By T.W.T. ATHERTON.
The existence of gold in the form of a natural sulphide in conjunction
with pyrites has often been advanced theoretically as a possible
occurrence, but up to the present time this occurrence has, I believe,
never been established as an actual fact.
During my investigations on the ore of the Deep Creek Mines, I have
found in them what I believe to be gold existing as a natural
sulphide. The description of this ore will, no doubt, be of interest
to your readers.
The lode is a large irregular one of pure arsenical pyrites, existing
in a felsite dike near the sea coast. Surrounding it on all sides are
micaceous schists, and in the neighborhood is a large hill of granite
about 800 ft. high. In the lode and the rock immediately adjoining it
are large quantities of pyrophylite, and in some places of the mine
are deposits of this pure white, translucent mineral, but in the ore
itself it is a yellow and pale olive green color, and is never absent
from the pyrites.
From the first I was much struck with the exceedingly fine state of
division in which the gold existed in the ore. After roasting and very
carefully grinding down in an agate mortar, I have never been able to
get any pieces of gold exceeding the one-thousandth of an inch in
diameter, and the greater quantity is very much finer than this.
Careful dissolving of the pyrites and gangue, so as to leave the gold
intact, failed to find it in any larger diameter. As this was a very
unusual experience in investigations on many other kinds of pyrites, I
was led further into the matter. Ultimately, after a number of
experiments, there was nothing left but to test for gold as a
sulphide.
Taking 200 grammes of pyrites from a sample assaying 17 ounces fine
gold per ton, grinding it finely, and; heating for some hours with a
solution of sodium sulphide (Na_{2}S_{2}), on decomposing the filtrate
and treating it for gold I got a result at the rate of 12 ounces gold
per ton. This was repeated several times with the same result.
This sample came from the lode at the 140 ft. level, while samples
from the higher levels where the ore is more oxidized, although
carrying the gold in the same degree of fineness, do not give as high
a percentage of auric sulphide.
It would appear that all the gold in the pyrites (and I have never
found any apart from it) has originally taken its place there as a
sulphide.
The sulphide is an analysis of a general sample of the ore:
Silica 13.940 p.c.
Alumina 6.592 "
Lime 0.9025 "
Sulphur 16.584 "
Arsenic 33.267 "
Iron 27.720 "
Cobalt 0.964 "
Per Ton.
Nickel Traces.
Gold 5 ozs. 3 dwts. 8 grs.
Silver 0 " 16 " 0 "
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99.969
Nambucca Head's Gold Mining Company, Deep Creek, N.S. Wales, Oct. 9,
1891.--_Chemical News_.
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