides of the fifth type occur in discordant, straight, contain excess arsenic and thus should coexist with other en echelon veins that transect primary layering, breccia arsenic-rich phases such as . Assemblages that zones and their mineral-filled vugs, and layers of contain arsenopyrite and loellingite also may contain mineralized chert. The veins contain quartz, some pyrrhotite. However, in the Cuyuna samples loellingite carbonate, stilpnomelane and sulfides including was not observed, and pyrrhotite occurs only rarely as chalcopyrite, pyrite, and arsenopyrite. The latter is an included phase in pyrite. Several models may explain rimmed by a second generation of pyrite. The veins also the apparent lack of arsenic in pyrite. contain anhydrite, apatite, and barite as inclusions in (1) Both pyrrhotite and arsenopyrite were formed early, quartz; veins commonly have alteration selvages of alkali and the pyrrhotite was subsequently sulfidized to produce feldspar. pyrite. The compositions of the sulfides were determined in only a preliminary way. No chemical differences exist Arsenopyrite l + pyrrhotite + sulfur -> arsenopyrite 1 + in the composition of iron sulfide in zoned sulfide lenses. pyrite Both massive-isotropic and porous-slightly anisotropic (2) Arsenic-rich pyrite and arsenopyrite were formed early phases are generally pure FeS2; arsenic «0.06 wt. and were subsequently metamorphosed to produce percent), cobalt (0.02-0.07 wt. percent), «0.01 arsenic-free pyrite. The freed arsenic was then taken up wt. percent), and «0.06 wt. percent) occur only in trace amounts. was detected in all samples at levels by the arsenopyrite. between 0.03 and 0.29 wt. percent. Iron sulfides, Arsenopyrite l + arsenic-rich pyritel + heat -> pyrite + including both pyrite and marcasite in other textural arsenic settings, have similar compositional attributes. Arsenic + arsenopyrite l -> arsenopyrite2 Arsenopyrite from a variety of textural settings has broadly similar compositions (Table 8). Analyzed (3) Arsenic-rich pyrite formed early and was subsequently samples are depleted in iron and sulfur and have variable metamorphosed to produce exsolved pyrite and Co (0.07-2.2 wt. percent) and Cu (0.01-1.3 wt. percent) arsenopyrite. contents. Analyses yield the general formula (Fe,Co)0.98. Arsenic-rich pryite l + heat -> pyrite + arsenopyrite3

33 (4) Pyrrhotite formed early and was converted to arsenic­ DISCUSSION rich pyrite by reaction with a sulfur- and arsenic-rich fluid phase. During a second stage the arsenic-rich pyrite A vast amount of literature exists regarding the exsolved to produce pyrite and arsenopyrite. geology and mineral resources of the North range. Grout and Wolff (1955) assumed that the Trommald Formation Pyrrhotite + As + S (vapor) -> As-rich pyrite of Schmidt (1963) was correlative with the Biwabik Iron As-rich pyrite + heat -> pyrite + arsenopyrite Formation of the Mesabi range, and therefore a chemical precipitate containing 15 to 30 percent iron. Although Regardless of its precise origin, arsenopyrite in some Harder and Johnston (1918) had suggested earlier that situations can be used as a geothermometer (Kretschmar much of the manganese was added to the iron-formation and Scott, 1976), because the As/S ratio in arsenopyrite from outside sources, Grout and Wolff introduced the is very sensitive to temperature changes. However, in idea that the original precipitate also contained 1-15 order to determine an equilibrium temperature, the percent manganese. They assumed that mafic igneous arsenopyrite must coexist with two other Fe- and S­ rocks that crop out near the south edge of the North bearing phases like pyrite and pyrrhotite, or aS 2 should range were intrusive in origin. Although metamorphic be known. In sulfide-bearing rocks from Gloria only changes were limited to contact processes near the pyrite was found to coexist with the arsenopyrite. intrusions, the igneous rocks were important in Grout Therefore a unique temperature cannot be determined, and Wolff's view because they were the source of (1) and only temperature ranges are possible. The range for high-temperature vein minerals such as quartz, acmite, the average As content (34 wt. percent) is 450 to 550°C, rhodochrosite, adularia, and stilpnomelane; and (2) the for the lower As contents (33 wt. percent) 400 to 490°C, energy needed to drive a ground-water system that and for the highest As contents (35 wt. percent) 480 to remobilized the manganese and iron ultimately 570°C. Log values for the activity of S2 thus range from concentrated into mineable ore bodies. -4 to -10, with probable values between -5 and -7. A Schmidt (1963) subsequently confirmed that the stable coexistence of pyrite and arsenopyrite restricts the principal ores were the residual concentrations of Fe and upper stability limit to 491°C; above that temperature, Mn oxides formed from iron-formation by solutions that arsenopyrite and pyrite break down to pyrrhotite and oxidized ferrous iron and leached silica, CO2, Mg, and liquid (Clark, 1959). Ca. Schmidt recognized two general types of ore-red Sulfides in sulfide iron-formation in the Rabbit Lake or reddish-brown, dominantly hematitic ores with relict Formation consist dominantly of pyrite, either as fine­ bedding textures, which were the most important ore grained crystals-in places having a framboidal components within thin-bedded rocks of the southern arrangement-or as interlocking grains in vaguely parts of the North range; and yellow to brown, dominantly laminated nodules that typically contain included goethitic ores that lack relict bedding, which formed ore subhedral pyrrhotite. Rotation of the nodules into the bodies in the northwestern part of the range. The direction of the metamorphic foliation implies a hematitic ores contain more boron (70-200 ppm) than predeformational origin. One sample (G 193) contains either the goethitic ores or the unaltered iron-formation idiomorphic crystals of arsenopyrite set in a matrix of (0-50 ppm). They also apparently predate the goethitic coarse-grained quartz. ores. Thus Schmidt proposed two stages of ore There is little if any difference in the composition of development. The first, a hydrothermal stage, was large euhedral pyrite blasts and fine-grained matrix spatially related to faults and igneous intrusions ~nd crystals. Arsenic (0.02-0.08 wt. percent) always is present involved deep hematitic oxidation by boron-carrymg in trace amounts, whereas nickel and values are solutions. A second, stage was characterized less than 0.01 wt. percent. Subhedral inclusions of by more irregularly distributed oxidation and leaching pyrrhotite have similar cobalt (0.03-0.09 wt. percent) and and by the development of widespread blanket-like ore arsenic (0.03-0.04 wt. percent) values. Microprobe bodies rich in goethite. In several places it is clear that analyses result in a stoichiometric formula of Feo.88S or the goethite ores formed from pre-existing hematitic ores Fe7S8. If formed as primary hexagonal pyrrhotite, an Fe and thus clearly represent a younger ore-forming event. content of 47 atom percent would indicate a temperature Renewed interest in the manganese potential of the of about 400°C, if pyrrhotite and pyrite were in North range has led to a re-evaluation of the origin of equilibrium (Scott, 1973). the manganese oxides. In particular, Schmidt's ideas

34 regarding hydrothermal processes on the North range have hot solutions ascending under high fluid pressure into a been critically re-evaluated. Uncommon minerals in the sequence of largely unconsolidated sediments. The thick-bedded rocks, such as hyalophane (McSwiggen and breccias are associated with a variety of sulfides­ others, 1994a), aegirine [acmite] (McSwiggen and others, dominated by pyrite, marcasite, arsenopyrite, and 1994b), Sr-rich barite (McSwiggen and others, 1995), chalcopyrite-that may well have precipitated from and tourmaline (Cleland and others, 1993), are consistent hydrothermal solutions early in the diagenetic history of with a fumerolic vent system that exhaled highly saline the sediment, after diagenetic magnetite had formed. and/or high-temperature fluids into sulfur-deficient Pyrite commonly carries included pyrrhotite and galena. seawater. We differ from Schmidt (1963) in concluding Remnant pyrrhotite implies solutions at very high that the hydrothermal system was active earlier rather temperature, whereas some sulfide textures imply than later in the depositional history of the iron-formation. quenching and rapid cooling. Temperatures assumed Precipitation of the Trommald Formation at the from the arsenopyrite geothermometer range from 400°C Gloria site started with deposition of AI-rich, Mn-poor, to 500°C. Relatively low Co/Zn values (average 0.79 in thin-bedded silicate (chlorite)-oxide (hematite, ilmenite) Zone A, range 0.29 to 2.5) also are probably compatible iron-formation. Primary hematite implies that with a hydrothermal origin. According to Toth (1980), sedimentation started under somewhat oxidizing hydrothermal deposits usually have low Co/Zn values, conditions. The lowermost beds of iron-rich strata contain whereas hydrogenous deposits have values >2.5. Co/Zn admixed volcanogenic or clastic material like that in the and ColNi ratios tend to increase upward within the Zones underlying Mahnomen Formation. Chemical A and B; this was also observed in the neighboring sedimentation subsequently was overwhelmed by an Merritt hole (McSwiggen and others, 1995). It probably influx of volcanogenic material leading to reducing reflects increasing hydrothermal alteration, because Col conditions where precursors to iron silicates, such as Ni values in some massive sulfide deposits shows good stilpnomelane and chlorite, and a variety of carbonates correlation with the intensity of hydrothermal alteration ranging in composition from Fe- and Mn-bearing (Almod6var and others, 1995). dolomite to ankerite and siderite, could be preserved. Much of the sulfide material is included within We speculate that during this early diagenetic stage, coarse-grained magnetite, which in turn is altered to accumulated organic material reacted with hematite to martite. This implies that reducing conditions first produce magnetite. Syndepositional sedimentary prevailed, but that they were soon superseded by structures, such as slumps, intraformational oxidizing solutions. conglomerates, and granular textures, imply periodic turbulence in what must have been a somewhat deeper­ Evidence for a Hydrothermal System water environment. High phosphorus values in many units of oxide The evidence above implies that the Trommald (hematite) iron-formation from around the world have Formation is basically a chemical , been attributed to organic processes operative in and composed mostly of iron and silica, admixed with a around an oxygen-producing photosynthetic zone. volcanogenic or detrital component. This precursor However, the lowermost oxide-rich part of the Gloria sediment was considerably modified during one or more core has some textural attributes that resemble those in hydrothermal events when Mn, Ba, and to a lesser extent the "sandy, clayey, and oolitic, shallow, island-dotted other constituents, such as Cu and Sb, were introduced. sea" iron-formation of Kimberley (1989a). According Supporting evidence for a hydrothermal event is to him, much of the phosphorus in this type of iron­ summarized here. formation can be attributed to inorganic processes associated with an iron-rich fluid. He did not specify a Rare earth element signatures source of that fluid, but a hydrothermal system is possible. Rare earth element concentrations in iron-formation Hein and others (1994) have shown that phosphorus in are controlled by the signatures of (1) a dilute modern marine hydrothermal deposits is commonly hydrothermal solution, (2) the precipitating agent­ scavenged by iron oxyhydroxides contained within commonly seawater, and (3) any contaminating sources, hydrothermal plumes. such as terrigenous detritus or volcanogenic material. Much of Zone A is marked by beds of breccia that Iron-formation that is admixed with considerable amounts resemble "hydrothermal explosion intervals," caused by of volcanogenic material typically has large REE totals,

35 mainly because of the relative abundance of these oxygen-deficient. Therefore hydrothermal solutions were elements in some silicate minerals compared to their the main source for REE and excess Eu in Archean absence in minerals such as oxides and carbonates that seawater (Danielson and others, 1992). The precipitated from seawater or hydrothermal solutions. In disappearance of a positive chondrite-normalized Eu modern environments, iron-rich sediments from anomaly in Early Proterozoic time is attributed to a hydrothermal fluids near submarine vents have decrease in amount of high-temperature fluid in the characteristically low REE concentrations, distinctly marine environment. negative Ce anomalies, and distinctly positive Eu Tourmaline in stratiform tourmalinites is generally anomalies. Such sediments have scavenged the REE from attributed to hydrothermal solutions operating at or just seawater, which has a negative Ce anomaly (Lottermoser, beneath the sediment-water interface (Cleland and others, 1992). The hydrothermal signature is masked by other in press). Tourmaline-bearing rocks in the Trommald components when the precipitate is mixed with material Formation are enriched in total REE (average 173 ppm) from a source that contains appreciable amounts of REE. and especially in the LREE (LalYbch5.5-16.9). However, Because they are contaminated in one way or another, it Ce or Eu anomalies, which would indicate a hydrothermal is only the combined low total REE values and positive component in these rocks, are probably masked by the Eu anomalies that suggests a hydrothermal origin for iron­ primary REE signature of the host rocks. formation in general (Danielson and others, 1992). Cerium anomalies result from diagenetic processes Discrimination diagrams and cycling (Derry and Jacobsen, 1990); modern oceans display a wide range of cerium behaviors, both Several discrimination diagrams employing major positive and negative, resulting from redox cycling across and trace element concentrations or ratios have been used anoxic-oxic boundaries in the water column or at the to distinguish between processes for chemically water-sediment interface. Small cerium anomalies in Late precipitated iron- and manganese-rich sediments. These Proterozoic iron-formation indicate the beginning of diagrams mainly distinguish between hydrothermal, global oxidative processes that removed cerium and hydrogenous, pelagic, or diagenetic environments in probably manganese and barium from the ocean system. modern geologic settings. A summary of the more Negative cerium anomalies in some manganese common diagrams is given in Bostrom (1983). The formations of Precambrian age in Africa are explained applicability of such plots to Precambrian iron-formations by the accelerated oxidation of Ce3+ either by rapid may be questioned because such rocks have fairly precipitation immediately after Mn-rich water reached restricted compositions. Nonetheless combined data from the oxygenated environment or by the lack of large several diagrams are useful in delineating possible amounts of biomass (Bau and Dulski, 1993). In the Gloria depositional environments and the role of hydrothermal core, samples that have large Mn and Ba values tend to solutions. have large REE values and negative Ce anomalies. This The silica vs. alumina diagram was used by Crerar implies that these samples originally had a low Ce/Ce* and others (1982) to distinguish hydrothermal from ratio, which can be attributed to a strong hydrothermal hydrogenous or deep-sea sediments. As with other iron­ component. An exception is sample G 481, which has a rich sequences, Gloria core samples scatter widely, distinctly positive Ce anomaly. especially when compared to the Biwabik Iron Formation Normalized to the NASC standard, iron-formation (Fig. 5). Samples of unoxidized thin-bedded iron­ from the Gloria site has positive Eu anomalies that formation that are relatively depleted in SiOz show a resemble or exceed Eu/Eu*NAsc anomalies in the Biwabik trend toward enriched Alz0 3 values, and consequently Iron Formation (Fig. 4). Eu anomalies form in toward the fields defined for hydrogenous or deep-sea hydrothermal systems like those associated with mid­ sediments. This trend likely reflects the presence of a ocean ridges, where Eu3+ is reduced to the Euz+ state distinct volcanogenic component (stilpnomelane), rather during MORB alteration at 300°C in the presence of HzS. than detritus accumulating in a deep-sea environment The discharging fluids are therefore enriched in Euz+ physically isolated from continental shelves. because of the reduced sorption of Eu2+ compared to The magnesium vs. sodium diagram of Nicholson that of Eu3+. In modern settings Eu is scavenged from (1992) was developed to discriminate between freshwater seawater by hydroxides at vent sites-a situation not and deep-water marine hydrothermal deposits with an likely in Archean time when deep ocean water was intermediate field for shallow marine, nonhydrothermal

36 thick-bedded iron-formation plots toward the fields 100 + o Zone A defined for shallow marine and freshwater deposits. hydrothermal Furthermore, the diagram emphasizes the greater Na20 + - Zones B and C + content of oxidized iron-formation than either unoxidized 80 + Zones D and E iron-formation or the Biwabik Iron Formation. + The uranium vs. thorium diagram in Figure 7 dates + Biwabik field back to Bonatti and others (1972) who suggested that the U/Th ratio might be useful in distinguishing t+ hydrogenous "'-"'60rP~.-.l If< hydrogenous Fe-Mn deposits from those of other origins. ! - + They state (p. 153) that "relatively high Th contents in o'" hydrogenous nodules are probably due to their extremely Vi 40 low rate of growth, during which some of the exceedingly low quantities of thorium contained in sea water can be preferentially incorporated in the iron-manganese minerals." Furthermore, once fixed, thorium remains in 20 largely insoluble compounds (Bostrom and others, 1979). However, uranium may be lost from the depositional system because of its greater mobility during . Therefore the usefulness of Urrh ratios o 5 10 15 in metamorphosed deposits is somewhat problematic. Al20 3 (wt %) Nonetheless Figure 7 shows that young metalliferous Figure 5. Silica vs. alumina diagram of Crerar and others sediments forming on active spreading centers can be (1982) showing the dominantly hydrothermal attributes of readily distinguished from slowly accumulating these constituents. Outlined area, field defined by manganese nodules by their larger U/Th ratios (generally samples from the Biwabik Iron Formation which lacks a > 1). Pelagic sediments define a field marked by small known hydrothermal component. Th-U values and UlTh ratios of less than one. Available data for the Biwabik Iron Formation define a unique field marked by very small amounts of U and Th and U/Th deposits (Fig. 6). A spread in MgO values in the marine ratios of one or less. Most of the samples from the field is characteristic of unoxidized thin-bedded iron­ Gloria core also plot in · the Biwabik field, whereas formation in both the Trommald Formation and the samples from Zones C and D have maximum UlTh ratios Biwabik Iron Formation. In contrast, oxidized thin- and

1.0 o ZoneA Marine 0 • Zones B to E 0.8 0 00 ~0.6 00 ~ '-' 0 00 0 o £,0.4 Z • 0 2 3 4 MgO (wt %) Figure 6. Magnesium vs. sodium diagram of Nicholson (1992) for manganese-rich chemical sediments.

37 100~------'------~~ * Sulfide iron-fonnation OZone E: thick-bedded iron-fonnation

• Zone D: Mn ores 10 • Zone C: Mn ores

OZone B+C: leached iron-formation

O.l~~~UU~~~~~~~~~~~~~~~~~--~ 0.001 0.01 0.1 I 10 100 Th (ppm) Figure 7. Uranium vs. thorium diagram (Bonatti and others, 1972).

of 17 and plot near and within the field defined by used. A matrix for 35 samples of iron-rich strata from hydrothermal deposits (e.g., the T AG-Mid-Atlantic the Gloria core is presented in Table 9, giving positive hydrothermal area). Uranium is not easily transported in or negative correlations and their significance (p). hydrothermal fluids, and is generally considered to be Inspection of these values indicates several features that quantitatively removed from seawater (Mills and others, are of significance. 1993). German and others (1993) explain large uranium (1) Significant positive correlations exist between the values (11-19 ppm) in iron-rich strata near the TAG elements AI, Ti, Mg, Ca, Na, K, P, Cr, Cs, Hf, Rb, Sc, hydrothermal mound, by the uptake of uranium released Th, Zr, and REE; variably positive correlations also exist from sulfide phases that were oxidized on the slopes of with V and Y. Unoxidized thin-bedded iron-formation the mound. Thus large uranium values in manganese-rich is enriched in these elements and most likely represents rocks of Zones C and D also may reflect the seawater volcanogenic material, probably ash that first was oxidation of sulfide-rich sediments in the basin or in the degraded into smectites and subsequently was sediment pile, resulting in enrichment of seawater in U recrystallized to form for the most part stilpnomelane. (and Sr, Ba, Ag?) and precipitation of these elements in Some of these elements-such as AI, Ti, Sc, Hf, Cr, Zr, exhalative Mn muds on the seafloor. Tourmalinites and REE-are geochemically immobile and will be used interbedded with iron-formation in Zone A are not enriched in subsequent discussion regarding mass-balance in uranium and straddle a field defined by sedimentary calculations using isocon plots. rocks. (2) Iron (measured as total iron) correlates weakly with most other elements, but correlates strongly with As, Correlation coefficient analysis Cd, Sb, and V. These constituents are typically associated with hydrothermal processes, or, in the case Spearman rank correlations (Rock, 1988) were used of V, with silicate or organic fractions. to statistically combine elements into possible groups. In (3) Manganese correlates weakly with iron, but correlates this scheme, the rank correlation coefficient is a measure strongly with Ag, Ba, Co, Mo, Sr, U, and Zn, elements that detects strongly monotonic relationships between that are typically enriched in hydrothermal fluids and! variables; it is not affected by analytical errors or outliers or in seawater. as happens when simple linear correlation methods are

38 Table 9. Spearman rank correlation cocffi<:ic.n~""'cr,,-N=35. Significance: 0.44-0.72 = >99%; 0.34-0.44 = 95-99%; 0.27-0.34 = 90-95%; and <0.27 = <90%.

81.. li _81. _B> _. l!I!'. ~ Ca Na K PLOt S Ag Jl.s Ba Be Cd Co Cr Cs Cu HI M 0 Ni Pb Rb Sb Sc Sr Th U V W Y Zn ?~_~-". _.-f." 1.00 ·0.21 -0.05 -0.67 ·0.45 0.16 -0.04 0.08 -0.03 -0.37 -0.43 -0.37 -0.32 -0.65 -0.40 0.23 -0.31 -0.41 0.12 0.11 -0.24 0.13 -0.18 ·0.35 -0.06 -0.24 -0.32 -0.18 ·0.61 -0.09 -0.35 -0.26 -0.14 -0.39 ·0.33 -0.29 -0.34 -0.17 Si 1.00 0.90 0.13 0.00 0.49 0.23 0.29 0.43 0.40 0.10 -0.51 0.03 0.06 0.12 0.02 0.34 0.15 0.63 0.35 -0.18 0.72 0.20 0.30 -0.18 0.49 -0.14 0.89 0.21 0.92 0.06 0.50 0.18 0.29 0.25 0.70 0.60 0.59 li 1.00 0.03 ·0.12 0.66 0.36 0.48 0.57 0.39 0.16 -0.60 -0.18 0.00 0.00 0.11 0.32 -0.65 0.67 0.50 -0.17 0.72 0.07 0.15 -0.31 0.64 -0.16 0.80 0.03 0.93 -0.17 0.40 0.09 0.20 0.10 0.61 0.47 0.47 AI 1.00 -0.04 -0.35 -0.12 -0.11 -0.18 0.25 0.06 0.13 ·0.14 0.53 0.01 -0.29 0.36 0.16 0.22 -0.09 0.06 -0.25 -0.21 0.19 -0.18 0.09 0.36 0.09 0.11 0.04 0.08 0.29 -0.09 0.28 0.07 0.18 0.08 0.01 Fe 1.00 0.06 0.13 ·0.02 0.22 0.04 0.55 0.02 0.67 0.25 0.48 0.14 0.06 0.57 -0.38 -0.02 0.23 -0.08 0.34 0.24 0.26 -0.18 -0.03 0.02 0.49 -0.10 0.39 -0.02 0.14 0.10 0.56 -0.04 0.18 0.02 Mn 1.00 0.63 0.57 0.72 0.38 0.38 -0.20 -0.25 ·0.30 -0.19 0.04 0.19 -0.09 0.35 0.64 -0.11 0.41 ·0.18 -0.06 -0.31 0.80 -0.17 0.43 -0.08 0.59 -0.36 ·0.18 0.28 0.06 -0.01 0.27 0.16 0.19 Mg 1.00 0.52 0.59 0.69 0.40 0.51 -0.21 -0.03 -0.02 -0.15 0.16 -0.07 -0.01 0.44 0.11 0.04 -0.14 -0.07 -0.23 0.58 0.24 0.09 0.23 0.27 -0.12 -0.21 0.03 0.27 ·0.04 0.14 0.16 0.07 Ca 1.00 0.66 0.38 0.34 0.00 -0.23 ·0.29 -0.30 -0.20 0.19 -0.20 0.23 0.50 -0.15 0.18 -0.31 -0.19 -0.24 0.41 -0.11 0.21 -0.30 0.45 -0.57 -0.28 ·0.07 -0.04 -0.16 0.12 0.00 0.00 Na 1.00 0.43 0.42 -0.41 -0.08 -0.07 -0.17 0.12 0.20 0.18 0.26 0.77 0.03 0.23 -0.11 0.09 -0.13 0.83 -0.04 0.33 -0.02 0.51 -0.30 0.00 0.01 0.20 0.12 0.14 0.20 0.10 K 1.00 0.32 0.21 -0.08 0.17 -0.05 -0.26 0.22 0.15 0.16 0.35 0.07 0.06 -0.15 0.10 -0.20 0.72 0.27 0.24 0.30 0.38 0.06 -0.06 -0.02 0.58 0.08 0.30 0.47 0.39 P 1.00 0.53 0.16 0.25 0.10 0.20 -0.02 0.37 -0.09 0.28 0.19 -0.07 -0.12 0.18 0.04 0.17 0.15 0.14 0.23 0.11 -0.03 -0.14 0.10 0.20 0.29 0.07 0.09 -0.05 LOI 1.00 -0.26 0.39 0.17 -0.13 -0.54 -0.36 -0.56 -0.45 0.76 -0.44 0.20 -0.10 0.46 -0.36 0.65 -0.55 0.38 -0.52 ·0.19 -0.45 -0.26 0.25 -0.50 -0.31 -0.12 -0.23 S 1.00 0.13 0.50 0.19 -0.18 0.53 -0.38 -0.32 0.24 0.01 0.62 0.33 0.45 -0.47 -0.09 0.09 0.55 -0.13 0.64 0.19 0.21 0.07 0.52 0.01 0.32 0.24 Ag 1.00 0.41 0.15 0.07 0.26 -0.03 -0.17 0.33 -0.06 0.28 0.26 0.22 -0.08 0.57 -0.03 0.49 -0.06 0.28 0.42 -0.05 0.41 0.17 0.10 0.25 0.08 Jl.s 1.00 0.14 -0.08 0.20 -0.41 -0.42 0.30 0.06 0.59 0.37 0.40 -0.20 -0.03 0.00 0.76 -0.07 0.54 0.33 0.04 0.11 0.31 0.18 0.25 0.09 Ba 1.00 -0.18 0.29 0.12 0.15 0.12 0.22 0.21 0.41 0.30 -0.34 -0.04 0.17 ·0.02 -0.02 0.16 0.35 ·0.24 -0.20 0.41 -0.12 -0.12 -0.17 Be 1.00 0.09 0.35 0.18 -0.30 0.35 -0.06 0.27 -0.36 0.86 -0.12 0.35 0.02 0.30 -0.10 0.28 -0.09 -0.07 0.37 0.24 0.04 0.05 Cd W \0 1.00 0.03 0.10 0.21 -0.03 0.19 0.63 0.29 0.06 0.15 0.26 0.33 0.03 0.50 0.35 0.15 0.29 0.67 0.10 0.32 0.27 Co 1.00 0.38 -0.32 0.59 -0.20 0.18 -0.41 0.33 -0.09 0.67 -0.36 0.69 -0.24 0.42 0.04 -0.01 0.06 0.46 0.16 0.23 Cr 1.00 -0.06 0.13 -0.37 -0.01 -0.34 0.73 0.10 0.40 -0.29 0.47 -0.36 -0.04 0.07 0.25 0.06 0.06 0.15 0.16 Cs 1.00 -0.30 0.31 0.33 0.30 0.36 0.47 -0.18 0.41 -0.25 0.28 0.05 0.03 0.31 0.21 -0.08 0.17 -0.01 Cu 1.00 0.31 0.25 -0.03 -0.03 -0.29 0.69 0.04 0.69 -0.13 0.47 -0.01 -0.04 0.23 0.53 0.31 0.34 HI 1.00 0.23 0.50 -0.24 -0.05 0.18 0.63 0.03 0.55 0.49 0.09 0.07 0.39 0.08 0.43 0.34 Mo 1.00 0.28 0.12 0.05 0.38 0.41 0.10 0.42 0.63 -0.22 0.11 0.74 0.22 0.15 0.09 N 1.00 -0.31 0.05 -0.18 0.32 -0.36 0.28 0.15 ·0.14 -0.06 0.15 -0.30 0.00 0.00 Pb 1.00 0.27 0.32 0.00 0.58 -0.10 -0.11 0.20 0.38 -0.03 0.22 0.25 0.29 Rb 1.00 -0.14 0.21 -0.13 0.00 0.04 0.18 0.51 -0.10 0.00 0.18 0.05 Sb 1.00 0.10 0.86 0.10 0.52 0.24 0.18 0.38 0.67 0.52 0.56 Sc 1.00 0.02 0.67 0.31 0.17 0.38 0.38 0.24 0.49 0.32 Sr 1.00 -0.18 0.31 0.23 0.29 0.05 0.72 0.56 0.57 Th 1.00 0.38 0.16 0.08 0.50 0.03 0.35 0.28 U 1.00 -0.18 0.12 0.45 0.34 0.30 0.29 V 1.00 0.12 -0.14 0.25 0.33 0.34 W 1.00 0.06 0.21 0.74 0.64 Y 1.00 0.07 0.20 0.09 Zn 1.00 0.39 0.36 Zr 1.00 0.94 La (4) Silica correlates negatively with Fe and Mn. Oxide-rich thin-bedded iron-formation that contains (5) Significant correlations of P, Ca, REE, Y, and Sr stratabound layers of manganese and iron hydroxide suggest the presence of apatite in much of the Gloria (Zone C) is 20-30 percent lower in mass on the basis of core. constant alumina than unoxidized rocks (Fig. 10). Individual hydroxide layers show a mass loss of 50 to Mass-balance calculations 60 percent. Some immobile elements do not plot on a well-defined isocon, with deviations of +64 percent (Ti) The hypothesis that unoxidized thin-bedded iron­ to -35 percent (Th) compared to AI. The silica average formation underwent major geochemical changes by is constant compared to Zone A, but, as in Zone B, Mg, hydrothermal fluids can be tested by the isocon methods Ca, Na, K, and P are depleted by 60 to 90 percent. Fe, of Grant (1986) using mass-balance calculations of V, Ni, and REE are enriched 50-100 percent; Co, As, Gresens (1967). This method graphically measures Cd, Y, and Zn are enriched as much as 200 percent; and changes in the concentration of various chemical Mn, Ba, Ag, Sr, and U are enriched 200 to 1300 percent constituents as brought on by metasomatic and with respect to AI. Thus, Zone C is enriched in elements hydrothermal processes. The assumption that the of the hydrothermal group with respect to Zones A and concentration of immobile elements remains unchanged B. provides a reference isocon against which other mass or Manganese-rich, thick-bedded iron-formation of Zone volume changes can be computed. The two most D yields similar results (Fig. 11). The Al isocon indicates commonly used elements for reference isocons are Al a mass loss of 40 to 50 percent with respect to Zone A. and Zr, and in many situations they plot on the same However, most immobile elements, except REEs, plot chord. In the investigated data set, however, of the two, close to the Al isocon (Zr +24 percent; Th -29 percent). Al has a smaller analytical error, and therefore was chosen Silica and iron plot close to the constant mass line; Mg, to define a reference isocon. Ca, K, P, and Na lost about half in mass compared to Changes in mass concentration associated with Zone C. One group of elements-the REE as well as sulfide mineralization in Zone A are illustrated in Figure Ni, Zn, Cu, Co, and As-plot on an isocon indicating 8, where the average composition of unoxidized sulfide­ approximately a 150 percent gain compared to Al and 45 free (S <50 ppm) iron-formation is compared to the percent if constant mass is assumed. A second group of average composition of unoxidized sulfide-bearing (S >50 elements, Mn, Ba, Pb, Sr, U, and Ag, shows immense ppm) iron-formation. On the basis of constant alumina, gains on the order of 1000 to 10,000 percent (Ba). The sulfide-bearing samples show a mass loss of 42 percent pronounced enrichment is of crucial significance, compared to sulfide-free samples. The "immobile" suggesting precipitation from a mixed seawater­ elements AI, Ti, Zr, Sc, and Th, as well as Cd, plot on hydrothermal component. the isocon. Si is enriched 37 percent, whereas elements Table 10 summarizes the mass-balance calculations. such as Fe, Mg, K, Ba, La are enriched 50 to 80 percent The isocon diagrams for Zones A to C (Figs. 8-10) are and plot on the line of constant mass. A group containing in accordance with a modification of thin-bedded silicate­ Mn, Y, and evidence of LOI is enriched 150-200 percent, carbonate iron-formation by hydrothermal fluids. Almost and As, Cu, Ca, Sr, Sb, P, and S are enriched 200-5000 identical enriched-element groups can be found in sulfide­ percent. Therefore sulfide mineralization was marked bearing Zone A as well as in Zones B, C, and D. Zones by the addition of As, Cu, and Sb and was accompanied B, C, and D are also characterized by extensive by the addition of carbonate (Mn-Ca-Sr-LOI) and/or destruction of carbonate, silicate, and apatite components. apatite (Ca-P-Y -Sr). Thus, it is likely that sulfur, the The Fe-As-V enrichment observed in Zones Band C is base metals, phosphorus, and manganese were transported absent in Zone D. Thick-bedded Mn-poor oxide iron­ in the same fluid. formation (Zone E) is different and cannot be derived Possible compositional changes between sulfide-free from modification of Zone A rocks. The differences unoxidized iron-formation and oxidized thin-bedded iron­ between Zone A and Zones B to D seem to result from formation of Zone B are shown in Figure 9. Na, Ca, changes in the oxidation potential of the solutions, rather Mg, K, P, and S are lost from the system, most probably than from changes in transported metals. In the lower because of the complete decomposition of sulfide parts of Zone A, solutions precipitated in a reducing minerals, carbonates, and silicates. However, the plot environment form sulfides and carbonates. The fact that shows that iron, along with V, Co, As, and U, has been introduced into the system.

40 35 35

A~ ..<:: 30 30 V+ () :3 ·c :3 I +Ca l» F~ e; V> V> IU V> 0' ;g 25 0' "0 25 V> V> V> ;; V> IU A~ .p.. "0 .p.. '" "0 0 -d N IU .g 20 <;>l ..cr 20 <;>l "0 ~ IU .D S ~ "0 .515 IU 15 -5 ;.0N "0 IU ·x ;.010N 0 10 ·x 0 ~ ;:l 5

0 0 5 10 15 20 25 30 o 5 10 15 20 25 30 unoxidized thin-bedded, sulfide-free unoxidized thin-bedded

Figure 8. Isocon diagram (Grant, 1986) of sulfide-bearing Figure 9. Isocon diagram (Grant, 1986) of thin-bedded oxide silicate-carbonate iron-formation vs. sulfide-free silicate­ .iron-formation (Zone B) vs. silicate-carbonate iron­ carbonate iron-formation (Zone A). formation (Zone A).

Ba Mn 35 35 A~ v+ :3 30 l» en 30 "0 en IU F~ "0 0' "0 S+ V> ] 25 U+ \ V> 25 b I N I Sr c Cd 0 "0 <;>l IU + £ + "0 gf 20 Mn+ ] 20 .D ·fii Zn+ ,;.::I IU () .D Ag+ I 15 :§ 15 ~ ..<:: :::s () "0 ·c IU N 10 blO ;.0 :::s ·x 0 5 5 K 0 0 0 5 10 15 20 25 30 0 5 10 15 20 25 30 un oxidized thin-bedded unoxidized thin-bedded Figure 10. Isocon diagram (Grant, 1986) of thin-bedded Figure 11. Isocon diagram (Grant, 1986) of thick-bedded Mn­ Mn-bearing oxide iron-formation (Zone C) vs. silicate­ bearing oxide iron-formation (Zone D) vs. silicate­ carbonate iron-formation (Zone A). carbonate iron-formation (Zone A).

41 Table 10. Summary of mass balance calculations for different zones of the Gloria core compared to unaltered Zone A average. (Mass and element group gains and losses in percent are calculated on the basis of constant alumina.)

Zone Mass Gains in Losses III ercent ercent A sulf* -42% Mn, Y, LOI 150 As, Cu, Sb, S, Ca, Sr, P >200 B -45% Fe, V, Co, As, U 100-200 Mg, Ca, Na, K 50-85 C -25% Fe, Ni, V, REE 50-100 Mg, Ca, Na, K, P 60-90 Co, As, Cd, Y, Zn 100-200 Mn, Ba, Ag, Sr, U >200 D -45% Ni, Cu, Zn, Co, As, REE 150 Mg, Ca, K 40 Mn, Ba, Pb, Sr, U, Ag, Mo >1000 P, Na 80 E 0% Si, Th, Zr 50-60 Ca, P, K, Mg, Na, Mn, Cr, Cd, Sc, Sr, Mo, U 100-300 LOI, Sb, Y, Co, Cu 50-100 *A sulf, sulfide-bearing Zone A iron-formation

many of the sulfides are mantled by magnetite, which in estimated volume changes, as calculated from the turn is rimmed and partly replaced by martite, implies thermodynamic data of Robie and others (1978) and that the solutions became oxidizing over time. The same Miyano and Klein (1989) include: enriched elements (with the exception of Sand Cu) were precipitated as oxides/hydroxides and minor carbonates (1) , minnesotaite -> goethite + quartz + in Zones B to D. The close association of the elements minor AI20 3 (approximate volume loss is 58 percent) Fe-As-V in Zones Band C is explained either as representing an addition of a hydrothermal component (2) Stilpnomelane -> goethite + quartz + AP+ + K+ + different from the Mn oxide factor, or as a result of Na+ + Ca2+ (approximate volume loss is 66 percent) simple mineralogical transformation of Fe silicates and (3) Fe-Mg-Ca-(Mn) carbonates -> goethite + CO2 + carbonates into goethite and hematite. Mg2+ + Mn2+ (approximate volume loss for Fe-rich Chlorite-rich schist in Zone A contains abundant carbonate to goethite is 30 percent). The reaction results tourmaline. Textural and geochemical evidence (Cleland in an increase in acidic conditions due to the secondary and others, in press) indicates that the tourmaline was reaction: derived when a boron-rich fluid reacted with AI- and Ti­ Fe3+ + 3H20 -> Fe(OH)3 + 3H+ rich sediments. This kind of tourmaline belongs to the replacement type of Slack (1992). The tourmaline­ (4) 3 magnetite -> 4 hematite (martite) + Fe2+ + 2e­ bearing rocks also contain some sulfides; Zn, Cu, Co, (approximate volume loss is 10 percent). and Ni contents are greater than found in sulfide-bearing iron-formation. The difference may reflect hydrothermal These reactions assume that the iron is oxidized and additions or simply reflect the original bulk composition incorporated into hydroxides which, based on magnetic of the precursor rock. Regardless, Co/Zn ratios are low evidence, formed at some temperature above the Neel (0.26-0.53) and point to involvement of a hydrothermal point. During these reactions silica would be fluid. Cu and Pb show positive correlations with boron. reprecipitated in situ as fine-grained quartz. Other Therefore it is probable that the boron-rich solutions were constituents, such as dissolved Mg, Ca, K, and Na, would part of the same hydrothermal event that produced the be transported over some distances and reprecipitated sulfides. elsewhere as secondary carbonates or incorporated into Textural evidence implies that much of the goethite manganese hydroxides. Zone C has chemical attributes in oxidized iron-formation of Zones Band C formed that are broadly similar to those of Zone B. It contains, from the decomposition of silicates and carbonates. however, layers and lenses of manganese and iron oxides Possible reactions for these transformations and their and is thus enriched in both constituents. It also is

42 enriched in hydrothermally derived constituents, such as exhalative sedimentary Mn oxides. Their absence implies Ba, Sr, Ag, U, Cd, and Y + Yb (HREE). their almost complete removal from the hydrothermal Zones A and B carry pervasive mineralized veinlets solutions before they reached the Gloria site. containing a complex mineralogy including quartz, In summary, geochemical attributes show that the silicate, Mn-Fe carbonate, sulfide, sulfate (barite, Trommald Formation is a mixture of precipitated anhydrite), and phosphates; alteration selvages of K-spar chemical constituents associated with ordinary iron­ indicate the relatively high-temperature addition of formation and detrital or volcanogenic material introduced potassium. Such vein lets postdate sulfide-mineralized to the basin during deposition, together with constituents breccia zones and stratiform sulfide lenses in Zone A, associated with hydrothermal fluids that also were active and also hydrothermal alteration of silicate and carbonate during deposition of the iron-rich sediments. Rare earth to goethite and hematite in Zone B. The vein lets probably element data point to oxidizing hydrothermal solutions. represent discharge zones of late hydrothermal solutions It is not clear whether the manganese and iron-rich oxides from a deep-seated source; a relation to the deformational formed from short pulses of hydrothermal brines venting events accompanying Penokean metamorphism cannot be onto the seafloor during ongoing deposition of thin­ excluded. bedded iron-formation or reflect metasomatic The change from Zone C to Zone D defines an abrupt modifications in unconsolidated iron-formation beneath change in sedimentary conditions where iron silicates the sediment-water interface. and carbonates in thin-bedded rocks are replaced by granular chert and hematite in thick-bedded rocks. That PALEOGEOGRAPHIC IMPLICATIONS change is marked by a thin unit of tuffaceous material indicative of tectonic activity. Unit D contains thick Geological and geochemical data and their strataform and stratabound layers of admixed manganese interpretations summarized above can be integrated into and iron oxides. The manganese-rich intervals have a general tectonic model. The North range group was complicated secondary textures that imply remobilization deposited on a rifted continental shelf, as first suggested and reprecipitation. The Mn-rich layers, however, are by Southwick and others (1988) and Schulz and others not the result of simple surficial processes. The effect of (1993). Stratigraphic relationships described by Morey such processes is assumed to be small, because sulfide­ and Southwick (1995) suggest that the shelf sedimentation rich beds of the Rabbit Lake Formation are not much was disrupted in places by one or more second-order rift affected by oxidation, and sulfides are still stable. basins in which "deeper water" thin-bedded iron­ Chemically, Zone D is characterized by abundant formation accumulated. Extraformational conglomerates manganese at the expense of iron, which on average is and numerous synsedimentary deformational structures, 20 wt. percent less than in Zones Band C, and around 5- as well as intraformational conglomerates and granular 10 wt. percent less than in Zone A. MnlFe averages 0.6 textures in thin-bedded iron-formation, are evidence of and in places is > 1. Mg and Ca are enriched in Zones B early tectonic instability. Discrete layers of intercalated and C and are present mainly as carbonates that fill pore tuff, together with components now incorporated in spaces between secondary iron-manganese ores. stilpnomelane in the iron-formation itself, are evidence Phosphorus contents are erratically higher than in Zones that tectonism was accompanied by volcanism. As Band C, but vary considerably. CaOIP20s ratios average sedimentation progressed and the tectonically induced 1.45. Tertiary seamount phosphorites known to be of troughs were filled with thin-bedded strata, the shelf diagenetic origin range from 1.5 to 1.9, further ultimately was flooded during a transgressive event that emphasizing the possibility of post-depositional formation led to broad areas of shallow marine conditions and the of apatite in parts of Zone D (Hein and others, 1994). deposition of thick-bedded strata. Volcanic contributions Overall the geochemical data imply that hydrothermal to the thick-bedded strata are minimal and imply more solutions contributed the elements Mn, Ba, Ag, As, Pb, or less stable tectonic conditions. and U to the depositional system, although other enriched Schmidt (1963) first recognized that manganese elements, Mg, Ca, and K, may have been derived either oxides accumulated in the interval between deposition of from decomposed iron-formation or from seawater. It thin- and thick-bedded rocks and in a place where tectonic may be of significance that the base metals Co, Cu, Ni, subsidence was greatest. The general stratigraphic and Zn, as well as , are depleted in Zones C and D. relationships described by Schmidt correspond to These metals typically are abundant in submarine- sedimentation in a second-order basin as envisioned by

43 Southwick and others (1988). The configuration of that I Chlorite-bearing iron-formation basin is unknown, but facies relationships imply a (1) Chlorite + dolomite + hematite + ilmenite + generally north-trending graben that deepened to the south quartz where Schmidt (1963) mapped a variety of "hypabyssal" (2) Chlorite + "biotite" + dolomite + siderite + intrusions. Some of these intrusions may be extrusive magnetite + hematite + apatite + quartz rocks deposited contemporaneously with iron-formation. (3) Chlorite + "biotite" + dolomite + magnetite Regardless, it is likely that a heat source or sources + quartz (+ hematite + goethite) driving a hydrothermal convection system was located II Stilpnomelane-bearing iron-formation to the south of the North range. (1) Stilpnomelane + siderite + dolomite + There is considerable mineralogical evidence that the magnetite + quartz hydrothermal system described here affected strata of (2) Stilpnomelane + siderite + magnetite + late Mahnomen to early Rabbit Lake age. Boron-bearing ilmenite + quartz solutions reacting with AI-rich detritus in the upper (3) Stilpnomelane + siderite + dolomite + Mahnomen and lower Rabbit Lake Formation produced magnetite + quartz + apatite + pyrite + tourmaline (Cleland and others, in press). In the arsenopyrite (sulfide lenses) Trommald Formation, hydrothermal solutions produced (4) Stilpnomelane + siderite + pyrite + a variety of textural and mineralogical attributes. Those chalcopyrite + quartz + goethite (sulfide solutions clearly were operative beneath the sediment­ veins) water interface, but they also may have reached the (5) Stilpnomelane + siderite + K-spar + rutile + sediment-water interface where manganese and iron pyrite + quartz + apatite + barite + anhydrite oxides were being precipitated directly on the seafloor (en echelon veins) from exhalative solutions in a shallow water environment. III Chlorite schist The uppermost part of the Trommald Formation also was (1) Chlorite + dolomite + (hydro)ilmenite + deposited under shallow-water conditions, but it lacks quartz + ankerite (blasts) + rutile + quartz + manganese oxides and contains the chemical signature monazite of a volcanogenic/detrital component considerably IV Tourmalinite different from that found elsewhere in the iron-formation. (1) Chlorite + tourmaline + quartz + rutile Abrupt changes in hydrothermal activity, together with (2) Chlorite + quartz + phengite + zircon + the abrupt appearance of clastic detritus suggest that tourmaline ± rutile renewed tectonic activity modified the hydrothermal plumbing system and ultimately produced regional The chlorite geothermometer of Cathelineau (1988), changes leading to epiclastic sedimentation in Rabbit which is based on the silicon content of the tetrahedral Lake time. site, indicates formation temperatures between 264 DC and 407°C. Interestingly, the averages of several analyses D CONCLUSIONS consistently increase with depth: 3l6 C (930 feet) and 390°C (1173 feet). These formation temperatures are It is clear that the North range group in general and consistent with the dolomite/ankerite + magnesite/siderite the Trommald Formation in particular contain ample geothermometer of McSwiggen (1993) which yields evidence of hydrothermal activity. It is also clear that temperatures in the range of 277°C to 380°C. much of that activity occurred in a depositional or early Temperatures derived from arsenopyrite geothermometry diagenetic environment. However, strict mineral proof are in the 400-500°C range. Minimum pressures of 2.5 that all of the hydrothermal activity predates Penokean kbar can be estimated using phengite compositions in deformation and metamorphism is lacking. Some "en schist of the uppermost Mahnomen Formation. echelon-type" veins filled with quartz, carbonates, Chlorite compositions of the five samples analyzed sulfides, and sulfates clearly postdate irregular sulfide­ from the lower part of the Trommald Formation, if taken bearing "hydrothermal" veins. Other stilpnomelane-rich at face value, indicate a high thermal gradient of about veins also are probably of metamorphic origin as are 50DC/l 00 m. However, if the lowermost sample (G 1173) veins filled mostly with white quartz. is omitted, and if the data are recalculated to true The following mineral assemblages observed in thicknesses, an even steeper gradient of 86°C/I00 m unoxidized, thin-bedded iron-formation suggest that would result. metamorphism was to the greenschist facies.

44 Alignment of carbonate grains in Zone A parallel to P-deposits: 16th International Colloquium on Africa foliation and discordant to bedding is evidence that Geology, Extended Abstracts: International Center metamorphism must have proceeded under reducing for Training and Exchanges in Geosciences: National conditions and with a high Peo2. It is not altogether Center for Scientific Research, University of Nancy, clear why the metamorphism did not affect the hydrous , p. 27-28. oxide phases, which are abundant in the core, especially Blake, R.L., 1965, Iron phyllosilicates of the Cuyuna above 900 feet. We suggest that the goethite that occurs district in Minnesota: American Mineralogist, v. 50, with secondary manganese oxide in obvious crosscutting p.148-169. relationship formed during a subsequent supergene event Bonatti, E., Kraemer, T., and Rydell, H., 1972, that may have occurred long after metamorphism had Classification and genesis of submarine iron­ ceased. We also suggest on magnetic grounds, that manganese deposits, in Horn, D., ed., goethite in Zones Band C is an early high-temperature Ferromanganes'e deposits on the ocean floor: mineral. However, it is well established that goethite in Washington, D.C., National Science Foundation, p. grains greater than 1 m in size transforms to hematite at 149-166. temperatures between 100°C (1 kbar) and 150°C (4 kbars; Bostrom, K., 1983, Genesis of ferromanganese deposits­ Murray, 1979). Either the hydrothermally formed Diagnostic criteria for recent and old deposits, in goethite survived through the regional metamorphic event, Rona, P.A., Bostrom, K., Laubier, L., and Smith, or there have been mineralogical changes along the line, K.L., Jr., eds., Hydrothermal processes at seafloor for example, hydrothermal goethite -> metamorphic spreading centers: N.Y., London, Plenum Press, p. hematite -> alteration goethite, that we can not recognize 473-489. on textural grounds. We have no evidence bearing on Bostrom, K., Rydell, H., and Joensuu, 0., 1979, the problem other than to recognize that the upper part U.ngban-An exhalative sedimentary deposit?: of the Gloria core has been affected by supergene , v. 74, p. 1002-1011. weathering processes. Cathelineau, M., 1988, Cation site occupancy in chlorites and as a function of temperature: Clay ACKNOWLEDGMENTS Minerals, v. 23, p. 471-485. Clark, L.A., 1959, The upper stability curve of the pyrite­ We thank B. Saini-Eidukat, North Dakota State arsenopyrite assemblage: Carnegie Institution of University, for carrying out X-ray diffraction analyses of Washington, Annual Report, Director Geophysics 10 samples, and for many helpful suggestions; Linda Laboratory, v. 58, p. 145. Dahl, USBM, for discussions and data supply; and Val Cleland, J.M., Morey, G.B., and McSwiggen, P.L., 1993, Chandler, MGS, for help with interpretation of Implications of tourmaline-rich rocks in the North susceptibility data. Range Group of the Cuyuna Iron Range, east-central Minnesota [extended abs.]: Institute on Lake REFERENCES CITED Superior Geology, 39th Annual, Eveleth, Minnesota, Proceedings, v. 39, pt. 1, p. 25-26, Almod6var, G.R., Saez, R., Toscano, M., and Pascual, Cleland, J.M., Morey, G.B., and McSwiggen, P.L., 1996, E., 1995, Co, Ni and "immobile" element behaviour Significance of tourmaline-rich rocks in the North in ancient hydrothermal systems, Aznalc6llar, Iberian Range Group of the Cuyuna Iron Range, east-central Pyrite Belt, Spain, in Pasava, J., Kribek, B., and Zak, Minnesota: Economic Geology, v. 91, in press. K., eds., Mineral Deposits: Rotterdam, Balkema, p. Crerar, D.A., Namson, J., Chyi, M.S., Williams, L., and 217-220. Feigenson, M.D., 1982, Manganiferous cherts of the Ayres, D.E., 1972, Genesis of iron-bearing minerals in Franciscan assemblage: 1. General geology, ancient banded iron-formation mesobands in the Dales Gorge and modern analogues, and implications for Member, Hamersley Group, : hydrothermal convection at oceanic spreading Economic Geology, v. 67, p. 1214-1233. centers: Economic Geology, v. 77, p. 519-540. Banerjee, S.K., 1991, Magnetic properties of Fe-Ti Dahl, L., 1992, Characterization of Minnesota manganese oxides: Reviews in Mineralogy, v. 25, p. 106-128. deposits for in situ mining: Duluth, Skillings' Mining Bau, M., and Dulski, P., 1993, Distribution of rare-earth Review, v. 81, no. 40, p. 4-9. elements in Precambrian sedimentary Fe-, Mn-, and

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47 Odom, J.E., 1984, Glauconite: Reviews in Mineralogy, reservoirs of the ore-forming elements: Abstracts, v. 13, p. 545-572. Southampton, Institute Mining and Metallography. Robie, R.A., Hemingway, B.S., and Fisher, J.R., 1978, Southwick, D.L., and Morey, G.B., 1991, Tectonic Thermodynamic properties of minerals and related imbrication and foredeep development in the substances at 298.15 K and 1 bar (l05 Pascals) Penokean orogen, east-central Minnesota-An pressure and at higher temperatures: U.S. Geological interpretation based on regional geophysics and the Survey Bulletin, v. 1452,456 p. results of test-drilling: U.S. Geological Survey Rock, N., 1988, Numerical geology: Springer-Verlag, Bulletin 1904-C, 17 p. Lecture Notes in Earth Sciences, v. 18,427 p. Southwick, D.L., Morey, G.B., and McSwiggen, P.L., Saini-Eidukat, B., Marozas, D., Blake, R., and Adamson, 1988, Geologic map (scale 1 :250,000) of the N., 1993, Implications of rock mineralogy and texture Penokean orogen, central and eastern Minnesota, and on the feasibility of in situ leach mining of Mn­ accompanying text: Minnesota Geological Survey bearing iron formations of central Minnesota, U.S.A.: Report of Investigations 37, 25 p. Applied Geochemistry, v. 8, p. 37-49. Stacey, F.D., and Banerjee, S.K., 1974, The physical Schmidt, R.G., 1963, Geology and ore deposits of the principles of rock magnetism: Elsevier, Cuyuna North range Minnesota: U.S. Geological Developments in solid earth geophysics, v. 5, 195 p. Survey Professional Paper 407,96 p. Toth, J.R., 1980, Deposition of submarine crusts rich in Schulz, KJ., Sims, P.K., and Morey, G.B., 1993, Tectonic manganese and iron: Geological Society of America synthesis, Lake Superior region, in Reed, le., Jr., Bulletin, v. 91, p. 44-54. and others, eds., Precambrian: Conterminous United Trendall, A.F., 1968, Three great basins of Precambrian States: Geological Society of America, The geology banded iron formation deposition: A systematic of North America, v. C-2, p. 60-62. comparison: Geological Society of America Bulletin, Scott, S.D., 1973, Experimental calibration of the v. 79, p. 1527-1544. sphalerite geobarometer: Economic Geology, v. 68, Trendall, A.F., and Blockley, lC., 1970, The iron p.466-474. formation of the Hamersley Group, Western Slack, J.F., 1992, , tourmalinites, and Australia, with special reference to the associated stratabound mineral deposits, in Foster, R.P., ed., crocidolite: Geological Survey of Western Australia, Mineral deposit modelling in relation to crustal Bulletin 119, 353 p.

48

Oxide-IF, thick-bedded, RBr, carbonate-rich

Mn-Fe oxide ore, catacJastic zone, partly RBrB interlayering, strong replacement, much rubble

Oxide-IF, banded, R, some Mn-rich layers, friable, altered, some chert beds

o Mn-Fe oxide ore, Mn-rich, granular, brecciated Oxide-IF, thick-bedded, RBrY, porous, decomposed, replaced by hematite Oxide-IF, thin-bedded, hm, R, with chert and Mnox beds Mn-Fe oxide ore, layered, gnarled ore Oxide-IF, thin-bedded, light, decomposed-argillic, RBrY Chert (2Ocm), massive Oxide-IF, thin-bedded, hm-ch, R, replaced by mottled goe + Mnox Oxide-IF, thin-bedded, goe-hm, R, decomposed

Oxide-IF, thick-bedded, hm, R, with massive Mn-rich zones ...... Oxide-IF, thin-bedded, hm-ch, R o Fe-Mn oxide ore, massive to nodular, B(R) Oxide-IF, thick-bedded, hm, R, chert-rich, replaced by goe ± hm Fe-Mn oxide ore, massive, B, wavy lamination (15 cm) Oxide-IF, lam., hm-goe, BrYG, , Mnox staining Oxide-IF, goe-hm, YRG, bx, with chert pebbles Oxide-IF, thick-bedded, hm-(ch), RY, Mnox replacement z vein Shale, G, clay coatings (altered mafic tuff?) Oxide-IF, hm-ch, R, replaced by massive goe (some Mnox) Mn-Fe oxide ore, layered, hm-goe-Mnox, BR, enveloped by chert at 580' Mn-rich Fe-Oxide ore, laminated, hm-goe, YR, partly brecciated Oxide-IF, thin-bedded, YR, chert-rich

u Oxide-IF, thin-bedded, light, RYG, II) massive replacement by goe + hm c ~ Oxide-IF, thin-bedded, replaced by hematite Oxide-IF, thin-bedded, YR, chert-rich, with hm-rich breccia zones Oxide are, brecciated, hm-goe, R, some Mnox o 10 20 0 0.5 wt% MnO cgs x W-3

53 600 ~-----r----P!2!:!!2!:!!!!:!!!q Oxide ore, banded, goe-hm, RY, veined by qz + oxides Oxide-IF, thick-bedded, YR, chert-rich, replaced by goe Oxide-IF, thin-bedded, goe-martite, Y, chert-rich, chert up to 2cm Oxide ore, hm-goe, with angular chert clasts « Icm), R 610 Oxide-IF, thin-bedded, hm, R, replaced by massive hm-goo, hm laminae separated by white qz (chert?); rare Mnox

Oxide-IF, thin-bedded, goe-hm, YR, veined by Qz, Mnox coatings 620 Oxide-IF, thin-bedded, hm-chert, R, replaced by goe (+Iate Mnox) Oxide ore, goe-Mnox, massive, fractured, BBr Breccia of massive grey Qz (vein?), chert + oxides Oxide-IF, thin-bedded, hm-goe, hm-rich replacement, R 630 Oxide-IF, thin-bedded, goe-(hm)-chert, with rare metallic layers, zoned chert, diss. martite, BrRY Oxide ore, banded, hm-goe, some green chert, RY Oxide ore, brecciated, hm-goe-(Mnox), R ...... 640 ¢:: Oxide ore, thick-bedded, Mnox-hm, BY '-' ~ Oxide ore, brecciated, goe-hm, with some Mnox, YR '0 u Oxide-IF, thick-bedded, hematite, R ~ Quartz vein (30 cm), vuggy 0 ~ 650 Oxide ore, goe clasts in hm matrix, distorted chert layers .0

Oxide-IF, thin-bedded, hm-goe, R, goe laminae, light bands 680 Oxide ore, hm, R, porous, brecciated, with angular chert clasts Chert breccia, 3 cm

Oxide-IF, goe-hm, chert-rich, Y Quartz vein, 8 cm 690 RY thin-bedded hm-goe-ch-IF Hm-(Mnox) zone, massive Oxide-IF, laminated, R, replaced by hm, goe

o 10 20 0 0.5 wt%MnO cgs x 10-3

54 Breccia ore, RY, hm clasts in banded Y sed, cut by massive goe-hm veins Oxide ore, laminated, goe-hm, R, friable Oxide-~ thin-bedded, hm-goe, RY 71 Porous cnert Oxide ore, banded, goe-(Mnox),YB Oxide-IF, thin-bedded, goe, Y-Br, discordant Mnox-filled joints Oxide-IF, thin-bedded, goe, Yr, Mnox pods

u Oxide-IF, thin-bedded, hm-martite, R, granular chert layers II) c ~ Oxide ore, banded, hm-Mnox, RB , brecciated towards top

Fe-Mn ore, massive, BR metallic, towards the top some primary mm-banding -< c Fe-Mn oxide ore, massive, hm-Mnox, BR metallic ::E Quartz vein Oxide-IF, thin-bedded, goe, Y-R, altered, Mnox replacement Fe-Mn oxide ore, very massive, black Oxide ore, massive, Mnox + goe+ hm, weak bedding Breccia zone, qz vein with hm+Mnox

Oxide-IF, massive, hm-goe, RB, with some Mnox? Oxide-IF, goe (martite), Y-R, strongly altered Coarse smoky quartz cemented by goethite Chert

Oxide-IF, thin-bedded, goe-martite, Y, porous with thin ~ II) decomposed chert; faulting

II) Altered layer; Mnox??, BY c o N Oxide-IF, goe-martite, Ybr Oxide-IF, thick-bedded, Rbr, martite-rich Oxide-IF, thin-bedded, goe, Y, cherty Oxide,-IF, thin-bedded, goe, YBr Quartz vein, 50 em Brecciated chert 10 Oxide-IF, coarse-grained, B-R, martite-rich ~ Oxide-IF, massive, hm-martite, R, with granular layers

wt%MnO cgs x 10-3

55 800 / ...... Chert-rich \(')

- - CQ Very martite-rich Oxide-IF, goe-ch-martite, Br, porous 810 - :r Oxide-IF, hm-martite, RY 1 10 - CQ Quartz vein with goe + Mnox? ...... Granular chert 820 - Oxide-IF, thin-bedded, hm-martite, R

'

840 M :2'-" - - Oxide-IF, thin-bedded, hm-ch, RBr, diss. martite CQ E ...... CQ Chert, decomposed '0 0 () t:: - J - 0 Oxide-IF, thin-bedded, hm-mt-ch, YR, porous, leached ~ N 0 '0 Oxide-IF, thin-bedded, hm, YR, leached, with Mnox? .0 850 - - (/J (/J Oxide-IF, thin-bedded, goe-hm, YBr 0 I veins wI apatite, rdc, qz, barite, Mnox ~ ) () - Oxide-IF, thin-bedded, hm-martite, R :s Oxide-IF, thick-bedded, hm-martite-chert, Bryg, porous 860 - ) - Oxide-IF, thin-bedded, hm-(martite)-chert, BrY ...... Oxide-IF, thick-bedded, hematite - - ...... Chert, granular C'l Oxide-IF, thin-bedded, hm-ch-(Mt), BrR, shale intercalations 870 - CQ Oxide-IF, massive, hm, shaly, friable Chert, vuggy, oxid., with Mnox - Oxide-IF, hm, R, shale, rubble Shale with green clay on surface 880 - Oxide-IF, massive, hm-martite, Br, shale intercal.

...... 5 cm altered chert, diss. martite - - Oxide-IF, massive to thin-b., hm-chert, Ryg ? Oxide-IF, massive, hm, R 890 - 3 cm granular chert

CQ - - Oxide-IF, laminated, hm, RG

900 I I I I I I I I o 10 20 0 0.5 wt%MnO cgs x 10-3

56

1000--r------.----...... --r-.,...------, SCM-IF, very massive, BG extensively veined (stilp-carb-qz) SCM-IF, granular, with 4cm chert \Q SM-IF, thin-bedded, dark GB, with 1010

:.'.:.:.:.'.:.'.:.:.:.', Chert, stilp-bearing (2 cm) 1020 V)

1040 ¢::'"""' Tourmaline-rich chlorite schist '-" ~ Qz-carb vein (4.5 cm) (3 () Schist, foliated, chlorite-carbonate, G; altered metatuff? ~ 0

Chert in double plunging fold (slumping?)

1070 :.:.:.:.:.:.:.:.:.:.:.:. Chert with FeCarb, sulfides (7 cm) SCM-IF, chert-bearing SCM-IF, lam., slumped, with cm-thick sulfide lens Chert pebble layers with rounded to subangular clasts 1080 SCM-IF, thin-b., Cherts with Fe-Carb rims+Sulfides Granular textures, in matrix of qz + sulfides

SCM-IF, wavy-lam., dark G, synsed. deformation

1090 «)

58