A Case Study from Fuka Contact Aureole, Okayama, Japan

328 Journal of Mineralogical andM. PetrologicalSatish－Kumar, Sciences, Y. Yoshida Volume and 99I. ,Kusachi page 328─ 338, 2004 Special Issue High temperature contact metamorphism from Fuka contact aureole, Okayama, Japan 329

The role of aqueous silica concentration in controlling the mineralogy during high temperature contact metamorphism: A case study from Fuka contact aureole, Okayama, Japan

* * ** Madhusoodhan SATISH－KUMAR , Yasuhito YOSHIDA and Isao KUSACHI

*Department of Biology and Geosciences, Faculty of Science, Shizuoka University, Shizuoka 422－8529, Japan **Department of Earth Sciences, Faculty of Education, Okayama University, Okayama 700－8530, Japan

The contact aureole at Fuka, Okayama, Japan is peculiar for the occurrence of extensive high－temperature skarn resulting from the intrusion of Mesozoic monzodiorite into Paleozoic marine limestone. The occurrence is also notable for the finding of ten new minerals, of which five are calcium－boron－bearing minerals, and scores of other rare minerals. Skarn formation at Fuka can be classified into three major types 1. Grossular－vesuvianite－ wollastonite endoskarn, 2. Gehlenite－exoskarns, and 3. Spurrite－exoskarns. Grossular－vesuvianite－wollastonite endoskarn forms a narrow zone (few centimeter width) separating the exoskarn and the igneous intrusion. It is also found, developed independently, along contacts of the younger basic intrusive dykes and limestone in the region. The gehlenite －exoskarns, in most cases, are spatially associated with igneous intrusion and are extensive (decimeter to meter thick). However, exceptions of independent gehlenite dikes are also observed. Retrogression of the gehlenite endoskarns results in the formation of hydrogrossular and/or vesuvianite. Accessory phases include schrolomite and perovskite. The predominantly monomineralic spurrite －exoskarn was formed in the outer zone of the gehlenite－skarn parallel to the contact as well as independent veins, dikes and tongues. The spurrite －exoskarn may extend tens of meters. Spurrite coexists with tilleyite or rankinite, although larnite is absent. Idiomorphic gehlenite and vesuvianite are the most common accessory phases observed. Retrograde hydration of spurrite to foshagite, scawtite and hillebrandite is commonly observed as veins and alteration zones within the spurrite exoskarn. Petrogenetic grids were constructed using “THERMOCALC” for the observed mineral assemblage in the －－－ spurrite skarn. Mineral fluid equilibrium in the CaO SiO2 CO2 system was computed, considering the metaso- － matic input of aqueous silica. Phase diagram analysis in the form of T XCO2 grids with varying silica activity indicated that the stability field of spurrite is strongly controlled by the activity of silica in the fluid. Optimum silica concentration in the fluid was between 9.1 × 10−4 and 4.5 × 10−3 mol/liter, above which wollastonite becomes stable, whereas further reduced silica activity will generate larnite. Appropriate temperature condition

for the formation of spurrite is between 850°C and 1080°C at an XCO2 fluid composition of 0.05 to 0.42. At tem-

perature conditions lower than 850°C, the spurrite stability field becomes narrow, with low CO2 activity. The formation of extensive spurrite－exoskarn suggests that the silica activity, temperature and fluid composition remained within the spurrite stability field. Petrogenetic analysis of phase diagrams suggests that the exoskarn formation at Fuka contact aureole was robustly controlled by the activity of silica in the high temperature metasomatic fluid.

Introduction skarn formation resulting from hydrothermal fluids ema- nating from granitic intrusions or those resulting from Skarn formation at igneous－limestone contact metamor- basic intrusions at high temperature but with less fluid phic environment is most suited for studies on metasoma- activity (see Kerrick, 1991 and references therein). How- tism, fluid flow and diffusion transport of elements. Most ever, there is still ambiguity in skarn formation mecha- studies till date focused on low to medium temperature nism and related processes at high temperature conditions

M. Satish －Kumar, [email protected] Corresponding (>800°C) accompanied by copious fluid flow at contact author aureoles. Temperature gradient in the crust and perme- I. Kusachi, [email protected]－u.ac.jp ability are crucial factors controlling reactive transport of 328 M. Satish－Kumar, Y. Yoshida and I. Kusachi High temperature contact metamorphism from Fuka contact aureole, Okayama, Japan 329

fluids as well as fluid flow. Further, higher temperature (Henmi and Kusachi, 1992) morimotoite (Henmi et al., conditions enhance element mobility. Understanding 1995), kusachiite (Henmi, 1995), takedite (Kusachi et al., metasomatism associated with high temperature skarn 1995), parasibirskite (Kusachi et al., 1998), and formation has wide implications on element recycling in okayamalite (Matsubara et al., 1998). Five of these new the crust, especially in volcanic arc provinces, wherein minerals are calcium boro －silicates. Preliminary extensive magmatic activity plays a key role in crustal geochemical investigations by Kusachi (1975) and growth. Further, skarn formations are often associated Kusachi et al. (1978) emphasized lattice diffusion of ele- with major economic mineral deposits. ments as the principal skarn forming mechanism. How-

CaO–SiO2–Al2O3–vapor system forms the simplest ever, not much is known about the physical conditions, chemical system at siliceous igneous rock －limestone fluid composition, distance and quantity of material input intrusive contacts. Wollastonite and grossular garnet are accompanying the skarn formation. Here we present the most common minerals in this system observed in field, mineralogical and petrogenetic analysis of the high－ contact aureoles of moderate temperature conditions and temperature skarn. The formation of high temperature high fluid activity. However, at high temperature condi- skarn minerals is evaluated using petrogenitic grids tions (>800°C) and silica under saturated conditions, min- involving aqueous silica with varying fluid composition

erals such as spurrite [Ca5Si2O8(CO3)], tilleyite and temperature conditions constructed with the aid of

[Ca5Si2O7(CO3)2] and rankinite [Ca3Si2O7] are expected in “THERMOCALC” (Powell and Holland, 2001) place of wollastonite, whereas gehlenite forms instead of grossular garnet. However, spurrite－gehlenite skarn has Geology around Fuka only limited occurrence in the world, e.g. Kilchoan, Scot- land (Agrell, 1965), Christmas Mountains, North America Fuka is located about 40 km west －northwest from (Joesten, 1974; 1976), Fuka, Japan (Kusachi, 1975; Kusa- Okayama city, southwest Japan. Geographically, Fuka is chi et al., 1978) and Apuseni Mountains, Romania (Pascal situated in the uplifted Kibi－pleateau with an elevation of et al., 2001; Marincea et al., 2001). By the rarity of natu- about 500 m, and the Nariwa River runs from west to east ral occurrence, studies on high temperature skarn has not and makes V shaped valley. The area that yields skarn is －－ much progressed, although the CaO SiO2 vapour system located between the slope of the south bank of the Nariwa triggers the transport of elements by means of decarbon- river and plateau face (Kusachi, 1975; Omae et al., 2002). ation－dehydration reactions in the crust. The basement rocks of Paleozoic age in this area com- Earlier studies on skarn formation at Fuka focused prises of pelitic rocks, psammitic rocks and chert, overlain on the mineralogic and geochemical aspects (e.g. Kusachi, by calcareous rock of the Nakamura Formation (Fig. 1). 1975). Ten new minerals were discovered from this Volcanic rocks of Mesozoic age covers the sedimentary occurrence. They are bicchuite (Henmi et al., 1973), rocks in the western part of the Fuka region. Several gen- fukalite (Henmi et al., 1977), oyalite (Kusachi et al., erations of younger andesitic dikes are found puncturing 1984), henmilite (Nakai et al., 1986), clinotobermerite the basement rocks and form few centimeter thick grossu-

Figure 1. Geological map around Fuka (simplified after Teraoka et al., 1996). Inset show the study area in western Japan. 330 M. Satish－Kumar, Y. Yoshida and I. Kusachi High temperature contact metamorphism from Fuka contact aureole, Okayama, Japan 331 lar – vesuvianite –wollastonite skarn assemblages. primary minerals observed across fractures, evidencing the retrogression. Unaltered spurrite is purple to grey in Field relations color.

Four classic skarn outcrops, where typical high－tempera- Fuka north outcrop ture skarn formation could be observed, are described briefly here. Skarn formation in this outcrop has been documented by

Fuka west outcrop

This outcrop is located at the western extremity of this metamorphic zone (location 3 in Figure 1). Here skarn crops out at the slope of a hill. The outcrop sketch is given in Figure 2a (after Kusachi et al., 1978). The skarn formation at this outcrop shows zonal pattern starting with quartz monzonite at the igneous side to gehlenite zone and then to spurrite zone. There is also a transition zone between gehlenite and spurrite zones, where both minerals coexist. Kusachi et al. (1978) observed that the skarn at this outcrop is formed by the intrusion of quartz monzonite and the primary minerals (gehlenite and spurrite) produced. At later stage, the skarn was altered by the intrusion of younger andesitic dike acted as a source of heat and fluids during retrogression. The gehlenite zone is dark in appearance, and shows prominent bleaching of

Figure 3. Examples of skarn formation observed inside the Fuka mine. (a) Photograph of mine wall at level 3 showing igneous rocks surrounded by spurrite exoskarn. (b) Broken tongues of igneous rocks surrounded by wollastonite－vesuvianite－grossular endoskarn and by gehlenite and spurrite exoskarn. (c) Sketch of skarn zones seen in photograph b. (d) Isolated igneous rock surrounded by all the three skan zones. The three zones represent the zonation of the skarn from the wollastonite －vesuvianite － Figure 2. Outcrop sketch of (a) Fuka West outcrop (after Kusachi grossular endoskarn to gehlenite exoskarn and spurrite exoskarn. et al., 1978) and (b) Fuka North outcrop (after Omae et al., (e) An example of a mine face in level 2 showing the three skarn 2002). zones. 330 M. Satish－Kumar, Y. Yoshida and I. Kusachi High temperature contact metamorphism from Fuka contact aureole, Okayama, Japan 331

Kusachi (1975) (location 4a in Figure 1). In a recent this outcrop also exhibits mineralogical zonation, similar study Omae et al. (2002) described the petrologic and pet- to the features observed in the outcrops described above. rochemical features of igneous rocks and xenoliths. Here also, the skarn shows a mineralogical zonation similar to Fuka mine Fuka west outcrop. Figure 2b is an outcrop sketch map, which shows that the skarn formation is related with two Fuka mine is located few hundred meters south from Fuka stages of igneous intrusion (Omae et al., 2002). The fist north outcrop (location 5 in Figure 1). Here, extensive stage of the intrusion is a monzodiorite, which formed the mining of highly crystalline pure limestone has been car- main high temperature metasomatic gehlenite －spurrite ried out toward the igneous contact in four tunneling skarn assemblage. During the second stage, two types of levels penetrating the hill. Inside the mine the retaining quartz monzonites, a melenocratic and a leucocratic, were pillars and quarry faces preserve fresh outcrops of skarn. intruded resulting in gehlenite －spurrite and grossular － Out of the four, first level is filled with water and impossi- vesuvianite－wollastonite skarns respectively (Omae et al., ble to access. Other three levels were mapped and sam- 2002). Two younger andesitic dikes cut across the entire pled. Younger andesitic dike that cuts across the longitu- sequence of earlier igneous and associated skran and alter dinal direction of the mine, serve as a marker to locate the the spurrite to tilleyite and gehlenite to hydrogrossular. sampling points three dimensionally. Kusachi (1975) observed that the grain size of tilleyite Several skarn occurrences inside the mine were stud- near the dike was much bigger than the tilleyite elsewhere ied in detail. Typical examples of skarn occurrence inside in the skarn, and concluded that the andesitic dike intru- the mine are shown in Figure 3. Generally, the skarns sion has resulted in the conversion of spurrite to tilleyite. consists of spurrite or its alteration products, surrounding Spurrite formed near the andesitic dikes has blue color. monzodiorite. The spurrite skarn is separated from the igneous intrusion by centimeter thick (3－5cm) grossular－ Fuka road cut wollastonite skarn that parallels the igneous contact. Patches and streaks (few centimeters in dimensions) of This outcrop is situated at the peak of the hill, about 200m gehlenite occur randomly inside the monomineralic spur- west of Fuka north outcrop (location 4b in Figure 1). rite skarn. Skarn dikes enclosing igneous rocks in the Continuous outcrop of road cut starting from the crystal- middle part of dike shows “eaten up” textures resulting line limestone through skarn extending to the igneous from the reaction between skarn forming fluids and earlier rock is exposed here (Omae et al., 2002). The skarn at igneous rocks (Fig. 3). Occasionally, monomineralic spurrite veins originate from a large skarn －igneous body Table 1. Mineral name, abbreviation and chemical composition of pinch out and form a network of veins and dikes. common minerals found in high temperature skarn at Fuka Skarn classification

Based on the mineralogical features and the relations with the parent igneous rock skarns at Fuka were classified into three types.

Grossular－vesuvianite－wollastonite endoskarn

This skarn is always associated with the igneous rocks. The thickness of the skarn varies from a few millimeters to few centimeters. At a single occurrence, however, the thickness always remains constant (Fig. 3). The modal mineralogy changes from more garnet－rich near the contact toward wollastonite －rich in the rim of the zone in contact with other skarns. Thin monomineralic wollastonite zone can be observed in some occurrences. The constant thickness of the grossular －vesuvianite －wollastonite skarn paralleling the igneous contact is probably Although larnite is not observed in the skarn, it was used to limit related to the direct material transport between the igne- the condition of formation of the skarn. ous rock and the limestone. 332 M. Satish－Kumar, Y. Yoshida and I. Kusachi High temperature contact metamorphism from Fuka contact aureole, Okayama, Japan 333

Table 2. Mineral assemblages observed in the Fuka skarns

Abbreviations are same as in Table 1. 332 M. Satish－Kumar, Y. Yoshida and I. Kusachi High temperature contact metamorphism from Fuka contact aureole, Okayama, Japan 333

Figure 4. Photomicrographs of typical skarn minerals. All photomicrographs are in crossed nicols. (a) Spurrite (Spu). (b) Cathodolumines- cence image of spurrite, which shows typical green luminescence compared to red of calcite. Calcite is dark because of brighter luminescence of spurrite. (c) Tylleyite (Ty). (d) Alteration products of spurrite comprising of fine－grained intergrowth of calcite (Cal) – Foshagite (Fos) – Hillebrandite (Hil). (e) Kilchoanite (Klc) enclosing relict of rankinite (Rnk). The veins observed in kilchoanite are fukalite (Fuk). (f) Vesuvianite accessory seen in sputtire exoskarn. (g) Gehlenite (Ghl) (h) Hydrogrossular (HGrs) is a common retrograde product of gehlenite. 334 M. Satish－Kumar, Y. Yoshida and I. Kusachi High temperature contact metamorphism from Fuka contact aureole, Okayama, Japan 335

Gehlenite exoskarn Skarn mineralogy

Gehlenite exoskarn appears either directly in contact or Several scores of skarn minerals have been reported from separated by a thin grossular －vesuvianite －wollastonite the Fuka contact aureole. These include the discovery of endoskarn with the igneous rock. The thickness of the ten new minerals and scores of rare minerals. List of gehlenite exoskarn varies from few centimenter to more common minerals found at Fuka is given in Table 1. than ten meters. At the Fuka north outcrop, gehlenite Salient mineralogical, textural and mineral chemical prop- exoskarn is in direct contact with monzodiorite and mela- erties of the minerals observed in the skarn are described nocratic quartz monzonite, whereas it is separated from below. The primary skarn mineral assemblages could be the leucocratic quartz monzonite by a thin zone of grossu- easily identified under microscope. However, the retro- lar－vesuvianite－wollastonite formed by the retrogression grade assemblages were difficult for identification, of the gehlenite exoskarn (Omae et al., 2002). In the because of the fine grained nature and similar optical quarry faces examined inside the Fuka mine, gehlenite properties. The mineralogy was reconfirmed by perform- zone is separated from the igneous intrusion by thin zone ing XRD on specific points in chips corresponding to the of grossular－vesuvianite－wollastontie. thin sections. The mineral assemblages found in the sam- ples investigated were broadly grouped in to primary high Spurrite exoskarns temperature minerals and secondary alteration products and listed in Table 2. Spurrite exoskarn forms the outermost and the thickest of Figure 4a shows the most common mineral in skarn, all zones. Spurrite exsoskarns are whitish to grayish in spurrite. Spurrite characteristically show greenish cath- color, and sometime spurrite turn purplish, especially in odoluminescence (Fig. 4b) (Long and Agrell, 1965). Til- Fuka west outcrop. Most commonly, spurrite skarn is leyite (Fig. 4c) is also found co－existing with spurrite at monomineralic, however gehlenite and other skarn miner- Fuka north outcrop. Hydrous alteration of spurrite pro- als occasionally occur. Thickness varies from a few cen- duces hillebrandite (Fig. 4d), foshagite and scawtite along timeters to tens of meters. The outer contact of spurrite with calcite. These hydrothermal products are very fine－ skarn with the marble is not essentially parallel the igne- grained and difficult to distinguish each other under ous－skarn contact. Tongues and veins of spurrite intrude microscope. Rankinite [Ca3Si2O7] is found occasionally the crystalline marbles. The spurrite exoskarn in contact (Fig. 4e). However, it is metastable under low tempera- with limestone is yellowish in color due to the prominent ture condition and alters to kilchoanite [Ca3Si2O7], which hydrothermal alteration of spurrite to low temperature is low temperature polymorph (Fig. 4e). Textural rela- hydrous minerals such as hillebrandite [Ca2SiO4･H2O], tions suggest that rankinite or kilchoanite is hydrother- foshagite [Ca4Si3O9(OH)2] and scawtite [Ca7CO3Si6O18･ mally altered to fukalite [Ca4Si2O6(OH)2(CO3)] along the

2H2O]. veins (Fig. 4f). Another characteristic high temperature mineral in the spurrite exoskarn is gehlenite [Ca2Al2SiO7]

Table 3. Representative microprobe analytical results high temperature skarn minerals

* Calculated. 334 M. Satish－Kumar, Y. Yoshida and I. Kusachi High temperature contact metamorphism from Fuka contact aureole, Okayama, Japan 335

(Fig. 4g). It occurs as isolated grains or veinlets within Table 4. List of selected reactions and pseudo invari- －－ spurrite skarn. However, gehlenite easily alters to other ant points in the CaO SiO2 CO2 system minerals during cooling and forms vesuvianite

[Ca19Al10(Fe,Mg)3Si18O70(OH)8] (Fig.4h), Bicchulite

[Ca2Al2SiO6(OH)2] or hydrogrossular [Ca3Al2

(Si, H4)3O12]. The rare skarn minerals at Fuka having Cu or Bi are usually found as druses in pure coarse －grained marbles and are considered to be associated with the late stage hydrothermal activity accompanying the skarn formation. Boron －bearing skarn minerals occur at three outcrops associated with spurrite－skarn (see Kusachi et al., 1999, for a review).

Mineral chemistry

Mineral chemical analyses were carried out using wave- length dispersive electron probe micro analyzer JEOL superprobe 733. Measurement conditions were 12 nA beam current, 15 kV and 20 sec measurement time for all elements. Minimum beam diameter was used for the measurements. Synthetic and natural standards were used and raw data were corrected using Bence and Albee (1968). Chemistry of major minerals in spurrite skarn, Mineral abbreviations are given in Table 1. spurrite, tilleyite and rankinite closely resemble the end member composition (Deer et al., 1992). Representative EPMA results are given in Table 3. The chemical features of alteration products and accessory minerals and those of gehlenite exoskarn are out of scope in this contribution and will be considered elsewhere.

Petrogenetic grids and its implications

The formation of grossular －vesuvianite －wollastonite endoskarn can be petrogenetically modeled in the simplified CASV system. The mineral assemblage of grossular + vesuvianite + wollastonite formed either by the retrogression of earlier high－temperature skarn minerals such as gehlenite or by direct contact between crystallizing magma and the calcitic marble at moderate temperature conditions. The fluid condition during the formation of

this skarn is considered to be low XCO2 at medium temperature conditions (Tracy and Frost, 1991). The formation of gehlenite exoskarn is considered as a result of the metasomatism of alumina and silica along the lithologic contacts of intruding magma. The petro- －－ gentic analysis of gehlenite skarn will be addressed else- Figure 5. T XCO2 reactions in the high temperature skarn forma- －－ where, because of the complex nature of the system tion in the system CaO SiO2 CO2. The numbers represent the involving alumia－bearing fluids. Here, our focus is on a reactions listed in Table 4 and the alphabets indicate the pseudoinvariant points. detailed analysis of CSV system, which represents the spurrite exoskarn. The formation of high temperature minerals in the 336 M. Satish－Kumar, Y. Yoshida and I. Kusachi High temperature contact metamorphism from Fuka contact aureole, Okayama, Japan 337

CSV system by progressive contact metamorphism sug- which are considered in the petrogenetic grids. － gests a mineralogic sequence of wollastonite → tilleyite Figure 5 shows a T XCO2 partial petrogenitic grid in → spurrite → rankinite → larnite with increasing temper- the CSV system calculated with a controlled aqueous sil- ature (See Tracy and Frost, 1991, and references therein) ica activity (ln a(Si(aq))) condition equal to －6.0. The based on experimental results. This sequence of mineral limiting condition of spurrite skarn mineralogy at Fuka is assemblage was observed at Christmas Mountains (Joes- the absence of prograde wollastonite and larnite. This ten, 1974; 1976). The petrogenetic grids used to deduce places constraints on the fluid composition and tempera- the observed assemblages were explained using reactions ture condition of formation of spurrite. Formation of involving quartz, calcite and the minerals in the sequence. spurrite is considered through the reaction between calcite However, at Fuka (and elsewhere in Apuseni mountains, and aqueous silica in the metasomatic fluid (Reaction 2, Pascal et al., 2001; Marincea et al., 2001) mineralogy and Fig. 5 and Table 4). The reaction occur at temperature textural features of the skarn does not suggest a progres- >1000°C at 1kbar and ln a(Si(aq)) of －6. Tilleyite can sive sequence of reactions involving quartz. The forma- form at temperature lower than that of spurrite by the tion of spurrite and tilleyite is suggestive of its formation reaction between calcite and aqueous silica, but at tem- with out early wollastonite. Hence, here we consider the perature less than 940°C wollastonite start forming formation of high－temperature CSV minerals along igne- directly (Reactions 3 and 4, Fig. 5 and Table 4). Larnite ous contact at Fuka invariably associated with the move- is formed at temperatures higher than 1080°C by the reac- ment of metasomatic fluids. With the advent of new tion between calcite and aqueous silica. Since larnite is internally consistent thermodynamic data for the construc- absent in the skarn at Fuka, the highest limiting tempera- tion of phase diagrams involving minerals, fluids and ture of formation of spurrite skarn is considered to be melts, it is now possible to construct partial petrogenetic 1080°C. grids involving aqueous species and minerals. We there- The stability fields of individual high －temperature fore apply the latest version of THERMOCALC v.3.1 skarn minerals are shown in Figure 6. At a given XCO2 and (Powell and Holland, 2001) using the internally consistent ln a(Si(aq)) condition larnite is stable at temperature con- data set of Holland and Powell (1998) for the construction of petrogenetic grids. The CaO–SiO2–CO2 system involving calcite, wollastonite, tilleyite, spurrite and rankinite, the major minerals found in spurrite skarn, were selected. In the present contribution, we ignore the minor occurrence of aluminous and titanium －bearing minerals such as vesuvianite, grossular, gehlenite or perovskite in the spurrite skarn. A fluid pressure of totalP of 1kbar is assumed considering the lithostratigraphy and earlier studies (Kusachi, 1975). Table 4 lists the selected stable reactions

Figure 7. The shift observed in the stability field of spurrite in aT －

XCO2 diagram at varying activities of aqueous silica. Dashed lines represent the movement of respective pseudoinvariant points in relation with changing silica activity. Note that spurrite is meta- Figure 6. Stability fields of Wollastonite, tilleyite, spurrite and ran- stable when silica activities are higher than －5.4, whereas at low kinite in the order of increasing temperature and/or decreasing silica activities spurrite can be stable at even lower temperature,

CO2 in the fluid. but at a very narrow XCO2 range only. 336 M. Satish－Kumar, Y. Yoshida and I. Kusachi High temperature contact metamorphism from Fuka contact aureole, Okayama, Japan 337

dition higher than spurrite. Rankinite and spurrite have present study. High temperature conditions for spurrite almost similar stability fields, however spurrite is stable and gehlenite skarn formation have been reported else-

in a broader XCO2 conditions than rankinite. Tilleyite is where as well. In a recent study on the formation of spur- stable at temperatures lower than that of spurrite and wol- rite－ and gehlenite－bearing skarn in Apuseni Mountains, lastonite forms at further low temperature conditions (Fig. Romania, Pascal et al. (2001) reported temperature condi- 6). If constant temperature is maintained, the stability tions of around 750°C at 750 bars and a maximum con- −4 fields of larnite is at XCO2 condition lower than that of centration of silica of 2.9 ×10 m. Their study also indi-

spurrite, whereas increasing CO2 activity stabilizes tilleyite cated that the variation of CO2 in the fluid is controlled by

and wollastonite, and finally calcite becomes the stable the activity of silica, since CO2 is produced during the phase. formation of spurrite, which in turn needs silica in the The effect of changing activity of aqueous silica on fluid to react with the calcite. Our results are also in the stability field of spurrite is shown in Figure 7. As an agreement with this, however the skarn formation at Fuka example, the stability field of the mineral assemblage occurred at a higher temperature condition that those in

between the invariant points IA, IB, ID and IF is considered. Apuseni. The most important factor controlling the spur- When ln a(Si(aq)) is greater than –5.4, the assemblage rite skarn formation at Fuka was the controlled activity of becomes unstable. A decrease in silica activity increases silica in the high－temperature aqueous solutions. the stability field of the spurrite －bearing assemblage. In summary, the petrogentic grids presented in this However, when ln a(Si(aq)) is les than –7.5, the reaction study suggests a strong control of silica activity on the

between pseudoinvariant points IB and ID, which form til- mineralogy of skarn, in addition to the temperature and leyite from spurrite become metastable. Also, the stability fluid composition. Using the grids presented here, the － field of spurrite becomes narrow with respect to the XCO2 physico chemical characterization of skarn formation at conditions. Hence, ln a(Si(aq)) between –7.5 and –6 is contact aureoles can thus be more accurately constrained, considered optimum for the formation of large quantities especially in terms of element mobility. of spurrite. This corresponds to a silica concentration of −4 −3 9.1 × 10 – 4.5 × 10 moles/litre of H2O. The tempera- Acknowledgments ture of formation of spurrite in the above silica concentration can be bracketed between 850°C and 1080°C and the MS－K thanks Prof. Wada for constant encouragement and － XCO2 condition between 0.05 and 0.42. enlightening discussion in the field. MS K acknowledges The mineral assemblages and their stability fields JSPS research grant (No. 15740302) and field research discussed above indicate that the spurrite skarn formation grant (No. 15403016; led by Prof. Hideki Wada). Mr. in Fuka resulted from the metasomatic movement of sil- Kawahara and Tsujimoto are thanked for assistance dur- ica －bearing high －temperature fluids. Spurrite, the most ing field work. We thank Dr. Santosh for the invitation to prominent mineral in the skarn formation at Fuka, formed contribute in this volume and for his editorial efforts. by the reaction of silica transported in aqueous solution from the crystallizing magma. High－temperature condi- References

tions, low XCO2 and controlled ln a(Si(aq)) were instru- mental in the formation of extensive spurrite exoskarn. Agrell, S.O. (1965) Polythermal metamorphism of limestones at The present of rankinite and tilleyite at Fuka west outcrop Kilchoan, Ardnamurchan. Mineralogical Magazine, 34, 1－15. Benece, A.E. and Albee, A.L. (1968) Empirical correction factors indicate a localized low XCO2 condition (Fig. 5) existing for the electron microanalysis of silicates and oxides. Journal when compared with the other occurrences. The forma- of Geology, 76, 382－403. tion of tilleyite near the andesitic dikeis considered to be a Deer, W.A., Howie, R.A. and Zussman, J. (1992) Rock Forming retrograde transformation of spurrite during the intrusion, Minerals. pp. 696, Longman, London. since the stability field of tilleyite is at a lower tempera- Henmi, C. (1995) Kusachiite, CuBi2O4, a new mineral from Fuka, Okayama Prefecture, Japan. Mineralogical Magazine, 59, ture and lower XCO2 conditions than that of spurrite (Fig. 5). 545－548. The temperature conditions prevailed during spurrite Henmi, C., Kusachi, I., Henmi, K., Sabine, P.A. and Young, B.R. skarn formation is in the range of 850°C to 1080°C. The (1973) A new mineral bicchulite, the natural analogue of high temperature condition of monzodioritic magma is gehlenite hydrate, from Fuka, Okayama Prefecture, Japan also revealed by the equilibrium mineral assemblages and Carneal, County Antrim, Northern Ireland. Mineralogi- cal Journal, 7, 243－251. observed in the xenoliths. Two pyroxene thermometry Henmi, C., Kusachi, I., Kawahara, A. and Henmi, K. (1977) applied to the xenolith assemblages indicate peak temper- Fukalite, a new calcium carbonate silicate hydrate mineral. ature range of 860°C to 930°C (Omae et al., 2002), which Mineralogical Journal, 8, 374－381. corresponds with the range of temperature deduced in the Henmi, C., Kusachi, I. and Henmi, K. (1995) Morimotoite, 338 M. Satish－Kumar, Y. Yoshida and I. Kusachi

2+ － Ca3TiFe Si3O12, a new titanian garnet from Fuka, Okayama Long, J.V.P. and Agrell, S.O. (1965) The cathodo luminescence Prefecture, Japan. Mineralogical Magazine, 59, 115－120. of minerals in thin section. Mineralogical Magazine, 34, 318－ Henmi, C. and Kusachi, I. (1992) Clinotobermorite, 326. － Ca5Si6(O,OH)18・5H2O, a new mineral from Fuka, Okayama Marincea, S., Bilal, E., Verkaeren, J., Pascal, M L. and Fonteilles, Prefecture, Japan. Mineralogical Magazine, 56, 353－358. M. (2001) Superposed parageneses in the spurrite －, tilley- Holland, T.J.B. and Powell, R. (1998) An internally consistent ite－, and gehlenite－bearing skarns from Cornet hill, Apuseini thermodynamic data set for phases of petrologic interest. mountains, Romania. The Canadian Mineralogist, 39, 1435－ Journal of Metamorphic Geology, 16, 309－343. 1453. Joesten, R. (1974) Local equilibrium and metasomatic growth of Matsubara, S., Miyawaki, R., Kato, A., Yokoyama, K. and Oka- － zoned calc silicate nodules from a contact aureole, Christmas moto, A. (1998) Okayamalite, Ca2B2Si2O7, a new mineral, Mountains, Big Bend region, Texas. American Journal of boron analogue of gehlenite. Mineralogical Magazine, 62, Science, 274, 876－901. 703－706. Joesten, R. (1976) High －temperature contact metamorphism of Nakai, I., Okada, H., Matsunomi, K., Koyama, E. and Nagashima,

carbonate rocks in a shallow crustal environment, Christmas K. (1986) Henmilite, Ca2Cu(OH)4[B(OH)4]2, a new mineral Mountains, Big Band region, Texas. American Mineralogist, from Fuka, Okayama Prefecture, Japan. American Mineralo- 61, 776－781. gist, 71, 1234－1239. Kerrick, D.M. (1991) Contact Metamorphism. Reviews in min- Omae, A., Kusachi, I. and Kobayashi, S. (2002) Petrology of the eralogy. pp. 847, Mineralogical Society of America, 26. igneous rocks forming high －temperature skarns at Fuka, Kusachi, I. (1975) Occurrence and genesis of skarns at the Fuka Okayama Prefecture. Japanese Magazine of Mineralogical North outcrop, the town of Bitchu, Okayama Prefecture. and Petrological Sciences, 31, 1－14. Bulletin of School of Education, Okayama University, 43, Pascal, M－L., Fonteilles, M., Verkaeren, J., Piret, R. and Marin- 71－105 (in Japanese). cea, S. (2001) The melilite－bearing high－temperature skarns Kusachi, I., Henmi, C. and Henmi, K. (1978) Gehlenite－spurrite of the Apuseni mountains, Carpathians, Romania. The Cana- skarns at the Fuka West outcrop, the town of Bitchu, dian Mineralogist, 39, 1405－1434. Okayama Prefecture. Bulletin of School of Education, Powell, R. and Holland, T.J.B. (2001) Course Notes for Okayama University, 48, 25－39 (in Japanese). “THERMOCALC Workshop 2001: Calculating Metamorphic Kusachi, I., Henmi, C. and Henmi, K. (1984) An Oyelite bearing Phase Equilibria” (on CD－ROM). vein at Fuka, the Town of Bitchu, Okayama Prefecture. Jour- Teraoka, Y., Matsuura, H., Makimoto, H., Yoshida, F., Kamitani, nal of Japanese Association for Mineralogists Perologists and M., Hiroshima, T., Komazawa, M. and Shichi, R. (1996) Economic Geologists, 79, 267－275. Geological map of Japan 1:200,000 Takahashi. Geological Kusachi, I., Henmi, C. and Kobayashi, S. (1995) Takedite, a new Survey of Japan. mineral from Fuka, Okayama Prefecture, Japan. Mineralogi- Tracy, R.J. and Frost, B.R. (1991) Phase equilibria and thermo- cal Magazine, 59, 549－552. barometry of calcareous, ultramafic and mafic rocks, and iron Kusachi, I., Takechi, Y., Henmi, C. and Kobayashi, S. (1998) Para- formations. In Kerrick, D.M. (ed) Contact metamorphism. sibirskite, a new mineral from Fuka, Okayama Prefecture, Reviews in mineralogy, Mineralogical Society of America, Japan. Mineralogical Magazine, 62, 521－525. 26, 207－289. Kusachi, I., Kobayashi, S., Henmi, C. and Takechi, Y. (1999) Cal- －－ cium borate minerals in the CaO B2O3 H2O system at Fuka, Manuscript received; 6 January, 2004 Okayama Prefecture, Japan. Mineralogical Journal, 28, 41 － Manuscript accepted; 27 April, 2004 46 (in Japanese).