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Volume 75, 1945-46
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The Scheelite Deposits at Glenorchy, New Zealand.

[Read before Wellington Branch, August 7, 1945; received by the Editor, August 8, 1945; issued separately, December, 1945.]

Abstract.

H. W. Fairburn (1942) has emphasised the desirability of petrofabric investigation of ore deposits, few of which have yet been examined in this way. The present paper embodies the results obtained by applying the methods of structural petrology in addition to the more usual methods to a field and laboratory study of the scheelite ores of Glenorchy, New Zealand, and the schists in which the ore bodies occur. Petrofabric investigation has shown that the steeply dipping attitude of the schists at Glenorchy is due to post-metamorphic tilting which has scarcely modified the internal fabric and which is provisionally correlated with Early Pliocene (?) tectonic movements. This fact has enabled a distinction to be made between scheelite-bearing quartz lodes and barren quartzo-feldspathie veins. The veins, which intersect the schistosity planes at high angles, are considered to result from segregation in tension joints formed parallel to the axis of the tilting movement: on the other hand there is good reason to believe that the lodes as well as the schists were affected by post-metamorphic tilting. Further, the quartzo-feldspathic veins show strong dimensional orientation of individual grains, a feature not seen in the lodes. The massive quartz lodes containing irregularly disseminated scheelite, calcite, arsenopyrite, pyrite, and traces of gold, occupy extensive crush-bands in the schist; they are believed to be due to the mineralising action of magmatic waters ascending from a subjacent batholith of granite (unexposed), intrusion of which occurred during the final stages of the Late Palaeozoic metamorphism after metamorphic deformation had died away. No definite and orderly sequence of crystallisation of the lode minerals could be established. The uniform and clear-cut patterns of the maxima in the quartz diagrams obtained by petrofabric study of two quartzose-schists are briefly discussed in connection with the problem of the mechanism of quartz orienting in metamorphism. Appendices contain chemical analyses and fluorescence tests of the scheelite and descriptions of special petrological procedure.

Contents.

Introduction and Summary of Geological History.

Schists.

A.

Petrography.

B.

Structural Petrology.

  • (i) Megascopic Fabricology.

  • (ii) Preferred Orientation of Quartz and Stilpnomelane.

  • (iii) Tectonic Synthesis.

  • (iv) Significance of Maxima in the Quartz Diagrams.

Scheelite-bearing Lodes.

Field Occurrence.

Petrography.

Preferred Orientation Phenomena.

Sequence of Mineralisation.

  • (i) Quartz-Scheelite Relationships.

  • (ii) Calcite-Scheclite Relationships.

  • (iii) Calcite-Quartz Relationships.

  • (iv) Pyrite Relationships.

  • (v) Conclusion.

Origin of the Scheelite.

  • (a) Lateral Secretion and Secondary Enrichment Hypotheses.

  • (b) Hypothesis of Magmatic Derivation.

Minor Quartz Veins.

Conclusions.

Acknowledgements.

Appendices.

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FIG. 1.—Locality map and geological setting of the Glenorchy district. 1, alluvium; 2, Late Tertiary igneous rocks; 3, Tertiary sediments; 4. Late Jurassic ultrabasic intrusions, 5. Mesozoic sediments; 6, chloritic schists; 7, greywackes, etc.; N.E. probably Mesozoic; N. W. S. and S. E. probably Late Palaeozoic; extreme S W; Ordovician graptolite-bearing slates; 8, “Fiordlund” complex of granites, diorites, norites, gneisses, etc. The province of Westland is 30 miles north of Glenorchy.

Introduction and summary of geological history.

The scheelite-bearing lodes described below are situated in the Glenorchy district, near the northern end of Lake Wakatipu, in the South Island of New Zealand (Fig. 1). They outcrop on the glaciated western slopes of the Richardson Range and in the narrow postglacial gorges incised into the flank of the range by westward flowing streams such as the Bucklerburn and Temple Creek (Fig. 2). Mining has been in progress intermittently since 1888, the maximum recorded annual production being 116 ¼ tons of concentrates in 1917. At the time of writing (August, 1944) the two principal mines—one near Glenorchy itself, the other at Paradise, some ten miles further north—are operated by the Mines Department of the New Zealand Government.

The geological history of the region is summarised as follows:—

1. Palaeozoic sedimentation. The oldest and most extensively developed rocks in the region are schists resulting from low-grade regional metamorphism of quartzo-feldspathic marine sediments (greywackes), which, judging from the work of F. J. Turner (1934,

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1935) and C. O. Hutton (1940) in adjoining areas, are probably equivalent to the Late Palaeozoic Te Anau Series and possibly include older rocks as well.

2. Metamorphism (? Late Palaeozoic) and mineralisation. Regional metamorphism has affected the older rocks throughout the whole of Otago Province, and in the Lake Wakatipu region has converted the greywackes to albitic and chloritic schists (Fig. 1), which in the Glenorchy mining area strike west of north and dip uniformly westward at high angles. The massive quartz lodes, within which scheelite occurs sporadically, occupy irregular fissures cutting obliquely across the schistosity of the metamorphic rocks. The schists also carry innumerable small, barren veins of quartz, usually less than one inch in width, which show a marked tendency to intersect the schistosity-planes parallel to the strike of the latter. The present steeply dipping attitude of the schists at Glenorchy and the main system of joints and quartz veins that traverse the rocks are the result of a general westward tilting movement subsequent and unrelated to the regional metamorphisin. Following A. M. Finlayson (1908), the writer believes that the scheelite lodes were formed from magmatic waters or gases ascending from a subjacent (unexposed) granitic batholith, the intrusion of which coincided with, and perhaps indirectly contributed to, regional metamorphism of the enclosing schists.* Scheelite-bearing lodes also occur at several localities (Macraes, Waipori) in eastern Otago, many miles distant from Glenorchy (Fig. 1). All these occurrences of scheelite, and the auriferous veins of Otago which reach their most spectacular levelopment in the eastern side of the Richardson Range (Park, 1908, pp. 74–94) presumably belong to a single period of mineralisation.

3. Hokonui orogeny (Late Jurassic?). The older rocks of New Zealand, including some of Upper Jurassic age, were almost everywhere involved, presumably in Late Jurassic times, in a powerful orogeny generally known in New Zealand geological literature as the Hokonui orogeny. Fifteen miles almost due west of the area mapped, a great sill of peridotite many miles in length was injected during the same period of tectonic activity (Hutton. op. cit., p. 10)—Fig. 1.

4. Peneplanation (Cretaceous). Widespread peneplanation followed the Hokonui orogeny throughout New Zealand, and had reached an advanced stage by middle Cretaceous times.

5. Marine transgression and sedimentation (Mid-Tertiary). In Central Otago, Upper Cretaceous beds including auriferous gravels derived from the gold-bearing lodes, lie unconformably upon the Cretaceous peneplane; but in the Lake Wakatipu district itself, the oldest members of this younger marine series as represented at Bob's Cove, some 16 miles south of Glenorchy, are probably Oligocene in age (Hutton, 1939b, p. 86).

6. Uplift, over-folding and thrust faulting (? Early Pliocene). The Bob's Cove beds, Oligocene and Lower Miocene, have been folded

[Footnote] * An alternative hypothesis of lateral secretion, deriving the scheelite from the schists themselves has been suggested by P. G. Morgan (1920), supported by P. Marshall (1918) for the comparable lodes in the Tuapeka (Waipori) district. J. Henderson (1935), although admitting the possibility of lateral secretion, favoured secondary enrichment of lean primary ores.

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deeply into the underlying schists (and thus preserved from subsequent erosion) by a tectonic movement which affected the whole of Otago, and indeed probably the greater part of the South Island, in the Miocene (Benson and Holloway, 1940, p. 3) or Mid-Pliocene (Wellman and Willett. 1942, p. 305) time. The tilting movement

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Fig 2.—Geological and structural map of the Glenorchy and Paradise (inset) districts. 1, chloritic schist (Chl3); the schists at Glenorchy form the western slopes of the Richardson mountain range: 2, Pleistocene gravels, sands and silts; 3. Recent ferrace-gravels and floodplain; 4, strike and dip of the schistosity planes; 5. outerop of the scheelite-bearing lodes; 6, height above sea-level. I. outcrop of Glenorchy lode; II. Paradise lode. Oriented specimens Nos. 7381 and 7382 were collected from positions X and Y respectively.

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responsible for the present steeply dipping attitude of the Glenorchy schists may well be co-eval with this period.

7. Late Tertiary Peneplanation. A second peneplane, still conspicuous in the strongly accordant summit levels of the great mountain ranges of western Otago, was cut in late Tertiary times across basement schists and Tertiary covering rocks alike (Benson, 1935), although much of the latter was thus removed by erosion.

8. Late Pliocene block-faulting. The present elevation of the mountain ranges of the South Island is largely due to block-faulting subsequent to cutting of the Late Tertiary peneplane. Local effects in the Glenorchy district were the elevation of the Richardson Range and development of the graben in which Lake Wakatipu is situated (Hutton, 1940, p. 10).

9. Pleistocene glaciation. Lake Wakatipu itself was the site of an immense glacier, and the mountain-sides to-day show conspicuous effects of glaciation to a height of at least 3,500 feet above the lake surface (Park, op. cit., pp. 25–44). The latter is 1,016 feet above sea level.

10. Post-glacial erosion, with incision of deep gorges into the glaciated slopes of the Richardson and other ranges in the country surrounding Lake Wakatipu.

Schists.

A. Petrography.

The Palaeozoic schists of Otago lie within the Chlorite Zone of regional metamorphism as defined by F. J. Turner (1933, pp. 237–256) for derivatives of greywacke, and belong therefore to P. Eskola's Greenschist Facies (1920). Four sub-zones of progressive chemical reconstitution—respectively designated by the symbols Chl1 to Chl [ unclear: ] —have been recognised by C. O. Hutton and F. J. Turner (1936) in the Chlorite Zone of Otago. The rocks of the Richardson Range lie for the most part in sub-zones Chl3 and Chl4 of Hutton and Turner's map, but within the Glenorchy area (Chl3) the grade of metamorphism rises perceptibly from west to east and is marked by increasingly pronounced lamination (“foliation” in the sense employed by A. Harker, 1932, p. 203) as alternating quartzo-feld-spathic and chloritic layers segregate parallel to the single well defined schistosity with increasing distinctness.

Petrographically the schists of the Richardson Range closely resemble those of similar metamorphic grade described by C. O. Hutton (1940, pp. 29–33) from adjacent areas of the Lake Wakatipu region. Quartzo-feldspathic types predominate, and for the most part consist of quartz, albite, epidote and chlorite, with or without actinolite. Some of the non-actinolitic schists contain white mica. Local departures from the dominant petrographic types are a quartzstilpnomelane-garnet schist, and a highly quartzose, non-chloritic schist with finely disseminated pyrrhotite. General absence of tourmaline in heavy mineral concentrates separated with bromoform from representative schists of the Glenorchy district is noteworthy in contrast with its wide distribution as an accessory constituent of most schists in other parts of Otago (cf. Hutton, op. cit.).

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B. Structural Petrology.

(i) Megascopic Fabric.

The principal elements in the megascopic fabric of the Glenorchy schists are a single steeply dipping schistosity (Fig. 2), prominent lineation [sometimes accompanied by a second and less pronounced linear structure inclined to the first at a high angle] (Fig. 3), and various sets of joints, many of which are filled with narrow veins of quartz. Each of these has been measured at 150 field stations, and the trend and pitch of lineation and the strike and dip of quartz veins have been verified by laboratory measurement of 104 accurately oriented hand-specimens from representative localities. Most of the schists are too fine-grained for petrofabric analysis; but preferred orientation of quartz and stilpnomelane was investigated fully in two specimens unusually rich in quartz and lacking mica.

Orientation of schistosity and lineation in the schists of Glenorchy is represented statistically in Fig. 4—a composite density-contoured equal-area projection of the lower hemisphere upon which each measured schistosity surface is first represented by a pole.* The density-maximum at S, representing a very strong concentration of poles about this point, brings out a very pronounced tendency for the plane of schistosity to strike N. 28° W. with a mean south-westward dip of 44°. A slight but definite elongation of maximum S., as compared with the projection of a small circle of 30° radius drawn about S. (broken ellipse in Fig. 4), shows that the dip is somewhat more variable than the strike of the schistosity. On the same diagram lineations are represented by poles distributed through the arcuate area K L M, in which two maxima may be recognised—a strong concentration at M corresponding to a trend of S 13–30° W. and pitch of 23–25° to the S.W., and a weaker concentration at K (trend N. 40° W., pitch 5° to the N.W.).

Several alternative explanations of these facts merit consideration:—

1. The present steeply dipping attitude of the schistosity could be referred to the period of metamorphism. On this hypothesis the trend of the principal lineation, which can be identified as the tectonic axis (b fabric axis) of metamorphic deformation, should coincide with the strike of the schistosity—the axis of folding synchronous with metamorphism. Actually the lineation in most schists dips fairly steeply and in the majority intersects the strike of the schistosity at an angle of about 60°. This explanation is therefore untenable.

2. Alternatively the present dip of the schistosity planes could be due to post-metamorphic folding or tilting:—

  • (a) The rock masses may have been tilted bodily with little or no internal penetrative movement, in which case the microfabric imprinted during metamorphism should likewise be tilted bodily; as will be shown below in the section headed Tectonic Synthesis, results of petrofabric analysis greatly favour this hypothesis.

[Footnote] * For method, see E. B. Knopf and E. Ingerson (1938, pp. 230–233).

[Footnote] † See E. Knopf and E. Ingerson (op. cit., pp. 216–218); E. Ingerson (1942, pp. 721–725).

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    Fig. 3.—Lineations visible in the Glenorchy and Paradise (inset) schists. Lines indicate direction, angles dip of the lineation in direction of the arrow.

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  • (b) Tilting or folding may, on the other hand, have been accompanied by penetrative movements (Knopf and Ingerson, op. cit., p. 33) affecting the individual component crystals of the rocks, and imprinting upon the latter a post-metamorphic fabric, whose principal symmetry axis (b) should then coincide with the strike of the schistosity. Such a movement could utilise pre-existing s-planes (schistosity) as re-activated slip-planes, and need not necessarily destroy completely the pre-existing metamorphic fabric. The presence of a recognisable second lineation, more or less parallel to the strike, in some schists, and of an accompanying slight crumpling of schistosity surfaces, would appear at first sight to support this last alternative, but analysis of the microfabric (see below) shows that these are comparatively minor features and that the microfabric as a whole has suffered little internal modification during the tilting movement.

The orientation of all conspicuous joints*, whether open or filled with quartz (veins), is shown in Fig. 5; quartz-filled joints alone are plotted in Fig. 6. In both figures there are two distinct maxima, corresponding respectively to planes (a) striking 326–345° and dipping (N.E.) at 25–45°, and (b) striking 265–270° and dipping (N) at 65°. The joints corresponding to the less distinct maximum (J Fig. 5) are nearly perpendicular to the mean trend of the lineation (cf. Figs. 4 M and 5 J) and perhaps are ac joints referable to the metamorphic fabric. The more conspicuous maximum, including the poles of most of the quartz-filled joints is unrelated to the lineation, but represents a set of joints perpendicular to the schistosity, intersecting the latter parallel to a common strike.

(ii) Preferred Orientation of Quartz and Stilpnomelane.

To test the relative validity of the two hypotheses, 2 (a) and 2 (b) above, the quartz and stilpnomelane fabrics of two suitable quartz-schists of simple composition (7381, 7382) , from localities 50 chains apart, were investigated by the usual universal-stage procedure with the following results:—

Specimen No. 7381.

Schistosity [= ab plane of fabric] strikes 326° and dips S.W. at 44°. Main lineation (= b) is parallel to the dip of the schistosity and coincides with the axis of slight micro-corrugation of some of the schistosity planes. Secondary lineation (= b1), inclined to b at 15°, is distinct on some individual surfaces of schistosity and perhaps represents a local departure from b in a slightly inhomogeneous fabric. The N.W. end of a, S.W. end of b and upper end of c axes are respectively designated as positive.

Fig. 7 is a contoured collective diagram showing the orientation of the optic axes of 400 grains of quartz measured in a section cut parallel to the ac plane of the fabric, the diagram so obtained being

[Footnote] * For method see E. Knopf and E. Ingerson (op. cit., pp. 230–233).

[Footnote] † Numbers refer to specimens in the collection of the Geology Department of the University of Otago, New Zealand.

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rotated into the ab plane. Homogeneity of fabric within the field of the microsection was established by plotting two partial diagrams, each for 200 grains, and comparing them with each other and with the collective diagram (Fig. 7). All were identical in pattern. A second set of 400 grains was measured in a section cut parallel to ab, and the resulting diagram (Fig. 8) was found to be substantially similar to Fig. 7, thus proving homogeneity of fabric within the field of the hand specimen as a whole. The possible significance of maxima in the quartz diagrams will be discussed in a later paragraph. Most important, however, is the presence in both diagrams of a strongly defined girdle, whose axis coincides neither with b nor b1 but occupies an intermediate position, thus confirming the previous suggestion that the discrepancy between b and b1 is due to slight inhomogeneity of the megascopic fabric, and not to imprint of two independent deformations. This inhomogeneity can most probably be correlated with minor oscillation of the direction of movement within individual slip-surfaces during the earlier stages of deformation.

Preferred orientation of stilpnomelane, occurring as sharply defined isolated flakes scattered through a mosaic of quartz grains in the same rock, is depicted in Figs. 10 and 11, constructed from measurement of the (001) cleavage of all accessible crystals in sections respectively parallel to ac and b1c. The single strong maximum at the pole of c in each diagram reflects a single s-plane—that of the schistosity (ab) itself. The apparent peripheral girdle patterns shown in both figures have no true counterpart in the fabric, but are due almost entirely to the tabular habit of the measured crystals and to the central blind spot of any orientation diagram for cleavage planes (Billings and Sharp, 1937, pp. 283–289; Turner and Hutton, 1941, pp. 231–233). However, the greater spread of poles from c towards the centre of the diagram in Fig. 11 than in Fig. 10 is certainly significant, and points to a definite though faint girdle pattern about an axis close to b, b1 or the axis of the quartz girdles. The quartz and stilpnomelane fabrics are therefore homoaxial or nearly so (Knopf and Ingerson, op. cit., p. 83).

Specimen No. 7382.

Schistosity (= ab plane of fabric) strikes 15° and dips W. at 25°.

Lineation (= b) trends N. 58° E. and pitches 18° S.W.; it intersects the strike of ab at 45°.

N.W. end of a, S.W. end of b and upper end of c axes are respectively designated as positive.

Preferred orientation of quartz (optic axes of 400 grains measured in a section parallel to bc) is shown in Fig. 9, which represents the bc orientation diagram rotated into the ab plane of the fabric.

(iii) Tectonic Synthesis.

The two petrographically similar quartz schists selected for petrofabric analysis were collected from localities 50 chains distant from each other. There is a difference of 49° in the strike of the schistosity at the two localities, and marked differences, too, in dip of schistosity and in trend and pitch of lineation. However, the

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patterns of the two orientation diagrams for quartz (Figs. 8 and 9) are strikingly similar if the fabric axes a, b, and c, not geographic co-ordinates, are taken as the datum for comparison. Furthermore, if the schistosity surfaces at the two localities are imagined each to be rotated about its strike into the horizontal plane, the trends of the two lineations (b) coincide to within a few degrees. This can only mean that large masses' of schist with initially subhorizontal schistosity have been bodily moved, probably by simple tilting rather than by folding, subsequently to metamorphism, without destruction or even perceptible modification of the internal fabrics of at least the quartzose members. In other words, the present steeply dipping altitude of schistosity and lineation is due entirely to postmetamorphic tectonic activity. Nor can it be argued that this tilting accompanied the final phase of complex metamorphism. Quartz is admittedly the most sensitive of common minerals to deformational influences, and the quartz fabric is generally assumed to express movement in the last stages of metamorphism. Since in the Glenorchy rocks the quartz fabrics have been tilted bodily without internal modification, the tilting must have been entirely post-metamorphic.

Petrofabric analysis in this way conclusively confirms the hypothesis stated under 2 (a) on P. 376.

While we are now justified in assuming that post-metamorphic tilting was not accompanied by notable modification of internal fabric in the rocks affected, the tilting movement seems, nevertheless, to have left a slight imprint upon the megascopic fabric, and presumably also upon the mica fabric in certain layers. A second lineation, trending more or less parallel to the strike of the schistosity, has been observed here and there on individual schistosity surfaces, and can now be interpreted as due to localised limited slip-movements on isolated s-surfaces during the tilting.

It has been stated above that post-metamorphic movement of the schists is regarded as one of tilting, not of folding. This view is supported by the uniformly westward dip of the schistosity; by the, at most, very limited extent of penetrative movement connected with this phase of tectonic activity; and finally by the steady increase in metamorphic grade across the strike from west to east, not only at Glenorchy but in the Lake Wakatipu district generally (Hutton, 1940, Map 2), the lowest beds now exposed being those of highest metamorphic grade. The mean strike of the schistosity planes, N. 28° W., gives the tectonic axis of the tilting movement. This latter may be provisionally correlated with Early Pliocene (?) movements involving folding and over-thrusting, the effects of which can be observed within 16 miles of Glenorchy in the Moonlight thrust-fault and the Bob's Cove Tertiary beds (Hutton, 1939b). It must be noted, however, that the earlier (Late Jurassic?) Hokonui orogeny has strongly folded and tilted Mesozoic rocks elsewhere in New Zealand, but the effect on the Glenorchy schist area is not clear. The most conspicuous set of joints, including nearly all quartz-filled joints, share a common strike with the schistosity of the rocks in which they occur. These are interpreted now as tension joints that opened parallel to the axis of tilting.

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(iv) Significance of Maxima in the Quartz Diagrams.

Although the possible genetic significance of individual maxima in the three quartz diagrams can be of no importance in the tectonic synthesis discussed in the preceding section, the patterns are so uniform and clear-cut as to merit brief comment in connection with the general, and as yet only partially solved problem of the mechanism of quartz orienting in metamorphism.

The maxima of Figs. 7, 8, and 9 coincide with none of the three principal fabric planes ab, bc, or ac, but occupy positions at first sight resembling those of maxima IV in B. Sander's synoptic diagram for quartz in S-tectonites (1930, D. 61). The resemblance is not accurate, however, for the maxima in the Glenorchy diagrams fall at 65° from a, and 38° from c as compared with 42° from a and 52° from c for B. Sander's maximum IV. Two possible explanations, neither regarded by the writer as satisfactory, suggest themselves:—

  • (1) Preferred orientation might conceivably be due to gliding in the ab fabric plane, upon the rhombohedron d (1012) as glide-plane, with the edge (d:d*) as glide-line, oriented parallel to a of the fabric. However, the writer hesitates to suggest a further addition to the number of crystallo-graphic glide-planes and glide-lines that have already been appealed to by various writers to explain observed orientation patterns in quartz.

  • (2) The quartz diagrams could also be interpreted as representing fabrics dominated by maximum IV, formed during the main late Palaeozoic metamorphism, but subsequently slightly modified by limited penetrative movement within the schistosity surfaces in a direction transverse to the original a axis of the main phase of deformation. This latter movement could conceivably be correlated with the general post-metamorphic tilting.

It is not proposed to discuss further the unsolved problem raised by the distribution of maxima in quartz diagrams of the Glenorchy schists.

Scheelite-Bearing Lodes.

Field Occurrence.

Massive quartz lodes, varying greatly in thickness (from several inches to 10 feet) and locally carrying irregularly disseminated scheelite and sulphides, occur at a number of localities in the Glenorchy and Paradise district (Fig. 2). A generalised diagram showing the relation of the scheelite-bearing lodes to the surrounding schists, is reproduced in Fig. 16. For the most part they occupy zones of shearing or faulting in the schist. In many places, especially where the outcrops have been exposed clearly by sluicing operations, the lodes have themselves been considerably affected by faulting, which in the Glenorchy mine has been observed to displace the scheelite orebodies within the enclosing quartz matrix.

[Footnote] * Following H. W. Fairburn (1941, p. 1270).

[Footnote] † For summary, see A. Heitenan (1938, pp. 107–111).

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The outcrops of the main lodes are shown in Fig. 2, and the poles of different parts of a number of lodes, measured at all available outcrops, are plotted in Fig. 12. From the latter it will be seen that the majority of the lodes strike in a northerly direction and dip to the north-east at high or low angles. Where the schists dip at comparatively low angles, at Paradise, the north-easterly dip of the scheelite-bearing lodes is steep, while at Glenorchy, where the schists are tilted steeply westward, the lodes dip gently to the northeast. This would suggest that the lodes originated as initially steeply dipping sheets and subsequently have been bodily tilted together with the enclosing schists into their present attitude.

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Fig 16.—Generalised diagram of a scheelite-bearing lode. S, scheelite; Q, quartz; CS. crushed schist; SCH, schist.

Although it is possible to trace individual outcrops of scheelite-bearing lodes for distances ranging up to a mile, the scheelite itself is most irregularly distributed. It occurs principally in small streaks and bunches a few inches in width, less commonly in more continuous seams and shoots separated by intervening masses of barren quartz. In some places quartz may be lacking, and the ore then occurs simply in bands of crushed schist.

Petrography.

The lode material consists principally of quartz, the other constituents—calcite, scheelite, pyrite, arsenopyrite and a minute amount of gold—together making up but a very small fraction of the whole. No feldspar was noted, a remarkable fact in view of the ubiquity of albite in quartzose joint fillings and segregation laminae in the surrounding schists; this is strong, indeed almost conclusive evidence against origin of the lode quartz by segregation (“lateral secretion”) from the enclosing schists. The quartz takes the form of irregular grains (less than 3 mm. in diameter in most sections), which locally have been reduced by crushing to aggregates of much smaller granules. Occasionally idiomorphism of quartz towards calcite or scheelite was noted. Undulose extinction is common but not universal. Scheelite occurs sporadically in pale brownish or greyish grains (1–4

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mm.), sometimes showing a trace of cleavage parallel to (111). Chemical analyses and fluorescence tests of the scheelite are given in Appendix I and II. Calcite takes the form of elongated tabulae (< 0μ5 mm.) or coarse, irregular grains, both of which usually show twin or glide-lamellae parallel to (0112). Pyrite is a common accessory mineral, especially in rocks containing seheelite. Pyrrhotite and arsenopyrite are less widely distributed, the latter being noted particularly in ore from the Paradise mine. In contrast with the gold-scheelite ores of Macraes, in Eastern Otago (Fig. 1), the Glenorchy scheelite ores are never sufficiently rich in gold to warrant being worked for that metal alone.

Preferred Orientation Phenomena.

Petrofabric analysis of a typical specimen of scheelite-bearing lode material (7383) from the Glenorehy lode gave the following results:—

The only directional structure visible in the hand-specimen is a pronounced lineation (corresponding in trend with that of the adjoining schist) in the crushed schist matrix which separates the lodequartz from the schist itself—a fact suggesting, though not proving, that the erush band now occupied by the lode came into existence during the closing stages of metamorphism. In the absence of schistosity and lineation within the lode quartz, three mutually perpendicular references axes, x, y (parallel to the strike of the lode) and z (parallel to the dip and also to the lineation shown in the crushed schist matrix) were selected to facilitate description of the fabric.

Fig. 13 shows the poles of 200 quartz axes measured in a single microsection, and reveals a complete lack of preferred orientation in the measured crystals. This indicates crystallisation of the lode quartz after deformation of the surrounding rocks (the main metamorphism) had almost ceased. Other sections of lode quartz, however, show minor cataclastic effects—granulation, undulose-extinction and local fracturing.

The scheelite fabric was investigated by measuring the optic axes of 100 grains (using a special method described in Appendix III), in each of three mutually perpendicular sections cut from the same specimen (7383)—Figs. 14, 15 and 17. Distinct concentrations of axial points are clearly recognisable in the central portion of each of these diagrams; these are mutually unrelated, however, and either could represent strong local preferred orientation of the crystal lattice in a highly inhomogeneous fabric (a most unlikely possibility) or else are to be attributed entirely to the influence of strongly marked crystallographic habit of scheelite grains oriented at random. Re-examination of the sections confirms this latter explanation; for in each section all grains with notably elongated outlines prove to be elongated approximately parallel to the optic axis Z. It would appear therefore, that preferred orientation (both according to space-lattice and to grain form) is lacking in the scheelite. but that the crystals have a marked tendency to be elongated parallel to Z, so that any microsection tends to include a high proportion of crystals cut nearly perpendicular to Z.

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Calcite takes the form of tabular crystals flattened transversely to the optic axis. The optic axis in each grain was located by the standard universal-stage procedure of alternate tilting and rotation into extinction, confirmed wherever possible by measuring and plotting visible cleavage planes (1011) and twin- or glide-lamellae (0112). A marked preferred orientation with the optic axes parallel to x (Fig. 18) is interpreted as due to growth of tabular crystals (flattened normal to 0001) with their greatest dimensions parallel to the plane of the vein wall (yz). But this is probably artificially accentuated in the orientation diagram (Fig. 18) by the influence of crystal habit; for crystals flattened transversely to a given microsection have a greater chance of being included in that section than have those which are flattened in the plane of the section itself (cf. Billings and Sharp, op. cit.).

Sequence of Mineralisation.

Possible criteria for determining a sequence of crystallisation for minerals in veins have been discussed at length by E. S. Bastin (1931) and C. S. Ross (1935, pp. 8–16). In the present study only two of these appeared to be generally applicable—viz.,

  • (a) the presence of veinlets of a later mineral cutting crystals of an earlier mineral, and

  • (b) enclosure of angular fragments of earlier in later minerals.

(i) Quartz-Scheelite Relationships.

The great abundance of quartz as compared with scheelite in most of the lodes strongly favours simultaneous crystallisation of the two minerals or else replacement of quartz by scheelite.

In a number of sections, quartz was found to be euhedral against scheelite (Figs. 19 and 20). This condition could result either from crystallisation of scheelite in drusy cavities around projecting crystals of earlier quartz, or alternatively from simultaneous crystallisation of the two minerals. In the latter case the euhedral form of the quartz might be correlated either with its superior hardness (Ross, op. cit., p. 12) or else, according to H. W. Fairbairn's recent analysis of the influence of ionic packing (1943, pp. 1341–1342), with its slightly higher “bonding index.” It is also conceivable, however, that the scheelite is actually in process of replacement by enclosed euhedral quartz grains.

There are several instances where quartz has continued to crystallise after scheelite, notably where individual crystals of scheelite are traversed by micro-veinlets of late quartz (Fig. 21). Lack of correspondence between opposite sides of some such veinlets (Fig. 21) suggests fracturing of scheelite [along the (III) cleavages] followed by replacement of scheelite by quartz, rather than simple infilling of fractures. Other illustrations of the textural relation of quartz to scheelite in the Glenorchy lodes are reproduced in Figs. 23 and 25. The angular scheelite grains enclosed in quartz in Fig. 25 suggest that here scheelite is the earlier of the two; in Fig. 26 the times relationship is ambiguous.

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It is concluded then, that in the Glenorchy lodes scheelite and quartz are products of simultaneous crystallisation, or else scheelite replaces earlier-formed quartz. In a few cases quartz has continued to crystallise after scheelite.

(ii) Calcite-Scheelite Relationships.

Where calcite occurs in tabular crystals (Figs. 26 and 27) it clearly is later than the scheelite with which it is associated. On the other hand, an equally abundant irregularly granular type of calcite with strongly marked effects of deformation (such as curvature of twin lamellae, Fig. 22) has seldom been observed in contact with scheelite, and its time relationship to the latter is unknown. One instance was noted where coarse granular undeformed calcite seems to be of later origin than enclosed angular scheelite (Fig. 25). A. M. Finlayson (1908, p. 117, Pl. 16c) has recorded replacement of corroded calcite by clear sharply bounded crystals of scheelite in the ores of Macraes, Eastern Otago, but nothing comparable to this has been observed in the Glenorchy ores.

(iii) Calcite-Quartz Relationships.

The euhedral nature of the quartz to the calcite shown in Fig. 19 has several possible explanations, including replacement of calcite by quartz. However, taking into account the great predominance of quartz over calcite in the lodes as a whole, it is probable either that the two minerals have for the most part crystallised simultaneously, or else that quartz has largely preceded calcite as indicated in Fig. 27.

(iv) Pyrite Relationships.

In hand specimens veinlets of pyrite are seen to cut both quartz and scheelite, while in microsections pyrite is clearly later than most of the calcite as well (cf. Fig. 19). An apparent exception is shown in Fig. 24, where calcite has crystallised adjacent to, and seemingly moulded upon, a grain of pyrite.

(v) Conclusion.

Investigation of the structural relationships between quartz, calcite, scheelite and pyrite in the Glenorchy ores fails to establish any definite and orderly sequence of crystallisation of these minerals. Perhaps the three principal constituents crystallised for the most part simultaneously; or possibly extensive crystallisation of quartz was followed by later development of calcite and scheelite. Local instances where quartz has crystallised after both scheelite and calcite, and others again where calcite is clearly later than scheelite are also known. Certainly pyrite is always one of the last minerals to crystallise.

Origin of The Scheelite.

Three hypotheses have been put forward to explain the origin of the Glenorchy scheelite ores:—

  • (a) A mechanism of lateral secretion favoured by P. G. Morgan (1920) and supported by P. G. Marshall (1918, p. 47) for the comparable lodes at Waipori.

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  • (b) Secondary enrichment of lean primary ores advocated by J. Henderson (1935, p. 26) who, however, admitted the possibility of hypothesis (a).

  • (c) Magmatic derivation, first suggested by A. M. Finlayson (1908) and applied by J. H. Williamson (1939, pp. 73–75, 105) to the deposits at Macraes.

The relative merits of these three alternative hypotheses are discussed below:—

Lateral Secretion and Secondary Enrichment Hypotheses.

These two hypotheses have much in common. Both postulate primary dissemination of tungsten, either in minute traces throughout the country rock (lateral secretion) or as a lean ore localised along the sites of the present lodes (secondary enrichment). In both cases, concentration of the scheelite in its present form is attributed to subsequent solution and redeposition by meteoric waters. In support of this contention it has been pointed out that the payable ore-bodies appear to thin out and ultimately to vanish with increase in depth below the present land surface—a fact suggesting that some relationship exists between the distribtion of ore shoots and the present ground-water level. Actually this interpretation of the facts is unwarranted. Most workings have been started at outcrops where scheelite was visible on the surface, and since the ore-bodies are known to be discontinuous and lensoid in form, it follows that in any working of this type the scheelite-ore must necessarily diminish and vanish with increasing distance from the surface outcrop. Furthermore, it is inconceivable that there can be any actual direct relationship between the distribution of scheelite within the lodes and distance below the present surface; for the existing topography is the result of Pleistocene and post-glacial erosion, and it is scarcely likely that the scheelite-bearing lodes can have come into existence during the short interval of time thus available. Moreover if, as appears almost certain, the scheelite and gold were introduced into the metalliferous lodes of Otago in a single metallogenetic epoch, the occurrence of alluvial gold in great abundance in the older gravels (Late Cretaceous and Eocene) that mantle the Lower Cretaceous peneplane throughout Otago, is clear proof that the Glenorchy scheelite lodes were already in existence by the close of the Cretaceous (Williamson, op. cit., p. 107).

There still remains the possibility that the scheelite has been concentrated by lateral secretion or secondary enrichment during the long erosion interval corresponding to the carving of the Lower Cretaceous peneplane. There is, however, no indication of any relationship between distribution of scheelite, and depth below the ancient peneplane surface.

Finally, it must be pointed out that no evidence has ever been brought forward that tungsten is disseminated, even in minute quantities, in the schists of Glenorchy. Nor has scheelite or any other tungsten mineral been found in any of the innumerable small quartzalbite veins which everywhere traverse the schists, and which are

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believed to have developed by a process akin to “lateral secretion” during post metamorphic movement (vide., p. 380).

The writer, therefore, believes that the hypotheses of lateral secretion and secondary cnrichment are totally inadequate to account for the Glenorchy scheelite ores, and are unsupported by positive evidence of any kind.

Hypothesis of Magmatic Derivation.

It has already been shown that the scheelite ores were in existence at least as early as the Cretaceous, and there is good reason to believe that the lodes, as well as the enclosing schists, were affected by post-metamorphic tilting (Early Pliocene?). On these grounds alone, the most likely period to which mineralisation can be assigned is that immediately following, and genetically connected with, the Late Palaeozoic metamorphism.

In 1908, A. M. Finlayson (op. cit.) suggested that regional metamorphism in Otago was accompanied by intrusion of a subjacent granitic batholith, from which emanated magmatic solutions which ultimately gave rise to the metalliferous (including scheelite-bearing) veins that are so widely distributed throughout the Otago schists. This hypothetical batholith nowhere reaches the surface in the Otago schist area, but granitic intrusions approximately co-eval with the period of regional metamorphism occur extensively in Fiordland and South Westland at distances of 14 to 30 miles from Glenorchy. And there is also petrographic evidence—notably widespread dissemination of tourmaline and local occurrences of piedmontite in the schists of Otago—strongly indicating the influence of granitic emanations in connection with regional metamorphism of the Otago schists (Hutton, 1939a). Here, then, is a possible, though not conclusively proved, source of tungsten for the Glenorchy scheelite lodes. This view is somewhat strengthened by the known occurrence of wolfram (Williams, 1934) in connection with the granites of Stewart Island (Fig. 1), which belong to the same intrusive series as the granites of Fiordland and Westland. On the other hand, it must be admitted that the noticeable rarity of tourmaline in schists of the Glenorchy district, and complete absence of this mineral in the lodes themselves, shows that there is no direct connection between introduction of tourmaline and development of scheelite in this part of Otago.

Apart from the problem relating to the possible sources of tungsten, there is also the problem of the origin of the great mass of quartz which makes up the Glenorchy lodes. Complete absence of albite—a mineral universally present in the swarms of segregation veinlets and laminae that characteristically seam the schists of Otago—is strong, indeed almost conclusive evidence against the origin of the lode quartz by segregation (“lateral secretion”) from the enclosing schists. Lack of obvious carbonatisation or other extensive metasomatism in the country-rocks adjacent to the lodes precludes the possibility that the quartz of the latter has been derived from the country rock by complimentary metasomatic exchange of CO2 for SiO2 as was proved by A. Knopf (1929, p. 45) in the case of the

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Mother Lode of California. There remains, then, the possibility—now strengthened to probability—that the quartz has been introduced from some external source—i.e., by the activity of post-magmatic siliceous solutions rising from a concealed granitic batholith.

Discussing the origin of tungsten-ores in general, R. H. Rastall (1918, p. 296) concludes that they are usually of magmatic origin. “Scheelite is found as a lode mineral, either with or without wolfram. A regular gradation can be traced from the tin-wolfram deposits, through the wolfram deposits without tin to the scheelite lodes. The genetic connection between scheelite and siliceous gold veins is also of significance.” A parallel seems to be offered by the occurrence of tungsten in Otago and Stewart Island; cassiterite and wolfram occur at a granite-schist contact in Stewart Island (Williams, op. cit.); a single occurrence of wolfram has been recorded by P. Marshall (1918, p. 40) in eastern Otago, while scheelite associated with gold is widely distributed in the Waipori district and Macraes (eastern Otago) and at Glenorchy (Fig. 1). The writer regards all these scheelite-bearing lodes as mesothermal deposits, genetically connected with intrusion of subjacent masses of granite (comparable with the exposed intrusions of Fiordland and Stewart Island) in the latest stages of the (? Late Palaeozoic) regional metamorphism responsible for the present condition of the schists of Otago. The relatively undeformed condition of the lode material as indicated by absence of recognisable tectonite patterns of preferred orientation in the constituent scheelite, calcite and quartz, is not incompatible with this hypothesis for it has been shown by F. J. Turner (1933, pp. 251–253) that the granites of Westland were emplaced in the later stages of, and in some cases immediately following, the main deformational phase of regional metamorphism in that province.

Minor Quartz Veins.

Throughout the Glenorchy district, as in many other parts of Otago (Hutton, 1940, p. 26), narrow veinlets of quartz, usually less than 5 mm. in width, and sometimes offset a few inches by minor faulting, cut the schists in vast numbers. They show a marked tendency to share a common strike with the steeply dipping schistosity planes which they consistently intersect at high angles, and in this respect conform to the trend of the most regular system of joints observed in the schists; similar observations have been noted for the quartz veinlets of the Lake Wakatipu region generally (Hutton, loc. cit.)

The principal constituents are quartz and albite, the former predominating, but the latter seldom insignificant. Many of the coarser grains of quartz (2 mm. or less in diameter) show undulose extinction, and here and there slight granulation; but otherwise the vein material is largely undeformed. Some veins contain pumpellyite (cf. Hutton, loc. cit., p. 27), which is invariably present in heavy mineral concentrates separated from crushed vein fillings. Pyrrhotite deposited along micro-fractures cutting across the vein material is a common additional accessory. Scheelite, although carefully searched for, was never found in heavy mineral concentrates.

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Much of the quartz is dimensionally elongated perpendicular to the vein walls (Figs. 28 and 29), but it is uncertain whether this feature, which is not shown by the lode quartz, is a result of infilling of open fractures, or of replacement of wall-rock adjacent to initial capillary joints. At least some replacement is suggested by the abundance of micro-inclusions of schist within the vein material (Fig. 28) and by the regular dimensional orientation of grains near the walls as contrasted with those near the centre of the vein.

From the general mineralogical similarity between quartzo-segregation bands formed by metamorphic differentiation in Otago schists (Turner, 1941), and the quartz veinlets just described, it is most probable that the vein filling has been derived largely if not wholly from the schists in which the veinlets occur (cf. Hutton, 1940, p. 26). The quartz-veinlets are considered to have developed along tension joints formed during a post-metamorphic tilting of Early Pliocene (?) time (vide, p. 380). It should be noted, however, that an earlier age may be indicated by the presence in the Glenorchy veinlets of a minute amount of pyrrhotite; this possibly has the same significance as pyrrhotite and chalcopyrite veins in the Lake Wakatipu district which have been genetically connected with injection of peridotites during the Late Jurassic (?) Hokonui orogeny (Hutton, 1942).

Conclusions.

(1) The schists of the Glenorchy district, like other low-grade schists of Otago Province, are products of regional metamorphism for which a Late Palaeozoic date is the most probable.

(2) Petrofabric investigation has shown that the steeply dipping attitude of the schists of Glenorchy is due to post-metamorphic tilting which has scarcely modified their internal fabric, and which is provisionally correlated with Early Pliocene (?) Tectonic movements. This fact has enabled a distinction to be made between scheelite-bearing quartz lodes and barren quartzo-feldspathic veins. The veins, which intersect the schistosity planes at high angles, are considered to result from segregation in tension joints formed parallel to the axis of the tilting movement. On the other hand there is good reason to believe that the lodes as well as the schists were affected by the postmetamorphic tilting. Further, the quartzo-feldspathic veins show strong dimensional orientation of individual grains, a feature not seen in the lodes.

(3) Massive quartz lodes containing irregularly disseminated scheelite, calcite, arsenopyrite, pyrite and traces of gold, occupy extensive crush bands in the schist. They are believed to be due to the mineralising action of magmatic waters ascending from a subjacent batholith of granite (unexposed), intrusion of which occurred during the final stages of the Late Palaeozoic metamorphism after metamorphic deformation had died away. No orderly and definite sequence of crystallisation of the lode minerals could be established.

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Appendices.

(1) Scheelite Analyses: Chemical analyses of scheelite from Glenorchy and the Mid-Wakatipu area were carried out by Dr. C. O. Hutton, with the following results:—

1. 2. 3.
SiO2 20.59 9.31 0.64
CaO 14.68 21.30 19.21
WO1 64.73 65.00 79.80
MoO4 0.05 0.08 trace
SuO2 nt.fd. nt.fd. trace
MgO 0.11 0.17 0.18
MnO 0.10 0.14 0.20
Fe2O3 0.13 0.12 0.09
CO2 4.20 nt.fd.
100.39 100.32 100.12
  • (1) Scheelite boulder, Twelve Mile Creek, Mid-Wakatipu S.D. Specific gravity 5.47.

  • (2) Scheelite boulder, Nine Mile Creek, Mid-Wakatipu S.D. Specific gravity 5.55.

  • (3) Scheelite, Glenorehy Mine. Glenorchy S.D. Specific gravity 5.98.

Qualitative chemical tests of the Glenorchy scheelite based on the spot test of F. Feigl (1937) had previously been undertaken by the writer. The very sensitive and specific colour reaction for molybdenum with potassium xanthogenate (Feigl, loc. cit., p. 320) yielded a negative result, as did the benzidene reaction for Cu (Feigl, p. 42). A slight trace of Mn was detected using the periodate and tetrabase test (Feigl, p. 103) while the titan yellow reaction (Feigl, p. 137) revealed the presence of a trace of Mg.

(II) Fluorescence: The recent investigation of R. Greenwood (1943) showed, on the basis of 54 spectrographic analyses of scheelite samples, that colour variation could be related to quantitative variation in certain impurities. The Mo content of scheelite examined by R. Greenwood was found to increase from a trace in the specimens fluorescing blue, to a maximum in material with a yellow fluorescence, Mn acting in a somewhat similar manner, while other elements showed no systematic variation. Samples from Glenorchy showed a constant whitish-blue fluorescence, and this would indicate that Mo and Mn are either absent or present in very slight amount, a result in agreement with the chemical analysis. No difference in fluorescence was observed between the two varieties of scheelite respectively coloured white and yellow in hand specimens.

(III) Measurement of Orientation in Scheelite. The usual method of measuring the orientation of an optically positive uniaxial mineral—e.g., quartz, by means of a universal stage, is to rotate the section on the inner circle of the stage about the vertical axis A1, until the slow direction Z' coincides with the E—W axis (A4) and then to tilt on the N—S axis (A2) until the crystal remains in extinction for any angle of tilt upon the E—W axis (A4). The optic axis (Z) of the crystal then coincides either with the E—W axis of the stage or with the vertical axis of the microscope

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Fig. 4.—A composite diagram (1) at S. 150 poles of schistosity planes. Contours 20, 16.5. 13, 10, 6.5, 3, 0.5 poles per 1% area; maximum concentration 24%; broken line represents a 30 [ unclear: ] circle drawn with the maximum concentration as centre. (ii) Distributed through the arcuate area K L M are 104 poles of Iineations. Contours 10, 8, 6, 4, 2, 1%; maximum concentration 11%.
Fig. 5.—150 poles of joints, both open or quartz-filled (reins). Contours 8, 6, 4, 2, 0.6%; maximum concentration 11%.
Fig. 6.—54 poles of quartz-filled joints (veins). Contours 18, 14, 11, 7, 4, 2%; maximum concentration 24%.
Fig. 7.—No. 7381, quartz, 400 grains measured in the ac section, the diagram so obtained being rotated into the ab position. Contours 4, 3, 2, 1, 0.25%; maximum concentration 5.25%.
Fig. 8.—No. 7381, quartz, 400 grains in ab section. Contours 4, 3, 2, 1, 0.25%; maximum concentration 7.25%.
Fig. 9.—No. 7382, quartz, 400 grains in ab section. Contours 4, 3, 2, 1. 0.25%; maximum concentration 5.5 %.

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Fig. 10.—No. 7381, Stilpnomelane, poles of (001) In 100 flakes in ac section.
Fig. 11.—No. 7381, Stilpnomelane, poles of (001) in 100 flakes in b' c section.
Fig. 12.—Poles of different parts of a number of scheclite-bearing lodes; triangles represent measurements of the Paradise lode, circles Glenorchy lodes.
Fig. 13.—No. 7383, quartz, optic axes of 200 grainy in xz section.
Fig. 14.—No. 7383, scheelite, optic axes of 100 grains in a y section. Confours 8, 6, 4, 1, 1%; maximum concentration 10%.
Fig. 15.—No. 7383, scheelite, optic axes of 100 grains in xz section. Contours 8, 6, 4, 2, 1%; maximum concentration 19%.

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Fig. 17.—No. 7383, scheelite optie axes of 100 grains in yz section. Contours 8, 6, 4, 2, 1%; maximum concentration 9%
Fig. 18—No. 7383. calcrte, optie axes of 39 flakes in [ unclear: ] z section.
Fig. 19.—No. 7650, scheelite (coarse stippling) and quartz (clear). calcite (line suppling) and pyrite (black). X 16
Fig. 20—No. 7383, scheelite (coarse stippling) and quaitz clear). X 29
Fig. 21—No. 7651, quartz (clear), scheclite (coarsc stippling) and calcite (line stippling). X 16.
Fig. 22—No. 7383, scheelite (coarse stippling), quartz (clear), and calcite (fine stipplint). X 29.

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Fig. 23—No. 7635. scheelite (coarse stippling) and quartz (clear). X 16.
Fig. 24.—No. 7283. pyrite (black). calcite (fine stippling), and quartz (clear). X 16.
Fig. 25.—No. 7654. Scheclite (coarse stippling), calcite (fine stippling) and quartz (clear). X 16.
Fig. 26—No. 7652. calcite (fine stippling), scheelite (coarse stippling) and quartz (clear) X 16.
Fig. 27.—No. 7652. calcite (fine stippling). quartz (clear), and scheelite (course stippling). X 16.
Fig. 28.—No. 7667, quartz-albite vein (clear) in schist. × 8.
Fig. 29.—No. 7668, quartz-albite vein (clear) in schist. × 8.

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Plane of Section after Tilting

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(A3). As a result of the great difference in refractive index between scheelite (1μ918–1μ934) and the glass of the covering hemispheres (1μ648), it is impossible to determine precisely the position of extinction during tilting, even for moderate angles of tilt, and the standard method mentioned above was therefore discarded. The following method proposed by Dr. F. J. Turner, and based upon a procedure described by S. Tsuboi (1937) for quartz, was found to be more satisfactory and was finally adopted:—

  • (1) All rotation-axes of the stage (Fig. 30) are first set at the zero position. The corresponding initial position of the optic axis (Z) of a crystal of scheelite of random orientation is shown in Fig. 31.

  • (2) The section is now rotated, to extinction about the vertical axis (A1) of the inner circle, so as to bring Z', the trace of the optic axis, into coincidence with the N—S axis (A2) of the stage (Fig. 32) and the reading of the inner circle (say 310°) is noted.

  • (3) The right-hand side of the stage is now tilted upward on A2, through a standard angle of 32° [the corrected angle—making allowance for respective refractive indices of scheelite and hemisphere-glass—equivalent to a true tilt of 30° for rays passing through the scheelite grain (Nikitin, 1936, Pl. 1)]—cf. Fig. 33. This brings the section out of extinction, except in the particular case where Z is in the plane of the section.

  • (4) The whole stage is next rotated about the main vertical axis of the microscope (A5), until a new trace of Z upon the plane normal to A5, is brought parallel to A2, when the section again goes into extinction. The angle of rotation (say 27°, clockwise) is noted (Fig. 34).

  • (5) From the above readings the position of Z can now be plotted upon an equal-area or a stereographic projection, as follows:—

    • (a) Set the index arrow at 310°, and mark off the great circle (broken line in Fig. 35) representing the original vertical plane Z′ZZ′ of Figs. 31–32 as tilted throtigh 30° (Z″ZZ″ Figs. 33–34).*

    • (b) Rotate the index arrow clockwise through 27°. The projection of Z upon the plane normal to the main microscope axis (A5′) with the section tilted 30° about A2, is given by P, the intersection of the broken are with the N—S diameter of the projection (Fig. 36).

    • (c) Rotate the index arrow back to the initial reading 310°. Now rotate P 30° to the left, along the appropriate small circle of the projection net, to give Z, the pole of the measured optic axis as projected upon the plane of the rock section (Fig. 37).

[Footnote] * With a little experience, it becomes unnecessary to plot more than a small are of this circle.

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Acknowledgments.

This paper was prepared in the University of Otago, New Zealand, under the direction of Drs. W. N. Benson and F. J. Turner, for whose assistance the writer expresses his gratitude. Dr. F. J. Turner also edited the manuscript of the paper. Part of the cost of cutting the oriented sections was defrayed by a Hutton Memorial grant of the Royal Society of New Zealand through Dr. F. J. Turner. Acknowledgment is made for the permission of Mr. R. W. Willett in allowing the use of part of an unpublished map; also for the courtesy extended by the Department of Mines during the writer's stay at Glenorchy.

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