Origin of Piedmontite-bearing Quartz-Muscovite-Schists of North-west Otago.
[Read before Otago Branch, May 7, 1946; received by Editor, May 14, 1946.]
Pale-pink laminated quartz-schists carrying muscovite and piedmontite occur at a number of localities throughout north-western Otago, especially in the mountainous region between Lakes Wakatipu and Wanaka. These rocks were noted by earlier writers as “bright pink micaceous quartzites” (McKay, 1882, pp. 82, 90, 91) and “red micaceous quartz-schist” (Park, 1906, pp. 42, 43). Their petrography and fabric have been described in various papers by C. O. Hutton and the writer (Hutton, 1940, pp. 19–23, 42, 43, 67, 68; 1943; Turner, 1933; 1936, pp. 221–223), of which Hutton's 1940 account is the most detailed.
Two additional occurrences, not previously recorded, have recently been brought to the writer's notice by Mr. H. W. Hockin and Dr. B. Mason respectively. The first is a typical north-west Otago quartz-muscovite-piedmontite-schist interstratified with thick beds of chloritic greenschist on Mount Joffre, a peak on the Main Divide at the head of the Matukituki Valley in north-western Otago. The second is a fine-grained quartz-piedmontite-schist with minor muscovite, ironore, accessory garnet and colourless tourmaline, found in stream boulders Waitahu River, Southern Nelson.
The writer has nothing to add to the careful petrographic and chemical account of the piedmontite-bearing schists of Otago given by Hutton (1940), but in discussing the origin of these rocks he would draw attention particularly to the following points which are believed to have special genetic significance:—
In every case where piedmontite-schists have been recorded in situ in Otago, they are immediately associated with chloritic greenschists, or else rocks of this latter class occur plentifully in the same general locality. These greenschists are metamorphic derivatives of basic lavas and tuffs.
Although greenschists are relatively plentiful in north-west Otago, the predominating rocks here, as elsewhere in the Otago schist belt, are quartz-albite-epidote-chlorite-muscovite-schists derived from sedimentary rocks of the greywacke family.
The mineralogical assemblage developed in the piedmontite-bearing rocks of Otago is remarkably constant: quartz is extremely abundant; muscovite and piedmontite are present
in about equal quantity; iron-ores and manganese-garnet are sometimes essential constituents or at least are present in accessory amount. Hutton has shown that colourless or pale tourmaline is a constant minor constituent occurring in noticeably greater quantity than in most other types of schist in this district, and has also identified barite as a relatively plentiful accessory in two of the piedmontiteschists which he examined. Also characteristic is a general lack of albite, which by contrast is usually an important constituent of other quartz-rich schists in Otago, while stilpnomelane and chlorite (also common minerals in Otago schists) are typically absent from the piedmontite-bearing assemblages.
As yet the piedmontite-schists have been found only in Subzone Chl. 4 of the Chlorite Zone of Otago; i.e., they occur only among the coarse-grained fully reconstituted schists of that subfacies of the Greenschist Facies wherein the low-temperature assemblage muscovite-prochlorite is stable.
The problem of the origin of Otago piedmontite-schists has been discussed by Hutton (1940, pp. 67, 68), who concluded that local concentration of magmatically derived boron-bearing solutions or vapours was an essential factor in the genesis of these rocks. This conclusion was based on the recorded occurrence of piedmontite in hydrothermally altered rocks in other countries, and on the relative abundance of tourmaline in the piedmontite-schists of Otago. The present writer agrees with Hutton that much of the tourmaline in Otago schists owes its origin to introduction of boron from subjacent granitic intrusions. But it cannot therefore be assumed that concentration of tourmaline in one particular lithological environment (as in piedmontite-bearing quartz-schist or in pelitic mica-schist) indicates that magmatic solutions or vapours have necessarily been specially active in the genesis of that particular type of rock. It would rather seem likely that, when tourmaline is plentifully associated with one type of rock in widely-scattered localities, its abundance is due to pre-metamorphic concentration of boron in the parent sediment.
The schists of Otago and Westland are the regionally metamorphosed derivatives of a thick series of geosynclinal sediments which accumulated in Palaeozoic times and which were metamorphosed towards the end of that era. Shallow-water feldspathic sandstones (greywackes) were the dominant lithological type in this series. Micaceous derivatives of pelitic and semipelitic sediments are subordinate, and calcareous rocks are rare. But in north-western Otago, greenschists of basic igneous origin are relatively plentiful, and apparently represent submarine flows and tuff-beds such as are so commonly associated with geosynclinal sediments in other parts of the world. Analysed specimens of Otago greenschists are not sufficiently rich in soda to be classed as spilites; but true spilite-tuffs and keratophyres occur among the late Palaeozoic rocks in the vicinity of Bluff (Service, 1937, pp. 191, 192, 199), while fragments of keratophyre have been recognised by the writer in the breccias of the Te Anau Series (also late Palaeozoic) west of Lake Wakatipu. The latter have
been shown to pass with increasing metamorphism into the fully reconstituted Otago schists of which the greenschists and piedmontiteschists under discussion are members.
In other parts of the world there is a remarkably constant tendency for highly siliceous cherts, often containing noteworthy manganese and iron, to occur in close association with basic lavas and tuffs (including spilitic rocks) in fillings of geosynelines. And it has been argued convincingly (e.g., by E. F. Davis, 1918; R. Ruedemann and T. Y. Wilson, 1936, p. 1550; N. L. Taliaferro, 1943) that much of the silica in such cherts has been precipitated, together with iron and manganese, from sea-water that has locally been enriched in these substances through activity of submarine springs connected with volcanism. It can scarcely be doubted that the piedmontite-bearing quartz-schists of Otago are the metamorphosed equivalents of geosynclinal cherts, genetically connected, in the manner just described, with the basic lavas and tuffs that have since been converted to greenschists. Their content of manganese, iron, boron, and in some cases noteworthy barium agrees perfectly with origin by precipitation of a silica gel from sea-water, though part of their silica may well represent accumulated skeletons of silica-secreting organisms such as are well known to flourish in siliceous sea-water. The muscovite ubiquitously present to the extent of 5–10% in Otago piedmontite-schists is believed to represent the fine detrital clay and silt fraction of the original chert, namely, sericite, clay minerals and adsorbed potash.
It is scarcely necessary to appeal to special hydrothermal conditions of metamorphism to account for the appearance of piedmontite in place of the manganese-garnet which is so much more widely distributed as the manganiferous mineral of Otago schists. In all probability, the bulk chemical composition of the rocks concerned is the deciding factor. The writer has not found it possible to demonstrate this by an Acf-diagram or any other three-component diagram, since the manganiferous and ferruginous schists of Otago are too complex to treat, even in simplified form, as three-component systems. Certain of the common mineral phases which appear in these rocks demand that Fe2O3, Al2O3, FeO, MnO, and MgO all be considered as independent components. It is significant, however, that in the many and varied types of rocks containing stilpnomelane (cf. Hutton, 1938, 1940) spessartite-rich garnet is the sole manganiferous phase—apart from stilpnomelane itself which sometimes carries several per cent, of MnO. Furthermore, the association spessartite-piedmontite is undoubtedly stable in the schists of western Otago. There is thus strong petrographic evidence that the assemblage stilpnomelane-piedmontite is unstable, and that its chemical equivalent would be garnet-magnetite-actinolite, which is a well-known association in ferruginous rocks of this district (cf. Hutton, 1940, pp. 38, 39). The stable assemblages recorded by Hutton for schists rich in iron and manganese in the Chlorite Zone of western Otago are:—
Quartz may accompany any of the above, and is, of course, the most plentiful mineral in rocks in which assemblages (1) and (2) are developed.
It has been shown by Hutton (1940, pp. 45, 46) that the highly ferruginous magnetite-garnet-actinolite-schists and garnet-stilpnomelane-schists have bulk compositions corresponding to those of impure sedimentary iron-ores. The writer suggests that they are metamorphic derivatives of ferruginous marine sediments, which like the associated cherts were formed in the first instance by chemical precipitation from sea-water into which iron, manganese, and silica had been introduced from submarine volcanic springs.
In conclusion, attention is drawn to the interesting problem of evolution of fabric in the quartz-piedmontite-muscovite-schists. During metamorphism these have changed from fine-grained products of chemical precipitation to relatively coarse-grained rocks, the individual quartz grains of which average 0.2–0.5 mm. in diameter and are distinctly elongated in the principal plane of schistosity. Preliminary investigation shows that preferred orientation of quartz, muscovite, and piedmontite is strongly developed. Obviously indirect componental movement, involving suppression of the majority of the original grains of quartz and survival and great enlargement of a very small minority, must have played an important role in the orienting process, and perhaps far outweighed the mechanical processes of direct componental movement. A record of petrofabric analyses of a few typical quartz-piedmontite-schists, carried out by Mr. J. B. Mackie, will appear shortly.
This paper was written while the writer was on the staff of the Geology Department, University of Otago.
—— 1940. Metamorphism in the Lake Wakatipu Region, N.Z.D.S.I.R. Geol. Mem., no. 5.
—— 1943. Piedmontite-bearing Quartz Schists from Black Peak, North-western Otago, N.Z. Journ. Sci. and Tech., vol. 23, no. 6B, pp. 231B–232B.
Park, J., 1906. Geology of the Alexandra Sheet, N.Z. Geol. Surv. Bull., no. 2.
Ruedemann, R., and Wilson, T. Y., 1936. Eastern New York Radiolarian Cherts, Bull. Geol. Soc. Amer., vol. 47, pp. 1535–1586.