of chert, iron oxides (magnetite, hematite), iron silicates (greenalite, minnesotaitc, stilpnomelane, riebeckite,. Fe mica), carbonates (calcite, ankerite, minor ...
Economic Geology Vol.92, 1997,pp. 87-97
Petrography andGeochemistry of the Paleoproterozoic HotazelIron-Formation, KalahariManganese Field, SouthAfrica: Implications for Precambrian Manganese Metallogenesis HARILAOS TSIKOSANDJOHNM. MOOnE Geology Department, Bhodes University, Grahmr•town 6140,SouthAfrica Abstract
Ne•vdataarepresented concerning the petrography andgeochemistry of the Paleoproterozoic Hotazel iron-formation oftheKalahari manganese field,SouthAfrica.Mineralogically andtexturally, theHotazelironformation isverysimilarto Paleoproterozoic banded iron-formations of theSuperior type,comprising laminae of chert,ironoxides (magnetite, hematite), ironsilicates (greenalite, minnesotaitc, stilpnomelane, riebeckite, Fe mica),carbonates (calcite,ankerite,minorsiderite),andpyrite.The majorchemical constituents of the Hotazelrocksare SiOs,Fe oxides,CaO, andMgO,whereasMnO, AlsO.•,Na•20,andK.,Ocontents are negligible (below1 wt %). Elementsof detritalderivation (Ti, Zr, Rb) andtransition metalssuchasZn, Cu, Ni, Co,andV arepresent in lowconcentrations, ontheorderof a fewtensof partspermillion. Stratigraphically higher,theHotazelbandediron-formation shows a conspicuous increase in CaOcontents thatiscoupled witha subtledecrease in bulk-rock Fe number ratios(FeZ+/(Fe •'++ Fe3+)),implying a gradual transition to a moreoxidizing environment. Shale-normalized rareearthdement(REE) patternsare similar to thoseof olderbandediron-formations of theTransvaal Supergroup, exhibiting lightREE depletion and negative Ce anomalies comparable to modernshallow-level seawater. Occasional weakpositive Eu anomalies in ironsificate-rich rocksindicateminoradmixtures of a hydrothermal component belowthe chemocline. Thesimilarities between theHotazeliron-formation andmostSuperior-type banded iron-formations, thelack of transition metalenrichments, andthe absence of strongly positiveEu anomalies in the Hotazelrocks challenge volcanogenic exhalative modelsproposed previously for the genesis of the interbedded Kalahari Mn deposits. Introduction
bandediron-formations(1,000 m) of the Asbesheuwels and
THE Paleoproterozoic manganese deposits of the Kalahari manganese fieldin thenorthern CapeProvince, SouthAfrica (Beukes, 1983;De Villiers,1983;Kleyenstiiber, 1984,1985; Nel et al., 1986;MiyanoandBeukes,1987;Beukeset al., 1995;Cornelland Schiitte,1995,1996;Gutzmerand Beukes,
Koegas Subgroups (Beukes andSmit,1987;Table1).These are,in turn,succeeded bya glacial diamietite (150m,MakganyeneFormation) andcontinental-type basaltic andesRe of the OngelukFormation (up to 900 m thick;Sehtitte,1999.; Cornell et al., 1996).
The HotazelFormation represents the youngest episode 1995;Beukes andGutzmer,1996;Gutzmer,1996),represent of iron-formation deposition in theTransvaal Supergroup. It
themajorworldresource of manganese ore.Theorigins of overlies lavas oftheOngeluk Formation andinitsupperparts thesedeposits arethesubject of debate, withepigenetic (De grades intodolomitie rocksof the Mooidraai Formation. The Villiers,1983),volcanogenic sedimentary (Beukes, 1983;Nel Hotazel and Mooidraai Formations constitute the Vo•lwater et al., 1986), and volcanicexhalative(Cornell and Schiitte, Subgroup, whichis bestpreserved asan erosional relictin 1995,1996)models havingbeenproposed. The manganesethe southernmost partof the Kalahariarea,locallyknownas ore is hostedby a unit of bandediron-formation knownas
Kalaharimanganese field(Fig.lB). the Hotazel Formation, which overlieslavasof the basaltic the Observations on borehole material from the Hotazel For-
andesitc-dominated Ongeluk Formation (Schtitte, 1992;Cor- mation have revealed four distinct units of banded iron-fornellet al.,1996).Mostprevious studies haveconcentrated on mationinterbedded with three manganese layers.In the themanganese oresthemselves andneglected theassociatedsouthern partsof the Kalaharifield,the manganese units iron-formations. Thisstudypresents mineralogical andgeo- consistof braunitcand Mn carbonateand lackfree SiO2(Nel chemical dataregarding the hostbandediron-formation of et al., 1986).The manganese horizonsattainconsiderable the HotazelFormation anddiscusses the significance that thicknesses (the lowermost economic Mn unit rangesfrom thesehaveconcerning thecontrasting genetic models. 15 to 45 m in places,the middleandupperunitsreach3 and10 m, respectively), areunaffected by anymetamorphic RegionalGeologicSetting processes, andbearrelatively lowmetalgrades(upto 40% ThePaleoproterozoic (2.7-2.2Ga)Transvaal SupergroupMn). The interbeddedbanded iron-formationunits exhibit in thenorthern CapeProvince (Fig.1A)formsa majorseg- thickness relations antithetie to those of the Mn ores and mentofthewiderTransvaal Supergroup in SouthAfricaand thisappliesparticularly to thebandediron-formation section adjoining partsof Botswana (Beukes, 1986).The sequencedireefiyoverlyingthe lowermostMn unit. This specific comprises a thicksuccession (1,600m) ofbasalplatform car- banded iron-formation unit attains a maximum thickness of bonates (Campbellrand Subgroup) whichare overlainby 40 to 45 lB in the areanorthof the Mamatwanmine(Fig.
0361-0128/97/1906/87-1156.00
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H. TSIKOS ANDJ.M. MOORE
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Kilometres
Kilometres
I
Fig.1. A. Geologic mapof theTransvaal Supergroup in thenorthern CapeProvince, sho•ving thedistribution of major lithostratigraphic subdivisions, structural features, andlocation of theKalahari manganese field(KMF,inset).B. Localib, mapof theKalahari manganese field,sho•ving thedistribution of majorstructural features, oretypes,andlocation of mines.
lB), wherethelowermost Mn horizondisplays a minimum currentlydefunctSmarttmine(Fig. lB). Mineralidentificathickness (15-20 m). In theimmediate vicinityof thepres- tionwasattainedby meansof X-raydiffraction (XRD) and ent mine site, however, this thicknessrelation is reversed petrographic studiesunderbothtransmitted andreflected (Fig. 2). light.Bulk-rockchemical analyses for majorandtraceelementswere undertakenby standardX-ray fluorescence Methodsof Study (XRF)procedures usinga PhilipsPW 1480X-rayspectromeFifty-sixquartered sections (approx15 cm long)of fresh terin theGeology Department at Rhodes University. Sample andhomogeneous bandediron-formation wereselected from preparation andanalytical procedures wereperformed acfourexploration boreholes drilledbytheSouthAfricanMan- cording to themethodof NorrishandHutton(1969),andall ganese Corporation (SAMANCOR) in thesouthernmost part analytical runswerecalibrated usinga varietyofinternational oftheKalahari manganese field,morespecifically in thearea and in-house standards. Whole-rock FeO determinations betweenthelow-grade Mamatwan manganese mineandthe wereperformed atthelaboratories oftheCouncilforGeosci-
TABLE1. Simplified Stratigraphy of the Transvaal Supergroup in the NorthernCapeProvince (lnodified afterBeukes andStair,1987) Approxim ate
Supergroup Transvaal
Group Postmasburg
Ghaap
Subgroup Voelwater
Formation
Majorlithology
thickness (m)
Mooidraai
Carbonates (dolomites), chert
Hotazel
Banded iron-formation, Mn ores
Ongeluk Makganyene
Andesitic volcanics Glacialdiamictites
Koegas
Siliciclastics,banded iron-formation
240-600
Asbesheuwels
Griquatown
Clastic-textured iron-formation
Kuruman
Microbanded
200-300 150-750
iron-formation
Campbellrand
Carbonates(dolomites),minor shales
Schmidtsdrif
Shales, quartzites, carbonates, lavas
250 5OO 50-150
1,500-1,700 10-250
HOTAZEL IRON-FORMATION, KALAHARIMn FIELD, S. AFRICA
89
200m
overprints of dispersed subhedral magnetite andadditional minoramounts of Fe silicates (greenalite, minnesotaitc), Mn carbonates (Mn dolomite, kutnohorite), andpyrite.Between the lowerandmiddleMn units,a veryfinegrainedchertgreenalite-minnesotaite-magnetite-microgranular sideriteassemblage with minorcalcite,ankerite,andp,vritepredominates.Conspicuous bandingis definedby rhythmicalternationof layeredchert-greenalite-minnesotaite with microgranular sideriteand/or.magnetite.The micro- to cryptocrystalline natureof thechert,greenalite, andsiderite D BIF indicatesa possiblerelationto the primaT' precipitates, whereas thecoarser grainedhabitof themagnetite andminI Mn-ore nesotaitc indicates recrystallization and/orreplacement under late diagenetic conditions. A characteristic assemblage of very fine grainedinterbandedchert-magnetite-carbonate-ferristilpnomelane with subordinate Fe silicates (minnesotaitc, greenalite, Fe mica) appears immediately belowandabove themiddlemanganese horizon. Stylolites are occasionally present parallelto the Fig.2. Simplified sketch of twoclosely spaced boreholes fromthevicinity of the Mamatwan Mn mine,displaying the antithetic thickness relations banding, enveloping riebeckite-bearing, chert-magnetite asbetween banded iron-formation and Mn ores in the Hotazel Formation. semblages. Depthsof intersection oftheHotazelFormation belowtheTertiaryKalahari A subsequent increase in carbonate contentcharacterizes cover are 'also indicated. virtually alltheremaining lithologies between themiddleand uppermanganese unitsandabovetheuppermanganese unit. Therespective mineralogy ischaracterized bythedisappearence,Pretoria, bymeans oftitrationagainst potassium dichro- anceof greenalite andsideriteandtheinterlayering of essenmate.Analyses of REE were undertaken by high-perfor- tiallypure,fine-grained idiomorphic magnetite withrecrystalmance ionchromatography using a Dionex4000iionchroma- lizedquartz-ankerite bandshostingminorminnesotaitc and tograph in theDepartment ofGeological Sciences, University pyrite.In addition, microcrystalline calcitebands withacicuof CapeTown,employing methods described by le Roex largrowths of ferristilpnomelane commonly intervene, formandWatkins(1990).Analytical results from12representativeinganunusual mineralogical association withregardto other samples selected fromthe largeroriginalsuiteof 56 samples iron-formations oftheTransvaal Supergroup. Froma textural from the Hotazel banded iron-formation(Tsikos,1994) are pointof view,the carbonate-rich bandediron-formation exsummarized in Table 2. hibitscoarser grainedmineralassemblages compared to the silicate-rich rock,irregular banding(podding), andfrequent, 66m
I
I
20m E
Results
Petrography The mineral constituents of the Hotazel banded iron-for-
soft-sediment deformation
structures similar to those re-
portedby Gross(1972). A thin (1-1.5 m) ferromanganiferous carbonate unit,enclosed within the banded iron-formation between the middle
mationcanbe subdivided intothreemajorcategories (after anduppermanganese beds(sample5, Table2), is marked James, 1954):(1)thesilicate group,whichcomprises (inran- byanabundance ofsparry calciteandcryptocrystalline, dusty domorder)theminerals chert(quartz), greenalite, minneso-hematitewithsubordinate chert,occasional euhedral pyrite taitc, stilpnomelane, riebeckite,and Fe-richmica;(2) the grains,and minorFe silicates (stilpnomelane, riebeckite). carbonate group,withmembers of the dolomite-ankerite se- This rockunit stronglyresembles the transitional banded ries,siderite,andcalcite;and (3) the oxide-sulfide group, iron-formation Mn lithologies, whichareparticularly richin comprising theminerals magnetite, hematite, andpyrite.Al- hematiteandcarbonate ("hematite lutites"of Kleyensttiber, ternating millimeterto centimeter-scale (usually fromI mm 1984),andit maycorrespond to a fourthincomplete episode to I cmthick)bands ofindividual phases (mainly magnetite)of Mn carbonate deposition. or,morecommonly, combinations oftheaboveminerals (e.g., Cyclicityin theHotazelFormation chert-greenalite-siderite-magnetite, chert-minnesotaite-greenalite,chert-ankerite-minnesotaite, calcite-stilpnomelane), deTheHotazelFormation reveals evidence of cyclicity onall finethe bandingin the Hotazeliron-formation. scales (Beukes,1983;Kleyensttiber, 1984;Tsikos,1994),a Mineralassemblages varyaccording to the stratigraphicfeaturetypicalof mostPaleoproterozoic bandediron-formalocation of the samples. Oxide-dominated faciesoccurin the tions. Here, three prominentmacrocycles consisting of lutite-Mn oxide-carbonlowerpartsof the HotazelFormation, whereas silicate-and banded iron-formation-hematite carbonate-dominated facies characterize themiddleandup- ate-hematite lutite-banded iron-formation mesobands are perpartsofthestratigraphy, respectively (Table3).A crypro- recognized across the HotazelFormation, whichcomprises crystalline rhythmically bandedhematite-chert assemblage a laterallypersistent megacycle. The Hotazelmegacycle is occurs belowthe mainlowerMn unit,withlatediageneticitselfdominated by oxidefaciesbandediron-formation at
90
H. TSIKOSAND J.M. MOORE
TABLE2. Bulk-Rock Geochemical Analyses of 12 Representative Samples fromthe HotazelBandedIron-Formation Sampleno.
1
2
3
4
5
6
7
8
9
10
11
SiO2 TiO.2 A[2Os
37.66 0.03
35.97 0.04
30.38 0.05
33.50 0.04
12.96 0.05
30.30 0.03
43.15 0.04
41.87 0.04
40.15 0.04
43.86 0.05
39.60 0.05
0.19
0.18
0.46
0.16
0.31
0.15
0.15
FeO
14.12
19.21
8.30
9.88
1.10
12.94
19.11
28.96
27.37
Fe•2Os
12
Weightpercent
0.36
0.16
0.49
0.04
28.63 0.05
0.32
31.15
26.38
17.82
28.54
19.26
22.84
23.41
28.73
9.66
21.09
24.56
13.81
20.59
7.13
20.16
MnO
0.15
0.37
2.21
1.38
7.17
0.55
0.12
0.20
0.16
0.40
0.52
5.24
MgO
2.22
2.49
2.18
3.04
1.39
2.40
1.90
2.19
2.13
2.83
2.64
2.81
15.62
4.60
2.06
2.33
2.54
2.58
4.44
0.01 0.01 0.07
0.01 0.04 0.10
0.02 0.02 0.13
0.01 0.01 0.22
0.01 0.01 0.09
0.01 0.01 0.18
0.02 0.02 0.07
CaO
12.95
7.58
15.77
10.74
0.01 0.01 0.05
0.01 0.01 0.11
0.05 0.07 0.05
0.02 0.01 0.08
L.O.I.
13.81
10.84
16.75
11.44
33.03
15.78
6.76
10.24
6.98
11.54
7.66
11.38
H.20-
0.23
0.30
0.37
0.38
0.36
0.35
0.20
0.40
0.32
0.53
0.29
0.37
Fe#
0.42
0.46
0.26
0.26
0.10
0.38
0.44
0.68
0.57
0.81
0.57
0.38
100.69
99.95
100.05
99.40
99.83
99.30
100.74
100.30
100.49
100.63
100.13
99.73
Na,20 K_oO P2Os
Total
33.69
0.03 0.04 0.04
Partsper million Zn Cu
11 19
17 25
17 20
31 24
9 17
13 22
12 23
17 23
15 21
15 21
16 27
37
Ni
39
43
42
43
35
41
45
43
44
38
54
52
6
6
9
