Fairchildite K2Ca(CO3)2 in carbonatites at Loolekop mine, Phalaborwa Igneous Complex

Zhitova L.M.*, Sharygin V.V.*, Nigmatulina E.N.*, Zhitov E.Yu.**

*V.S.Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia; **Novosibirsk State University, Novosibirsk, Russia

 

Fairchildite and buetschliite, the high- and low-temperature polymorphs of K2Ca(CO3)2, were described in wood-ash clinkers formed during forest fires, firstly in western United States (Milton, Axelrod, 1947) and then in Canada (Dawson, Sabina, 1958). At present day, there are three natural occurrences of fairchildite: the North America forest fires and slags of Rosenberg mine, W.Germany (Schnorrer-Kohler, David, 1991). High-temperature K2Ca(CO3)2 was indicated in burning (>600oC) ash products of biomass and agricultural waste, in different glasses and cement clinkers. The experimental study of the system Na2CO3-K2CO3-CaCO3 at 1 kb (Cooper et al., 1975) has shown that fairchildite is stable from its melting point (809C) down to 547C, where it inverts into buetschliite, and the nyerereite-fairchildite solid solutions are real. All this strongly assume the possible appearance of fairchildite in carbonatites, but it has never observed previously in them. Nyerereite enriched in K is the major mineral in natrocarbonatites of Oldoinyo Lengai and occasionally occurs as daughter phases of mineral-hosted inclusions in calciocarbonatites and related silicate rocks (Le Bas, Aspden, 1981; Kogarko et al., 1991, Veksler et al., 1998; Zaitsev, Chakhmouradian, 2002; Kogarko, Turkov, 2007; Stoppa et al., 2008). Here we report fist occurrence of magmatic fairchildite found in magnetite-hosted inclusion of carbonatite from the Loolekop mine, Phalaborwa. The previous inclusion studies for carbonatites and silicate rocks of this complex were provided by Aldous (1986) and Solovova et al. (1998).

The Phalaborwa igneous complex (age >2060 Ma), located in the Archean Shield of northeastern Transvaal, South Africa, is the unique in that its carbonatites are host of an economic deposit of copper ores with considerable resources of magnetite, apatite, vermiculite, uranothorianite and baddeleyite (Hanekom et al., 1965, Palabora MC, 1976). The complex (area - 16.5 km2, depth - up to 5 km) is a ring shaped pipe-like structure, which resulted from the four intrusive cycles: 1 - pyroxenites; 2 - syenites; 3 - phoscorites and banded carbonatites; 4 - transgressive carbonatites. The specific feature of all rocks of the Phalaborwa complex is low abundance of Na2O. Studied samples were collected from deep horizons of the Loolekop mine, where phoscorites and early carbonatites form a banded ore body, and represent coarse-grained combined phoscorite-carbonatites. They contain variable amounts of coarse-grained magnetite, partially serpenitized olivine (Fo80), fluorapatite and phlogopite in fine-, medium-grained matrix, consisting of Mg-calcite and dolomite. Sulfide mineralization is represented mainly by chalcopyrite and bornite. Brucite, chondrodite, baddeleyite, uranothorianite, Ba-Sr-carbonates, barite, celestine, Na-REE-apatite are minor or accessory.

Magnetite forms coarse grains (up 3 cm) and contains abundant solid inclusions different in mineral composition and origin. Host magnetite in chemical composition approaches ideal FeFe2O4 with minor TiO2 (1.0-1.7), MgO (1.2-1.7), Al2O3 and MnO (up to 0.2 wt.%). Solid decay microstructure of magnetite-ilmenite-ulvospinel and individual large lamellae of picroilmenite (TiO2 - 58.3-58.6; FeO -22.2-28.0, MgO -12.1-16.9, MnO - 1.1-2.5 wt.%) are common. Discrete euhedral grains (size - 100-300 m) and intergrowths of fluorapatite, dolomite, calcite, olivine, baddeleyite, and phogopite also occur. Unlike solid inclusions, multiphase primary inclusions have rounded or irregular shape (size - 10-300 m) and vary in mineral composition from carbonate to carbonate-silicate-oxide. Dolomite, calcite, apatite, picroilmenite, phlogopite, magnesite, brucite are essential minerals in such inclusions. Minor phases are interstitial in respect to the essential minerals. According to scanning microscopy, there are baddeleyite, uranothorianite, strontianite, witherite, barite, celestine, siderite, Ca-Ba-carbonate, bastnaesite-(Ce), Na-REE apatite, halite and Fe-Ni-Co-Cu sulfides. The carbonate inclusions usually vary in calcite-dolomite ratio and calcite sometimes contains perthitic dolomite. In addition, minute (5-10 m) inclusions enriched in halite, sylvite and Mg-Fe chlorides occur in magnetite. They are similar to secondary inclusions described in apatite of the Loolekop phoscorites by Solovova et al. (1998).

 

Figure 1. BSE image and elemental maps for fairchildite-containing inclusion in magnetite, phoscorite-carbonatite.

 

Подпись: Fe

 

 

Note: Fair - fairchildite; Dol - dolomite; Phl - phlogopite; Ilm - geikielite-ilmenite; Brc - brucite; With - witherite.

In the core of magnetite crystal we have been found a multiphase inclusion (size - 60 m) with K-Ca-carbonate K2Ca(CO3)2. This inclusion also contains dolomite, picroilmenite (geikielite-ilmenite), phlogopite, brucite, witherite and halite (Fig. 1, Table 1). The relationships of phases within inclusion have shown that K-Ca-carbonate crystallized after phlogopite and picroilmenite and before dolomite. Estimations for the magnetite-ilmenite and dolomite-calcite pairs gave equilibrium temperatures higher than 650-700oC. The study of fluid and salt inclusions in apatite of the Loolekop phoscorites (Solovova et al., 1998) has shown homogenization temperatures in the 630-750oC range. Judging mineral relations and temperature estimations, K-Ca-carbonate seems to be high-temperature polymorph of K2Ca(CO3)2 - fairchildite. The existence of brucite and phlogopite in multiphase magnetite-hosted inclusions suggests the presence of some water in the initial carbonatite melt, what can provoke the fairchildite-buetschliite inversion with decreasing temperature. However, experiments of Cooper et al. (1975) has shown that neither presence of water nor pressure changing lead to a drastic shift in the fairchildite-buetschliite inversion temperature. Fairchildite may convert slowly to buetschliite at room temperature and in air conditions (Mrose et al., 1966). Possibly, the same phenomenon of fairchildite decomposition was observed by us during one month after opening of the inclusion on the surface.

 

Table 1. Chemical composition (EMPA, in wt.%) of minerals of fairchildite-containing inclusion and host magnetite, phoscorite-carbonatite, Phalaborwa complex.

 

Phase

n

SiO2

TiO2

Al2O3

FeO

MnO

MgO

CaO

BaO

SrO

Na2O

K2O

F

Total

Mgt-host

1

0.00

0.95

0.10

90.34

0.16

1.74

0.00

0.00

0.00

0.00

0.00

0.00

93.29

Ilm

3

0.00

58.60

0.00

21.58

2.98

16.55

0.00

0.00

0.00

0.00

0.00

0.00

99.70

Phl

2

38.27

2.85

12.11

5.91

0.13

27.13

0.59

0.13

0.00

0.12

10.91

0.76

98.90

Dol

6

0.00

0.00

0.00

1.87

0.17

21.19

29.48

0.07

0.47

0.02

0.00

0.00

53.28

Fair

5

0.00

0.00

0.00

1.47

0.01

0.00

23.15

0.00

0.00

0.63

38.54

0.00

63.80

Brc

3

0.00

0.00

0.00

3.80

0.22

65.85

0.31

0.00

0.03

0.02

0.00

4.22

74.45

 

Thermobarogeochemical studies of Solovova et al. (1998) have shown that the Phalaborwa carbonatites and phoscorites have been formed at 850-870oC and 4-4.5 kb and calciocarbonatite melt contains up to 7 wt.% SiO2, 11.5 wt.% P2O5, 0.8 wt.% K2O and very low concentration of Na2O (<0.1 wt.%). In phoscorite-carbonatites the traces of Na2O are fixed as an accessory Na-REE-apatite (groundmass, inclusions) and as halite in salt inclusions. Unlike calciocabonatite worldwide, the drastic predominance of K2O over Na2O was promoted the appearance of fairchildite instead of nyerereite during crystallization of the Phalaborwa calciocarbonatite melt.

This study was financially supported by RFBR (grant 07-05-00685).

 

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