Characteristics of the Veins.
The auriferous belt consists of a zone of crushed and fissured slate of varying width (maximum, about two miles), and it is to be observed that this zone lies parallel with the line of strike of the slates, with the granitic intrusions on either side of it, and with the observed outcrops of the older Devonian rocks to the east, while it practically corresponds with the line of occurrence of the altered diabases. These coincidences have a structural significance.
If the intrusion of diabases was the first phenomenon, it is probable that they caused a line of weakness along the strike of the slates. Later, with the granitic intrusions and the intense crumpling of the slates between, this line of weakness would be the locus where crushing and faulting would be concentrated. The junction between the soft slates and the hard Devonian cherts would also be a line along which movement would readily take place—in fact, the belt of resistant Devonian rocks, which were indurated and folded prior to the deposition of the slates, would evidently act as a central buffer during the later folding movements, and would
further concentrate movement and shattering into a comparatively narrow zone in the slates.
The absence of veins in the Devonian rocks is not surprising, as they would not be amenable to extensive fissuring, more especially as the relief of strains would be readily effected by faulting of the weaker slates. Moreover, fissuring and faulting would take place where the stress due to folding was greatest—that is, in the apex of the main syncline of the slates.
Throughout the width of the zone of disturbance there are places where more defined fissuring has taken place, giving access to thermal solutions. It is in these places that the veins proper occur, and the position of the chief veins is indicated by the position of the mining claims.
Turning to the individual veins, if they can be so regarded, a vein consists of a series of lenses of quartz, with constriction of the vein - walls between. The lenses follow each other more or less continuously to considerable depths, with short barren patches of “vein-formation” between the lenses. The vein, as indicated by its walls and the included band of crushed slate which contains the lenses, conforms as a rule to the bedding of the country rock, though frequently it may cut across it.
The lenses vary considerably in dimensions in different parts of the field, the smallest occurring at Boatman's and the largest in the Inkerman Mine. In most cases they show a steep pitch to the north along the, strike of the vein. The gold, of fineness 960, and worth over £4 an ounce, is free-milling, and varies from coarse to fine. The former is caught on the battery-tables, and the latter recovered by cyaniding the tailings. It is scattered for the most part through the lenses, though occasionally it lies in shoots on the walls. Of sulphides, auriferous pyrite is always present, the pyritic concentrates being worth approximately £20 per ton. Stibnite is also very frequently present, some veins carrying a notable proportion of it. It is mostly low grade, and mingled with quartz. When present in quantity it occurs in seams and bunches. As an accessory it is scattered sporadically through the stone, and greatly increases the consumption of cyanide. To remedy this, the Keep It Dark Company now treat their tailings with a solution of caustic soda previous to cyaniding.
Metasomatic Action.—The results of analyses made of the fresh and altered slates show a considerable loss of silica and alkalies; the effect of the ore-bearing solutions on the slate has been sericitization, with a decrease of specific gravity. The effects are thus quite analogous to those which I have described in the case of the veins of Otago.* Both Reefton and Otago, it may be noted, carry a very similar class of vein and ore throughout, although most of the Otago veins cut across the bedding of their country rock.
[Footnote] * This volume, p. 82.