Mineralogy and Liberation Characteristics of Western Mesabi Range Oxidized Taconites

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Bureau of Mines Report of Investigations/1983

Mineralogy and Liberation Characteristics of West ern Mesabi Range Oxidized Taconites

By Rolland L. Blake

UNITED STATES DEPARTMENT OF THE INTERIOR Report of Investigations 8813

Mineralogy and Liberation Characteristics of Western Mesabi Range Oxidized Taconites

By Rolland l. Blake

UNITED STATES DEPARTMENT OF THE INTERIOR James G. Watt. Secretary

BUREAU OF MINES Robert C. Horton, Director Library of Cong:ess Cataloging In Publication Data:

Blake, Rolland Law,s Mincrnl<>gy ,Inti liberation charnclcrislics of WeSlern Mesabi R,;lngc 0xirli/.cd (DCOlli(l'~

(Report of in,'estignllOns ; 8813) Uibliogrnphy; p.2')·2(,. SJ[)[. of Docs. nn.: 1 28.23:8813. L Tncnnile-M;nllcsOl;l--Ivlesabi rZilnge. I. Title. II. Series; IZe port of ilHestigatinns (United States. Oureau of Mines) ; 8813.

Tl\23,U43 rqE390.2.17!il 0220: 1549',23] R~-600235 CONTENTS

Abstract ....•....•...•..••..•...... •...... ••...• 1 Introduction •••••• 2 Objective •••• 2 Background ...... 3 Acknowledgments .•....•••...... •••....••••...... ••..•.....•...••••...••••....•• 5 General geology of the Wes tern Mesabi Range •••••••••••••••.•••••••••••••••••••• 5

Area 1 ••... 11 •••• " ...... 6 Area 2. 6 Area 3. 7 Bulk sampling methodology and locations •••••••••••••••••••••• 8 Methodology ....••...... •...... •...... •...... a Geographic location ••••••••••••••••••••••••••••••••••••••••••••••••••••••• 9

Stra tigraphic location ...... II ...... 9

Characterization of bulk samples .•...... ••.... 11 ••••••••••••••••••••••••••••• 12 Macroscopic texture and grain size •••••••••••••••••••••• ~ •••••••••••••• 12 Area 1...... •.•...•...... •...... •...... •.... 13 Area 2...... •...... •.. ,...... •.. 14 Area 3 •• 14 Mineral compos i tiona ••... ,...... 14 Area 1 •• ...... ,. .' ...... 14 Area 2 •• 15 Area 3 •• 15 Discussion of mineral composition and element distribution ••••••••••• 15 Microscopic textures, alteration, and liberation •••••••••••••••••••••••••• 17 Beneficiation results and estimate of Western Mesabi Range oxidized taconite resources .....•..•.•..•.....•..•••...... •...... 23 s U lD.IIla r y • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 24 References ...... •...... •...... •...... •...•...... •... 25

ILLUSTRATIONS

1. Location of Western Mesabi Range, MN, and oxidized taconite areas 1, 2, and 3 ...... ,...... 2 2. Outcrop map of Biwabik Iron-Formation in areas I, 2, and 3 ••••.••••••••••• 4 3. Longitudinal section of Biwabik Iron-Formation in areas 1, 2, and 3 ••••••• 4 4. Structures that affect oxidation of taconite in area 1 •••••••••••••••••••• 7 5. Stratigraphic location of bulk sample of oxidized taconite from area 1 •••• 9 6. Stratigraphic location of bulk samples of oxidized taconite from areas 2 and 3 ...... •••• "•....•••....•...•...... •...... 10 7. Hand specimens of area 1 showing variations in textures and structures •••. 11 8. Hand specimens of area 2 showing variations in textures and structures •••• 12 9. Hand specimens of area 3 sho\o[ing variations in textures and structures •••• 13 10. Mineral composition versus size distribution of area 1 bulk sample as minus 10411esh feed to flotation grinding circuit. ••••••••••••••••••••••• 16 11. Photomicrographs of polished surfaces of area 1 bulk sample taconite •••••• 18 12. Photomicrographs of polished surfaces of area 2 bulk sample taconite •••••• 19 13. Photomicrographs of polished surfaces of area 3 bulk sample taconite •••••• 20 14. Photomacrograph showing alteration of taconite by weathering, including solution, redeposition in cavities, and direct replacement. ••••••••••••• 21 15. Photomicrograph of polished surface showing extensive weathering along quartz grain boundaries...... 21 TABLES

1. Bureau of Mines published results of beneficiation tests on bulk samples of WMR nonmagnetic taconite...... 2 2. Mineral distribution of WMR bulk samples...... 15 3. Chemical analyses of WMR 1. 2. and 3 bulk samples...... 15 4. Degrees of taconite alteration recognized from microscopic analysis...... 22 5. Liberation characteristics of lron oXldes in bulk sample WMR-l...... 23 6. Beneficiation results for WMR oxidized taconite bulk samples...... 23 7. WMR oxidized taconite tonnage estimate compared with Marsden's OXYBIF estimate. •• ••••• .•• •• ••• • •••• • • •• •• • • • ••• •• • ••• • ••••••••••••••••••••••••• 24

UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

°c degree Celsius 11 m micrometer

cm centimeter mlt million long tons

ft foot rnm mi.llimeter

ft 3 /lt cubic foot per long ton pct percent

km kilometer ppm parts per million

m meter wt pct weight percent

mi mile MINERALOGY AND LIBERATION CHARACTERISTICS OF WESTERN MESABI RANGE OXIDIZED TACOI\IITES

By Rolland L. Blake 1

ABSTRACT

The Bureau of Mines investigated the mineralogy, tonnage, and bene ficiating characteristics of oxidized (nonmagnetic) taconite iron re sources of the Western Mesabi Range as part of its program to insure adequate future domestic mineral supplies. Magnetic taconite currently provides about 95 pct of Mesabi production, but the large tonnages of oxidized taconite between the magnetic deposits will be needed to help meet future domestic demand for iron ore. Three bulk samples that represent large tonnages from the Western Mesabi Range were selected to determine their mineral composition and liberation by light optical microscopy and to test their beneficiating response.

This report describes the mineralogy and liberation characteristics of the three samples of the Biwabik Iron-Formation for 26.5 mi (42.7 kID) from Keewatin to Grand Rapids, MN. (Details of response to beneficia tion were reported previously.) The samples represent mostly material from the Lower Cherty Member of the Biwabik- Iron Formation, although one sample included some Upper Cherty Member. Minerals of all three samples were quartz (chert), hematite, goethite, and minor to trace amounts of magnetite, iron carbonate, and iron silicates.

It was concluded that the oxidized taconites of the Western Mesabi Range represent a subeconomic indicated resource that can be selectively mined and beneficiated as a future large source of iron feed for steelmaking.

1supervisory geologist, Twin Cities Research Center, Bureau of Mines, Minneapolis, MN. 2

INTRODUCTION

OBJECTIVE the beneficiation testing and response (1-6).2 Table 1 lists these publications This report des cri bes the geology, min u~der the area sampled and type of eralogy, tonnage potential, and micro beneficiation. scopically observed liberation character istics of oxidized taconite on the 2Underlined numbers in parentheses re Western Mesabi Range (WMR). Six previous fer to items in the list of references at Reports of Investigations have described the end of this report.

TABLE 1. - Bureau of Mines published results of beneficiation tests on bulk samples of WMR nonmagnetic taconite i

WMR-l WMR-2 WRM-3 Flotation •••.••••••••••• RI 8482 (3) RI 8505 (4) RI 8552 <-~) . Reduction roasting •••••• RI 8549 (~) RI 8572 (6) None. WHIMS2 ••••••.••••••••••• None ••••••• RI 8325 (I) None. lSee figures 1 and 2 for locatl.on of WMR areas 1, 2, and 3. 2Wet high-intensity magnetic separation. 930 920

R26W R24W R22W I R20W RI8W RI6W RI4W \ RI2W RIOW R8W -+_~OqCH!CH~NG _ ~

~---+---r--Areas-

I J----+--t ~~-+~~-r-I T56N

-+~~--+-~~-+---r--T-~'--I I I i T54N &;; TI42N T-- 1}y TI41N ---+------=t:i=i==t=t==t=~=+=t==t=t=t= ---- T~ Ha~9orS~47° 47"'-+---t-AITKIN COUNTY ,/

I ------j r-~~--_r--T--4--~ N I Scaie,mi 10 5 0 5 10 I ••' MINNESOTA o 5 1015 Scale,km FIGURE 1. - Location of Western Mesabi Range, MN, and oxidized taconite areas 1,2, and 3. 3

BACKGROUND even material that would forme r ly be classified as one of the natural ores To expand the U.S. iron reserve base, will be found in small to large amounts the Bureau of Mines has undertaken re and included as feed to the plant. search to develop beneficiation processes for the oxidized taconites of the WMR Oxidized taconite is presently uneco (4). In 1980 domestic production of 69.6 nomical to treat, but it is readily mIt (million long tons) of iron ore sup accessible in the subglacial outcrop plied 70 pct of U.S. requirements (98.9 area of the WMR. The tonnage and bene mlt) (7). The Mesabi Range (fig. 1), the ficiating characteristics of this mate only producing iron ore district of the rial were not well known. Recently, oxi three (Mesabi, Cuyuna, and Vermilion) in dized taconites at Tilden, MI, have been Minnesota, supplied 45.2 mlt (65 pct) of treated successfully by the selective 1980 domestic iron ore (7). Each year flocculation-deslirning-cationic-flotation magnetic taconite (first- produced in process that was developed jointly by the 1952) supplies more and more (about 95 Bureau's Twin Cities Research Center and pct in 1980 (7» of ~esabi Range produc The Cleveland-Cliffs Iron Co. (9). tion because-pellets made from taconite Tilden oxidized taconite, in general, concentrates have superior grade and contains mostly hematite and little goe structure for blast furnace feed compared thite, whereas Mesabi oxidized taconite with natural iron ores, wash ores, and contains hematite plus considerable goe gravity concentrates. Although the open thite. Both ores contain fairly coarse pit Mesabi Range magnetic taconite depos quartz, and some slime problems are its are limited, surrounding or between caused by earthy hematite from Tilden and them are the oxidized (nonmagnetic) taco also by earthy goethite from the WMR. nite resources. The concentrate produced in the jointly developed process makes pellets of chemi For this report, oxidized taconite is cal quality equal to that of pellets made Biwabik Iron-Formation material that has from magnetic taconite concentrate. How been altered enough that the dominant ever, more thermal energy is required for iron minerals are now hematite and/or heat-hardening of pellets made from the goethite, but alteration has not pro hematite-rich Tilden concentrate, and ceeded far enough to have produced the they are of lesser physical quality than natural iron ores. Oxidized taconite was those from magnetic taconite concentrate. formed from (1) magnetite taconite in which most to all magnetite has been The area exami ned in this study extends altered to hematite and/or goethite or 26.5 mi (42.7 kID) from Keewatin to Grand (2) iron carbonate-iron silicate taconite Rapids, MN (fig. 2). Most work was done in which most to all of the iron on samples from the Lower Cherty and some carbonate-iron silicate has been altered from the Upper Cherty Members of the to goethite and/or hematite. Oxidation Biwabik Iron-Formation 3 (fig. 3). This refers to the fact that most of the Fe 2 + part of the range was divided into three has been oxidized to Fe 3 +. One objection blocks (fig. 2) for sampling based on to the term "oxidized taconile" is that apparent differences in mineralogy and on it excludes primary hematite taconite, availability of bulk samples representing such as the Red Basal Taconite of the large tonnages of taconite. Area WMR-l Mesabi Range or metamorphic hematite of extends for 13 mi (20.9 km) along the some other iron ranges. As used here, strike of the Biwabik Iron-Formation oxidized taconite is similar to that defined by Bleifuss (8), except that he 3The material studied here was oxidized acknowledged only a little secondary fer taconite involved mostly in early stages ric oxide enrichment, whereas the present of alteration rather than taconite in author recognizes somewhat more. This volved with more extensive alteration may be a moot point because when oxidized that formed semitaconites, or wash and taconites are utilized in the future, heavy-media ores. 4

Scole,mi o I 2 3 o I 2 3 4 5 Scale, km

FIGURE 2, . Outcrop map of Biwabik Iron~Formation in areas 1,2, and 3.

700[ VlrQlnlO Formarlon 600 200

UFPER SLATY MEM8E 500 --L----r--- 150 \ 400 UFPER CHERTy MEMBER \ \ \ \ 300 100 200 BI ..... obrk Iron-Forma1lon 50 100 Red basal tacant'" o o . R 25 w R 24 W R 23 w R22W R 21 W @00nd~ ~a~h""a.u1.1 POk&CjJoma Formation WMR- 3~+------WMR- 2-·------+------'--- -WMR- 1- - --

Scole,mi o I 2 3 4 5 ~ ;w-o; ______012345618 Scale. km

FIGURE 3. - Longitudinal section of Biwabik Iron-Formation in areas 1,2, and 3 (modified from White (U)).

and within the subglacial outcrop. Area plus bench-scale and pilot plant WMR-2 extends for 10 mi (16.1 kID), and beneficiation experiments, and reported area WMR-3 is 3.5 mi (5.6 km) long. (12-17) on nonmagnetic taconite prior to Petrographic samples were collected from the CUrrent series of tests. Gundersen drill core, from open pit bank samples, and Schwartz (~) investigated the miner and from three representative bulk sam alogy, textures, and stratigraphy of the ples (several hundred tons each). Pol hard magnetic taconite from the Eastern ished and thin sections were made for Mesabi Range. examination by light optical microscopy. Mineral and rock fragments were examined A comprehensive study of nonmagnetic in refractive index oils; some minerals oxidized taconite of the WMR from Hibbing were identified by X-ray diffraction. to Coleraine, MN, a distance of 30 mi along the strike (fig. 1), was made by The mineralogy (10) and stratigraphy the GNR (Great Northern Railway),4 but (11) of the Biwabik-rron-Formation and of their 1963 company report remains unpub the soft iron ores derived from these lished. Part of the GNR work was pub rocks were investigated by earlier work lished by one of the investigators (~). ers. The Bureau has conducted fairly continuous sampling since about 1956, 4Now Burlington-Northern. 5

A recent report by Marsden (19) tabu oxey-bif). "OXYBIF" refers only to the lates estimated Mesabi Range iron ore oxidized Biwabik Iron-Formation, the non reserves by ore class and by geographic magnetic taconite that is considered to range unit. The reserve estimate was be potentially concentratable material designed to include all taconite material and that may yield commercial-quality that can be produced to a no-loss situa concentrates or pellets. OXYBIF is the tion. Iron ores are classed as natural hematitic or goethitic cherty iron- iron ore, four grades of magnetic tac formation that was formed by the oxida onite ore based on weight recovery of tion of the magnetite-rich, cherty, iron magnetite and the iron content of the formation and that, if unoxidized, should concentrates, and OXYBIF (pronounced be taconite or taconite lean ore.

ACKNOWLEDGMENTS

This work was made possible by the aided in understanding the problems en cooperation and courtesy of employees of countered. Helpful discussions are also Mesabi Range iron mining companies. acknowledged by personnel of the Uni Geologists, mining engineers, and metal versity of Minnesota and the Minnesota lurgists discussed problems, guided sam Geological Survey. pling tours, provided bulk samples, and

GENERAL GEOLOGY OF THE WESTERN MESABI RANGE

The WMR trends west-southwest (fig. 1); unoxidized taconite. Most of the minor west of Grand Rapids, MN, it turns south folding and gentle southerly dip are be and is suspected from drill core evidence lieved related to the regional meta to correlate with the Emily and Cuyuna morphism. A more recent event that districts (11). The iron ores and mag affects mining was the deposition of netic taconites are derived from the Pleistocene glacial drift on the Western Biwabik Iron-Formation, a metasedimentary Mesabi Biwabik Iron-Formation outcrop unit of the Animikie Group of Middle Pre area to a depth of about 40 ft at the cambrian age. The iron-formation dips north contact with the underlying quartz gently southeast and is underlain con ite and 160 to 200 ft at the south con formably by the Pokegama Quartzite and tact with the argillite (11). The drift overlain conformably by the Virginia must be removed for open-pit mining of Argillite Formation. The minerals and the taconite. Weathering of the iron textures indicate that the Biwabik Iron formation to the present time has pro Formation was originally deposited mostly duced increasing amounts of iron ores and as a chemical precipitate (10), perhaps oxidized taconite by forming hematite and as a noncrystalline gel, high--in silica, goethite at the expense of magnetite, iron, and carbonate, and containing con iron carbonate, and iron silicates and by siderable water. The three sedimentary removal of silica. formations were involved in burial, lith ification, and finally regional metamor Stratigraphically the Biwabik Iron phism, an event dated at about 1.76 bil Formation was observed by Wolff (22) and lion years ago (20). The underlying confirmed by other geologists and mining sandstone was cemented to a quartzite, engineers to contain four members, from and the overlying shale was altered to an oldest to youngest the Lower Cherty, argillite. The Biwabik Iron-Formation Lower Slaty, Upper Cherty, and Upper lost much water and formed iron oxide Slaty (fig. 3). Each member contains (mostly magnetite), iron carbonate, and both cherty and slaty layers, but the iron silicates stilpnomelane, minne units are named for their predominant sotaite, greenalite, and chamosite (10- rock type. The term "cherty" refers to a 1.1, ll)· These minerals are found in the dominance of chert as the matrix for 6

other minerals; "slaty" refers to iron study area. The section is drawn looking silicate and carbonate-rich layers that north, and information was projected to a may contain some chert. These rocks ex plane along the subglacial south contact hibit no slaty cleavage, only bedding with the Virginia Formation. On the W}~ parting and jointing, and are thus argil the Lower Cherty is the thickest member, lites and not slates. ranging from 225 to 330 ft (70 to 100 m), and contains most of the ore bodies and Oxidation of the iron-formation to iron the most consistent magnetic taconite ore has been thought from early Mesabi layer. The basal 20 to 30 ft (6 to mining days to be controlled by struc 9 m) (10) of the Lower Cherty is called tures such as noses of folds, joint sys the "red basal taconite" layer from its tem intersections, and faults, because original hematite content. This layer is these more porous and permeable paths usually too lean or does not liberate allow access of ground water to leach and well enough to be considered an iron otherwise alter the original minerals. reserve or resource. The Lower Slaty Major structures were d2scribed by Gruner Member usually ranges from a few feet to (10) for the entire range and by White 40 ft (12 m) thick; the lower part is (11) for the Wes tern ~lesabi. Both in either unoxidized black shale or has been vestigators found that vertical or oxidized to paint rock, a slippery steeply dipping joint systems are ori feeling, red kaolinite-hematite and/or ented in all directions, but three joint goethite-bearing material. It is usually 0 sets are predominant: N 10 E, N 45° W, too lean in Fe, too high in A1 2 0 3 , and/ and N 80° w. or too fine grained to be treatable.

AREA 1 Recent magnetic taconite exploration and mining in area 1 have enabled Owens, Area extends for 13 mi (20.9 km) Trost, and Mattson (23) to study struc along the strike of the Biwabik Iron tural control of taconite oxidation and Formation and includes the western third to guide mining operations to provide of R 21 W, all of R 22 W, and the eastern fairly homogeneous feed for two magnetic half of R 23 W (figs. 2-3). This is from taconite beneficiating plants. Struc 2 mi (3 km) east of Keevatin, MN, to 1 mi tures such as folds, joints, faults, and (1.6 km) west of Snowball Lake, MN. The fracture zones have affected the iron east and west limits of area 1 were resources, reserves, and mining in area 1 selected because this block of the forma (fig. 4). These structures have con tion was believed to be fairly consistent trolled ground water circulation and throughout in containing substantial therefore the location of higher iron amounts of both magnetic and oxidized grade areas where silica has been re (nonmagnetic) taconite, whereas west of moved. These structures affect the min this block extensive oxidation has left ing process, partly by dictating where very little magnetic taconite. The sub enriched material is located, but also by glacial outcrop of the Biwabik Iron requiring careful mining operation plan Formation in area 1 (fig. 2) is between 1 ning to remove the valuable iron ore and and 2 mi (1.6 to 3.2 km) wide. Its north taconite with the lowest cost of removing and south limits are irregular because of lean rock. This requires extensive the shallow dip and minor folding. The development drilling, as well as field average dip is 6° south, but local dip geologic mapping. undulations and monoclines may reach 45° to 70°. Shallow reverse dips to the AREA 2 north are sometimes encountered in mining. Area 2 extends for 10 mi (16.1 km) along the strike of the Biwabik Iron- Figure 3 is a stratigraphic, longi Formation and includes the western tudinal section, which shows thickness half of R 23 W and all of R 24 W from variations of the four members in the 1 mi (1. 6 km) west of Snowball Lake to 7

R 23W R22W R21 W / Granite, greenstone, and Quartzite

Harrison Monocline- Monocline

~------WMR - I ------+/

KEY Scale, mi 0 I 2 3 U ----T 0 Fault (dip angle, direction, and movement) ! ! 0 2 3 4 5 --~ - Monocline Scale, km = Fracture zone Dike T • Bedding (dip, direction, and angle) 6

FIGURE 4... Structures that affect oxidation of taconite In area 1 (after Owens (23)).

Coleraine, MN (figs. 2-3). This block is area 1, but fewer and of lesser in composed mostly of oxidized (nonmagnetic) tensity, will probably be observed. taconite; only very small amounts of mag Stratigraphically (fig. 3) area 2 is netic silicate-carbonate iron-formation similar to area 1, and structure is again remnants occur west of the Draper Mine thought to be the major ore control. Fault, west end of figure 4 (23). The subglacial outcrop of the Biwabik Iron AREA 3 Formation in area 2 (fig. 2) ranges from 1 to 1.5 mi (1.6 and 2.4 km) wide, and Area 3 extends for 3.5 mi (5.6 km) its north and south limits are somewhat along the strike of the Biwabik Iron more regular than in area 1 because of Formation and includes the eastern half less structural deformation. The average of R 25 W. This block is composed mostly dip of the beds is again about 6°. Only of oxidized (nonmagnetic) taconite, and one monocline is known; it is north of most of the characteristics described in Marble and Calumet, MN. Most structural area 2 apply here also. Stratigraph information of area 2 is based on diamond ically (fig. 3) the Lower Slaty Member drilling, whereas in area 1, it was sup starts to thin and pinch out in this in plemented by current mining of magnetic terval; the Lower Cherty Member continues taconite; the most westerly taconite to thin slightly, a trend starting in processing plant is near the west end of area 2. The structure in area 3 is area 1. When future oxidized taconite mainly the slight dip of the beds averag mining proceeds in area 2, local struc ing about 6° SE, and no other structures tures similar to those observed in have been reported. 8

BULK SAMPLING METHODOLOGY AND LOCATIONS

METHODOLOGY valuable information along with that ob tained during open pit mining. One pos Pilot plant testing of ores requires sible source of a large sample is the large samples to reach a steady state, natural ore and taconite open pit mine operate at steady state, and shut down. banks. However, most material does not Also, changes made tD refine equipment represent oxidized taconite between the and processes require additional testing richer ore bodies because the alteration on the same type of sample for valid com that produced these deposits was grada parison. Therefore, bulk samples of 350, tional, and economical mining limits are 600, and 800 tons were collected from determined by falloff in grade and recov-· areas WMR 1, 2, and 3, respectively. ery. Thus, the enriched halo that sur rounds open pit mines usually would not "Magnetic" taconite on the WMR for this represent the leaner material sought study is Biwabik Iron--Formation rock t".hat between the mines. However, it is pos retains most or all of its original mag sible to select a bank location at an netite. It usually also retains its open pit mine where mineralogy, texture, original iron carbonate and iron silica~e chemical analysis, and beneficiation minerals. "Oxidized" taconite is rock in tests have shown the oxidized taconite to which magnetite is absent or a minor con be similar to the average material be stituent, either from lack of original tween open pits. The parameters of this magnetite or from oxidation that removed material between pits would be kown from the original magnetite or replaced it drill core logging, chemical analysis, with hematite and/or goethite. Oxidized beneficiation tests on the core, and per taconite contains minor to trace, or sonal experience. none, of the original iron silicates and carbonates because the oxidation Iron-mining-company exploration geol alteration processes that replace magne ogists, mining engineers, and research tite by hematite or goethite also replace laboratory metallurgists who have carried carbonates and silicates. Oxidized tac out exploration drilling and benefici onite may vary between being rich in, or ation testing of areas between mines are devoid of, hematite and/or goethite based the best people to suggest where to find upon its original mineralogy and the a representative sample of substantial degree and type of weathering it was tonnage of nonmagnetic taconite. Also, exposed to. At the iron-rich end, oxi they can plan the mining of a large sam dized taconite would approach the wash ple when weather is favorable and heavy and heavy-media ores; richer still would equipment and access roads are available. be direct-shipping ores. At the lean Bulk samples for areas 1, 2, and 3 were end, oxidized taconite would approach an obtained in this rru.'lnner. The area 1 bulk i ron-f ree rock. Oxidized taconi te amen- sample is more representative of the able to beneficiation could be in the material in that area than are the area 2 range of 25 to 40 pct Fe and with a tex and 3 bulk samples for their areas. TIl is ture such that the ircn oxides could be is because considerable research to ob separated from the gangue minerals. Sat tain such a sample had already been done isfactory recovery and grade would result before the Bureau asked the mining com from processes other than magnetic sepa pany for such a sample. However, the ration or from those used on wash and personnel at companies in areas 2 and 3 heavy media ores. also worked hard to provide representa tive samples. Providing a large metallurgical sample that represents oxidized taconite in each Hand specimens that represent the vari area requires considerable knowledge of ous macroscopically different types of the Biwabik Iron-Formation. Drill cores rock comprising a bulk sample were could not furnish enough material, but selected with help from a company their appearance and analyses provided geologist. It was not possible to 9 estimate the weight or volume pe"ccent taken in accessible aceas. Subject to of each type of rock in the sample. this limitation, the sample~ consisted of When fine material was present in sig- the best mixture to represent the oxi nificant amounts, a separate sample dized taconite based on extensive drill was taken to compare with the hand ing, core analysis, and laboratory specimen. testing. GEOGRAPHIC LOCATION STRATIGRAPHIC LOCATION Geographic location of bulk samples is Generalized stratigraphy of area 1 tac limited by the fact that they had to be onite is shown in figure 5. Geologists Virginia Argillite Formation 650···· 200

600 Upper Slaty Area I 601-180 1 bulk sample 500 Thickness-j------wt pct 150

1 Upper Cherty A 70 30 1 1 50 100 400 J ______..- Lower *Slaty E .. _1 ______1- 51 Biwabik -I I I- 1 Iron 100 I D..... B 35 20 Oxidized Formation D.. W 300 -'f~------taconite layer 5251-650 1 W 0 C _134 ______41 _ 701 o 1 D -f------30 9 Total 170 1 100 200 Lower Cherty Member 50 1 1 Magnetic 265 - 340 taconite layer 140~2201 100 Nonmagnetic I 1 slaty taconite -----.~..-51-1 0 1 1 Lean magnetic~ ..... 15 -20 1 o taconite Red basal taconite 25 o Pokegama Quartzite Formation

FIGURE 5. - Stratigraphic location of bulk sample of oxidized taconite from area 1. 10 and metallurgists have determined that of 70 wt pct from the Lower Cherty hor the Lower Cherty Member is the most amen izons plus 30 wt pct from the Upper able of the four members to proposed pro Cherty horizon as shown in figure 5. cessing of oxidized taconite because of These weight proportions were chosen by its predominantly coarse texture and its mining company geologists and engineers moderate to extensive oxidation. In area as an accurate blend of Upper and Lower I the magnetic layer is presently being Cherty Members that represents the amen mined where its magnetite content is suf able materials above the currently mined ficient for treatment by magnetic separa magnetic layer. By design the weight tion. Below that layer, the lean tac percent figures are not porportional to onite and lean red basal taconite have the vertical interval. Typical hand low potential amenability. The upper specimens were selected from the separate part of the Lower Cherty, which is re stockpiles before the materials were moved by stripping to expose the magnetic mixed to form the composite sample. taconite, is of primary interest for non magnetic processing. The Upper Cherty The area 2 bulk sample was 600 tons of Member has some potential amenability, material taken from a 20-ft layer at the but the two slaty members are normally top of a I85-ft-thick section of Lower too fine grained, too high in Al 20 3 , or Cherty material. As shown in figure 6, too unoxidized (contain iron carbonate the 185-ft section was above the red and iron silicates) to be of interest. basal taconite (which does not liberate iron oxides well and contains undesirable The area I bulk sample is a 350-ton A1 20 3 ) and does not include the top 60 ft composite of material selected to consist which varies from wash ore to retreat ore

Virgi nia Argi IIi te Farm atian 600

Upper Slaty Virginia Argillite Formation 50'-140'

500 Upper Slaty 65'-110' 150

Upper ICherty Upper ICherty 400 50'-210' 170'-210' E Biwabik I Iran I r 100 I (L j Formation CL LL1 ~ 300 500'-600' 0-25' o Biwabik Iron 60't Natural wash are, and retreat ore Formation 550' 200 ===~~r£J~-]l{ t I 50 Area 2 Lower Cherty Lower Cherty bulk sample 260'- 360' 240'-280' I represents 185' I 100 this 185' of [ Lower Cherty Taconite

o Red basal taconite 20'-25' Red basal taconite 20' o Poke\jlama Quartzite Formation Poke\jlama Quartzite Formation Are(J 2 Area :3 FIGURE 6., Stral igroph ic local ion of bu I k samples of ox id ized lacon ite from areas 2 and 3. 11 to lean sand. The Upper cherty material taconite. The sample represents 2S0 ft in area 2 was not considered because it of Lower Cherty Member, except for 20 ft is leaner and is not expected to yield an of red basal taconite (fig. 6). Fewer acceptable recovery. The area 2 Lower drill holes in this area resulted in less Cherty stratigraphic equivalence of the certainty as to how well the sample magnetic taconite layer of area 1 is represents the entire strata, but it is mostly oxidized and so is represented by the best sample available from area 3, the bulk sample WMR-2. Although the sam which has had little mining activity. As pling interval was only 20 ft based on shown by Bleifuss (8) (fig. 1), some accessibility, drill core analyses and strata in the Lower Cherty Member of the testing of core indicated to the mining WMR originally contained carbonate-chert company personnel that this material was or silicate-chert and no magnetite, while fairly representative of the 18S-ft sec other strata contained magnetite which tion. There are leaner and richer layers mostly altered to martite (hematite). and zones within the strata, but these The carbonate-chert and silicate-chert are also present in the 20·-ft sampling strata altered so as to contain goethite interval. as the dominant iron oxide. Westward the amount of original magnetite dimin The area 3 bulk sample consisted of 800 ished until in area 3 it was almost tons taken from a 17-ft-thick layer insignificant. between 44 and 61 ft above the red basal

o 4

,--I -'----'--.~ Scale. em

FIGURE 7. - Hand specimens of area 1 showing variations in textures and structures. 12

C~CTERIZATION OF BULK SAMPLES

Macroscopic and microscopic examination MACROSCOPIC TEXTURE AND GRAIN SIZE of the specimens selected from each stockpile for the bulk sample of area The macroscopic appearance of unoxi 1 showed almost as rruch variation in dized taconite usually reveals cherty and mineralogy, texture, alteration, and lib slaty layers that are interbedded (figs. eration rock types in anyone stock 7-9). Each layer or lamina consists of pile as among stockpiles (fig. 7). Typi rather specific proportions of two or cal sample differences within a stockpile more of the minerals already mentioned. included varying proportions of hematite, Slaty layers usually range in thickness of goethite, and of total iron oxides to from less than 1 to 10 mm. Cherty taco-· quartz. Other differences included the nite layers are usually from 2 mm to over thickness and continuity of chert-rich 10 cm in thickness. The lateral extent versus iron oxide-rich layers. Further of the layers ranges from a few centi differences include the range in degree meters to several tens of meters. Layers of weathering that resulted in removal of are frequently parallel, but many cherty silica and migration of small amounts of layers are wavy, some pinch out, and a iron to form lean and porous rocks at one few bifurcate and may rejoin around nod place, enriched concentration of iron ules that formed during lithification. oxides nearby, and almost unweathered Minerals in cherty layers commonly occur taconite at another place. as separate grains within granules in a

o 4

I Scole. em

FIGURE 8 •• Hond specimens of area 2 showing variations in textures and structures. 13

4 LJ.J ... LJ Scale, r:m

FIGURE 9. - Hand specimens of area 3 showing variations in textures and structures.

matrix of finer mineral grains. Granules The Upper Cherty (A of fig. 5) stock are 0.2 to 2 mm in longest dimension, and pile sample showed macroscopically that ovoid to elliptical to distorted in some rocks had a cherty matrix with shape. Minerals in slaty taconite are scattered or clustered 0.1- to 1.0-mm too fine grained to be observed with the grains of goethite and/or hematite. Tac unaided eye, and these layers are usually onite coherency ranged from hard, un devoid of granules. leached material to extensively leached and friable rock, the latter containing Area 1 earthy goethite. Some oxidized taconite showed a cherty matrix with partly decom Three (A, B, and C) of the four stock posed products from iron silicate and piles (excluding D of fig. 5) of area 1 iron carbonate. Some taconite is mas sample showed similarity in their overall sive, hard goethite with either scattered degree of oxidation and contained only quartz inclusions or frequent cavities. trace amounts of magnetite, iron carbon a te, and iron silicate, indicating that The upper 35-ft layer (! of fig. 5) of the original minerals were either re the Lower Cherty Member is an oxidized placed or leached. The fourth stockpile taconite derived from a carbonate-chert (D of fig. 5) appeared more like the material containing only small to trace underlying magnetic taconite. It varied amounts of magnetite. This layer usually from an unoxidized to a partly oxidized is lean (23 pct Fe) and contains some condition. wash ore and retreat ore. The oxidized 14

taconite contains hard goethite and a sample. Original layering has been small amount of hematite as scattered partly destroyed by weathering. small grains to irregular networks with a cherty matrix. Some goethite occurs in Area 3 solid layers up to 5 cm thick. Rock coherency ranges from hard to friable. Macroscopically the area 3 taconite sample (fig. 9) is similar to the area 2 The middle layer (C in fig. 5) of the sample (fig. 8) in that it has a white Lower Cherty sample of area 1 is composed chp.rty matrix with dis semi nated iron of mostly oxidized taconite with a cherty oxides, hematite, and goethite. Some matrix containing goethite and/or hema iron oxide occurs as clusters or isolated tite. Most iron oxide is in scattered grains from 0.5 to 10 rum in diameter. A small grains, while some forms an inter small amount of iron oxide occurs in connecting network through the cherty solid layers from 0.25 to 3 cm thick. matrix, and some occurs as solid iron Original layering is preserved in some of oxide layers from 0.5 to 2 cm thick. the rocks. Most of the oxidized taconite is hard and coherent, but some is leached and fri MINERAL CmJPOSITION able. Occasionally, thin layers that are magnetite-rich occur between granule tac Area onite and a solid goethite layer. The mineral content of bulk sample WMR- The lowermost layer (D of fig. 5) is a is based on a point count of sized magnetic taconite with-olive green iron fractions briquet ted and polished and on silicate and buff-colored iron carbonate chemical analyses of three different head matrix containing magnetite as scattered samples, plus other analyses on selected grains and rarely as layers. Where oxi size fractions. Quartz is the most abun dized, the taconite shows that goethite dant nilneral at near-ly 45 wt pct (table and hematite (brown and red, respec 2). Magnetite is present in the overall tively) have replaced the magnetite. bulk sample at 4 pct, while iron carbon Sedimentary layering (bedding) is well ate is 2 pct and iron silicates are only preserved in the less oxidized parts, 0.3 pct. The low iron carbonate, iron whereas it is partly destroyed in the silicate, and magnetite content results oxidized material. from the more oxidized nature of 91 pct of the sample (A, B, and ~ of fig. 5). Area 2 Most of the magnetite, iron carbonate, and iron silicate are found in the lean Macroscopically the area 2 taconite oxidized 9 pct of the sample (0 of fig. sample is partly coherent rock with a 5). Figure 10 shows the distrTbution of white cherty matrix containing scattered these minerals in various size fractions iron oxide, mostly hematite and partly screened from minus 10-mesh pilot plant geothite, and trace amounts of magnetite feed after the sample had been partly (fig. 8). The iron oxide occurs mostly ground. The three most abundant minerals as irregular clusters of subhedral to are present in nearly uniform amounts for euhedral grains, the clusters ranging the three coarsest size fractions. Below from 0.2 to 1.5 rom in diameter. Scat 100 mesh, quartz content increases, which tered through the cherty matrix are small suggests that goethite and hematite are amounts of earthy goethite and hematite more readily broken to finer sizes. that would be lost during beneficiation. Below 10 ~m, quartz and the ferrous ox This sample is fairly homogeneous when ides are present in the same amounts. compared to the highly variable area 1 15

TABLE 2. - Mineral distribution of wrffi bulk samples, weight percent

WMR-l I Mineral Chemical I WMR-2: I WMR-3: Petrographic! analysis Petrographic3 Pet rographic4 2 I calculations Quartz ••••••••••••.•.•••• 44.8 44.8 40.0 49.0 Hema tit e ••••••••••••••••• 25.9 23.0 41 34 Goethite ••••••••••••••••• 22.8 25 16 14 Subtotal • ••••••••••• 48.7 48,0 57.0 48.0 Magnetite •••••••••••••••• 4.1 5.5 .9 1 Carbonate •••••••••••••••• 2.1 1.6 .7 Trace Iron silicate •••••••••••• .3 .1 1.0 2 Total ••••••••••••••• 100.0 100.0 100.0 100.0 -- J Bas_dp on pOlnt count of slzed fractlons brlquetted and polished and an average of 3 chemical analyses. 2Based on chemical analysis totaling 97.6 pct and normalized to 100 pct. 3Based on average of 2 chemical analyses and visual estimate of polished briquets of screened fractions. 4Based on point count.

Area 2 ferric oxide (table 2). The area 1 sam ple also contains about 5 pct magnetite Minerals of area 2 sample (table 2) for a combined iron oxide content of show about equal amounts of quartz (40 about 53 pct. The area 3 sample contains pct) and hematite (41 pct), 16 pct goe the least combined iron oxide (49 pct). thite, and about 1 pct each of magnetite, The ratio of hematite to goethite in each iron carbonate, and iron silicate. The bulk sample is of interest, Early in the extensive oxidation of area 2 sample is indicated by this low magnetite, iron TABLE 3. - Chemical analyses of WMR 1, 2, silicate, and iron carbonate content. and 3 bulk samples, percent

Area 3 Chemical WMR-l WHR-2 WMR-3 Si 0 2 ••••••••••• 43.7 44.5 49.0 The area 3 sample shows a mineral con Al 20 3••••••••• • .65 .64 .45 tent (table 2) of 49 pct quartz, 34 pct FeO •••••••••••• 2.9 .7 1.5 hematite, and 14 pct goethite. As in Fe 203 •••••••••• 47.4 50.9 44.6 area 2, the iron carbonate and iron sil TiO 2 ••••••••••• .25 .26 .14 icate are nearly destroyed because of MgO •••••••••••• .19 .26 .14 extensive oxidation. CaO ••• _.•••••••• .52 .41 .19 K20 •••••••••••• .03 .05 • 1 Discussion of Mineral Composition Na 20 ••••••••••• .03 .05 .1 and Element Distribution tv'irlQ •••••••••••• .61 .10 .10 Sulfur ••••••••• .018 .01 .012 Referring to table 2 for mineral com Phosphorus ••••• .039 .013 .01 position and to table 3 for chemical Carbon ••••••••• .17 .11 .23 analyses of the bulk samples, quartz con LOI at 400 0 C .• 2.26 1.6 2.8 tent is 45, 40, and 49 wt pct for bulk LOI at 1,000 0 C 3.17 2.4 3.4 samples 1, 2, and 3, respectively, a mod To tal ••••• 101.94 99.00 102.77 erate variation indicating that sample 2 J Jacobs and Colombo (3). should be richest in iron oxide as evi 2Jacobs and Colombo (4). denced by its combined ferric oxide (hem 3Jacobs (~), modified-by eliminating 11 atite and goethite) content at 57 pct. trace elements. Areas 1 and 3 each have 48 pct combined 16

80 -

u -a. 70 KEY -~ 0 Quartz w I:l. Hematite N en 60 "V Goethite 0 Magnetite w .....J 0 Carbonate + silicate 0 o Particle size distribution .....- 50 0::::

PARTICLE SIZE, mesh (fLm as shown)

FIGURE 10. - Mineral composition versus size distribution of area 1 bulk sample as minus 100 mesh feed to flotation grinding circuit. study, it was believed that area 1 sample considerably more goethite than hematite. would have considerably more hema These ideas were based on the predominat tite than goethite, that area 2 sample ing color of the samples (red for hema would have more goethite than hematite, tite and brown to yellow for goethite). and that area 3 sample would have Also oxidation and hydroxylation that 17 would favor goethite formation over hema arsenopyrite. Phosphorus is probably tite was thought to increase westerly. from apatite or related phosphates, which The area 1 bulk sample contained about are difficult to recognize in the taco equal amounts of hematite and goethite. nite. LOI up to 400 0 C represents water The area 2 sample contained 2.6 times as adsorbed on mineral surfaces and (OH) in much hematite as goethite, and the area 3 the goethite crystal structure. LOI sample contained 2.4 times as much hema between 400 0 C and 1,0000 C represents tite as goethite. When these ratios were both CO 2 from carbonate and (OH) from observed by light optical microscopy, iron silicate structures, as these miner there was concern that softer goethite als were thermally decomposed. might pluck out of the polished section more readily than hematite, thus giving a MICROSCOPIC TEXTURES, ALTERATION, falsely low goethite content. However, AND LIBERATION chemical analyses supported the ratios indicated. Earthy goethite and earthy Petrographic examination of taconite is hematite do not polish well, and some is carried out with light optical microscopy lost during sample preparation. The using polished briquets, occasional thin amounts of hematite and goethite are im sections, and fragments in refractive portant because hematite is richer in index oils. Mineral identification, lron (70 pct) than goethite (63 pct) and grain size, texture (grain-to-grain rela a concentrate grade is thus richer if it tionships), alteration, and liberation contains more hematite. Also, the min characteristics (size and type of locking eral beneficiation response may be dif of ore to gangue minerals) can be deter ferent. The slime problem during bene mined by these techniques. Concentrates, ficiation is greater with goethite and middlings, and tailings can be observed earthy hematite since they appear to to let the mineral benefication process break down more readily to finer par engineer know if liberation has been ticles than the coarser crystalline achieved and if the separation process hema tite. was successful. Most of the process problems were related to separation effi The various chemical constituents of ciency; few resulted from lack of the bulk samples (tables 2 and 3) are liberation. considered to be associated with the min erals as follows: Ferrous iron is dis Unoxidized and partly oxidized taconite tributed among magnecite, siderite, and contains iron silicates (minnesotaite, iron silicates, and ferric iron among stilpnomelane, and possibly greenalite), hematite, goethite, and magnetite. (Two siderite, and magnetite. Some of this thirds of the iron in magnetite is Fe 3 +,) magnetite is partly replaced by hematite Si0 2 is present mostly as quartz or (martite) along magnetite grain bound chert, with a very small amount (0.1 wt aries and along crystallographic planes pct) attributed to iron silicates, A within magnetite. Most of this magnetite small amount of the A1 20 3 is present in occurs in clusters of grains with overall the iron silicate st~lpnomelane. Prob size ranging from 30 to 300 ~m in di ably most of the Al 20 3 is present as ameter and averaging about 100 ~m. Indi small quantities of painty material such vidual magnetite grains range from about as kaolinite stained with fine hematite 100 ~m to just visible (several ~rn). and/or goethite. The kaolinite is a Liberation of most magnetite would be weathering product of stilpnomelane. The about 30 to 40 ~m because the magnetite small amounts of CaO, MgO, and Mn are breaks easily through the clusters and present in both carbonate and silicates less easily at magnetite-gangue grain substituting for Fe. The small amount of boundaries. Figures 11-13 show typi sulfur probably results from weathering cal textures in samples from areas 1, 2, of trace amounts of pyrite and possibly and 3. 18

o 100 Scale, I-'m

FIGURE II. - Photomicrographs of pol i shed surfaces of orea 1 bulk sample taconite. It, Light -gray hematite and medium-gray goethite replacing magnetite euhedra. Dark gray is quartz or chert matrix, black spots are cavi ties in the surface. 8, Light-gray hematite in coarse crystals replacing magnetite and in irregular blades re placing iron silicates surrounded by massive goethite (medium gray) that partly filled cavities. Present cavi ties in surfacearedark gray toblack. C, Hematite (white) ond goethi te (light gray) replacing gronules that were magnetite. Medium gray is quartz matrix, and black is covitles in surfoce. D, Light-gray/ fresh magnetite in a thin layer, alang a fracture, and in isolated euhedra in 0 mottled gray matrix of iron carbanate, iron silicate, and quartz. Black is cavities in surface. White is hematite in initial stage of replacing magnetite. 19

t j '.: . I•- • ,:

o 100 Scale,l-lm

F I GU R E 12. - Phot om i crog raphs 0 f pol i shed surfaces of area 2 bu I k sam p Ie tacon i teo A, Whi te hema t i te ha s re placed magnetite euhedra :n granule texture. Light-gray goethite contains coarse and minute cavities and has poor liberation, Medium-gray quartz shows moderate alteration by solution along grain boundaries that in places leaves coarse cavities (black to dark gray). B, Whi te hematite and I ight-gray goethite replace magneti te clusters of anhedra to euhedra. Magneti te rei ics may be seen as lightest gray in centers of several coarse clusters of hem atiteandgvethite. Gray quartz matrix shows black cavities. C, White hematite and light-gray goethite havere placed magnetite. Goethiteal so occurs in irregu lor shapes fill ing cavities in the gray gangue that is mostly quartz, itself containing bllJck cavities in the surface. f), Medium-gray goethite in a sol id layer (upper half) containing a few small gray quartz inclusions. Lower half shows leaner goethi te layer with more disseminated quartz and very poorliberationcharacteristJcs. Afractureacrossthelayersis partly filled with chert and iron carbonate. Black to dark gray is cavities in prepared surface. 20

a 100 Scale, \-1m

FIGURE 13, - Photomicrog-aphs of poli shed surfaces of area 3 bulk sample taconite. A, White hematite in medium to fine grains replacing magnetite. Light-gray goethite mostly filling cavities. Hemat i te has better liberation characteristics Ihan goethite in this texture. Dark gray is quartz or chert, and b l ack is holes in the polished sur face. 15, Light-gray massive goethite contains minute hemati te (white) i nclusions and fine to coarse gangue (me diumgray), Cavities in surface are black. C, Coarse h ematite (light gray) is matrix for roarse goethite (medium gray). Black spotsarecavities, and dark-gray areas are iron carbonates and/ or iron si licates. f),Whitehematite has poor Ii berati 011 charac ter i sti cs in th i s quartz and iron s i I icate gray matr i x. BI ack spot s are surface cavi ti e s. 21

Increasing oxidation causes magnetite quartz or chert and iron silicates, to be pseudomorphously replaced, mostly leaving cavities. Some cavities are left by hematite and partly by goethite, thus open, giving the rock a porous and fri preserving the original shape and size of able texture, Some cavities are filled the octahedral magnetite. The last rem with redeposited Si02 , and some are nants of magnetite occur as a central filled with iron oxide, mostly goethite core within grains or clusters of grains. and a small amount of hematite. The Iron carbonate is occasionally replaced degree of oxidation and alteration is pseudomorphously, but more often it is fairly obvious both macroscopically (fig. leached away and the cavity is filled by 14) and microscopically. ~licroscopically goethite and occasionally by hematite in both oxidation and leaching occur first an irregular massive form. Iron sili along grain boundaries and then work, cates are replaced by goethite more fre somewhat irregularly, deeper into grains. quently than by hematite. Much of the Various degrees of alteration recognized taconite appears to be good ore macro microscopically are summarized in table 4 scopically, but microscopy shows a cherty and illustrated in figure 15. matrix with disseminated goethite and/or hematite replacing scattered iron carbon The liberation size of goethite should ate or silicate. Such material is too vary considerably more than that of hema lean and would have an extremely small tite because most hematite replaces liberation size. Other layers altered from iron silicates are painty; they con tain the clay minerals nontronite and kaolinite. Therefore, a large percentage of the oxidized taconite would either be left in place or removed as lean rock stripping.

Alteration of the original taconite also includes leaching of Si02 from

:] t 2: :s 4 L-L-:--. .L -..l ~al • • c"..

o 50

I Scale, IJ.m

FIGURE 14. - Photomacrograph showing alteration of FIGURE 15. - Photomi crograph of pol i shed surface taconite by weathering, includ ng solution, redeposition showing extensive weathering along quartz grain in cavities, and direct replacement. boundaries. Medium-gray quartz has I ighter gray goe thite occurring both as fine and coarse cavity fi II ing Unfilled cavities are dark gray to black. N N

TABLE 4. - Degrees of taconite alteration recognized from microscopic analysis

Original taconite Alteration Ini tial Moderate Extensive Pervasive minerals and features process DEGREE OF TACONITE ALTERATION OBSERVED MICROSCOPICALLY

Magnetite •••••••••••• Oxidation and Grain edges, cleav- Partly to mostly Magnetite rel- No magnetite. pseudomorphic age planes alter to altered to ics at center replacement. hematite. hematite and/or of ferric ox- goethite. ide grains.

Iron carbonates and Solution of Slight alteration Much altera- No relics. iron silicates. carbonate and to hematite and/ tion to hema- leaching of or goethite if tite and/or silica. much carbonate goethite. or silicate. QUARTZ MATRIX SHOWING LEACHED QUARTZ-QUARTZ CONTACTS Quartz, •••••••••••••• Leaching of <10 pc t ...... 11 to 30 pct •.• " 31 to 70 pct •• >70 pct. silica.

Iron silicate ••.••••• ·...... If much iron sili- If little iron No relics. cate, Si0 leached silicate, Si0 2 I 2 and the iron is leached and oxidized. the iron is oxidized.

Cavities, none ••••••• · ...... Occasional •••••••••• Few ••••••••••••• Freq uen t •••••• Many, inter- connected.

H.. ock, coherent ••••••• · ...... Coherent .••••.•••••• Mostly coherent. Moderately Very friable. friable. CAVITIES AND CAVITIES FILLED BY FERRIC OXIDES Cavities for filling Secondary enrich- <10 pc t ...... 11 to 30 pct •••• 31 to 50 pc:: •• >50 pct. by iron oxides, ment of ferric none. oxide by filling cavities. 23 magnetite in its original size and shape, TABLE 5. - Liberation characteristics of whereas much goethite fills cavities in iron oxides in bulk sample WMR-1 irregular shapes. Screened fractions of WMR-1 bulk sample were examined in Estimated liberation polished briquets and as fragments in from gangue minerals refractive index oils on a slide to esti Screen fraction in size fraction, pct l mate the percentage of each iron oxide Hema- Goe- Magne- liberated from gangue minerals (table 5). tite thite tite Liberation of the iron oxides from each 10 by 20 mes h •••• 3 20 10 other was not required, as all contribute 20 by 35 mes h •••• 10 25 15 positively to concentrate grade of iron. 35 by 65 mes h •••• 20 35 20 Results confirmed the expectation that 65 by 100 mesh ••• 25 40 30 goethite is more completly liberated than 100 by 200 mes h •• 30 50 40 hematite or magnetite at coarser sizes. 200 by 325 mesh •• 50 60 70 At finer sizes no real difference in the 325 by 400 mesh •• 65 65 80 amount of liberation was apparent for the 400 by 30 IJm ••••. 80 75· 85 three iron oxides. Oxidized taconite 30 by 20 ~ m •••••• 90 85 95 from all three areas was essentially sim 20 by 10 ~m •••••• 95 95 95 ilar microscopically except for the in Minus 10 II m•••••• 98 98 98 creased amount of magnetite in area 1 I Estlmated' by vlsual observatlon uSlng sample. Oxidation and alteration effects light optical microscopy of polished and liberation were essentially similar grains in briquets and as fragments in in the three areas. refractive index oils on glass slide. BENEFICIATION RESULTS AND ESTIMATE OF WESTERN MESABI RANGE OXIDIZED TACONITE RESOURCES Beneficiation results of pilot plant over the anionic process on the basis of testing of WMR oxidized taconite are reagent costs. for areas 1, 2, and 3, summarized in table 6 from three ref respectively, the concentrates averaged erences (2-4). An estimated 3.25 billion 61.7, 63.5, and 62.8 wt pct Fe and 5.1, long tons of oxidized taconite amenable 4.3, and 4.7 wt pct Si02 • The Fe recov to either cationic or anionic "selec ery was 67, 85, and 72.3 wt pct, respec t i ve-f locculat ion-deslimi ng-f lotation" is tively, showing that area 2 sample available in WHl{ areas 1, 2, and 3. The responded best. Chemical treatment of crude feed composite averaged 32 to 36 wt process water resulted in recovery of pct Fe. The cationic process is favored about 85 pct of the total water used. TABLE 6. - Beneficiation results for WMR oxidized taconite bulk samples

Cationic flotation Anionic I WMR-1 WMR-2 WMR-3 : flotation Estimate of oxidized taconite I resourcesl •••••• billion long tons •• 2.0 0.75 0.5 NAp He rna tit e - go e t hit era t i o ••••••••••••• 1.1 2.6 2.4 NAp Crude iron ••••••••.••••••••• wt pct •• 35.3 36.1 32.3 NAp Concentrate, pct: Feed 2 ••••••••••••••••••••••••••••• 38.5 48.1 37.2 37.7 I ron ...... 61.7 63.5 62.8 63.0 Si02 ···············,,············· 5.1 4.3 4.7 5.3 Iron recovery ...... 67 85 72.3 73.5 Tails or slime + tails •••••• pct Fe •• 18.8 10.7 14.2 13.7 Reclaimed water ••••••• pct of total •• 83 88 88 86 NAp Not appllcable. IJacobs (2); Jacobs and Colombo (3-4); sum 3.25 billion long tons. 2Feed composite: 93 pct minus 400 mesh. 24

The poorest response of the three samples pits. All Lower Cherty material in areas was for WMR-l with 67 pct of the Fe in 2 and 3, except the red basal taconite, the feed recovered. Two factors may have was considered oxidized. In area 1 only entered here: The lower layer of partly Lower Cherty oxidized taconite above and magnetic taconite (layer D of fig. 5), below the magnetic layer was considered, mixed in with more oxidized- layers above and only some of the Upper Cherty tac it, was probably not efficiently recov onite where it had been mixed in with ered by flotation. Also, WMR-l was the bulk sample 1. A tonnage factor of 14 first sample treated in the pilot plant, ft 3 It 0as used to convert volume to and process experience and refinements weight. No atte"mpt was made to account made during the study favored better con for lean rock, poor liberation, or other ditions for recovery from WMR 2 and 3. problem material. According to the joint Additional experiments to supplement flo U.S. Bureau of Mines and U.S. Geological tation by reduction roasting (5-6) and/or Survey Mineral Resource Classification WHIMS (wet high-intensity magnetic sepa System (24), this material would be con ration) (1) indicate that further im sidered a "subeconomic indicated re- provements in beneficiation are possible source. Table 7 lists these tonnages over straight flotation treatment. and compares them with the corresponding amounts from Marsden's (19) OXYBIF ton In table 6 the estimated amounts of nages, which were modified' to correspond oxidized taconite resource (2.0, 0.75, to the same geographic range units. and 0.5 billion long tons for areas 1, 2, and 3, respectively) are based on the TABLE 7. - WMR oxidized taconite ton best judgment of mining company personnel nage estimate compared with Marsden's as the amount of oxidized taconite that OXYBIF estimate the bulk samples represented. Because these estimates do not cover the entire Oxidized taco- OXYBIF, WMR areas 1, 2, and 3, they are conserva Area nite, billion billion tive. In the present study an estimate long tons 1 metric tons 2 was made of the amount of oxidized taco 1 •••••••••• 3.2 2.9 nite available in these three areas based 2 •••••••••• 3.2 2.9 on the geology. It considered a 6° S 3 •••••••••• 1.5 1.1 average dip and material within the out Total ••. 7.9 6.9 crop area to a vertical line at the south 1Long ton = 2,240 lb. outcrop contact of the Biwabik Iron 2Marsden (19); metric ton = 2,204.6 lb; Formation, and did not substract the modified bY- corresponding geographic large amount of iron ore already removed ranges. in scattered underground mines and open

SUMMARY

The geology and mineralogy of three Lower Cherty Member contain a magnetite bulk samples used for testing the rich layer presently being mined and mag selective-flocculation-flotation bene netically beneficiated. Above this mag ficiating response of oxidized taconite netic layer is oxidized taconite which of the Western Mesabi Range have been was mixed in a representative proportion studied. A strike distance of 26.5 mi with some of the Upper Cherty Member to (42.7 kID) from just east of Keewatin, MN, provide the best sample for testing oxi to Grand Rapids, MN, was divided into dized taconite response to flotation and/ three large blocks, and one bulk sample or magnetic roasting for future exploi from each was tested in a pilot plant for tation. In area 1, not all Upper Cherty amenability to flotation processes. oxidized taconite is amenable to bene-" ficiation because the liberation size of Within the outcrop area of the Biwabik iron oxides is too small and some areas Iron-Formation the area 1 rocks of the are too lean for reasonable recovery. 25

In areas 2 and 3 extensive oxidation Petrographic analyses of the feed and had changed most of the Lower Cherty Mem products of selected separation stages ber to a nonmagnetic or oxidized taco are important in helping the mineral ben nite. In these areas, a large thickness eficiating engineer know the mineralogy, of the Lower Cherty Member was selected the type and degree of locking, and the on the basis of drill core and beneficia behavior of the material in the process. tion testing as probably amenable to flo Taconite mineralogy is fairly simple, but tation processing. A large representa the variation in grade and in grain or tive sample was chosen from each area. particle size is closely related to its The Upper Cherty Member in areas 2 and 3 response. Some parts of the taconite was considered too lean to be of interest have coarse layers or clusters of iron at this time, but future testing may oxides, which liberate well; other parts locate areas where its iron content could of the taconite contain very fine, dis furnish enough recovery to be utilized. seminated, or tightly locked minerals In all three bulk samples the major min that cannot be economically separated erals were quartz (or chert), hematite, physically. Flotation alone, or in com and goethite. Magnetite was a minor min bination with magnetic roasting and/or eral in area 1 and recoverable with the WHIMS of some products, has been found to ferric oxides. Iron silicates and iron produce concentrates of acceptable grade carbonate were also present as minor to and recovery if advanced drilling, test trace amounts in oxidized taconite and ing, and selective 'mining are utilized. were usually rejected by the process. Therefore, the oxidized taconites of the Liberation of all three iron oxides was WMR represent a subeconomic indicated found to be similar in the three bulk resource that can be selectively mined samples. Most problems with beneficia and beneficiated as a future large source tion processing were found to be with of iron feed for steelmaking. separation and not with liberation.

REFERENCES

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-crU,S, GOVERNMENT PRINTING OFFICE:1983-605-015/75 INT.-aU.OF MINES,PGH.,PA. 27123