(7.) The Barewood Claim.
This is the best-known vein in the Taieri Gorge district, which includes several veins prospected at Hindon, Matarae, and elsewhere.
The country rock is a quartz-mica-schist, lying almost horizontally. The vein strikes north-west and south-east, and dips north-east at an angle of about 60°. It is worked by an underlay shaft, which cuts the vein at a depth of 130 ft. At 180 ft. and 240 ft. crosscuts open up Nos. 2 and 3 levels, while a winze from No. 3 level has been sunk for a further distance of 30 ft. This is one of the few veins now being worked on the goldfield. It averages from 4 ft. to 5 ft. in width, but widens out to 15 ft. in the upper levels. It is composed of solid quartz throughout, divided by subordinate clay heads or partings parallel to the well-defined walls. The foot-wall is uninterrupted, but the hanging-wall carries small leaders (from 6 in. to 12 in. wide), which wedge out a short distance up. These leaders generally carry good gold.
Slickensides are often well developed, generally on the hanging-wall. In places the quartz adjoining the walls, and also the adjacent wall-rock, are highly brecciated. This has been seen on both walls in No. 3 level, and on the hanging-wall in No. 1, while it is absent in No. 2.
Horizontal Distribution of Gold.—The gold, so far as workings have disclosed, is uniformly distributed along the strike, and shows as yet no tendency to occur in localised shoots.
Vertical Distribution of Gold. — The present depth of workings has disclosed an interesting variation in value from the No. 3 level winze to the surface. The accompanying section shows the values at different points, taking the average value through the whole cross-section of the vein:—
From the surface to a depth of 50 ft. the value rose from ½oz. to 1 oz. per ton. From here to the intermediate level the value fell to 16 dwt., and then more slowly, till at three stopes below No. 1 level it was 14 dwt. From here to No. 2 level the mean value became very low—approximately, 5 dwt. Between Nos. 2 and 3 there was an equally rapid rise, and at No. 3 it varies from ½ oz. to over 1 oz. per ton, while at the foot of the winze the assay value is uniformly over 1 oz., and rich specimen-stone, particularly the brecciated variety, may be picked up showing much coarse gold. It is peculiar also that at this point the gold is often dark in colour and rusty.
At No. 3 level, below it, and for some distance above it, the gold is pretty evenly distributed across the vein. Rising to No. 2 level, a barren block of glassy quartz comes in, and the seamy gold-bearing quartz is pushed over to the two walls, and divided by clay partings from the barren centre block, which corresponds practically to a “horse” of country rock. This block wedges out when followed in either direction along the strike, and it also has an easterly pitch or dip along the strike of the vein. Both these features may be seen on the accompanying plan of the workings,
where it will be observed that in each level the foot-wall and hanging-wall seams have been driven on, leaving the centre block intact.
Thus in the wider portion of the vein the gold has been deposited in two shoots along the walls. The gold-bearing quartz differs from the barren “dog's-tooth” quartz in being seamed and mottled with pyritic mullock, and under the microscope is finer in grain. These facts point to a certain amount of replacement along the walls, while the barren block has been formed by simple deposition in an enlarged fissure.
In places in Nos. 2 and 3 levels there occur peculiar siliceous concretions (Plate II, 3), cavernous and irregular in form, with a fine chalcedonic banding. They are dark, and coated thickly with very fine pyrites. In appearance they suggest “clinkers” in coal, or fossil forms.
Associated Minerals.— The dominant sulphide is pyrite, in fine crystals and grains. It is absent in the clear glassy quartz, and thickly distributed in the auriferous quartz. In the No. 3 level there was found within the vein, and near the hanging wall, a narrow cavity containing a cluster of large stalactites of pyrite. Stibnite, galena, and scheelite occur occasionally. The only one of importance is stibnite, which is becoming common in the deeper levels.
Habit of the Gold.—The gold is largely free, the assay value of pyrite being low. It is probable, however, that at a greater depth the ore will become refractory.
Alteration of the Wall-rock.—The country rock is little altered, except for about 2 ft. from the vein-walls, where the alteration is considerable, the rock being soft and “mullocky.” There is, further, frequently found on the walls a type of greasy yellowish rock, devoid of pyrite, and very similar to the corresponding rock described above in the Nugget and Cornish Mine. It accompanies the brecciated ore, the payable seams being well developed in No. 3 level and the winze.
The following analyses indicate the alteration of the normal 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. Unaltered rock.||——|
|2. Altered rock.||-24.90|
|3. Altered rock, recalculated on a basis of constant alumina.|
|4. Gains and losses.|
The alteration in this case is more intense than at Skipper's, although more local, and the loss of material greater. It is, however, of a similar nature—namely, sericitic.
Analyses made of the yellowish or bleached variety of wall-rock show that, like that at Skipper's, it is a kaolinized type of the ordinary sericitic rock, evidently formed by the action of descending waters.
Ore-shoots of Primary Origin.—The characteristic hanging-wall and foot-wall seams described above doubtless originated during the primary deposition of the gold, through the influence of the wall-rock.
Ore-shoots of Secondary Origin—Secondary Sulphide Enrichment.— A number of facts indicate that the No. 3 level winze has encountered a zone of enriched sulphides (pyrite and stibnite). These are as follows: (1.) The occurrence of the gold in the free state, and its frequent rusty colour. (2.) The occurrence of stalactites of pyrites. (3.) The peculiar siliceous concretions, probably due to solution and redeposition by subsequent leaching processes. (4.) The brecciation, signifying subsequent movement, would give readier access to descending solutions. (5.) The kaolinization and bleaching of the yellowish wall-rock, and the absence of pyrite in it, are evidently due to descending surface waters, and it is notable that it occurs associated with the brecciated ore and with the richest seams. (6.) Finally, the impoverishment in the upper levels, supported by the above data, points to the work of secondary enrichment, which has largely leached the gold out of the upper levels and redeposited it with sulphides in the zone now being opened
up. It is probable that when this zone is passed through the ore will become lower grade and refractory, as at Bendigo and on the Carrick Range.