THE WEST AUSTRALIAN NATURAL HISTORY SOCIETY.
THE GOVERNMENT GEOLOGIST ON “GOLD.”
At the last monthly meeting of the West Australian Natural History Society, the Government Geologist read the following paper on “Gold” :—
CHEMICAL AND PHYSICAL PRROPERTIES.
Gold is always found in the native or metallic state, but generally alloyed with silver, copper, or iron, and although one of the most widely distributed and earliest worked metals it is comparatively rare, owing to the fact that it mostly occurs in small quantities requiring a great deal of labour to win it. It has been always highly prized owing to its beautiful colour, the ease with which it can be worked, the fact that it does not tarnish when exposed to the action of air or water, and so far it has been almost universally adopted as the standard of exchange.
In the early part of the 4th century the Alchemists spent their lives in seeking what was called the Philosophers stone which would give them the power of melting the baser metals in certain proportions, and thus transforming them into gold. It has now been generally decided by chemists that it is an element, or in other words that it cannot be split up into any more elementary substances, neither can it be manufactured. In the pure state its specific gravity is very high, being [figure unclear] times as heavy as water, which physical character is taken advantage of in separating it from other minerals, for beside platinum and two or three of the very rare metals, an equal bulk of it is heavier than any other substance.
This metal melts at 2000deg. Farenheit, [sic] and can also be volatilised at very high temperatures. In the pure state its colour and streak (a mark made by it on porcelain) is a deep golden yellow, but in the finely divided state it is either red or black, whilst by transmitted light it is green. It is the most malleable and ductile metal, as a grain of it can be beaten out large enough to cover 54 ¾ square inched of 1—280,000th of an inch in thickness, whilst Faraday calculated that the gold from four sovereigns, if drawn into wire, would be long enough to reach round the earth at the equator.
It does not readily enter into chemical combinations with the other elements, and when it does the resulting salts are very unstable being decomposed when brought in contact with other metals, metallic salts, organic substances or by exposure to the action of light and air. It is not acted upon by any simple acid, but is dissolved by chlorine in solution or by nitro-hydrochloric acid forming auric chloride, one of its most stable salts.
Pure gold is so soft that it can be scratched by the finger nail, therefore it has to be alloyed with other metals to increase its hardness, silver or copper being the most generally employed. An alloy it must be understood is not a chemical compound, as no chemical action takes place, but simply a mixture of two or more metals in any proportions, the English gold standard being an alloy of 11 parts of gold to one part of copper, but as this is not hard enough for jewelry the proportion of copper is greatly increased. The fineness of those alloys is spoken of as so many carat gold. Pure gold is expressed as 24 whilst in lower standards the number of parts of pure gold in the 24 is mentioned, a sovereign is 22 carat or 22 parts gold and 2 copper.
As gold used in jewelry is mostly of certain standards as 22, 18, 15 or 12 carats it is not necessary to melt it up to tell its fineness, but this can he done by marking with it on a black basaltic stone (called a touchstone) then treating the mark with dilute nitric acid and comparing it with golds of known standards similarly treated.
Gold also has the property of forming an amalgam with mercury at ordinary temperatures, when it forms a soft slimy mass in which the gold and mercury are found to be perfectly mixed This affinity of mercury for gold is taken advantage of in its extraction.
For testing the presence of gold in very minute quantities the mineral is finely pulverized and agitated with an alcoholic tincture of iodine, into this solution a piece of filter paper is dipped and then burnt, when if the colour of the ash is purple, it indicates the presence of gold; but this should be confirmed by evaporating the alcoholic tincture to dryness and treating the residue with nitro-hydrochloric acid, and again evaporating, dissolving the residue in water, and dropping in a drop of a mixture of stannous and ferric chloride, when a deep purple colour will be seen (Purple of Cassius), which confirms the presence of gold.
Gold may be distinguished from iron pyrites, copper pyrites, and mica, by the ease with which it will cut with a knife. Iron pyrites, being as hard as quartz, will not cut; copper pyrites will cut, but it yields a greenish powder ; whilst mica splits off in shining scales. Another method, where the specks are too small to try the with a knife, and acids are not at hand, is to make the stone red-hot, and either let it cool or drop it into cold water, when the iron pyrites will turn red, the copper black, and the mica lose its lustre, whilst the gold will remain unaltered.
There need be no fear of melting the gold, as it requires a much higher temperature than that of an ordinary fire. Gold, besides being valuable as a medium of exchange, is one of the most useful metals. For jewelry it cannot be surpassed, owing to its beautiful colour. the fact that it does not rust, and the ease with which it can be worked. It is also used largely for plating and gilding, in both of which processes gold leaf was originally used, but now it is found much more economical, when the article to be plated is metal, to deposit a thin coating of gold from solution by means of an electric current, by which a very thin film of gold can be evenly deposited over a large surface.
Gold is used also for colouring glass, the beautiful reds called ruby glass being due to the pretence of it in small quantities. In photography it in also of great value, owing to the permanency of the beautiful tones that can be obtained by replacing the silver in the original print with it, and a great variety of shades are produced, varying from black, blue, pink to brown, according to the salt used in the solution.
ITS OCCCURRENCE.
Gold, as was before said, is always found in nature in the metallic state, but mostly alloyed with small quantities of other metals. It was formerly supposed to be always associated with quartz which was considered to be an indication of it, but this idea has exploded, as it has now been found with calcite, serpentine, diorite, and granite, and associated with the ores of lead, iron, antimony, copper and tin.
It is true certainly that quartz commonly occurs with it, but the white quartz reefs which were
first worked are now not thought so much of as the more mineralized veins. Although always found in the metallic state it is highly probable that it also exists in nature as a sulphide, but as this salt is so unstable it would be decomposed before this could be determined.
Gold occurs in nature in two forms namely alluvial gold and reef gold, in the former state it is found in the stream beds and deep leads and has been derived directly from the reefs by the weathering action of the stream, the gold being left behind at the bottom of the gully owing to its great specific gravity, whilst the lighter minerals have been washed away. Reef gold occurs in veins, lodes or dykes, the term reef or vein being used to describe a lode where there is a predominance of an earthy mineral, lode where an ore or metallic mineral is in the larger proportion, and a dyke when it owes its origin to plutonic action and is infilled from below. These reefs and veins mostly occur in the older Palaeozoic formation, the rocks being generally clay slate or schist. Veins are fissures or faults which have been infilled by mineral matter in solution, either from small cross fissures and leaders, or directly from the side of the vein itself, which has been mostly the case in this colony where, as a rule, the reefs have one good wall coated with a greasy impervious caseing, whilst on the other side the country is much broken, and many small veins and leaders or feeders strike away from it into the country, and the rock on this side being as a rule more pervious than next the good wall.
Although only found in workable quantities in the mineral veins amongst these older rooks gold occurs in minute quantities throughout the whole geological series, and the sea water in some places contains an appreciable quantity of it in solution. It was upon this fact that the theory of the latteral infilling of mineral veins was based, as previously the presence of gold could only be accounted for by the theory that all reefs were filled from below, the vein stuff which carried the gold being thrown up in a molten state. But now that we know that gold is soluble under certain conditions, it is highly probable that highly heated water, charged with certain salts, might have dissolved the minute traces of gold from out of the rock through which it passed, depositing it in the fissures.
In what state the gold was when in solution it is impossible to say, as we have no means of experimenting with it, at enormous temperature, and under the tremendous pressure that existed when it was deposited; but it is highly probable that it was in the form of a double sulphide. The silica was also probably in the form of soluble silicate, whilst the other metals would be in the form of sulphides. The water charged with the mineral matter would gradually find its way into these fissures, on the sides of which it would deposit the mineral in layers, which would cause the banded appearance which is so commonly met with in veins, whilst owing to the peculiar physical character of minerals which, when deposited from solution, sort and arrange themselves, the gold, instead of being all through the stone in an inappreciable form, would be deposited in smaller or larger specks or particles according to the richness at the time of the infilling solution.
ITS EXTRACTION.
To extract gold the stone must first be pulverized. This is done either by batteries or mills, the former of which although a very rude contrivance is found to answer the purpose better than any more complicated machine.
A battery consists roughly of a number of pestles or stampers (generally five) working in a mortar or box into which a stream of water is conveyed by a pipe, and the stone fed in from the back whilst at the front there is a fine wire grating or screen through which when fine enough the stone is carried by the water on to the table. This is an inclined board with amalgamated copper plates and a number of troughs filled with mercury, whilst at the lower end of the table are blankets; owing to the greater density of the gold it sinks into the mercury, or when in very fine particles is caught by the amalgamated plates, whilst the blankets catch the coated gold which will not amalgamate and other ores which may be rich in the precious metal.
Mills are of various patterns, but the general principle on which they work is to grind the stone with mercury which amalgamates with the gold whilst the earthy minerals are worked away as sand and mud. The amalgam resulting from both these processes is heated in a retort, which drives off all the mercury, leaving behind a spongy cake of gold, which is then melted and cast into ingots. The blanket sands are generally roasted and ground with mercury, when the resulting amalgam is treated as above.
In other processes, like the chlorination, the stone is first pulverised and then put into vats. These vats are closed down and charged with chlorine gas which attacks the gold, forming a soluble chloride. When this part of the process is complete it is dissolved out with water from which the pure gold is precipitated. This process is largely used when the gold is very finely divided, as at Mount Morgan, but it would not pay where the simple modes of extraction suffice.
Alluvial gold is mostly found associated with a more or less free wash or dirt (pay dirt) in a gutter at the bottom of a stream bad or lead (old watercourse), but sometimes it is cemented by lime, magnesia, silica or iron, which makes it impossible to work without first crushing it, whilst at other times the wash contains so much stiff clay that it is necessary to puddle it before it can be washed.
There is very little trouble as a rule in working alluvial ground, except in places like Victoria where the leads are covered by basalt often of great thickness, and the gold can be easily separated from the dirt by either washing or dry blowing. The washing is done in many ways, the principals of which are the sludge, the cradle, and the dish. The first of these consists of a long inclined box open at the lower end with a false bottom and ban or ledges, called a Long Tom, through which a stream of water is run and into which the dirt is thrown, the whole being cleaned up at certain intervals when the gold is found under the false bottom and ledges, whilst all the stones and rubbish have been washed out at the lower end.
A cradle is a box on rockers with a coarse screen at the top on which the dirt is put, water in small quantities poured over it and then rocked, which sifts out the coarser stones, which are thrown away, whilst the finer pass over a number of inclined boards with ledges in the inside of the cradle and finally, minus the gold, are discharged from a small shoot at the bottom. In this, as in the sluice, the gold is found along the ledges of the inclined trays.
The dish most commonly used in these colonies is round with a flat bottom, sloping sides and a little groove on one side to prevent the gold running away with the water and sand. Into this dish the dirt is put with water, the gold being settled to the bottom by a peculiar circular motion. The dish is then inclined and partially submerged in water, by which the lighter material is washed off until only the gold in left.
Dry blowing is only resorted to when water is very scarce. This consists of a winnowing process. First one dish is placed upon the ground in as windy a place as possible, then another is taken full of dirt and poured into the first from as great a height as the blower can reach. After this has been repeated several times and the larger stones picked out, the reduced quantity of heavy material is treated in one dish, from which it is thrown into the air in a particular manner after being first shaken in the pan. When this is sufficiently reduced it is finished by picking out or blowing the small remaining stones away from the gold which is then fairly clean.
AURIFEROUS BELTS OF THIS COLONY.
There are four auriferous belts in this colony, the first of which runs nearly North and South, about 200 to 250 miles from the coast in the southern portion of the colony, and it is on this belt that Yilgarn and the Murchison goldfields are situated.
In the Ashburton the belt runs North West and South East, but is probably the extension of the Yilgarn belt, which ends at the head of this river. The Roebourne belt strikes East and West along the North West coast from the Nicol River to the DeGrey River, disappearing beneath the sandy tableland to the eastward. The Kimberley belt strikes in a North East and South West direction, and is very probably the extension of the Roebourne belt, re-appearing on the North-Easterly side of the sandy tableland. These belts carry gold for a great length, the reefs as a rule are of great size and very rich, and wherever they have been tested they have proved to be good.
A large quantity of stone has been crushed from the different fields, which has averaged 1oz. to the ton of stone, whilst alluvial patches of great value are still being worked. Gold mining here is in its infancy, not yet being ten years old ; but as during this time it has made great progress, especially during the last year or two, there is every prospect that this colony, before long, will be one of the chief gold producing countries of the world.
