From 1875 to 1877 Mr. Thureau held a position as Lecturer at the Bendigo (Victoria) School of Mines, of

"Geology as applied to Mining,"  Mineralogy; also practical

Mining, the Administrative Council of that institution

arranged during each winter for a series of public lectures on Popular Science, and at such he elaborated a series of lectures

upon the hydrothermal origin of the famous Bendigo Quartz

Reefs.  He was subsequently elected,

upon unsolicited nominations and recommendations, as a

Fellow of the Geological Society of London, which honourable position he still holds and treasures.

The discovery of silver in the ash or mud, adds, for the

first time, this metal to the list of elementary substances

observed in the materials ejected from volcanoes, and the

addition derived some special interest from the fact of this

ash having come from the greatest volcanic (active) vents of

that great "argentiferous" zone of the Andes. Small as

would be the proportion of silver, it must represent a very

large quantity of that metal ejected during the eruption, in

view of the vast masses of volcanic ash, etc., distributed over

the large area which is indicated by the fall of argentiferous

ashes at a distance of 102 miles from the central crater to

Bahia de Caraguez.

Mr. Thureau states that, if silver, lead, iron, manganese, titanium, chlorium,

mercury and other less important metals occur in volcanic

ash or mud shown by frequent analyses, as derived,

inter alia from the immensely rich argentiferous formations

which that gigantic "vent" cotopaxi protrudes; a similar

occurrence here on a smaller scale, within a well-known

"auriferous zone" is not only feasible, but can be, or is now,

demonstrated to be a fact. The only, and to us most valuable

difference, is, that the South America ejecta expelled the

silver in its ashes, whilst, with our "Iron Blow" the ash or "mud" is still retained within the "dead" vent or closed

fissure, and happily for the colony at large, it is comeatable,

and it can be extracted by future systematic mining operations,

followed by skilful treatment for the rich gold it is reported

same contains

Thureau, G

University of Tasmania Open Access Repository

THE "IRON BLOW" AT THE LINDA GOLDFIELD.By a. Thiteeau, F.a.S.In the recently issued printed Papers and Transactions ofthe Royal Society of Tasmania, on page 216, are publishedsome notes by Mr. R. M, Johnston, F.L.S., an esteemedmember of the Society, on the " Iron Blow " at the LindaGoldfield, his conclusions having been based upon theexamination of some rocks and specimens from that localityreceived from Mr. Grotty and Mr. Belstead, the Secretary forMines.It is upon that remarkable gold-deposit that I desire tooffer a few remarks, at the same time embracing theopportunity of supplementing and elaborating my report, No.146 of 1886, presented to Parliament.In the following remarks, I shall exclusively confine myselfto the question of the probable origin of this unique goldformation in furtherance of my theory of its being due to** volcanic agency," and not, as Mr. Johnston contends, tolocal decomposition, especially so far as the dark colouredand pulverulent masses are concerned. I may likewiseobserve that in my report to the Government such questionsas these concerning and referring solely to the more scientificaspect, must of necessity be very brief, because the largerquestions as to the present or ultimate value of any mineralor metalliferous discovery, are of more immediate practicalTalue as affecting directly the progress of the community atlarge.In the first place, it appears that the Secretary for Minesobtained the specimens in question from Mr. Grotty, thediscoverer of that " Iron Blow." Subsequently, Mr. Johnston,aided by Mr. Ward, the Government Analyst, concluded thatthe soft purply black and so highly auriferous mineral wasthe result of decomposition of some of that immense bed orvein of solid j)yrites (iron) filling the greater width of thefissure on its " hanging wall," or about 225 feet out of a totalwidth of 280 feet between walls of that chasm.Dismissing all speculations as to whether it has beenprudent to base any reliably practical opinion, such as to thequestion of origin of that valuable deposit, upon the examina-tion of "specimens" only, even though, such was to somedegree supported by chemical analyses, it further appearsfrom the late Mr. C. P. Sprent's report that, but a verycursory examination of that deposit, in situ, had been madeduring that gentleman's and associates' tour from the Ouse2 THE "iron blow" AT THE LINDA GOLDFIELD.to the West Coast. Thus, on the whole, a settled andreliable opinion as to the causes governing the past geologicalhistory of the "Iron Blow," accounted for by Mr. Johnstonas a process of deconip)Osition of materials at hand, in opposi-tion to the theory of volcanic agencies which I have advancedin my report, deserves to be treated in detail, as involvingimportant issues.Decomposition is, I believe, a chemical process by whichthe destruction of one or more substances leads to the sub-stitution and depositing of quite different matters, therebybringing about the rearrangement of the former originalsubstances in quite different forms.In this case it has been attempted to be proved that thosemassive beds of pyrites on their decomposition fromlocal causes, were replaced by that highly interestingpulverulent mass reported so rich in gold. Now, I havebefore me two letters from the Government Analyst, viz.:one dated November, 1824, and the other October,' 1886, inwhich the results of the analysis of "solid pyrites" fromthat "Iron Blow" are given thus:—In the first letterMr. Ward states : " I have carefully tested the mineralsreceived .... have not been able to detect thepresence of tin or any other metal of commercial value;"in the second he says : " In none of the samples forwardedfor assay have I been able to find more than traces of gold,"To these may be added those examples cited in Mr. Johnston'spaper, viz.: No. 9, "A sample of Iron Pyrites in which goldis not mentioned as being present, and in No. 7 the sampleonly shows "fine specks of gold just visible to the eye," butthis is not from pyrites, but from the soft purply pulverulentmass, which is about 66 feet wide.On page 219, the author states: "Whether we supposethat the ' Iron Blow' is due to hydrothermal agency or not,there is nothing in the composition of the iron pyrites or thedark purplish rock which necessitates their having beenoriginally formed in the way of volcanic mud." It is moreprobable that the four principal elements, iron, barytes,sulphur and gold, were originally precipitated from solution."Leaving out the references made in the paper in questionas to the production of gold elsewhere as foreign to the subjectunder discussion, and which, however, are not altogetheraccurate, I beg to direct your attention to the facts uponwhich I join issue with Mr. Johnston's theory of origination.The analyses of Mr. Ward, cited by Mr. Johnston andmyself, conclusively prove the almost total absence of gold inthe pyrites, veins, or beds, which may be described as verydense and excessively solid, and which undoubtedly haveresisted both decomposition and dissolution for ages ; how isBY G. THUREAU, F.G.S, 3it possible then, I may ask, that these almost non-auriferousiron bi-sulphides produced on their supposed (inert) decomposition that peculiar purple mineral, assaying, as reported,•considerably above 170ozs. of gold per ton ? Again, those sovery solid pyrites contain no barytes, which latter minerals Ifirst discovered as the necessary adjunct to the gold. " Exnihil aut nihilo fit."It may also be fairly questioned how it is that these veinsor beds of pyrites, so dense in character, must have undoubtedly withstood atmospheric influences for immeasurableperiods, on decomposition (?) filled, with new substancesresulting from that process, over 50 feet in width by over amile and a half in length, and to unknown depth of an openfissure with a "solution" only. Such a fissure or chasmwould have collapsed at the sides long before the decompositionprocess had even been initiated, as the adjacent and super-incumbent rocks could not have withstood the lateral andvertical pressure their own great gravity would produce, hadnot the walls of that fissure been kept apart by some heavyfilling material of a homogenous kind, exerting in itself asufiiciently powerful resistance to the overhanging walls ofthis fissure.Supposing, however, decomposition was the cause and effectof this rich aggregation of minerals and metals, or, in theauthors own words : " That it (the Iron Blow) is the result ofoxidation of pyrites similar to that now so largely associatedwith it ; the hydrated oxide first formed, being subsequentlymetamorphosed sufficiently to get rid of its combined vajjourand produce the slight change in the form of disseminated'particles of harytes, as revealed by the microscope ; or, thisprocess may have occurred during the process of oxidation,"etc., etc.It will therefore be necessary to bear in mind that, asproved from analysis, we have, firstly, a nearly non-auriferousbi-sulphide of iron (pyrites) to deal with, containing no barytato speak of ; and secondly, that water is assumed to haveproduced the rich pulverulent gold rock by means of thedecomposition of the former, and contemporaneously or subse-quently by means of infiltration filled the fissure, and thatsmall (?) disseminated particles of baryta appeared eitherbefore (whence ?) or during the process of oxidation.Now, it is a fact that baryta is the " matrix" of that purplerock, exceeding " thirty (30%) per cent, of the whole of thevein-matter, being disguised by coatings and linings" ofspecular iron, and exhibiting gold in fine crystalline andfilagree forms ; that auriferous rock likewise exhibits a dis-tinctly recognisable vesicular structure, the cells and cavitiesbeing now, however, filled by means of similar rock of a4 THE IRON BLOW AT THE LINDA GOLDFIELD.denser kind and of a darker colour, as, in all probability, theresult of these ore-deposits having become saturated withsteam or hot-vapours, and by means of segregation andexpansion of these high-pressure volcanic emanations, thecavities or cells were firstly formed and subsequently filled,thus explaining the so-called "schistose" appearance, which,from all appearances was principally due to the gradualcooling of a seething mass of volcanic mud or ash which wasejected in combination of several kinds of metallic vapours,such, as for instance, specular iron, which not only forms aconspicuous constituent of that volcanic material, but alsooccurs quite frequently in the wall-rocks of that immensefissure. In my opinion everything in connection points to amore drastic process of origination than simple and quiescentdecomposition only.That there is strong evidence of the former ebullition andbelching forth of metalliferous and mineral vapours at hightemperatures within certain ejective points of discharge withthe volcanic muds and ashes, is clearly demonstrated by theoccurrence of elongated or spherical nodules in these mudsand ashes, which nodules on examination are found tocontain, within hard crusts of " Limonite "—sesqui-oxide ofiron—nuclei of pure iron pyrites, thus pointing the way howthe decomposition of pyrites under precisely similar circum-stances has actually occiirred, and caused the formation of asecondary and hydrated iron ore, and not of purple rock,though in very close contiguity to the massive pyrites veinand beds referred to. Those nodules, it is submitted, present,neither more nor less, former gaseous bubbles surchargedwith vaporous sulphuretted solutions of iron, becoming rigidwhen nearer the cooler atmosphere, and which from compres-sion by the surrounding muds, etc., assumed their presentcharacteristically elongated forms.When it is borne in mind that geologists have concludedthat " the nature of vapours evolved depends on the tem-perature or degree of activity of the volcanic orifices ; chlorineand fluorine emanation indicating the most energetic j)haseof eruptivity, sulphurous gases, a diminishing condition andcarbonic acid (with hydro-carbons) the dying out of thatactivity, and that sublimed by volcanic heat or chemical re-actions, causing the decomposition of metals and minerals fromcondensing vapours along crevices and surfaces wherein theyreach the outer air and are cooled ; and further that, besidessulphur there are chlorides, and in a lesser degree, iron,copper, and lead ; also free sulphuric acid, sal amonia,specidar iron, oxides of copper, boracic acid, alum, sulphateof lime, baryta and others, are formed whilst at very hightemperatures, and in connection with simultaneously en-BY G. THUBEAU, F.G.S. 5gendered electric currents" it becomes clear to the close andcareful observer of these tinique gold deposits, in situ, thatdynamical geology can alone account for these, strictlyspeaking, volcanic j^roducts.Having myself had opportunities for examining active" mud volcanoes " in 1877, near Carson City, State of Nevada,U.S.A., these " Steamboat Springs " were most interesting,and I can therefore speak with some authority upon thesubject. There, as is held by American geologists, thesevolcanic "vents" occur on the line of continuation of thefamous Comstock Lode (silver-gold), and each spring orgeyser is indicated at the surface to the visitor, at a distanceby a thin column of white steam. When more closelyapproached, it is found that the discharges of heated mudand vapours are intermittent, and that previous to each ofsuch discharge a greyish semi-liquid mass rises slowly withinthe mouth of the '* fumaroles " below, and en reaching thetop of the respective orifices, the carbonic, sulphu.retted andother gases encompassed beneath, cause, through pressure, adome-like expansion of the " volcanic mud," which, however,with increasing subterranean pressure eventually bursts, andallows the " mud " again to subside. Each discharge, it isnoted, however, leaves a thin deposit or lamina in the "cups"at the surface, which, after hardening, was found on analysisto be chiefly charged with silica (quartz), and to also containa sensible percentage of gold and silver. This process iseven now in active progress, and as it assimilates a great dealto what can be seen in its " dead state " at our " Iron Blow "—if baryta is substituted for silica as matrix in the latter case—the question of origin as to both metalliferous deposits isnot only, in my opinion, very suggestive, but forms the onlypossibly true solution of the case.By way of further analogy, I would likewise draw attentionto the fact of Senor Santos having found "Lead" in the"volcanic ash" from the eruption of Cotopaxi, of August23rd, 1878, and in a paper read before the Eoval Society ofEngland, on January 6th, 1887, Mr. J. W. Mallet, M.D. andF.R.S., etc., I'eports upon the " Occurrence of Silver in VolcanicAsh, from the Eruption of Cotopaxi, Ecuador, of July 22ndand 23rd, 1886."A condensed extract may prove of interest :—He, Dr.Mallet, received a specimen of volcanic ash from SenorJulian E. Santos, of Ecuador, which was collected at hisresidence, Bahia de Caraguez, about 102 miles nearly duewest from Cotopaxi. This is the highest and most mightyof the active volcanoes of our globe ; it erupted on the 22ndof July, and the ash began to fall at Bahia de Caraguez nextmorning, to a depth of several inches, thus representing an6 THE "lEON BLOW' AT THE LINDA GOLDFIELD.enormous cliscliarge of volcanic and metalliferous as well asmineral matter. The specimens consisted of a finely dividedpoivder, mobile and soft to the touch, hrownish grey in colour.Under the microscope, the following minerals could bedistinguished in the granules and spicules, viz.: quartz, twofelspars (one Avhite and one pink), augite, magnetite (stronglymagnetic, and scales of deej^ red specular iron oxide. Aftersubjecting this ash to several experimental tests, it was, as apreliminary, found to possess a specific gravity of 2'64 at 18°C, as compared Avith water at tlie same temperature. Ananalysis of tlie material taken, as a whole, i,e., without anyprevious mechanical separation of its constituent minerals,and without previous digestion with water or acid, but dried up100 C, gave no less than sixteen separate ingredients,,amongst which were traces of silver. That metal wassu.bsequently obtained by icet assay; and it was also after-wards found that it could be obtained from the ash by furnaceassay—fitsion with pure lead carbonate, sodium carbonateand a little cream of tartar, and cupellation of the lead buttonso obtained or produced, which gave a minute bead ofmetallic silver ; the same reagents were tested in largerquantities, leaving out the ash, when negative results followed.It was subsequently ascertained that silver could b'e extractedfrom this volcanic ash by boiling it with a solution ofammonia, or of potass, cyanide, or of sodium sulphate."The discovery of silver in the asla or mud, adds, for thefirst time, this metal to the list of elementary substancesobserved in the materials ejected from volcanoes, and theaddition derived some special interest from the fact of thisash having come from the greatest volcanic (active) vents ofthat great "argentiferous" zone of the Andes. Small aswould be the proportion of silver, it must represent a verylarge quantity of that metal ejected during the eruption, inview of the vast masses of volcanic ash, etc., distributed overthe large area which is indicated by the fall of argentiferousashes at a distance of 102 miles from the central crater toBahia de Caraguez.There cannot be, it is submitted, much difference of opinionthat, if silver, lead, iron, manganese, titanium, chlorium,mercury and other less important metals occur in volcanicash or mud shown by frequent analyses, as derived,inter alia from the immensely rich argentiferous formationswhich that gigantic " vent " cotopaxi protrudes ; a similaroccurrence here on a smaller scale, within a well-known*' auriferous zone " is not only feasible, but can be, or is now,demonstrated to be a fact. The only, and to us most valuabledifference, is, that the South America ejecta expelled thesilver in its ashes, whilst, witli our " Iron Blow " the ash orBY G. THUKEAU, F.G.S. 7"mud" is still retained within tlie "dead" vent or closedfissure, and happily for the colony at large, it is comeatable,and it can be extracted by future systematic mining operations,followed by skilful treatment for the rich gold it is reportedsame contains.With regard to the opinion I have had occasion to expressin my report to the Grovernment, I may add that the miningoperations carried on since still expose rich ores at times, andas Mr. Johnston concludes his Paper by saying : It—thehydrothermal theory—had also been adopted by Mr. Thureauin respect to such mineral formations as the Iron Blow at theLinda, although the latter " seems to be unaware of the factthat the mode of origin of the more common quartz reefs arealso frequently ascribed to the hydrothermal agency."I may be permitted to state that, in the years 1845 to 1848,when a student at the Royal School of Mines, Clausthal,Hannover, Germany, I studied under several eminent pro-fessors of geology, and at that time no less than five or moretheories—including what is now termed hydrothermal—weretnown, recognised, and applied practically. Since then Ihave been, and am still, an ardent student of mining geologyin several countries, so that it is not likely that I am ignorantof so important a portion of that science.When I held, in 1875 to 1877 inclusive, the position asLecturer at the Bendigo (Victoria) School of Mines, of" Geology as applied to Mining," Mineralogy ; also PracticalMining, the Administrative Council of that institutionarranged during each winter for a series of public lectures onPopular Science, and at such I elaborated a series of lecturesupon the hydrothermal origin of the famous Bendigo QuartzEeefs, without controversy. It appears that at those lectures,—illustrated by models, diagrams, geological sj)ecimens, andanalysis,—visitors from England, 'New Zealand, and Americaattended, and as one result of the interest they must havetaken in the subject dealt with, I was subsequently elected,upon unsolicited nominations and recommendations, as aFellow of the Geological Society of London, which honoui'-able position I still hold and treasure.

The "iron blow" at the Linda goldfield

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The "iron blow" at the Linda goldfield

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