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1973 Description and origin of carbonate minerals in the upper and lower cherty members of the Biwabik Formation, Minnesota Leslie R. Honeyman University of North Dakota
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Recommended Citation Honeyman, Leslie R., "Description and origin of carbonate minerals in the upper and lower cherty members of the Biwabik Formation, Minnesota" (1973). Theses and Dissertations. 142. https://commons.und.edu/theses/142
This Thesis is brought to you for free and open access by the Theses, Dissertations, and Senior Projects at UND Scholarly Commons. It has been accepted for inclusion in Theses and Dissertations by an authorized administrator of UND Scholarly Commons. For more information, please contact [email protected]. DESCRIPTION AND ORIGIN OF CARBONATE NEIERALS IN THE UPPER. AND LOWER CHER.TY ?•IF1'-fBERS OF THE :BIWABIK FORE,\.TION, HI:l:\1{ESCTA
Leslie R. Honeyman, Master of Science
The University of North Dakota, 1973
Faculty Advisor: Professor Frank Karner
This study was done for the Erie Mining Company after periodic problems with the quality of pellets from taconite of the Biwabik Fonna tion had been linked to the presence of carbonate minerals. The carbon ates fouad in the cherty members of the Precambrian Biwabik Formation are predominantly side~ite, ankerite, and calcite. Siderite is the only primary carbonate mineral and occurs in three forms: microgran ular, granular (oolitic), and crystalline aggregates (recrystallized).
Ankerite is a secondary mineral occurring as crystalline aggregates and alteration rims on granular siderite. Calcite is found only in fractures and fault zones. Siderite formed from the original precip itate and early in the diagenetic history of the rock. The alteration of siderite to fine-grained, euhedral magnetite appears to be the major part of the pellet-quality problem at Erie. The fine-grained magnetite causes small amounts of siderite to be incorporated with magnetite in the magnetic separation process. This siderite breaks down when the pellet is roasted and carbon dioxide is given off causing the pellet to disintegrate. Ankerite formed in two periods: the ankerite rims on siderite granules formed early in epigenesis and the crystalline ankerite formed much later, Calcite formed late in the epigenetic
l history of the formation. It appears that the carbonates of the cherty members of the Biwabik :b'ormation can be used as indicators of an envi- ronment of deposition having reducing conditions and as indicators of the extent of metamorphism caused by the intrusion of the Duluth Gabbro.
2 DESCRIPTION Al\l) ORIGIN OF CAIGONATE HINEPALS I:~ THE
UPPER AND LOWER CHZRTY MEHBERS OF TEE BIWABIK
FORM.AT IO?:;, l'IINNESOTA
by
Leslie R. Honeyman
Bachelor of Philosophy, University of North Dakota, 1969
A Thesis
Submitted to the Graduate Faculty
of the
University of North Dakota
in partial fulfillment of the requirements
for the degree of
Master of Science
Grand lorks, North Dakota
August 1973 This thesis submitted by Leslie R. Honeyman in partial fulfill ment of the requirements for the Degree of Master of Science from the University of North Dakota is hereby approved by the Faculty Advisory Committee under whom the work has been done.
/- ( Chainnan) ·
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'·
ii DESCRIPTION Ai).:'D ORIGIN OF CARBONATE r-IINER...\.LS IN THE UPPER Al\'1J LOWER CHERTY }I™BERS OF THE BIWABIK
Degree ------Master of Science
In presenting this thesis in partial fulfillment of the requirements for a graduate degree from the University of North Dakota, I agree that the Library of this University shall make it freely r2vailable for inspection. I further agree that permission for extensive copying for scholarly purposes may be granted by the professor who supervised my thesis work or, in his absence,·by the Chairman of the Depart ment or the Dean of the Graduate School. It is understood that any copying or publication or othe:c use of this thesis or part thereof for financial gain shall not be allowed with out my written permission. It is also understood that due recognition shall be given to me and to the University of North Dakota in any scholarly use which may be made of any material in my thesis.
Date
iii ACKNOWI ... EDGHE1"'T S
The three members of my committee, particularly my com.rnittee
chairman Dr. Frank Karner, are gratefully acknowledged for their con
structive criticism and suggestions during the course of this work.
The other two members are ffrs. Walter L. Moore and Lee Clayton.
Erie Mining Company, Hoyt Lakes, Minnesota supplied funds for
field support, laboratory analysis, and thin sections. Wayne Tuttle,
Forrest Boyee, and Joe Evers of the Erie Mining Company supplied
immeasurable help and assistance while I was on the Erie property.
iv T.ATJLE CF COl.TTENTS
ACK?WWLEDGHm!TS iv
LIST OF TABLES vi
LIST OF FIGURES vii
ABSTRACT ix
INTRODUCTIQ1(l 1
STR..4.TIGRAPHY 4 General Lower Cherty Hember Lower Slaty 11er:iber Upper Cherty Member Upper Slaty Hember
METHODS • 14
CARBONATE PETROGP..APHY • 2S Basic Definiticns Siderite Ankerite Calcite
DIFFERENCE IN CONCENTRA.TION OF CARBONATE MINERALS • • • • • • • 42
Lower Cherty Member Upper Cherty Member
INTERPRETATIONS OF CHANGES IN CARBONATE CONCE!:.'TRATION AND OCCURRENCE • • • • • • • • • • • • • • • • • • • 48 Siderite Ankerite Calcite
CONCLUSIONS 52
APPE'rlillIX. Weight Per,::entage for Sample Interval 57
REFERENCES ...... 64
V LIST OF TABLES
Table Page
1. Comparison of Hineral Abundances Obtained by X-ray and Point-Count Methods. . · 23
2. Precision of X-ray De.termination of Peak Intensities 24
3~ Weight Per~entage for Sample Interval. • • • • • • • • • 58
vi LIST OJ? FI·:;:aRES
1. Location and Geologic Map of Study Area Modified from Erie (1967) ..... 3
2. Geologic Cohu-nn for the Eastern Hesabi Range. from Erie (1967) ...... 6
3. Photograph of Interbedded Carbonate and Magnetite Rich Chert in Outcrop .••..••••. 15
4. Locations of Drill Hole and Pit Samples from Areal; Area 6; Area 3; and Area 2-West • • . • 18
5. Photomicrographs Showing the Occurrences of Siderite as (A) Hicrogranular Siderite; (B) Granular Siderite with Hicrogranular Side.rite Cement; (C) Granular Siderite with Chert Cement; and (D) Crystalline Aggregates of Siderite (Recrystallized) . . • . •••••. 28 ,,. o. Photomicrographs Showing the Alteration or Replacements of Siderite to (A) Recrystallized Siderite; (B) Magnetite; (C) Ankerite and Hagnetite; (D) Ankerite; (E) Minnesotaite; (F) Stilpncmelane; ((;) Quartz; and (R) Stilpnomelane and Magnetite 31
7. Photomicrographs Showing the Oc.currences of Ankerite as (A) talline Rhombs; and (B) Alteration Rims o~ Siderite Granules. • • • ••• 37
8. Photomicrograph Showing Ankerite Altered to or Replaced by Magnetite •... 39
9. Photomicrograph Shot,ing a Calcite Vein •• 41
10. Triangular Diagrams Showing Vertical and Lateral Changes in the Concentrations of Siderite, Ankerite, and Calcite in the U, T, S, and R Layers of the Lower Cherty Member 44
11. Triangular Diagrams Showing the Vertical and Lateral Changes in the Concentrations of Siderite, Ankerite, and Calcite in the L, K', K, and J Layers of the Upper Cherty Member ••..•...•.• 47
vii 12. The Precipitation and Deposition of Carbonate }faterial in the Biwabik Fon,,.ation (I·fodified from DiElC:Qth and Chauvel, 1973) .. 55
viii This study was done. for the Erie Hining Company after periodic problems with the quality of pellets from taconit2 of the Biwabik Forrr.~- tion had been linked to the presence of carbonate minerals. The. carbon- ates found in the cherty members of the Precambrian Biwabik Formation are predominantly siderite, ankerite, and calcite. Siderite is the only primary carbonate mineral and occurs in three forms: nicrogran ular, granular (oolitic), and crystalline aggregates (recrystallized).
Ankerite is a secondary mineral occurring as crystalline aggregates and alteration rims on granular siderite. Calcite is found only in fractures and fault zones. (siderite formed from the original precip itate and early in the diagenetic history of the rock. The alteration of siderite to fine-grained, euhedral magnetite appears to be the major part of the pellet-quality problem at Erie. The fine-grained magnetite causes small amounts of siderite to be incorporated with magnetite in the magnetic separation process. This siderite breaks down when the pellet is roasted and carbon dioxide is given of£ causing the pellet to disintegrate. Ankerite formed in two periods: the ankerite rims on siderite granules formed early in epigenesis and the crystalline ankerite formed much later. Calcite formed late in the epigenetic history of the formation. It appears that the carbonates of the cherty members of the Biwabik Formation can be used as indicators of an environment of deposition having reducing conditions and as indicators of the extent of metamorphism caused by the intrusion of the Duluth Gabbro. ix INTRODUCTION
This study was done for the Erie. Hir1ing after periodic
problems with the quality of pellets from taconite of the Biwabik. For-- matio.::1 h2.d been linked to the presenc2. of carbon.:cte minerals. The purpose was to determine the of carbonate minerals present, the
relationships of the carbonate minerals to each other, and ultimately
to the environ.,.~ent of deposition of the taconite.
The area of study is the Erie Mining Company property, extend
ing from the western edge of Area 1, just north of Aurora, 1'1innesota, to the eastern edge of Area 8, along the western edge of Birch Lake, where the Duluth Gabbro Complex transects the Biwabik Formation
(Figure 1).
Vertical and lateral differences in carbonate mineralogy were studied in four geologic zones within each of the two cherty members of the Biwabik. Carbonate minerals have been altered to various sili cates, oxides, and other carbonate minerals; the difference in carbon ate minerals is critical in its effect on pellet quality of the high- vollli~e taconite reining operations.
A better understanding of the carbonate mineraology and occur nmce will lead to a better interpretation of the depositional environ ment of the iron-fonnation and the geologic history of the Precambrian
Era.
1 2
Fig. 1.--Location and geologic map of study area modified from Erie (1967). // ERIE PLAIJT
0 I 2 c::r-=r=::c::i l.C' SC~\LE H,J fAll.ES
till ~'ii DULUTH GA8Li110 ~ VIRGINIA SLATE ~ UPPER SUHY UPPEF< CHERTY MINNESOTA LI] LOWEF? SUHY f:izi LOWER CHEHTY E3 Pat< EGAM>'.\ GUATZITE l::il 1<.N!FE LAKF. SLA.TE L.OCAL GEOI_OG Y [g3j Gf,ANITE General
The Biwabik Formation is underlain by the Pokegama Quartzite
Forrr.ation and is overlain by the Virginia Slate Formation. The
Biwabik Formation is considered to be late Precambrian in age and has been dated from 1,7 to 2.0 x 109 years before present (Goldich and others, 1961). The detailed stratigraphy of the Biwabik Forma tion have been incl.uded by Wolff (1917), Gruner (1924, 1946), Grout and others (1951), White (1954), and Gunderson and Schwartz (1962).
The Biwabik Formation is divided into four members, in ascending order.: The Lower Cherty, Lower Slaty, Upper Cherty and
Upper Slaty Members. Each of the members is divided into several layers (Gunderson and Schwartz, 1962) (Figure 2). The Upper and
Lower Cherty Memb2rs are the primary ore zones and are the members of interest in the carbonate study. These members are described from the lowest layer upward.
The members of the Biwabik Formation have been divided into a series of layers that are recognized by most workers in the eastern
Mesabi district. The units are referred to as "submembers 11 by some workers, but they will be referred to as "layers" in this paper.
The description of the members were taken primarily from
Gunderson and Schwartz (1962) and the Erie Mining Corn.pany (1967).
The descriptions the layers within the members were determined from drill cores. 4 s
Fig, 2. column for the Eastern ·Mesabi Range from Erie (1967). G' LOGIC COLUMN ERA FOR~A:HION LAYER Dl'.:SCR1P,iON
I- GL,\CIAL Tl LL
DULUTH ME::\/LJ!',l T;) <..UAHSE (:,R.A,"-.!t:J l> >~ ·. C.OM?<:ts:::r: -Jr= F[P.P.DMAC.r•lESiAN GA3BRO . . . At'1Ci ?._A:n'):.._ASE. FC: ... VS?i\R + <\< 1' +
1/IRGINiA - gQN-MAYNE:'T1C, 6RE Y TO BL.ACK SLATE' SLATE
,. ~---'-,i ~RE'f-W"ITE. C.OA'l,E • GRA!NEO LIMES TONE nc '..H:CRT AND I:".'),.,\ - SIL!C.ATES C SL.:. T 'f ANO CHERT'f ;.},;l)S
0 E f;l&H PHOSPHORUS J.AY ER
F MAC,NeTIC., 1NTEl!9E00li.D SLAT'( AND 6EOS
HIGHLY MAUN€TtC... M ..O..GNETITE O(G.UP.~ A<:ii G GRANULAFI cor,,CENTRAT10N$ ANO oEDS
H f-11C.HLY MAGNETIC., 'i';AVY Bi:DS QF MA!,NETITE
ALGAL $TRUC.TuR£
MAGNETITE IN CHeRT ANO l?.QN SlLJCATC: MA";i!IX
2 K z 0 DIABASE SILL COMPOSITION SH.\lLA>I TO &Ail BRO I- K' SAM< AS "K" LAYER <::l: <{ ~:::.; c:: 0: 0 ro lL.. 141G~L"1 TO $UG:,~HL"f MA.GNE.":':C. "-'\l)(E,',) 1.J,J~\JY ,,_~s ANO STRA\Gl-H BEQS OF MAGNETITE------1 ~ M00€R..\.TE.L'f M,._,UNKf\C.. THiC..'.4. LA~(£R~ OP C..h2P.T <( I') 5E?,'<\'R."TING M.\GNET1C LA.IE"?.:> co {.) <{ ----- NON-MAGNETIC. THIN Bal1Dl'.D. ()A?;K·GA!''!' TO !ILACJ.s w 3,: SLATE ARvilllTl':S a:: al a... Q SIM!LIAR TO ·p'" L/\'l[R
~U6H·rL "'f TO HU:,kL 'Y MA~ti~TtC , '¥\IIOi!lY S?AC.£ 0 M...:.>ltTI ft e.t.OS ,N <'..>4!'.RT AND IRO"l $1Ll<'..A1E )- ";;-- MArF\ll\ 1- C: w s . •. :i::: 0 T Cl: w HH,HL'f MA(,NHIC, 'c,TRAlul-lT BEDS Or MA(.,r-11:TITE ;,: u ,N AN IRON SILICATE MA.TRI:<. 0 ..J \I w POWCG4Nll\ QUARTZITE ALGO>;<"N GRAN1TE ///,r///, KNt~E LAKE ;j~j-,:_:~~ M[TASEV1'.'lENT~~'1 ROC.KS 3L<\TE /////// LAl.i'lEMTlAN 'LJ-RAH!TE El..Y GREE,.STO>IE 7
In the io11s a o~ mineral must mPKe uo more tha11 10 pe1.. c,er1.~ of th.,2 rock.. .1.\. L:1I:.or -r;:.ineral m~.1-:::.!s u.p lt}ss tl12..n 10 per cent. The members are described from the lowest , strat raphica1ly, upward.
rrhe Lo~ver Che_ l·!ember contains magnetiferous chert 'with dif- fering amounts of the carbonate minerals siderite, ankerite, and cal- cite and iron silicates minnesotaite, stilpnomelane) and chamosite.
The member is generally 40 to 50 m thick. The Lower Cherty Member is divided into layers: V, U, T, S, and R (Gunderson, 1958) and is generally 40 to m thick.
The lo,-;er contact of the Lower Ch2.rty Member with the Pokegama
Formation marked by a 0.25 to 1 m conglomeratic zone. The conglom- erate is co1:1posed of chert, magnetite, and hematite pebbles in a chert matrix. The conglom2ratic zone is often referred to as the "basal conglomerate" or W layer of the Lower Cherty Member.
Mineralogy Layer Description Ninor
V The major part of this unit is ch2rt chert minnesotaite with thiti. beds of slate. Conglomer magnetite stilpnomela:.1e atic beds of magnetite and hematite hematite chamosite pebbles are found in the chert zones. siderite The presence of minnesotaite, stilpno melane and chamosita give part of chert zones a color. Siderite is closely associated with dissemi nated magnetite. This unit is slightly magnetic and ranges in thickness from 1 to 2 ra. 8
U This unit is a magnetite-rich chart. magnetite siderite The bedding is horizontal with mag chert ankcrite netite interbedded with chert and ferroan chamoslte iron silice..tes. The lower part of hypersthene the unit may contain some slate grunerite with thin, maroon colored beds of jasper. Carbonate minerals occur as variable beds and lenses. The unit is moderately magnetic and ranges from 1 to 3 min thickness.
T This is an interbedded magnetite-rich magnetite ankerite chert zone. It has wavy to blotchy chert calcite magnetite beds. The magnetite beds siderite minnesotaite have indefinite boundaries grading stilpnomelane into the intermediate chert beds. grunerite Carbonate material occurs as mot chamosite tled zones with minor amounts of calcite and ankerite. This unit is highly magnetic and ranges from 0.5 to 1 min-thickness.
S This unit is a granular, interbedded, magnetite ankerito magnetite-rich chert with magnetite chert minnesotaite occurring as granules in distinct siderite cum.'11lingtoni te beds. Closely associated to the fayalite magnetite are irregular beds and lenses of pink and brown siderite and ankerite. Minnesotaite is closely associated with the magnetite with fibers radiating outward from the magnetite. This layer is highly magnetic and ranges in thickness from 1 to 3 m.
R This unit is an interbedded magnetite magnetite siderite rich chert. The magnetite occurs in chert ankerite. widely spaced beds in a chert and iron iron sili fayalite silicate matrix. The chert zones have cates ferroan amphi been altered to ferroan amphibole and bole fayalite giving the chert a green color. The carbonates occur in the lower 2 meters of this unit. This unit ranges from slightly to highly magnatic and ranges in thickness from 1 to 6 m. 9
The I..01:,;rar ]Yl2.mber is unit is and is charact9rized by th~n-bedded, black slate
(argillite). The upper boundary as described by Gunderson and Schwartz
11 (1962) is ••• the appearance of abundant magnetite-rich beds or granule structures the well defined, bedded quartzose pre- sumed to belong wi .:hin the Upper Ch,~rty Member. 11 The upper contact with the Lower Slaty is at.the base of the Q layer of the Lower Slaty
}fembar. The Q layer is kncnro as the !!Intermediate Slatet1 and is trace- able throughout the Mesabi Range, The Q layer contains nor.magnetic> t~.lnly bedded, black slate with small fractures filled 1;,;rith pyrite, quartz, or calcite.
This unit has been divided into two laye.rs, the Q and P, which are similar to the Interrnediate Sl-'lte.
Mineralogy Layer Major Minor
Q This unit is black to dark gray, ferroan cummingtonite fine-grained argillite. The rock hypersthene fayalite appears massive \vith indistinct chert biotite bedding. The rock contains small pyrrhotite fractures containing sulfides and pyrite carbonates. This layer is non calcite magnetic to slightly magnetic and ranges in thickness from 5 to 10 m.
P This unit is a bl~ck slate with the ferroan magnetite iron silicates making up the bulk of hyparsthene. fayalite the rock. CUiTu'1lingtonite occurs as chert cumm.ingtonite both acicular and prismatic grains. This unit shows evidence of metamor phism with relic granule structure still evident. The unit is slightly magnetic and ranges irt thickness from 4 to 12 m. 10
}fember.
The Upper Cherty Hember is di:vide.d into the. following
O, I1, L, K, J, I, 2nd H. Generally these l2.y2rs cont::ain pebbles of locally derived ccnglomeratic quartz and have thin beds rich in rJ.a;:;- netite. The chert layers a.re much thicker than in the other r:::.err.oers of the formation. The chert has distinct bedding and relic granule structure throughout. This member is cut by a diabase sill that is
1.5 m thick. This sill is believed to be related to the intrusion that formed in the Duluth Gabbro Complex. The average thickness of the unit is about 60 m thinning to the east. The upper contact is recognized by the disappearance of magnetite beds and the appearance of nonmagnetic slate (argillite) beds.
Mineralogy Description Minor
0 This unit is an interbedded magnetite magnetite ankerite rich chert with a considerable amount chert minnesotaite of magnetite granules present. Chert siderite occurs as thick zones separating mag netite beds. Within the chert zones are pebbles of magnetite. The base of this unit is marked by a thin con glomerate. The layer is moderately magnetic and ranges from 3 to 5 min thickness.
M This unit is a bedded magnetite-rich magnetite fayalite chert. The magnetite occurs in wa~rf chert stilpnome.lane distinct beds. Some granular struc minnesotaite ture is present, the granules contain cuiraningtonite fine-grained magnetite, chert, and minnesotaite. Minor amounts of sid erite are closely associated with the granular zones. The unit ranges from slightly to highly magnetic and is 2 to 6 m thick. 11
L This ttnit is a ~-:ra-.r/ b 2.dd 2d, magnetite rich chert with te occurring as ch2rt minr:esotz.ite sharply-de.fin2.d, wz.vy beds. This unit si K This unit is a ·wavy bedded, magnetite magnetite minnesotaite rich chert,· '.rhis is transacted by a ch12:rt stilpnomelane diabase sill with the part of the unit siderite balow the sill referred to as the K'. The magnetite occurs in distinctly td gradationally bedded zones. The unit is slightly conglomeratic with the interbe.dde.d magnetite and chert zones being closely spaced in the upper part of the unit a:i.d more widely spaced ·in the lower p~rt. Siderite occurs in close association with euhedral mag netite and anke:;:-ite. The layer is moderately magnetic and ranges in thickness from 3 to 5 m. J This unit is magnetite-rich chert magnetite siderite with wavy bedded, granular magnetite chert minnesotaite interbedded with chert and ferroan ferroan stilpnomelane silicates. Side.rite is found in hyparsthene mottled zones altering to minnesota gru:1erite ite and stilpnomelane. The central part of the unit is characterized by magnetite-rich pebbles and intra clasts of an intraformational con glomerate. This layar is highly magnetic and ranges in thickness from 1 to 3 m. I This unit is characterized by the magnetite ankerite presence of algal structures. Also chert siderite found within this zone is a minor jasper amount of conglomerate. The algal structures are outlined by dissemi nated, fine-grah1ed magnetite and jasper. This layer is slightly mag netic and ranges in thickness from t\ r:: ,_.,... ,., ...... 1? -'-- H This unit is 2. wavy bedded, magnetite- ms.g1;.etit2 CUTI1Tilin3to::".. l t2 . ; rich chert with yellow to darl:: green chert siaer:ir:e... amphibole lages occurring in actinolite thin, fine-grained layers amo~g the hornblande magnetite-rich zones of the unit. The magnetite consists of fine-grained zones of irregularly bedded lenticular lamellae. The magnetite zones cccur as groups of closely spaced beds inter layered with actinolite and hornblende. This unit is highly magnetic and ranges in thickness from 2 to 5 m. G This unit is a magnetite-rich chert magnetite fayalite with the magnetite occurring in hematite ferroan indistinctly bedded, granular zones. hypersthene The i:r.agnetite is interbedded with abundant hematite and iron silicates in a chert matrix. This unit is highly magnetic and ranges in thick ness from 3 to 6 m. Member The Upper Slaty Member has been. divided into six layers: F, E, D, C, B, and A. The contact between the Upper Cherty and the Upper Slaty Members is marked by the gradational increase in the amount of bedding. The thinly bedded zones become thicker, more regularly bedded, and more numerous (Gunderson and Schwartz, 1962). The average thickness of the Upper Slaty Member is about 35 to 40 m. This member contains less magnetite than the Upper Cherty Hember although it has a relatively high magnetite content. The extremely small grain size of the magnetite and its intimate association with chert and iron silicates makes present economic utilization of this member impossible. 13 Desc; F This unit has thici<:, regularly bedded magnetite siderit:e zon2s of e, which cuar:ningtonite are se?arated by thin zones of granu- chert lar c:-iert. It is slightly a.nd in thickness from 2 to 5 m- E This unit is a e-rich che·ct. magnetite siderite It contains numerous magnetite-bearing chert granular structures and some magnetite- laminated zones. Presence of quartz- ose, septarian structures is charac- teristic of this unit. The unit is moderately magnetic and ranges in thickness from 2 to 4 m. D This unit is an evenly bedded magnet magnetite minnesotaite ite-rich chert with the magnetite chert ferroan occurring as thin stringers. The hypersthene chert zones have small amounts of stilpnomelane iron silicates, which make the rock cum1ningtonite a distinctive green. The unit is actinolite slightly magnetic and ranges from 1 to 2 m. C This unit is p::edominantly chert. magnetite chert The predominant characteristic is cl!lnmingtonite pyrite alternating slate (argillite) zones fayalite and chert zones. Magnetite occurs hypersthene as thin layers. Many silicates are associated with this layer. The unit is slightly magnetic and ranges from 1 to 2 min thickness. B This unit is nonmagnetic and consists pyroxene of irregularly bedded masses of pyro amphibole xene and am:phibole. This layer chert ranges in thickness from 1 to 2 m. A This unit is the uppermost unit of the calcite Biwab:Lk Formation. It consists of a marble coarse-grained calcite and is typic ally described as marble. This layer ranges in thickness from 2 to 3 m. NETHODS The carbonate minerals of the Upper and Low'C:r Che.rty ~·1embers are the n:aj or topic of this paper. The detennination of carbonate: minerals was made from thin section, by x-ray diffraction, by cherdcal anB.lysis, and by electron microprobe data (Figure 3). More than four hundred samples were x-rayed with a Philips high angle diffractometer using Cr filtered, Fe k-alpha radiation for the determination of carbonate minerals present. The samples were scanned from 26 to 42 degrees two-theta. One hundred and forty samples were thin-sectioned to determine the occurrence of carbonate mineral types. Two hundred samples were chemically analyzed for CaO, MgO, and total CO2 and were used in the interpretation of the data obtained by the thin section and x-ray studies. The samples to be x-rayed and chemically analyzed came from stored diamond-drill core and from pit samples collected on a grid pattern in th2 pit areas (Figure 4). The thin-section samples were taken from the pit samples collected on the grid pattern and .from seyeral drill cores. The data from the x-ray study was used to determine the con centration of each of the five major minerals (quartz, magnetite, siderite, ankerite and calcite) present in the sample. The concen tration was determined using the method employed by Karner (1968). The cl-spacings in Angstroms of each of the minerals were: quartz, 3.34; magnetite, 2.97; siderite, 2.79; ankerite, 2.89; and calcite, 14 15 Fig. 3.--Photograph of interbedded carbonate and magnetite rich chert in outcrop. Sample taken from Erie Area 1. Bar is 0.5 m long. 16 17 Fig. 4.--Locatio:ns of drill hole and pit samples from Area 1, page 18; Area 6, page 19; Area 3, page 20; and Area 2-~est, page 21. AREA l SAMPLE LOCATIONS \ ---·.~~--I 1 I I I I --l- -+-- ,- ;---i 1 I ------I I ------I I I \ I I I I I I ii· 5 I I I I I CD I IJ-4 I I I I I 1 \ 11-151 1!-10 I I I I I I I I 9 C:\.3 I I I I I I I I I I I \ I ~-9: I I I I I I I 11-141 (iJ 1 CR-2 1 I· 351 I 1 1 I 1-2ol CD , '11-6 I Q 11-I ~ I I j I I I O I 1\-301 I I O I \ II· I-. I I I I I I I I (f) I 1-25 I 1 l I ~-7: q:> Fi " Q I l ; 1 1 1 I 1!66d ,l-40 I T G) I 1-19, 1 341 !l-6 I 11-60: : : : I : &. CD I &· 11-2~ I I O I . 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