The Significance of Tourmaline in the Otago Schists.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 68, 1938-39, Page 599

The Significance of Tourmaline in the Otago Schists. By C. Osborne Hutton, M.Sc., Ph.D., F.G.S., N.Z. Geological Survey. [Read before the Otago Branch, September 13, 1938; received by the Editor, September 16, 1938; issued separately, March, 1939.] During a recent detailed petrographic investigation, tourmaline has been found to be a widely distributed and often abundant constituent of the metamorphic rocks of western Otago. In view of Goldschmidt and Peters' recent work (1932) the question has arisen whether we can justifiably consider this mineral to have formed as a result of permeating boron-bearing vapours originating from a deeply-buried granite batholith, or to be due to the boron content of the original unmetamorphosed sediments. The excellent work of these writers has shown that the boron content of clay sediments is often sufficient to bring about crystallization of tourmaline when these sediments suffer dynamic metamorphism and that it is not necessary to look to the younger acid intrusives for the origin of the boron. However, bearing this in mind, the writer feels that there is sufficient evidence to support the suggestion that the tourmaline present in the Otago schists owes its origin, at least in the majority of cases, to the presence of a granitic intrusion deep beneath the schist area. The origin of the tourmaline may be considered in the light of the following facts: (1) Tourmaline occurs in albite-epidote-chlorite or albite-epidote-actinolite-schists which lack micas or more than accessory amounts of quartz and hence appear to have been derived from basic igneous rocks. (2) In an unusual green schist of tuffaceous origin, tourmaline is very abundantly developed as irregular patches in chlorite; it also occurs in sinuous lines in calcite and in groups of prisms concentrated within albite. (3) Pale tourmaline is plentifully developed in a peculiar albitemuscovite-schist outcropping within a belt of crushed and pulverised rocks, the tourmaline-bearing schist itself not having suffered cataclasis. (4) Veins of metallic sulphides occur throughout the region, and tourmaline has been observed in the adjacent rocks (Hutton, 1934). (5) Granites, diorites and norites of the Fiordland Complex come to the surface 25–26 miles west of the head of Lake Wakatipu. (6) Pale or completely colourless tourmaline is always developed, often abundantly, in a series of quartz-muscovite-piedmontite-schists. (7) Quartz-albite-epidote- (+ muscovite) schists are not comparable to clay sediments in bulk composition or mode of origin.

(8) Scheelite occurs in varying amount in quartz veins throughout the Otago Schist Complex and Finlayson (1908) attributed its origin to solutions bearing tungstic acid ascending by way of lodefissures from deep-seated magmas, largely granitic in character. The presence of tourmaline in albite-epidote-chlorite or albite-epidote-actinolite-schists, in dense patches in a green schist of tuffaceous origin and in the albite schist in the shear zone would certainly seem to suggest pneumatolytic origin. The occurrence of clots of tourmaline forming sinuous lines in calcite is unusual, but Turner (1933, p. 224) described a comparable case in a rock from southern Westland, and considered pneumatolytic vapours responsible for the introduction of tourmaline. Nevertheless the possibility that the tourmaline is due to recrystallization and redistribution from a tourmaliniferous pebble of sedimentary origin must not be excluded, though if this were the case, it is difficult to understand why the mineral should occur in this curious manner. The tourmaliniferous albite-muscovite-schist from the crushed zone has a curious bulk composition, for it is high in Na2O, K2O, Al2O3 and must contain appreciable B2O3; in the writer's opinion pneumatolysis alone can be held responsible for its unusual composition. The widespread occurrence of tourmaline in most quartzofeldspathic schists is also believed to be due to permeating boronbearing vapours, rather than to any original boron content of the sediments, for as pointed out in (7) above, their mode of origin and their composition is not comparable to that of clay sediments. Nevertheless it is true that these schists are derived from sediments which have originated by rapid disintegration of basic to intermediate rocks with some admixture of argillaceous and arenaceous impurities; but the boron content of the argillaceous material would have to be fairly high to account for the not inconsiderable quantity of tourmaline usually present. In the piedmontiferous quartz-muscovite-schists, colourless tourmaline is a constant and important constituent, forming “nests” or strings of minute idioblastic prisms, sometimes completely confined within waterclear xenoblastic grains of quartz. The tourmaline in these schists has the properties corresponding most closely with those of elbaite, the refractive indices being α = 1.617 and γ = 1.637; γ—α = 0.020. The mineral is so plentiful in some thin slices that there would seem little doubt that it is the result of pneumatolysis. The production of piedmontite is, in the writer's opinion, closely associated with the presence of tourmaline in these quartz-muscoviteschists. The micaceous quartz-schists in this region can be divided into two groups:— (1) Quartz-muscovite-garnet-schists without plentiful pale or colourless tourmaline, and (2) Quartz-muscovite-piedmontite-schists with some manganese garnet but plentiful pale tourmaline.

If the only condition necessary for the formation of piedmontite was the presence of manganese, then why should it not be present in a quartz-muscovite-spessartite-schist without tourmaline? Clearly therefore, there appears to be a connection between the presence of piedmontite and much tourmaline. In recent work on several American occurrences of piedmontite (Lausen, 1927; Mayo, 1932; Short, 1933; Guild, 1935; Simonside, 1935) the authors all agree in believing that either hydrothermal solutions or proximity to igneous masses must be held responsible for the formation of this mineral. Furthermore H. von Eckermann (1936, p. 192) considers that close proximity to a sill of the Risberg granite is responsible for the development of crystals of manganese-epidote in a sericite-cordierite-schist in the Loos-Hamra Region. The present writer believes that the piedmontite in the western Otago schists is primarily a pneumatolytic mineral. On the evidence of the scheelite veins in several localities in Otago, Finlayson (1908, pp. 119–120) put forward the suggestion that a batholith of a granitic type underlay the Otago region and there seems no good reason to doubt this view. An investigation of the south Westland area by Turner (1933) has shown that a granite mass becomes less deeply buried towards the west, the rocks becoming more and more metamorphosed till a zone of hornfels and contact gneisses is reached. Hence on this evidence alone it would appear reasonable to suppose that in the Central Otago-Wakatipu area this batholith is still present, but much more deeply buried. It is possible that apophyses of this intrusion have reached, or at least come very close to, the surface in Eastern Otago. Scheelite has been found at Saddle Hill, Barewood, Hindon, and Waipori. Furthermore, Turner and the present writer (1936, pp. 266–267) have found andalusite and kyanite in heavy residues of Tertiary sediments from the Dunedin area itself. Minerals such as kyanite, exhibiting very good cleavages, will not survive transportation over any great distance, hence this mineral at least would seem to have been derived from a nearby zone of intense metamorphism now buried under the Tertiary sediments or submerged off the present eastern shore-line. Further, it is clear that the watershed of streams that laid down these Tertiary beds must have included an area of thermally metamorphosed rocks, as indicated by the presence of andalusite. To sum up, therefore, the present writer is of the opinion that in most cases the tourmaline present in the metamorphic rocks of the Otago area is due primarily to vapours and solutions given off from a granite batholith, the depth of burial of which becomes less and less as the western sea-coast is approached. Literature Cited. von Eckermann, H., 1936. The Loos-Hamra Region, Geol. Forens. Forhand., vol. 58, no. 405, pp. 129–343. Finlayson, A. M., 1908. The Scheelite of Otago, Trans. N.Z. Inst., vol. 40, pp. 110–122.

Goldschmidt, V. M., and Peters, Cl., 1932. Zur Chemie des Bors II, Nachrichten von der Gesellschaft der Wissen., Gottingen, Mat. Phys. Klasse, iii, no. 28, iv, no. 31, pp. 528–545. Guild, F. N., 1935. Piedmontite in Arizona, Amer. Min., vol. 20, no. 10, pp. 679–692. Hutton, C. O., 1934. Metallic Sulphides in the Shotover River District, N.Z. Jour. Sci. and Tech., vol. 16, no. 3, pp. 154–155. — and Turner, F. J., 1936. The Heavy Minerals of Some Cretaceous and Tertiary Sediments from Otago and Southland, Trans. Roy. Soc. N.Z., vol. 66, pp. 255–274. Lausen, C., 1927. Piedmontite from the Sulphur Spring Valley, Arizona, Amer. Min., vol. 12, no. 7, pp. 283–287. Mayo, E. B., 1932. Two New Occurrences of Piedmontite in California, Amer. Min., vol. 17, no. 6, pp. 238–247. Short, A. M., 1933. A Chemical and Optical Study of Piedmontite from Shadow Lake, Madera County, California, Amer. Min., vol. 18, no. 11, pp. 493–500. Simonside, R. R., 1935. Piedmontite from Los Angeles County, California, Amer. Min., vol. 20, no. 10, pp. 737–738. Turner, F. J., 1933. The Metamorphic and Intrusive Rocks of Southern Westland, Trans. N.Z. Inst., vol. 63, pp. 178–284.