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Volume 78, 1950
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3. Petrology

(a) Petrography of Serpentinites. The serpentine minerals used in this paper—chrysotile, antigorite, bastite and serpophite—follow the usage of earlier investigators in New Zealand as summarized by C. O. Hutton (1936, pp. 241–242). It should be noted that Lodoch-

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nickow (1933), however, citing evidence obtained by Russian workers of the very wide variation in optical properties of the serpentine group of minerals has proposed a purely textural classification of serpentine minerals.

The Mossburn serpentinites can be divided into mesh serpentinites with or without bastite, and sheared serpentinites. The mesh serpentinites are macroscopically dark green in colour, frequently slickensided and with a dull to enamel-like lustre. Bastite pseudomorphs are recognisable in several specimens (7729, 7722). In thin section, the serpentinites generally display a mesh or lattice structure of pale yellow chrysotile fibres less than 0·5 mm. in length, and this structure appears comparable with the mesh types described by C. G. Hutton from North-west Otago (1936, pp. 242–243). Occasionally yellowish green fibres with negative elongation radiate from cores of pale yellow serpophite (7724, 7728). Bastite pseudomorphs are present in many rocks (2719–22, 7729, 7730) as crystals generally less than 4 mm. in length and usually they display pleochroism ranging from yellowish green (X) to pale green (Z). Iron ore occurs as scattered irregular grains 0·2 mm. in diameter and as fine granules marking the mesh boundaries. In some instances, the coarse grains are chromite (7719). Vein serpentine in one specimen (7719) has a medium birefringence and a moderate pleochroism according to the scheme, X—light yellow, Z—dark golden yellow; this type would appear to be closely related to bowlingite (Winchell, 1933, p. 437).

Two sheared serpentinites were noted, one (7749) being a white variety from the south end of Black Ridge and the other (7733) a contorted dark green rock from the ridge east of Windy Hill. In thin section, both display a marked cataclastic structure, the main constituent of which is a low birefringent serpentine (serpophite?). Finely scattered iron ore is abundant in one specimen (7733).

  • (b) Petrogenesis of Serpentinites. Except where localised marginal shearing of the serpentinite has occurred, the Mossburn serpentinites are massive rocks in which mesh structure is dominant and chrysotile the most plentiful serpentine mineral. Relicts of peridotite have not been observed, but the presence of chrysotile serpentine with and without bastite suggests that the original peridotite had the composition of dunite and harzburgite. Asbestos veins have not been developed to any extent. The absence of other adjacent igneous intrusions from which solutions could emanate suggests that here, as in many other areas in New Zealand, serpentinisation probably results from autometasomatism of the original peridotite by concentration of magmatic waters during the consolidation of the magma after injection (Benson, 1918; Turner, 1933; Hess, 1933; Lodochnickow, 1933).

  • (c) Petrography of altered dolerites and gabbros. The altered basic intrusions associated with the serpentinites are massive, black, amphibole rocks veined with white prehnite. The amphibole is dominantly uralite occurring in fibres 1 mm. in length, and with a pleochroism ranging from colourless (X) to deep green (Z) Rarely a brown pleochroic amphibole has been observed, occasionally as separate crystals (7855), but more often as central cores to the green uralite (7861). A few relict prisms of augite remain (7859, 7876, 7875), and these are being replaced by uralite. Colourless prehnite

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  • is present both interstitially and in veins less than one inch in width. The prehnite sometimes displays a radiating spherulitic structure between crossed nicols (7889, 7857), and in this respect resembles the prehnite from the Baltic described by P. Eskola (1934, p. 134). Many rocks (7849, 7862, 7884) contain abont equal amounts of uralite and prehnite; in others (7856, 7878, 7883) feldspar laths 3 mm. in length are essential constituents (5–30 per cent.). Although refractive index measurements were hindered by the presence of saussurite inclusions several determinations indicate that the feldspar is now albite (7856, 7883, 7878). Sphene is a common accessory mineral and its presence points to an original ilmenitic iron ore. Locally abundant are interstitial clear quartz (7877) and pale green chlorite (penninite) with anomalous blue interference tint (7876). Shearing of the basic rocks has occurred along the margins of the intrusions (8005).

  • (d) Petrogenesis of altered dolerites and gabbors. The occurrence of unfractured feldspar crystals and relicts of pyroxene indicate that basic intrusions were dolerites and gabbros which have now been converted into uralite-prehnite rocks. Earlier workers (Benson, 1918, 722–733, 1926, p. 43; Grange, 1927; Turner, 1930, p. 198, 1933, p. 271) have shown that this uralitization and prehnitization is probably brought about by the metasomatic effect of aqueous solutions emanating from the basic magma and as a result of the serpentinization of lime-bearing minerals in the original peridotite.