TasNat 1927 Vol2 No3 Pp9-14 Lewis OutlinesGeology

7/28/2019 TasNat 1927 Vol2 No3 Pp9-14 Lewis OutlinesGeology

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TRE TASMANIAN NATURALIST. 9we all felt that we had lost a father, and it will readily be seenfrom this narrative that after Cook's death the spirit of dis-covery, the decision, and the indomitable courage, were gone."

Conclusion.Much Qf the greatest interest has had to be Qmitted, but thevalue of the work has been shown. I t and "The V Qyage Qf theEndeavQur," by PrQfessQr G. ArnQld WQQd, should be meat anddrink for every normal Australian child. I t WQuid be very diffi-cult to. Qverrate their interest and attractiQn. Surely such bQQksmight be included in the curriculum Qf Qur schQQls?

I must acknQwledge my g r a t i ~ u d e to. Professor Ernest SCQttfQr my first intimatiQn as to. the wQrk frQm which I have qUQted.My remarks follQW the lines Qf an article Qf his published somemQnths ago. in the MelbQurne "Argus."

Outlines of Tasmanian Geology(CQntinued)SECTION 23.

ORE DEPOSITS.This chapter may be well cQncluded with a few remarks Qn

the mQre valuable Qf Qur rQck types. I t must, hQwever, alwaysbe bQrne in mind that there is nQthing unusual in an Qre depQsitQr occurrence Qf coal Qr Qther eCQnomically valuable rQck.They are Qnly types Qf rQck which have SQme particular valueQf themselves Qr from the uses to. which they may be put, andbQth in structure and mode Qf Qccurrence, and also. in Qriginthey cQnfQrm to. the same general principles as do. any Qthertypes Qf rocks.

Mining is chiefly a questiQn of CQsts and different types Qfrock have to. possess a certain standard Qf purity to. make it payto. mine them. Many Qther cQnsideratiQns also. affect thequestion. But these prQblems do. nQt cQnfrQnt the geologist.He cQnsiders an Qre deposit sQlely from the questiQn Qf cQntent,origin and nature and leaves the questiQn Qf CQsts to. the miner.Thus. to. a geolQgist a lQde Qf pure Qre is in the same categQry asone with the smallest trace Qf Qre, prQvided the mQde Qf Qriginis the same. There exixsts every PQssible variatiQn and mQstfrequ ently there is no. ore in the fQrmatiQn at all. This makesno. difference to. the geQlogical classificatiQn Qf types Qf depositand the enunciating Qf general rules applicable to allQccurrences.


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10 THE TASMANIAN NATURALIST.An ore may be defiued as a mineral either being or, fromwhich there may be extracted by a suitable and economically

p o s ~ i b l e process, some metal or allied substance. We have seen,under the heading of Mineralogy, what mineral species theseores may belong to and their several characteristics. Thefollcwing table may be of use when reading accounts of miningfields

MetalAluminium

AntimonyArsenicCadmiumChromiumCopper

GoldIron

LeadMercuryMolybdenumNickel

PI a tiniumSilver

Tin

Zinc

CommonestOres

CorundumBauxiteStibniteArsenopyriteGreenockiteChromiteChalcociteBorniteChalcopyriteCupriteNative GoldPyritePyrrhotititeHematite

M a ~ n e t i t eLimoniteSideriteGalenar;nnabarMolybdeniteMilleriteNiccoliteNative Plati-num, rl!entiteCeaT!!yriteGalenaCassiteriteStanniteSphalite

Composition of OresAluminium + oxygenAluminium + oxygen +

waterAntimony + sulphurIron + Ar"enic + sulphurCadmium + sulphurIron + chromium + oxygen

C o p p e ~ + sulphurCopper + iron + sulphurDittoCopper + oxygenPure gold + alloys of gold

groupIron + sul phurDittoIron + oxygenDittoIron + oxygen +waterIron + carbon + oxygenLead + sulphurMercury + sulphurMolybdenum + sulphurNickel + ~ u l p h u rNickel + AntimonyPure PlatinumSilver + SulphurSilver + chlorineOften some silver with leadand sulphurTin + oxygenTin + copper + iron+ antimony + sulphurZinc + sulphur


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THE TASM


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12 THE TASMANIAN NATURALIST.rounding rocks. Also, during differentiation, the heavier nativeelements will tend to sink and thus concentrate in the ultrabasic zones.In this way we commonly find that the sulphide minerals,originally accessories of the magma, have passed out into surrounding rocks and that such ores are rare as a part of an igneousrock, that the gold group and platinium.iron group are common minor accessories of ultra-basic igneous rocks, and thatoxides, particularly tin and iron oxides, are frequently found asa constituent of a completely differentiated plutonic igneousrock-most commonly granite.The consolidation (crystalization) of the magma expellsthe water constituents and other lighter minerals as gases.TheRe are superheated to a very considerable temperature anltin that state possess the ability to dissolve variou minerals. Thesulphides are particularly susceptible to this action. Silica ,calcite, flourite and other minerals are so to a less degree. Thegases pass upward through fissures, cracks, etc . in the overlyingrocks, often dissohing minera1s from the rocks passed through.The effect of this dissolution often affects the solution so thatit becomes saturated with one or more of the minerals which itcarries and so deposits this mineral. At other times the solution passes upwards into cooler rocks until the temperaturedrops to a point at which it can no longer dissolve some of itsmineral c ~ n t e n t s , and it then deposits them.

The process may continue along the same fissure for along while during the period of differentiation. The gases maytherefore draw off different mineralogical products at differentstap:es of differentiation. Each product will be acted upondifferently by the alteration of the solution by further absorbtion of minerals and by decreasing temperatme. In this waya mineral vein may contain a series of different mineralsdeposited at different times. This difference may be shown inhorizontal banding, the earlier deposits being found on theoutside of the vein and the later ones Ioradually blocking up thefissure, or the difference may be apparent in a vertical arrangement. the nature of the ore changing with depth. As the rulesof mineral deposition are well known the laws of paragenesis,or tl-te times at which under varying differences of the composition of the mineral solutions and differences of temperaturesdifferent minerals will be deposited, give a valuable guide towhat may be expected in depth from the discovery of a certai ,1mineialogical content of a surface outcrop.

Form of Ore Deposits.Ore-bodies, originating in the way described above may beclassified according to their form, as fol lows:-


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THE TASMANIAN NATURALIST. 131. Deposits of Volcanic Origin.Dep

gypsum, sometimes accumulate round fumeroles of active volcanoes as sublimates. Such deposits sometimes attain sufficientsize to be w o r k ~ d commercially. Only those of relatively recentorig:.n can persist.2. Stockwork orebodies.

1his is the name applied to a mass of rock traversed by alarge number of small veins of ore that mutually intersect oneanotler, but are each too small to be worked independently.The veins seldom possess clear walls, but merge into the country rock, which is often impregnated with mineral matter to aless extent. Frequently the veins are barren except at their intersection. These ore bodies are frequently of large extent butthe proportion of ore won to rock removed is so small that theymust be classed as low grade propositions, and can only beworked economically when they can be quarried as an open cut.This is one of the commonest forms of ore bodies, and adoptedby all sulphide ores. I t is often an accompaniment as an outerzone of richer forms. It is to be found on many of the miningfield3 of the West Coast.3. Contact Ore Bodies.f.:ontact deposits originate from emanations of metaliferousgases, etc., which have been driven off from the magna, but

which have been trapped by the surrounding rocks. They arealways an accompaniment of an igneous intrusion, and areusually found well away from the igneous rock towards theouter limit of the metamorphic zone, but never outside this.They usually adopt the form either of stockwork bodies or ofreplacement bodies, and apart from their definite mode of originthey possess no special form to distinguish them.4. Replacement Ore Bodies.

In most cases the ore body has replaced the country rock.The ascending solution has dissolved the whole of some constituents of the rock through which it has passed, and hascarr:ed them away, but in so doing it has affected the saturation

p o i n ~ of the mixture sufficiently to cause the deposition of thisparticular ore we now find. The replacement zone usuaU"follows certain well defined zones of fracture or crush connectedwith either an igneous intrusion or regional deforming movemen' b. The commonest occurrence is at the fault junctionbetwt'en a hard and a soft rock, the latter of which has bee 1crushed against the former. The solutions have ascended thefault fracture, and have invaded the crush zone. Often the


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14 THE TASMANIAN NATURALIST.material o.f the crush zo.ne has been replaced grain by grain, so.that the stratified 0.1' o.ther o.riginal structure is a c c u r a ~ e l y preserved. I t is o.ften apparent that a small quantity o.f material0.1' o.ne particular mineral o.r the cement is disso.lved first and isreplaced by a little o.re. The pro.cess is co.ntinued with increas-ing po.wer until the replacement o.re bo.dy is o.f co.nsiderable sizeand purity.

So.me o.f the largest and mo.st impo.rtant o.re bo.dies in thewo.rld are fo.und in this fo.rm. And the mo.st impo.rtant o.reswhich are so. fo.rmed are the pyritic (co.pper and iro.n pyriteswith their asso.ciated minerals). The Rio. Tinto. co.pper lo.de inSpain, the Bro.ken Hill galena lo.de in New S o . ~ t h Wales andthe Mo.unt Lyell co.pper o.re bo.dy in Tasmania are amo.ngst thebest kno.wn examples.

(To. be Co.ntinued.)

A Trip to the Moulting Lagoon.BY A CLUB MEMBER.Having listened to. an excellent address by Mr. M. S. Shar-land o.n bird life at the Mo.ulting Lago.o.n, my wife and I deter-

miued that this interesting lo.cality sho.uld be selected fo.r o.urnext ho.liday excursio.n. With an affectio.n fo.r o.ur featheredfrieJIds, this o.uting fo.rmed the playgro.und fo.r jo.yful anticipa-tio.n, and it was with light hearts that we fo.und o.urselves packedup o.n a Sunday mo.rning ready fo.r the ro.ad. Our little carlo.o.ked like a travelling circus with its impedimenta strappedo.n every available space, and the springs sho.wed that we had arnaxjmum lo.ad up.

Travelling via Brighto.n, Tea Tree and Richmo.nd, by no.o.nwe had reached the Pro.sser River, and as the beach at Orfo.rdis seldo.m witho.ut great interest fo.r the bird lo.ver, we decidedto spend the night there and explo.re the har and .beach. Anumber o.f hero.ns were paddling up sand wo.rms, while the gullswere everywhere in sco.res. The next mo.rning the parro.ts camedo.wn to. the ho.neysuckle clump where we were camped, andgreeted us .with their cheery chatter. The friendly little wrensho.pped aho.ut us, picking up the cnunbs we scattered fo.r them.We sho.uld have liked to. have stayed lo.nger, but having in mindall that we were o.ut to. see we packed o.ur tent and baggage andagain started on 1he mad.It wo.uld -be wro.ng to. pass o.n witho.ut sayi.ng at least so.me

t h i n _ ~ o.f t.he beauty o.f the Pro.sser River and its surro.undings.We ' will never fo.rget o.ur first glimpse o.n that particular mo.rn-

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TasNat 1927 Vol2 No3 Pp9-14 Lewis OutlinesGeology