American Mineralogisl, Volume 66, pages 278-289, I98I Carbonatesand pyroxenoidsfrom the manganesedeposit near Bald Knob, North Carolina' Gnnv A. WrNrBn,2 ERrc J. ESSENEeNo Doxero R. Peecon Department of Geological Sciences,University of Michigan Ann Arbor, Michigan 48109 Abstract Studiesof minslals from a metamorphosedmanganese deposit at Bald Knob, North Caro- lina, yield estimatesof temperaturesand local fluid compositionsat the peak of metamorph- ism. Electron microprobe analysesof Mn-Ca carbonatesdemonstrate that they span the rho- dochrosite-kutnahorite solvus without a break, suggesting metamorphic temperature >550oC. The adjacent lid-nmphibolite facies metamorphism implies temperaturesnot greatly exceedingthis value. Pressureis estimatedat 5tl kbar basedon nearby occurrences of aluminosilicates.Compositional data on coexistingrhodonite and pyroxmangite sriggests that a narrow miscibility gap separatesthe two phasesat theseP-Tconditions. The assem- blage rhodonite/pyroxmangite-rhodochrosite-quartz requires water-rich fluids during meta- , morphism. The common occurrencesof the accessorymins16fu cattierite (CoSr), alabandite (MnS), and pyrophanite (MnTiOr) at Bald Krob require a relatively reduced and sulfidizing local metamorphic environment. Occasional finds of rhodonitg,/pyroxmangite-alabandite- quartz and rhodonite/pyroxmangite-tephoite-alabanditerestrict fO2/fS2 to a narrow band between l0-'e/10-3 and l0-t6/100. Conditions were probably butrered throughout meta- morphism by reaction of a sulfur-rich, oxygen-poorprotolith. Introduction sediment composed primarily of quartz, manganese The manganesedeposit near Bald Knob, North carbonates, sulfides, and oxides, which was sub- Carolina, is located in Alleghany County, in the sequently metamorphosedto yield the present assem- northwestregion of the state.Sparta, North Carolina, blages.This conclusion is supported by data showing fies about 3 miles southwest,and Bald Knob is a half that the temperature for the deposit is compatible mile northwest. The deposit is enclosed by Pre- with that inferred from the local metamorphic assem- cambrian-early Paleozoicamphibolites, misa schists, blages. and garnet-mica schistsof the Piedmont terrane. Two parageneses,carbonate-rich and quartz-rich, The deposit was first describedby Ross and Kerr exist at Bald Knob. The carbonate-rich rocks are prinarily (1932)who reported two new minerals, galaxite and now undersaturated in SiO, and consist of alleghanyite.More recently, Peacoret al. (1974) de- Mn-Ca carbonates, alleghanyite, sonolite, mangan- jacobsite, scribed another new mineral, kellyite, from this de- humite, tephroite, galaxite, and kellyite, posit. The idealized formulae of these minerals and with accessoryalabandite, cattierite, cobaltite, apa- pyrophanite. quartz-rich those mentioned below are given in Table l, along tite, caryopilite, and The py- with their structure types. Although Ross and Kerr rocks contain quartz, spessartine,rhodonite, ascribeda hypogenevein origin for the manganese- roxmangite, Mn-Ca carbonate, tirodite, with minor rich body, the depositprobably representsan original pyrophanite and galaxite.The rocks are banded,and the constituent grains show typical interlocking metamorphic textures. The banding results from con- tContribution No. 362 from the Mineralogical Laboratory, De- centrations of one or more of the phases,probably partment of Geological Sciences,University of Michigan, Ann Ar- bor, Michigan 48109. reflecting original sedimentary variations. These ob- 2Presentaddress: Tennessee Valley Authority, P.O. Box 2957, servations lend support for a metamorphic origin of Casper,Wyomng82602. the deposit, as opposed to the vein origin suggested w03-0o4x/ 8| /0304-0278$02.00 278 IryINTER ET AL.: CARBONATES AND PYROXENOIDS. NORTH CAROLINA Table 1. Unusual Bald Knob minerals, their ideal formulae and at varioustemperatures at a total pressureof l0 kbar. structure types Their exploratory runs in the pressurerange 2 to 15 kbar indicate that the efect of pressure on the rela- l"1ineral Formula Stru cture tions is small, and in our study their results are as- sumedto be directly applicableto assemblageswhich equilibrated at -5 kbar. The solvus in the Mn-rich pyrophanite MnTi03 i lmenite part of the CaCOr-MnCO, system widens slightly galaxite MnAlr0O spinel with increasingMgCO, as it extendsinto the ternary jacobsite MnFe2 04 spinel system.At 600"C there is completesolid solution of alabandite MnS hal i te Ca-Mn-Mg carbonatesin the Ca-poorhalf of the sy- cattierite CoS, pyr i te tem for those compositionswith less than l7 mole cobaltite CoAsS arsenopyrite percent MgCOr. These systemsmay be used to esti- alleghanyite (MnrSi0O)rMn(0H,F),chondrodite mate the temperature at which natUral Ca-Mn car- manganhumite (Mnzsi04)3Mn(0H,F)2humite bonates equilibrated. Carbonates from the Bald sonoli te (Mn2si04)4Mn(0H,F)zcl inohumite Knob deposit were analyzed and compared to the tephroite Mn^Si0, ol ivine Z+ phase relations in order to restrict their temperature spessartine MnrAl i ga ,S ,0.,, rne! of equilibration. nhodonite MnSi0, rhodonite Exploratory X-ray diffraction studies of many pyroxma ng i te MnSi 03 pyroxmangite Bald Knob carbonatesshowed a wide range of Ca- ( kutna hori te CaMn COr ), dol omite Mn solid solution (Stein, written communication). ti rodi te MnrMSi (0H tremo US,0r, ), l i te Subsequentpartial analysesby electron microprobe yi ke11 te MnoAlOSir0.,O(0H), serpentine showed that Mn and Ca are the major cations in caryopi I i te MnUSiUOrU(0H)10* friedelite thesecarbonates, with Mg and Fe presentin only mi- nor amounts. Eleven thin sections with carbonate compositionsthat would span the general range of *Peacorand Essene(l 980) values found previously were chosenfor more com- plete analysis.A total of 67 grains from thesesections was analyzedon an ARL-EMXelectron microprobe at by Ross and Kerr. Our discussionis predicated on The University of Michigan. Details of the analytical the assumptionthat the Bald Knob deposit was in- proceduresare given in the appendix. deed equilibrated during the peak of regional meta- morphism. The experimental and thermodynamic data on manganesecarbonate, silicate, and sulfide Discussion systemsare applied to determine the metamorphic After converting the carbonate analyses to mole conditions at which the Bald Knob assemblages percent and extracting the carbonate components, formed. the carbonatesrange from CcrRcrr'to CcrrRc",with less than l07o (Mg,Fe)COr. These compositionsare Carbonatethermometry plotted on a ternary diagram (Fig. l), where FeCO, Quantitative electron microprobe analysesof car- is combinedwith MgCOr becausetheir effectson the bonateninerals from the Bald Knob depositsuggest solvusare approximatelythe sameat low concentra- that they can be approximated by the ternary system tions. Increasingthe MgCOr component of Ca-Mn CaCOr-MnCOr-MgCOr. Goldsmith and Graf carbonateswidens the solvusextending out from the (1957)investigated subsolidus relations in the system CaCOr-MnCO, binary and presumablyraises its up- CaCOr-MnCO, and found a solvus in the Mn-rich per temperaluls limi{ to someextent. Bald Knob car- half of the system.They found complete solid solu- bonate compositions are apparently unintemrpted by tion betweencalcite and rhodochrositeabove 550'C. the solvi in the system,and we infer that the mini- One limb of the solvusapproaches a l:l Ca:Mn mum temperatureof equilibration of these carbon- compositionat about 450oC,indicating a high degree ates was greater than 550'C, the peak of the kut- of cation ordering below this temperature corre- nahorite-rhodochrositesolvus (Goldsmith and Graf, sponding to kutnahorite, CaMn(COr)r. Goldsmith 1957). Country rocks are muscovite-oligoclase- and Graf (1960) also investigated subsolidus rela- tions in the ternary systemCaCOr-MnCOr-MgCO, 3 Cc = calcite (CaCO3),Rc = rhodochrosite(MnCO3). 280 WINTER ET AL.: CARBONATES AND PYROXENOIDS, NORTH CAROLINA Table 2. Analyses of Bald Knob carbonates in weighr perc€nt oxide component and mole p€rc€nt carbonate end member Sanple weight% li{ol % BK- Mno Cao lu1g0 Feo *C0Z !1nC03 CaC03 M9C03 FeC03 4-t 32.13 23.ll 53 I.13 40.42 49.29 44.85 4.14 l.7l 4-2 34.81 20.33 47 1.25 39.91 54.09 39.97 4.03 1.92 4-4 27.08 18.99 27 0.57 39.63 58.03 37.59 3.50 0.88 4-7 3t .10 23.62 77 0.78 40.22 47.95 46.06 4.81 1.18',l 77 0.87 39.4? 68.74 25.0t 4.89 .35 4-10 43.71 12.57 ',| 4-12 39.77 16.53 7't 0.16 39.62 62.24 32.72 3.87 .18 39.25 17.67 57 0.22 38.97 62.48 35.58 ',I.581.60 0.35 6-2 37.99 18.89 57 0.18 39 12 60.25 37.89 0.29 4t.88 t5.17 58 0 24 38.66 67.21 30.79 1.62 0.39 6-4 44.40 13.21 67 0.32 38.83 70.94 26.70 1.87 0.51 8-2 22.56 31.48 0.88 2.08 40.92 24.19 60.36 2.34 3.ll 8-6 55.45 3.57 0.61 1.21 38.5789.10 7.26 1.72 1.91 91.25 6.65 0.95 I .15 MqCq + ftCO3 Mn C03 8-7 56.61 3.26 0.34 0.72 38.45 8-l 2 54.96 3 .41 0.78 2.51 39.12 87.06 6.82 2.19 3.93 8-18 32.02 20.74 1.85 4.40 40.84 48.61 39.84 4.95 6.60 Fig. l. Triangular composition diagram superimposedon the 8-19 27.28 27.10 1.06 2.41 40.85 41.41 52.05 2.84 3.70 phase diagram of the CaCO3-MgCO3-MnCO3slstem, showing 10-2 the solvi at 500oC(after Goldsmith and Graf, 1960).Bald Knob 24.84 26.58 I .59 2.52 39.55 38. 96 52.75 4.39 3 . 90 l3-t 29.53 I 6.49 0.53 0.24 38.l8 64 2t 33.89 1.52 0.38 carbonate compositions are plotted. FeCO3 is combined with t3-2 39.73 16.60 0.55 0.21 38.41 64.17 33.92 l.s8 0.84 MgCO3 in this plot.