tellurium minerals from that locality approach nearest the calaverite and so-called yellow ore of Nagyag; they usually contain selenium. A dark coloured telluride, closely resembling nagyagite is also found. Those tellurides occur in small veins within the formation lodes, and also as impregnations.
The free gold in the upper levels of lodes and reefs was originally only partly deposited as such. A large percentage of it only became liberated after the decomposition of sulphides, and in some few cases of tellurides. The decomposition of auriferous pyrites under meteoric agencies within a lode generally causes a natural gold concentration. The auriferous pyrites by gradual access of air and surface water are chemically affected. During the succeeding process ferro-sulphates (Fe. SO4) and free sulphuric acid are formed, and sulphuretted hydrogen evolved in the meantime. Ferro-sulphate under presence of air oxidises into ferri-sulphate, Fe.3 (SO4) 3. This chemical compound: according to Le Conte and others, dissolves gold slightly, whereas ferro-sulphate precipitates this metal. The remnant of the pyrites, after this decomposition, is an insoluble hydrated peroxyde of iron, which generally imparts to the lodes a ferruginous character. In consequence of the removal of the pyrites the quartz or vein-stone is left in a honeycomb condition. By this process gold became liberated from the inclosing pyrites, and it is most probable that the same reactions have caused a natural gold concentration within certain portions of the lodes. The decomposition of lodes within the interior gold region often attains depths of 200 feet and more.
(b) Cement Deposits.
In various localities of the interior gold region, and usually in close vicinity to auriferous lodes and reefs, certain more or less horizontal beds have been found to contain gold. The material of such beds, even in one deposit, is often found to vary. In some instances it is more or less of felsitic and dolomitic nature, with a considerable admixture of ferruginous clay; in others it is sandstone, with transits into conglomerates. Silicious sinter, silicious ferro-siderite, and travertine are occasionally met with.
The absence of water-worn gravel within such beds excludes a pure and simple alluvial origin, and so does the gold found within those beds. The latter shows crystalline development on some of the larger grains, and the minute leaves and threads resemble vein gold far more than water-worn drift gold.
Small rounded and smoothened quartz pebbles of the size of beans are found embedded in the so-called cement, together with large sharp-cornered fragments of vein-stone and country rock.
In gneissic granite country the rock of such beds is generally sandstone and the cementing matrix of those sandstones is a crypto-crystalline silica, which frequently becomes ferruginous and sometimes felsitic.
In greensone country such beds are generally tuffaceous.
The so-called cement beds are in cases infiltration deposits, in others surface accumulations derived from disintegrated auriferous deposits, which accumulations were ultimately united into auriferous breccia, and they occur also as tuffaceous and silicious beds, in which the material of the beds, as well as the gold contained in them, are precipitates of one and the same mineral solution.
The small rounded quartz pebbles are similar to those which are found in bubbling springs, and they point decidedly to a thermal emanation of the auriferous solutions. In some instances auriferous cement deposits are found in close vicinity to outcropping gold-bearing lodes or reefs. The generic connection in some of those cases is recognisable, and discloses the fact that the overflows of auriferous solutions have formed surface gold deposits, the origin of which is similar to that of genuine lodes.
(c) Secondary Gold Deposits.
Some of the secondary gold deposits within the interior gold region are found to occur in lacustrine alluvium, and others again owe their origin to æolian surface accumulation but in any case they are never far from lodes or reefs, in which their gold was primarily contained.
Areas containing primary gold deposits were subject to decomposition; hydraulic denudation being of rare occurrence, accumulation of the detritus on the surface took place. In this, disintegrated portions of primary gold deposits were enclosed. (the gold being partially liberated from the lode-stone partially contained in its fragments). Æolian removal of the lighter parts of the detritus taking place, the gold, in consequence of its chemical and physical properties, remained behind, and so accumulated on the surface, together with bulky fragments of quartz and silicious ironstone. Such gold deposits are, in consequence of their origin, of a shallow depth, and even the attainment of that shallow depth is caused by occasional heavy rainfalls, which have softened the underlaying detritus so far that the gold particles sank in consequence of their weight.
The elevation of the larger portion of the interior gold region above sea was probably effected at the end of the palæozic era. Since then the same forces and agencies of nature have been active in that region, which we still observe in our time, and it is only the degree of their intensity which has since undergone changes. Erosion and hydraulic denudation must have had a by far wider field of action than at present. The greenstone hills and ranges are only insignificant remnants of once gigantic mountains and mountain chains, and it is almost certain that the bulk of their material was transported into the lacustrine flats by hydraulic agencies. At the present time æolian denudation and the formation of vast loëss beds are predominant.
It is not at all improbable that during the earlier periods of the interior gold region, secondary gold deposits, similar to those in other parts of Australia, were formed. If so, they are already covered not only by lacustrine alluvium, but also by subaërial formations.
