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The Pamphlet Collection of Sir Robert Stout: Volume 75

Analyses of Old Mine-Timbers

Analyses of Old Mine-Timbers.

Sample. Description and Locality. Part Analyzed. Gold Found.
I. Prop from the Tipperary Gold Mining Co., Macetown, Otago, much decomposed. The part analyzed had been under water for many years. Outside of the prop. None.
II. Do. Inside of do. None.
III. Prop from an old tunnel in an alluvial terrace, near Skipper's Point, Otago. The part analyzed had been under water for probably 20 years. Outside of prop. None.
IV. Do. Inside of do. None.
V. A prop from the Premier mine, Macetown, Otago. Water had been running over it for 2 years at least. Outside of prop. 0.0038 grain.
VI. Rotten wood from the lower part of a rejected prop from the Long Tunnel Gold Mining Co., Walhalla. From a part of the mine where a richly auriferous reef had been worked. Outside of do. 0.0097 grain.*
VII. Do. Inside of do. None.
VIII. Decayed portion of lath brought up from the deep workings of the Northern Star Gold Mining Co., Ballarat. Had evidently been saturated with water for many years. No portion was sound; all was burned and analyzed. None.
IX. Portion of sleeper used in a tramway, Northern Star Gold Mining Co., much decayed. A large quantity of water had been running over it for at least a year. No portion was sound; all was burned and analyzed. 0.0129 grain.
page 41

Search for Gold in Mine-Waters.—It was soon found useless to examine mine-waters which contained much iron, and in which the ferrous salts had not been oxidixed to ferric salts by exposure to the atmosphere. The analyses were consequently restricted, for the vadose region, to waters containing a very small percentage of iron salts, and chalybeate waters which have been thoroughly oxidized (the latter being apparently exceptional, even in that region), and, for the deep circulation, to waters containing so little iron that they do not act as precipitants of gold.

This chapter treats of mine-waters under the conditions of temperature and pressure now encountered. What might be effected by these waters under other conditions does not concern us at this stage.

If gold be present in mine-waters at all, it is likely to be in very minute proportions. Hence large quantities of water must be operated upon. The method of evaporation for the assay of the residue was too tedious, especially in view of the limited time at my disposal in each locality. I therefore availed myself of the well-known action of sulphides and organic matter in precipitating gold from solution.

A filter was constructed, consisting of a tinned-iron cylinder, about 3 inches in diameter and 6 inches long, terminating below in a funnel, inside of which was placed a filter of glass wool, and above this the reducing agents (animal charcoal, artificial iron and lead sulphides, roughly powdered). The upper part was connected by a rubber tube with the tap supplying the water to be tested.

The following preliminary test proved the efficiency of this filter: A solution in 400 gallons of water (from the Dunedin water-mains) of 28 pounds of common salt, 8 ounces of magnesium sulphate, 8 ounces of ferric chloride and 0.1 grain of gold in the form of auric chloride, was allowed to trickle slowly through the filter, the operation taking about 48 hours. The mixture of sulphides and charcoal was then removed, roasted and assayed, when 0.0926 grain of gold was recovered, showing that practically all the gold had been precipitated.

In examining mine-waters, the water was first collected in an iron tank, and powdered alum was added to it, completely precipitating all suspended matter. After standing some hours, page 42 it was tapped from 2 or 3 inches above the bottom of the tank, and filtered as above described.

Four samples were thus treated; but in no case was gold found.

The first sample (about 500 gallons) was water from the high-level tunnel of the Tipperary G. M. Co., which had percolated down from the surface through the lode-fissure for from 200 to 500 feet, and flowed in a large stream from the tunnel-mouth. This practically represented the vadose circulation only.

The second sample (about 500 gallons) was from the pump discharge of the New Chum Railway Co., Bendigo, Victoria, and represented the whole drainage of the mine from the surface to a depth of 2850 feet.

The third sample (about 700 gallons) was from the Long Tunnel G. M. Co.'s mine, "Walhalla, Victoria, near a very rich lode, and represented the whole drainage between the adit (700 feet below the summit of the hill) and a level about 2300 feet below the surface, or over 1500 feet below the adit.

The fourth sample (about 500 gallons), from a deep tunnel of the Premier mine, Advance Peak, Otago, driven on a lode which had proved richly auriferous in places, represented the whole drainage from the surface to probably 2000 feet below it

The negative results of these tests are the more surprising to me, since other examinations, hereinafter described, afforded strong evidence that solution and re-precipitation of gold have taken place in the vadose region.

It is, of course, possible that gold may have existed in these samples in some form from which it was not precipitated by the reagents used. This is suggested, indeed, by my experience (see Chapter VII.) in attempting to precipitate gold from sea-water. The question can only be decided by the evaporation of samples (first freed from suspended matter) in larger amount and number than mine, and the assay of the residues, I venture to recommend such an inquiry to those who live id the vicinity of rich mines and have time for the work.

Does the Gold of the Vadose Region Contain Generally Less Silver than That of the Deep Circulation in the Same District?

Several investigations have seemed to prove that, on the average, the gold of the alluvial deposits in Australia and elsewhere page 43 is appreciably finer in quality than the vein-gold. The matter has been discussed chiefly in connection with the origin of nuggets;* but, so far as I know, no comparison as to fineness has been made between the vadose and the deep vein-gold.

The observed difference between placer-and vein-gold may be held to show, either that the oxidized mineral-bearing waters running in the ancient drift-deposits dissolved out part of the silver with which the gold was alloyed, or else that these waters dissolved both gold and silver, the gold being again precipitated, alloyed with less silver than before. Similar reasoning might be applied to an observed difference between the vadose and the deep zone, in the quality of vein-gold. I have therefore made comparative assays of this character in a number of cases.

The fineness of the gold may vary considerably even in the same level, and within a few feet. This is true even in such districts as Bendigo, Ballarat and Otago, where the percentage of silver is comparatively low, while in districts like the Thames, N. Z., the variation observed is sometimes extraordinary. On the whole, however, the average quality of the gold won in districts of the former class varies little.

Five localities were chosen (chiefly by reason of facilities for obtaining specimens from near the surface), namely, (1) the Nenthorn gold-field, in mica-schist, in eastern Otago; (2) the Tipperary and Premier mines, in mica-schist, Macetown, central Otago; (3) the Dart river, in northern Gippsland, representing the Upper Silurian of Victoria; (4) the Bendigo field, representing the Lower Silurian; and (5) the Thames district of the North Island, in altered Lower Tertiary andesite.

In a few instances the analyses were made of gold picked out of the reef; but the majority were assays of vein-stone. The percentage of silver was in each case obtained by difference, and represents the loss per cent, of the prill after quartation.

page 44
The results were as follows:
Relative Fineness of Vadose and Deep Vein-Gold.
A. Surface Samples from Nenthorn Gold-Field, Otago, N. Z.
No. Percentage of Gold. Percentage of Silver.
1 91.85 7.15
2 89.27 10.73
3 90.43 9.67
4 89.19 10.81
5 92.01 7.99
6 89.38 10.62
7 91.46 8.54
8 90.79 9.21
9 90.07 8.53
10 92.00 8.00
Average fineness of 10 samples, 90.645 per cent. Range of fineness, 89.19 to 92.01 = 2.827 per cent.
B. From Deeper Levels of Nenthorn District (the Bulk of the Gold was Held by Pyrite).
No. Percentage of Gold. Percentage of Silver.
1 89.86 10.14
2 91.43 8.57
3 87.17 12.83
4 90.45 9.55
5 89.07 10.93
6 92.01 7.99
7 90.17 9.83
8 88.75 11.25
9 91.03 8.97
10 90.30 9.70
Average fineness of 10 samples, 90.024 per cent. Range of fineness, 87.17 to 92.01 =4.84 per cent.
C. Surface Samples from Tipperary, Premier and Sunrise Gold Mining Companies. All near Macetown, Central Otago, N. Z.
No. Percentage of Gold. Percentage of Silver.
1 96.12 3.88
2 93.37 6.63
3 94.19 5.81
4 92.87 7.53
5 95.00 5.00
6 94.58 5.42
7 93.23 6.77
8 95.84 4.16
9 95.93 4.07
10 94.89 5.11
Average fineness of 10 samples. 94.602 per cent. Range of fineness, 92.87 to 96.12 = 3.25 per cent.
D. From Deep Levels (1000 to 1500 Feet Below Surface) in Tipperary and Premier Mines, Macetown.
No. Percentage of Gold. Percentage of Silver.
1 95.39 4.61
2 94.48 5.52
3 91.87 8.13
4 96.29 3.71
5 93.06 6.94
6 94.31 5.69
7 93.80 6.20
8 94.73 4.27
9 93.90 6.10
10 94.10 5.90
Average fineness of 10 samples, 94.293 per cent. Range of fineness, 91.8 to 96.29 = 4.42 per cent.
Note.—A sample taken from a bar of 194 ounces from deep levels of Premier Gold Mining Co. contained 94.45 per cent.

These analyses of 76 samples go, on the whole, to prove that the average fineness of the gold in the vadose region is appre

page 45
E. Surface Specimens from Richly Auriferous Lode in the Vicinity of the Dart River, North Gippsland, Victoria.
No. Percentage of Gold. Percentage of Silver.
1 91.87 8.13
2 94.96 5.04
3 93.08 6.92
4 92.00 8.00
5 92.63 7.37
6 94.73 5.27
7 93.45 6.55
8 95.01 4.99
9 94.91 5.09
10 92.84 7.18
Average fineness of 10 samples, 93.548 per cent. Range of fineness, 91.87—95.01 = 4.14 per cent.
F. Unoxidized Specimens from the Same Reef, Dart River, North Gippsland, Victoria.
No. Percentage of Gold. Percentage of Silver.
1 91.50 8.50
2 90.43 9.57
3 87.69 12.31
4 Note (a) 96.28 3.72
5 91.17 8.83
6 Note (b) 84.64 15.36
7 88.93 11.07
8 95.03 4.97
9 90.37 9.63
10 90.80 9.20
Average fineness of 10 samples, 90.984 per cent. Range of fineness, 84.64—96.28 = 12.36 per cent. Note (a).—Sample assayed consisted of arsenopyrite and pyrite. Note (b).—Sample assayed composed chiefly of galena.
G. Surface Samples from Bendigo Gold-Field, Victoria.
No. Percentage of Gold. Percentage of Silver.
1 94.76 5.24
2 95.03 4.97
3 94.05 5.95
4 94.27 5.73
5 94.10 5.90
6 95.00 5.00
Average fineness of 6 samples, 94.535 per cent. Range of fineness, 94.05 to 95.03 = .98 per cent.
H. Samples from Deep Levels (1200 Feet to 3000 Feet) of Bendigo Gold-Field.
No. Percentage of Gold. Percentage of Silver.
1 93.08 6.92
2 95.04 4.96
3 94.00 6.00
4 91.27 8.73
5 91.30 5.70
6 94.19 5.81
7 93.96 6.04
8 92.95 7.05
9 92.37 7.63
10 94.70 5.30
Average fineness of 10 samples, 93.586 per cent. Range of fineness, 92.37 to 95.04 = 2.64 per cent.

ciably greater than in the deep circulation in the same district, and also that the vadose gold is considerably more regular in quality.

It seems to be indicated that considerable solvent action must have been exercised by water percolating through the rocks of page 46
I. Samples from Vadose Region of Various Parts of Thames District, N. Island, N. Z.
No. Percentage of Gold. Percentage of Silver.
1 63.72 36.28
2 79.34 20.66
3 52.90 47.10
4 57.39 42.61
5 67.01 32.99
6 59.98 40.02
7 65.55 34.45
8 73.07 36.93
9 60.08 39.92
10 51.19 48.81
Average fineness of 10 samples, 63.023 per cent Range of fineness, 51.19 to 79.34 = 28.15 per cent.
K. Samples from Deep Circulation of the Thames District.
No. Percentage of Gold. Percentage of Silver.
1 63.76 56.24
2 71.14 28.86
3 59.93 40.07
4 53.01 46.99
5 64.25 35.75
6 43.17 56.83
7 66.98 33.02
8 51.30 48.70
9 69.04 30.96
10 48.75 51.25
Average fineness of 10 samples, 59.133 per cent. Range of fineness, 43.17 to 71.14 = 27.97 per cent.
the vadose region, as the denudation of the surface has gradually lowered the water-level, converting the deep circulation of former times into the vadose of to-day.

* Prof. Liversidge, in the paper already cited, gives a convenient summary of the literature of this subject.

Surface samples are hard to get in Bendigo. Nearly all the companies are mining in the deep region, and have long ago exhausted the pay-quartz above. I took many samples from outcrops of reefs, but found in the majority little or no gold. The results given below are those in which a prill of appreciable size was obtained by assaying 1000 grains of vein-stone.