Mineralogy and Mineral Chemistry of Oxide- Facies Manganese Ores of the Postmasburg Manganese Field, South Africa

Mineralogy and mineral chemistry of oxide- facies manganese ores of the Postmasburg manganese field, South Africa

J. GUTZMERAND N. J. BEUKES Department of Geology, Rand Afrikaans University, P.O. Box 524, Auckland Park, 2006, South Africa

Abstract The diagenetic to very low-grade metamorphic manganese ores of the Postmasburg manganese field provide a unique example of oxide-facies manganese ores in a Palaeoproterozoic palaeokarst setting. The ores are composed mainly of braunite group minerals, including braunite, partridgeite and bixbyite, with rare braunite II and Ca-poor, silica-depleted braunite. Iron-poor partridgeite is distinguished from Fe-rich bixbyite and the occurrence of Ca-poor, silica-depleted braunite is reported for the first time, Braunite and partridgeite formed during early diagenesis but remained stable under greenschist facies metamorphic conditions. In contrast, bixbyite is apparently a product of metasomatic remobilisation under peak metamorphic conditions. It is suggested that local variations of the metamorphic mineral association reflect variations of the host rock composition and that they are not related to changing P-T conditions of metamorphic alteration, a model promoted by previous authors. The phase chemistry of braunite, braunite II and bixbyite is explained by the existing polysomatic stacking model for the braunite group. However, the chemical composition of partridgeite and Ca-poor, silica-depleted braunite can only be explained by introducing a distinct module layer, with partridgeite composition, to the existing polysomatic stacking model.

KEVWORDS: braunite, bixbyite, partridgeite, Campbellrand Subgroup, Transvaal Supergroup, manganese ore, Postmasburg, South Africa.

Introduction oxides (Ramdohr and Frenzel, 1956; Frenzel, 1980). This paper describes the chemical composi- TIlE manganese ores of the Postmasburg manganese tion of the various ore-forming minerals in greater field (Fig. IA) are subdivided into ferruginous ores of detail and relates compositional variations to specific the Westem Belt and siliceous ores of the Eastern paragenetic and geological settings (Figs 1 and 2). Belt (Fig. 1B). Ferruginous ores form most of the ore Special attention is given to the crystal chemistry of reserve and were mined extensively from 1922 to the the braunite group of minerals. late 1980s at Bishop, Lohatlha and Gloucester in the central part of the Western Belt (Fig. 1B). Siliceous Analytical techniques ores occur locally in the Wolhaarkop Breccia and were mined on a small scale in a number of localities Identifications of ore and gangue minerals were (Fig. 1B) (Hall, 1926; Nel, 1929; De Villiers, 1960). based on a combination of X-ray powder diffraction Although the petrography of the manganese oxide analysis and ore petrographic studies. Electron ores of the Postmasburg manganese field has been microprobe analyses were carried out on a studied in some detail (Schneiderh6hn, 1931; De CAMECA CAMEBAX 355 microprobe with an Villiers, 1960, 1983; Plehwe-Leisen, 1985) very little aftached LINK ExLII energy dispersive spectrometer is known about the chemical composition of the operated at 15 kV and 10 nA (on brass). Data minerals. This is so because previous studies relied acquisition was 80 sec per point analysis and all on the optical identification of ore and gangue results were ZAF corrected. Several point analyses minerals, a procedure unreliable for manganese were performed for each mineral and average Mineralogical Magazine, April 1997, Vol. 61, pp. 213-231 Copyright the Mineralogical Society 214 J. GUTZMER AND N. J. BEUKES

compositions determined to ensure the representative diaspore and kaolinite with variable amounts of character of the results. Mineral formulae were hematite and minor amounts of illite, anatase and calculated with Minfile, version 3-88 (Afifi and rutile. Some of the minerals present in the manganese Essene, 1988). shales and ores are listed in Table 1. Fine and coarse-grained varieties of ferruginous Geological setting manganese ores are present in the Western Belt. Fine grained bedded ores are composed of partridgeite, The Postmasburg manganese field is situated on the braunite, diaspore and ephesite (Fig. 2) and Maremane dome (Fig. 1C) defined by dolomites of characterized by abundant sedimentary and dia- the Campbellrand Subgroup and iron-formation of genetic structures such as intercalations of shale or the Asbestos Hills Subgroup of the early hematite pebble conglomerate, often showing Palaeoproterozoic Transvaal Supergroup (Beukes, contorted sedimentary bedding. In contrast, coarse 1986; Grobbelaar and Beukes, 1986). Only the grained ores have a vuggy or massive idioblastic eastern half of this dome is exposed (Fig. IC). To appearance with abundant veining and brecciation. the west the Transvaal Supergroup is overlain The coarse grained ores form irregular pods within unconformably by red beds of the Gamagara the fine grained bedded ores and differ from the latter Formation of the late Palaeoproterozoic (2.2-1.9 in that they contain abundant coarse grained iron-rich Ga) Olifantshoek Group. Further to the west older bixbyite in association with very coarse grained rocks belonging to the Transvaal Supergroup, diaspore and ephesite (Fig. 2). including iron-formation, diamictite and andesitic The geological setting of the manganese deposits lavas, are thrust over the Gamagara Formation along strongly suggests accumulation as residual palaeo- the Black Ridge thrust fault (Fig. IC) (Beukes and Smit, 1987). Manganese ores, and closely associated hematitic TABLE 1. List of some minerals encountered in the iron ores (Grobbelaar and Beukes, 1986; Van Postmasburg manganese field Schalkwyk and Beukes, 1986) occur along or immediately below the pre-Gamagara unconformity. The iron ores, derived from hematitization of Mineral Name Formula Asbestos Hills iron-formation, and its reworked equivalents in the Doornfontein conglomerate, are jacobsite MnZ+Fe204 preserved as infills of large karstic sinkhole hausmannite Mn2+Mn3+O4 depressions (Van Wyk, 1980; Van Wyk and braunite Mn2+Mn63+SiOi2 Beukes, 1982). partridgeite* Mn203 Siliceous manganese ores of the Eastern Belt are bixbyite a-(Mn,Fe)203 manganite 7-MnOOH hosted by the Wolhaarkop chert breccia thought to 4+ represent a subsurface solution collapse breccia that roman6chite Ba~_2Mn80t6.xH20 cryptomelane Kl_2Mns4+O~6.xH20 accumulated as Asbestos Hills iron-formation 4+ slumped into karst depressions in the dolomite of manganomelane** (Na, K,Ba)j_zMn80I6"XH20 lithiophorite LizMnZ+Mn4+AI40j 8.6H2 O the Campbellrand Subgroup (Fig. 3). Small lenses, pyrolusite 13-MnO2 pods, and sheet-like bodies of high-grade, siliceous ramsdellite MnO2 manganese ore occur in the lower part of the diaspore c~-A1OOH Wolhaarkop breccia near the contact with the gamagarite Baz(Fe3+,Mn)[(OH,H20)/(VO4)2] underlying dolomite (De Villiers, 1960). The ephesite (Na,Ca)Alz[(OH)ffAl(A1,Si)Si2Olo] braunite-rich ores are massive, fine to medium amesite (Mg,Fe)zAI2[(OH)8/(Si2,AI2)Olo] grained and dense. Ferruginous ores of the Western Belt form the base of the Gamagara Formation in the central part of the * Partridgeite is not a mineral recognized by the IMA Gamagara ridge where the succession unconformably (International Mineralogical Association), see discus- overlies manganese-rich dolomite of the Reivilo sion in text. Formation (Fig. 4). Aluminous shales or hematite ** Following the suggestion by Frenzel (1980) the complex~roup of minerals with the generalized formula pebble conglomerates of the Gamagara Formation Al_2Mn80i6"xH20 (A = Na,K,Pb,Ba) is in this study rest conformably on the ferruginous manganese ore referred to as the manganomelane group. Several that is confined to karstic depressions in the compositional end-members are distinguished in this Campbellrand dolomite. The conglomerates and group namely manjiroite (Na-rich), cryptomelane (K- _shales were deposited in an alluvial floodplain rich), coronadite (Pb-rich) and roman~chite (Ba-rich). -environment (Van Schalkwyk and Beukes, 1986). Phases of intermediate composition are referred to as The aluminous shales are composed of pyrophyllite, manganomelane. OXIDE-FACIES MANGANESE ORES 215

A B. 23o

Sishen I~ ,~;L,vELg/~ i ~ "DEMANENG

/K~G $ \ ~ '~\ /__| r, \0~, o..o.. I ~ MOKANINC~ I.

280 ~/; g CO

Z l.U I-- Z Gamagara Formation w 0 x Unconformity

~:~ Asbestos Hills I.F. k!, __ _.1 Campbelkand Dolomite ,.i

o15'

Legend:

WESTERN eE'r

F~-'] EASTERN BELT farm boundary m vH~,g~ - -~ ,/ 230 ,u 1 0 km

FIG. 1. Regional map of the Postmasburg manganese field showing the location of the deposits of fermginous manganese ore along the Gamagara Ridge (Western Belt) and the occurrence of Wolhaarkop Breccia (and associated manganese ores of the siliceous type) in the Klipfontein Hills (Eastern Belt), the Gamagara Ridge and in the Aucampsrust area (modified after De Villiers, 1960). karst sediments during the period of intensivc Olifantshoek Group (Grobbelaar and Beukes, 1986). weathering that preceded the deposition of the The karstic manganese and iron-rich dolomites of the sediments of the late Pataeoproterozoic Campbellrand Subgroup not only host, but apparently 216 J. GUTZMER AND N. J. BEUKES

Mineral Detrital Diagenetic Metamorph Hydrothermal Supergene

Chert l I

Chalcedony Quartz

Hematite

) J Speeularite i

Goethite P

Hausmannite

Jacobsite

Braunite E

Partridgeite

Bixbyite

Pyrolusite

Ramsdellite

Lithiophorite

Cryptomelane Roman~chite l ] Wad I i O Diaspore I

Barite

Gamagarite

Apadte

Acmite

EphesRe

Amesite

Muscovite i i ! Albite I ' I ! t t I I

FxG. 2. Mineral paragenetic table for the manganese ores of the Postmasburg manganese field (after Gutzmer, 1996). OXIDE-FACIES MANGANESE ORES 217

LEGEND

Recent "alluvial canga

q- F Recentwad

-~ Sishenshale Doomfontein hematite pebble conglomerate Blinkklip I.-F. breccia

Asbestos Hills I.-E o= 2 km Wol opb occia

1 Silice~ manganeseore ~.~'~ -F Solution collapse unconformity ~ ~ [~ Campbellranddolomite

2 km

Fie. 3. Model for the origin of the Wolhaarkop Breccia and associated siliceous manganese ore of the Eastern Belt. (A) Wolhaarkop chert breccia accumulates as residual sediment in karst caves into which portions of hematitized Manganore Iron-Formation subsequently slump. Fluvial conglomerates of reworked iron-formation cover the slumped iron-formation. (B) Following burial and greenschist facies metamorphism, renewed terrestrial exposure and erosion lead to relief inversion. The sinkhole infills are positively exposed against the surrounding dolomite host. Alluvial canga deposits accumulate along the footslope of the hills (modified after Gutzmer, 1996).

also sourced the manganese deposits (Grobbelaar and Petrography Beukes, 1986). Indications are that siliceous manganese ores of the Eastern Belt accumulated in karst The siliceous manganese ores, situated in the caves intimately associated with the Wolhaarkop Wolhaarkop chert breccia (Fig. 3), are composed of subsurface solution collapse breccia (Fig. 3). In braunite, with minor amounts of partridgeite, contrast, ferruginous manganese ores of the Western hematite, authigenic quartz and recrystallized chert Belt were deposited together with lateritic clays fragments (Fig. 5A). The latter were apparently (lower aluminous shales of the Gamagara Formation) derived from the dissolution of underlying dolomite in subaerial sinkhole depressions (Fig. 4). (De Villiers, 1960). Braunite occurs as equigranular Postmetamorphic lenses and veins of low tempera- mosaic-textured aggregates composed of subhedral ture hydr0thermal baryte and coarse grained spec- grains 5 to 1000 ~tm in diameter. Braunite crystals ularite crosscut the manganese orebodies (Fig. 2). often show distinct concentric colour zonation from Subaerial exposure in geologically recent times brown to brownish-grey (Fig. 5B). Lamellar and resulted in relief inversion and positive weathering spotty intergrowth of different braunite varieties and of manganese and iron-ore bodies against the of braunite with partridgeite are present. Needle- surrounding dolomites (Figs. 3 and 4). Supergene shaped and platy, anhedral grains of hematite are manganese oxides such as roman~chite, cryptome- associated with braunite-partridgeite aggregates. lane, lithiophorite and pyrolusite formed at this time Rare prismatic apatite crystals with a diameter of (Fig. 2). up to 250 lam (De Villiers, 1945a) are present and 218 J. GUTZMER AND N. J. BEUKES

lw LEGEND

Recent alluvialcanga T Recent supergene wad Marthaspoort quartzite ! Sishen shale Doornfontein hematite I pebble conglomerate 500 m Conglomeratic interbed Massive to vuggy pods of coarse crystalline bixbyite-rich ore

i Bauxitic clay-wad sediment/ferruginous ore Campbellrand dolomite (ReiviloFormation)

i 700 m

FIG. 4. Model for the origin of the ferruginous manganese ores of the Western Belt. (A) A layered succession of mixed bauxitic clay-ferromanganiferous wad with interbedded clay lenses and conglomeratic beds is deposited in open karst depressions of the Reivilo Formation and buried below hematite conglomerates and lateritic clays of the Gamagara Formation. (B) Greenschist facies metamorphism is succeeded by terrestrial exposure and erosion resulting in renewed karstification, brecciation of the manganese orebody, formation of romanb,chite crusts and manganese wad and accumulation of alluvial canga deposits (modified after Gutzmer, 1996).

known to be replaced by braunite. Sheaf-like the braunite matrix of the breccia display no evidence aggregates of Ba-muscovite occur in supergene of replacement by braunite (Fig. 5A). altered siliceous ore. Coarse grained baryte occurs The fine grained bedded ferruginous manganese in irregular patches in the Wolhaarkop breccia and ores of the Western Belt are composed of braunite, encloses large euhedral braunite crystals. Aggregates partridgeite, hematite and diaspore, with variable of needle-shaped acmite and compact crystalline amounts of ephesite and amesite. Braunite is masses of coarse grained albite are present in intimately intergrown with ephesite and amesite, braunitic ores from Sishen and Mokaning (De whereas partridgeite is more abundant in oxide and Villiers, 1945a). diaspore-rich beds. Finely laminated and massive The Wolhaarkop breccia, which hosts the siliceous beds are present in the fine grained ore. Early ores, is composed of recrystallized chert fragments diagenetic structures include ovoidal microconcre- set in a fine grained matrix of braunite, hematite and tions (Fig. 5C), botryoidal layered crusts and authigenic euhedral quartz. Many of the recrystal- anastomosing veinlets, all composed of braunite lized chert fragments are overgrown by authigenic and/or partridgeite. Fine laminae are defined by quartz, partly replaced by fine grained braunite. In slightly differing concentrations of braunite or contrast small authigenic quartz crystals developed in partridgeite, grain size and/or degrees of supergene OXIDE-FACIES MANGANESE ORES 219

FIG. 5. Reflected light photomicrographs. (A) Small, authigenic quartz crystals in mosaic textured braunite matrix. Braunite is replaced along grain boundaries by supergene manganomelane (white) (oil immersion, MAN238); (B) Concentrically zoned euhedral braunite crystals in vug filled by supergene roman~chite (white) (oil immersion, BEE208); (C) Diagenetic partridgeite microconcretions (light grey) that are partly replaced by supergene roman~chite (white) in a massive braunite matrix (grey) (oil immersion, GLOI87); (D) Braunite lamellae (grey) intersect partridgeite grain (light-grey), in a matrix of finely intergrown Si-depleted braunite and braunite. Partridgeite and Si-depleted braunite are replaced by roman~chite (white) (oil immersion, GLOI87); (E) Flame structures of Si-depleted braunite (medium grey) extending from the surface of a large partridgeite grain (light grey) into braunite (grey). Roman~chite appears white, diaspore black (oil immersion, GLO187); (F) Large cubes of bixbyite (light grey) with small hematite inclusions (white) surrounded by hematite (white), lithiophorite (dark grey) and diaspore (black, short laths) (oil immersion, LOH79). alteration. Massive beds are composed of aggregates partridgeite-braunite grain boundaries into the of braunite with well developed equigranular surrounding braunite (Fig. 5E). Small anhedral mosaic texture and massive to idioblastic partrid- hausmannite grains occasionally replace partridgeite. Intimate intergrowths of braunite with geite. Ephesite and diaspore occur as gangue partridgeite are common (Fig. 5D). In one sample minerals. Micaceous ephesite plates, up to 100 ~tm from Glosam mine, flame-shaped intergrowths of long, and up to 500 itm long diaspore laths define Ca-poor silica-deplete braunite protrude from distinct laminae or irregular clusters. 220 J. GUTZMER AND N. J. BEUKES

The coarse grained ferruginous manganese ores of in the ferruginous manganese ores as replacement the Western Belt are composed of cubic bixbyite product of diaspore and ephesite (Figs. 6B and C). idioblasts that are up to 30 mm in diameter. The Minor amounts of manganite, pyrolusite, ramsdellite, bixbyite crystals are intimately intergrown with goethite, and X-ray amorphous wad occur as coarse grained diaspore, ephesite, and amesite (Fig. products of supergene alteration. Pyrolusite is 5F), mosaic-textured aggregates of braunite and rare common as vug or vein fill in some of the supergene plates of hematite. Bixbyite cubes are characterized ores especially (Fig. 6D). Some ramsdellite is also by lamellar twinning (Schneiderh6hn, 1931; present in the supergene ores and may be easily Ramdohr, 1969) and concentric growth zonation mistaken with pyrolusite (Plehwe-Leisen, 1985). along {100}. Oriented intergrowth inclusions (Ramdohr, 1969) of thin braunite and braunite II Mineral chemistry lamellae (De Villiers, 1992) (10 to 100 ~tm long) and up to 100 lam long diaspore laths appear along { 100} Braunite. Three compositionally distinct varieties and { 111 }, illustrating the cogenetic formation of of braunite occur in the Postmasburg manganese these minerals with bixbyite. Gamagarite, in a unique ores, namely braunite, braunite II, and Ca-poor silica- occurrence from Glosam mine, occurs as clusters of depleted braunite. Major element electron microp- needle-shaped crystals (up to 1 cm long) in bixbyite- robe analyses of braunite sensu stricto (from now on rich ore (De Villiers, 1943b). simply called braunite) indicates a generally Fe-poor Supergene ores are mainly composed of mangano- composition with a stoichiometric SiO2 content close melane group minerals (Frenzel, 1980). to 10 wt.% (Table 2) and the formula Roman6chite, cryptomelane and manganomelane of (Mno.7-1Cao-o.2)2+ 2+ (Mns.5-5.9Alo-o.2Feo-o.4)3+ 3+ intermediate composition replace partridgeite, bran- (Sio.9-1.1Tio-o.l)Ol2. The contents of Fe, Ti, and Ca nite and bixbyite (Fig. 6A). Lithiophorite is abundant are apparently controlled by the coexisting mineral

FIG. 6. Reflected light photomicrographs (A) Botryoidal roman6chite (BIS244). (B) Large bixbyite cube replaced by fine crystalline roman6chite (white) and lithiophorite (dark grey) (LOH79); (C) Close up of previous photograph. Note that different generations of manganomelane/roman6chite are characterized by different colour shades. Note also polishing scratches and strong bireflectivity on coarse crystalline lithiophorite (oil immersion); (D) Coarse crystalline supergene pyrolusite fills in vugs in braunitic manganese ore BEE210). OXIDE-FACIES MANGANESE ORES 221 assemblage. Braunite in close association with A1203 hematite contains 2.51 and 5.41 wt.% Fe203 whereas braunite in assemblages devoid of hematite contains Si'depleted between 0.1 and 2.21 wt.% FezO3. Concentrations of TiO2 vary between 0.41 and 1.05 wt.% TiO2 in I k.~ auni~aunite braunite associated with aluminous shales, but are less than 0.1 wt.% TiOz (Table 2, Fig. 7) in braunite from ferruginous and siliceous manganese ores. SiO2 contents decrease with increasing TiO2 contents, strongly suggesting substitution of Si by Ti (Table 2). CaO appears to be enriched in braunite in ferruginous manganese ores (1.85-3.09 wt.% CaO), as compared with braunite hosted by aluminous shales (0.3-0.54 wt.% CaO) and siliceous manganese ores (0.70-1.62 wt.% CaO). A1203 contents are apparently indepen- dent of the coexisting mineral assemblage (Table 2). Braunite II (a variety of braunite containing only FIG. 7. Triangular diagram Al203-TiOz-CaO to distin- 3-5 wt.% SIO2, a similar amount of CaO and about guish braunite formed in different geological 20 wt.% Fe203) was first described from the Kalahari environments. manganese field (De Villiers, 1945a; De Villiers and Herbstein, 1967). De Villiers (1992) described lamellar braunite II that contains 4.8 wt.% SiOz, 12 wt.% Fe203 and 4.64 wt.% CaO as inclusions in consistently contain more than 10 wt.% FezO3 bixbyite from Glosam mine in the Postmasburg (Table 3). The mineral name partridgeite is therefore manganese field. Rare flame-shaped growths of retained in this study to distinguish iron-poor anisotropic another variety of braunite, i.e. Ca-poor silica- cvMn203 from iron-rich isotropic bixbyite. Chemical depleted braunite (Figs. 5D and 5E), contain as formulae of (Mn3~o_l.96Alo.o2_o.osFe3~l_o.o4)O3 for . _3+ 3+ little as 1.1 wt.% CaO, between 3.3 and 4.2 wt.% pm/ndgelte and (Mnj.52-1.64, F%.24-0.37, A10.05-0.09)O3 SIO2, and 0-3.54 wt.% FezO3, but high concentra- for bixbyite are derived fi'om the microprobe data tions of A1203 (2.71-3.99 wt.%) (Table 2). (Table 3). Analytical totals of Ca-poor silica-depleted braunite The minor element composition of partridgeite and add up to somewhat more than 100 wt.%. This bixbyite is very similar. MgO, CaO, BaO and alkali suggests that some of the manganese may be present contents are well below 0.5 wt.% (Table 3). A1203 as MnO and not as Mn203. This is in contrast to concentrations vary between 0.45 and 2.80 wt.% in braunite II in which the divalent cation position is partridgeite (Table 3) and appear on average a little entirely occupied by Ca (De Villiers and Buseck, larger in bixbyite (1.41-2.90 wt.%) (Table 3). This is 1989). The compositional data of the Ca-poor silica- also true for TiO2 with partridgeite containing less depleted braunite lead to the formula than 0.15 wt.% TiOz and bixbyite containing 3+ 3+ Ca0.~Mn6.sFe0.1Al0.sSl0.4Ol2. between 0.06 and 0.68 wt.% TiO2. Bixbyite and partridgeite. Iron-poor partridgeite, Hausmannite. Microprobe analyses of hausman- containing less than 5 wt.% Fe203, and iron-rich nite were impeded by the fact that it is virtually bixbyite, containing 10 to 22 wt.% Fe203, are present. always replaced by supergene roman~chite. Only one Partridgeite was described by De Villiers (1943a) as a sample with sufficiently fresh hausmannite was new mineral species from the Postmasburg manganese encountered (Table 3). Minor element contents of field. However, Mason (1944) and Roy (1981) hausmannite are low and within the range reported in chaUenged the significance of the name partridgeite the literature (Frenzel, 1980; Roy, 1981). A13+ and and it became custom to refer to it as iron-poor Fe 3§ are likely to substitute for Mn3+ in the bixbyite (Plehwe-Leisen, 1985; Roy et al., 1990). hausmannite structure, suggesting a formula of 2+ 3+ 3+ 3+ During this study abundant partridgeite was examined Mn l.os(Mnl.sAlo. 1Feo.os)O4. microscopically and compared with associated iron- Manganomelane group and pyrolusite. The Ba- rich bixbyite. The two phases are easily distinguished. rich member of the manganomelane group, roma- Similar to the descriptions by De Villiers (1943a) and n6chite, contains between 13.3 and 15.6 wt.% BaO, De Villiers (1960), partridgeite appears anisotropic less than 1 wt.% Na20 and KzO and less than 0.5 whereas bixbyite is isotropic. Microprobe analyses wt.% MgO, CaO and SiO2 (Table 4). The Fe203 and confirmed that those grains identified optically as AlzO3 contents are below 2 wt.%. Cryptomelane partridgeite are in fact iron-poor and contain less than contains significantly more K20 (4.92-5.42 wt.%) 5 wt.% Fe203 whereas examples of bixbyite than BaO (0.43-1.99 wt.%) or Na20 (0.18-0.45 222 J. GUTZMER AND N. J. BEUKES

t~ x~

,.Q

o~ o + r e-,

O x, + ~dd~N ~d~dd~ Ns O

OOOO~OOOO

M ~

O .1

O .<

E .E < c- o

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o ~o do~MM~dd~ dd e,,

oo ~~ ~ ddd~M~ddMM dd

e-i 06 o ~l ej o .< z~ ei 6 o~ .1 II1 Z -I- [... ~.~ .~..~ooOooo o .~ OXIDE-FACIES MANGANESE ORES 223

dd~d~d d d

dd~dg ddMd d

ID o ~D m. e~

~D ~2

ell a:l m O ~2 e~ .,.., 0

O t~

~D 0 M

O e- l

m. r

:5 o.0 oooo0o c~ ~ oo -- ,.-.1 09 e~ o ~o ~o ~ N~

e~ < r~

d ~+

,..., O ~ e-,

O o d r~. Z -I-

m ~z 224 J. GUTZMER AND N. J. BEUKES wt.%). A1203 concentrations range between 1.33 and 1.32 to 1.47 wt.%, but detected only 0.11-0.5 wt.% 3.50 wt.%; FezO3, MgO and CaO contents are below Mn203. Electron microprobe analyses were per- 1 wt.% in the cryptomelane. The manganomelane of formed to resolve the obvious disagreement between intermediate composition is characterized by variable the results of these two studies (Table 5). Fe203 concentrations of minor elements, containing contents range between 0.3 and 0.54 wt.% and 0.07-3.67 wt.% BaO, 1.12-1.58 wt.% Na20, Mn203 contents between 0.2 and 0.6 wt.% (Table 5). 0.23-3.06 wt.% K20, 0.0-0.89 wt.% FezO3, and Considerably larger Fe concentrations reported by 1.21-2.01 wt.% A1203 (Table 4). Analytical totals of Chudoba (1929) and Nel (1929) may be due to the roman6chite, cryptomelane and manganomelane presence of minute hematite inclusions in the range between 93.3 and 97.2 wt.%, indicating the diaspore crystals. Unit cell constants calculated for presence of variable amounts of absorbed and the diaspore from Guinier camera exposure~ of a structural water (Frenzel, 1980). single crystal fragment are a = 4.401 +0.002 A, b = Microprobe analyses of coarse grained pyrolusite 9.428__+0.002 A, c = 2.845___0.001 A (D. Nyfeler, indicated it to be essentially MnO2, with negligible written communication 1994), in very good agree- amounts of other elements (Table 4). The presence of ment with unit cell constants of diaspore reported in absorbed water in the pyrolusite is indicated by the ICDD (1994) database. X-ray and backscattered analytical totals of between 97 and 98 wt.% (Frenzel, electron images of single crystal fragments oriented 1980). into different crystallographic orientations, indicated Lithiophorite. De Villiers (1945b) described the a homogenous distribution of Mn and Fe. It is occurrence of coarse botryoidal lithiophorite from the suggested that the small amounts of Fe3+and Mn3+ Glosam mine. A small piece of the original sample substitute for A1 in the diaspore lattice. The presence analysed by De Villiers (1945b) was used for of small amounts of Mn may also explain the unusual microprobe analysis and atomic absorption spectro- colour of the diaspore crystals. metry (Li20) (sample GLOLi01, Table 4) to compare Analyses of ephesite, the Na-rich analogue of its composition with that of the more abundant fine margarite (ROsler, 1984) confirm the formula grained lithiophorite. Results compare very well, (Nao.92Lio.a7)(A12.o4,Feo.13,Mno.14,Mgo.os)[(OH)z/ with A1203 and MnO2 predominating and small Al(Alo.92Sio.08)Si2Olo] (calculated for sample amounts of Fe203, MgO (1.61 wt.% and 2.48 wt.%) LOH176, Table 5). Amesite (ROsler, 1984), an and Li20 (about 3 wt.%) present (Table 4). The aluminous serpentine mineral, has a composition results obtained in this study are also in very good (Table 5) of Mg3.96All.97Mno.03Feo.olTlo.m[(OH)8/2+ 3+ . agreement with the original analysis by De Villiers (5i2.o6,All.94)O1o ] (sample BIS241) or (1945b). The general formula LizMn2+Mn4+ Mg3.s7AI 1.95Mno.osFeo.o3[(OH)s/(Si2.16All.84)O1 O] AI4018.6H20 given by De Villiers (1945b) for the (sample GLO197). Ba-Al-muscovite, a mineral lithiophorite is thus essentially correct. previously unknown in the PMF, was identified by Gamagarite, apatite and baryte. Microprobe microprobe analysis (Table 5) and has a composition analyses of gamagarite were performed on a small corresponding to (Bao.6Ko.32Nao.06MnZ~,sFe~3) piece of the type sample described by De Villiers AI1.92[(OH)z/(Si2.44AIl.56)Om] if a stoichiometric (1943b). Results (Table 5) compare well with water content is assumed. analyses by De Villiers (1943b) and Harlow et al. (1984), suggesting the formula Baz(Fe3+,Mn)[(OH, Discussion H20)/(VO4)2]. Apatite, present in the siliceous manganese ore, was found to have a hydroxy-apatite Origin and stability of ore-forming minerals. composition as no CI or F were detected by Members of the braunite group are known to be microprobe analysis. The composition of baryte is stable in various geological settings. Braunite and fairly uniform, with 0-1.5 wt.% SrO substituting for bixbyite form under metamorphic conditions ranging BaO (Table 5). from lowest greenschist facies to granulite facies Diaspore, ephesite, amesite and Ba-Al-muscovite. (Roy, 1981; Abs-Wurmbach et al., 1983; Dasgupta The presence of coarse grained ephesite, diaspore and and Manickavasagam, 1981). Several authors have amesite in the manganese ores has attracted the presented convincing evidence for the early diage- attention of several earlier workers (Chudoba, 1929; netic formation of braunite as a reaction product of Hall, 1926; Coles-Phillips, 1931). Mn and Fe-bearing poorly crystalline Mn4§ and quartz diaspore crystals, up to 10 cm long and with a distinct (Ostwald and Bolton, 1990; Roy et al., 1990; whisky brown to brownish red colour, were first Ostwald, 1992). Others have advocated its sedimen- described as rhodonite by Hall (1926) and later tary (Serdyuchenko, 1980; Hou, 1994) or even analysed by Chudoba (1929) who reported concen- supergene origin (Ostwald, 1992). Partridgeite, or trations of 1.96 wt.% Fe203 and 4.32 wt.% Mn203. Fe-poor bixbyite, is only known from lower Nel (1929) found similar Fe203 concentrations of greenschist facies metamorphic assemblages (Roy OXIDE-FACIES MANGANESE ORES 225

Ox ox

~ 0

o o

2 0

t- 0

u.tq~ O

0 0 ~d 0

oo o

,.m8 o

O~ 0 ~

d Z on ~e~ ~.~.,;A-~o _~ o~ o~- o~,~0 ~ Oo~~ ~.~ ~'~ o~ .~ ~,N .~ N~z 226 J. GUTZMER AND N. J. BEUKES

k~D ~D cq O= c~

02,,~ r 0 ,~1

Q~ ..... ~J c~ ..=

OC) "~ I"- ~0 kO 0 er rr

0 V ~ 0 %) r

,-< .=_

C~ oo ~8 Ch ~) ~D Oa

0 .=_ Lu E 0 ~J >, o u~ dd 4n d 6 o

o ~

...-~ ;~0 ~ ~)

' ='~ ~ =0 ~176176 i I-.1 e~ ~ et o6~Z ~<~~ ~~>~ OXIDE-FACIES MANGANESE ORES 227

et al., 1990; Gutzmer, 1996), although experimental The coexistence of pyrophyllite and diaspore in the studies suggest the stability of the association aluminous shales of the Gamagara Formation braunite-partridgeite at high temperatures and suggests formation from kaolinitic clays at tempera- pressures of up to 600~ and 7 kbar at oxygen tures between 300 and 350~ (Anovitz et al., 1991) fugacities controlled by the MnO2/Mn203 buffer and a pressure of 3 to 4 kbar (Fig. 8). The (Abs-Wurmbach et al., 1983). Apart from its crystallinity of illite in the Gamagara shales confirms occurrence in the Postmasburg manganese field, greenschist facies metamorphic conditions with peak silica-depleted braunite has as yet only been metamorphic temperatures below 400~ (Plehwe- recognized in the lower-greenschist facies meta- Leisen, 1985). Nell et al. (1994) showed that the morphic oxide ores of the Kajlidongri mine, India bixbyite in the ferruginous manganese ore formed in (Ostwald and Nayak, 1993), and as a major a temperature range of between 315 to 375~ i.e. constituent of hydrothermally altered Wessels-type during peak metamorphism. Diaspore, ephesite and ore of the Kalahari manganese field (Kleyensttiber, amesite are known to be stable under greenschist 1985; Gutzmer and Beukes, 1995). facies metamorphism (B~irdossy, 1982; Feenstra, Braunite, silica-depleted braunite, braunite II, 1996; Anovitz et al., 1991). Acmite has recently partridgeite and bixbyite from the Postmasburg been reported as a minor constituent of the manganese field occupy non overlapping fields in greenschist facies manganese-rich iron-formations the SiO2-FezO3 diagram (Fig. 9). The predominance of the Cuyuna Range, USA (McSwiggen et al., of braunite in siliceous and partridgeite or bixbyite in 1994). the silica-poor ferruginous ores indicates that the Meteoric fluids are thought to be responsible for distribution and composition of the different braunite the replacement of the greenschist facies manganese group minerals in the manganese ores of the ore mineral assemblage. The abundance of roma- Postmasburg manganese field are controlled by the host rock composition. Petrographic evidence suggests that braunite, partridgeite and Ca-poor silica-depleted braunite formed diagenetically in the manganese ores of the Postmasburg manganese field and were locally recrystallized during greenschist facies metamorphism. Very fine grained manganese ores preserved details of sedimentary and early diagenetic structures. Layered aggregates with fine fibrous botryoidal texture, now composed of braunite and partridgeite, closely resemble Mn4+-oxihydroxide - crusts described in Mesozoic unmetamorphosed 4 ....:.j/,r KKyanite karst-hosted wad deposits by Brannath and Smykatz-Kloss (1992). Concentric zonation in braunite was ascribed to diagenetic replacement processes by Ostwald (1982) and Plehwe-Leisen (1985). Replacement of authigenic quartz by braunite further supports a diagenetic origin for some of the ~ :.~V--:-:-:-:-:-:-:-:-:~tPle~e-~ise. braunite in the siliceous ores. Sedimentary and early diagenetic textures are /f 11"--'4 Nell et al. obscured in the more coarse grained manganese :.~i!ii ~:l I (1994) I/ I ores. Mosaic textures that are present in massive 300 400 500 braunite-partridgeite ores are typical of metamorphosed manganese ores (Ramdohr, 1969; Roy, T (~ 1981). This suggests that recrystallization of early Fro. 8. Stable equilibria in the system A1203-SiO2-H20 diagenetic braunite-partridgeite assemblages took (Anovitz et al., 1991). Hatched area outlines peak place during metamorphism. The occurrence of metamorphic conditions suggested for the manganese bixbyite is restricted to isolated pods of very ores and associated sediments of the Olifantshoek Group coarsely recrystallized ferruginous manganese in the Postmasburg manganese field. Lower greenschist ores. Textures such as veining, brecciation and the metamorphic conditions are in agreement with the abundance of vugs indicate that bixbyite-rich ores temperature of formation of bixbyite (Nell et al., formed as a result of localized metasomatism, with 1994) and metamorphic conditions estimated by hydrothermal fluids most probably expelled from Plehwe-Leisen (1985) for the Postmasburg manganese immediate host rocks. field. 228 J. GUTZMER AND N. J. BEUKES

12.0 -~-~BBraunite sio2f Symbols Braunite (siliceous ore) Braunite (ferruginous ore) /k Braunite (shale) O Si-depletedBratmite 8.0 Braunite 11 (De Villiers, 1992) Partridgeite 4= Bixbyite

Braunite II

4.0 ~ ~OOsilica_depleted Braunite

--~xbyite ~ artridgeite

0.0~~ , I -~.~ ~- , 0.0 5.0 10.0 15.0 20.0 25.0 Fe203 Fro. 9. SiO2-Fe203 diagram to discriminate different members of the braunite-bixbyite group of minerals. Note clear separation between partridgeite and braunite.

n6chite and cryptomelane indicates that considerable layers of the partridgeite module with two layers of amounts of barium and potassium were introduced the braunite module. during supergene alteration. Iron is apparently fractionated into braunite in The crystal chemistry of the braunite group. The cogenetic braunite-partridgeite pairs (Fig. 10A) but crystal chemistry of three of the members of the into bixbyite in braunite-bixbyite pairs (Fig. 9) braunite group present in the Postmasburg manga- (Dasgupta et al., 1990). The observed reversal in nese field, namely braunite, braunite II and bixbyite, the fractionation is at present not fully understood but can be explained by the polysomatic stacking model may be related to the stabilization of bixbyite (Fe- of De Villiers and Buseck (1989). This model rich et-Mn203) at decreasing metamorphic oxygen envisages stacking of layer modules of braunite fugacities by increasing substitution of Fe for Mn Mn22+ Mn73+ SIO24, bixbyite (Mn,Fe)I6024, and brau- (Abs-Wurmbach et al., 1983). nite II CaMn3~SiO24. However, the chemical Differences in the minor element composition of composition of the other two minerals of the braunite braunite, partridgeite, bixbyite, Ca-poor silica- group, namely partridgeite and Ca-poor silica- depleted braunite and braunite II reflect small depleted braunite, cannot be explained without differences in their crystal chemical properties. The adding a partridgeite layer module to the existing enrichment of TiO2 in shale-hosted braunite is related model. The partridgeite module, expressed as to the presence of a tetrahedrally coordinated site, Mn16024, most probably has lower symmetry than occupied by Si, in the braunite layer module the Fe-rich bixbyite module, as is indicated by the (Fig. 10B). The presence of unusually high concen- optical anisotropy of partridgeite in the manganese trations of CaO in braunite II (Fig. 10C) reflects the ore. Partridgeite in the Postmasburg manganese field presence of a distinct Ca-bearing module in the is thought to be essentially composed of this new structure of braunite II (De Villiers and Buseck, module. The composition of Ca-poor silica-depleted 1989). The CaO concentrations in braunite and Ca- braunite could then be explained by stacking three poor silica-depleted braunite are lower than in OXIDE-FACIES MANGANESE ORES 229 , TiO2 ~' Fe~O~ A@ B. 6.0 - (wt (wt %) 1.0 -

4.0 -.... o 0.5 i .$

2.0

-... ~. m '2 I T ea "r I

CaO Braunite II Ce ,AI~O, (wt %) (wt %) D@ 4.0 4.0

~o ._= '2

v~ 2.() 2.0 "2 = g .O

FIG. 10. (A) Fe203 content of braunite and partridgeite. Coexisting pairs are connected by tie lines. Ranges of the contents of TiO2 (B), CaO (C) and AI203 (D) in the different members of the braunite-bixbyite group of minerals.

braunite II but considerably larger than those in grade metamorphic conditions. The peak meta- partridgeite and bixbyite (Fig. 10C), suggesting morphic temperature was not greater than 375~ at substitution of Ca for Mn2§ in braunite group 3-4 kbar pressure. The metamorphic oxide assem- minerals (Baudracco-Gritti, 1985; Abs-Wurmbach blage reflects a regionally uniform lower greenschist et al., 1983). A1203 concentrations (Fig. 10D) are facies metamorphic overprint for the entire deposit, greatest in silica-depleted braunite. A1203 contents in unlike suggested by SchneiderhOhn (1931), De bixbyite and partridgeite are below those in Ca-poor Villiers (1960) and De Villiers (1983). silica-depleted braunite but much larger than in Five members of the braunite group are present in braunite (Fig. 10D). The fractionation of aluminium the Postmasburg manganese ores, namely braunite, into Ca-poor silica-depleted braunite needs further braunite II, Ca-poor silica-depleted braunite, partrid- investigation since AI~§ apparently substitutes for geite and bixbyite. Iron-poor partridgeite is a distinct Mn3+ and not Sin§ in the braunite group of minerals phase that is chemically and optically easily (Abs-Wurmbach and Langer, 1975). distinguished from Fe-rich bixbyite. The phase chemistry of braunite, braunite II and bixbyite is in agreement with the polysomatic stacking model for Conclusions the braunite group proposed by De Villiers and The manganese ores of the Postmasburg manganese Buseck (1989). However, the composition of field were apparently derived from a manganese wad partridgeite and Ca-poor silica-depleted braunite precursor in a karstic setting under diagenetic to low- can only be explained in terms of this polysomatic 230 J. GUTZMER AND N. J. BEUKES stacking model if a module layer with a partridgeite Mineral. Mag., 22, 482-5. composition, Mnj6024, is introduced. Chudoba, K. (1929) Uber 'Mangandiaspor' und Manganophyll von Postmasburg (Griqualand West, SiJdafrika). Neues Jahrb. Mineral., Beilagen Band 64 Acknowledgements A, 11-8. The authors are greatly indebted to the geological Dasgupta, H.C. and Manickavasagam, R. (1981) staff of ASSMANG (Beeshoek), ISCOR (Sishen), Chemical and X-ray investigation of braunite from SAMANCOR (Hotazel) and Rio Tinto Exploration the metamorphosed manganiferous sediments of for their permission to visit their properties in the India. Neues Jahrb. Mineral. Abh., 142, 2, 149-60. Postmasburg manganese field. Special thanks to Dasgupta, S., Bannerjee, U., Fukuoka, M., Bhattacharya, Willem Grobbelaar, Technical Manager at P.K. and Roy, S. (1990) Petrogenesis of metamor- Beeshoek Iron Ore Mine, for logistical support and phosed manganese deposits and the nature of the many interesting discussions during field work in precursor sediments. Ore Geol. Rev., 5, 359-84. 1993 and 1994 by J.G. We want to express special De Villiers, J. (1960) Manganese Deposits of the Union thanks to Andreas 'Hasselblad'-Pack, for his expert of South Africa. Geol. Surv. S. Afr., Pretoria, 280 pp. advice on how to obtain high quality black and white De Villiers, J.E. (1943a) A preliminary description of photomicrographs and to Daniel Nyfeler (Bern) for the new mineral partridgeite. Amer. Mineral., 28, information on the manganese-bearing diaspore. We 336-8. are also grateful to J. Ostwald whose thorough review De Villiers, J.E. (1943b) Gamagarite, a new vanadium helped to improve the quality of the final manuscript. mineral from the Postmasburg manganese deposits. Amer. Mineral., 28, 329-35. De Villiers, J.E. 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