Art. IV.—Some Observations on the Schists of Central Otago.
[Read before the Otago Institute, 8th October, 1907.]
1. Denudation Forms.
Many of the Central Otago ranges are capped by vast assemblages of rock hummocks or buttes. These are well displayed on the Dunstan, Old Man, Carrick, and Rough Ridge Ranges, also at Barewood and Macrae's. As we approach the coast these hummocks become more numerous and smaller, till they finally disappear. They are best seen near mature river development, while sufficient erosion removes them altogether. They are thus not enduring features of the landscape, but are brought into existence, and again destroyed, by erosive activity.
These peculiar forms have been remarked by several observers, notably the late Captain Hutton* and Mr. T. A. Rickard†; but the only one who discusses their nature is Rickard, who studied them at Barewood. As he observes, they are generally composed of more siliceous and resistant portions of the rock. Basins and cavities are frequently developed near their base,
[Footnote] * F. W. Hutton, “Geology of Otago” (Dunedin, 1875), p. 91.
[Footnote] † T. A. Rickard, “Goldfields of Otago,” Trans. Am. Inst. Min. Eng. vol. xxi, p. 411.
and these are seen to face generally to the north—i.e., to the midday sun. The formation of these cavities is, therefore, probably due to changes of temperature, and to freezing, and consequent disintegration.
No writer, however, has sufficiently emphasized the importance of joints in the production of these hummocks, for to this factor their formation is chiefly due. This is evident from a study of them in any one locality, where they are seen to be roughly square in plan, with their corresponding sides parallel.
Castle Rock, on the Dunstan Range, in the form of two large turrets, shows very conspicuously the effect of the jointing (fig 2).
The buttes are seen to best advantage in districts of horizontal strata, and this is why they are so conspicuous along the summits of many of the ranges, where the bedding-planes of the schist are generally horizontal. Where the dip increases they become irregular; and with a nearly vertical dip, as on the fault-line at the south end of the Pisa Range, they appear as nearly upright minarets or “bayonet peaks” (fig 3).
It thus appears that the amount of dip is the chief cause of their varying form, the jointing of the rocks the cause of their existence.
The combined effects of spheroidal weathering and of spliting along joint-planes have been the cause of the numerous resemblances in the rock-hummocks to human forms, such as the Monk on the Carrick Range, and the Celebrities on the Skipper's Road.
2. Fracture Cleavage.
This phenomenon, hitherto undescribed, is well displayed in the lower schists at Alexandra and on the Dunstan Range. Here we find a series of cross-fractures filled with quartz, and inclined to the foliation or flow-cleavage planes at an angle of 45° (Plates XI and XII). The veins thus formed are widely spaced and discontinuous.
This is a typical example of fracture cleavage developed by shearing in the zone of rock-fracture. Its mode of origin has been pointed out by C. R. van Hise, who says, “In the zone of rock-fracture, where the differential stress surpasses the ultimate strength of the rock, there may be produced a fissility in two sets of intersecting planes equally inclined to the greatest pressure.”* In Otago one set is generally emphasized to the exclusion of the other. The name “fracture cleavage” is due to C. K. Leith, who has discussed its nature at length in his monograph on “Rock-cleavage.”†
In some of the upper members of the schists, shearing-planes occur frequently along the foliation-planes, and there result slip-bands marked by a line of crushed and broken rock. These are well seen in some of the railway-cuttings in the Taieri Gorge.
It thus appears that the effect of shearing-stress differs according to the depth of the rocks affected, since fracture cleavage in the lower schists gives place to slip-bands along the foliation-planes in the upper schists.
3. The Chlorite-Schists.
Distinctive types of chlorite-schist occur generally near the base of the mica-schists, notably the coarse chlorite-schist on the Dunstan and Pisa Ranges, and the granular chlorite-schist in younger beds at Cowcliff Hill, near Gibbston. These have all the characteristics of metamorphosed igneous rocks, an origin which was suggested by Hutton for some bands of chloriteschist near Queenstown.‡
The two types referred to are interbedded with the micaschists, in bands varying from 50 ft. to 300 ft. thick. They are frequently underlain by thin distinctive bands of micaceous quartz-schist, which may represent altered contact rock.
[Footnote] * C. R. van Hise, “Principles of North American Pre-Cambrian Geology,” U.S. Geol. Survey, 16th Annual Report, part i, p. 643.
[Footnote] † C. K. Leith, “Rock-cleavage,” Bulletin No. 239, U.S. Geol. Survey (1905), p. 119.
[Footnote] ‡ F. W. Hutton, “The Foliated Rocks of Otago,” Trans. N.Z. Inst. vol. xxiv (1891), p. 360.
1. Chlorite-schist, Dunstan Ra.; Anal., J. S. Maclaurin (Bull. No. 1, N.Z. Geol. Surv., 1906, p.42).
2. Chlorite-schist, Gibbston; Anal., A. M. Finlayson.
3. Chlorite-schist, Klippe, Sweden; quoted by Roth, Gesteinsanalejsen, 1884, p. 8).
4. Epidote-schist from diabase, South Mountain, Pa.; C. H. Henderson, Trans. Amer. Inst. Min. Eng., xii, p. 82.
5. Diabase, Point Bonita, Calif.; F. L. Ransome, Bull. Geol. Dept. Univ. Calif., i, 106.
The features of the two Otago types are the low silica percentage, and the high proportions of lime, magnesia, and notably titanium. These figures indicate a basic igneous rock. Analyses 3 and 4, of chlorite-schist and epidote-schist respectively, show analogous features. No. 5, of a typical diabase, is inserted for comparison, and shows close correspondence in respect of the main constituents.
Under the microscope, type No. 1 (Plate XII, 2) shows a mass of chlorite fibres and scales imbedded in elongated granules of quartz, the structure being perfectly schistose. Rutile is abundant in elongated crystals; plagioclase and magnetite are accessories, though the last is frequently very coarse and strikingly developed in large and thickly clustered octahedra. Calcite and epidote are very abundant, and the rock is sometimes so highly epidotised as to constitute an epidote-schist. Some specimens carry pyrite in large flattened cubes. The altera-
tion of the rock is too intense to determine whether this constituent is original, but it was at least introduced prior to the dynamic metamorphism of the schist, the large size of the individuals being the result of recystallization during metamorphism.
Type No. 2 (Plate XII, 3a) is less schistose, and preserves more of its original structure. It is composed of a mass of labradorite and quartz crystals thickly grouped, the interspaces being occupied by fibres of chlorite, a good deal of calcite and epidote, with rutile plentiful and magnetite accessory. The feldspars are roughly rectangular, and simple or once twinned. The absence of polysnthetic twinning indicates secondary recrystallization. Both feldspars and quartz are crowded with crystals of epidote having a marked centric arrangement. The rock is practically a feldspar-schist.
The specific gravity of these rocks varies from 2.9 to 3.2.
Judging from the above lines of evidence, there is no doubt that the schists described are altered flows or sheets of basic igneous rocks, contemporaneous with the associated mica-schists of sedimentary origin.
4. Igneous Intrusions.
The So-called Porphyrites of the Carrick Range.
Both Hutton and Ulrich refer, in their “Geology and Goldfields of Otago,” to dykes of porphyrite, or hornstone-porphyry, on the Carrick Range, in the vicinity of the Carrick reefs.* Careful examination failed to locate these dykes, and I can only conclude that both these authorities have been misled into calling dykes some outcrops of dark iron-stained gossan near old Carricktown. These have frequently a brecciated structure, and the resulting appearance resembles a porphyritic rock with phenocrysts of quartz. The outcrops are, however, simply the barren caps of lodes.
Two magnesian dykes occur in Central Otago which were unknown to Hutton, and which have not been hitherto described.
This occurs across the Kawarau River from Gibbston, about half a mile up the left branch of the Springburn, a tributary of the Gentle Annie. The schist in the neighbourhood of the intrusion has been highly crushed and disturbed. The dyke
[Footnote] * Hutton and Ulrich, “Geology of Otago” (Dunedin, 1875), pp. 31 and 157.
is composed of altered olivine rock, and sections show the characteristic mesh structure of serpentine derived from olivine-Where least altered, it is a black serpentine rock, showing occasional good cleavage-surfaces of hypersthene. The serpentine in more-altered portions graduates into talc, and the rock is traversed by veins of calcite and chrysotile asbestos.
The surrounding rock is a fine-grained mica-schist with a band of fine chlorite-schist. For a distance of 6 ft. from the contact the mica-schist has been altered into a highly quartzose schist, with a striking development of biotite blades arranged across the foliation-planes. The chlorite-schist shows, as a result of the intrusion, numerous actinolite needles. This development of the magnesia minerals, biotite and actionolite, is a characteristic contact-effect of magnesian intrusions.
Moke Creek Dyke.
This occurs on the right bank of Bushy Creek, 300 yards above its junction with Moke Creek, between Kilpatrick and Moke Lakes. It lies on approximately the same line of strike as the Moke Creek copper lode, and is, like the other, a serpentinised olivine rock. The outcrop is very obscure, and highly weathered into a talcose serpentine, with remnants of massive dark-green serpentine.
This dyke is particularly interesting, in that an analysis of the serpentine showed it to contain 0.075 per cent. of copper. Copper - ores are frequently associated with magnesian rocks, and this proximity of a copper- bearing dyke to a copper lode strongly suggests that the ore in the lode has been formed from a previous concentration of the ore in an ultrabasic magma beneath.
5. On the Presence of Segregated Gold in the Schist.
The majority of writers—notably Hector,* Ulrich,† McKay,‡ and Rickard§—in order to account for the vast amount of alluvial gold in Otago, claim that the schists carry gold segregated in the quartz laminæ. In the first place, the contention is unnecessary, as is evident from a careful study of the lodes in Otago. In the second place, only two examples have
[Footnote] * Sir J. Hector, “Outline of New Zealand Geology” (Wellington, 1886), p. 83
[Footnote] † Hutton and Ulrich, “Geology of Otago” (Dunedin, 1875), p. 157.
[Footnote] ‡ A. McKay, “Gold-deposits of New Zealand” (Wellington, 1903), p. 68.
[Footnote] § T.A. Rickard, “Goldfields of Otago,” Trans. Am. Inst. Min. Eng. vol. xxi, p. 442.
been recorded and both of these rest on unsatisfactory evidence.
McKay records the occurrence of gold in laminated quartz at Green's Reef, Ophir.* Both Ulrich† and Park‡ have conclusively shown that there is here a zone of crushed rock on the line of a fault, through which mineralising solutions have risen. The crush-zone is penetrated by cross-veins and “flats” of quartz carrying pyrite and gold, and leaving no doubt as to the secondary origin of the metal. This instance must therefore be rejected.
H. A. Gordon states that gold has been found in the schist near the Bullendale lode, Skipper's.§ He did not describe or figure the specimen, nor did he, apparently, take any precautions to observe from where it was taken, which was very necessary in the case of a wide mullocky lode like that at Bullendale, where a broad belt of country has been intersected by several parallel fissures, and the intervening rock impregnated with auriferous pyrite.
I examined a reported instance near Butcher's Gully, Alexandra,∥ which proved to be on the line of a crush-zone, highly mineralised, the rock being penetrated by “flat” veinlets of quartz resembling laminated quartz.
My conclusion is that the occurrence of gold in the schist laminæ is not borne out by observation.
The presence of gold in the schist would therefore require to be tested by careful analysis, and, in view of Wagoner's recent researches on the presence of gold in various rocks,¶ it is quite probable that the mica-schists of Otago may carry minute quantities of gold. It is, however, inconceivable that the quantity present could ever induce one to claim such as the main source of the alluvial gold of Otago.
The first investigator who opposed the view that the alluvial gold of the drifts was derived from segregated gold in the schists was Professor James Park, in his report on the Alexandra Subdivision,** and to him I am greatly indebted for the many facilities and opportunities he gave me during my work with him on the Cromwell Subdivision.
[Footnote] * A. McKay, “Gold-deposits of New Zealand” (Wellington, 1903), p. 68.
[Footnote] † “Handbook of New Zealand Mines” (Wellington, 1887), p. 75.
[Footnote] ‡ J. Park, Bulletin No. 2, N.Z. Geol. Surv., 1906, p. 29.
[Footnote] § “New Zealand Mining Handbook” (Wellington, 1906), p. 33.
[Footnote] ∥ Hutton and Ulrich, “Geology of Otago” (Dunedin, 1875), p. 157.
[Footnote] ¶ Luther Wagoner, “Detection and Estimation of Small Quantities of Gold and Silver,” Trans. Am. Inst. Min. Eng., vol. xxxi, p. 798.
[Footnote] ** Bulletin No. 2, N.Z. Geol. Surv., 1906.
Fracture cleavage in rock-face, lower schists, Alexandra. (Photo by Professor Park.)
Fracture cleavage in boulder, Bannockburn Bridge. (Photograph.)
Section of chlorite-schist, Dunstan Range. Magnetite (black) marks the foliation-planes. Chlorite (cloudy) and quartz (clear) are present. × 32 diameters.
Section of granular chlorite-schist, Gibbston. Shows large recrystallized feldspars and quartz. × 32 diameters.
Same negative as 3a; printed deep, to show centric arrangement of epidote and twinning of feldspar.