American Mineralogist, Volume 66, pages 87-99, l98I High-grade metamorphic Archean banded iron-formations, Western Australia: assemblages with coexistingpyroxenes * fayalite MnnuN J. Gorn' AND CoRNELTS KLErN Department of Geology, Indiana University Blo omington, Indiana 4 7 405 Abstract At Heaney's Find, Meier's Find, and Queen Victoria Rocks in the greenstone belt terrain (-2.6-2.7 Gyr) of the Yilgarn Block of Western Australia, bandediron-formationshave been regionally metamorphosed. The primary metamorphic assemblages in the iron-formations contain quartz, grunerite, clinopyroxene, orthopyroxene, magnetite,and minor hornblende and pyrrhotite. Fayalite is also present at Heaney's Find and Queen Victoria Rocks. Fe- shales interbanded with the iron-formations contain hornblende,alrnandine, and minor bio- tite, plagioclase, ilmenite, magnetite,and pyrrhotite. Smooth, curved grain boundaries be- tween various combinations of the above minerals suggest that they are equilibrium assem- blages. Actinolite and some grunerite with ragged boundaries against other minerals are interpretedas having formed after the primary assemblages. Asbestiform grunerite, ragged magnetite, and greenalite are retrograde. Based on (a) two-pyroxene geothermometry, (b) oli- vine-orthopyroxene geothermometry, and (c) the absence of evidence for the crystallization of pigeonite, the peak metamorphic temperature is estimated to have been670 + 50"C. Using Smith's (1971) quartz-olivine-orthopyroxene geobarometer, the composition of ortho- pyroxenecoexisting with fayalite + quartz in the iron-formation assemblages indicatesthat the pressure during metamorphism was 3-5 kbar. A somewhat lower pressure, asindicated by experimental data for the quartz + olivine --+ orthopyroxene reaction (Bohlen et al. 1978), is also consistent with the geologyof these three areas. Introduction Metamorphosedbanded iron-formations from three locations, Meier's Find, Heaney's Find, and QueenL Victoria Rocks (Fig. l) in the greenstone belt terrain of the Eastern Goldfields Provinceof the Yil- garn Block, Western Australia (Rb-Sr isotopic ageof 2.6-2.7 Gyr; Arriens,l97l) containcoexisting ortho- and clinopyroxene. Two ofthese occurrences contain fayalite t qruafiz as well. Comparison of the compo- sitioas of coexisting orthopyroxene, clinopyroxene, and fayalite with experimental data allows an estima- tion of metamorphic temperature and pressure. These results are of particular interest, as estimates of pressure during high-grade metamorphism of the volcanogenic supracrustal(greenstone) belts in the Yilgarn Block are uncommon, due to a scarcity within the greenstone successions of pressure-sensi- tive metamorphic assemblages such as metapelites. rPresent address: Department of Geology, Georgia State Uni- versity,University Plaza, Atlanta, Georgia 30303. mn3-0o4x/ 8 | /0 I 02-0087902.00 Assemblages containing orthopyroxene, clinopy- roxene, and fayalite are uncommon in metamor- phosed iron-formations. Suchassemblages have been reported from the highly metamorphosed portions of the Biwabik (Bonnichsen, 1969, 1975)and Gunflint Iron Formations (Simmonset al., 1974)where they are the result of contact metamorphismby the Du- luth Complex, and in iron-formations metamor- phosed by the Nain Complex in Labrador (Berg, 1977). Suchassemblages containevidence of inverted pigeonite whereas those from the Yilgarn Block do not. Fayalite-bearingassemblages lacking ortho- pyroxene have been reported from the moderately metamorphosed portions of the Biwabik (Morey el al.,1972),and Gunflint Iron Formations(Floran and Papike, 1978), the Negaunee Iron Formation (Haase and Klein, 1978), and from iron-formations at Koolanooka in the Yilgarn Block (Baxter, 1965; Gole, 1980a). Such orthopyroxene-free assemblages provide few constraints for estimating metamorphic pressure. Fayalite-orthopyroxene-bearing assem-
Transcript
American Mineralogist, Volume 66, pages 87-99, l98I
High-grade metamorphic Archean banded iron-formations, Western Australia:assemblages with coexisting pyroxenes * fayalite
MnnuN J. Gorn' AND CoRNELTS KLErN
Department of Geology, Indiana UniversityBlo omington, Indiana 4 7 405
Abstract
At Heaney's Find, Meier's Find, and Queen Victoria Rocks in the greenstone belt terrain(-2.6-2.7 Gyr) of the Yilgarn Block of Western Australia, banded iron-formations have beenregionally metamorphosed. The primary metamorphic assemblages in the iron-formationscontain quartz, grunerite, clinopyroxene, orthopyroxene, magnetite, and minor hornblendeand pyrrhotite. Fayalite is also present at Heaney's Find and Queen Victoria Rocks. Fe-shales interbanded with the iron-formations contain hornblende, alrnandine, and minor bio-tite, plagioclase, ilmenite, magnetite, and pyrrhotite. Smooth, curved grain boundaries be-tween various combinations of the above minerals suggest that they are equilibrium assem-blages. Actinolite and some grunerite with ragged boundaries against other minerals areinterpreted as having formed after the primary assemblages. Asbestiform grunerite, raggedmagnetite, and greenalite are retrograde. Based on (a) two-pyroxene geothermometry, (b) oli-vine-orthopyroxene geothermometry, and (c) the absence of evidence for the crystallizationof pigeonite, the peak metamorphic temperature is estimated to have been 670 + 50"C. UsingSmith's (1971) quartz-olivine-orthopyroxene geobarometer, the composition of ortho-pyroxene coexisting with fayalite + quartz in the iron-formation assemblages indicates thatthe pressure during metamorphism was 3-5 kbar. A somewhat lower pressure, as indicated byexperimental data for the quartz + olivine --+ orthopyroxene reaction (Bohlen et al. 1978), isalso consistent with the geology of these three areas.
Introduction
Metamorphosed banded iron-formations fromthree locations, Meier's Find, Heaney's Find, andQueenL Victoria Rocks (Fig. l) in the greenstone beltterrain of the Eastern Goldfields Province of the Yil-garn Block, Western Australia (Rb-Sr isotopic age of2.6-2.7 Gyr; Arriens,l97l) contain coexisting ortho-and clinopyroxene. Two ofthese occurrences containfayalite t qruafiz as well. Comparison of the compo-sitioas of coexisting orthopyroxene, clinopyroxene,and fayalite with experimental data allows an estima-tion of metamorphic temperature and pressure.These results are of particular interest, as estimates ofpressure during high-grade metamorphism of thevolcanogenic supracrustal (greenstone) belts in theYilgarn Block are uncommon, due to a scarcitywithin the greenstone successions of pressure-sensi-tive metamorphic assemblages such as metapelites.
rPresent address: Department of Geology, Georgia State Uni-versity, University Plaza, Atlanta, Georgia 30303.
mn3-0o4x/ 8 | /0 I 02-0087902.00
Assemblages containing orthopyroxene, clinopy-roxene, and fayalite are uncommon in metamor-phosed iron-formations. Such assemblages have beenreported from the highly metamorphosed portions ofthe Biwabik (Bonnichsen, 1969, 1975) and GunflintIron Formations (Simmons et al., 1974) where theyare the result of contact metamorphism by the Du-luth Complex, and in iron-formations metamor-phosed by the Nain Complex in Labrador (Berg,1977). Such assemblages contain evidence of invertedpigeonite whereas those from the Yilgarn Block donot. Fayalite-bearing assemblages lacking ortho-pyroxene have been reported from the moderatelymetamorphosed portions of the Biwabik (Morey elal.,1972), and Gunflint Iron Formations (Floran andPapike, 1978), the Negaunee Iron Formation (Haaseand Klein, 1978), and from iron-formations atKoolanooka in the Yilgarn Block (Baxter, 1965;Gole, 1980a). Such orthopyroxene-free assemblagesprovide few constraints for estimating metamorphicpressure. Fayalite-orthopyroxene-bearing assem-
\\\
\Sou lhe rn\ Cross / . ^ <TERN
a )
^ < , K o t g o o f l t e J
r 1 A ^ ^ < . : / 1 o . 1
" -F;Y" .
- .QueenFind V ic lo r io
Meier ! F ind Rocks
EXPLAN AT I ON
ff i Suprocrustot rocks
t_] Gronitoids
88 GOLE AND KLEIN: ARCHEAN BANDED IRON-FORMATION
Fig. 1. Map of part of the Eastern Goldfields Province of theYilgarn Block, Western Australia, showing the distribution ofsupracrustal rocks (greenstone belts) and granitoids and thelocations of Heaney's Find, Meier's Find, and Queen VictoriaRocks.
blages from lithologies other than iron-formationsare briefly reviewed in Jafe et al. (1978).
Geologic setting
The assemblages in the banded iron-formationfrom Heaney's Find and Meier's Find have pre-viously been described by Miles (1943, 1946a, b).Samples for this study were obtained from two dia-mond drill holes at Heaney's Find (sample 4 fromD.D.H. 3, others from D. D.H. l: Miles, 1946a) andfrom one diamond drill hole at Meier's Find (D.D.H.2; Miles, 1946a). The diamond drill holes at Heaney'sFind are 140 m apart and approximately equidistantfrom a poorly exposed granitoid-gre€nstone contact.At both Heaney's Find and Meier's Find the dia-mond drill holes are located near the contact of a 2-4km-wide greenstone belt and a large granitoid plu-ton. The greenstone belt represents the eastern limbof a regional-scale anticline with the granitoid occu-pying the core (Fig. l). Miles (1946a,b) described thelocal geology at both locations.
At Heaney's Find intrusive granitoids crop out 500m west of the banded iron-formations. The iron-for-mations are steeply dipping, 5-20 m thick, and occur
within a sequence of hornblende + plagioclase +augite + biotite and tremolite + anthophyllite + talcschists. Relict pillows within the schists indicate thatthey are probably the metamorphosed and deformedequivalents of mafic and ultramafic volcanics, respec-tively. Intercalated with these schists are thin bandsof argillaceous metasediments. The area has been in-truded by numerous pegmatitic dikes.
The local geology at Meier's Find is very similar tothat of Heaney's Find except for the lack of ultrama-fic rocks (Miles, 1946a). At Meier's Find, the bandediron-formation is locally in contact with cross-cuttinggranitoid bodies, and granitoid was encountered atdepths of 40-60 m in several diamond drill holes(Miles, 1946a).
At Queen Victoria Rocks (Fig. l), a small (prob-ably 3-6 km') outlier of highly metamorphosedmafic, ultramafic, and metasedimentary rocks occurswithin the granitoid. Outcrop is almost non-existent,and the relationship between rock types is not clear.
Heaney's Find, Meier's Find, and Queen VictoriaRocks are all in the dynamic-style high-grade meta-morphic domain mapped by Binns et al. (1976). Assuch, the metamorphic assemblages at these localitiesare part of a regionally extensive metamorphic envi-ronment that extends well away from granitoid con-tacts. Contact metamorphism related to granitoidemplacement, which in places is superimposed on theregional metamorphic pattern, forms distinct, narrowaureoles of fine-grained hornfelses (Binns et al., 1976,p. 306). The assemblages at Heaney's Find, Meier'sFind, and Queen Victoria Rocks do not have horn-felsic textures. but instead show textures similar tothose in lithologies in the high-grade domain wellwithin the greenstone belts (R. A. Binns, personalcommunication, 1980). Thus, despite the close spatialrelationship between the greenstone lithologies andthe intrusive granitoids at these localities, the meta-morphic assemblages are not the result of contactmetamorphism. Binns et al. (1976) [see also Archi-bald et al. (1978)l suggest that the heat source re-sponsible for the regional metamorphic pattern alsocaused the generation of the intrusive granitoids byreactivating granitoid gneisses which formed thebasement to the greenstone cover (Archibald andBettenay,1977).
Analytical methods
Analyses of bulk samples were obtained by X-rayfluorescence spectroscopy, except that Na was ana-lyzed by atomic absorption spectroscopy, FeO by ti-tration against potassium dichromate, H'O and CO'
GOLE AND KLEIN: ARCHEAN BANDED IRON-FORMATION 89
by gravimetric means, and S with a Leco furnace.Electron microprobe analyses were made on foursamples from Heaney's Find, four from Meier'sFind, and eight from Queen Victoria Rocks. Thesesamples contain the most diverse assemblages col-lected. Analyses were made on a 3-spectrometer EtecAutoprobe, with the data reduction procedure ofBence and Albee (1968). Alpha factors used werethose of Albee and Ray (1970). Analyses with thesame sample number but different letters were ob-tained from different mesobands or Fe-shale bandswithin the same sample. The majority of the analysesin Tables 3 to 7 are averages of 3 to 6 individualanalyses, as are the points in Figures 3, 4, and 5.
Lithologies
Two Fe-rich rock types, banded iron-formationand Fe-shale, occur at Heaney's Find, Meier's Find,and Queen Yictoria Rocks. Barided iron-formationgenerally consists of alternating 2-30 mm thickmesobands, although somej iron-formation is mas-sive. Successive mesobands are distinguished by (a)different mineral assemblages, (b) diflerences in therelative proportions of minerals, (c) the presence orabsence of microbands, and (d) different spacing be-tween microbands. Microbanding in these highlymetamorphosed iron-formations is uncommon and isbest preserved in quartz-magnetite-rich assemblageswhich, although they are recrystallized, show little orno reaction between minerals (Fig. 2a). In iron-sili-cate- and originally carbonate-rich mesobands, meta-morphic reactions and recrystallization have de-stroyed or severely modified earlier structures.' The bulk compositions of the banded iron-forma-tions (Table l) are characterized by SiO, + Fe,O, +FeO totals over 90 wt. percent, by low but significant(<4 wt. percent) contents of MgO and CaO, and byvariable but small AlrO, contents. Despite their lowconcentrations, these minor components have an im-portant efect on assemblages in the iron-formations.Other components generally occur in trace amounts(<0.31.wt. percent).
Fe-shale occurs as l-30 cm-thick bands within theiron-formation. These bands are massive, and miner-als are generally evenly distributed throughout them.In hand specimen, they are distinguishable frombanded iron-formation by their brownish-greencolor, which is a result of the dark green hornblende+ almandine + biotite they contain. The bulk com-positions of Fe-shales are highly variable, althoughthey have some cornmon chemical characteristics.They are Fe-rich, containing more than 20 wt. per-
Table l. Bulk chernical analyses of iron-formations and Fe-shales.Analyses are from Tables 2 and 3 of Gole (1980b)
cent total Fe, and their minor components, particu-larly ALO. and TiOr, are significantly higher than inthe banded iron-formations (Table l). The very lowmetamorphic-grade equivalents of the Fe-shales arevery fine-grained and finely laminated. These essen-tially unmetamorphosed Fe-shales contain chamositeor chlorite, stilpnomelane, carbonate, magnetite, py-
Table 2. Summary ot ".t"-It;_t;t"t:,iron-formations
and
sample Cata logN o ' N o ' a *
Assed lage
1 A1 B234 A4 B4 C4 D
I O1 1t2A1 2 8I 3 A
I 41 51 6
H e a n e y ' s F i n d
20A39 fe l roa W-euI -g run - fay -nag -hb Id2 0 8 3 9 f e r r o a u g - g ! u n - e u I - q t z - f a y - ( m a g ) - ( p o ) - ( n a 9 * ) - ( g r e e n * )2 0 8 4 1 q t z - n a 9 - g r u n - f a y - ( p o )2 0 8 4 3 q t z - f e r r o s a l - g r u n - f a y - n a g - ( q r u n r ) - ( n a g * )2 0 8 5 4 h b l d - b i o t - a l m - q t z - p l a g ( A n 3 5 ) - ( z i r c o n )2 0 8 5 4 h b l d - a 1 n - m a 9 - p o - ( n a r c a s i t e i )20654 fe r roaug-euI -g run- fay-hb1d-q tz -mag- (po)2 0 a 5 4 f e r r o a u g - f a y - g r u n - h b l d { a g - ( p o ) - ( m a g r )
M e i e r ' s F i n d
20444 a lm-hb1d- fe r rosa l -nag- ( i Im)2 0 8 4 5 g r u n - f e r r o a u g - u 1 - n a g2 0 8 5 0 f e r r o s a l - g ! u n - f a c t - h a 9 - ( p o ) - ( c p y )2 0 e 5 1 q t z - b i o t - c u n n - p 1 a q ( A n 5 2 )
Queen V ic to r ia Rocks
A49 '19 c l lm- fe r roaJg- fe r rohyp-nag{ tz - ( rc ) - (cpy)
8 4 9 8 5 g r u n - f e r r o a u g - e u l - ( q t z ) - ( p o ) - ( m a g . ) - ( q r e e n * )8 4 9 8 6 g r u - f e r r o a u g - f a y - h b I d - ( q t z ) - ( g r u n r ) - ( m a g * )8 4 9 8 7 f a y - g r u n - q t z - f e r r o a u g - n a g84981 fe r roaug-e u l - fay -q t z -9 run - ( po ) - (mas )I 4 9 9 0 fe ! roaug-eu1 -9 run-q tz -hb 1 d -ma9
8 4 9 9 0 h b l d - f e r r o s a l - g r u n - ( p o )84991 hb ld -a lm-c( lm-b io t -nag- j , Im81992 qEz-mag-grun8 4 9 9 8 q t z - p i a s ( A n 3 4 ) - b i o t - h b l d
Mine!a1 abbrev ia t ions
a lm = a lnand ine fe r rosa l = fe r rosa l i teb io t = b io t i te g leen = greena l i tec p y = c h a l c o p y r i t e g r u n = g r u n e r i t ecum = cwing ton i te hb ld = hornb lendee u L = e u l i t e i l m = i l n e n i t efay = faya l i te mag = nagnet tefac ts = fe r loac t ino l i te p laq = p lag ioc tasefer roaug = fe r loaug i te po = pyr lho t i tefe r rohyP = fe r rohypers thene q tz = quar tz
t t C a t a l o g n , n b e r s i n x h e c o l T e c C i o n o f c h e D e p a r t n e n t
o f c e o 1 o 9 9 , u n l v e r s j t g o t w e s t e t n e u s t f a l i a .
E i n e E a f s a t e l i s t e d i n d e c r e a s i n q r e l a E T v e a b o n d a n c e .
P a r e n t h e s e s n e a n s t t a c e a a o u r t . ' s i g n i f i e s r e t . o g t a d e
n i n e r a f . A s s e n b T a g e s a n i t a l i c s t e p z e s e n t p e i i t i c n e t a -
GOLE AND KLEIN: ARCHEAN BANDED IRON-FORMATION
Fig. 2. Textures and structures in iron-formations. Mineral abbreviations are listed in Table 2. (A) Thin mesobands composed ofmagnetite and quartz with rninor grunerite. Some of the quartz-rich mesobands contain poorly-defined microbands Queen VictoriaRocks, sample 15. Plane polarized light. (B) An aggregate of equant magnetite grains rimmed by fayalite in a grunerite-ferroaugite-richassemblage. Fayalite contains inclusions of fine-grained magnetite along fractures and some grain boundaries. Ferroaugite (cpx)contains fine-grained ragged inclusions offerroactinolite. Heaney's Find, sample lA. Plane polarized light. (C) A large ferrosalite grain
GOLE AND KLEIN: ARCHEAN BANDED IRON.FORMATION
rite, pyrrhotite, quartz or chert, potassium feldspar,ilmenite, and carbonaceous material, and are com-monly interbanded with both Archean and Pro-terozoic banded iron-formations (Gole, 1980b).
Assemblages
The iron-formation assemblages at Heaney's Findand Queen Victoria Rocks are very similar (Table 2),with clinopyroxene, orthopyroxene, grunerite,quartz, minor magnetite, and hornblende the mostcommon constituents. Fayalite-bearing assemblagesare uncommon and may or may not contain ortho-pyroxene or quartz. Pyrrhotite is present in traceamounts in many assemblages. Similar assemblagesoccur at Meier's Find, although orthopyroxene is un-common and fayalite is absent. Magnetite is not amajor constitutent of the iron-formations at any ofthe locations, and is generally restricted to quaftz-rich Fe-silicate-poor assemblages. The minerals inFe-silicate-rich mesobands are evenly distributed,with the following exceptions: (a) where quartz oc-curs in aggregates in some mesobands defining acrude layering roughly parallel to mesoband con-tacts, (b) where magnetite is rimmed by fayalite as insamples lA and lB (Fig. 2b), and (c) where retro-grade minerals are present. Nevertheless, the pro-grade minerals listed in Table 2 are in mutual contactsomewhere within a small area of a thin section.
Fe-shale bands are composed predominantly ofhornblende and almandine and may contain variableamounts of biotite, ferrosalite, grunerite, and ferroac-tinolite. Quartz, andesine, fayalite, magnetite, ilne-nite, and pyrite or pyrrhotite are minor minerals andapatite and zircon are trac€ constituents. The miner-als are evenly distributed throughout Fe-shale bandsexcept in one hornblende-rich band where gruneriteoccurs as rims around fayalite.
Based on textural relations, the following mineralsare clearly not part of the high-grade metamorphicassemblages: (a) very fine-grained asbestiform gru-nerite that projects into quartz grains along mutualcontacts with Fe-silicate, (b) fine-grained, raggedmagnetite distributed along fractures, cleavages andgrain boundaries in many assemblages, and (c)greenalite, which occurs in veinlets.
Quartz is abundant in most samples of iron-forma-tion but occurs in small amounts or is absent in mostfayalite-bearing assemblages and Fe-shale bands. Inquartz-rich mesobands, quartz aggregates form thinlaminae separated by laminae of Fe-silicates or mag-netite or both. The laminae probably represent relictmicrobanding.
Orthopyroxene occurs as coarse to very coarse an-hedral grains (0.2-10 mm and locally >2 cm in diam-eter). The large grains contain inclusions of magne-tite, quartz, sulfides and, less commonly, hornblendeand ragged fine-grained grunerite. Grain boundariesare generally smooth and curved against other pro-grade minerals (Figs. 2d and e), except in some sam-ples where grain boundaries with grunerite are some-what irregular. Extremely fine (<l-2 trrm) exsolutionlamellae of calcic pyroxene parallel to (100) are vis-ible in many grains. The Fe'z*/(Fe'?*+Mg+Mn) ratioof orthopyroxene ranges ftom 0.762 to 0.831 (Table3, Figure 3). Within a mesoband, compositions aresimilar regardless of adjacent minerals. Ca values arerelatively uniform which suggests that the exsolutionlamellae have not biased the electron probe analysestowards either the composition of the host or the ex-solution lamellae. Orthopyroxenes from Heaney'sFind and Meier's Find have considerably higherMnO contents (1.34-2.27 wl. percent) than thosefrom Queen Victoria Rocks (0.18-0.51 wt. percent;see Table 3, Fig. 4). Estimated FerO. values are lowand variable and may not reflect the actual ferriccontents of the pyroxenes.
Clinopyroxene is texturally very similar to ortho-pyroxene. In some assemblages clinopyroxene con-tains abundant inclusions of fine-grained actinolite,ragged grunerite, and minute grains of magnetite(Fig. 2c), whereas coexisting orthopyroxene is gener-ally inclusion-free., Exsolution lamellae parallel to(100) are common and are more pronounced thanthose in orthopyroxene. Exsolution lamellae parallelto other crystallographic directions are rare, however.For the averaged analyses of clinopyroxene coexist-ing with orthopyroxene (Table 3), the 'Wo' content,defined as the ratio Ca/(Ca+Fe+Mn+Mg), rangesfrom 0.427 to 0.439, although the range betweensingle analyses is greater, reflecting the presence of
with an intergrowth of grunerite and ferroactinolite along its boundary. Ferrosalite (cpx) contains inclusions of very fine-grainedmagnetite and ragged ferroactinolite. Meier's Find, sample 7. Doubly polarized light. (D) Coexisting ferroaugite (cpx), eulite (opx),grunerite, quartz, and magnetite. Queen Victoria Rocks, sample l3A. Doubly polarized light. (E) Coexisting fayalite, eulite (opx),ferroaugite (cpx), grunerite, and quartz. Note the smooth, curved grain boundaries between all minerals. Queen Victoria Rocks, samplel2B. Plane polarized light. (F) Fayalite-grunerite-quartz assemblage in which grunerite mantles fayalite grains so that quartz andfayalite are not in contact. Grunerite-quanz boundaries are somewhat angular. Heaney's Find, sample 3. Plane polarized light.
92 GOLE AND KLEIN: ARCHEAN BANDED IRoN-FoRMATIoN
Table 3. Representative electron microprobe analyses ofcoexisting and single pyroxenes in iron-formation and Fe-shales
H e a n e y t s F i n d M e i e r ' s F i n d Queen V ic to r ia Rocks
1 A 3
C P X OPX CPX CPX
t 2 B 9
O P X C P X O P X C P X
1 2 A
CPX
1 3 8
C P XoPx C P X c P x c P x
s i o , 4 6 . 8 3T l o t 0 . 0 0A 1 2 O 3 0 . 2 1F e O * 4 4 . 6 8u n o 2 . 2 1M g o 4 . 7 0C a O 0 , 8 9N a 2 O 0 , 0 0K z o o . o osum TE-:67
exsolution lamellae. The 'Wo' content of clinopyrox-enes from orthopyroxene-free assemblages tends tobe larger than this range with the exception of onesample (Fig. 3, no. 4D), where exsolution lamellaemay be present. Clinopyroxenes from Fe-shale as-semblages (Fig. 3, samples 5 and l3B) have relativelyhigh 'Wo'contents compared to those from bandediron-formations. Clinopyroxenes have the lowest Mncontents (Fig. a) and lowest Fel(Fe+Mg) ratios ofany of the Fe-silicates in the assemblages. EstimatedFerO, contents are variable and are, on the average,slightly higher than those in orthopyroxene (Table3).
Amphiboles may form up to 40 percent of the iron-formations and as much as 100 percent of the Fe-shales. The iron-formation assemblages containgrunerite with minor hornblende and generally onlytrace amounts of actinolite, whereas Fe-shale assem-blages are dominated by hornblende (Table 2).Poikiloblastic grains of grunerite up to 0.5 cm longenclose qvattz, pyroxene, fayalite, magnetite, andhornblende. Most grunerite, however, occurs as in-terlocking tabular grains. The poikiloblastic andmany of the tabular grunerite grains have smoothcontacts with adjacent mineqals and appear to bepart of the high-grade assemblage (Figs. 2b, d, ande). In a few samples (particularly no. 3), grunerite hasangular boundaries, especially against quartz, andoccurs as rims between quartz and fayalite (Fig. 2f),which suggests that this grunerite may be para-
genetically later than most of the assemblage. Thereis, however, no clear-cut textural distinction betweenthese later grunerites and the grunerite that is part ofthe high-grade assemblage. Most actinolite occurslate in the paragenetic sequence and is closely associ-ated with clinopyroxene (Fig. 2c). Similar late-stageamphiboles have been described from the metamor-phosed Biwabik and Gunflint lron Formations (Bon-nichsen, 1969, 19751, Morey et al., 1972; Simmons etal., 1974; Floran and Papike, 1978). Very fine-grained, fibrous to asbestiform grunerite is clearlylater than the prograde assemblage, because it cutsacross quartz grain boundaries and projects intoquartz from orthopyroxene and other gruneritegrains. This variety of grunerite is probably of retro-grade origin, as noted by Bonnichsen (1969, 1975)and Immega and Klein (1976). Hornblende occurs asisolated polygonal grains within banded iron-forma-tion assemblages and forms an almost mono-mineralic interlocking mosaic in many of the Fe-shale bands.
Members of the cummingtonite-grunerite seriesand actinolite are chemically simple and over 95Vo oftheir compositions can be represented in the Ca-Mg-(Fe+Mn) quadrilateral. One exception is cum-mingtonite from an Fe-shale band (Table 4, sample14) that contains significant amounts of Al and Na.Although very fine exsolution lamellae are visible inmost grains, CaO contents of grunerite are generally<l wt. percent. The most Fe-rich grunerite occurs in
GOLE AND KLEIN: ARCHEAN BANDED IRON-FORMATION
H E A N E Y . S F I N D M E I E C S F I N D
OPX Presen l OPX Absent Al I Assembloges
O = l A
= 1 8
r = 4 C
QUEEN V ICTORIA ROCKS
OPX Present
CPX
o = 9a = l O
I = 1 2 8
v = 1 3 A
sample 3, which contains the three textural varietiesof grunerite described above. Grunerites interpretedto be part of the earliest and somewhat later assem-blages (the latter occurs as rims between fayalite andq\artz, Fig. 2{) cannot be distinguished by their com-positions, suggesting that both crystallized under
OPX Absent
similar conditions. Fibrous retrograde gruneritemore Fe-rich than the earlier varieties (Tablesample 3).
Hornblende shows a wide range of Al'* sub-stitution for Sio* in the tetrahedral site (Table 4). Oc-cupancy of the A site by Na* and K* is highly vari-
MSO 70 80 eo 80 eo FeO*MnOMole 7"
Fig. 3. Graphical representation of electron microprobe analyses of pyroxenes, fayalite, grunerite, and ferroactinolite in iron-formations and Fe-shales. Mineral abbreviations are listed in Table 2. Tie lines join coexisting pyroxene pairs. Total Fe is taken as FeO.
able, although this is greatly influenced by theamount of Fe estimated to be Fe'*. These amphi-boles fall within the field of hastingsitic and ferroede-nitic hornblende to ferrotschermakitic and ferrohorn-blende according to the classification of theInternational Mineralogical Association (Leake,le78).
Fayalite grains are generally equant, anhedral, anduniform in size (0.2-3.0 mm in diameter, Fig. 2c)
within a mesoband, except where they occur as rimson magnetite grains (Fie. 2b; samples 14 and l0). In-clusions consist of quartz and extremely fine-grainedmagnetite. [n several samples, fayalite containsabundant ragged magnetite along fractures and grainboundaries. This magnetite, and perhaps the fine-grained magnetite inclusions, are retrograde in ori-gin. Where fayalite occurs with grunerite or pyrox-ene, it is only rarely enclosed by these minerals. Thisis in contrast to fayalite in the metamorphosed Biwa-bik and Gunflint Iron Formations, where it is com-monly mantled by other Fe-silicates (Bonnichsen,1969, 1975; Morey et al., 1972; Sirnmons et al., 1974;Floran and Papike, 1978). In the Heaney's Find andQueen Victoria Rocks samples, where fayalite ismantled by grunerite, the latter occurs betweenfayalite and quartz (Fig. 2f) and rarely betweenfayalite and hornblende.
Within a single mesoband, fayalite compositionsare uniform and independent of the presence or ab-sence of qaartz and associated minerals. Fayaliteshows a relatively narrow range in Fel(Fe+Mg) ra-tios (Table 5). The most Fe-rich fayalite occurs in avery quartz-rich, orthopyroxene-free assemblage(Table 5, no. 3; Fig. 2f). Fayalite contains the highestMn content of coexisting Fe-silicates (Fig. a) and the
Table 4 Representative electron microprobe analyses of coexisting and single amphiboles in iron-formations and Fe-shales
H e a n e y ' s E i n d M e i e r ' s F l n d Q u e e n V i c t o r i a R o c k s
3 4 c 1 8
cRUN cnu"l cRUN HBLD GR,N
1 A ' t 5
GRUN HBLD GRUN FACT HBLD
L 2 B I 3 A l l B 1 4
GRUN GRIN HBLD HBLD HBLD CUM4
s i o , 4 4 . 1 8T r o Z 0 . 0 0A L 2 O 3 0 . Q sF e O * 4 3 . 0 4M n o L . 4 2M g O 3 . 1 1C a O 0 . 6 3N a 2 O 0 . 0 0K r O 0 . 0 0T o t a l 9 7 . 0 3
r o t a l 1 4 . 9 9 1 I 4 . 9 9 7 I 5 . 0 0 3 1 5 . 5 0 s 1 5 . 0 0 2 1 5 . 0 0 4 r 5 . r 2 3 r 5 . 0 9 5 r 5 . 3 3 7 1 5 . 0 9 1
GOLE AND KLEIN: ARCHEAN BANDED IRON-FORMATION 95
most manganiferous fayalite ssnlains more Mn thanMg (Table 5, no. 3).
Magnetite is abundant in quartz-rich assemblagesand is a minor constituent of most other iron-forma-tion assemblages. It is a trace constituent of Fe-shalebands. Magnetite is generally finer-grained than thesilicates and'commonly occurs as inclusions in theFe-silicates. In the banded iron-formations, it is es-sentially pure FerOo. Ilmenite oocurs with magnetitein several of the Fe-shale bands, where both oxidesare minor constituents. In sample 5 ilmenite andmagnetite.occur as independent and separate grains,whereas in sample 14 they occur as an adjoining pairof intergrown grains that originally may have been asingle homogeneous phase. All ilmenite and magne-tite have essentially the end-member composition, al-though magnetite from sample 14 contelns Q.l2floAlrO, and 0.l2%o Cr.rO, (Table 6). These composi-tions suggest that the coexisting itnenite and mag-netite equilibrated at temperatures below 500'C(Buddington and Lindsley,1964). However, the com-positions of associated silicates indicate considerablyhighsl metamorphic temperatures (see below) andthe low equilibration temperature indicated by thecoexisting ilmenite-magnetite is surprising, espe-cially in sample 5 where the oxides occur as isolatedgrains. Possibly these oxides were not in mutual equi-librium during metam orphism.
Garnet occurs only in Fe-shale bands and is gener-ally subhedral with abundant inclusions. Composi-tionally it is >60 percent almandine with the remain-der mostly grossular, spessartine, and lesser pyrope
Table 5. Reprcsentative €lectron microprobe analyses of fayattefrom Heaney's Find and Queen Victoria Rocks
OueenVi.ctor ia Rocks
Table 6. Electron microprobe analyses of coexisting ilmenite andmagnetite in Fe-shales from Meicr's Find (no. 5) and Queen
* A 1 1 F e a s F e O . * * C a f c u f a X e d a c c o r d i n q X o
t h e m e t h o d o f C a r m i c h a e l ( 1 9 6 7 ) ,
(Table 7). Garnet compositions are uniform bothwithin single grains and within individual Fe-shalebands.
Biotite also occurs only in Fe-shale bands and israrely abundant. Biotite and ilmenite, where present,contain most of the TiO, (Table 8) reported for thebulk analyses of Fe-shales (Table l).
Other minerals include apatite, zircon, and pyrrho-tite, much of which is altered to marcasite. Theseminerals generally occur as inclusions in other min-erals, although pyrrhotite may form polygonal aggre-gates with silicates. Plagioclase is present in one Fe-shale band. Greenalite forms rare retrograde veinlets.
The textural relations in the iron-formation andFe-shale assemblages at Heaney's Find, Meier'sFind, and Queen Victoria Rocks indicate that theminerals crystallized together, within the exceptionof late-stage grunerite and actinolite and retrogademinerals. The primary metamorphic minerals arethus likely to be in chemical equilibrium. Most im-portantly, the smooth, curved grain boundaries in theeulite-fayalite-quartz-bearing assemblages (sampleslB and l2B) suggest that these are equilibrium as-semblages that do not appear to have reequilibrated(except for exsolution phenomena) after they formed,presumably at the peak of metamorphism.
Discussion and conclusions
Reasonable estinates of the peak metamorphictemperature make it possible to estimate the meta-morphic pressure by comparing the composition of
* e J l F e a s ? e O . A s s e n b f a g e s l i s t e d i n ? a b 7 e 2 .
96 GOLE AND KLEIN: ARCHEAN BANDED IRON-FORMATION
Table 7. Representative electron microprobe analyses ofgarnet inFe-shales
H e a n e y ' s F i n d
QueenM e i e r ' s V i c t o r i a
Find Rocks
roxene compositions that are averages of severalsingle electron probe analyses with a range of 'Wo'
contents may mean that the difference in temper-ature shown in Figure 5 between Heaney's Find andthe other two localities is not real. When the pyrox-ene compositions are corrected for estimated Fe'*contents, both the Wood and Banno and the Wellsgeothermometers yield marginally higher temper-atures (0-5oQ higher), whereas the range in temper-atures from Ross and Huebner's method is widenedslightly (-600'-720'C). Each geothermometer, withthe possible exception of Ross and Huebner's, in-dicates very similar temperatures at all three local-ities. However, temperatures obtained with the Wellsgeothermometer are 27-60"C higher than those withthe Wood and Banno method. which in turn are con-siderably higher than those from the Ross and Heub-ner geothermometer.
Sack (1980) proposed an updated orthopyroxene-olivine geothermometer. This geothermometer (Fig.6, Sack, 1980) is strictly applicable for I atm only, al-though Sack (personal communication, 1980) sug-
Table 8. Electron microprobe analyses of biotite in Fe-shales
3 . 9 0 9 3 . 6 2 80 . 5 3 1 0 . 7 1 50 . r 5 s 0 . 4 6 2L . 4 4 4 I . 2 2 Iffi ffi9 . U J Y O . U Z O
H e a n e y ' s-b rno
4 A
M e i e r ' s
-b lncl
5Total L 5 . 9 7 6 1 5 . 0 3 9 1 6 . 0 2 6
* A 1 l F e a s F e O . * * F e ? O , a s s u m i n g B s i x e= 4 . 0 0 0 .
orthopyroxene coexisting with fayalite + quartz tothe experimental data of Smith (1971). Three lines ofevidence can be used to constrain the estimate of thepeak metamorphic temperature: (a) two-pyroxenegeothermometry, @) orthopyroxene-olivine geo-thermometry, and (c) the absence of evidence for thecrystallization of pigeonite.
The Wood and Banno (1973) geothermometer forcoexisting pyroxenes (assumming all Fe as FeO)yields temperatures of 783-803' at Heaney's Find,805' at Meier's Find, and 790-818'C at Queen Vic-toria Rocks. The Wells (1977) pyroxene geother-mometer yields temperatures of 810-840', 849o, and82+871'C for these locations. Ross and Huebner's(1975) graphical geothermometer (Fig. 5) for pyrox-ene pairs indicates temperatures of approximately660-720"C for Meier's Find and Queen VictoriaRocks, whereas the pyroxene pairs from Heaney'sFind indicate somewhat lower temperatures (-620-650"C). Possible errors introduced by using clinopy-
i ons on t he bas i s o f 22 oxvqens
s io2TiO2A 1 2 O 3FeO*MnOM9oCaON a 2 OKro' r ' oEa l
3 4 . 9 22 . 0 3
1 5 . 6 33 0 . 6 0
0 . 2 44 . 0 00 . 0 00 . 0 98 . L 7
95.3-9"
2 . 4 26---T-0
0 . 5 30 . 2 44 . 0 90 . 0 30 . 9 5':6-4
0 . 0 00 . 0 3L . 6 ' lT:76
1 5 . s 4
3 4 . 0 60 . 2 6
1 6 . 7 02 6 . 0 8
0 . 3 68 . 4 10 . 0 00 . 0 8
93-:fO
5 . 4 42 . 5 58.TO
0 . 5 90 . 0 33 . 4 90 . 0 52 . 0 06 . 1 5
0 . 0 0o . o 2I q e
r.60-
L 5 . 7 6
3 5 . 1 82 . 5 3
2 2 . 0 40 . 0 4
r 0 . 4 90 . 1 0
6 . 9 8v l . r o
5 . 3 82 . 6 2E-.O0-
0 . 4 90 . 2 94 . 4 4
0 . 0 02 . 3 95 . 9 9
o . o 20 . I 6I . 3 6T-J-Z-
1 5 . 5 3
S i
I
AIT iFeMNMgI
CaNaKI
Total
* A l I F e a s F e O
GOLE AND KLEIN: ARCHEAN BANDED IRON.FORMATION 97
gests that the method is probably not very sensitiveto pressure in the range of 0-8 kbar. Ko,""_-*r (OL-OPX) and XF" (OL) from four eulite-fayalite pairsplotted in Figure 6 of Sack (1980) yield temperaturesin the range of approximately 620-700oC. The eu-lite-fayalite pairs are from samples 14 [Ko,.*",u,(OL-OPX) : 3.259, XF"(OL):0.9231,18 (3.740, 0.951),and 4C (3.792, 0.953) from Heaney's Find, andsample l2B (3.711, 0.949) from Queen VictoriaRocks.
The absence of pigeonite or orthopyroxene formedby the inversion of pigeonite in the iron-formationassemblages at Heaney's Find, Meier's Find, andQueen Victoria Rocks suggests that the peak meta-morphic temperature was below that at which pi-geonite forms in very Fe-rich bulk compositions.Relevant experimental data are scarce, but can beused to indicate a probable maximum temperaturefor the assemblages under discussion. Experimentaldata pertinent to the formation of pigeonite have re-cently been reviewed by Ross and Huebner (1979).From their discussion and data, a likely minimumtemperature for the formation of pigeonite in bulkcompositions having Fel(Fe+Mg) : 0.75 at 3 kbar isabout 800oC. Lindsley et al. (1974) determhed thatat2.5 kbar the addition of 5 mole percent MnSiO, topyroxenes with a bulk composition of Fel(Fe+Mg): 0.75 stabilizes pigeonite to about 750"C, or 50olower than its minimum stabitty in an Mn-free sys-tem. Pyroxenes from Heaney's Find and Meier'sFind contain between 2 and a little over 4 mole per-cent MnSiOr, and their bulk compositions are some-what more Fe-rich than Fel(Fe*Mg) : 0.75. Theabsence of pigeonite from these assemblages thus in-dicates a maximum metamorphic temperature of750-800'C. A maximum temperature of about800"C is indicated at Queen Victoria Rocks, wherethe MnSiO, component in pyroxenes is very low(Table 3).
The Ross and Huebner and the orthopyroxene-olivine geothermometers indicate very similar peakmetamorphic temperatures. Based on the estimatesfrom these methods the probable metamorphic tem-perature is 620-720"C, which is consistent with amaximum temperature of 750-800'C indicated bythe absence of pigeonite. Other studies have shownthat both the Wells and the Wood and Banno geo-thermometers yield temperatures that are system-atically 1s6 high, particularly for low-pressure ter-rains (Bohlen and Essene,19791,Dahl, 1979; Hewins,1975; Stormer and Whitney, 1977). Our results sup-port these conclusions.
( M g . F e . ) S i O . Or thopyroxene Or tho ter ros i I i i eFeSiO3
Fig. 5. Coexisting pyroxenes plotted on the two-pyroxenegeothermometer ofRoss and Huebner (1975).
Applications of the orthopyroxene-olivine-quartzgeobarometer of Smith (1971) to natural assemblagesrequires that the effect of components in addition toMg, Fe, and Si be considered. The most importantadditional components are Ca and Mn, which areboth partitioned preferentially into orthopyroxenerelative to fayalite and which stabilize orthopyroxeneinstead of olivine + quartz (Bohlen and Essene, 1977;Jaffe et al., 1978; Lindsley et al., 1974; Smith, 1972).Thus, for almost all natural orthopyroxenes the defi-nition of Fs as Fel(Fe+Mg) will result in an over-estimation of pressure when compared to Smith's(1971) data for synthetic orthopyroxene. For thenatural orthopyroxenes we have expressed Fs by twodifferent values, Fel(Fe+Mn+Mg) and Fel(Fe+Mn+Mg+Ca), in an attempt to take account of thepresence of Mn and Ca in a similar although slightlydifferent manner to Jafe et al. (1978). The resultingFs values for two eulites coexisting with fayalite andquartz bracket a small pressure range at 620-'120"C(Fig. 6). The pressure derived for the orthopyroxenefrom Queen Victoria Rocks, which is Mn-poor [Mn/(Fe*Mn+Mg) : 0.0031, is slightly higher than forthe Mn-bearing eulite [Mn/(Fe+Mn+Mg) : 0.037]from Heaney's Find.
A minimum pressure estimate can be obtainedfrom the orthopyroxene-bearing but olivine-freeassemblages at Meier's Find (sample 6). Eulite fromthese assemblages is Mn-bearing [Mn/(Fe+Mn+Mg)= 0.0251, and its Fs values [Fel(Fe+Mn+Mg) :0.787, Fel(Fe*Mn*Mg*Ca) : 0.7691 yield pres-sures about I kbar lower than those indicated by theHeaney's Find eulite, which is also Mn-bearing.
Co(Mg.rFe.r) SirO"Hedenbergife
CoFeSi206
+ Heoneys F ind' i i leier's Find. Queen V ic to r io
98 GOLE AND KLEIN: AR,CHEAN BANDED IRON-FORMATION
Temperolure, "C
600 700 800 900 1000
Fig. 6. P-T diagram (after Jaffe et al., 1978) with isoplethsshowing the composition of orthopyroxene coexisting with olivineand quartz, based on experimental work in the MgO-FeO-SiO2system by Lindsley (1965) and Smith (1971). The Fs,- isoplethdetermined by Bohlen et al. (1978) is also shown. The shadcdareas bracket the estimated metamorphic temperature ranges. TheFs values were calculated as Fel(Fe+Mn+Mg) (upper boundary)and Fe,/(Fe+Mn+Mg+Ca) (ower boundary) for eulites fromHeaney's Find (1B) and Queen Victoria Rocks (l2B).
The difference in pressures indicated by eulitecompositions from Heaney's Find (and perhapsMeier's Find) and Queen Victoria Rocks may, how-ever, not be real, or at least not as large as indicatedin Figure 6, because Mn is treated as having the samepressure-lowering effect as Mg in the calculation ofthe Fs values. This is unlikely. The effect of Mnshould be less than that of Mg because in theHeaney's Find and Queen Victoria Rocks assem-blages Mn [Ko,". M")(OL-OPX) : 1.00-1.19] is frac-tionated less than Mg [Ko..,"ns)(OL-OPX) : 3.26-3.791 between fayalite and eulite (see also Berg, lp77;Jaffe et al.,1978). The Fs values we have used shouldthus yield pressures that are sorrewhat low.
A further uncertainty in using the orthopyroxene-olivine-quartz geobarometer is introduced by thediscrepancy between the position of Smith's (1971)and Bohlen et al.'s (1978) P-T cuwe for the reactionFS,oo : Fa,oo * Qtz (Fig. 6). Bohlen et sl.'s data sug-gest that the Fs,. isopleth should be almost I kbar
lower at 700'C than that of Smith. If all Fs isoplethsare lowered by a similar amount, then the shadedareas in Figure 6 would be correspondingly lower.
The P-T conditions (670i50'C, 3-5 kbar) de-picted in Figure 6 for Heaney's Find and Queen Vic-toria Rocks are similar to those estimated for thehigh-grade regional terrains in the Eastern Gold-fields Province of the Yilgarn Block (Binns et al.,1976; Binns and Groves, 1976). Lower pressures, asperhaps required by the experimental data of Bohlenet al. (1978), are, however, also consistent with thegeology at these localities and would indicate a rela-tively high thermal gradient during metamorphism.
AcknowledgmentsWe thank Spargos Exploration N.L. for permission to sample
diamond drill core from Queen Victoria Rocks. Part of this studywas undertaken while M.J.G. held a Commonwealth PostgraduateResearch Award at the University of Westem Australia. Furtherfunding was provided by NSF grant EAR 76-11'140 (to C.K.). Theelectron microprobe used in the study was obtained on NSF grantGA-37109 (to C.K.), with joint funds from the Indiana UniversityFoundation. We thank W. H. Moran, R. T. Hill, and J. R. Tolanfor the drafting of the illustrations; Mrs. Thea Brown for the typ-ing of the manuscript; D. Weaver for his upkeep of the electronmicroprobe; and R. A. Binns and R. F. Floran for their critical re-view of the manuscript and their constructive comments.
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Manuscript received, April 24, 1980;accepted for publication, August 18, 1980.
