Traditionally, gold prospectors noted that the possible association between gold and pyrite and the development of this type of auriferous deposit is different. Basically, wherever masses of pyrites are decomposing, the quantity of sulphuric acid found is very great. A large number of other minerals are present, which with sulphuric acid at once brings about a chemical change; thus giving rise to the mineral sulphates found in gold veins and deposits. After these reactions are satisfied, any surplus of sulphuric acid is left free to act upon the common salt contained in the water percolating the deposits, and hydrochloric acid is formed, which, in turn, coining into contact with an oxidizing agent, evolves free chlorine.
Pyrites almost invariably contains gold in relatively minute particles; the decomposition of pyrites, with subsequent liberation of chlorine in close proximity to this gold, may cause the immediate formation of a gold chloride soluble in water. The reactions bringing about the liberation of free bromine and iodine, and the formation of soluble gold salts, take place from similar causes and in the same manner as explained in the case of chlorine. Before closing the subject of solubility, it may aid the general understanding of the case to describe briefly how these solvent agencies affect such other metals as are generally associated with pyrites. Although chemical geology is extremely interesting, it is almost an interminable subject, therefore a number of intermediate reactions which must take place.
Dealing with the only important metals affected by the breaking down or decomposition of auriferous pyrites deposits: any calcium carbonates, magnesia or barite, are at once converted into sulphates by the action of sulphuric acid; iron is converted into oxide, sulphate, and carbonate; lead into sulphate and carbonate; arsenic enters into combination with iron and calcium; copper becomes a sulphate or carbonate; silver sulphide decomposing under the conditions enumerated, and in the presence of salt water, forms silver chloride. All these reactions, in all probability, occur before the liberation of free chlorine, bromine, or iodine takes place.
In cases where all the sulphuric acid evolved is consumed in bringing about the first-mentioned reactions, there is nothing left to form solvents for gold, therefore the metal remains as it was and where it was prior to the decomposition of the pyrite, and the gold prospector will find it in the gossan of the deposit. That the foregoing reactions take place in nature the following tends to prove, on certain goldfields where the rocks are covered with little or no surface soil, it may be noticed that long lines of brown iron ore (hydrous oxide of iron) cross the field in the direction of the strike of the lodes, sometimes running along the outcrops, in other cases following fissures or cracks on the true strike of the rocks containing the lodes; this iron oxide, in many instances, may be traced to quartz gold veins, and is a product of decomposing pyrites. Careful examination of the lodes or veins reveals that on and near the surface iron oxide is found in hard compact masses, sometimes botryoidal or stalactitic; this as far as it extends is called the cap of the lode. Below the cap, the quartz lode proper is met, and on examination it will be found that the joints and surfaces are stained and coated with compact iron oxide, which also fills some of the cavities in the stone. Deeper down the lode, the vascular rock, highly charged with iron oxide and known as gossan, is found in the joints and cavities, in a spongy and friable form; it is the valuable mineral