History of West Australia/Appendix 1
THE INTERIOR GOLD REGION OF WESTERN AUSTRALIA.
By S. GÖCZEL,
Mining Engineer and Metallurgist.
I.—SITUATION AND EXTENT.
Not far inland from the western seashore of Australia, the great West Australian tableland rises, more or less abruptly, to elevations of over 1,000 feet. In the south-western portion of the colony this tableland commences with the Darling Ranges. The latter run almost parallel with the western coast line, from the Southern Ocean to Geraldton, and the plateau itself extends from north to south throughout the continent.
Proceeding from the coast eastwards, in the gneissic-granite of the Darling Ranges, archæan strata are encountered for the first time. The eastern slopes of those ranges sink into a number of erosive valleys, where, favoured by fertile soil and a copious rainfall, the principal agricultural settlements of the colony are thriving. Beyond these valleys the country is undulating. The eastern extent of the plateau approaches in the southern latitudes the 123rd meridian; it passes this meridian in the middle, and goes considerably beyond it; then it again recedes westwards in the northern latitudes.
The basis and nucleus of this extensive elevation consists of archæan gneissic granite.
From this nucleus coastwards, and more or less parallel with the coast-lines, belts of palæozoic, mesozoic, and tertiary sedimentary strata have found a greater or lesser development.
The recent coast features are chiefly estuary deposits and sand-dunes.
The eastern declines of the tableland sink gradually into desolate sandy depressions, in which mesozoic strata rise to the surface.
The level of those depressions is sometimes only 700 feet above sea.
The interior gold region of Western Australia comprises the central and eastern portions of this tableland.
The presence and great development of palaeozoic sedimentary rocks in the northern and north-western portion of this plateau, and the insignificant occurrence of such rocks in the interior gold region, form conditions for a natural and traceable boundary of that region north and north-westwards.
That portion of the plateau which is situated to the west and south-west of the interior gold region—although to a great extent of physical and geological similarity to it—shows sufficient distinction for the demarcation of the boundaries between the two.
In the western and southern portions of the plateau the archæan strata have been subjected to a general fracturing and folding process, in a similar manner as within the gold region; but the palæozoic greenstones and igneous rocks in general are of limited occurrence.
Contrary to this, the archæan strata within the gold region are traversed by gigantic divisional fractures, running more or less parallel in a north-westerly direction. Greenstone ranges and hills extend along these fractures, and the occurrence of gold also follows their course.
Eastwards, the previously indicated termination of the tableland coincides with the eastern boundary of the interior gold region, which, accordingly, would occupy an approximate area of 120,000 to 140,000 English square miles.
The physiographical designations adopted in Western Australia do not correspond with the usual nomenclature; thus the term mountains is very often applied to hill ranges of no great extent and elevation. Most of the so-called rivers are a succession of waterholes, contained in flood channels of great length, and the so-called lakes are saline marshy depressions, with an occasional shallow sheet of water. During dry seasons they usually appear as arid flats, covered by glittering sheets of salt efflorescences.
Elevations topped by extensive undulating sand plains, granite hills covered by wool-bag-shaped boulders, steep gneissic and granitic cliffs, and huge dome-shaped and rounded granite rocks, are features of the archæan strata.
Within the gneissic-granitic areas, erosion has often imparted a precipitous character to ravines, intervening between alternating plain levels. Most of those ravines are old lacustrine strands, and in the northern portions of the gold region their height often reaches 50 feet, and sometimes more. Their gradual decrease into hardly perceptible rises of the country is frequent.
The table tops, as locally named, are remnants of denuded portions of high plains; they are flat-topped elevations, bordered all round by more or less precipitous descents. Such ravines and granite outcrops often follow fissures and faults.
Proceeding from the north, in a southerly direction, the borders of archæn elevations assume successively more and more the forms of gentle slopes, which occasionally are interrupted by massive granite outcrops, these being generally the more resistive portions of the archæan strata along spices of monoclinal folds.
During the gradual recession of the palæozoic ocean towards the south and south-east, the waves of that shallow ocean had a longer time in which to obliterate the unevennesses of the archæan rocks situated in the southern and south-eastern portion of the gold region, than they had in the northern.
Comparatively low and rugged ranges and hills, consisting chiefly of altered schists and of massive and schistose greenstones follow the more or less irregular courses of divisional fractures in the archæan lythosphere.
Within depressions, following also chiefly divisional breaks, occur numerous saline flats and so-called salt lakes, some of which occupy hundreds of square miles.
As this region has hardly any drainage towards the sea, those saline depressions receive the bulk of the meteoric water, which descend within their respective water-sheds. The collected water forms generally extensive shallow sheets, and during dry seasons the largest part, if not the whole of it, disappears through evaporation and absorption.
The strata forming the bottom of such salt lakes are usually permeable for water, and the water level recedes, in some cases to a considerable depth underground, leaving behind crusts of salt efflorescences which cover the bottom of the lake basins.
The collected water reaches those lakes partly in flood channels, and partly in subterranean conduits, and by percolation. In some of the smaller lacustral basins a new supply of rainwater remains potable for some time.
The saltness of the lacustrine depressions, and also the saltness of the bulk of the subterranean waters in this region, is attributable to some extent to oceanic leavings, but chiefly to the salt formed by the decomposition of the rocks, and especially the salt liberated by the disintegration of rock-forming minerals. In nearly all the quartz, feldspar, hornblende and augite-crystals of the igneous rocks, visicular cavities, containing salt solutions, or even common salt in cubic crystal form, occur.
The lacustrine depressions are usually surrounded by argillaceous reddish-brown coloured löess flats. Their soil is of æolian origin, and they occupy a very large portion of the interior gold region.
The archæan elevations, as well as greenstone hills and ranges, rise above those löess flats, and have supplied, and still supply, the material of which the löess strata are built.
The elevations, consisting of solid rocks, are usually surrounded and often covered by strata of accumulative decomposition, the material of which strata consists of the unremoved detritus derived from the decomposition of those rocks.
Most of the surface formations are neogene, of subærial, fluviatile, lacustrine, and æolian origin.
In depressions, neogene features probably overlay, comformably, lacustral and fluviatile formations of the mesozoic era.
As there exists hardly any drainage towards the sea, the meteoric water-supply is fully balanced by evaporation, and also by absorption in the continuous process of mineral alteration.
The annual rainfall is not a large one (about nine inches), and the surface strata are very porous. There are no running rivers and rivulets. Creeks and gullies, eroded in declines of ranges and elevations, are short, and all traces of them are soon lost in sandy flats. Their beds are mostly dry, but in some cases water may be found for some considerable time after a rainfall in intervening deepenings. Such water reservoirs occur more frequently in the northern parts.
Massive granite outcrops contain water collecting and retaining basins and rock holes, which have gradually weathered out in places where the granite had less resistibility against decomposition. Such rock holes are locally called "namma holes."
"Native wells and native soaks" are usually situated at the base of outcropping granite rocks, and roughly deepened into rock fissures. Water collected in such fissures, and in higher situated detritus standing in communication with them, will drain into these wells and soaks until the subterranean supply down to the level of the well-bottom becomes exhausted.
Fresh-water pools into which the rain-water of an adjacent area is collected, and which in consequence of the imperviousness of the clayish beds is retained for some time, are locally termed "clay pans."
Elevated sand plains, extensive löess flats, saline depressions, and salt lakes—the latter often bordered by sand-dunes—succeed one another with tiresome monotony throughout this region; a monotony which is only scantily relieved by low and rugged greenstone ranges and by a few larger granite outcrops.
Eucalyptus forests and mulga bush cover the largest portion of the gold region. The former predominate south, the latter north, of the 30th degree S. Lat. Elevated sand plains and sand drifts bear usually a stunted scrub vegetation, and are always covered with prickly spinifex.
The occurrence of nutrious grasses in the southern portion is confined chiefly to the soil which surrounds the outcropping granite rocks (which are also the areas of rock holes and soaks), and also to the bases of greenstone hills and ranges.
The vegetation of saline depressions and salt-lake borders consists chiefly of dense pale-green and reddish-green salt bush scrubs, which seldom reach a height of more than three feet. Several species of the dreaded Western Australian poison plants (gastrolobium and oxylobium) occur on the high sand plains and in the surroundings of granite outcrops.
III.—ROCKS OF THE INTERIOR GOLD REGION.
Archæn Gneissic-Granites, although mostly covered by subaërial formations, are the most predominant rocks within the interior gold region. Their outcrops appear sometimes as genuine gneiss, with a more or less developed parallel arrangement of their mica-scales, and also as huge, often dome-shaped, massive granite rocks. In some places, especially where fractures and dislocations have occurred, the surface decomposition has produced the well-known wool-bag-shaped blocks. The mica of these rocks is usually biotite (magnesia mica) of a dark green or black colour. Sienitic features, in which the mica, or portion of it, is replaced by hornblende, occasionally occur. Various gradations between micaschist and typical gneiss, and between the latter and massive granite, are frequently met with.
The gneisso-granitic rocks of the interior gold region belong to the oldest strata of the geologically known earth-crust.
Palæozoic Greenstones.—These rocks are highly crystalline, partially of schistose and partially of massive texture. They form mountains and hill ranges.
Amongst the schistose greenstones the feldspar-amphibolites, or diorite schists, are of most general occurrence. Occasionally they overlay the gneissic-granite directly; sometimes beds and seams of quartzite of sienitic gneiss, or of hornblendic schist, appear interpolated. The diorite schists are found both coarse and fine grained, sometimes almost perfectly stratified, in other cases breaking up in cubes, thin or massive slabs, and even in stalk-like pieces. The constituting minerals of this rock are plagioclase or triclinic feldspar and hornblende.
From the surface down to a considerable depth the diorite schists are usually altered, their feldspar being more or less kaolinised, and their hornblende transformed into a chloritic mineral. The removal of lime and sodium in form of soluble carbonate by percolating waters is the cause of this alteration. The outcrops of diorite schists appear therefore often as chloritic schists. Diabase schists occur in several places.
Porphyroides, or schistose porphyroides, form dykes, breaking through diorite schist country-formation. In such instances the dyke rock has assumed the schistose texture of the country rock. There is no contact alteration observable.
Striped ferruginous ribbon jaspers form sometimes precipitous declines of hill ranges. They generally indicate lines of fissures and dislocations. These jaspers are the result of a hydrothermal alteration. They usually adjoin the dyke fissures. The overheated waters forming part of the igneous dyke-magma have forced their way into the adjoining schistose country rock. Less resistance was offered along cleavage planes. By the introduction of silica, and partly re-crystallisation of the original rock material, the mineralogical constitution of the original rock was completely altered, and the schistosity maintained, if not more accentuated. In their crosscuts such jaspers show stripes of different colours, principally reddish-brown, grey, green, yellow and white. This metamorphosis does not extend far from the fracture, and in several instances the successive transition from ribbon jasper into dioritic or chloritic schist can be observed.
All the massive greenstones of the interior gold region belong either to the diorite or diabase group. Felsite porphyries are the youngest igneous rocks of this region, and are considered here conjointly with the greenstones.
Diorite and diabase, in connection with their corresponding schistose features, are the principal material of which nearly all the greenstone ranges and hills of the interior gold region consist. Porphyrites occur in dykes. They also form extensive banks, and low massive complexes of rounded hills. Felsite porphyry also forms dykes and banks.
Tuffs.—Beds of greenstone and felsite tuffs, and also calcareous and dolomitic tuffs, are abundant and extensive. The greenstone tuffs form banks and layers of dense, and occasionally schistose, rock material, within which larger greenstone fragments are embedded. They are usually of a dirty greenish colour. The felsite tuffs consist of a sandy and ash-like material, in which spherulites, from the size of a pea to that of a hen's egg, are embedded. Cross-sections of such spherulites present successive spheres, which are less decomposed towards the centre. The central part in larger pieces consists mostly of felsite porphyry, and occasionally of a dark porphyritic rock. In some instances amphibolite, diorite, hornstone, quartz, feldspar, and hematite, in fragments of half-an-inch to an inch diameter, are completely enclosed in the centre of such sperulites. These tuffs have in all likelihood a similar origin to that of the volcanic tuffs of the present day. They were either hot mud ejections or scattered lava, ejected by means of steam and gas explosions which took place in the channels below the vents. The ejected material was scattered in form of volcanic ashes, sands, lapilli, and sperulites. The rock and mineral fragments, enclosed in the above-mentioned sperulites, were unfused particles in the ejected lava mass. Some of the tuff-beds show that they have originated as hot mud emanations; others, again, are in all probability thermal deposits. Calcareous and dolomitic tuff-beds were formed, and are formed even in recent times, within the great lacustral depressions. Their material is chiefly derived from concentrated mineral solutions. These solutions were meteoric waters, which have entered and passed through the silicate rocks. On their journey they have performed their work of rock decomposition, and have charged themselves with soluble mineral matter. When these mineral solutions have entered into the highly concentrated salt water of lacustral depressions, most of their lime and also magnesia became precipitated as carbonates. Such precipitates, intermixed with pulverulent gypsum, common salt, and detrital matter transported and deposited by wind, have formed extensive tuff-beds within great lacustral depressions.
The tuff-beds were formed by the aid of water. Their material is in most cases partially decomposed, and some of them are stratified.
Sandstones and Contact Conglomerates.—In the northern parts of the interior gold region, gneisso-granitic elevations are sometimes overlaid by beds and cappings of ferruginous grits, sandstones and conglomerates, which hardly ever attain any considerable thickness. Similar conglomerates occur as contact formations between gneissic granite and overlaying diorite cappings. Those strata are bare of palæontological proof. Probably they are representatives of the Cambrian section.
Neogene sandstone-beds occur also within the northern portion of the gold region. Occasional rainfalls assume here already a more tropical character, and probably succeeding accumulations and evaporations of surface water have been the means of cementing loose sand deposits. Those sandstone-beds are more or less water-bearing, and therefore of importance in regard to water supply.
Argillaceous Sand covers archæan rocks as surface stratum. It is generally the result of their accumulative decomposition.
Ferruginous Clay is formed in situ by accumulative decomposition of palæozoic greenstones.
Stony Deserts are formed in situ by surface accumulation of more resistive and bulky rock fragments during hydraulic or æolian denudation.
Löess.—Fine grained, argillaceous, mostly reddish-brown coloured strata, which cover the largest portions of the flat, trough-shaped depressions within this undulating region. The bulk of their material is derived from elevations where æolian denudation is taking place. Sand and dust resulting from the general decomposition of rocks is taken possession of by winds. Under more favourable conditions, the former soon settles, forming stretches of so-called sand plains. The finer dust is carried to a much greater distance till it is retained between the roots of the vegetation, principally when there are rainfalls.
The material of which the löess strata are built was subjected in this manner to a natural dry-dressing process. These strata are therefore usually even-grained, and the grains are always sharp-cornered. In this place it may be mentioned that by careful observation of the löess surface it is possible for the traveller to determine the direction in which the nearest outcrop of granite is situated, and from the quality and size of the sand grains conclusions as to its probable distance can be arrived at.
During my early travels in this region this knowledge was of great use to me in the search for water; namma holes and native soaks being confined to outcropping granite rocks.
Sand-Dunes are of frequent occurrence along the shores of salt lakes.
Banks of Pulverulent Gypsum are frequently met lacustrine features.
Saline Clay-Beds are lacustrine formations, of which the last series can be considered as lacustrine alluvium.
Fluviatile Alluvium is limited to banks of watercourses, and to sandy flats in which creeks disappear.
Ironstone Beds and Ironstone Gravel are secondary formations derived from the decomposition of greenstones, and accumulated on the surface during an æolian denudation.
1. The Archæan Strata.—In the geological structure of the West Australian plateau, the gneissic granites appear as oblong blocks of huge dimensions, some of them showing widths of twenty and more miles.
The archæan earth-crust, consisting of the above-named rocks, was broken through by numerous fissures of a more or less west of northerly course, and the crust blocks produced in that manner have gradually adjusted themselves into their present positions.
The West Australian tableland must have originated as an arch-shaped bulging of the archæan earth-crust. Lateral pressure (explained in geology as a result of secular cooling and contraction of the earth-spheroid) may have caused that effect by acting in an east-west direction.
At a later period this archæan arch gave way, separating during its downbreak into the above-described crust-blocks.
On the tableland surface, the archæan strata appear as oblong elevations, tending longitudinally towards west of north. Those elevations enclose the previously described flat, trough-shaped valleys, and their rise above the lacustrine and löess strata is either gradual or it takes place abruptly, forming rocky ravines and cliffs.
The stratification of the archæan rocks on the plateau surface is either horizontal, undulating, or slightly inclined, and accordingly they form elevated plains, or they indicate monoclinal folds.
2. The Palæozoic Greenstones occur in several distinct belts of country, of more or less parallelism, west of northerly longitudinal extent. These belts are hundreds of miles in length, and follow the courses of gigantic fractures in the archæan strata. Each of these fractures consists of many fissures or systems of fissures, through which the material of the palæozoic greenstones has risen in a state of aqueo-igneous or hydrothermal fusion, and within which fissures it became solidified by successive cooling. The fracturing of the archæan earth-crust, as well as the magma emissions, were apparently due to lateral pressure.
The palæozoic greenstones within the interior gold region occur in the following forms:—
(a) Schistose diorites, bordered on the east and west by horizontal or inclined strata of gneissic granite. They generally occupy enormous-sized fissures and spaces of V-shaped cross-section within those strata. Where shafts have entered those schistose diorites to depths of 200 and 300 feet, it has generally been found that the schistosity gives place to a massive and jointed texture. Along contacts with gneissic granites, those diorites enter in form of apophyses and wedge-shaped dykes, the former rocks, and they also enclose some of their detached portions.
(b) Dioritlc, diabasic, porhyritic, and also felsitic dykes occur within the schistose diorites, and also within the archæan strata. They are derived apparently from the same magma reservoir as the diorites in which they occur. In several instances the dyke has received the same schistose texture as the country rock.
(c) Coarse crystalline massive greenstones, forming rounded hills and hill-complexes, which point out strongly that they are igneous material, solidified by gradual and slow cooling within the interior of the earth-crust. They are solidified magma reservoirs exposed by denudation.
(d) Massive diorite banks and cappings, overlaying archæan strata. In cases of direct contact between such diorite banks and granite, the diorite assumes aphanitic, and the granite granophyric texture.
(e) Successions of massive and schistose diorite strata in the structure of hill and mountain ranges. The massive strata increase in number and size on relative higher levels.
The large fractures and fissures within the archæan strata have served as vents for enormous submarine emissions of aqueo-igneous magma, which became deposited on the bottom of a palæozoic ocean. The difference of temperature at different times and different places in the ocean, as well as the occasional quantity of emitted magma, gave cause to a faster or slower cooling, and, accordingly, to a fine or coarse crystalline texture.
Spaces of V-shaped cross-section, formed by the tilting of archæan crust-blocks along fissures, have received the first emissions of aqueo-igneous magma which rose through those fissures. Those spaces have also preserved from denudation large portions of the rocks, into which the magma became solidified. The process of solidification took place in a centripetal direction, and during its progress the solid magma-crusts were subjected to the continuously acting lateral pressure. A general schistosity and numerous fissures and faults within that rock-crust were natural results of that force. The lower magma portions, remaining for long periods in a state of aqueo-igneous fusion, were forced by the same pressure through newly created fissures upwards.
The schistose diorites within the interior gold region have apparently originated as such solidified magma-crusts, and the numerous dykes which they enclose are vents of the rising magma, which ultimately became solidified within, and forms now the present dyke rocks. Thus lines of probably submarine volcanoes have originated along distinct lines of fracture in the archæan earthcrust. During undeterminable periods denudation has destroyed the bulk of eruptive features, which were formed in a like manner; but still, to a careful observation, many of the remnant portions disclose an undeniable similarity to recent volcanic structures, in form as well as in material.
The ejection of huge masses of eruptive material has necessarily left large cavities underneath; subsequent fractures and subsidences of rock-crusts which have formed the roofs of such cavities, have caused depressions. Within some of the great salt lake areas series of such downbreaks are even now recognisable.
The differences between elevations and depressions must have been once very considerable in this region. Denudation, lasting through geological ages, has reduced the probably once gigantic elevations to the size of mere hills and hill ranges, and the denuded material was made use of for the gradual filling in of great depressions.
In recent times this work is performed chiefly by subaërial, æolian, and chemical agencies, and it is very probable that during the mesozoic era hydraulic agencies have chiefly served the same end.
The depressions within the hydrographic basins of the interior gold region are generally occupied by marshy salt flats or so-called salt lakes. The latter are areas of surface collection and evaporation. The water influx into those lacustrine basins is chiefly subterranean, although during heavy thunderstorms a few flood channels may also contribute considerably. The salt accumulations within these areas are due to an excessive evaporation. During the dry seasons, almost all water within the salt lakes disappears, and the damp and muddy lacustrine beds become covered with hard, sandy salt crusts and salt efflorescences. In the vicinity of flat shores, such sandy salt crusts usually cover a permeable pulverulent stratum of lacustrine sediment. Here the subterranean water influx can be observed after a heavy rainfall. This influx causes a pressure on the sandy, salt crust from below, and forces it into calotte-shaped elevations, sometimes above an already accumulated shallow sheet of water. Finally, the tops of those elevations burst, and water flows out of the crevices. Part of the salt within the lacustral basins pursues a circular current. In dry seasons salt dust is lifted from the 'dried-up lake bottom by the winds and deposited on higher levels. Then, during wet periods, it is re-dissolved and returned to the lake by rain. The extension of a saline vegetation, far beyond the borders of a lake, and above its level, is due to the salt being transported and deposited in the above-described manner.
The annual average rainfall for this region has not yet been sufficiently ascertained, but it scarcely will exceed nine inches. Of this, a large portion falls in brief showers, which hardly soak a few inches of the loose, sandy surface soil. The result is that it becomes partly absorbed by the vegetation, and partly re-evaporated during the next few days. Only heavy thunderstorms contribute towards subterranean storage.
The subterranean waters within this region are either percolating or resting. In both those conditions they are found either potable or mineralised and salt.
Water under artesian pressure is here non-existent, the reasons for this being as follow:—
(a) There are no interchanging impermeable and permeable strata, of which one of the latter is confined by two of the former, and all of which are regularly dipping under a low, flat country.
(b) There is no elevated water-collecting area within the interior gold region which could sufficiently supply a subterranean stream, even if the strata necessary for its reception and conduct would be in existence, which they are not.
(c) The numerous and extensive outcrops of the archæan gneissic granites exclude absolutely every supposition that regular stratified rocks could reach this region from outside.
The only spaces where subterranean water, under some slight hydrostatic pressure, could occur, are the monoclinal folds within the archæan rocks. Some of those folds may contain stratified beds, which, if they exist, would be like the mesozoic strata near the south coast, almost horizontal. Water is certain to have collected within the permeable rocks which fill the spaces of V-shaped cross-section, in the archæan monoclinal folds; but as the hydrographic basins enclosing such folds (like the rest of this region), have hardly any drainage towards the sea, such water, if tapped, will be more or less salt.
There are neither running creeks nor springs within this region and even water accumulations in flood channels during occasional thunderstorms generally disappear underground as soon as they enter certain sandy flats.
A certain part of the rain precipitations, after soaking through the porous, sandy surface soils, enters the pores, joints, and fissures of the underlaying rocks.
The rain-water, passing through the atmosphere, becomes charged with carbonic acid, and being provided with that powerful agent, is enabled to perform far-reaching alterations within the percolated rocks.
The upper strata of the gneissic granites have generally suffered such alteration; they are decomposed. This decomposition reaches various depths, and often consists in a complete kaolinisation of the feldspars, and in the more or less advanced change of the mica (biotite) into a pale, talc-like hydrous mineral, the quartz grains being hardly affected.
The greenstones are subjected to a similar alteration, in which the feldspars become also kaolinised, and the other component minerals, hornblende or augite, transformed into varieties of hydrous minerals.
In very large portions of the decomposed greenstones within this region, hornblende and augite are altered into chlorite and delossite; therefore, the original rocks into chloritic rocks.
With the kaolinisation of the feldspars in the upper strata of the archæan rocks, and the alteration of the feldspar amphibolites (diorite schists) into chloritic schists, easily soluble mineral constituents of the country were replaced by more resistive ones, and the rocks, after alteration or decomposition, became far more permeable for water than they were in their primary state.
During the process of decomposition or alteration, large quantities of mineral matter were removed, and, as this process continues, are still continuously being removed towards and into the water-collecting depressions of the various hydrographic basins. As the rocks have retained their original texture, their storage capacity for water has necessarily largely increased. Decomposition reaches the greatest depths along fissures and lodes. In portions of country not traversed by fissures and lodes, decomposition does not reach great depths, but the latter generally increases in approaching the next lode or fissure.
The subterranean physiography of the surface of the un-decomposed and still solid country rock prescribes the movements of the percolating waters until they reach a level of complete saturation, and from thence the outfall towards the collecting depression of the hydrographic basin decides their further movements.
The rain-water entering such decomposed porous rocks, soaks centripetally downwards till the underlaying, solid, impermeable rock, or a plane of complete saturation within the former, is reached.
Various degrees of resistibility against decomposition, as well as number and size of joints and fissures, chiefly influence the subterranean physiography of the still solid rocks.
Supposing the decomposed rocks were removed, the so exposed surfaces of the still solid impermeable rocks would present elevations and depressions (ridges, flat-topped elevations, channels, and basins). The percolation towards the collecting basins will necessarily follow such depressions. Conduits may spread over a wide area of the subterannean solid rock, or they may be confined to narrow channels. Basin-form reservoirs are also certain to occur.
In the schistose greenstones the decomposition reaches generally greater depths than in the gneissic granites. The latter form more massive elevations, and are less fractured. Water tapped within the areas of those rocks is therefore generally less salt, and in higher portions even potable.
The general rest level of the subterranean waters within the principal hydrographic basins does not greatly vary, but gradual rises from south-east towards north-west are apparent.
A general subterranean drainage, if existent, would tend accordingly south-eastwards.
The heights above sea of most of the great salt lakes, as Lake Cowan, Lake Lefroy, Lake Barlee, Lake Deborah, Lake Austin, and that of the lakes to the east of Lake Barlee are between 900 and 1,100 feet. Lake Cowan approaches most the former, and Lake Austin the latter height.
The decomposition, and with it the permeability of the gneissic rocks, extends locally down to depths of 200 and more feet from the surface. Within the schistose greenstones it is more general and often attains depths of more than 300 feet.
The subterranean conduits extend chiefly along lodes and fissures The water struck in many of the numerous mining shafts is percolating, and the seeming water level may be many feet above the general rest level of the hydrostatic basin, within which such shafts are sunk.
Some of the subterranean reservoirs contain tolerably potable water. Generally, currents of percolating waters from higher levels pass these reservoirs on their journey towards the general collecting basin. The permeable rocks within the reservoirs are already leached, and the water which drains into them is still fresh. Such reservoirs are numerous within the interior gold region, although they are not always to be found in the immediate vicinity of mining camps.
(a) Primary Gold Deposits.
The gold deposits within the interior gold region occur in stretches of country, the longitudinal axes of which strike in various directions, but in travelling over this region the prevalence of a general north-westerly tend can hardly remain unobserved.
It is not a matter of accident, but of cause and effect, that most of the gold deposits are situated in the vicinity of saline depressions and so-called salt lakes; they usually occur along lines of fracture and subsidence.
The ejection of huge masses of aqueo-igneous magma to the surface has necessarily left large cavities underneath. The subsequent fracture and subsidence of portions of earth-crust, which have formed the roofs of such cavities, have caused the depressions which appear now as saline flats and salt lakes.
The lines of fracture present generally systems of faults and fissures, along which one portion of the broken country has remained in a relatively higher position, whereas the other has subsided.
Gneissic granites rise occasionally to the surface forming contacts with palæozic greenstones; they are generally part of the relative higher portion of country along the line of fracture.
Some of the fissures have served as vents for igneous magma, which ultimately became solidified within. They now form dykes; others were filled with quartz simply by secretion; others again became vents for solfatara solutions, which have risen under steam and gas pressure. Those solutions have filled their fissures chiefly with quartz, with auriferous quartz and with metalliferous minerals, which usually are associated with gold.
During subsequent movements in the earth-crust, complete quartz lodes, as well as dykes, were broken through, and sometimes dislocated by newly-formed fissures, and in some instances the original lode or dyke-fissure was partly reopened.
Fractured portions of such broken reefs and dykes gave easy access to circulating mineral solutions. If those solutions were derived from deep-seated solfatara action, and were auriferous, these conditions favoured the formation of rich gold shoots and auriferous ore columns in otherwise poor or barren quartz reefs or dykes. Along contacts between gneissic granites and greenstones, odes or reefsare usually found to occur in both formations, and occasionally passing from one into the other.
We may suppose with many modern geologists, that the rocks of the archæan era are the original seat of the gold. Rivers crossing formations consisting of such rocks generally produce gold sand deposits, although no reefs or lodes occur in their vicinity; such deposits may be technically worthless, in consequence of their small yield, but they give support to the supposition, and make it appear very probable that gold occurs in a metallic state, finely distributed in the archæan rocks. In accordance with that supposition, it becomes possible to explain the extraction and transportation of gold, and also the formation of auriferous deposits by natural agencies, which have left their traces of action behind.
Archæan gneissic granites underlie the interior gold region and the material of which the palæozoic greenstones were formed has risen to the surface or to higher levels (in a state of aqueo-igneous fusion), through fissures which occurred in the former. Such magma ejections have taken place along great lengths of those fissures, and apparently were accompanied by overheated steam and gas explosions (no matter if steam and gas were originally contained in a compressed state in the pyrosphere, or were developed under access of oceanic waters, which through fissures have reached the fiery interior of the earth).
When such fissure-ejections ceased to occur frequently, the largest portions of the fissures became closed by solidified magma, and the igneous actions within this region became more like those which can be observed in recent volcanism.
Crater eruptions have succeeded fissure emanations, and after the cessation of the latter, fumarales, solfataras, and mud volcanoes were the last manifestations of igneous agencies within this portion of the earth's surface.
It appears probable that the active magma ejections have occurred during the later periods of the palæozoic era, and that commencing in the western parts of the present gold region, they have proceeded with a simultaneous recession of the palæozoic ocean in an easterly and south-easterly direction.
At the beginning of the mesozoic era a general elevation of this region above sea was completed, and the formation of most of the primary gold deposits has probably taken place during the early periods of that era.
Doubtlessly the formation of lodes commenced with the first fracture of the archæan lythosphere, but the formation of most of the primary gold deposits within this region is due to hydrothermal gold emanations. The latter have succeeded the greenstone eruptions for which the present dyke fissures have served as vents.
The effects of solfatara action on rocks can be observed in volcanic regions, and it is obvious that such action must be much more intense when deep-seated. Its highest degree is hydrothermal fusion.
To such deep-seated action of various degrees immense masses of archæan strata must have been subjected along the great divisional fractures of the interior gold region. Gases under pressure as well as overheated steam and water could easily pass through rocks in such a state. Soluble chemical combinations became dissolved, the waters became charged with them; and forced by their own steam and gas pressure those solutions entered upon their journey towards the surface.
If gold existed in archæan rocks which have suffered such subterranean solfatara leaching, it could enter into solution.
Alkaline sulphides, ferri-sulphate, solutions of sodium carbonate and sodium silicate, are solvents of gold in a higher or lesser degree.
Solfatara solutions rising out of silicate rocks generally contain alkaline carbonates and silica. Alkaline sulphides, in which metallic sulphides are slightly soluble, are often found to occur in conjunction with them. Probably the gold has entered such solutions as gold sulphide, and in some cases as telluride.
The solfatara solutions have risen to the surface through fissures, but certain channels within the fissures, where there was least resistance, have served as principal vents.
During their upward journey the solfatara solutions became gradually relieved of their steam and gas pressure, and they also were subject to gradual cooling. In consequence thereof, the less soluble silica was deposited within the fissures along with sulphides (chiefly iron sulphides) and metallic gold. Sulphide of gold, being a very unstable chemical compound, became dissociated at the moment of its precipitation, and metallic gold was deposited either as minute particles enclosed in sulphide of iron (pyrites) or as gold free in quartz.
Telluride of gold has probably played a similar role as sulphide, but being a stable chemical compound became deposited as such.
All the primary auriferous gold deposits within the interior gold region are more or less connected with fissure eruptions. Dyke fissures, extending sometimes for miles, have served as eruption channels, until the rising magma within became solidified by successive cooling. The igneous rock within such dyke fissures appears occasionally interrupted, and its place is taken by quartz. So-called quartz blows generally disclose such occurrences. Magma has not entered those portions of the fissures, and the open spaces became successively filled either by lateral secretion or by deposits from circulating solutions. If such solutions were auriferous gold deposits were formed.
Quartz lodes or reefs formed within such empty spaces of dyke fissures, where igneous magma did not enter, generally contain very low-grade ore; still phenomenally rich gold shoots have been found within some of those lodes or within their branches.
The country formations through which the dykes have broken are chiefly dioritic and chloritic schists, but along contacts of such schists with archæan rocks they occasionally enter the latter also.
In some instances portions of dyke rock (diorite, diabase, porphyrite, and felsite-porphyrite) are traversed by numbers of more or less irregular auriferous quartz veins. The fissures which have served for the formation of the latter are generally contraction fissures, due to the cooling of the dyke rock. Solfatara solutions rising along the dyke walls have permeated the dyke rock within such fractured portions, and have deposited auriferous quartz and auriferous pyrites.
A complete kaolinisation of such dyke portions is of frequent occurrence; it generally affects the adjoining country formation, and reaches down to depths of 200 and more feet.
Such complete kaolinous decomposition is generally confined to true fissures and to the vicinity of such. This kaolinisation is apparently a sequence to the cessation of activity within hydrothermal vents.
The general or surface alteration of the palæozoic greenstones has chiefly produced chloritic features.
Portions of dykes containing auriferous quartz veins can be considered as compound ore deposits, inasmuch as lodes of igneous origin (the dykes) contain within their body numerous veins of aquæous origin. In many instances the dyke rock, and in some cases the adjoining country formation, have been impregnated with gold and iron pyrites. The latter impregnate the rocks in form of countless small hexahedrons; in decomposed portions such hexahedrons are hematite pseudomorph after iron pyrites.
In some cases of similar occurrences not only the smaller auriferous veins within the dyke rock, but also the latter, and perhaps even some of the adjoining country rock, will constitute payable gold ore.
Eruption fissures and faults are generally accompanied by numbers of parallel, lateral, and branch fissures, which gave occasion for the formation of auriferous lodes and quartz reefs.
The various occurrences of auriferous ore in reefs are as follow: The auriferous quartz in some of the reefs forms a continuous body, the width of which does not vary considerably; cross-fissures and faults, as a rule, cause increased richness in gold.
The auriferous quartz appears in form of lenticular bodies along a fissure. In the intervening spaces the fissure narrows greatly, and contains only fractured country rock. In such cases the edges of the lenticular quartz bodies have been found to be of greater richness. The dip of such quartz bodies may follow the underlay of the lode fissure, or else it may be inclined along the course of the lode.
The so-called formation lodes generally occur in schistose country formation, and their course follows more or less the strike of the schistosity. They are almost confined to the vicinity of eruption fissures, and contain in their decomposed levels, besides chloritic talcose and serpentinous gangue, loose and porous gossan and silicious sinter. The fissures of the formation lodes are not clean open fissures, but rather plains along which rock movements have taken place. Solutions under steam and gas pressure rising through such fracture planes have altered the country rock for a certain distance; they have caused its ore impregnation, and their deposits in conjunction with crushed country rock have formed lode breccia. The country alteration and ore impregnation, even in the absence of lode breccia, retains a true lode appearance. Several such formation lodes have been opened up in the southern portion of the Kalgoorlie lode system, and their size and richness in gold have attracted considerable attention. The gold in the upper levels of the Kalgoorlie formation lodes occurs almost entirely as free gold, but in two allotropic modifications, namely, as the usual yellow gold with metallic lustre, and also as amorphous gold of brown, red, and purple colour. Samples of the latter assume by heating and also by rubbing in the agate mortar the usual gold colour. This modification is more resistive against amalgamation, but dissolves easily in chlorine and cyanide of potassium. In some of those lodes at lower levels very rich tellurides of gold are found. The 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.