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Axinite Mineral Group in Low-Grade Regionally Metamorphosed Rocks In AmericanMineralogist, Volume 65, pages 1119-1129, 1980 Axinite mineral group in low-graderegionally metamorphosedrocks in southem New Zealand leN J. PnrNcre AND YosuKE KAwAcHI Geology Department, Univerity of Otago Dunedin, New Zealand Abstract In southern New Zealand, minerals of the axinite group are widespread in vein assem- blages in regionally metamorphosed rocks of the prehnite-pumpellyite, pumpellyite-acti- nolite, and chlorite zone greenschistfacies. Fe, Mg-axinites, approachingend-member fer- roaxinite in composition, along with quartz and often prehnite, pumpellyite, iron-rich epidote, and chlorite fll veins in spilitized volcanic and greywacke lithologies. Tinzenite and more cofilmonly manganaxinites occur in quartz veins in nearby femrginous and manga- niferous cherts. Chemical analysesof vein axinites are presented,as w€ll as analysesof four porphyroblastic ferroan manganaxinites which occur as rock-forming minerals at widely sep- arated localities. Compositional variability in published axinite analysesalong with those of this study can be attributed partly to formation temperatures.A low-temperature miscibility gap may exist in the axinite group. Tinzsnile or manganaxinite and ferroaxinite are stable in low-grade metamorphic rocks of appropriate compositions, whereas ferroan manganaxinites and man- ganonn ferroaxinites occur in some pegmatites, skarns, and regionally metamorphosedrocks which equilibrated at more elevated temperatures. For a wide lange of burk compositionsin biotite and garnet zone greenschistfacies and amphibolite facies rocks of southern New Zealand, tourmaline is the only observedborosili- cate phase. At metamorphic conditions typical of these grades,axinite minerals would be re- stricted to relatively Ca-rich lithologies by a reaction of the form: 3 ferroaxinite + 2 chlorite + 2 albite * 5 quartz : 2 tourmaline * 4 epidote + 2 actinolite * 5 water Introduction spread as being mainly due to host rock composition. The general formula for the axinite group pro- Minerals of the axinite group have long been rec- posedby Lumpkin and Ribbe (1979)is: ognizedas typical phasesin manganese-oredeposits, skarns, and pegmatitesand associatedhigh-temper- [(Mn,Fe'*,M g,Zn Al)(Car-,Mn ) (Alz-,Fef*)]]I atureenyironments. Ozaki (1970,1972\ examined the (oH,_-o-x(Brs*--Al,)I'o3ol relationship between mode of occurrence of axinite and chemical composition. He distinguished five dif- where w < 1, r ( l, y 11 l, z 11 l, and VI and IV ferent categories:manganese and femrginous ore de- represent co-ordination of the cations. Sanero posits,pegmatites, contact metamorphic and metaso- and Gottardi (1968) have clarifed the nomen- matic ore deposits, regionally metamorphosedrocks, clature and defined the end members ferroaxinite and veins in igneous and sedimentary rocks. When [Fe2*CarAlrBSioO,r(OH)]and manganaxinite axinite analysesfrom each occurrence are plotted on [Mn'*CarAlrBSLO,r(OH)]. The name ferroaxinite is a Mn-Ca-Fe diagram the manganesecontents of ax- applied to thoseaxinites with Ca > 1.5and Fe ) Mn, inites in each rock type are shown to decreasein the and manganaxinitewhere Ca > 1.5 and Mn > Fe. above order, although considerable overlap is ob- Tinzenite includes those axinites with Ca < 1.5 and served(Fig. l). Ozaki interpreted this compositional Mn > Fe which approach the empirical composition ffi03-.00/.x/80/ I r 12-l l 19$02.00 ll19 I 120 PRINGLE AND KAWACHI: AXINITE \ l. manganese and ferrugrnous ore deposits. 2. pegmatites 3. contact netamorphic and metasomatic ore dePosits 4, regional metamorphic rocks 5. veins in igneous and sedinentary rocks. 30 lln Irc / \ Fig. l. Triangular Mn-Ca-Fe diagram after Ozaki (1972), showing compositional variations of axinites in different [thologies. MnrCaAl,BSLO,s(OH).Sanero and Gottardi (1968), records of axinite from veins in regionally metamor- followed by Ozaki (1969, 1972) and Lumpkin 6d phosed rocks of grads higher than zeolite facies have Ribbe (1979), showed that substitution in the group been reported. The majority of these are in rocks is principally in two series, one from ferroaxinite to 6pafaining mineral assemblagescharacteristic of the manganaxinite, the other fron manganildnite to- amphibolite facies and lower grade. wards tinzenite. Recent workers on low-grade metamorphic rocks Axinite in low-grade regionally metamorphosed in southernNew Zealand have recognizedminerals of rocks was first reported by Kojima (194) in the the axinite group at various localities in several dis- Sambagawametamorphic belt in central Japan. Here tinct terranes(Coombs et al., 1976)flanking the axis axinite occurs in a stilpnomelane-bearing band in of the Haast Schist(e.9. Mason, 1959;Mansergh and greenschist.The only extended account ofaxinite as Watters, 1970;Read and Reay, l97l; Andrews et al., a fock-forming mineral in schistsis by Nureki (1967), 1974; this study) (Fig. 2). Four occurrenoesare as who described and partially arralyzedmanganaxinite rock-forming minerals, others are confined to veins, from transitional blueschist-greenschistfacies rocks but all are restricted to rocks of the chlorite zone of the Sangunmetamorphiczonein soutlwest Japan, gteenschist facies, pumpellyite-actinotte facies, and where it occurs as inclusion-studded porphyroblasts prehnite-pumFellyite facies. We suspectthat such and as clear grains in nearby quartz-albite-ildnite occurrencesare relatively abundant and that miner- veins. The manganaxinite-bearing assemblagescon- als of the axinite group are more coilrmon in region- tain stilpnomelane * chlorite + epidote + albite + ally metamorphosed sequencesthan has generally qrraftz + muscovite + calcite + sphene + opaque. been appreciated. Examples of axinite-bearing veins in regionally metamorphic rocks are more numerous. Simonen Field relations and petrography and Wiik (1952) describe ferroaxinite-quartz-calcite veins in amphibolites and basic intrusive rocks in Torlesseterrane and adjacentHaast Schistterrane Finland. In a review of axinite oocurrencesin the These terranes consist of Late Paleozoic to Meso- Norwegian Caledonides,Carstens (1965) emphasized zoic greywackes and argillites together with'rare the association of epigenetic axinite-bearing veins cherts, spilitized pillow basalts, limestones and con- with metabasite lithologies. Worldwide, over forty glomerates, and their schistose derivatives. Axinite PRINGLE AND I(AWACHI: AXINITE tl2l Fig. 2. Axinite localities in the southern half of the South Islant axinite occurrences:(a) Lord Range(Andrews et al-,1914);(b) Pertl (d) Humboldt Mountains (Bishop et al.,1976)-Dots are localitiesof PareoraGorge (OU a672); (3) Kirkliston Mountains (OU 33912) 35524,15597, 35526, 35523, 35528); (5) Hawkdun Range (Gradv, 35595,35596,28558,33366,28n$;Q) Akatore(Read and Reay,l97l) (OU 25686)' hema- minerals along with other low-grade metamorphic matic tourmaline, manganoanstilpnomelane, phasesoccur in veins at numerous locatties. Axinite tite, and abundant calcite' porphyroblasts, however, are more restricted in dis- At lower grade, in pumpellyite-actinolite facies man- tribution. rocks, veins containing quartz and pale brown Near Dansey Pass, thick axinite-bearing quaftz ganaxinite (OU 35527) occur in massive hematitic veins cut a schistosemetachert in chlorite zone green- metacherts. Pinkish brown and pale purple ferroaxi- veins schist facies rocks of textural zone IIIA (Bishop, nites are cofilmon constituents of anastomosing 35524' 1972). TtLe axinite, a manganaxirdte (OU 25336),' in nearby metabasitelithologies (OU 35523' in thick- coexists with hematite and manganoan brunsvigite 35525,35526, 35597). The latter veins vary in ad- which contains 4.04.3 weight peraent MnO. Minor nessfrom a fe* mm to severalcm and contain, of cal- tourmaline, along with spessartineand muscovite, is dition to ubiquitous quattz, variable amounts present in the host rock but not in the axinite-bearing cite and iron-rich epidote (Psru-Psr.).lron-rich cm veins. Elsewherein the chlorite zone, subhedral man- epidoteis often concentratedas border zones'l-3 ganaxinite porphyroblasts(OU 35528)studded with wide in thicker ferroaxinite-bearing veins. Albite, minute inclusions predominantly of quartz and hem- pumpellyite, chlorite, asbestiform actinolite, sericite, phases' atite occur within stilpnomelane-chlorite-epidote la- pyrite, and sphene are common accessory minas in a fine-grainedhematitic metachert.Also, an Mg-pumpellyite containing 2.5 weight percent MgO impure marble contains small grains of ferroan man- coexistswith ferroaxinite in OU 35526. ganaxinite (OU 25335) mantled by manganoan Manganaxinites in veins from two other localities 70 km brunsvigite. Other phases in this rock include pris- were analyzed. In the northern Malvern Hills, west of Christchurch, buff-brown manganaxinite #1676)fills thin veinlets along with quartz in a I numbers are Geology Department, UniversitY of (OU Sample metachert. The host Otago catalogue numbers. rhodonite-pyrolusite-bearing tr22 PRINGLE AND KAIIACHI: AXINITE rock forms part of a 600-m-thick sequenceof me_ inclusions of quartz and hematite which causea pink tabasites, hematitic and manganiferous metacherts, to pale brown coloration of cores in some grains and marbles metamorphosed to prehnite_pump_ (Fig. 3). ellyite facies. The Torlesse rocks pass gradationally into the A large boulder of pumpellyite-bearing metachert Haast Schist terrane with an increasing development in river gravels of Station Stream, which drains the of penetrative schistosity. Metamorphic grade in eastern Kirkliston
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