PAPERS
Department of Geology • The University of Queensland
VOLUME 12 number 3
Editor: S.H. HALL
Zeolites in the Main Range Volcanics, Queensland D.J. DRYSDALE P.264-268
Bracewellite and the origin of "Merumite" D.J. DRYSDALE P.269-277
Perlitic texture and other fracture patterns produced by hydration of glassy rocks D.J. DRYSDALE P.278-285
Lithium aluminium silicate minerals and pollucite from Meldon, Devon and San Piero in Campo, Elba D.J. DRYSDALE P.286-293
Polytype 2H molybdenites with high rhenium contents D.J. DRYSDALE P.294-303
Date of publication: December 1991 ZEOLITES IN THE MAIN RANGE VOLCANICS, QUEENSLAND
by D.J. Drysdale
ABSTRACT. Amygdaloidal zeolites from the Main Range Volcanics, Queensland, in the collections of the Queensland Museum and the Geology Museum, Queensland University, are dominated by chabazite and natrolite. Stilbite and analcite are the only other species represented. Zeolites are present in a number of amygdaloidal horizons at several levels in the pile and there is no evidence for vertical zonation of species.
INTRODUCTION
Zeolites present in amygdales in basalt lavas that build up thick piles of more or less constant composition are characteristically distributed in depth related zones that are regional in scale and defined by the incoming and outgoing of marker species. This zonation is interpreted as reflecting heat flow in the lava pile during zeolitization and is supposed to be a measure of regional geothermal gradient. High heat flow, as at mid-ocean ridges and spreading centres in continental environments, is the main factor that controls the nature and extent of regional zonation. Local zonal distributions of zeolite species have also been observed, related to local heat flow domes about individual volcanic centres, active geothermal centres, dyke swarms, highly fractured zones, shallow intrusions, and the roots of deeply eroded major volcanoes. No study has been made of the zeolites present in the pile of the Main Range Volcanics, Queensland.
THE MAIN RANGE VOLCANICS, QUEENSLAND, AND THEIR ZEOLITES
Stratigraphy of the Main Range Volcanics is described by Stevens (1969), Grenfell (1984) and Ewart and Grenfell (1985). The volcanics are late Oligocène to early Miocene lava flows, products of anorogenic volcanism, that cover an area of 4300 square kms. and attain a maximum thickness of about 900m. Dips are shallow, generally less than 3 degrees and most commonly westward. The southern Main Range (south of a line from Laidley to Allora) comprises two formations of subequal thickness: the Governors Chair Volcanics (lower), a mildly alkaline basalt - comendite association with several thick trachyte horizons (including the Steamers Trachyte Member); and the Superbus Basalt (upper), exposed in most of the southern Main Range, with a maximum recorded thickness of 440 m of predominently hawaiite lavas with alkaline olivine basalt and mugearite. The Toowoomba Basalt of the northern Main Range passes laterally into the Suberbus Basalt.
Pap. Dep. Geol. Univ. Qd., 12(3): 264-268, Dec., 1991. 265
Some zeolites listed in Whitehouse (1937) are from the Main Range Volcanics: chabazite associated with stilbite in basalt, Toowoomba; chabazite in decomposed basalt. Freestone Creek; chabazite associated with calcite and natrolite in basic volcanic tuff. Spring Bluff - Harlaxton railway cutting; and chabazite associated with calcite, magnesite and natrolite in decomposed basalt. Mount Davidson. Stevens (1965, 1969, 1984) notes species at a number of localities: at about 975m on Mount Mitchell and Mount Cordeaux an amygdaloidal zone rich in chabazite indicates an upper margin of a basalt flow, for 75m above this the lavas are highly amygdaloidal and generally altered, and at higher elevations amygdales occur but are less abundant and confined to flow tops; zeolites occur in basalt 1km past the Boonah border gate on White Swamp Road; porphyritic basalt with chabazite occurs below the Steamers Trachyte Member on the road to Queen Mary's Falls and just below Dagg's Falls; at Harlaxton there is natrolite-bearing basalt; and at Teviot Falls there are amygdaloidal basalts with natrolite and analcite. Stevens also notes markedly amygdaloidal basalts containing chabazite are found at just over 915m between Mount Cordeaux and Acacia Plateau and at lower levels to the west of this line but it is not known whether these occurrences are at the same stratigraphic horizon. The collections of the Queensland Museum and the Geology Museum of the University of Queensland house 45 specimens of zeolitic amygdaloidal basalts identifiable as coming from the Main Range Volcanics in the Ipswich and Warwick 1: 250,000 sheet areas. Chabazite dominates (present in 34 specimens), followed by natrolite (9 specimens). Locality information is detailed but difficulties with the lateral conelation of lavas make it impossible to work out a stratigraphy for the museum specimens. Mount Mitchell (5 specimens). Mount Cordeaux (5 specimens), the Toowoomba district (15 specimens). Mount Superbus (3 specimens) and the Killamey district (9 specimens) dominate the collections. Main Range localities represented in the Australian Museum collections have only chabazite and natrolite (pers. comm. Dr Lin Sutherland).
The work of Stevens and Grenfell suggests it may be that zeolitic amygdaloidal basalts are restricted to a few highly amygdaloidal horizons in the lava pile. The Dagg's Falls and Queen Mary's Falls road occurrences are in basalts not far below the Steamers Trachyte Member, the Boonah border gate and Teviot Falls localities are probably also below the Steamers Trachyte Member, the zeolite occunences on Mount Cordeaux are well up in the Superbus Basalt, the occurrence near Acadia Plateau is in the Governors Chair Volcanics below the Steamers Trachyte, those on Mount Mitchell are also in the Superbus Basalt, and the Harlaxton locality is in the Toowoomba Basalt.
DISCUSSION
The Main Range assemblages differ from those observed in most other thick piles of basalt lavas in two respects: 266
1. The range of species is limited relative to assemblages from the classic localities of Ireland, Iceland, and the Faeroes (Cunie 1910, Walker 1962 and references therein, Betz 1981, Mehegan et al. 1982 and references therein). Table Mountain, Colorado (Kile and Modreski 1988), and Paterson, New Jersey (Peters et al. \91Sy, and assemblages from the Parana Basin, Brazil (Murata et al. 1987), the Deccan Basalt, India (Jeffery et al. 1988), Nova Scotia, Canada (Bujak & Donohoe 1980), and the Siletz River Volcanics, Oregon (Keith & Staples 1985).
2. There is no zonation of species as described from Ireland, Iceland, Nova Scotia, and Brazil (references as above). For other localities zonation is suspected but not proven: Betz (1981) notes that it appears that zeolite zoning is present in the Faeroes but does not describe a scheme; at Table Mountain, Colorado, locality dependant variations in individual species descriptions and occurrence that are analogous to regional variations in Ireland, Iceland, the Faeroes, India and Brazil are noted by Kile and Modreski (1988) but lack of complete locality information for many specimens makes a comprehensive evaluation of regional variation uncertain; and Jeffery et al. (1988) found the zones distinguished by earlier workers in the Deccan basalts to be absent from an area of 15,000 sq. kms. and the zeolites to be unzoned or zoned according to a pattern not yet recognised.
Acccounts of lava piles lacking zonation of zeolites are rare. The Siletz River Volcanics, Oregon, (Keith & Staples 1985) are a thick pile of zeolitized flows devoid of any regional zonation, and in the volcanics of north-eastern Azerbaijan, Iran, (Comin - Chiaramonti et al. 1979), the distribution of zeolite species does not show any relationship to the volcanic stratigraphy. The Siletz River Volcanics is the only well documented example of a thick pile of lavas in which the zeolite species lack a vertical zonal distribution. Keith and Staples (1985) suggest zeolitization here occuned during a low temperature (60 - 70°) hydrothermal event, or by reaction of cold (circa - 10°) meteoric water with basalt over a long time, and that the concept of localised heat flow from sources such as individual eruptive centres seems to be a reasonable explanation of the inegular distribution of the zeolites. Murata et al. (1987) suggest a fundamental cause of the lack of zonation here may be low heat flow in the region, a result of the subduction of oceanic plate under this part of the North American plate, and that the cavities contain different assemblages of zeolites distributed at random because sealing off of cavities from one another through precipitation of minerals (in this 267 case montmorillonite) at different times led to their minerals crystallizing in closed systems. There may be other examples of lack of vertical zonation of zeolite species in basalt sequences. For example, Birch (1988) notes there has been no evaluation of zonation, either vertically or horizontally, in the distribution of zeolite species in the Older Volcanics, Flinders, Victoria, but Hall (in Birch, 1989) observes the zeolites are restricted to exposures along the coast and have not been detected in the flows further inland, the majority are found in relatively thin and markedly vesicular flows which are amongst the youngest in the sequence, and montmorillonite as a vesicle lining is the most common secondary mineral and the first mineral to have formed in the cavities. Coulsell (1980) suggested crystallization from percolating groundwater solutions for the zeolites at West Head, Flinders, and crystallization from groundwater solutions in preexisting and probably partly weathered surface basalts reheated by new overlying flows for the zeolites at Cairns Bay and Little Bird Rock, Flinders.
Restricted species ranges are less rare in the literature. They are reported, for example, for a number of zeolitized basalt flows in continental environments, including the Massif Central, France, in references cited in Robert et al. (1988). In the Main Range Volcanics the paucity of species, and the lack of species variation, suggest zeolitization was not intense - the number of species in Ireland and Iceland increase with increasing intensity of zeolitization.. The zeolites appear to occur mainly in a few highly amygdaloidal horizons. Zeolitization may have been by percolating meteoric water and have taken place at low temperatures.
ACKNOWLEDGEMENTS
To Dr N.C. Stevens for specimens, help, advice and his interest and to the Queensland Museum for information about their collections.
REFERENCES
BETZ, V., 1981. Zeolites from Iceland and the Faeroes. Mineral. Rec., 12:5-26. BIRCH, W.D., 1988. Zeolites from Phillip Island and Flinders, Victoria. Mineral. Rec., 19:451-460. BIRCH, W.D.(Ed), 1989. Zeolites of Victoria. The Mineralogical Soc. of Victoria, Special Publication , No.2. BUJAK, J.P. & DONOHOE, H.V., 1980. Geological highway map of Nova Scotia. Atlantic Geoscience Soc., Special Publication, No.l. COMIN-CHIARAMONTI, P., PONGILUPPI, D. & VEZALLINI, G., 1979. 2Leolites in shoshonitic volcanics of north-eastern Azerbaijan (Iran). Bull. Mineral., 102:386-390. COULSELL, R., 1980. Notes on the zeolite collecting area of Flinders, Victoria. Australian Mineralogist, 1:163-167. CURRIE, J.R., 1910. The minerals of the Faeroes. Trans. Edinburgh Geol. Soc., 9:1-68. 268
ENGLAND, B.M. & SUTHERLAND, F.L., 1988. Volcanic zeolites and associated minerals from N.S.W., Mineral. Rec. 19:389-406. EWART, A. & GRENFELL, A.T., 1985. Cainozoic volcanic centres in southeast Queensland, with special reference to Main Range, Bunya Mountains, and the volcanic centres of the northern Brisbane coastal region. Pap. Dep. Geol. Univ. Qd., 11:1-57. GRENFELL, A.T., 1984. The stratigraphy, geochronology and petrology of the volcanic rocks of the Main Range, southeastern Queensland. Unpublished Ph.D. thesis, Univ. Qd. JEFFERY, K.L., HENDERSON, P., SUBBARAO, K.V. & WALSH, J.N., 1988. The zeolites of the Deccan basalt - a study of their distribution. Geol. Soc. India Mem.li}, K.V. Subbarao (Ed.):151-162. KEITH, T.E.C. & STAPLES, L.W., 1985. Zeolites in Eocene basaltic pillow lavas of the Siletz River Volcanics, Oregon. Clays & Clay Miner., 33:135-144. KILE, D.E. & MODRESKI, P.J., 1988. Zeolites and related minerals from the Table Mountain lava flows near Golden, Colorado. Mineral. Rec., 19:153- 184. MEHEGAN, J.M., ROBINSON, P.J. & DELANEY, J.R., 1982. Secondary mineralization and hydrothermal alteration of the Reydarfjordur drill core, Eastern Iceland. J. Geophys. Res., 87:6511-6524. MURATA, K.J., FORMOSO, M.L.L. & ROISENBERG, A., 1987. Distribution of zeolites in lavas of southeastern Parana Basin, Brazil. J. Geol., 95:455- 467. PETERS, T.A., PETERS, J.J. & WEBER, J, 1978. Famous mineral localities: Paterson, New Jersey. Mineral. Rec., 9:157-179. ROBERT, C., GOFFE, B. & SALIOT, P., 1988. Zeolitization of a basaltic flow in a continental environment : an example of mass transfer under thermal control. Bull. Mineral., 111:207-223. SHANNON, D.M., 1983. Zeolites and associated minerals from Horseshoe Dam, Arizona.Mineral. Rec., 14:115-117. STEVENS, N.C., 1965. The volcanic rocks of the southern part of the Main Range, S.E. Queensland. Proc. Roy. Soc. Qd., 77:37-52. STEVENS, N.C., 1969. The Tertiary volcanic rocks of Toowoomba and Cooby Creek, South-east Queensland. Proc. Roy. Soc. Qd., 80:85-96. STEVENS, N.C., 1984. Queensland Field Geology Guide Geol. Soc. Australia, Qd. Div. WALKER, G.P.L., 1962. Garronite, a new zeolite from Ireland and Iceland. Mineral. Mag., 33:173-186. WHITEHOUSE, M.J., 1937. Zeolites of Queensland. Proc. Roy. Soc. Qd., 49:71- 81.
Dr. D.J. Drysdale Department, of Geology & Mineralogy The University of Queensland Queensland, 4072. BRACEWELLITE AND THE ORIGIN OF "MERUMITE"
by D.J. Drysdale
ABSTRACT. A review of the chemical and crystallographic characteristics and the occunences of bracewellite, guyanaite, grimaldiite and eskolaite, the principal Cr minerals constituting "merumite", and of syntheses of guyanaite, grimaldiite and eskolaite, suggests these minerals crystallized hydrothermally from Cr(III), Al(ni), Fe(lII) containing solutions or gels under oxidizing conditions that converted some Cr(III) to Cr(VI). The presence of Fe(III) in its diaspore structure stabilizes bracewellite. Guyanaite and eskolaite synthesize more readily than bracewellite and dominate the merumite assemblage; grimaldiite is also easily synthesized and is a minor component of merumite, perhaps forming at a later stage in vugs.
INTRODUCTION
"Merumite" was defined by Milton et al. (1976) as an aggregate of (mainly) eskolaite, three hydrous chromium oxide minerals (bracewellite, guyanaite, and grimaldiite), a copper-chrome oxide mcconnellite, and chrome gahnite, with quartz, gold and chrome pyrophyllite, occuning as grains in placers in the Merume River valley, Guyana, and thought to be derived from the adjacent Robello Ridge. "Merumite" is of interest as bracewellite, guyanaite, grimaldiite and eskolaite are the only high Cr content minerals known other than chromite, and "merumite" the only occurrence of a high Cr assemblage that may be of hydrothermal origin other than some chromites in serpentinized ultrabasics described by Swaddle et al. (1971).
THE "MERUMITE" MINERALS
Milton et al. (1976) observe that "merumite" consists of a small number of minerals and their relative amounts, crystal habits and relationships to one another vary greatly from specimen to specimen. Guyanaite, eskolaite and perhaps bracewellite are the major components of "merumite". Guyanaite seems to be at least equally abundant generally to eskolaite. The three minerals may be intergrown. Eskolaite occurs in massive aggregates that may (rarely) be free of admixed minerals. Bracewellite occurs as prismatic crystals lining vugs and cracks in "merumite" and crystalline masses admixed with other minerals. Some "merumite" grains consist largely of bracewellite, others of bracewellite with a little guyanaite, or of bracewellite, eskolaite and minor guyanaite, or of bracewellite and eskolaite. Grimaldiite/mcconnellite intergrowths occur in vugs, chromian gahnite crystals are frequently associated with the grimaldiite/mcconnellite polycrystals, chromian pyrophyllite and quartz are equally common accessory minerals, and gold occurs as a primary mineral. Their colours, chemistry and crystallography are detailed in Table 1.
Pap Dep. Geol. Univ. Qd., 12(3): 269-277, Dec., 1991 Table 1 The properties of the principal chrome minerals in merumite,
extracted from Milton et al. (1976)
BracewcUitc Guyana! te Grimaldiite Eskolaite
Colour Deep red to black light green to Deep red Green
greenish black;
dark reddish brown
Chemistry Fe variable and Fe Al low Fe low Pure Cr2O3
characteristically with guyanaite
high; Al variable Fe rich with
bracewellite
Crystallography
Space group Pbnm Pnnm R3m R3c
4.492 A 4.857 A 2.99 A 4.958 A a
9.860 A 4.295 A b
2.974 A 2.958 A 13.4 A 13.6 A c
Syntheses None hydrothermal hydrothermal Thermal
reduction of reduction of decomposition of
CrO2; chromate solutions; chromates or
hydrothermally grimaldiite
hydrothermal from CRIII
oxidation of solutions
hydrous Cr III & hydrous
oxide gels Cr III oxide gels 271
Eskolaite and the three hydrous chromium oxides remain rare minerals. Bracewellite is known only from Guyana. Guyanaite has one other known occurrence, at Outokumpu, Finland, (Vuorelainen et al. 1968), where golden brown to greenish brown guyanaite pseudomorphs eskolaite. Grimaldiite has two other known occurrences. From China it is reported from ultrabasic rocks (Wang 1983); from Bolivia, Livingstone et al. (1984) describe grimaldiite partially coating cracks and cavities in penroseite. Eskolaite has five other known occunences. At Outokumpu (Kouvo and Vuorelainen, 1958) it occurs in chrome bearing tremolite skarn, pyrrhotite veins, skarn copper nickel ore, ore footwall quartzites, and chlorite seams in the mine country rocks. In the Transvaal, (Forster 1960) eskolaite is associated with chromite in a layered ultrabasic complex with the paragenesis olivine-gold. In Yugoslavia, Grafenauer (1961) found chromites which when etched showed eskolaite separated in the chromite. Oppenheim et al. (1977) reported eskolaite in small pebbles of quartz greywacke within boulder clay, and Moore et al. (1978) found eskolaite in placers in Brazil.
The various occunences have some similarities. The Guyana and Brazil occunences are in placers and the Irish occurrence in pebbles; there is an association with gold in Guyana, Transvaal and Brazil; with nickel, cobalt and copper mineralization in Transvaal and Outokumpu; there are ultrabasic rocks in the vicinity in Guyana, Transvaal, Brazil, Yugoslavia and China; and the chromium mineralization is believed to be hydrothermal at Guyana, Outokumpu, Bolivia and Brazil. Both the new occurrences of grimaldiite are pink brown and Fe low, with significant AI2O3, like the Guyana material.
SYNTHESES OF MERUMITE' MINERALS
There is a very large chemical literature on the hydrous chromium oxides because of their importance as catalysts. This literature is confusing as grimaldiite is designated alpha CrOOH, guyanaite beta CrOOH, and bracewellite appears nowhere. It would seem more satisfactory to keep nomenclature consistent with the Al, Fe and Mn oxyhydroxides and reserve alpha CrOOH for bracewellite, the form isostructural with diaspore, goethite and groutite. This convention is used here.
The syntheses data in Table 1 are from Laubengayer and McCune (1952), Thamer et al. (1957), Laswick and Plane (1959), Laswick (1962), Snyder and Ibers (1962), Tombs et al. (1964), Carruthers et al. (1968), Torokin et al. (1968), Shibasaki et al. (1970), Swaddle et al. (1971), Shibasaki (1972), Shibasaki et al. Christensen (1976), and Fenerty and Sing (1976). Synthetic guyanaite is olive green, and grimaldiite can be synthesized in blue grey and red violet varieties. Guyanaite can be prepared via topotactic reactions by hydrothermal reduction of chromates at 200 to 400° and by hydrothermal oxidation and crystallization of hydrous Cr (III) oxide gels at 200 to 250°. This latter reaction may proceed via chromium dioxide as Shibasaki et al. (1970) and Shibasaki (1972) note the ease of oxidation of chromium hydroxides and oxyhydroxides 272 arising from similarity in structure between chromium dioxide and grimaldiite. Dehydration of the hydrous oxides under oxidising conditions was accompanied by some oxidation of Cr(III) to Cr(VI), and Cr(VI) decomposes above 300° to Cr2O3. Red grimaldiite can be synthesized by hydrothermal reduction of chromate solutions at 300 to 360°; blue grey grimaldiite by hydrothermal crystallization from Cr(III) solutions at 150 to 300°, and crystallization of hydrous Cr(III) oxide gel at up to 415°. Cr(VI) was produced from Cr(III) solutions if conditions were oxidizing. Although zinc chrome spinels have not been synthesized, spinels MCr2O4 (M = Mg, Mn, Fe, Co) have been made hydrothermally at 300° from chromium hydroxide and M hydroxide (Swaddle et al. 1971).
Colour of the synthetic products is influenced by particle size and/or presence at the surface of Cr in different oxidation states (Alario - Franco and Sing 1972). Ratnasamy and Leonard (1972) found regions in Cr hydroxide gels wherein the short range structure is similar to grimaldiite, eskolaite and (CrO4)2-. Red colours suggest the presence of some Cr(VI) during the synthesis.
Bracewellite has not been synthesized in the experiments cited. The chemistry of the "merumite" minerals suggests crystallization in the Cr2O3 - Fe2O3 - AI2O3 - H2O system and synthesis of bracewellite was attempted in this and the Cr2O3 - Fe2O3 - H2O system. In the first group of experiments, in the system Cr2O3 - Fe2O3 - H2O, mixed hydroxides of composition (Cr 0.5 Fe 0.5) hydroxide were coprecipitated from Cr(III) and Fe(III) nitrate solutions with sodium hydroxide and/or ammonium hydroxide (Christensen and Jensen 1967 found the precipitating agent influences whether ScOOH with diaspore or boehmite structure forms in the SC2O3 - H2O system). The mixed hydroxides were crystallized hydrothermally at 2000 bars and 320° in sealed gold capsules in externally heated cold seal pressure vessels. Products identified by XRD were grimaldiite and haematite. The result is like that of Zolotovskii et al. (1978), who obtained grimaldiite, goethite and haematite by aging Fe(III) and Cr(III) mixed hydrogels and found no Cr-Fe substitution. Failure to synthesize bracewellite is not surprising. The alpha MOOH minerals are notoriously difficult to nucleate and grow (with the exception of goethite). Synthesis may require introduction of extraneous cations (e.g. Li or F in tohdite and Cl in akaganeite), or specific routes in preparation of starting materials to provide suitable precursor structures, or presence of seed crystals, or use of very high pressures. The latter is not an option for bracewellite synthesis as CrOOH takes the InOOH structure at very high pressures. The second group of experiments were carried out in the system Cr2O3 - Fe2O3 - AI2O3 - H2O, using mixed hydroxide precipitates seeded with diaspore crystals from Chester, Mass., and with synthetic alpha FeOOH. The bulk composition of charges was (Cr 0.80 Fe 0.10 Al 0.10)OOH. Relative changes in the amount of diaspore - like phase were determined by comparing XRD patterns of reactants and products. The method combines the techniques of Maurel (1967) and Biais et al. (1972). The results show growth of a phase (Cr, Fe, A1)OOH isostructural with diaspore. 273
DISCUSSION
Milton et al (1916} show the weight of evidence indicates "memmite" originated in the local sandstones and conglomerates, which together with shale, jaspilite, quartzite and tuff, form the Robello Ridge, an outlier of Roraima Formation. The Formation is fully oxidised red bed facies and is intruded by a vast amount of tholeiitic magma represented by the dykes and sills of the Roraima Intrusive Suite, dated at 1700my, and by the Minor Dyke Suite, dated at 221my. The rocks of the ridge show mild hydrothermal metamorphism. Their model suggests "memmite" was formed in a low to moderate temperature hydrothermal type of deposit. In the model Cr is precipitated at a late stage, after quartz, along with gold, and prior to chromian pyrophyllite. The Cr has been concentrated and precipitated as oxide - hydroxide, perhaps with an early stage in which the proto - merumite was an uncrystallized gel.
Bums and Bums (1975) observe the mineralogy of Cr is dominated by the Cr(III) and chromate ions. The colours of the synthetic and natural minerals, the starting materials for syntheses, and synthesis conditions and routes suggest two possibilities for the formation of "merumite": (1) by hydrothermal reduction of chromate solutions, at 200 to 400°, in a model analogous to the formation of montroseite and paramontroseite from vanadate solutions; (2) crystallization from Cr(III) solutions or from hydroxide gels under oxidising conditions at 150 to 400°.
In Guyana an origin by reduction of chromate is unlikely if "merumite" was deposited in the fully oxidised red bed facies of the Roraima Formation. Natural chromates are absent from Guyana. Elsewhere they are limited to the oxidation zone of lead bearing ore deposits and to desiccated saline deposits, environments very different from that of "merumite".
Frequently the source of Cr for chromates is obscure, but at the Florence Mine, Arizona (Williams & Anthony 1970) chromates occur where the oxide ores are near an altered diabase dyke carrying 0.14% CrO3. An analogous source for Cr cannot be established for "merumite" as no Cr content data are available for the Roraima Intrusive Suite or the Minor Dyke Suite and a source for Cr remains problematical.
Origin from Cr(III) oxide hydroxide gels crystallizing under oxidising hydrothermal conditions is more likely. Under such conditions oxidation of some Cr(III) is likely. Some Mn mineral assemblages formed under similar conditions: at Volcano, Eolian Islands, goethite, haematite, manganite, feitknechtite and braunite are being deposited in recent sediments from hot, acid, hydrothermal fluids containing Fe(II) and Mn(II) produced by submarine volcanic activity aniving in an oxidising basic environment (Monaco & Valette 1976). 214
The mineral syntheses point to temperatures of crystallization between 150 and 400°. A lower temperature limit is suggested by the post-CrOOH formation of chrome pyrophyllite as Papezik and Keats (1976) found pyrophyllitization of rhyolite at 2000 bars or less in acid solutions in the temperature range 260 to 280°.
The presence of the three polymorphs of CrOOH in "merumite" suggests metastability, not unusual in assemblages of the oxyhydroxide minerals: similar assemblages of AIOOH, FeOOH and MnOOH minerals are common and unremarkable. The close association of some of these minerals reflects their close crystallographic similarities, as in the groutite/manganite association described by Gruner (1947). Bracewellite and guyanaite are similarly crystallographically closely related.
Contrasting compositions of eskolaite associated with bracewellite and associated with guyanaite, differences between the compositions of typical bracewellite and guyanaite, and between the compositions of typical grimaldiite, bracewellite and guyanaite suggest the diaspore structure of bracewellite may be stabilised by incorporation of fenic iron. There is support for this from the literature on syntheses of aluminium hydroxides and oxyhydroxides. Montel (1960) and Saraswat et al. (1980) found ferric hydroxide orients crystallization of aluminium hydroxide towards bayerite in amorphous coprecipitates of Fe and Al hydroxides; Gout and Kouadio (1973) formed ferriferous diaspore by grinding goethite + hydrargillite mixtures at 110 to 210® and describe in natural bauxites ferriferous diaspore side by side with purely aluminous boehmite; Maurel (1967) synthesized diaspore + haematite from the oxides in presence of 1 to 5% Fe2O3 and some crystals of natural diaspore at 425° and 2500 bars for 30 days; and Biais et al. (1972) produced ferriferous diaspore by hydrothermal treatment of mixed Al-Fe hydroxide coprecipitates at 180° and saturated vapour pressure for 24 hours. Aluminous goethite intervenes as an initial support to start the germination of diaspore. Precursor structures are important: Maurel (1966) made feniferous boehmite from mixtures of the oxides at 250° and 1000 bars for 21 days and the starting materials showed the XRD reflections of poorly crystallized boehmite.
CONCLUSIONS
The geographical association of "merumite" placers with the Robello Ridge suggests a genetic relationship with the fully oxidized red-bed facies Roraima Formation. The assemblage of CrOOH minerals with mcconnellite, chrome gahnite, pyrophyllite, gold and doubly terminated quartz crystals suggests a low to moderate temperature hydrothermal origin, and synthesis experiments show these minerals could result from hydrothermal reduction of chromate solutions, or hydrothermal crystallization of Cr(III) containing solutions or Cr oxide hydroxide gels. The absence of a reducing environment suggests the latter. The red brown colours in some synthetic products and the natural minerals points to the presence during formation of some Cr(VI) and formation under oxidising 275 conditions. There is evidence in natural and synthetic oxide hydroxides that extraneous cations can stabilize some structures, and synthesis routes and conditions can influence the reaction outcome. The association in "merumite" of Fe rich eskolaite with Fe rich bracewellite and the low Fe contents of guyanaite and associated eskolaite and of grimaldiite both here and in their few other known occurrences point to stabilization of bracewellite by incorporation of Fe into its diaspore structure.
"Merumite" is an interesting pointer to the potential for low temperature Cr-rich hydrothermal mineralization that may not involve chromite, but where the Cr came from in Guyana and why the merumite minerals are so rare remain enigmas.
ACKNOWLEDGEMENTS
To Professor Charles Milton, Quadam Research Professor of Mineralogy, George Washington University, for his interest and information about his work on merumite; Professor W.S. MacKenzie for making available the high pressure experimental facility of the University of Manchester and Mr Douglas Kidd for running the experiments; Dr Sat Narain of the Guyana Geology and Mines Commission for information; Dr P.J. Moore of the Institute of Geological Sciences for help with their records of the Mazaruni district and for information about the Roraima Formation; and the Department of Geological Sciences, University of Durham for library and laboratory facilities where part of this work was carried out while a Visiting Scientist.
REFERENCES
ALARIO-FRANCO, M.A. & SING, K.S.W., 1972. Interconversion of orthorhombic chromium oxyhydroxide and chromium dioxide. J. Thermal Anal. ,4:47-52. BIAIS, R., BONNEMAYRE, A., DE GRAMONT, X., MICHEL, M., GIBERT, H. & JANOT, C., 1972. Etudes des substitutions Al-Fe dans les oxydes et hydroxides de synthèse. Preparation de diaspore ferrifere. Bull. Soc. Fr. Miner. CrZsiaZ/ogr. ,95:308-321 BURNS, V.M. & BURNS, R.G.,1975. Mineralogy of chromium.Geochim. Cosmochim. Acta 39:903-910. CARRUTHERS, J.D., FENERTY, J. & SING, K.S.W. 1968. Bulk and surface properties of chromium oxide gels. Reactivity of solids Proceedings International Symposium 6th. '.121-135. CHRISTENSEN, A.N., 1976. Hydrothermal preparation and magnetic properties of CrOOH.Jc/a Chem. 5'ca/2i/.,A30:133-136. CHRISTENSEN, A.N. & JENSEN, S.J.,1967. Hydrothermal preparation of ScOOH.Jcia Chem. Scand.,21:121-126 276
FENERTY, J. & SING, K.S.W.,1976. The thermal decomposition of hydrous chromium oxides. Proceedings European Symposium Thermal Analysis 75/.:3O4-3O5. FORSTER, LF.,1960. Occunence of primary gold in ultrabasic rocks of Lowveld. NeuesJb. Miner. Abh.,94'.22^-266. GOUT, R. & KOUADIO, L.K.,1973. Effects du broyage sur des melanges hydrargillite-goethite.C./?.y4ca
SHIBASAKI, Y.,1972. Synthesis of orthorhombic CrOOH. Mater. Res. Bull., 7:1125-1133. SHIBASAKI, Y., KANAMARU, F., KOIZUMI, M., ADO, K. & KUME, S., 1970. Synthesis of CrO2 by oxidation of Cr(OH)3. Mater. Res. Bull., 5:1051-1058. SHIBASAKI, Y., KANAMARU, F. & KOIZUMI, M.,1973. Conversion of CrO2. Mater. Res. Bull., 8:559-564. SYNDER, R.G. & IBERS, J.A.,1962. O-H-O potential energy curves for chromous acid./. Chem. Phys., 36:1356-1360. SWADDLE, T.W., LIPTON, J.H., GUASTELLA, G. & BAYLISS, P.,1971. The aqueous chemistry of chromium III above 100c. Synthesis of chromium spinels. Can. J. Chem., 49:2433-2441 THAMER, B.L., .DOUGLAS, R.M. & STARITSKY, E., 1957. The thermal decomposition of aqueous chromic acid and some properties of the resulting solid phases. /. Am. Chem. Soc., 79:547-550. TOMBS, N.C., CROFT, W.J., CARTER, J.R. & FITZGERALD, J.F., 1964. A new polymorph of CrOOH. Inorg. Chem., 3:1791-1792. TOROKIN, A.N., AVERBUKH, T.D., BELOVA, N.V. & REMPEL, P.S., 1968. The polymorphism of chromium hydroxides obtained by reducing sodium chromate with hydrogen. Russ. J. Inorg. Chem., 13:13-15. VUORELAINEN, Y., HAKLI, T.I. & KATAJA, M., 1968. A hydrated oxide of chromium as a pseudomorph after eskolaite, Outokumpu. Geol. Soc. Finl. Bull., 40:125-129. WANG, B., 1983. Study of grimaldiite discovered for the first time in China. Chem. Abstr. 99,198165 (Original not seen) WILLIAMS, S.A. & ANTHONY, J.W.,1970. Hemihedrite, a new mineral from Arizona. Am. Miner., 55:1088-1102. ZOLOTOVSKII, V.I., KRIVORUCHKO, O.P., BUYANOV, R.A. & ZAIKOVSKII, V.I., 1978. Study of the mechanism of crystallization of coprecipitated iron and chromium hydrogels during aging in the mother liquor. Kiner. Katal., 19:1022-1028.
Dr. D. J. Drysdale Dept, of Geology and Mineralogy The University of Queensland Queensland, 4072 PERLITIC TEXTURE AND OTHER FRACTURE PATTERNS PRODUCED BY HYDRATION OF GLASSY ROCKS
by D.J. Drysdale
ABSTRACT. Hydration of extrusive and intrusive glassy rocks initiated either during early cooling of the glass or later at lower temperatures variously results in development of perlitic texture, large scale perlitic structures here termed mega-perlite structures, or sheath and core structure. Which structure results appears to be dependent on the temperature at which hydration began and the degree of undercooling of the glass. Perlitic texture is favoured by a low temperature of hydration and a high degree of undercooling. Mega perlite cells also appear to form at low hydration temperatures but with a lower degree of undercooling of the glass. Sheath and core structure forms at a high temperature of hydration and a low degree of undercooling and is associated with devitrification.
INTRODUCTION
Perlitic texture and sheath and core structure are well known in the literature (reviewed in Lacy 1966 and Drysdale 1979). Mega-perlite cells, or large scale perlitic structures, have not been described before. The properties and relationships with one another of these fracture patterns produced by hydration of glassy rocks and the nature of the water in the glasses may signify different hydration and thermal histories for the glassy rocks in which they occur and lead to estimates of original magmatic water contents, rates of cooling, and temperatures of hydration.
PERLITIC TEXTURE
Perlitic texture characteristically develops with cells ranging from 0.3 mm to a few millimetres in diameter and is predominantly associated with extrusive sequences of glassy lavas and welded tuffs, or with dome-like complexes shallowly intruding such sequences. Most perlites are felsic glasses with SiO2 contents of 70-75% and water contents of the order of 4-6% although higher (6.77%) and lower (1.16%) water contents are known. The formation of perlitic texture is characteristic of low temperature hydration of rapidly chilled glasses. Lacy (1966) has described the structural mechanisms involved in the development of perlitic cracking. At first, hydration at low temperature produces OH's bonded to Si or Al, interstitial OH's, and hydrogen bonds. A second stage of low temperature hydration forms further hydrogen bonds. Slow imbibition of surface water stretches the glass structure and is followed by the formation of hydrogen bonds, shrinking and cracking. The process has been replicated in the laboratory. Contraction over a two year period of an experimentally hydrated glass is similar
Pap. Dep. Geol. Univ. Qd., 12(3): 278-285, Dec., 1991 279 in magnitude to the contraction component of the obsidian-perlite transition. Hydration reactions are sensitive to the thermal history of the glass as well as to its composition, so pitchstones characteristically lack perlitic cracking because hydration begins at a temperature sufficiently high to form terminal OH groups within a sufficient number of tetrahedral groups. The easy transference of these groups to alternative loci permits configurational adjustments that prevent the build up of contractile stress during subsequent low temperature hydration which results in the formation of hydrogen bonds. The hydration rate is a function of glass composition, previous hydration history, and temperature.
Some intrusive glasses are perlitic. Examples are known from pitchstone dykes and the chilled pitchstone margins of felsite intrusions throughout the Scottish Tertiary Province and from Iceland (Judd & Cole 1883, Cole 1888, Harker 1904 and 1908, Carmichael 1966). Some perlites are not felsic glasses. Examples are from Golden Door, Arizona - Nevada (Longwell 1963), Three Forks and Wolf Creek, Montana (Robinson & Marvin 1967), Bihar, India (Rao and Purushottam 1962), several localities in the Scottish Tertiary Province (Richey and Thomas 1930, Rao 1959, Ridley 1971), and from Iceland (Carmichael 1966). In the Scottish Tertiary Province tachylytes, the glassy selvages of basaltic dykes, are often perlitic (Judd & Cole 1883, Cole 1888, Harker 1904 and 1908). All perlites are, however, rapidly chilled glasses hydrated at low temperature.
SHEATH AND CORE STRUCTURE
Sheath and core structure was discussed most recently by Drysdale (1979). The structure is restricted to intrusive glassy pitchstones (Figure 1). Glassy rocks with sheath and core structure characteristically lack perlitic texture. Examples include the Eskdalemuir dyke, Dumfriesshire; the pitchstone sheets of the I^ch Scridain district and the Rudh a Chromain sill, Carsaig, Mull (Anderson & Radley 1915, Buist 1961); and dykes in the Cowal distrcit (Gunn et al. 1897, Tyncll 1917). Simons (1962) noted the sheath and core structure of Anderson and Radley is very similar to his devitrification "dykes" in vitrophyric welded tuff from Klondyke, Arizona. Sheath and core structure is caused by hydration and subsequent but related devitrification beginning in relatively slowly cooled intrusive glasses at relatively elevated temperatures. This suggests that sheath and core structure and perlitic texture should not occur together. They do so at Ben Hiant, Ardnamurchan, Scotland, where the pitchstone lavas of the south-west vent furnish an effusive example of a magma that on Mull is represented as an intrusive phase in mineralogically and chemically identical pitchstone sills. Richey and Thomas (1930) observe that perlitic cracks are a noticeable feature of the glassy base in the lavas of Ben Hiant but devitrification of the glassy base is well marked only when these rocks come within the influence of some later intrustion. So unlike the intrusive sheets of sheath and core structure pitchstone on Mull, stoniness is not developed along the joint planes in these chemically identical pitchstone lavas except as a secondary characteristic in the Ben Hiant pitchstones where they come into contact with intrusive masses. 280
Whether perlitic texture or sheath and core structure develops is a function of thermal and hydration history. Mull intrusive pitchstones have developed sheath and core structure whereas their compositional and mineralogical effusive equivalents have developed perlitic texture at Ben Hiant.
MEGA - PERLITE STRUCTURES
Fracture patterns with perlitic characteristics in which the cells are very much larger than those seen in normal perlites are known from a number of localities. Good examples occur in some of the pitchstone sheets of the Loch Scridain district, Mull, which are characterised by sheath and core structure. A typical example occurs at Beinn Chreagach am Ormsaig (Figs 1 and 2). Here the large perlite cells appear within the topmost part of a 3m thick intrusive pitchstone sheet. The isolated cells are clearly defined by ovoid, concentric fractures that cut the cells off from the sunounding pitchstone. Subcells within the large cells are marked out by curvilinear fractures. Sheath and core structure is not developed at the level of the large cells, but the pitchstone below the mega-perlite cells is broken into cores separated by thin stony sheaths. In thin section the pitchstone is full of very fíne crystallites in scopulite, trichite and stellate groupings set in a glassy base that constitutes about 60% of the rock. Glass from the topmost part of the sheet shows, in thin section, uncommon curvilinear cracking, producing very poorly developed incomplete microscopic perlite cells. Activation energies for dehydration determined as in Drysdale (1979) show that glasses from the large perlite cells contain about 2% water acquired by low temperature hydration with activation energies for dehydration of SKcals per mole and 13Kcals per mole for hydrogen bonded water and hydrogen bonded OH and, in contrast to glasses from cores below the large cells, little water held as pairs of OH's bonded to glassformers and acquired by hydration at higher temperatures.
The fracture patterns forming the mega-perlite cells are strongly reminiscent of the curviplanar primary fractures bounding blocks of glass that contain numerous concentric perlitic fractures, the outermost of which parallel the margins of the isolated blocks, described by Noble (1968). Similar fracture patterns have been described from Rudh an lasgaich and Knock, Skye, by Clough (in Harker 1904 and original field slips and notes), although these were not at the time recognised as perlite-like. However, Hawkes and Harwood (1932) did identify large perlitic ellipsoids (20 x 15 x 8 cm) in the 0.3 to 1 m thick chilled glassy selvedge of an inegular dyke in Iceland. The selvedge of a dyke at Allt Thuill, Skye, (Harker 1904) also has perlitic forms more than 2.5 cm in diameter. The occurrences have some common features. Development of mega-perlite cells has taken place at each locality in the marginal parts of intrusions; at Skye and Mull the cells appear within the marginal zone but not right at the intrusion contact; at Skye normal perlitic texture occurs in the selvedge outside the mega perlite cell zone; and on Mull traces of perlitic texture appear at the outer margin. There is a range of compositions. On Mull the glasses are andesitic and at Iceland the glass is the chilled selvedge of a felsic, high level dyke. 281
Perlitic texture in chilled selvedge
Megaperlite structures
Sheath and core structure
Figure Sheath and core structure, perlitic texture, and megaperlite structures in a 1. Loch Scridain, Mull, pitchstone sheet. "Reproduced by permission of the editor(s) of Geological Magazine"
Figure 2. A mega-perlite cell in plan view.
"Reproduced by permission of the editor of Geological
Magazine" 282
DISCUSSION
The mega-perlite cells are localized within the topmost part of intrusive pitchstone sheets at Mull and in analogous positions elsewhere, within outer selvedges of perlitic glass. This suggests that the mega-perlite cells originated by low temperature hydration of glass chilled less rapidly than that producing normal perlite on hydration. This is substantiated by the presence of water of low temperature hydration origin together with the absence of water acquired by hydration at relatively high temperatures.
The degree of chilling or undercoolling of a glass is measured by the difference between its actual temperature and its equilibrium temperature, as the condition of a glass corresponds to a condition of equilibrium at some temperature within its annealing range. Tool (1946) coined the term "fictive temperature" for the temperature at which the glass would be at equilibrium if suddenly brought to it from its given state. The properties of a glass are affected by changes in either its actual or fictive temperature. However, the fictive temperature characterization of a glass has limitations because of subtle effects caused by its thermal history.
Perlitic texture seems to result from low temperature hydration of high fictive temperature glasses, and mega-perlite structures from low temperature hydration of glasses with lower fictive temperature.
No determinations exist of fictive temperature for natural glasses. Harmand and Zimmerman (1976) determined from continuous t.g.a. curves for some felsic glasses the temperature at which the rate of liberation of water was maximal. These temperatures were taken to be the temperatures at which water was at equilibrium with the silicate melt, and to indicate the temperature of beginning of solidification . The temperature was 1160° for obsidian, 520° for perlite, and 440° for pitchstone. They concluded that the water of the perlites and the pitchstones had been acquired by the molten silicates at the time of their outpouring and at temperatures of the order of 440-550°. However it is most unlikely that these felsic glasses were molten at these temperatures, and Lacy (1966) has shown that 400° is too high a temperature to produce the characteristic features of low temperature hydration, and to allow the formation of perlite. He hydrated synthetic glasses corresponding to the syenitic and granitic minimums at 400° under pressure in the laboratory and found on dehydration a 17Kcal per mole activation energy plateau and a sharp increase in activation energy for dehydration after removal of 3.5% water as the remaining 1% water was lost. However, dehydration of a natural intrusive glass with a water content of more than 7% showed the activation energy for dehydration rose from low values to 17Kcal per mole during the loss of the first 2% by weight of water, remained constant then to 4.5% loss, and then suddenly rose to 20 Kcal per mole until 7.3% water had been expelled. These dehydrations represent the reversal of the low temperature hydration processes. The contrast between the results for the synthetic glasses 283 hydrated at 400® and the natural glass shows that 400° is too high a temperature for hydration that will reproduce all the features of low temperature hydration. Harmand and Zimmerman's equilibrium temperatures can be interpreted as fictive temperatures for their glasses; the temperatures are in the right sense, decreasing from obsidian through perlite to pitchstone.
CONCLUSIONS
The evidence of the occunences of the various structures and of the activation energies for dehydration of the glasses points to obsidian, glasses with perlitic texture, glasses with mega-perlite structure, and pitchstones with sheath and core structure lying on a continuum.
The 0.3-0.5% water held with high activation energies for dehydration in obsidians, perlites, mega-perlites and cores of sheath and core structure is thought to be original magmatic water now trapped in the glass structure in isolated monomeric OH groups bonded to glassformer Si and Al. Later hydration of the solid glassy rock can have one of three effects:
1. Lacy (1966) and Drysdale (1979) showed hydration beginning at moderately elevated temperatures in a solid but still warm and relatively slowly cooled intrusive pitchstone produced terminal OH groups which prevented perlitic cracking but which were shown by Marshall (1961) to lead to devitrification after hydration. This devitrification after hydration produced pitchstone sheets with sheath and core structure.
2. Low (down to ambient) temperature hydration by meteoric water or under subaqueous conditions of obsidians and very rapidly chilled glasses produced perlitic texture, seen in the obsidian/perlite pairs called "marekanites" and in frozen selvedges to minor intrusions, via the structural adjustments detailed by Lacy (1966).
3. Similar low temperature hydration of less rapidly chilled glass produced mega-perlite structure. The rarity of mega-perlite structures may be a result of their loss by exfoliation weathering from the pavement exposures where they show up best; exfoliation often also destroys the evidence of "marekanites" on the obsidian/perlite genetic relationship by removal of the perlite "onion skin" shell from around the obsidian balls.
Perlitic texture, mega-perlite structure, and sheath and core structure form in glassy rocks but may be preserved as ghostly relicts in completely devitrified rock, leaving a record of their cooling, hydration, and devitrification history.
Measurement of fictive temperature, the temperature at which the glass structure became "frozen in", is a measure of the rapidity of chilling and a useful parameter in describing the condition of a glass. 284
ACKNOWLEDGMENTS
To the Institute of Geological Sciences, Murchison House, Edinburgh, for access to the original field slips and notes of C.T. Clough.
REFERENCES
ANDERSON, E.M. & RADLEY, E.G., 1915. The pitchstones of Mull and their genesis. Q. J. Geol. Soc. London 71:205-217. BUIST, D.S., 1961. The composite sill of Rudh'a'Chromain, Carsaig, Mull. Geol. Mag., CARMICHAEL, I.S.E., 1966. The pyroxenes and olivines from some Tertiary acid glasses. J. Petrol., 1:309-336. COLE, G.A.J., 1888. Some additional occurrences of tachylyte. Q. J. Geol. Soc. London., 44:300-307. DRYSDALE, D.J., 1979. A note on sheath and core structure in the Mull pitchstones. Geol. Mag., 116:99-104. GUNN, W., CLOUGH, C.T. & HILL, J.B., 1897. The geology of Cowal. Memoir Geological Survey Scotland. HARKER, A., 1904. The Tertiary igneous rocks of Skye. Mem. Geol. Surv. G.B. HARKER, A., 1908. The geology of the Small Isles of Inverness-shire. Mem. Geol. Surv. G.B. HARMAND, C. & ZIMMERMANN, J.L., 1976. Etudes des elements volatils contenus dans quelques verres volcaniques acides. C. R. Acad. Sc., Paris 5er. £>.,282:1391-1394. HAWKES, L. & HARWOOD, H.F., 1932. On the changed composition of an anorthoclase - bearing rock glass. Mineralog. Mag., 23:163-174. JUDD, J.W. & COLE, G.A.J., 1883. On the basalt glass (tachlyte) of the Western Isles of Scotland. Q. J. Geol. Soc. London., 39:444-465. LACY, E.D., 1966. The hydration and dehydration of aluminosilicate glasses. Vetro Silic. ,10:5-9. LONGWELL, C.R., 1963. Reconnaissance geology between Lake Mead and Davis Dam, Arizona - Nevada. US Geol. Surv. Prof. Paper, 374-E. MARSHALL, R.R., 1961. Devitrification of natural glass. Geol. Soc. Amer. Bull., 72:1495-1520. NOBLE, D.C., 1968. Stress - corrosion failure and the hydration of glassy silicic rocks.Amer. Mineral., 53:1756-1759. RAO, M.S., 1959. Minor intrusions and dykes of the Lamlish - Whiting Bay region, Arran. Geol. Mag., 96:237-246. RAO, C.S.R. & PUROSHOTTAM, A., 1962. Pitchstone flows in the Rajmahal Hills, Santal Parganas, Bihar. Records, Geological Survey of India, 91:341-348. RICHEY, J.E. & THOMAS, H.H., 1930. The geology of Ardnamurchan, North west Mull and Coll. Memoir Geological Survey Scotland. 285
RIDLEY, L, 1971. The petrology of some volcanic rocks from the British Tertiary Province: The Islands of Rhum, Eigg, Canna and Muck. Contrib. Mineral. & Petrol., 32.251-266. ROBINSON, G.D. & MARVIN, R.F., 1967. Upper Cretaceous volcanic glass from Western Montana. Geol. Soc. Am. Bull., 28:601-608. SIMONS, F.S. 1962. Devitrification dykes and giant spherulites from Klondyke, Arizona. Amer. Mineral. 47:871-885. TOOL, A.Q.,1946. Relation between inelastic deformability and thermal expansion of glass in its annealing range. J. Am. Ceram. Soc., 29:240-253. TYRRELL, G.W.,1917. Some Tertiary dykes of the Clyde area. Geol. Mag., 54:301-315 & 350-356.
Dr. D.J. Drysdale Dept, of Geology & Mineralogy The University of Queensland Queensland, 4072. LITHIUM ALUMINIUM SILICATE MINERALS AND POLLUCITE FROM MELDON, DEVON AND SAN PIERO IN CAMPO, ELBA
by D. J. Drysdale
ABSTRACT. Petalite and spodumene occur at Meldon and San Piero in Campo as pocket minerals in miarolitic aplites and pegmatites respectively. This form of occunence is rare for petalite and not common for pollucite. At Meldon spodumene apparently succeeds petalite and does not result from the more usual isochemical breakdown of petalite or replacement of spodumene by petalite. Meldon is also established as one of the few localities where all three lithium aluminosilicate minerals are present. The sequence of crystallization from late stage fluids is unusually complete here, from petalite through kunzitic spodumene to replacement of spodumene by eucryptite + albite intergrowths. The sequence of crystallization at Elba extends from petalite to the zeolites.
INTRODUCTION
The A. W. G. Kingsbury Collection in the Mineralogy Department, British Museum (Natural History), houses a large number of fine specimens from the Meldon aplite quany, Okehampton, Devon. This aplite has been much studied and over forty minerals are known from the quarry. Other collections hold excellent specimen material from the pegmatite veins of San Piero in Campo, Elba, another famous mineral locality from which more than thirty minerals are known. The two sets of specimens present unusual parageneses variously of petalite, spodumene, eucryptite and pollucite. British Museum specimen numbers for the material looked at in this work are listed in the appendix.
PETALITE, SPODUMENE, EUCRYPTITE AND POLLUCITE FROM THE MELDON APLITE AND FROM ELBA
Petalite in the Meldon aplite was described by Kingsbury (1966) as present in autometasomatized brown aplite, in pegmatite veins and in coarse, complex pegmatite lenses showing evidence of several stages of mineralization. He also noted spodumene and pollucite in the pegmatitic lenses. The pollucite was most recently described among a group of rare-element minerals by von Knorring and Condliffe (1984) and the petalite and spodumene were most recently considered by Drysdale (1985). A new examination of material in the Kingsbury Collection found clear colourless petalite, pale pink or lilac kunzitic spodumene as a pocket mineral in crystals several centimetres long showing some crystal faces and not associated with petalite, pollucite as colourless, glassy, clear crystals and as masses enclosing spodumene and intergrown with montmorillonite, and one specimen of eucryptite intergrown with albite but paragenesis otherwise unknown. Analyses of the petalite and spodumene appear in Drysdale (1985).
Pap. Dep. Geol. Univ. Qd., 12(3): 286-293, Dec., 1991 287
A pollucite analysis is reported here (Table 1), and the presence of eucryptite/albite intergrowth has been confirmed by XRD and chemical analysis. MacFadyen (1970) had previously noticed many of the accessory and later formed minerals listed by Kingsbury, including petalite, occur well crystallized in druses, especially in the pegmatite lenses, and Durrance and Lamming (1982) noted mineralogical banding is well developed and scattered small vein-like structures carry coarser crystals, including petalite. Chaudry and Mahmood (1979) found the Meldon petalite to be of metasomatic origin, the process responsible for its formation being operative only at restricted points in the intrusion. Metasomatism and formation of pegmatites they attribute to the influence of the volatile-rich (F,B,H2O) residuum of the crystallizing aplite. This may have collected towards the top of the intrusion as the present erosion surface just intersects the upper extremities of the body, which may form a much larger mass at depth. Von Knorring and Condliffe (1984) cite evidence for a slow rate of cooling and their observation of native arsenic and lollingite within small cavities is indicative of low temperature hydrothermal conditions.
TABLE 1
Analysis of pollucite, British Museum specimen number K/Mel.6O3 from the Old Aplite Quarry, Meldon, Okehampton, Devon.
SiO2 46.35% AI2O3 17.04% Cs2O 29.70% Fe2O3 0.07% total MnO 0.02% CaO 0.07% MgO 0.05% Na2O 1.92% K2O 0.61% Rb2O 0.84%
Other of the Museum's collections hold a suite of specimens from the Speranza vein, Elba, one of a number of pegmatitic veins of the Monte Capanne granodiorite stock and its contact. The minerals of the Elba pegmatites are described by Orlandi and Scortecci (1985), but little has appeared on petalite and pollucite (originally called "Castor and Pollux" at this locality because of their constant association) since the descriptions of Comucci (1915), Grill (1920) and Carobbi and Minguzzi (1948). Small vuggy pockets are present in these veins and miarolitic cavities have yielded crystals of pink, yellow and green tourmaline. 288 aquamarine and morganite, petalite and pollucite. The collection contains prismatic, transparent, colourless and corroded petalite crystals and petalite is constantly associated with pollucite; pollucite is clear, glassy and corroded, with distinct crystals lying on orthoclase or in close association with lepidolite, or yellow tourmaline, or foresite (stilbite + cookeite).
DISCUSSION
Petalite, spodumene and eucryptite, and in lesser degree pollucite, at these localities and in these specimens present some unusual features:
1. Petalite rarely occurs as a pocket mineral but does so at Meldon and Elba. The only other example found in the literature is from Kazakhstan (Aramakov et alA915\ where a quartz-petalite-pollucite assemblage occurs as discrete pockets in the axial zone of vein bulges. Large blocky quartz-microcline units are also confined to these positions and these two assemblages have probably crystallized from a gas phase. Conversely, a quartz-spodumene-albite assemblage is confined to the vein periphery in bulge areas and extends across the whole vein width elsewhere and probably crystallized from a melt.
Stewart (1978) noted Meldon and Elba suggest that the components of petalite are readily transported in a gas. This has now been verified in experiments growing petalite crystals and replacing spodumene with petalite at 400o and 2kb by hydrothermal transport of petalite components in an aqueous fluid phase in contact with spodumene containing pegmatite assemblages. The experiment designs are modified from Holdaway (1966) and Liou (1971). Such experiments may monitor stable or metastable equilibria. For example, Thompson (1970) monitored by quartz crystal weight change the high pressure reaction laumontite=anorthite+2 quartz+4 water taking place metastably in the stability field of the low pressure reaction sequence: laumontite=wairakite+2 water and wairakite=anorthite+2 quartz+2 water. So it is possible that in the absence of eucryptite, replacement of spodumene by petalite is taking place metastably in these experiments. As Heinrich (1978) notes, several workers have suggested that late stage petalite replacing spodumene, stalky fissure filling petalite, and petalite as a vugh mineral has originated from hydrothermal solutions under low P/T Alpine-vein conditions and petalite can form metastably below its lower temperature limit from such solutions. The phase diagram of London (1984), confirmed in these new experiments, precludes these petalites being metastable.
Why these petalites are preserved into the stability fields of spodumene+quartz and eucryptite+quartz is uncertain. Meldon is cited by Cerny and London (1983) as an example of the preservation of petalite in near surface environments as a result of the instability of eucryptite in quartz saturated environments above low temperatures and pressures. Metastable preservation of petalite formed at low confining pressures in miarolitic pegmatites may be because: 289
(a) With decreasing pressure petalite becomes stable to progressively lower temperatures where reaction rates become imperceptible.
(b) Reduced solubility of aluminosilicate components in late stage subsolvus H2O - CO2 fluids that are CO2-rich may result in petalite being metastably preserved whereas petalite in contact with an H2O-rich phase may react to spodumene+2 quartz or eucryptitc+3 quartz.
2. Where petalite and spodumene both are present in the same pegmatite their relationship is usually one of two types:
(a) Petalite isochemical breakdown to spodumene+2 quartz, often pseudomorphing the primary petalite, has been observed at nineteen localities.
(b) Petalite replaces spodumene. There are six occurrences in the literature and possibly one moreithe account is ambiguous.
At Meldon neither relationship is observed. Drusy kunzitic spodumene crystals are probably post-petalite and a crystallization product of the fluids that earlier crystallized petalite and went on to crystallize spodumene and pollucite and then alter spodumene to eucryptite+albite or to crystallize eucryptite.
3. With the finding of eucryptite, Meldon joins Bikita, Tanco, Siberia, Soviet Central Asia and the Mistress Mine, Zimbabwe, as a locality where all three lithium aluminosilicate minerals occur. At the first three localities petalite breaks down to spodumene + quartz; at the latter two, petalite replaces spodumene; at Meldon spodumene probably succeeds petalite. Relationships between petalite and eucryptite and between spodumene and eucryptite at Meldon are unknown.
Elsewhere eucryptite is known mainly from eucryptite + albite and eucryptite + quartz intergrowths and it may always be formed by breakdown of spodumene or petalite. London and Burt (1982) show the commonly observed replacement of spodumene by eucryptite + albite is a result of subsolidus metasomatic reaction in a relatively high Na, low P and F environment in low silica systems. Low silica activities exist within large spodumene crystals and saline residual pegmatitic fluids dominated by Na may have emerged from cleavelandite complexes. It seems more likely that eucryptite/albite intergrowths formed in this way at Meldon than that here eucryptite is a late stage crystallization product as at Haapaluoma (Lahti et al. 1982), where it occurs with other lithium minerals but crystallized after these from post-magmatic fluids that had decomposed earlier formed spodumene crystals. Nevertheless, Meldon and Elba present unusually complete crystallization sequences from late stage fluids, from petalite and spodumene to pollucite and the zeolites. 290
4. Pollucite at Meldon occurs both as a pocket mineral and as described by von Knorring and Condliffe (1984). Their account is similar to one occunence in Siberia, where pollucite forms aggregates confined to the boundaries between textural and mineralogical complexes and fills the spaces between and conodes other minerals in an unzoned pegmatite (Filippova 1970), and one occurrence in Soviet Central Asia where pollucite is finely disseminated and fills spaces between other minerals (Melentyev 1970). Meldon and Elba pocket pollucites resemble those from Hebron and Greenwood, Maine; San Diego, California; East Sayan; the Pamirs; Kazakhstan; and Afghanistan. Euhedral crystals are known in miarolitic cavities in Hebron, Greenwood and Kazakhstan as well as Elba.
5. Meldon and Elba fit London's (1986) model for the origin of some miarolitic pegmatites in which inhibition of tourmaline crystallization until the late stages of pegmatite consolidation leads to a local generation of quantities of water, vésiculation and pocket formation accompanying tourmaline crystallization. At Meldon elbaite is low in the blue and white aplites, increases in brown aplite, and increases markedly in much altered rocks. It is most abundant in pegmatite rich areas. In the Elba pegmatite bodies schorl is almost universally present, and elbaite, lepidolite and the rare species are all pocket minerals.
London's (1986) model, developed from a study of fluid inclusions in minerals of the San Diego, California and Kulam, Afghanistan, pocket pegmatites, seems widely applicable. Examples include miarolitic pegmatites in the Pamirs, Sayan and Kazakhstan. In the Pamirs (Durnev et al. 1973) miarolitic cavities occur only in an axial quartz-albite-microcline unit containing elbaite; cavities are absent where the unit lacks elbaite. In Sayan (Melentyev 1963) in a zoned pegmatitic vein coloured and polychromatic tourmalines appear with pollucite and spodumene in drusy segregations at the boundary between the axial quartz segregations and the quartz-microcline-spodumene zone. In Kazakhstan, Aramakov et al. (1975) found rubellite appears in quartz-petalite-pollucite pockets in the axial zone of vein bulges.
ACKNOWLEDGEMENTS
To Mr. R. F. Hall for analyses and to the Mineralogy Department, British Museum (Natural History) for access to collections and donation of material.
REFERENCES
ARAMAKOV, I.G., ZYUZ, S.D., MELENTYEV, G.B., REMEZ, V.K. & CHEREPIVSKAYA, G.A. 1975. New data on pollucite-bearing pegmatite of Kazakhstan Doklady of the Academy of Sciences, USSR, Earth Science. Sect. 220:113-116. 291
CAROBBI, G. & MINGUZZI, C. 1948.1 constituenti minori della pollucite elbana e della petalite dell Elba e di Uto. Atti Accademia nazionale Lined. Rendiconti. Classe scienze fisiche, matematicke e naturalli. Series 8, 4:653-658. CERNY, P. & LONDON, D. 1983. Crystal chemistry and stability of petalite. Tschermaks Mineralogische und Petrographische Mitteilungen 31:81-96. CHAUDRY, M.N. & MAHMOOD, A. 1979. Types of distribution of the minerals of the Meldon aplite, Devonshire. Mineralog. Mag. 43:307-309. COMUCCI, P. 1915. Sopra la petalite elbana. Rendiconti Accademia nazionale Lined, Roma Series 5a, 24:1141-1146. DRYSDALE, D.J. 1985. Petalite and spodumene in the Meldon aplite, Devon. Mineralog. Mag. 49:758-759. DURNEV, V.F., MELENTYEV, G.B., SOKOLOV, V.A., POKROVSKIY, Y.N. & CHEREPrVSKAYA, G.A. 1973. First find of pollucite in pegmatite of the Pamirs. Doklady of the Academy of Sciences, USSR, Earth Science Sect. 213:117-119. DURRANCE, E.M. & LAMMING, D.J.C. 1982. The geology of Devon. University of Exeter. FILIPPOVA, Y.I. 1970 A new paragenetic type of tantalum and cesium-bearing pegmatite. Doklady of the Academy of Sciences, USSR, Earth Sciences Sect. 192:123-126. GRILL, E. 1920. Sulla pollucite elbana. Atti della Société Toscana di Scienze Naturali Processi Verbali 29:No.4. HEINRICH, E.W. 1978. Mineralogy and structure of lithium pegmatites. Journal de Mineralogía, Recife 7:59-65. HOLDAW AY, M. J. 1966. Hydrothermal stability of clinozoisite+quartz. Amer. J. Sci. 264:643-667. KINGSBURY, A.W.G. 1966. Some minerals of special interest in south-west England. In: Present views of some aspects of the geology of Cornwall and Devon. K.G.F. Hosking & G.J. Shrimpton, Eds., Royal Society of Cornwall'.2^1-2A(). LAHTI, S.P. KALLIO,P. & VON KNORRING, O. 1982. The composition, physical properties and occurrence of eucryptite from the Haapaluoma pegmatite, Finland. Bull. Geol. Soc. Finland 54:5-13. LIOU, J.G. 1971. P-T stabilities of laumontite, wairakite, lawsonite and related minerals. J. Petrol. 12:379-411. LONDON, D. & BURT, D.M. 1982. Chemical models for aluminosilicate stabilities in pegmatites and granites. Amer. Mineralog. 67:494-509. LONDON, D. 1984. Experimental phase equilibria in the system AISÍO4-SÍO2- H2O: a petrogenetic grid for lithium-rich pegmatites. Amer. Mineralog. 69:995-1004. LONDON, D. 1986. Formation of tourmaline-rich gem pockets in miarolitic pegmatites. Amer. Mineralog. 71:396-405. MACFADYEN, W.A. 1970. Geological Highlights of the West Country. Butterworths, 61-62. 292
MELENTYEV, G.B. 1963. The first find of pollucite in the granitic pegmatites of Sayan. Doklady of the Academy of Sciences, USSR, Earth Science Sect. 141:1287-1289. MELENTYEV, G.B. 1970. First find of pollucite-bearing pegmatite in Soviet Central Asia and new data on the mode of concentration of cesium. Doklady of the Academy of Sciences,USSR, Earth Science Sect. 192:177- 180. ORLANDI, P. & SCORTECCI, P.B. 1985. Minerals of the Elba pegmatites. Mineralog. Rec. 16:353-363. STEWART, D.B. 1978. Petrogenesis of lithium-rich pegmatites. Amer. Mineralog. 63:970-980. THOMPSON, A.B. 1970. Laumontite equilibria and the zeolite facies. Amer. J. Sci. 269:267-275. VON KNORRING, O. & CONDLIFFE, E. 1984. On the occunence of niobium tantalum and other rare-element minerals in the Meldon aplite, Devonshire. Mineralog. Mag. 48:443-448.
Dr D.J. Drysdale Dept, of Geology and Mineralogy The University of Queensland Queensland, 4072 293
APPENDIX
Specimens from the Department of Mineralogy, British Museum (Natural History).
Meldon aplite: Russel (Meldon) 10375 K. Mel. 602 & 603 K. Mel. Not catalogued. Box 35, tray 13 1923/98,1963/682,1968/109,1968/112
San Piero in Campo: 1908/202,1913/313,1914/310, 1914/312, 23290, 35210, 36448, 40406, 46363, 46364, 46365, 46366, 48484, 49193, 51610, 51910, 54783, 71558, 81476, 86869, 87083, 94690. POLYTYPE 2H MOLYBDENITES WITH HIGH RHENIUM CONTENTS
by D.J. Drysdale
ABSTRACT. Rare polytype 2H molybdenites with the high Re contents characteristic of the presence of some 3R molybdenite are described from relatively high temperature deposits that are not of porphyry or pipe style, often with low Mo grades and possibly early development of high Re/Mo ratios. So not all high Re molybdenites are 3R-containing or associated with porphyry-style mineralization. Whether these constitute a new class of molybdenites or only demonstrate varying Re content with no apparent effect on polytype crystallization is uncertain. Molybdenites from the uniquely Australian pipe style of Mo mineralization are low Re 2H or high Re 2H+3R.
INTRODUCTION
Molybdenite occurrences have diverse styles. Newberry (1979a) recognised porphyry and related copper deposits, porphyry molybdenum deposits, copper molybdenum and tungsten molybdenum skarns, and pegmatites/quartz veins/greisens. Ayres (1974) recognised molybdenites in acid and alkaline rocks (including disseminations in granite, porphyry copper and porphyry molybdenum deposits, in pegmatites and aplites, and in pipes), in veins (simple quartz veins and quartz veins in granite), in impregnations in gneisses and schists, in skarns, and in base metal sulphide deposits.
Molybdenite can occur in 2H and 3R modifications and different polytypes characterize the various occunence styles. Newberry (1979a,b) showed molybdenites from pegmatites/quartz veins/greisens are predominantly 2H. Frondel and Wickman (1970) also found the majority of 2H came from quartz veins or pegmatites associated with granitic rock or granite contacts, but 2H has been found in every geological environment known for molybdenite. For specimens that were mixtures of 2H and 3R, in half 2H was dominant and these came generally from quartz veins, pegmatites, or granite contacts. Similarly, Ayres (1974) found for Australian and P.N.G. molybdenites that 2H is present in all geological environments and is the only polytype present in pegmatites, quartz pegmatite pipes, simple quartz veins, skarns, and highly metamorphosed ores. Mixed polytypes occur as disseminations or quartz veins in granite, in porphyry copper deposits and in quartz porphyry and garnet quartz pipes.
Newbeny (1979a,b) found that 2H molybdenite is stable with respect to 3R but 3R is kinetically favoured at high impurity contents as a result of impurity related non-equilibrium growth processes. Samples with more than about 500ppm
Pap. Dep. Geol. Univ. Qd., 12(3): 294-303, 1991. 295 impurities contain some of the 3R polytype in consequence of growth by addition onto screw dislocations. Re is the most common impurity element but high contents of Ti, Sn, Bi, W and Fe also correlate with presence of 3R so 3R is associated with tin-bearing greisens, with high titanium deposits, and with bismuth and bismuthinite. Other elements can also be important for polytype occurrence. In molybdenum mineralization of porphyry style of the Sardinian batholith, (Bralia et al. 1983, Guasparri et al. 1985), 2H was found to be the most prevalent structural arrangement. However, pure 3R and 2H+3R mixtures were present in all samples from the Perda Major! mineralization. Analysis of selected samples of the two polytypes revealed the presence of trace elements such as Co, Cu, Zn, Ag and Pb whose concentrations varied between the different polytypes, whereas other elements, in particular Re and W, were present in equally low concentrations in both cases.
Newberry (1979a,b) stressed hypogene alteration can leach Re from high Re 3R and recrystallize the Re depleted molybdenite to Re poor 2H associated with sericitic alteration. Supergene leaching of 3R is at too low a temperature to promote recrystallization to 2H and produces low Re 3R. Such processes can affect the early close relationship between Re content and 3R abundance, partial leaching and recrystallization causing confusion and apparently conflicting data.
No class of high impurity 2H molybdenite is recognised by Newbeny. His model is based on samples largely from porphyry copper and molybdenum, copper molybdenum skarn, and tungsten molybdenum skarn deposits. Molybdenites from pegmatites/quartz veins/greisens do not feature prominently in his data and for some of the evidence he presents on these deposits the literature is in conflict: Ayres (1974) and Mandarine and Gait (1970) are cited by Newberry as describing 3R associated with bismuth and bismuthinite at Kingsgate and Whipstick, N.S.W., but the original works record only 2H at Kingsgate and Watson (1970) found high Re at Whipstick. Paradoxically, Ayres' listing of average Re abundances shows molybdenites from disseminations or quartz veins in granite, which may be mixed polytype, are average Re poorer than those from simple quartz veins, pegmatites and aplites, and skarns, which are 2H only.
OLD DATA REVISITED AND SOME NEW OBSERVATIONS
Molybdenite occurrences of known mineralization style and paragenesis for which polytype and Re content are available were extracted from the literature and from data on the collections of the British Museum (Natural History) and Queensland and Durham Universities' museums. New determination were made for some specimens for which this data was lacking in the museums' records. Polytype determination was by the method of Frondel and Wickman (1970) as modified by Ayres (1974), with some determinations by the method of Chivas (1978). Re content was determined as > or < 300ppm, the lower limit of detection 296
Table 1 High Rhenium 2H & Low Rhenium Mixed Polytypes
Locality Polytype Re ppm Deposit Type Accessory Ore Minerals
Shap, England 2H+3R <300 Quartz veins in Nil 9:1 adamellite
Elsmore Hill 2H+3R 67 Quartz veins & wo, gl, st, cs, bi, cp NSW 1:4 disseminations in granite
Bald Knob, 2H+3R bdl Disseminations cs NSW in pegmatite
Mt. Douglas, 2H<3R 9.2 Quartz veins cp, bm, si Korong Vale, Vic.
Yea, Vic. 2H 695 Quartz veins wo, py, bm, cp
Wog Mt., NSW 2H 444 Pegmatites, po, py, cp, fmo aplites & granite
Yetholme, NSW 2H 900 Metasomatic pw, sch, fmo, py, cp, skarn ap, sp
Agarak, 2H to 1000 Hypothermal cp, py, and other Armenia sulphides
Kadzharan, 2H to 1650 Hypothermal cp, py, and other Armenia sulphides
Kaier, 2H 800 Pegmatitic/ py and other sulphides Armenia pneumatolytic 297 for Re with the EPMA available. Table 1 lists finds of high Re 2H and low Re 2H+3R in non-porphyry style mineralizations. High Re 2H molybdenites occur at Yea, Victoria, Wog Mountain and Yetholme, N.S.W., and at three localities in Armenia. Low Re mixed polytypes occur at Shap, England, Elsmore Hill and Bald Knob, N.S.W., and Mount Douglas, Victoria. Table 2 shows molybdenites from the various styles of the uniquely Australian pipe mineralization are all either low Re 2H or high Re 3R+2H. Data are from Khurshudyan (1967), Riley (1967), Morgan et al. (1968), Khurshudyan et al. (1969), Mandarino and Gait (1970), Watson (1970), Liddy (1971), Ayres (1974), Knight (1979) and the writer.
Other 2H molybdenites are anomalously high in elements other than Re and so do not appear in Table 1. Melnikov et al. (1978) reported ferrimolybdite, jordisite and molybdenite all containing Fe 5.51-8.82% and all 2H, and Santosh (1988) found Fe 196-1078ppm (mean 615ppm) in 2H molybdenite from disseminated mineralization in a granite pluton and in pegmatite and quartz veins.
DISCUSSION
Very few occurrences do not fit the pattern of relationship between polytype abundance, impurity content, style of mineralization and vein type, and hypogene hydrolytic and supergene alteration described by Newberry (1979a,b).
For the low Re mixed polytypes of Table 1 their apparently anomalous character can be explained as an artefact of mis-classification or lack of mineralogical detail:
1. Reclassification of the Shap, England, occurrence adds it to the class of mixed polytypes from veins without sericitic alteration in porphyry-style associations (Fortey 1980). The low Re/low 3R mixed polytype is from quartzymolybdenite veins, lacking alteration and other sulphides, that may be transitional A-B veins in Newberry's (1979b) nomenclature, or there may have been some weathering of the molybdenite: the sample used was dull grey whereas fresh molybdenite, abundant at the stage of quarrying of several years ago but no longer available, was steely blue grey with a metallic lustre when freshly exposed.
2. Mineralogical detail places Elsmore Hill and Bald Knob, N.S.W., in the class of 3R associated with tin-bearing greisens. At Elsmore Hill molybdenite occurs in the more siliceous parts of vughy quartz and quartzose mica containing veins. Cassiterite is found near the vein centres (Cotton 1909, Weber et al. 1978). At Bald Knob, molybdenite is disseminated in pegmatitic granite and quartz in an area of greisen and there are tin stockworks in greisen nearby (Weber et al. 1978). 298
Table 2 Pipe Deposits
Locality Polytype Re ppm Pipe Type Accessory Ore Minerals
Wolfram 2H 1.7 Quartz bi, wo, py, ap, sch, Camp fl, bm, joseite
Bamford 2H 6.3 Quartz wo, bi, py, cp, sch, gl, fl. pw
Wonbah 2H 73-161 Quartz cp, py, gl, sp
Kingsgate 2H 0.25 Quartz or bi, bm, wo, ap, py, pegmatite po, cp, gl, cs
Deepwater 2H 13.2 Quartz and bi, bm, wo, pegmatite cp, gl, ds
Jingera 2H 38 Quartz, quartz bi, bm, uraninite feldspar
Wilson's 2H 10.2 Quartz cs, wo Downfall
Everton 3R>2H 530-630 Quartz porphyry py. cp
Whipstick 3R>2H 580 Garnet quartz bi, bm, ap, py, cp, fmo, uraninite, joseite
Abbreviations: ap arsenopyrite, bi bismuth, bm bismuthinite, cs cassiterite, cp chalcopyrite, fl fluorite, frao fenimolybdite, gl galena, pw powellite, py pyrite, po pynhotite, sch scheelite, sp sphalerite, sn stannite, wo wolframite 299
3. Mount Douglas, Victoria, is classed by McKenzie (1976) both with his Type 1 occurrences (bundles and lenses of flakes in quartz veins in granite) and with his Type 2 occurrences (disseminated flakes in aplite and pegmatite dykes and in granite). Mineralogy suggests it fits the group of 3R associated with bismuth and bismuthinite: Whitelaw (1921) observed molybdenite is associated with quartz veins deposited in the final stages of the secondary silicification of granite, is in siliceous vughs in and disseminated through the granite, and veins and granite carry splashes of sulphides containing Cu, Fe, Bi, and Mo.
The high Re 2H molybdenites of Table 1 are:
1. At Yetholme, N.S.W., metasomatic mineralization occurs in a skarn. High Re 2H molybdenite occurs with powellite, fenimolybdite, scheelite, pyrite, chalcopyrite, arsenopyrite and sphalerite. Powellite carries about 25% of the molybdenum (Weber etal. 1978).
2. At Yea, Victoria (Mitchell 1970), and Wog Mountain, N.S.W. (Weber et al. 1978), molybdenite is minor. The Wog Mountain occunence is molybdenite, pyrite, pyrrhotite, chalcopyrite and ferrimolybdite disseminated in granite; the Yea occurrence is in massive quartz veins.
3. At Agarak and Kadzharan in the Zangezur orefield, Armenia (Khurshudyan 1967, Khurshudyan et al. 1969), Cu-Mo mineralization of quartz-molybdenite-chalcopyrite association is present as stockworks and disseminations. The stockworks occur as zones of hydrothermally altered batholithic rocks. Khurshudyan (1967) described Kadzharan as comparatively hypothermal; Khurshudyan et al. (1969) described deposits in the Zangezur region as hydrothermal high or medium temperature but ascribed presence of 3R at Agarak to its lower temperature of formation. At Kaier, also in the Zangezur ore region, pegmatitic-pneumatolytic molybdenum mineralization is high Re 2H.
These relatively high Re 2H molybdenites may constitute a new class of molybdenites or may not demonstrate anything other than varying Re content with no apparent affect on polytype crystallization.
If there is a class of high Re 2H molybdenites why they should crystallize is not clear, although there are some possibilities: 300
1. Growth under conditions not conducive to growth by addition onto screw dislocations. Synthetic molybdenites prepared under conditions favouring growth by addition onto screw dislocations, such as growth from a flux or hydrothermal crystallization, are 3R (see reviews in Khurshudyan et al. 1969, Newberry 1979a). Where growth conditions militate against growth by addition onto screw dislocations 2H results: Rekharskiy et al. (1985), and references therein, carried out experiments that indicate that the presence of the 2H and 3R polytype modifications is governed not by the Re content but by the physicochemical conditions of their formation. However, there is no evidence that natural molybdenites are not products of hydrothermal crystallization. Early and later molybdenite-containing veins in granite were found by Bloom (1984) to contain hypersaline and less saline brines respectively; Thomas et al. (1988) found saline brines in the aqueous fluid phase of the low Re 2H containing Tanco pegmatite. There is no reason to suppose that high Re 2H molybdenites of similar paragenesis were not deposited from similar solutions and by similar mechanisms.
2. Growth of the stable 2H modification because of crystallization at relatively high temperatures and despite high impurity levels. The deposits in which they occur are pegmatitic-pneumatolytic, hypothermal, and skarn; in the Zangezur orefield high Re hypothermal and pegmatitic-pneumatolytic molybdenites are 2H but high Re molybdenites from epithermal deposits are 3R, and in the Ayotsdzor orefield veins intermediate between mesothermal and epithermal carry high Re 3R molybdenite. However, ores with low Re 2H and high Re 3R molybdenites were deposited over a wide range of temperatures (Hulen et al. 1978, Wesolowski and Ohmoto 1981, England 1985). Crystallization of 2H molybdenite with >500ppm Re cannot be ascribed to high crystallization temperatures, just as the many attempts to link crystallization of 3R to low temperatures failed.
3. Growth of high Re 2H molybdenite in a high Re/Mo environment developed unusually early in the deposit history. Newberry (1979b) found porphyry deposits characterized by A-type veinlets are high grade Mo/low Re. Their molybdenite is low Re 2H, deposited because of a drop in K/Na ratio of the fluid. Deposits characterized by B-type veinlets are low grade Mo/high Re. Their molybdenite is high Re/abundant 3R, deposited because of temperature fall. The Re-content of molybdenite is an inverse function of the amount of the mineral deposited. In porphyry deposits, 3R resulted from development of a sufficiently high Re/Mo ratio at the stage of B-type veinlet 301
formation. High Re 2H molybdenites appear to be from low grade deposits. At Yetholme, N.S.W., grades range from 0.1 to 0.14% Mo and about one quarter of the Mo is in powellite; Yea, Victoria, and Wog Mountain, N.S.W., were insignificant producers; and at least some of the Armenian deposits carry only a little molybdenite. In these low Mo content deposits high Re/Mo ratios may have developed early, leading to crystallization of Re rich 2H molybdenites.
CONCLUSIONS
The data assembled show molybdenite polytypes from Australian pipe deposits join polytypes from porphyry copper, porphyry molybdenum and copper skarn deposits in fitting exactly Newbeny's model for the origin of polytypes. A small group of high Re 2H molybdenites has been found, characteristically from low grade Mo high Re deposits in which high Re/Mo ratios probably developed at an early stage in the crystallization history and at relatively high temperatures in molybdenites disseminated in granites, pegmatites and aplites, in quartz veins in stockworks in granites, and in skarn deposits. Re contents in excess of 500ppm that in other styles of occurrence cause metastable growth of some 3R have not led to appearance of 2H+3R molybdenite. There are practical consequences: not all 2H-only molybdenite deposits are low Re and not all high Re molybdenites are associated with porphyry and skarn styles of mineralization containing some 3R.
ACKNOWLEDGEMENTS
To Durham University for laboratory and library facilities and access to museum specimens while resident there as a Visiting Scientist, when part of this study was carried out, and to the British Museum (Natural History) for access to records.
REFERENCES
AYRES, D., 1974. Distribution and occunence of some naturally occuning polytypes of molybdenite in Australia and P.N.G. J. Geol. Soc. Australia 21:273-278. BLOOM, M.S., 1984. Hydrothermal fluid compositions in molybdenum mineralised granites : an application of fluid/mineral equilibria. Australian Geological Convention 7th, Sydney, Geol. Soc. Australia Abstracts No.l2:62-63. BRALIA, A., GUASPARRI, G., RICCOBONO, F. & SABBATINI, G., 1983. Polytypism of molybdenite in the molybdenum deposits of the Sardinian batholith. Periódico di Mineralogía 52:235-251. CHIVAS, A.R., 1978. Porphyry copper mineralization at the Koloula igneous complex, Guadalcanal, Solomon Islands. Econ. Geol. 73:645-677. 302
COTTON, L.A., 1909. The tin deposits of New England, N.S.W.Part 1. The Elsmore-Tingha district. Soc. N.S.W. Proc., 34:733-781. ENGLAND, B.M., 1985. Famous mineral localities: The Kingsgate mines. Mineral. Rec., 16:265-289. FORTEY, N.J., 1980. Hydrothermal mineralization associated with late Caledonian intrusions in northern Britain. Trans. Inst. Mining Met. Sect. B, 89:173-176. FRONDEL, J.W. & WICKMAN, F.E., 1970. Molybdenum polytypes in theory and practice. 2. Some naturally occurring polytypes of molybdenite. Am. Miner., 55:1857-1875. GUASPARRI, G., RICCOBONO, F. & SABATINI, G., 1985. Hercynian molybdenium mineralizations of porphyry style in the Sardinian batholith. A discussion on the genesis and a comparison with other deposits of the family.Rend. Soc. Ital. Mineral. Petrol., 39:629-648. HULEN, J.B., NIELSON, D.L., GOFF, F., GARDINER, J.N. & CHARLES, R.W., 1987. Molybdenum mineralization in an active geothemal system, Valles Caldera. Geology, KHURSHUDYAN, E.K., 1967. Mode of origin of the rhombohedral modification of molybdenite. Doklady, 171:149-151. KHURSHUDYAN, E.K., ARUTYUNYAN, L.A. & MELIKSETYAN, B.M.,1969. Origin of the polytypes of molybdenite. Geochemistry, 6:942-949. KNIGHT, N.D., 1979. Australian molybdenite deposits. Aust. Bur. Miner. Resour. Geol. Geophys., Report No.9. LIDDY, J.C., 1971. Molybdenite in eastern Australia. Mining Mag., 124:41-53. MCKENZIE, J.A., 1976 in DOUGLAS, J.B. & FERGUSON, J.A. (Eds.) Geology of Victoria. Geol. Soc. Australia, Special Publication No.5:452-453. MANDARINO, J.A. & GAIT, R.I., 1970. Molybdenite polytypes in the Royal Ontario Museum. Can. Mineralogist, 10:723-729. MELNIKOV, I.V., GORSCHKOV, A.I., STRELTSOV, V.A., IVANOVA, O.A., KORUVUSHKIN, V.V., BORONIKHIN, V.A. & SOBOLOVA, S.V., 1978. Crystallochemical characteristics of finely dispersed structures of molydbenum disulphide. Chem. Abstr., 88,173602t. (Original not seen). MITCHELL, B.D., 1970. Molybdenite at Yea. Record, Geological Survey of Victoria 4:429. MORGAN, J.W., LOVERING, J.F. & FORD, R.J., 1968. Rhenium and non- radiogenic osmium in Australian molybdenites. J. Geol. Soc. Australia, 15:189-194. NEWBERRY, R.J.J., 1979a. Polytypism in molybdenite. 1. A non-equilibrium impurity related phenomenon. Am. Miner., 64:758-767. NEWBERRY, R.J.J., 1979b. Polytypism in molybdenite. 2. Relationships between polytypism, ore deposition, alteration stages and Re contents. Am. Miner., 64:768-775. REKHARSKIY, V.I., DIKOV, Y.P., ZHUKHLISTOV, A.P., ZUYAGIN, B.B., GORSHKOV, A.I., SAVELYEV, L.V. & TRUBKIN, N.V., 1985. Effects of time and temperature on the phase characteristics and structure of sulphides of the MoS2-ReS2 system. Int. Geol. Rev., 27:615-624. 303
RILEY, G.H., 1967. Rhenium concentration in Australian molybdenites. Geochim. Cosmochim. Acta, 31:1489-1497. SANTOSH, M., 1988. Granite-molybdenum system of Ambalavayal, Kerala. Part 2. Nature of mineralization, sulphur isotopes, fluid characteristics and a genetic model. J. Geol. Soc. India, 32:191-213. THOMAS, A.V., BRAY, C.J. & SPOONER, E.C.T., 1988. A discussion of the Jahns-Burnham proposal for the formation of zoned granitic pegmatites using solid-liquid-vapour inclusions from the Tanco pegmatite, S.E. Manitoba, Canada. Trans. Roy. Soc. Edinburgh: Earth Sciences, 79:299- 315. WATSON, D.P., 1970. Rhenium. Mineral Ind. Geol. Surv. N.S.W., 46:15pp. WEBER, C.R. PATERSON, I.B.L. & TOWNSEND, D.J.,1978. Molybdenum in N.S.W. Geol. Surv. N.S.W., Mineral Resources, No.43:218pp. WESOLOWSKI, D. & OHMOTO, H., 1981. Remobilization of molybdenum from W-Mo skarns by late meteoric fluids at King Island, Tasmania. Geol. Soc. Amer., Abstracts with Programs'.SIS. WHITELAW, O.A.L., 1921. Mount Douglas molybdenite mine, Korong Vale. Geol. Surv. Victoria, Record 4:430-434.
Dr D. J. Drysdale Dept, of Geology & Mineralogy The University of Queensland Queensland, 4072.