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Volume 41, 1908
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(2.) Skipper's and Macetown Veins.

The veins of this district are mineralised shear-zones rather than fissureveins. The country rock is a soft, finely laminated mica-schist, traversed by broad belts of fracturing. Along these belts the rock is sheeted or divided by several parallel fissures, the intervening schist being crushed and contorted, and more or less altered. These planes of fracture served as channels for the mineralising solutions, which caused the formation of segregated lenses or blocks of quartz. These blocks are of varying size and value. The gold is mostly fine and free, and the adjacent shattered schist or lodeformation is impregnated with pyrite.

The Shotover, or Nugget and Cornish, Vein.—The country rock strikes north and south, and dips to the west at from 30° to 45°. The vein, striking north-west and dipping south-west at about 60°, crosses the Shotover River about two miles above Skipper's Point. At the river-bank there are two veins, the eastern and the western, about 100 ft. apart. These two merge into one a short distance up the hill, and the single fissure-line has been traced across the ranges for some miles to the north-west. On the southeast side of the river the two outcrops are distinctly seen, but only the eastern has been traced for any distance. This runs over the dividing-range, apparently in line with the Premier reef of Macetown.

The vein is typical of its class, two main fractures constituting respectively the hanging and foot walls, with a parallel sheeting of the intervening belt by subordinate fractures. In the western reef four blocks of quartz have been stoped out. At the junction of the two veins a large block (the No. 1) was stoped for a depth of 250 ft. below the surface. The blocks are generally lens-shaped, and limited on all sides. They are generally bounded by thin clay partings or selvages, but not infrequently these are absent, and there

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Fig. 1.

a. Quartz shoot. b. Vein-formation of crushed schist.

is a gradual transition from quartz to lode formation. In such cases the quartz “makes” gradually out of the lode-formation, and passes over to a parting or wall, where it wedges out (fig. 1).

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The lode-formation varies in width from 8 ft. to 20 ft., and has generally defined walls with tough clay casings. The value of quartz varies from 5 dwt. to 20 dwt. per ton. The gold is fine, hackly, and free, the extraction being about 80 per cent. by mill amalgamation. The pyrite is auriferous, but not by any means rich enough to warrant the treatment of the pyritic lode-formation, as some promoters would have us believe.

In places, more especially near the surface, the lode-formation contains bands of a soapy yellowish-grey rock, especially, near quartz, and devoid of pyrites.

The following analyses indicate the normal mode of alteration:—

[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]

1. 2. 3. 4.
H2O 2.06 2.48 2.13 +0.07
SiO2 53.05 54.12 46.30 -6.75
Al2O3 10.31 11.94 10.31
Fe2O3 9.74 5.63 4.83 -4.91
FeO 8.60 6.54 5.60 -3.00
CaO 5.17 3.93 3.37 -1.80
MgO 0.72 1.65 1.41 +0.60
K2O 4.74 4.73 4.05 -0.69
Na2O 2.90 4.31 3.69 +0.79
MnO2 0.51 0.22 0.18 -0.33
TiO2 1.06 0.39 0.32 -0.74
CO2 1.15 1.51 1.30 +0.15
FeS2 3.43 2.94 +2.94
100.01 100.88 86.43 +4.64
-18.22
-13.58
1. Unaltered country rock.
2. Altered lode-formation.
3. Altered lode-formation, recalculated on a basis of constant alumina.
4. Gains and losses of altered rock.

These figures show—(1) that a good deal of replacement has occurred in connection with the segregation of quartz; (2) that the type of rock alteration may be regarded as partial sericitization.

The following analyses indicate the nature of the yellowish altered rock:—

[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]

1. 2. 3. 4.
H2O 2.06 2.60 1.56 -0.50
SiO2 53.05 46.90 28.14 -24.91
Al2O3 10.31 16.46 10.31
Al2O3 9-74 6-67 4-00 -5-74
FeO 8.60 7.26 4.35 -4.25
CaO 5.17 7.45 4.47 -0.70
MgO 0.72 1.97 1.18 +0.46
K2O 4.74 4.07 2.44 -2.30
Na2O 2.90 2.08 1.25 -1.65
MnO 0.51 0.23 0.14 -0.37
TiO2 1.06 0.27 0.16 -0.90
CO2 1.15 3.98 2.19 +1.04
FeS2
100.01 99.94 60.19 +1.50
1. Country rock. -41.32
2. Yellow rock.
3. Yellow rock, recalculated with constant alumina.
4. Gains and losses. -39.82
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It is evident from these figures that this altered rock is a kaolinized variety of the normal lode-formation. The bleaching and the total removal of pyrite are probably due to the secondary processes of descending surface waters, accompanied by the formation of kaolin.

Present mine-workings have not yet shown what factors regulate the occurrence of the quartz blocks. It is probable, however, that they are connected with some local structural features. The No. 1 block, for instance, occurred where the two reefs junction.

A common occurrence of gold in this and other veins of the district is as fine “paint” coating the clay selvages. This is probably due to processes of secondary enrichment, the clay partings acting as a filter to the gold-bearing solutions. In other words, this seems to be an instance of adsorption—the process recently studied by Kohler.*

Microscopically the quartz occurs in coarse granules, with patches of fine-grained quartz studded with pyrite crystals. Such patches evidently indicate portions where replacement has occurred.

Other Veins.—The Invincible, fifteen miles up the Rees Valley from Glenorchy; the extensive group of veins round Macetown; some veins near Arrowtown; and the Bullendale or Phœnix vein, up Skipper's Creek, as well as other smaller veins in the Shotover Basin, all belong to this type, and have the same characteristics.

[Footnote] * E. Kohler, “Zeitschrift fur Praktische Geologie,” 1903, p. 49.