Quartz Reefs Research Articles

Regional setting and nature of gold mineralization in Tanzania and southwest Kenya

Abstract The most productive gold deposits in Tanzania and southwest Kenya are in the Nyanzian greenstone belts (2700 Ma) and the Ubendian System (2000 Ma). The deposits occur in the form of massive or disseminated Fe-sulphide-rich bodies in banded iron formation (BIF) or tuffs, mineralized shear zones in mafic rocks and shear-zone-controlled quartz reefs. Mineralization is mostly interpreted as epigenetic, although for some deposits a syngenetic origin is possible. There are also base-metal-rich massive sulphide deposits of probable syngenetic origin in the Nyanzian greenstone belts. Iron-rich mafic rocks are the most favourable hosts in the lower Nyanzian belts as well as in the Ubendian System, whereas BIF is the dominant host rock in the upper Nyanzian System. To date about two-thirds of the gold production has come from shear-zone-controlled mineralization, and about one-third from stratiform-stratabound deposits.

Turbidite-hosted gold mineralization at the Dolaucothi gold mines, Dyfed, Wales, United Kingdom

The Dolaucothi gold deposit occurs in thin bedded turbidites of Upper Ordovician to basal Silurian age deposited near the southern margin of the lower Paleozoic Welsh basin. The host rocks were deformed during the Caledonian orogeny (390-410 Ma) into a series of inclined asymmetrical, commonly overturned, tight southeast-facing folds with associated cleavage. Reverse dip-slip faults accompanied the folding and the whole area is cut by late tensional faults. The mineralization is controlled by structures produced during the Caledonian orogeny, in particular a thrust fault which formed the host to the Roman lode, a flat-lying auriferous quartz reef up to 6 m thick.Gold occurs in a variety of hosts including pyritic shales containing both euhedral and framboidal pyrite, quartz-carbonate stringer veins, in steeply or gently inclined veins associated with shear zones and in planar quartz veins associated with the Roman lode. The mineralogy is simple, with pyrite and arsenopyrite being the dominant sulfides. Ankeritic carbonates, hydromuscovite, and the lithium silicate, cookeite, are also present.It is proposed that, during prograde metamorphism associated with early stages of the Caledonide orogeny, fluids circulated through the basement below the southeast margin of the Welsh basin and leached gold, and associated metals, from rocks of igneous or volcanic origin. These overpressured fluids were released to high crustal levels during later orogenic movements and uplifts, perhaps related to reactivation of such basement faults as the infra-Tywi fault. Geothermometric studies indicate that the host rocks experienced upper anchizone facies metamorphism whereas the temperatures of the mineralizing fluids were in the range of 345 degrees to 450 degrees C. Isotopic studies point to a common origin for the sulfides in both the shales and the veins and also indicate that the host rocks provided the source of the carbon now incorporated in the carbonates.

Structural control on gold mineralization at Walhalla, Victoria

Gold mineralization associated with quartz reefs is related to the structural history of the Early Devonian, Walhalla Group. These reefs are situated in the Walhalla Synclinorium, developed during the Middle to Late Devonian Tabberabberan Orogeny. A pervasive north‐south‐trending axial planar cleavage and two styles of folding were produced during regional east‐west compression. The first are upright, open to close folds with sub‐horizontal fold axes. The second are plunging inclined, close to tight folds with fold axes that plunge steeply to the north and south. An extensional event is associated with the emplacement of the Woods Point Dyke swarm and a set of normal faults that offset all earlier structures. High‐angle reverse faults, which post‐date the folding and the emplacement of the dykes, were utilized as conduits for hydrothermal fluids and preferentially localize mineralization to laminated quartz veins. En echelon vein arrays formed during initial stages of reverse faulting became deformed duri...

Hydrothermal alteration, stable isotopes, and fluid inclusions of the Golden Cross epithermal gold-silver deposit, Waihi, New Zealand

The Golden Cross epithermal gold-silver deposit is located 120 km north of presently active geothermal systems of the Taupo volcanic zone in the Waihi district, Coromandel Peninsula, New Zealand. Gold and silver of economic grades (1-20 g/metric ton equiv gold over 7 million metric tons) occur in zones dominated by quartz reefs, veins, and silicified hydrothermal breccias with adularia common in these zones. The highest precious metal concentrations occur in an inferred fluid up-flow zone. Gold and silver are thought to have precipitated from cooling hydrothermal fluid largely of meteoric origin, in direct response to hydraulic fracturing and subsequent boiling.Alteration minerals are dominated by clay, quartz, adularia, calcite, and pyrite. Hydrothermal clays show a distinct zonation with the sequence smectite, interstratified illitesmectite, illite, and chlorite corresponding to a regional increase in temperature with depth. Kaolinite overprints the above mineral assemblage to at least 400 m below the present ground surface. Quartz and adularia appear to be good indicators of high permeability in the past, whereas bladed calcite indicates local boiling. Fluid permeability was enhanced by repeated hydraulic fracturing. Alkali-chloride fluids with near-neutral pH were responsible for the dominant alteration described above, whereas the downward percolation of a near-surface acid-sulfate fluid, as hydrothermal activity waned, precipitated kaolinite + or - hematite + or - jarosite.Fluid inclusion studies reveal that in main-stage quartz, type I liquid-rich inclusions with an average T h of 200 degrees C and T (sub m ice ) of -0.5 degrees to -1.6 degrees C are dominant. Type I inclusions within late precipitated calcite show a T h decrease from 210 degrees to 150 degrees C from the center to the periphery of a large calcite body. Early (?) type Ia liquid-rich inclusions within quartz have a T h > 350 degrees C and T (sub m ice ) values of -7 degrees to -10.2 degrees C. Type II (vapor rich) and type III (two-liquid, vapor rich) inclusions within quartz with a T h 270 degrees C have T (sub m ice ) values typically from -25.4 degrees to -29.7 degrees C. Type III inclusions have not previously been described from epithermal deposits in New Zealand and possibly contain organic liquid(s).Oxygen isotopes of hydrothermal quartz exhibit distinct horizontal and vertical gradients in an east-west cross section through the deposit. The delta 18 O SMOW values from 7.0 to 11.7 per mil are inferred to represent a gradient in permeability (variable water/rock ratios) with higher fluid flowthrough in the west of the cross section, where precious metals are concentrated. Calcite delta 18 O SMOW values of 5.2 to 7.8 per mil combined with fluid inclusion homgenization temperatures are consistent with a model whereby calcite was precipitated late and over a relatively short period of time. Calculated values of delta 18 O SMOW of water for main-stage quartz and late-phase calcite indicate a meteoric source of -5 to -6 per mil for the hydrothermal water. Values for delta 34 S CDT from hydrothermal pyrite of -1.4 to +4.8 per mil reflect, in part, late acid-sulfate fluid. Alteration assemblages, inferred fluid compositions, temperature, apparent salinity, and water isotopic compositions of the Golden Cross epithermal gold-silver deposit are analogous to those for some active geothermal systems of the Taupo volcanic zone. Golden Cross may thus have been a part of a much larger geothermal system, not unlike those of the zone today.

An integrated approach for ground water investigation in part of Fatehpur Sikri of district agra-a case study

The Fatehpur Sikri area infringing the western UP, Rajasthan border was investigated for its water supply crisis in respect of poor quality and quantity as well. The study has indicated presence of two parallel quartz reefs running in NE-SW direction. Geohydrological and photogeological studies have revealed the possibility of subsurface extension of quartz reefs. However, this weathered area along reefs is promising water bearing zone. Paleo channels are also marked during studies and the area found suitable for ground water exploitation. A map showing various categories is also presented.

Delineation of potentially auriferous quartz reefs by analysis of the bark of Pinus radiata (Monterey pine)

Rocks and bark samples of Pinus radiata from Union Hill, Waihi, New Zealand were analyzed for 19 elements to establish whether the elemental content of bark could be used to delineate subterranean auriferous quartz reefs and veins. Three elemental concentrations in bark (chromium, nickel and sodium) show sufficiently great differences ( P < 0.05) between populations growing on or off the reefs (i.e. on quartz reefs or on andesites), to be diagnostic of subterranean potentially auriferous quartz. Gold in the bark not diagnostic although it was correlated with arsenic and iron. It is concluded that the technique should be applicable elsewhere in New Zealand or overseas where P. radiata is to be found growing over potentially auriferous ground.

Some observations on the geochemistry of gold mineralization in the weathered zone at Norseman, Western Australia

Abstract At Norseman Au mineralization is hosted by parallel easterly dipping quartz reefs in a westerly dipping sequence of Archaean basaltic flows and As is strongly correlated with this mineralization. Near-surface exploration is hampered by an extensive cover of deeply weathered soils and a veneer of silty calcareous material, probably of aeolian origin, which, because of its low Fe content of The Au values in the drill samples were very low and poorly correlated with the extrapolation to surface of the Mararoa shear. However, the absence of a Au halo (at a 10 ppb detection limit) may be due to the shear being barren near surface, or may be due to the samples being taken from the strongly leached zones of a laterite profile. In the area of the test traverse at the southern Mararoa area, the upper horizons of the original laterite profile have been eroded away and the remaining horizons covered by a veneer of calcareous material. There is a better correlation of the Au values of the shallow drill samples with the projection to surface of a porphyry, which hosts minor low-grade Au mineralization. The results for Au dispersion around the Mararoa shear contrast with dispersion at the nearby Hinemoa Mine where gold mineralization crops out. Channel sampling in costeans showed that an Au anomaly (threshold 40 ppb) extends 50 to 150 m into oxidized wall rocks. This well defined halo may be due to the secondary dispersion of Au from the quartz reef into the ferruginous zone of the laterite profile. Gold in the leaves and twigs of Melaleuca pauperiflora F. Muell. from the traverse at the southern end of the Mararoa shear was determined by neutron activation analysis and found to be broadly correlated with the projected Au mineralization. However, the lack of correlation with the soil Au values to 10 m may be due to the plants taking up most of their water from below the depth of sampling.

Fluid inclusion systems in metaplaya deposits and their relationships to mineralization and tectonics

Fluid inclusions have been analysed along traverses through the Upper Proterozoic Damara Orogen, South West Africa/Namibia. Two genetically different fluid systems of regional importance could be distinguished: (1) a fluid system with 25–50% gross salinity (NaClKClCaCl 2), source: continental evaporitic sequences; and (2) another fluid system with 4–16% gross salinity (NaCl), source; metamorphic fluids derived from pelitic geosynclinal sediments. Further fluid systems of local importance are bound to numerous late-tectonic granite and alaskite intrusions. Residual hypersaline solutions caused mineralization of stratiform Cu deposits and numerous giant quartz reefs. The two fluid systems have played an important role in the development of thrusts and nappes.

Geochemical and stable isotopic studies of the Crown and Mararoa reefs, Norseman, Western Australia

The quartz reefs of Archean age at Norseman, Western Australia are restricted to the flatter-dipping sections of reverse faults and the gold mineralization is mainly stratabound. Structural and petrographic data indicate that the reefs were emplaced prior to the peak of metamorphism, but gold distribution and wallrock mineralogy were modified by metamorphism and subsequent deformation events. Bucky' quartz crosscuts laminated quartz and was deposited during an episode of hydraulic fracturing. Gold and sulfides are concentrated on fractures in the laminated quartz reefs. Wallrock assemblages of the Crown and Mararoa reefs require two temporally distinct phases of alteration synchronous with the emplacement of laminated and bucky quartz reef. The first involved the deposition of hydrothermal actinolite and biotite, whereas chloritic overprinting of these assemblages characterizes the second phase of alterarion. Both potassic and chloritic assemblages were modified by the crystallization of metamorphic actinolite and tremolite. In general, single stage alteration is characterized by the addition of volatiles, K 2 0 and CaO, and the loss of Na 2 O. Changes in whole-rock geochemistry during chloritic overprinting of the potassic assemblages include losses of CaO, Al 2 0 3 , Na 2 0 and K 2 0, and a gain of MgO. The δ 18 O values of the Norseman wallrooks range from 5.8 to 9.1 ‰ with a mean value of 7.2 ±0.9 ‰ (16 sampes ). Reef quartzes also exhibit a narrow range of δ 18 O values (11.7 ± 0.4°‰, 26 samples). The wallrock assemblages are cornpatible with a mineralization temperature ≥ 400°C. The calculated δ 18 O value of the water in equilibrium with the quartzes at these temperatures is 7‰. Waters involved in the subsequent metamorphism have identical δ 18 O values which suggest they have a common source, connate fluids and hydrous minerals in the greenstone pile. The observed timing relations and isotope systcmatics suggest that the ore fluids were connate brincs which were mobilized during tectonic dewatering of the volcano-sedimentary pile and channelled up early faults.

Resistivity Investigations for Ground Water in Metamorphic Areas Near Dhanbad, India

ABSTRACTDespite sufficient rainfall, large parts of Eastern India suffer from water scarcity. Ground water occurs in weathered or semiweathered/fractured layers in hard‐rock areas whose thickness varies, in general, from 5 to 20 m.Ground‐water studies have been carried out in several areas in and around Dhanbad (Lat. 23°48′N, Long. 86°24′E) in Bihar State of India. The area covers Pre‐Cambrian hornblende and feldspathic gneisses, schists, granulites, quartzites, metabasics and pegmatities. The area forms a part of ENE‐WSW trending Satpura orogenic belt. The quartz reefs formed as fault fillings act as barriers to the flow of ground water. Geophysical investigations, using electrical methods, with Schlumberger configuration using AB up to 300 m were carried out at most of the locations. Conventional resistivity meters were used for this purpose.The data from 78 Schlumberger soundings has been analysed. Both A and H type curves were obtained. Spectrum of resistivity values has been prepared to study the over‐all variation of resistivity values in the area. It is observed that the curves with arithmetic mean values are quite representative. The values obtained from spectral and regression analyses are nearly the same.The results of geoelectric soundings have been compared with the geological section wherever available. It is inferred that the thickness of the weathered layer as deduced from Schlumberger soundings includes partly the unweathered/fractured layer as well. An attempt has been made to find an empirical relationship between the daily yield of water in gallons/day and the longitudinal conductance (s = h/p) of the weathered layer. Two empirical relationships have been obtained, one for the winter months, December‐January and the other for the summer months, June‐July. A suitable explanation for the two curves has been given.

Gold mobilization during cleavage development in sedimentary rocks from the auriferous slate belt of central Victoria, Australia; some important boundary conditions

The development of differentiated cleavage in metasedimentary rocks from the auriferous slate belt of central Victoria has made it possible to quantify the amounts of Au, Ag, As, Sb (determined by neutron activation analysis) Cu, and Zn released during metamorphism of the sedimentary rocks. This has been done by assuming that Ti, Zr, Y, and Ir, which are approximately twice as high in the cleavage layers as in the adjacent uncleaved layers, were little affected relative to other species during cleavage development and hence can be used to ascertain the gains and losses of the more mobile species. The quantities of the ore-forming metals released from the sedimentary rocks during cleavage development were controlled by their pyrite contents, the main host for these metals. Although 70 weight percent of the pyrite in the cleavage layers was dissolved, these layers constitute only 15 percent of the rocks and therefore the net loss of sulfur from the metasediments was 10.5 percent of their original sulfur contents. Similarily, although their average precleavage gold content was 5.0 ppb Au, only 0.53 ppb Au was released during metamorphism. It is improbable that this gold was deposited in the saddle reefs and other reef structures which are typical of the slate belt. For example, a column of sediments 40 km deep would have been required to have supplied the 42,466 kg of gold produced from the Clunes gold field. It is probable that the bulk of the gold released during cleavage development left the system well before the formation of the quartz reefs. This (along with the other constituents released) may have formed lower temperature epigenetic deposits in the tectonically younger rocks which overlaid the deformed sediments. Examples of precious metal deposits which formed in this way may be the Carlin-type deposits which are characterized by a low-temperature suite of elements. A Victorian example may be the gold-antimony ores at Costerfield.This investigation emphasizes the importance of both the availability of metals in rocks and the timing of their release from the rocks, if they are to be source rocks for economic concentrations of metals.

Efficiency of regression techniques for ore valuation at Kolar Gold Fields, India

The Kolar Gold Fields are some of the best known gold deposits in India. An example of ore valuation utilizing 49 ore blocks of the Oriental lode of the West Reefs, explored and developed in the Nundydroog mines, is given. In this reef system, there are large ore reserves of sulfidebearing quartz reefs, and the gold distribution is erratic both along strike and downdip. Ore valuation at present is based on the arithmetic mean of samples taken at peripheral positions of the blocks. Samples taken from internal portions of the blocks give a totally different picture of the value. To correct this discrepancy, normal regression and lognormal regression of internal block and total block values, over peripheral block values have been used to evaluate the deposits. The valuation efficiency criterion shows the logarithmic variance for distribution of ratios of unregressed and regressed block values with the corresponding arithmetic mean of internal stope values as observed inside the blocks. The studies have shown that the logarithmic variance is minimum if the logarithmic regression is used, thereby indicating maximum efficiency. Further, the undervaluation and overvaluation of low- and high-grade blocks is less for the logarithmic example. With help of the logarithmic regression equation an effective pay limit of 177.8 in.-dwt has been found for selective mining, for peripheral block values corresponding to the official pay limit of 240 in.-dwt.

Resistivity profiles over quartz reefs of Dhanbad

A number of resistivity profiles over quartz reefs have been taken using Wenner and Schlumberger configurations around Dhanbad in order tofind the potential of these for water accumulation. These reefs give rise to both resistivity highs and lows as compared to the background. The results are discussed in the light of known theories. Some suitable sites for water accumulation have been delineated.

The Structural setting of the Mt Greenland Gold Mine

Abstract The Mt Greenland Mine was situated at an altitude of about 1,900 ft on the south-eastern slopes of Mt Greenland (2,968 ft), five miles from Ross, between the Totara and Mikonui Rivers, Westland (inset, Fig. 1). The auriferous quartz reefs of the area were discovered in the early “eighties”, and Downey (1928) has detailed their mining history until the conclusion of operations by the Mt Greenland Gold Quartz Mining Co. Ltd. in 1926. Up to this time 2,032 oz of gold was obtained from a lode mainly between 2 and 4, but up to 16 ft thick, which averaged just over an ounce of gold per ton.

A tectonic analysis of the quartz reefs in the metamorphites of Tilatanr, Tetulmari (Dhanbad)

Numerous quartz-filled faults occur in the metamorphites comprising mica-schists, granulites, injection gneisses, migmatites, amphibolites and hornblende gneisses of Dharwar age in the Tilatanr area. The metamorphites have been folded into a pitching anticline with drag folds on its limbs due to strong folding movements and deformation during Dharwar period. Quartz-filled faults occur not only as individual ones but also as zones. Various structures have been noticed in the quartz of the quartz-filled faults, namely, box-works, honeycomb-texture, platy quartz, shear fractures, tension joints and joint drags. Faults are later than folds and hence they are incongruous with the folds. On tectonic analysis it has been found that there are two types of faults, namely, high angle shear thrusts (reverse faults) and normal faults. Normal faults are later than the shear thrusts and they represent the latest adjustment of rocks to deformation. Shear thrusts are parallel to the strike of the beds and the regional structure and they are reverse faults. Associated with the shear thrusts occur shear fractures, tension joints and joint drags. These minor structures associated with the shear thrusts have been analysed and their relation to the structural axes have been described on the lines of Sander, and others.

The structural environment of the Bendigo gold field [Victoria

The Bendigo, Vic., goldfield has produced over 16,000,000 ounces of gold, chiefly between 1851 and 1915. It is one of a number of gold-mining districts in southeastern Australia in a region underlain by a thick series of Ordovician slates and sandstones that is closely folded and intruded by Devonian (?) granitic rocks. At Bendigo gold-bearing quartz reefs occur close to the axes of several anticlines, especially on domes.The folds, faults, and quartz reefs are discussed and the conclusion reached that all three were developed during a long period of compressive stresses that affected Bendigo and a large surrounding region. The slates and sandstones yielded at first by folding but later by reverse faulting as well, especially in the axial portions of the folds. The connecting system of faults along each anticline provided an independent channelway and locus of deposition for ore solutions. Evidence is presented that compression and faulting continued during deposition of the reefs. It is believed that the reefs grew mainly by accretion. Tensional rifts roughly parallel to the axial planes of the anticlines and later than the quartz deposits are occupied by lamprophyre dikes that have been considered Tertiary but may belong to the Devonian (?) period of folding, intrusion, and mineralization.

Research Items

Stone Implements of Types new to Southern India.—Six stone implements of types not hitherto recorded in India are figured and described by K. Sripada Rao in the Journal of the Mysore University, vol. 4, pt. 2, July 1930. They are selected from among four hundred and fifty implements collected on geological excursions of the Central College, Bangalore. It is suggested that of the six types, four, from Reddipalle, Cuddapah District, of Cuddapah quartzite, represent an advance on the Lower Palæolithic (Acheulean) culture of Biligere, Mysore, while the last two, from Trichinopoly and Ranganathpur, Mysore, belong to the Middle Palæolithic. The first implement is triangular with incurved sides. One surface is concave and shows no sign of working the other has a small central triangular platform of which the sides are parallel to the sides of the implement, giving it the appearance of a truncated triangular pyramid. Similar specimens were collected from widely separated areas such as Satyavedu, Alicoor Hills, and Kibbanhalli (Mysore). The nearest parallel is a ‘tribrach’ from the Isle of Wight, described by Sir John Evans, and compared by him to specimens from Yucatan and Russia. The second implement is roughly triangular with rounded angles and has one surface chipped in three broad triangular faces longitudinally disposed. It closely resembles the Stellenbosch ‘cleaver’. The third implement is horseshoe shaped. The worked surface is covered by six broad flaked faces, the three big ones forming, with the back, the three straight cutting edges, of which the main one is opposite the curved edge of the horseshoe. The fourth implement is roughly rectangular. The worked surface (one side only) shows four big flakings, of which the one longitudinally disposed forms the cutting edge. Of the two remaining implements, the one from Trichinopoly, of yellow cherty material, is crescent shaped. The convex edge, 83/4 in. in length, is thin and sharp, the concave edge is also sharp. One end is pointed, the other has a ridge, and it is suggested that this was inserted in a handle to enable both edges to be used. The sixth implement, from Ranganathpur, is of white and greasy-looking quartz reef, stained red by iron oxides. It is of the shape of an ox-head with ledge-like notches in the place of the ears. These make the implement probably unique in India and perhaps indeed elsewhere. It is suggested that the rounded and sharpedged butt, which affords no hand-hold, was intended to fit in a slotted handle.

GEOLOGY OF THE BARBERTON GOLD-MINING DISTRICT

THE Geological Survey of the Union of South Africa has issued an important memoir on the geology of the Barberton gold-mining district. This district is made up essentially of the Older Granite and the Swaziland System, probably of pre-Cambrian age, and underlying the Transvaal System, the latter being of importance mainly as determining the great escarpment of the Drakensberg; it may be noted that the latter contains auriferous deposits, both reef and alluvial, that have been worked for some thirty-five years. The tectonics of the Barberton district are very complex, intense folding, in various regions, such as the Sheba Hills, having been brought about by the intrusion of the great masses of granite. One of the most interesting features of this report lies in the conclusions reached respecting the genesis of the auriferous deposits of the Barberton district. Apart from the alluvials, auriferous deposits of two types are recognised, namely, pyritic quartz reefs and zones of impregnation. The former occur mainly in the granite of the De Kaap valley, and in some of the older rocks, and in many cases the results obtained from thei'- exploitation have been, upon the whole, disappointing. The latter include some of the best known deposits, such as the famous Sheba Mine; the deposits do not show definite walls, and the auriferous rock does not differ from the surrounding country except by its impregnation with iron pyrites and with gold, often very finely disseminated, so that the'workable limits of the deposits can be established only by continual assays. It is pointed out that the zone of contact between the granite and the adjoining stratified rocks is the area within which most of the important gold-bearing deposits are situated, and it is suggested that “gold occurrences are far more likely to be expected within the sphere of influence of the intrusive granite,” this forming a belt of country averaging about three miles in width. Furthermore, in prospecting, it should not be forgotten that many of the payable deposits of the Barberton district take the form, not of the well-defined quartz reef, with which most prospectors are familiar, but of “mineralised zones of impregnation, sometimes almost indistinguishable from country rock.”

Earth-Currents and the Occurrence of Gold

GOLD has been so large a factor in the prosperity and greatness of Australia, that the interesting subject of the origin of gold drifts and reefs must always possess to us something more than a purely scientific attraction. In the earlier days of the goldfields there was among the diggers much speculation, of a scientific and semi-scientific nature, as to the processes by which Nature had produced the accumulations of coarse and fine gold dust which it was their business to extract from the alluvial drifts. The most obvious explanation, of course, was that the grains of gold had an origin similar to that of the debris and detritus of various characters which made up the alluvium itself; and this explanation seemed to harmonize so completely with the general processes of Nature that at one time it was almost universally accepted as the correct one. But many thoughtful mining authorities had their doubts upon the subject, and these doubts were not founded, as so frequently happens, upon mere prejudice, but were fortified by the fact that certain phenomena characteristic of the occurrence of drift gold were not only not explained by the “detrital hypothesis,” as it is called, but were absolutely inconsistent with it. Chief among these objections may be mentioned the undoubted generalization that drift gold is nearly always purer than the gold in the reefs of the neighbourhood in which it occurs. No explanation as to the long distances to which grains of gold might be conveyed, or to the possible purifying effects of natural chemical action, made up any satisfactory explanation of the known facts, and accordingly under the detrital theory these facts had to remain shrouded in mystery. Then, again, it was a frequent occurrence for gold to be found so peculiarly embedded in pieces of wood, or in conjunction with natural crystals of minerals, such as the sulphides, that those who were constantly being brought into contact with such phenomena were firmly convinced that at all events there was a certain proportion of the gold found in alluvial drifts which had its origin in some other source than the breaking down of quartz reefs by the ordinary processes of Nature. The majority of those who held to this belief had at first but little scientific knowledge of natural reactions; and when questioned as to their theory on the subject, they were accustomed to say of the alluvial drift-gold, that it appeared to be actually growing—a statement which sometimes provoked, not unnaturally, a smile of pity for misplaced credulity.

Mineral Statistics of Victoria

FOR some years past the yield of gold in the colony has been steadily decreasing. In 1868 the quantity of the precious metal obtained from alluvial deposits amounted to 1,087,502 ounces, and from quartz-veins 597,416 ounces, making in all 1,684,918 ounces of gold. Last year the quantities were respectively—alluvial, 293,310; quartz, 465,637: making a total of 758,947 ounces. The comparatively rapid diminution in the supply from alluvial sources is quite intelligible, as these would necessarily be soonest exhausted, though it is important to observe that in 1879 for the first time for eleven years the return from this source shows a decided advance on that of the preceding year, which is attributed to a better supply of water for sluicing operations, and to the opening up of deep mining ground. It is to quartz -mining, however, that the colony must look for the further development of her gold-fields. There has been a gradual decline in the yield from quartz-mines since 1872, when the amount obtained was 691,826 ounces. But the Secretary for Mines in his recent report speaks hopefully of the probable future of this important industry. Up to the end of 1879 the total quantity of gold raised in Victoria is estimated to have been 48,719,930 oz. 11 dwts., valued at 194,879,722l. The proportion of gold in the quartz varies considerably in different districts. Thus, last year at Castlemaine the average yield of each ton of quartz was 5 dwts. 18.45 grs., while in Gippsland it amounted to 1 oz. 2 dwts. 18.66 grs. The quartz of the latter locality is by much the most auriferous in the colony. The decrease in the supply of gold has been accompanied by a falling off in the number of miners. The men who found employment in gold mining in 1874 was 45,151; last year they numbered 37,553, which was an increase, however, of 917 over the number for 1878. The mining population includes an industrious and unpopular contingent of Chinamen, who last year amounted to 9,110, or 528 fewer than in the previous year. Taking the total annual yield of gold and dividing its value among the miners employed, the earnings of an alluvial miner are rated last year at 48l. 10s. 11/2d. per annum., while those of the quartz miners are given as 118l. 6s. 9d. Deep mining in quartz reefs continues to make pro-press, and the mines are becoming every year deeper. Some shafts are now more than 2,000 feet deep. The revenue derived by the colony from the gold districts amounted last year to 15,641l. 16s. 9d., being a slight advance on that of 1878.