Ecandrewsite – Zincian Pyrophanite from Lujavrite, Pilansberg Alkaline Complex, South Africa

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The Canadian Mineralogist Vol. 42, pp. 1169-1178 (2004)

ECANDREWSITE – ZINCIAN PYROPHANITE FROM LUJAVRITE, PILANSBERG ALKALINE COMPLEX, SOUTH AFRICA

ROGER H. MITCHELL§ AND RUSLAN P. LIFEROVICH¶ Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario P7B 5E1, Canada

ABSTRACT

Ilmenite-group titanates are relatively common accessory minerals in agpaitic nepheline syenite (lujavrite) in the Pilansberg alkaline complex, South Africa. They crystallized in three generations and form a continuous ecandrewsite – pyrophanite solid- solution extending from (Zn0.81Mn0.12Fe0.07)TiO3 to (Mn0.93Fe0.7)TiO3. The earliest generation of ferroan manganoan ecandrewsite of widely varying composition (Zn0.81–0.53Mn0.33–0.12Fe0.11–0.04)TiO3 is replaced by zincian pyrophanite (Mn0.82–0.34Zn0.50– 0.20Fe0.13–0.06)TiO3. The latest generation is pyrophanite with only traces of zinc, showing a compositional evolution toward ferroan pyrophanite. The three generations of Zn–Mn titanates recognized on the basis of their paragenesis and composition crystallized during the subsolidus alteration of the lujavrite by evolving deuteric alkaline fluids. The enrichment in Zn, Mn, and Ti in these fluids resulted from the decomposition of earlier-formed magmatic-stage Zn- and Mn-bearing titano- and zirconosilicates. This is the first reported occurrence of a near-complete ecandrewsite – pyrophanite (ZnTiO3 – MnTiO3) solid- solution series.

Keywords: ecandrewsite, pyrophanite, solid-solution series, subsolidus alteration, lujavrite, Pilansberg, South Africa.

SOMMAIRE

Les titanates du groupe de l’ilménite sont des minéraux accessoires relativement répandus dans les venues de syénite néphélinique agpaïtique (lujavrite) du complexe alcalin de Pilansberg, en Afrique du Sud. Ils ont cristallisé en trois générations et définissent une solution solide continue entre ecandrewsite et pyrophanite, allant de (Zn0.81Mn0.12Fe0.07)TiO3 à (Mn0.93Fe0.7)TiO3. La génération la plus précoce d’ecandrewsite ferreuse et manganésifère, de composition très variable, (Zn0.81– 0.53Mn0.33–0.12Fe0.11–0.04)TiO3, se voit remplacée par la pyrophanite zincifère, (Mn0.82–0.34Zn0.50–0.20Fe0.13–0.06)TiO3. La génération tardive de cette série est faite de pyrophanite avec seulement des traces de zinc, et montre une évolution compositionnelle vers la pyrophanite ferreuse. Ces trois générations de titanates de Zn–Mn, établies selon des critères paragénétiques et compositionnels, ont cristallisé au cours de l’altération subsolidus de la lujavrite en présence de fluides alcalins déuteriques en évolution. L’enrichissement de ces fluides en Zn, Mn, et Ti est le résultat de la déstabilisation de titanosilicates et de zirconosilicates précoces contenant du Zn et du Mn. Nous établissons pour la première fois la présence d’une solution solide quasiment complète entre ecandrewsite et pyrophanite (ZnTiO3 – MnTiO3).

(Traduit par la Rédaction)

Mots-clés: ecandrewsite, pyrophanite, solution solide, altération subsolidus, lujavrite, Pilansberg, Afrique du Sud.

INTRODUCTION a contact between granitic and noritic units of the Bushveld complex. Shand (1928) initially described the The Pilansberg alkaline complex, located ca. 50 km geology of the complex. Lurie (1986) briefly consid- north–northwest of Rustenburg, in the Republic of ered its economic potential, and aspects of the mineral- South Africa, is one of the largest and least-studied dif- ogy of the eudialyte-bearing rocks were studied by ferentiated intrusions of agpaitic sodic syenite. The par- Olivo & Williams-Jones (1999). During a comprehen- tially unroofed ring complex (~25 24 km), was sive study (Liferovich & Mitchell, in prep.) of the min- emplaced at an intraplate extensional structure in the eralogy of the eudialyte nepheline syenite, i.e., the green Middle Proterozoic (1250 ± 50 Ma; Retief 1963), along syenite of Shand (1928), occurring at Pilansberg, we

§ E-mail address: [email protected] ¶ Permanent address: Geological Institute Kola Scientific Centre, Russian Academy of Science, 14 Fersman Street, RU–184200 Apatity, Russia 1170 THE CANADIAN MINERALOGIST recognized the occurrence of ecandrewsite, zincian erals. The rock also contains evenly distributed creamy pyrophanite and pyrophanite in hypersolvus astro- fine-grained pseudomorphs after euhedral crystals of phyllite nepheline syenite or lujavrite (sensu Brögger trigonal or ditrigonal habit. These are aligned parallel 1890). In this paper, we describe the paragenesis and to the feldspar prisms and comprise 12–15 vol.% of the origin of these rare ilmenite-structured titanates, and initial assemblage. The habit of the pseudomorphs provide the first description of a near-complete matches that of early generations of eudialyte described ecandrewsite – pyrophanite (ZnTiO3 – MnTiO3) solid from the Khibina and Lovozero complexes by solution. Kostyleva-Labuntsova et al. (1978). The pseudomorphs consist of titanite, zoned subhedral zircon, optically THE GEOLOGICAL CONTEXT unidentifiable hydrous Na–Zr silicates(s), analcime, natrolite, late-forming secondary strontian eudialyte, The Pilansberg complex consists primarily of pho- allanite, pyrophanite, siliceous pyrochlore, oxides of nolitic-to-trachytic pyroclastic and lava flow sequences Mn, and very small crystals of diverse oxides and sili- intruded by large arcuate plugs of agpaitic feldspa- cates (see below). In less-altered lujavrites from thoidal syenite, together with a semicircular dike of Pilansberg, primary eudialyte is partially replaced by a tinguaite. From the periphery to the center, the complex similar mineral assemblage, indicating that the pseudo- is composed of red nepheline syenite, strongly altered morphs in the sample studied are formed after primary amphibole foyaite, tinguaite, white foyaite (subsolvus eudialyte. Accessory minerals present include: calcian nepheline syenite), lujavrite (or hypersolvus trachytoid niobian loparite-(Ce), manganoan astrophyllite, titanite, green nepheline syenite), and red foyaite. Inwardly dip- fluorapatite, sphalerite, and banalsite. Primary albite is ping dikes of diverse alkaline porphyry, microsyenite not present, and opaque minerals occur as rare single and tinguaite also are present. The emplacement of grains or fine-grained aggregates. The texture and min- lujavrite was subsequent to that of the red and white eralogy of the samples examined are similar to those foyaites, and the cone sheets of tinguaite, but was prior exhibited by lujavrites from the Lovozero, Ilímaussaq to the late dikes of tinguaite (Shand 1928, Olivo & Wil- and Poços de Caldas agpaitic complexes (Gerasimovksy liams-Jones 1999). et al. 1966, Ulbrich & Ulbrich 2000, Markl 2001, Pekov The agpaitic character of the Pilansberg rocks is in- 2001). dicated by the presence of the sodic minerals: eudialyte, Postmagmatic processes have resulted in the corro- lamprophyllite, astrophyllite–kupletskite, and mosan- sion and subsolidus replacement of the primary mineral drite. Intense postmagmatic processes have left their assemblage throughout the whole rock by deuteric al- imprint on all the syenites and generated a complex kaline fluids. Secondary voids are not observed in the mineralogy by alteration of the primary mineral assem- altered rocks, implying that the alteration was “volume blages. These processes superimposed exotic low-tem- conserving” and similar to the autometasomatic alter- perature mineralization involving Na, F, P, Mn, Zn, Y, ation described by Markl & Baumgartner (2002) in the light rare-earth elements, Zr, Nb, Sr, Pb, U (Lurie lujavrite from Ilímaussaq. 1986) upon pre-existing rocks, and is analogous to the Ecandrewsite, zincian pyrophanite and pyrophanite late-stage alteration recognized in the similar Ilímaussaq occur in altered lujavrite. In these rocks, the assemblages and Lovozero agpaitic sodic complexes (Khomyakov of secondary phases forms four paragenetic groups dif- 1995, Markl & Baumgartner 2002). fering in terms of the alkalinity modulus of Khomyakov (1990, 1995), which in order of formation and decreas- MINERALOGY OF ECANDREWSITE-BEARING ing alkalinity are: LUJAVRITE (1) Moderately agpaitic assemblages containing ecandrewsite (sensu lato), together with allanite-(Ce), The inequigranular samples of melanocratic lujavrite zircon and Si-bearing pyrochlore. studied were collected from a hillside outcrop in the (2) Hyperagpaitic assemblages containing zincian south-central segment of the Pilansberg complex. This pyrophanite formed after ecandrewsite together with lujavrite has a well-defined trachytic texture, is medium ferroan kupletskite, stronalsite, strontian fluorapatite, to fine grained, and homogeneous on a hand-sample unidentified REE-bearing- and Na–Zr-bearing silicates. scale in terms of the distribution of mafic minerals. The (3) Moderately alkaline assemblages containing Zn- rocks consist of euhedral microcline prisms (25–30 poor, ferroan pyrophanite formed after lamprophyllite vol.%) together with analcite or natrolite pseudomorphs or kupletskite together with secondary Sr–Nb-bearing after subhedral nepheline (15–20 vol.%), set in a matrix eudialyte, unidentified Pb–Mn and Zn–Mn oxides, and of flow-aligned aggregates of thin prisms of green spherulitic zircon. aegirine. The pseudomorphs after nepheline contain (4) Low-alkalinity assemblages of a modally sili- numerous very small prismatic needle-like crystals of ceous stage containing silica, allanite-(La), bastnäsite- aegirine. Brown-to-bronze astrophyllite occurs as 0.5– (La), sulfatian monazite-(La), manganoan biotite, Nb- 2 cm laths comprising 8–10 vol.% of the rock. Sodalite and Zr-bearing titanite. (8–12 vol.%) fills the interstices between the felsic min- ECANDREWSITE – ZINCIAN PYROPHANITE SERIES, PILANSBERG COMPLEX 1171

Associated with the above are a plethora of very hypersolvus lujavrite is shown in Figure 1. The mineral small (typically <5 m) phases. These include: U–Si- compositions and textural relationships suggest that al- bearing pyrochlore, weloganite-like Sr–Zr silicate, teration involved at least two stages of subsolidus over- zirconian astrophyllite, K-bearing zirconosilicates, printing by fluids released from cooling nepheline REE-arsenates and phospho-arsenates, gasparite-(Ce), syenitic magma. These fluids in terms of their composi- retzian-(Ce), coronadite-like Pb–Mn, Pb–Zn–Mn and tion followed the “alkalinity wave” concept introduced Pb–Zn–Th oxides, huttonite, and olekminskite. Many by Khomyakov (1991, 1995) and refined by Markl & of the minerals occur in association with natrolite or are Baumgartner (2002). A detailed discussion of this pro- found as inclusions in acicular aegirine. cess (Liferovich & Mitchell, in prep.) is beyond the A generalized paragenetic sequence for the mineral scope of this paper. assemblages found in this metasomatically altered

FIG. 1. Generalized sequence of formation of hypersolvus trachytic nepheline syenite (lujavrite) in the Pilansberg complex, South Africa. Minerals given in parentheses were replaced during postmagmatic alteration (as inferred by comparison with less-altered lujavrite). Markl & Baumgartner (2002) gave details of the evolutionary path of pH in the sodic fluids related to lujavrites. 1172 THE CANADIAN MINERALOGIST

PARAGENESIS OF ECANDREWSITE pyrochlore. Zincian pyrophanite can be distinguished AND ZINCIAN PYROPHANITE from the earlier-forming ecandrewsite only in back- scattered electron images. Zinc-poor to zinc-free Ecandrewsite and zincian pyrophanite are opaque or pyrophanite was the latest titanate to form (Figs. 2b, c). orange-brown in thinner fragments. Ecandrewsite pri- It has been observed to corrode and replace ferroan marily forms euhedral plates (0.2–0.4 mm in length) that kupletskite and lamprophyllite (Fig. 2d). are strongly corroded and mantled by aggregates of All three generations of Zn–Mn–Fe titanates became zincian pyrophanite (Fig. 2a). Smaller subhedral grains unstable during the later stages of alteration and were of ecandrewsite are enclosed by kupletskite or analcime. corroded and partially replaced by allanite-(La), titanite Rarely, very small grains of well-preserved ecandrews- and manganoan biotite (Fig. 2b). A Si-bearing ite fill the interstices of the thin prismatic crystals of pyrochlore also replaces niobian pyrophanite. At the aegirine within the analcime. Although the relationships contact with natrolite, the pyrophanite is commonly re- of ecandrewsite to the primary minerals have been ob- placed by titanite and rarely by a lamprophyllite-like scured by the autometasomatic alteration of the rock, phase, together with plumbopyrochlore. the absence of inclusions of primary minerals in ecandrewsite and its association with analcime replac- ANALYTICAL METHODS ing nepheline suggest a subsolidus origin. Zincian pyrophanite postdates ecandrewsite (Figs. Mineral compositions were determined at Lakehead 2a, b) and encloses manganoan astrophyllite, ferroan University by energy-dispersion X-ray spectrometry kupletskite, hilairite, allanite-(Ce) and Si-bearing (EDS) using a JEOL JSM–5900 scanning electron mi-

FIG. 2. Back-scattered electron images of some characteristic modes of occurrence of ecandrewsite, zincian pyrophanite and pyrophanite in lujavrite. a) Ecandrewsite cemented by zincian pyrophanite; b) ecandrewsite and zincian pyrophanite in interstices of prismatic aegirine, in association with hilairite; c) latest laths of pyrophanite growing on aegirine crystals (the latter have entrapped irregular grains of titanite); d) aggregates of pyrophanite growing on aegirine, which, in turn, covers a cur- rently decomposed large crystal of primary silicate (lamprophyllite?). Note that the black areas are low-atomic-number phases, mostly zeolites and other silicates. I ecandrewsite, II zincian pyrophanite, III pyrophanite, Ac analcime, Aeg aegirine, Bt biotite, Hil hilairite, L pseudomorphs after lampropyllite, Ln LREE silicate [lucasite-(Ce)?], and carbonates, Ph siliceous pyrochlore, T titanite, Z latest zeolite (natrolite). ECANDREWSITE – ZINCIAN PYROPHANITE SERIES, PILANSBERG COMPLEX 1173 croscope equipped with a Link ISIS 300 analytical sys- to 10% R2+, regardless of analytical method. Similar tem incorporating a Super ATW light-element detector cation deficiencies are evident in data given for (133 eV FwHm MnK). Raw EDS spectra were ac- ecandrewsite by Whitney et al. (1993). quired for 130 s (live time) with an accelerating voltage of 20 kV and a beam current of 0.475 nA on a Ni stan- MINERAL COMPOSITIONS dard. Spectra were processed with the Link ISIS SEMQUANT software package with full ZAF correc- Representative compositions of members of the tions applied. The following well-characterized mineral ecandrewsite – pyrophanite series are given in Table 1. and synthetic standards were used: jadeite or loparite Figure 3 illustrates all data obtained in terms of the Zn– (Na), orthoclase (K), corundum (Al), ilmenite (Fe,Ti), Mn–Fe and Mg–Mn–Fe systems, with comparative data periclase (Mg), zircon (Zr), wollastonite (Ca), lueshite for ilmenite-group titanates from carbonatite, kimberlite (Nb), loparite (REE), barite (Ba), SrTiO3 (Sr), Zn metal; and other agpaitic syenites. Although the compositional Mn metal, pyroxene glass DJ35 (Si, Na). For compara- fields of ecandrewsite (field I), zincian pyrophanite tive purposes, ecandrewsite was also analyzed by stan- (field II), and pyrophanite (field III) do not overlap, dard wavelength-dispersion methods using a Cameca these grade nearly continuously from ecandrewsite with SX–50 electron microprobe at the University of ~80 mol.% ZnTiO3 to Zn-free late pyrophanite. Manitoba. All of the titanates show considerable variation in Ferric and ferrous iron were calculated from micro- terms of the FeTiO3 component, and only some cores probe-obtained values of FeOT using the method of of relatively Zn-poor ecandrewsite and their Zn-rich Droop (1987). This method can be used to assess only margins have no detectable Fe. Al is typically below 3+ the minimum Fe content and assumes that Mn is the detection limit (0.2 wt.% Al2O3) and does not ex- 2+ present only as Mn and that no cation vacancies are ceed 0.4 wt.% Al2O3. Nb is present and distributed ir- present. These assumptions might not be correct, as the regularly within and between crystals. The abundance minerals of the ecandrewsite – pyrophanite series from varies from trace to: 2.5%, 2.9%, and 4.1 wt.% Nb2O5 Pilansberg exhibit cation deficiencies at the A site of up (average) in ecandrewsite, zincian pyrophanite and 1174 THE CANADIAN MINERALOGIST pyrophanite, respectively. Minor Zr is present (<0.5 pyrophanite (<7.4 mol.% ZnTiO3; Fig. 3). This mineral wt.% ZrO2), and Mg is below the limits of detection is overgrown by material transitional between manga- (<0.2 wt.% MgO). The incorporation of Nb (and Zr) in noan ecandrewsite and zincian pyrophanite (41–57 the structure does not follow any of the schemes sug- mol.% ZnTiO3; Fig. 3), and is associated with natrolite, gested by Chakhmouradian & Mitchell (1999). calcite, and strontianite. All titanates at Poços de Caldas Ecandrewsite and zincian pyrophanite are composi- have low iron contents (<5 mol. % FeTiO3). tionally zoned. The least-altered grains of ecandrewsite Manganoan ilmenite and ferroan pyrophanite are in aegirine interstices have a zinc-enriched margin typical minerals of agpaitic and hyperagpaitic pegma- (Figs. 2a, 3), and evolve from a core of (Zn0.56Mn0.35 tite, related metasomatic rocks and postmagmatic zeo- Fe0.06)(Ti0.99Nb0.01)O3 to a rim of (Zn0.66Mn0.27Fe0.04) lite-rich parageneses in the Lovozero and Khibina (Ti0.98Nb0.01Zr0.01)O3. The most zincian compositions complexes (Khomyakov 1995, Pekov 2001, 2002), at vary from interstitial Fe-free manganoan ecandrewsite Gordon Butte, Montana (Chakhmouradian & Mitchell to ferroan manganoan ecandrewsite occurring in aggre- 2002), and at Mont Saint-Hilaire (Horváth & Gault gates cemented and partially replaced by pyrophanite 1990). Pyrophanite has been described as a common (Figs. 2a, b). accessory phase in mineralized albite-dominant tuffs in Zincian pyrophanite shows an opposite pattern of the Pilansberg complex and was considered as a poten- zonation, with Zn depletion toward the margins of crys- tial source for Nb by Lurie (1986), although data on its tals. Compositional evolution is from manganoan composition were not provided. ferroan ecandrewsite to low-Fe pyrophanite with less Agpaitic nepheline syenite complexes and their sodic than 5 mol.% ZnTiO3 (Fig. 3). Isolated grains of hyperagpaitic derivatives are typically enriched in zinc ecandrewsite (Zn0.72Mn0.19Fe0.09)TiO3 and zincian compared to the lithosphere (Gerasimovksy 1969, Markl pyrophanite (Mn0.82Zn0.12Fe0.06)TiO3 occur within inter- 2001). According to Pekov (2002), the common min- stices among aegirine crystals. Corrosion, replacement eral of zinc in peralkaline rocks is Fe–Mn-bearing and cementation textures are absent, suggesting that sphalerite. Early postmagmatic Zn mineralization is also zincian pyrophanite did not form by the replacement of present as H2O-free minerals such as willemite, ecandrewsite, and thus represents a distinct generation genthelvite, landauite, murataite, nordite and osumillite. of Zn–Mn titanate. Subsolidus, low-temperature autohydration processes result in sequestration of Zn in clay minerals, zeolites DISCUSSION and labuntsovite-group minerals. These observations are at variance with the mineralogy and geochemistry of Ecandrewsite has not previously been described zinc in silica- and alumina-saturated systems (Neumann from igneous rocks. Ferroan manganoan varieties have 1949). In the latter, thermodynamic data indicate that been recognized from diverse regionally metamor- VIZn2+ is more stable than IVZn2+ at high temperature, phosed sedimentary rocks (Birch et al. 1988, Whitney and that Zn is sequestered in gahnite (ZnAl2O4) rather et al. 1993). The most Zn-rich example reported to date than as ZnTiO3 (Whitney et al. 1993). From field and occurs in a pelitic schist found in Death Valley, Califor- textural evidence, Plimer (1990) argued that zincian il- nia, where it forms a solid solution extending from menite – ecandrewsite solid solutions form by reaction ferroan ecandrewsite with 84 mol.% ZnTiO3 to near of titanium dioxide(s) or ilmenite with zincian clays or end-member composition ilmenite with 0.9 wt.% MnO carbonates during prograde metamorphism. This pro- and traces of Zn (Whitney et al. 1993). cess results in disequilibrium crystallization with sig- Manganoan zincian ilmenite, with up to 5.4 wt.% nificant intergranular variations in the Zn to Fe ratio of ZnO and 9.0% MnO, has been described from miarolitic the products. Both sphalerite desulfidation and break- cavities in a peralkaline rhyolite from Cape Ashizuri, down of zinc-bearing silicates and oxides during retro- southwestern Japan (Nakashima & Imaoka 1998). Suwa grade alteration of kyanite schists have also been et al. (1986) have also described minerals of the zincian reported to result in formation of ecandrewsite in pyrophanite – ilmenite series with up to 7.6 wt.% ZnO metasedimentary rocks (Whitney et al. 1993). Zincian also in a miarolitic paragenesis from alkaline granite ilmenite in association with aegirine and arfvedsonite is occurring at Kuiqi, Fuzhou, in China. This latter occur- also known to precipitate directly from hydrothermal rence represents a rather limited natural example of solid solutions exsolved from peralkaline rocks (Suwa et al. solution between ecandrewsite and pyrophanite, extend- 1987, Nakashima & Imaoka 1998). ing from 1.3 to 15.4 mol.% ZnTiO3 (Fig. 3). In the absence of experimental data on the partition- Recently, during a comparative study of trachytic ing of Zn into pyrophanite relative to complex silicates, subsolvus nepheline syenites from the Poços de Caldas we cannot assess quantitatively the conditions of for- peralkaline complex, in Brazil, we recognized a further mation of ecandrewsite and zincian pyrophanite. More- occurrence of Zn–Mn titanate (Liferovich & Mitchell, over, these minerals, at Pilansberg, show very wide in prep.). An early generation of this phase, in associa- intergranular variations in composition, indicating non- tion with manganoan sphalerite, fluorite and an uniden- equilibrium crystallization. In addition, Whitney et al. tified Na–Al–Zn silicate, is zincian to zinc-free (1993) showed disagreement between the compositions ECANDREWSITE – ZINCIAN PYROPHANITE SERIES, PILANSBERG COMPLEX 1175 of cation-deficient ecandrewsite and zincian ilmenite by the abundance of cognate volcanic rocks. The cool- and thermodynamic calculations of their relative stabil- ing rate and crystallization path of this hypersolvus ity. Unfortunately, because of the extensive postmag- lujavrite did not permit either fractional crystallization matic alteration of the ecandrewsite-bearing lujavrite, or segregation of volatile-rich fluids, which might un- there are no equilibrated mineral pairs of silicates and der other circumstances have given rise to discrete oxides suitable for geothermobarometry of the pegmatites and late hydrothermal veins. ecandrewsite and zincian pyrophanite parageneses. Experiments by Piotrowski & Edgar (1970) showed With respect to the genesis of ecandrewsite in that the H2O-saturated solidus of lujavrite lies at 450°C agpaitic rocks, none of the hypotheses described above for a H2O pressure of 0.1 GPa. Recently, Markl (2001) are directly applicable. For example, there is no textural showed that the temperature of the nepheline-to-anal- evidence in favor of its formation by replacement of cime conversion in hydrous albite-free lujavrite is in the sphalerite at Pilansberg. Although this mineral is present range of 420–200°C at a(SiO2) < 0.1 and aH2O ranging in less-altered lujavrite at Pilansberg, it is absent in the from 1.0 to 0.5. As ecandrewsite and zincian pyropha- ecandrewsite-bearing rock. Hence, it is possible that the nite appear to have formed contemporaneously with extensive deuteric alteration of the lujavrite has resulted analcime replacing nepheline, we assume that the titan- in the complete replacement of primary sphalerite coin- ates must have formed at temperatures below 400°C, cident with the breakdown of eudialyte and conversion i.e., they are subsolidus phases. of nepheline to analcime. However, in opposition to this Markl & Baumgartner (2002) have developed a hypothesis, sphalerite at Poços de Caldas seems to co- quantitative model for the evolution of the alkalinity of exist in equilibrium with ecandrewsite. In the absence fluid-rich sodic agpaitic syenites. They infer that the of other evidence, we conclude that the sources of Mn, Na:Cl ratio of the fluid buffers the alkalinity, such that Zn and Ti must lie in the decomposition of pre-existing the higher the aCl–, the lower the pH. The “volume-con- Ti–Mn-bearing silicates (eudialyte, lamprophyllite, serving” replacement of nepheline by analcime in a kupletskite, etc.) followed by concentration of these el- more-or-less closed system results in successively in- ements in the residual hydrothermal fluids. creasing alkalinity of the fluid, and a dramatic decrease Core-to-rim enrichment of Zn in ecandrewsite grains in pH in the final stages due to the sequestration of Na indicates increasing activity of Zn during the agpaitic in sodic species with Cl accumulating in the residual stage (Fig. 1). The opposite sense of zoning of zincian solutions. This model explains the formation of zircon, pyrophanite, formed during the hyperagpaitic stage, in- allanite-(Ce), Si-bearing pyrochlore and ecandrewsite dicates competition for zinc between coeval pyrophanite during the incipient stages of alteration of the lujavrite, and Na–Zn silicates (Fig. 1). The sequestration of Zn in as the pH is buffered until all interstitial sodalite and pyrochlore, Na zirconosilicates and eudialyte rather than primary Cl-bearing eudialyte is decomposed. Zinc is coeval pyrophanite suggests that Mn–Zn diadochy in the incorporated only in ecandrewsite and zincian ilmenite structure is not feasible at relatively low tem- pyrophanite at this stage. After complete replacement peratures, i.e., there is not a complete ecandrewsite– of sodalite and primary eudialyte, alkalinity increases pyrophanite solid solution at low temperatures. This as nepheline is replaced by Cl-free analcime, and at the conclusion is supported by our experimental data on the highest alkalinity Na–Zn silicates are deposited in pref- solid-solution series Mn1–xZnxTiO3, which suggest the erence to zincian pyrophanite. maximum solubility of Zn to be about 0.8 apfu at low pressure (Mitchell & Liferovich 2004). CONCLUSIONS As noted above, the evolution of the Pilansberg lujavrite can be divided in several superimposed stages The formation of zinc titanates belonging to the il- of differing alkalinity (Fig. 1). This evolution essentially menite structural group in lujavrite at Pilansberg is due follows that occurring in the most evolved rocks of the to subsolidus reaction of deuteric fluids with the primary Lovozero, Khibina and Ilímaussaq peralkaline sodic magmatic minerals. The titanates form a near-complete complexes (Khomyakov 1995, Markl & Baumgartner solid solution between ecandrewsite and pyrophanite. 2002). In these complexes, the postmagmatic hyperag- Changes in the composition of the titanates reflect paitic fluids have typically segregated as discrete changes in the alkalinity of the fluids involved. Inter- pegmatites and hydrothermal veins and not permeated estingly, there is only limited solid-solution among and altered the host rocks. Pilansberg differs in that the ecandrewsite, pyrophanite and ilmenite, even though highly evolved assemblages of postmagmatic minerals these minerals are isostructural. are superimposed directly on the magmatic agpaitic rocks. The textures and replacement relationships indi- ACKNOWLEDGEMENTS cate that the development of an in situ volatile-rich zeolite-dominant paragenesis is the result of retention of an This work is supported by the Natural Sciences and orthomagmatic volatile-rich fluid exsolved from the Engineering Research Council of Canada and Lakehead cooling agpaitic magma. That the fluid-rich lujavritic University. Alan Mackenzie and Anne Hammond are magma was emplaced at a shallow level is illustrated thanked for assistance with the analytical work and 1176 THE CANADIAN MINERALOGIST

1988); 2 San Valentin Pb–Zn Mine

egmatites (Kostyleva-Labuntsova

ian & Mitchell 1999); 8 kyanite schists,

et al.

rrows show the compositional trends. Other

rch

1987); 13 Poços de Caldas lujavrite, Brazil (our data).

et al.

1993); 9 Cape Ashizuri, Japan (Nakashima & Imaoka 1998); 10 worldwide kimberlite (Mitchell 1986); 11 Jacupiranga carbonatite a

et al.

1988); 3 Lovozero nepheline syenite and alkaline pegmatites (Pekov 2001; our data); 4 Khibina nepheline syenite and alkaline p

et al.

. 1978); 5 Kovdor magnesiophoscorite; 6 Seblyavr calciocarbonatites; 7 International’naya kimberlite pipe, Yakutia (Chakhmourad

occurrences: 1 type-locality ecandrewsite from siliceous metasedimentary rocks, Melbourne Rockwell mine, Little Broken Hill (Bi (Birch al Death Valley, California (Whitney jacupirangite (Gaspar & Wyllie 1999); 12 Kuiqi alkaline granite, Fujian, China (Suwa

. 3. Composition (mol.%) of ecandrewsite–pyrophanite from Pilansberg lujavrites. Generations are marked in Roman numerals; red a

F ECANDREWSITE – ZINCIAN PYROPHANITE SERIES, PILANSBERG COMPLEX 1177 sample preparation, respectively. Dr. R.C. (Jock) & KOZYREVA, L.V. (1978): Mineralogy of the Khibina Harmer is thanked for assistance with the collection of Complex II. Nauka, Moscow, Russia (in Russ.). the Pilansberg syenites. The Director of the Pilansberg National Park is thanked for permission to undertake LURIE, J. (1986): Mineralization of the Pilanesberg alkaline geological investigations in the park area. Ian Coulson, complex. In Mineral Deposits of South Africa 2 (C.R. Anhaeusser & S. Maske, eds.). The Geological Society of Gregor Markl and Bob Martin are thanked for construc- South Africa, Johannesburg, South Africa (2215-2228). tive comments on the initial draft of this paper. MARKL, G. (2001): A new type of silicate liquid immiscibility REFERENCES in peralkaline nepheline syenites (lujavrites) of the Ilimaussaq complex, South Greenland. Contrib. Mineral. BIRCH, W.D., BURKE, E.A.J., WALL, V.J. & ETHERIDGE, M.A. Petrol. 141, 458-472. (1988): Ecandrewsite, the zinc analogue of ilmenite, from Little Broken Hill, New South Wales, Australia, and the ______& BAUMGARTNER, L. (2002): pH changes in San Valentin mine, Sierra de Cartegena, Spain. Mineral. peralkaline late-magmatic fluids. Contrib. Mineral. Petrol. Mag. 52, 237-240. 144, 331-346.

BRÖGGER, W.C. (1890): Die Mineralien der Syenit-pegma- MITCHELL, R.H. (1986): Kimberlites: Mineralogy, Geochemis- titgänge der Südnorwegishen Augit- und Nephelinsyenit. try, and Petrology. Plenum Press, New York, N.Y. Z. Kristallogr. Mineral. 16, 339-350. ______& LIFEROVICH, R.P. (2004): Pyrophanite – CHAKHMOURADIAN, A.R. & MITCHELL, R.H. (1999): Niobian ecandrewsite solid solutions: crystal structures of the ilmenite, hydroxylapatite and sulfatian monazite: alterna- Mn1–xZnxTiO3 (0.1 ≤ x ≤ 0.8) series. Can. Mineral. (in tive hosts for incompatible elements in calcite kimberlite press). from Internatsional’naya, Yakutia. Can. Mineral. 37, 1177- 1189. NAKASHIMA, K. & IMAOKA, T. (1998): Niobian and zincian il- menites in syenites from the Cape Ashizuri, southwest Ja- ______& ______(2002): The mineralogy of Ba- and Zr- pan. Mineral. Petrol. 63, 1-17. rich alkaline pegmatites from Gordon Butte, Crazy Moun- tains (Montana, USA): comparison between potassic and NEUMANN, H. (1949): Notes on the mineralogy and geochem- sodic agpaitic pegmatites. Contrib. Mineral. Petrol. 143, istry of zinc. Mineral. Mag. 28, 575-581. 93-114. OLIVO, G.R. & WILLIAMS-JONES, A.E. (1999): Hydrothermal DROOP, G.T.R. (1987): A general equation for estimation Fe3+ REE-rich eudialyte from the Pilanesberg complex, South concentrations in ferromagnesian silicates and oxides from Africa. Can. Mineral. 38, 653-663. microprobe analyses, using stoichiometric criteria. Min- eral. Mag. 51, 431-435. PEKOV, I.V. (2001): Lovozero Massif: History of Investigations, Pegmatites, Minerals. Zemlya Press, Moscow, Russia (in GASPAR, J.C., & WYLLIE, P.J. (1983): Ilmenite (high Russ.). Mg,Mn,Nb) in the carbonatites from the Jacupiranga Com- plex, Brazil. Am. Mineral. 68, 960-971. ______(2002): New zincian minerals and genetic aspects of the crystal chemistry of zinc in hyperalkaline pegmatites. GERASIMOVSKY, V.I. (1969): Geochemistry of Ilímaussaq In New Approach to the Study and Description of Minerals Alkaline Massif, S.W. Greenland. Izdat. Nauka, Moscow, and Mineral Formation Processes. Moscow, Russia (35- Russia (in Russ.). 37).

______, VOLKOV, V.P., KOGARKO, L.N., POLYAKOV, A.I., PIOTROWSKI, J.M. & EDGAR, A.D. (1970): Melting relations of SAPRYKINA, T.V. & BALASHOV, YU.A. (1966): Zinc. In undersaturated alkaline rocks from South Greenland, com- Geochemistry of Lovozero Alkaline Massif (V.V. pared to those from Africa and Canada. Medd. Grønl. Scherbina, ed.). Publisher, city (273-277; in Russ.). 181(9).

HORVÁTH, L. & GAULT, R.A. (1990): The mineralogy of Mont PLIMER, I.R. (1990): The ilmenite–ecandrewsite solid solution Saint-Hilaire, Quebec. Mineral. Rec. 21, 284-359. series, Broken Hill, Australia. Neues Jahrb. Mineral., Monatsh., 529-536. KHOMYAKOV, A.P. (1990): Mineralogy of Hyperagpaitic Alka- line Rocks. Nauka, Moscow, Russia (in Russ.). RETIEF, E.A. (1963): Petrological and Mineralogical Studies in the Southern Part of the Pilanesberg Complex, ______(1995): Mineralogy of Hyperagpaitic Alkaline Transvaal, South Africa. Ph.D. thesis, Univ. of Oxford, Rocks. Oxford Science Publications, Clarendon Press, Ox- Oxford, U.K. ford, U.K. SHAND, S.I. (1928): The geology of Pilansberg in the Western KOSTYLEVA-LABUNTSOVA, E.E., BORUTSKY, B.E., SOKOLOVA, Transvaal. Trans. Geol. Soc. S. Afr. 31, 91-156. M.N., SHLYUKOVA, Z.V., DORFMAN, M.D., DUDKIN, O.B. 1178 THE CANADIAN MINERALOGIST

SUWA, K., ENAMI, M., HIRAIWA, I. & TANG, TAI-MING (1987): WHITNEY, D.L., HIRSCHMANN, M. & MILLER, M.G. (1993): Zn–Mn ilmenite in the Kuiqi granite from Fuzou, Fujian Zincian ilmenite – ecandrewsite from a pelitic schist, Death Province, East China. Mineral. Petrol. 36, 111-120. Valley, California, and the paragenesis of (Zn,Fe) TiO3 solid solution in metamorphic rocks. Can. Mineral. 31, ULBRICH, H.H. & ULBRICH, M.N.C. (2000): The lujavrite and 425-436. khibinite bodies in the Poços de Caldas alkaline massif, southeastern Brazil: a structural and petrographic study. Received November 7, 2003; revised manuscript accepted Revista Brazileira de Geociencias 30, 615-622. July 10, 2004.