THE MOUNT LYELL COPPER DISTRICT OF TASI^IANIA.
By Chester G. Gilbert and Joseph E. Pogue,Of the Department of Geology, United States National Museum.
INTRODUCTION.The present paper is based upon a study of a representative col-lection of rocks and ores received in 1910 by the United StatesNational Museum from Robert Sticht, manager of the Mount LyellMining & Railway Co. (Ltd.). The geologic and technical informa-tion is derived largely from the writings of Gregory and of Sticht (seeBibhography), which embrace the most authoritative expressions onthose subjects. ^Ir. Sticht has also generously assisted the under-taking by supplying copies of his papers and communicating unpub-Ushed data. LOCATION.The Mount Lyell mining district, comprising the Mount LyellIVIine, the North Mount Lyell Mine, the South Tharsis and RoyalTharsis Mines, and other workings of less importance, occupies anarea of 9 square miles situated 18 miles inland from Macquarie Harboron the west coast of the island of Tasmania. The field is reachedover a railway 28 miles in length from Strahan, the harbor port, toQueenstown, the mining center. The region is wild and inliospitable,is covered with a dense undergrowth, and is scarcely penetrableexcept along streams and where cleared by man or forest fire. Thecolony is under British rule and lies off the southernmost point ofAustraha, from which it is separated by Bass wStrait, about 150 milesin width. HISTORY.Discovered in 1642, but not settled until 1803, Tasmania attractedno mining interest until the early sixties of the nineteenth century,when gold was discovered in the western interior. In 1881 prospec-tors came across gold-bearing alluvium near Mount Lyell, and in 1883the ferruginous outcrop of the Mount LyeU ore body was encountered.Proceedings U. S. National Museum, Vol. 45?No. 2005.80459??Proc.N.M.vol.45?13 39 609
610 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.45.Good values in gold were found here and although the underlyingpyritic mass was soon detected, its significance was not recognized,and attention was confined to the oxidized ore, the valuable portionof which was depleted within a few years. In 1891 lagging interestin the deposit was revived by the recognition of the copper poten-tiahties of the pyritic mass, and a syndicate was organized for workingthe ore on a systematic scale. In 1893 the company was incorporatedas the Mount Lyell Mining & Railway Co. (Ltd.), wloich to thepresent has retained control of practically the entire field. At itsinception this company was fortunate in securing the services of aneminent metallurgist, E. D. Peters, whose favorable report andrecommendations, coupled with a very timely find of rich silver-copper ore, insured the financial backing necessary for the success ofthe enterprise. Development work was at once actively prosecutedand a coast-to-mine railway projected and started. Robert Sticht,an American engineer, was engaged to direct the undertaking, and tohis administrative ability and scientific skill the subsequent successof the company is largely due. In 1895 the open-cut system wasadopted for working the ore body and a system of pyritic smeltingwas planned and inaugurated whereby the sulphur and iron of theore served as fuel for its smelting, admitting of an economical recoveryof copper and the precious metals. Success amply rewarded theintroduction of this treatment, which at the time was little under-stood and had not been tested on a large scale upon regular copperores.In the meantime an independent company was developing a prop-erty (the North Mount LyeU) to the north of the pyritic mass, butencountered little ore until, in 1897, a rich body of siliceous bornitewas accidentally located. The treatment adopted, however, provedunsuccessful, and in 1903 the company was merged with the MountLyell Mining & Railway Co. (Ltd.)?a happy combination, for theores of the two deposits were metallurgically complementary andwere amenable to more economical treatment than could be appliedto either alone. Many other mining companies have operated in thefield, but all the important ones have been added by the principalcompany to the original holdings.The history of the Mount Lyell district is unique and affords anexample of notable success in the face of great natural obstacles.The profitable exploitation of a low grade deposit in a remote andisolated region, where labor costs and difficulties are at a maximum,is an important acliievement. The secret of tliis success is to besought in the application from the outset of the highest type oftechnical, scientific, and administrative abiUty to the problem.
NO. 2005. MOUNT LYELL COPPER DISTRICT?GILBERT & POGUE. 611GEOLOGY.The geology of the Mount Lyell district is shown on the accom-panying sketch map. A narrow belt of deeply dipping schistoserocks is bordered on the east by an area of quartzitic conglomerates,while to the west lies a sedimentary series of Silurian age, composedof sandstones, quartzites, and clay slates with limestone intercala-tions. The ore is confined to the schists, lying along their contactwith the conglomerate, which is marked by an important overthrustfault. Igneous rocks, both acid and basic, are rather extensivelydeveloped near the western coast of the island and approach in bulk
!/ ' \\Giac>a.\ L^-^l '^""'f* tSketch map of the Mount Lyell district. (Modified feom Geegoey. )to within 1^ miles of the deposits; dikes of diabasic character comenearer, though none are in contact with the ore. The region showssigns of glaciation.The conglomerate is younger than the schists, containing inclusionsof the latter, and normally overlies them except near the mines,where the relations are reversed by the overthrust fault. The for-mation is predominantly reddish in color and is composed of roundedto subangular pieces of quartz and quartzite, ranging from small
612 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.45.grains to bowlders a foot or two in diameter, and consolidated bysiliceous cement. Its lowest member is a typical quartzite, andother quartzite beds are intercalated through the series. The forma-tion is unmineralized except in proximity to its schist contact, whereit shows pyrite, chalcopyrite (in places some bornite and chalcocite),and hematite, all in unimportant amounts. Specmiens near thefault plane show results of the fault pressure by a schistose tendency,and under the microscope by undulatory extinction and shatteredcondition of some of the quartz grains.The schist series forms a belt three-fourths to 1^ miles wide, withnorth-south course. The strike is N. 50? W. to N. 60? W., wath dip of60-80? to the southwest. The rocks range in color from light grayishgreen to dark blue and in structure run from moderately schistoseto highly schistose. The light-colored members are typical seri-cite schists, resolvable under the highest power of the microscopeinto a dense quartz mosaic, knit with shreds of sericite in parallel align-ment. The darker members, which have the greater distribution, arechloritic schists showing under the microscope predominant chlorite,accompanied at times with sericite, enwrapping grains and mosaicmasses of quartz. All sections are mineralized with pyrite in minutegrains and crystals, which are scattered through the quartz, chlorite,and sericite alike. As shown by the lack of pressure effects due tocrystal growth and the frequent presence of pyrite crystals inter-cepting otherwise continuous laminse, the pjTite is judged to havebeen deposited mainly through replacement. Gregory, who hasmade a detailed petrographic study of the schists, finds that theygrade from those showing no original structure to those definitely ofigneous origin, "formed by the alteration of quartz-porphyrites andprobably also of acid volcanic tuffs." From a study of several thinsections, comparison wdth similar rocks of Virgmia and North Caro-lina,^ and careful consideration of the chemical analyses given byGregory, the writers believe that it can be affirmed with considerablecertainty that the schist series represents the mashed equivalent ofvolcanic rocks of acid to intermediate character.Faulting has been profound in the region, but is largely localized inthe major overthrust already referred to and attendant cross faultsof minor development. The ore deposits are confined to the faultzone, and the principal deposition has taken place in the angles formedby the intersection of the cross faults with the major fault. Thelatter may be traced along the surface and is marked b}" outcrops ofhematite which, however, according to Sticht, bear no relation tothe pyrite and is not its gossan.
1 The sericite and chlorite schists of Mount Lyell are strikingly similar, both in appearance and in micro-scopic detail, to analogous copper-bearing schists of the Virgilina, Gold Hill, and Cid districts in Virginiaand North Carolina, which have resulted from the dynamic metamorphism of volcanic rocks.
NO. 2005. MOVVT LYELL COPPER DISTRICT?GILBERT d POGVE. 613THE MINES.Two mines only need be considered in detail, as these represent themajor development of the field and embrace the features of interestshown by the minor deposits.Mount LyeU mine.?This mine has opened up a pyritic mass of re-markable size, carrying a low copper content and values in gold andsilver. The ore is fine-grained, homogeneous pyrite, bearing chalco-pyrite and including evenly distributed gangue of quartz and baritein extremely subordinate amount. The ore body lies entirely withindeeply dipping schists, adjacent to and approaching on the foot-wallside within a few inches of the conglomerate fault contact. Its con-figuration is that of a horn-shaped body, tapering downward, withmaximum dimensions of 270 by 660 feet at the 400-foot level. It hasbeen mined en masse by the open-cut system, and in 1906 had beendeveloped to a depth of 730 feet. At present the mining is carriedon underground, the open cut nearing completion.The body as a whole is low grade, the copper averaging from 0.6to 0.75 per cent, with silver running from 1.10 to 1.75 ounces andgold from 0.06 to 0.08 ounce. The mass is singularly free fromdeleterious elements, containing only about 0.25 per cent arsenic, lessthan 0.17 per cent antimony, no bismuth, and traces only of seleniumand tellurium. Along the contact and about the periphery the orebody is locally richer from deposition of higher percentage of chalco-pyrite and addition of tetrahedrite and enargite bearing areas.North Mount LyeU mine.?This mine is the richest property andlargest producer in the district. It differs from the Mount LyeQ inthat the predominant ore is bornite, which, together with subordinatechalcocite and some tetrahedrite, pyrite, and chalcopyrite, formslenticular masses in sericitic and chloritic schists. These minerals,with considerable sihca and some barite, have been deposited asstringers and lenses following the lamination of the schistose rocks,forming impregnated zones or "fahlbands" of ill-defined limits,representing combined replacement and interlaminal deposition.The ore is worked by underground mining and the operations havepenetrated to a depth of 1,100 feet. The values in copper run muchhigher than in the Mount Lyell mine, averaging from 5 to 7 per cent,though the precious-metal content is less. Owing to the siliceouscharacter of the ore it forms an efficient flux for the pyritic ore ofMount Lyell, permitting the lowest grades of the latter to be profitablyworked.
614 PROCEEDINGS OF THE NATIONAL MV8EVM. vol.45.THE ORE MINERALS?MOUNT LYELL MINE.?The ore minerals at the Mount Lyell mine, in the order of theirobserved prominence, are pyrite, chalcopyrite, enargite, tetrahedrite,sphalerite, galena, bornite, and chalcocite.Pyrite.?The Mount Lyell ore is dominantly pyritic and the averageore specimen appears to consist purely of densely granular pyrite withaccessory quartz. Where the development of quartz is sufficient tobe conspicuous its distribution is not uniform, but tends towardsegregation into bands. In polished sections the banding becomesmore apparent and presents a pUcated and distinctly schistoseaspect. (See fig. 1, PI. 48.) Under magnification the pyrite appearscharacteristically granulated and is not intergrown with other sul-phides. Its bearing toward quartz is both that of contemporaneousand of later development.Ghalcopyrite.?The copper content of the Mount Lyell ore islargely due to the presence of chalcopyrite, which permeates thepyritic ore in the most intimate fashion. It is imperfectly distin-guishable in hand specimens, but under the microscope is seen totraverse the pyrite in stringers and form a network enmeshing grainsand cementing fractured iadividuals. (See fig. 3, PI. 49.) Its devel-opment is especially marked along the quartzose bands, and themclosing fUaments decrease outward from such areas, in some in-stances leaving the denser pyrite of the section entirely free fromobservable chalcopyrite. Such pyrite, free from visible chalcopyriteeven at a magnification of 200 diameters, however, was found to reactfor copper.Enargite.?^The occurrence of this mineral is highly localized.Where present it does not permeate the ore after the fashion of chal-copyrite, but occurs as relatively large, irregular, confluent areasuiclosing breccialike pieces of pyrite. (See fig. 2, PI. 48.) Its mostinteresting microscopic feature is an ever-present impurity in theform of chalcopyrite, which is disseminated throughout the enargitein minute patches, networks, stringers, and disconnected points. (See
1 The microscopic work was done on polished sections with a metallographic microscope using veritcalillumination from an acetylene light, at magnifications ranging from 30 to 200 diameters. The mineralswere identified by noting their characteristics, such as color, hardness, structure, tarnish, and etching effects,upon areas sufficiently large to furnish fragments for blow-pipe tests ; by which means criteria were obtainedfor the identification of these minerals even where microscopically developed. In making the photographsit was found desirable to increase the color contrasts by developing tarnishes by brief treatment with acid;for this purpose nitric acid was most useful.A useful hardness test, applicable to the determination of the relative hardness of adjacent grains Inopaque sections, was developed during the course of the study, and may be applied as follows: Havingcentered the microscope tube on the contact between two mineral grains, place a small metal straightedgeon the section and move until its edge intersects the two grains; then remove the section from the microscopestage, holding firmly the straightedge in place, and, by means of a knife point, draw a line across the twograins, being careful to press uniformly throughout. Replace the section under the microscope and notethe size of the channel as it passes from one grain to the other. The larger channel, of course, will lie inthe softer mineral. This method is sensitive to within a half degree of hardness in the customary scale andis applicable to grains as small as 0.5 mm. in diameter.
NO. 2005. MOUNT LYELL COPPER DISTRICT?GILBERT d POOVE. 615figs. 1 and 2, PL 50.) A noteworthy feature of this included chal-copyrite is that it is everywhere richer toward the margin of the enar-gite areas and is not infrequently strongly concentrated close to orat the border. It nowhere crosses into the pyrite, nor does it seem tobe related to the chalcopyrite already referred to as disseminatedthrough the pyrite ore.Bornite, chalcocite, tetraJiedrite.?These minerals, while important ascorrelating the Mount Lyell ores with those from North Mount Lyell,are exceedingly limited both in quantity and in extent, and since thekmode of occurrence is analogous to that observed in ore from. NorthMount Lyell they may be reserved for discussion under that head.SpTialerite and galena.?These minerals represent highly localizedphases of the ore and are nowhere prominently developed. They areconfined to the pyrite and were in no place observed in associationwith the copper sulphides. Sphalerite is much more prevalent thangalena and wherever the latter does occur it is in intimate associationwith the former. Occasional sections show both together, or sphal-erite alone, as disseminated grains in the pyrite ore; but commonlytheir occurrence is as veinlets traversing the section.THE ORE MINERALS?NORTH MOUNT LYELL MINE.In the North Mount Lyell workings the same minerals are to befound as at Mount Lyell, but in relative proportion so different as toproduce ores of entirely divergent character. Here the minerals, inorder of their importance, are bornite, chalcopyrite, chalcocite, tetra-hedrite, and pyrite; and these form mineralized zones in the schistsand not a great sulphide body as at Mount Lyell, where pyrite isdominant.Bornite.?The ore mineral of widest development and greatest sig-nificance at North Mount Lyell is bornite. It occurs alone, m associa-tion with pyrite, and admixed with other copper sulphides. (1 ) Whereoccurring alone it forms lenses within the schists and presents nomicroscopic features of note. (2) In association with pyrite it is con-fined to quartzose patches and channels of megascopic proportionswithm a pyrite-quartz rock similar to the typical ore of Mount Lyell.(3) The third type of occurrence is the dominant one. The borniteis in close association with chalcopyrite, or chalcocite (with or withouttetrahedrite), or both, and the ore forms lenticular areas and stringerswithin the inclosing schists. In polished sections scattering pyritegrains show up in ore and gangue alike, and in places granular pyriteaggregates are visible. Toward chalcopyrite, bornite has a variedbearing. While the two are often developed in intimate association,as if intergrown (fig. 2, PI. 51), there is in other sections a distmctlynoticeable tendency for the chalcopyrite to associate itself with granu-lar pyrite aggregates where such occur in the section, and for thebornite in a general way to envelop the association as a whole. In
616 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 45.one section the bornite grains, when examined at 200 diameters, areseen to be bordered by chalcopyrite, or by tetrahedrite, or both. (Seefig. 3, PI. 51.) The relation between bornite and chalcocite is oftenthat indicative of contemporaneity; the boundaries are ordinarilyintricate and clear-cut, and good examples of graphic intergrowthswere observed. (Figs. 3 and 4, PI. 50.)Chalcopyrite.?In addition to the associations with bornite justdescribed, and minor interstitial development in pyrite, chalcopyriteat North Mount Lyell occurs alone inclosed in sericitic or chloriticschists. In polished sections under the microscope the chalcopyriteshows in every proportion from the merest development (fig. 1, PI. 49)to a solid opaque body with only a scattering of gangue (fig. 2, PI. 49).Chalcocite.?So far as studied chalcocite is a rather minor constit-uent of the ore and is always in close association with bornite. Insections across such specimens the bornite occupies relatively largeareas, with the chalcocite occurring here and there in smaller patcheseither as sharply defined individuals but with marginal lines variouslyembaying, and embayed in, bornite in the most completely intimatefashion, or rarely as graphic intergrowths. (See fig. 4, PL 50.)*Such chalcocite is clearly of contemporaneous development withbornite. Sections of this chalcocite, when etched by immersing afew minutes in dilute nitric acid, develop characteristic cleavage lines,as shown in figure 4, Plate 49. In two of the sections studied rela-tively large areas of bornite were found which assumed a granularcharacter toward their margins, and were encased in pure chalcocite.This structure is suggestive of secondary chalcocite, but no furtherexamples were found and even the ones in question were destroyedwhen the sections were repolished preliminary to more detailed study.It may be safely said that chalcocite deposited by descendmg surfacewaters is an ununportant constituent of the North Mount Lyell ore.Tetrahedrite.?This mineral is somewhat analogous to enargite atMount Lyel] in that its development is highly localized. Like enar-gite, too, it is intimately associated with extremely fine chalcopyritediscernible only under high magnification. On the section tetrahedrite appears both as irregular patches up to 10 mm. in diameter,and as sharply defined, exceedingly narrow, marginal zones si.rrounding bornite grains. In many instances the bounding zone willconsist in part of chalcopyrite, the two alternating and together forming a beautifully sharp, irregular zone completely encasing the bornitearea. (See fig. 3, PI. 51.) This mode of occurrence for chalcopyriteis confined absolutely to tetrahedrite-rich areas, and its relationship tobornite is totally different from that in the conventional tetrahedrite-free ores of the North Mount Lyell mine.
1 These figures closely resemble crystallographic intergrowths of bornite and chalcocite in the copperores of Virgilina, Virginia. See Laney, Proc. U. S. Nat. Mus., vol. 40, 1911, pi. 68.
NO. 2005. 3I0VNT LYELL COPPER DISTRICT?GILBERT & POGVE. 61 YPARAGENESIS OF ORE MINERALS.The one mineral whose genetic relationships are everywhere sharplydefined is pyrite. Its bearing toward the other sulphides is clearlythat of a mineral of prior development. In most sections, however,there are certain examples of interassociation which suggest that thesulphide development from pyrite onward was one of sequential stagesrather than of distmct isolated periods.Among the copper minerals proper there is nowhere any sharplydefined order of sequence such as exists between the group as a wholeand the pyrite. Indications pohit strongly, however, to chalcopyriteas having been the first to follow the lead of the iron sulphide. Itswide diffusion, the intimacy of its occurrence everywhere with themanifestly earlier pyrite, and the frequency with which it is to befound inclosing and enmeshing pyrite clusters with the whole engulfedin bornite, by themselves would be conclusive. Elsewhere, however,chalcopyrite and bornite occur intimately intergrown (see fig. 2,PL 51) as if of contemporaneous development. From these twotypes of ? relationships the inference would be that a period of clialco-pyritization passed into one productive of chalcopyrite and bornitetogether.Of the rich copper minerals bornite shows itself not only the onemost extensively developed, but the one most intimately associatedin order of continuity with chalcopyrite. In some instances it isintergrown with chalcopyrite (fig. 2, PL 51); in other instances it isintergrown with chalcocite (fig. 4, PL 50) ; there are also numerousintermediate examples of its occurrence independently of chalcopyriteor chalcocite. These associations point strongly to a period of bornitedevelopment that was inaugurated while chalcopyrite was still form-ing, continued through a period of its own, and closed with simulta-neous precipitation of bornite and chalcocite. Certain relations ofthe latter mineral tend further to indicate that it continued to formfor a while after bornite ceased depositing.Tetrahedrite, occurring as replacement rims to bornite grains, isdistinctly later than that mineral. Though never associated withchalcocite so as to indicate relationship, tetrahedrite is judged to besubsequent to it also, since chalcocite is m part contemporaneouswith bornite. With tetrahedrite occurs chalcopyrite, in minute pro-portions, having analogous bearing toward bornite; this chalcopyriteis of course also later than bornite and represents a second generationof chalcopyrite. This chalcopyrite and the tetrahedrite show everyindication of synchronous deposition. Their formation is due eitherto descending waters or is the result of further changes in the primaryore-bearing solutions, dependent upon some specialized condition.Tetrahedrite at North Mount Lyell is paralleled by enargite atMount Lyell. Both minerals are of localized occurrence and are
618 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.45.characterized by a close association with chalcopyrite, of specializeddevelopment. The enargite forms an apparent fracture-filling in'massive pyrite and incloses microscopic ramifications of chalcopyrite.These inclosures do not pass into the adjacent pyiite, nor do theypossess any arrangement suggestive of subsequent penetration of theenargite by chalcopyritc-bearing solutions. On the contrary, theypresent rather strong evidence of simultaneous development alongwith the enargite, as a kind of a residual crystallization as a result ofthe molecular adjustment forming enargite. As the enargite has notbeen aft'ected by the other mineralizing processes it may be inferredthat its formation represents a late stage of the depositional epoch.The microscopic study points unmistakably to the formation of theores through replacement of the minerals of the schists; and the seri-citic and chloritic components have been the first to be attacked andsubstituted. Gradual transitions from unmineralized rock to solid oreare often seen. In many places a schistose pattern delineated byresidual quartzes has been inherited by massive pieces of ore. (Seefig. 1, PL 50.) In most sections unreplaced shreds and fragments of theordinal schists may be detected. One section disclosed a hexagonalquartz crystal with embayments filled with pyrite and enargite, show-ing in striking manner the corrosive effects of sulphide solutions evenupon that mineral. (See fig. 1, PI. 51.) Accompanying dominantreplacement a certain amount of interlaminal deposition is also evi-denced, but few examples are free from some replacement as well;and this process is merely a preliminary to the dominant one.The study leads also to the conception that the ore deposition tookplace during a distinct mineralizing epoch marked by solutions pro-gressively changing in composition and depositing a series of sulphideminerals in sequential and transitional stages. The order of deposi-tion, as evolved, runs from cupriferous pyrite through chalcopyrite,bornite, and chalcocite, to the tetrahedrite-enargite group (accom-panied by chalcopyrite of a second generation). From this it mustnot be inferred that the formation of any one ore mineral was con-fined to any one period, or that the sequence was absolute; on thecontrary there is ample evidence of transitions and overlappings, andmany complications undoubtedly intervened to make the process evenmore involved. What is strongly manifest, however, is that the de-position of any one of the sulphide minerals, in so far as it is a primeessential in the ore as a whole, was confined to some given period inthe evolution.A further generality, so persistently applicable as to seem not with-out significance, is one involving a relation between three broad fea-tures of the principal ore minerals, namely, the proportion betweentheir respective iron and copper contents, the order of deposition evi-denced by them, and the extent of their individual participation in
NO. 2005. MOUNT LYELL COPPER DISTRICT?GILBERT d- POGVE. 619the mineralization. The succession of deposition, as evolved, is inexact harmony with the order of increasing copper content and withthat of decreasing ii-on content. Noting this apparent agreement asthe work with the metallographic microscope progressed, a sample ofpyrite free from included chalcopyrite, so far as could be detected witheven the highest power objective, was tested qualitatively and foundto give a copper reaction.^ While it is by no means certain from thisthat the copper present in the pyrite is not due to chalcopyrite of sub-microscopic order, it is of importance in coordinating the true pyritewith the chalcopyrite; for a dissemination of chalcopyrite so fine asnot to be distinguishable under the highest magnification, must havebeen present during solidification of the pyrite, and from this sub-microscopic chalcojjyrite there is every gradation up to the mega-scopically prominent chalcopyrite of the pyrite ore, referred to a po-sition consequent to joyrite in the order of crystallization. Accord-ingly, starting with what may reservedly be termed cupriferous pyrite,which is at once the most extensively developed and the earliest ofthe ore group, and passing successively through chalcopyrite, bornite,and chalcocite, the tendency is so marked as to make the differentmineral species seem indicative of successive points in a steadily di-minishing iron content and increasing copper content in solutions ofconstantly diminishing quantity.SECONDARY ENRICHMENT.This subject can be discussed only in a general way, because fewonly of the specimens available showed characteristics referable tothis process. Also the writings of Sticht and Gregory, while makingfrequent reference to enrichments in the ore bodies, do not in everyinstance present criteria suitable for discriminating whether suchenrichment is due to descending surface waters, or is merely a specialphase of primary deposition; indeed, such criteria are difficult toobtain and a problem of this kind could be successfully attacked onlythrough a metallographic study of specimens collected with this endin view. As the enriched portions of the ore are the ones naturallymost completely worked out, such an attainment is obviouslyimpossible.In general, it may be said that enrichment is more prominent inthe Mount Lyell body than in the North Mount Lyell deposit. Thelatter is marked by a uniformity of its mineral associations exceptinga slight increase in proportion of chalcocite to bornite in the lower
1 This result is the reverse of that obtained by Laney (Bull. 21, North Carolina Geol. and Eeon. Survey,1910, p. 92), Simpson (Econ. Geol., vol. 3, 1908, pp. 628-?35), and Finlayson (Econ. Geol., vol. 5, 1910, p.420), from metallographic study of "cupriferous pyrite" from Gold Hill (North Carolina), Butte (Mon-tana), and Huelva (Spain), respectively; all of whom found that in these ores the copper content is due todefinite copper minerals recognizable under the microscope, and where such are not visible the ore is copper-free.
620 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.45.levels. The pyrite mass, however, is not homogeneous throughoutm values. Its footwall portion is richer in copper, gold, and silverthan its hanging-wall portion, and several places about its periphery,but especially on the footwall side, are characterized by bonanzasformed of important admixtures of copper or silver sulphides, orboth. Such areas of higher values are principally border phenomena;in the heart of the ore body only one such occurrence has been noted.This was a small, pipelike zone running from 3 to 6 per cent copper,due to chalcopyrite, and extended vertically from about the 400 to500 foot level.In the ore sections studied evidences of secondary enrichment werelargely lacking. In certain specimens, however, chalcocite of prob-able secondary deposition was recognized as different from other andpredominant chalcocite, which was in part of contemporaneous forma-tion with the bornite and ever in sequential genetic relation with theother primary sulpirides. Certain areas of tetrahedrite and chalcopy-rite also showed relations suggestive of secondary origin.GENESIS OF DEPOSITS.To recapitulate, the ore deposits are of two kinds: (1) Great lens-shaped masses of nearly pure sulphide ores, the Mount Lyell type,and (2) mineralized bands of schist (fahlbands), the North MountLyell type. Microscopic study of the ores shows that this differenceis one of degree and not of kind, and indicates that the same set ofore-bearing solutions gave rise to both kinds of deposition. Gregory ^has discussed at length the ore genesis, and his conclusions may bebriefly summarized as follows: Alkaline ore-bearing waters, risingalong fault planes during the period of faulting, absorbed heat gen-erated by these earth movements. With decreased pressure andlowered temperature incident upon approach to the surface, they de-posited their content, forming falilbands in the less shattered portionsof the schists and producing replacement masses in the highly shatteredand extremely permeable areas adjacent to fault loci. The deposi-tion, therefore, is regarded as "due to tectonic and not to igneousaction." Gregory does not explain the ultimate origin of the solu-tions nor of their metallic contents.Consideration of the microscopic features of the ores has led thewriters to believe that Gregory's explanation is not entirely adequate,and that the ore-bearing solutions were a deep-seated developmentfrom a differentiating mass of igneous rocks ^ and that these solutionsrose along structurally developed channels, changing gradually incomposition from the beginning to the end of the depositional epoch.
' Australian Inst. Min. Eng., vol. 10, 1905, pp. 145-15C.2 A similar conception was developed by Spurr in 1907 (A theory of ore deposition, Econ. Geol., vol. 2,1907, pp. 781-795) and later further elaborated by him (Econ. Geol., vol. 7, 1912, pp. 485-492). Finlaysonapplies a somewhat similar explanation to the origin of the Huelva pyrite deposits.
NO. 2005. 3I0UNT LYELL COPPER DISTRICT?GILBERT & POGUE. 621That the ore deposition was conditioned by structural features andrepresented a combination of replacement and impregnation, withpredominance of the former, seems conclusive and needs no furtherelaboration here. ANALOGOUS DEPOSITS.Cupriferous pyritic deposits of the Mount Lyell type play so impor-tant a part in the world's copper reserve ^ and present features ofsuch general interest that it may be profitable to review briefly thesignificant geological characteristics of the leading representatives.Ural Mountains.^?Numerous lenses and sheets of massive cupriferouspyrite, occurring in schists and greenstones of the Ural Mountains,contribute to the copper output of that region. The pyritic ore runsabout 3 per cent in copper, due to later interstitial chalcopyrite,which is associated with some sphalerite and galena, and in onemine, bornite. Both the wall and ore are cut by joint seams carryingenrichments of chalcopyrite and tennantite mixed with white veinquartz and sometimes barite; these are distinctly later than thepyrite, though not necessarily attributable to the action of de-scending waters. The masses are replacement deposits in theschistose rocks.^Norway.?Copper-bearing pyi'itic ores are extensively developedin this country, the districts of greatest import being the Sulitelma,north of the Arctic circle, and the Roros and the Meraker near Trond-hjem in central Norway. These deposits, which are notably similar incharacter, are lenticular masses of pyrite, with admixed chalcopyrite,averaging from 2^ to 3 per cent copper, and occurring within crystal-line schists alongside intrusive masses of gabbro, or soda granite.The bodies are comparatively small in horizontal dimensions, rarelyexceeding 60 feet in width, but extend downward to great depths.According to Vogt* they originated from solutions which were expelledfrom the intruding gabbroid and related gi-anitic masses, anddeposited their metallic content along the slipping planes of theschists durmg their metamorphism.Rammelsherg.?The oft-discussed deposits of Rammelsberg, in theHartz Mountains of central Germany, consisting of pyrite with chalco-pyrite, galena, sphalerite, arsenopyi-ite, barite, etc., intercalatedin metamorphosed clay slates, have been cited ^ as analogous to theMount Lyell occurrence. According to Lmdgren and Irving,? how-
' The importance of this type of copper deposit is apt to be underestimated in the United States wherecopper is obtained largely from ores of an entirely different order. Abroad the dominant type of copperdeposit is pyritic.2 Turner, Mining Mag., June, 1912.3 Turner, Econ. Geol., vol. 7, 1912, p. 709. Knox, idem, pp. 295-297.
* Trans. Amer. Inst. Min. Eng., vol. 31, 1901, p. 141. (See also, Weed, The copper mines of the world,1907, pp. 103-106. Beck-Weed, The nature of ore deposits, 1905, vol. 2, pp. 462-465.) *
' See Gregory, Australasian Inst. Min. Eng., vol. 10, 1905, pp. 179-1816 Econ. Geol., vol. 6, 1911, pp. 303-313.
622 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.45.
ever, the ore itself has been dynamically metamorphosed, the sulphidemass (excepting the pyi-ite which, being too hard, has suffered shat-tering only) having flowed like " thick muck.
"
Huelva, Spain}?The greatest single copper-producing districtabroad, ranking fourth among those of the world, occupies an east-west mineralized zone, lying mainly in the Province of Huelva,southern Spain, and including the well-known deposits at Tharsisand Rio Tinto. The ore is massive, homogenous pyrite, carryingchalcopyrite and subordinate associated galena and sphalerite, withlocal enrichments of chalcocite and minor bornite. The ore-bodies arelenticular masses, many in number and ranging up to enormous sizes(largest about 3,000 by 600 by 1,500 feet) and are mainly inclosedwithin those portions of Paleozoic slates and intrusive porphyries thathave suffered severe dynamic metamorphism and been convertedinto schists (chiefly sericitic). The ore bodies are in proximity to ex-tensive intrusions of acid and basic igneous rocks, showing advanceddifferentiation, and are usually located along lines of more or less faultmovement. Microscopic examination of the ores by Finlaysonresults in the following conclusions i^ (1) The copper occurs as adefinite mineral in the ore and is not chemically combined with thepyrite. (2) The primary mmerals have deposited in the foUowingorder: pyrite, chalcopyrite, sphalerite, galena. (3) The ore bodieshave been secondarily enriched; in the lean deep ores chiefly by achange of chalcopyrite to chalcocite, and in the richer, or shallowerores, chiefly by a deposition of chalcopyrite followed by chalcocite.Bornite occurs in very subordinate amounts, and appears interme-diate in formation between chalcopyrite and chalcocite. (4) Theonly primary copper mineral is judged to be chalcopyrite; the othercopper minerals appear to have resulted from descending solutions.The deposits are conceived by Finlayson to be due to the replace-ment of altered rocks by solution, rising along structural planes ofmaximum permeability, and originating through concentration byborder segregation in deep-seated igneous rocks prior to their finalintrusion and consolidation.Shasta County, California.^?The deposits of this important copperdistrict comprise numerous and extensive lenses of cupriferous pyriteinclosed within mashed alaskite-porphyry, which near the ore bodiesis practically a schist, containing considerable sericite (with parago-nite) and chlorite. The ore, which averages 3 to 3^ per cent copper,is pyrite with chalcopyrite and subordinate sphalerite and in lessamounts still, galena, bornite, and chalcocite; the gangue is quartz,calcite, and barite. The deposition is attributed to replacement ofthe schistose rock by solutions, expelled from nearby igneous rocks
1 Finlayson, The pyritic deposits of Huelva, Spain. Econ. Geol., vol. 5, 1910, pp. 356-372 and 403-437.2 Idem , p. 420.8 Graton, Bull. 430, U. S. Geol. Survey, 1910, pp. 71-111.
NO. 2005. MOUNT LYELL COPPER DISTRICT?GILBERT d POGUE. 623upon their final consolidation, which found congenial conditions forprecipitation in the more schistose phases of the alaskite-porphyrywhere this rock presented maximum permeability and surface ofattack.Of the ore minerals pjrrite was the first to crystallize in most in-stances; sphalerite, on the whole, is later. Chalcopyrite is the young-est of the important minerals, forming vemlets around and in othersulphides; it prefers the company of sphalerite to pyrite and showsalso an affinity for quartz and barite. Primary bornite is commonlyassociated with chalcocite, and the two either take the place of chal-copyrite or are intimately associated with it. Secondary borniteand chalcocite are also present, but are readily distinguished fromthe former.Ducktown, Tennessee}?These important deposits, which arenotable for successful working of low-grade ore, form lenticular totabular masses, inclosed within deeply dipping schists of sedimentaryorigin, and represent limestone intercalations which have sufferedreplacement by ore-bearing solutions from probable magmaticsources. The primary ores consist of pyrrhotite and pyi'ite withchalcopyrite and subordinate sphalerite; these are associated withspecularite, magnetite, actinolite, calcite, tremolite, quartz, pyroxene,garnet, zoisite, chlorite, mica, graphite, titanite, and feldspar. Sec-ondary enrichment has proved of importance only near the surface,where chalcocitization has taken place in a narrow zone rarely overa few feet in thickness, between the gossan and primary sulphidezone. The Ducktown occurrence is not a strict analogue of theMount Lyell deposit, though presenting many features m common .^SELECTED BIBLIOGRAPHY.Fawns, Sidney.Some notes on the Mount Lyell Mine, Tasmania. Inst, of Min. & MetalL, vol. 4,1895-96, pp. 279-289.Describes briefly geology of deposits and treatment of the ores.Gregory, J. W.The Mount Lyell Mining Field, Tasmania. With some account of the geology ofother pyritic ore-bodies. Trans. Australasian Inst, of Min. Eng., vol. 10, 1905,pp. 26-196. 29 figs., 17 plates, 1 geol. map.Gives results of thorough geological investigation, including detailed petrography. Containscomplete bibliography to year of publication.Mount Lyell Mining P'ield. Australasian Mining Standard, 1905 (17 numbers).Same as preceding.
1 Emmons and Laney, PreUminary report on the mineral deposits of Ducktown, Tennessee. Bull. 470,U. S. Geol. Survey, 1910, pp. 151-172.2 From the foregoing review it is seen that the great cupriferous pyrite deposits of the world are strikinglyalike in their geologic relations, mineralogic content, and origin. It may therefore be generalized that anyregion of schistose rocks (especially sericite and chlorite schists) that is intruded by a differentiated seriesofigneous rocks, is one favorable to the occurrence of such deposits, and where in such regions gossans arefound, which in their outcrops show values in gold and silver and the presence of barite, lenses of cupriferouspyrite at depth may be anticipated with considerable confidence.
624 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.45.Peters, E. D.The Principles of Copper Smelting. New York, 1907.Chapter on pyritic smelting (pp. 213-338) deals extensively with practice at Mount Lyell. (See alsoPeters, Modem Copper Smelting.)Sticht, Robert.Ueber das Wesen des Pyrit-Schmelzverfahrens. Metallurgie, Halle, a. S. 1906,52 pp.Metallurgical.Stand der Betriebe der Mount Lyell Mining und Railway Company (Ltd.), amSchlusse des Jahres 1905. Metallurgie, Halle, a. S. 1906, 55 pp., 13 figs.Gives geology, method of mining, and treatment of ores.Mining and Smelting at Mount Lyell, Tasmania. The Mineral Industry during1907, vol. 16, pp. 385-442.Outlines the geology of the deposits, describes the mines and their development, and gives anaccount of the reduction works and metallurgical treatment of the ores.Weed, W. H.The Copper Mines of the World. New York, 1907, pp. 163-169.Gives geological description of the Mount Lyell deposits.Text-figure: Sketch map of the Mount Lyell District. (Modified from Gregory.)EXPLANATION OF PLATES.Plate 48.Fig. 1. Polished section of typical cupriferous pyrite ore from Mount Lyell Mine,showing schistose pattern resulting from replacement. Natural size. Cat.No. 77549.2. Polished section of pyrite (light) with seams and veins of enargite (dark).Mount Lyell Mine. Natural size. Cat. No. 77552.Plate 49.Fig. 1. Incipient stage of replacement. Chalcopyrite (white) subordinate to gangue(gray). X 30. North Moimt Lyell Mine. Cat. No. 77571.2. Advanced stage of replacement. Chalcopyrite (light) predominating overgangue (dark). X 30. North Mount Lyell Mine. Cat. No. 77571.3. Chalcopyrite (dark) cementing gi'anulated and shattered pyi'ite (light). X 30.Mount Lyell Mine. Cat. No. 77549. Shows that the chalcopyrite wasdeveloped later than the pyrite.4. Area of primary chalcocite, showing characteristic cleavage developed byetching with dilute nitric acid. X 30. North Mount Lyell Mine. Cat. No.77593. Plate 50.Fig. 1. Enargite field (light) including minute ramifications of chalcopyrite (dark).Pyrite grain to left of section. X 30. Mount Lyell Mine. Cat. No. 77552.2. Portion of enargite field of figure 1, enlarged to 180 diameters, showing the in-cluded chalcopyrite (dark). The latter, though appearing so in the photo-graph, is probably not a fracture filling in the enargite, but of contempora-neous development.3. Bornite (dark) and chalcocite (light), showing crystallographic intergrowthbetween the two, indicative of simultaneous development. X 30. NorthMount Lyell Mine. Cat. No. 77593.4. Crystallographic intergrowth of bornite (dark) and chalcocite (light). Portionof figure 3 with magnification X 120. North Mount Lyell Mine. Cat. No,77593.
NO. 2005. MOUNT LYELL COPPER DISTRICT?GILBERT & POGUE. 625Plate 51.Fig. 1. Quartz crystal partly replaced by enargite (light) and pyrite (dark). Theblack areas are irregularities in the section. X 40. Mount Lyell Mine.Cat. No. 77552.2. Intergrowth of bornite (dark) and chalcopyrite (light), indicative of simultane-ous development. North Mount Lyell Mine. X 40. Cat. No. 77601.3. Grain of bornite (fe) in gangue (g), surrounded by border of tetrahedrite andchalcopyrite (white). The tetrahedrite and chalcopyrite can not be differ-entiated in the photograph, but their relations to bornite are similar; theywere probably developed simultaneously and are later than the bornite.X200. North Mount Lyell Mine. Cat. No. 77597.80459??Proc.N.M.vol.45?13 40

U. S. NATIONAL MUSEUM PROCEEDINGS, VOL. 45 PL. 48
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Polished Sections of Ore.For explanation of plate see page 624.

U. S. NATIONAL MUSEUM PROCEEDINGS, VOL 45 PL. 49
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Photomicrographs of Polished Ore Sections-FOR EXPLANATION OF PLATE SEE PAGE 624.

U. S. NATIONAL MUSEUM PROCEEDINGS, VOL. 45 PL. 50
Photomicrographs of Polished Ore Sections.For explanation of plate see page 624.

U, S. NATIONAL MUSEUM PROCEEDINGS, VOL. 45 PL. 51
Photomicrographs of Polished Ore Sections.For explanation of plate see page 625.