The Limestone Ores of Manhattan, Nevada

ECONOMIC GEOLOGY

WITH WHICH IS INCORPORATED

THE AMERICAN GEOLOGIST.

Vo•,. XVI. JANUARY, i92i. No. i

THE LIMESTONE ORES OF MANHATTAN, NEVADA.

HENRY G. FERGUS0N. 1

INTRODUCTION. Mostof theore deposits c•fthe Manhattan district are gold- bearingveins of a simpletype and presentno unusualfeatures, but other ores inclosedin a single thin bed of limestoneoffer in- terestingproblems. Manhattan is situatedin the southernpart of the northeasterly trending Toquima Range, about 35 miles north of Tonopah. The Toquima Range is one of the lessprominent of the many narrow, isolated mountain ranges which are such notable fea- tures of the Great Basin topography. The range is borderedby desert'valleys--Ralston Valley and Monitor Valley on the east and Big Smoky Valley on the west. On both sides,but particularly on the west, the boundarybetween rock in placeand valley fill is irregular, in marked contrastto the sharp lines of demar- cation on the easternfront of the Toyabe Range, which borders Big Smoky Valley on the west. The district was first visited by the writer in the summerof x9x5- At that time lodemining had suffereda collapseand little could be seenof the ore deposits. Work, therefore, was chiefly directedto a study 6f the complexitiesof the geologicstructure x Published by permission of the Director, U.S. Geological Survey.

I HENRY G. FERGUSON. and observationson the placers? The district was revisitedin' x9x9 and the geologyof the principaldeposits studied in some detail;another shorl• visit madein June,x9zo. The writer de- siresto acknowledgethe courtesiesextended to him by all the miningmen of the district. The detailedgeologic maps of the surfacegeology and undergroundworkings of the White Caps mine,made by Messrs.O. McCraneyand J. L. Dynan, were of •specialvalue. The writer gratefully acknowledgesthe assist- ancein criticismsand helpful suggestionsfrom his colleagues, particularly A. C. Spencerand W. T. Schaller. The ore depositsof the presentManhattan district date from April, x9o5, when "specimenore" was discoveredby John C. Humphreyon April Fool Hill. An old mine about4 milessouth of the presenttown of Manhattanhad, however,been worked in the late sixties. The discoveries of rich surface ore with plentiful free gold were made within a few hundredfeet of the road leadingwest from the old camp at Belmont. That such ore shouldbe passedunnoticed for 40 years must be attributed to the inconspicuouscroppings which characterizethe later dis- covered deposits. During x9o6 the district was in a stateof excitementand rich discoverieswere constantlybeing reposed. •The 'winter of x9o7-8 was markedby depressionfollowing the deflationof the boom,but the hard times,which led to temporarycessation of quartz mining, turned the attentionof the minersto the placers of Manhattan Gulch. For several years lode mining was con- ductedspasmodically, and generallythe placeroutput exceeded that of the lode mines. The best placer ground has, however, been exhaustedsince x9x5, and sincethat time the lode output has beenthe more important. In x9x7 the discoveryof rich ore in the lower levelsof the White Caps mine led to another boom,which, thoughshort lived, led to a considerableamount of exploratorywork. At the presenttime a small amountof 2 Ferguson. H. G., "Placer Deposits of the Man.hattan District, Nevada," U.S. Geol. Survey Bull. 640, pp. •63-x93, x9•7. LIMESTONE ORES OF MANHATTAN, NEVADA. 3 explorationwork is being carried on, but the productionis principally from the shallow surfaceworkings of lessees.

GENERAL GEOLOGY. The rocksof the southernportion of the Toquima Range include Paleozoicsediments, intrusive granodioritemasses of Cre-

F•c. z. Geologic map of the southern part of t'he T'oquima Range.

taceousor Eoceneage and Tertiary lavas and sedimentsand in- trusives. In the productiveportion of the Manhattan district HENRY G. FERGUSON.

(fig. 2) the older sedimentsare of Cambrianor Ordovicianage, intenselyfolded, cut by many faults and everywheremore or less metamorphosed. Paleozoic Sediments.--The Cambrian sediments, in which are the productiveore bodies,consist principally of micaceousschists. Lenticular beds of white quartzite and dark fine-grainedsand- stone are common. There are also several limestone beds, of which three in the lower part of the formation, and two near its top are sufficiently well defined to be traced on the surface. Other smaller beds are less persistent. Since many of the deposits occur as replacementsin limestone,these beds have been well exposedby prospecting,and consequentlythe structureof this part of the district could be worked out in detail. The threelower beds are but slightlyaltered and consistof blue-grayor white, fine-grainedcrystalline limestone. The lowest, locally known as the Pine Nut limestone, is about •o feet thick and is the only one which showsany developmentof silicate minerals, and that only in scatterednodules. The next, known as the Morning Glory limestone, is •5 to 20 feet thick and consistsof pure crystallinelimestone. The third bed, the White Capslimestone, is 30 to 35 feet thick and forms the wall rock of someof the most.important deposits of the region. Ex- ceptwhere altered by mineraliz!ngsolutions, it is a pure crystalline limestone, light blue in color and essentially free from silicates. The two limestonesin the upper part of the Cambrian terrane, on the other hand, show a considerabledevelopment of silicatesand range from white crystalline limestonesto rocks consistingessentially of diopside. The total thicknessof Cam- brian sedimentsexposed in the district probably exceeds3,00o feet. Above the Cambrian and tentatively assignedto the Ordo- vician is a fine-graineddark schist,about 400 feet thick. Above the dark schists-isa seriescomposed principally of a dark-gray thin-beddedlimestone, associated with which are varying amounts of black jasper. This seriesis probablyless than 400 feet thick. The first fossiliferous horizon is found above this dark lime- LIMESTONE ORES OF MANHATTAN, NEVADA. 5 stone. There are lenticular bedsof dark quartzite, overlain by a few feet of slatescarrying well-preservedgraptolites, indicat- ing a horizon equivalentto a part of the upper Pogonipforma- tion of the Eureka district s and to the Palmetto formation of the Silver Peak quadrangle.4 Above this is a dark jasperylime- stone,a few feet ir• thickness,identical in characterwith that below the quartzite. Above this is a considerablethickness of chloriticschist similar to that found directlyabove the Cambrian, but here diversifiedby occasionalbeds of impure brown limestoneand dark blockyslate. An isolatedoutcrop of Carboniferous(Pennsylvanian) sand- stone was found about a mile northwest of Manhattan. The relations to the other sediments are undetermined. Granitic Rocks.--Large massesof granitic rocks occupy a considerablearea in the Toquima range. The rock is in general coarselyporhpyritic, with numerousorthoclase phenocrysts, up to a coupleof inchesin length. Microscopicexamination shows that it rangesin compositionfrom a soda-graniteto a granodiorite, more commonlythe latter. There are also many dikesof very fine-grainedaplite. Theseconsist essentially of quartz and orthoclasewith varyingdmounts of muscoviteand 'a small amountof pyrite, apparentlyoriginal. In placesquartzose segre- gationsin the aplitesform a connectinglink with the .quartz veins. Spurr• describesan occurrencein the nearby Belmont district,in whichan aplitedike grades into a metalliferousquartz vein. Thenearest large exposure of graniticrock lies about 2 miles to the south of the productivemines. But it is probablethat there is a nearer granite massto the north, beneaththe Tertiary lavas, sincepebbles of granite are commonin the lower rhyolite 8 Walcott, C. D., Mon. U.S. Geol. Survey, vol. 8, p. 284, •884. Hague, A., Mon. U.S. Geol. Survey, vol. 20, p. 58, I892. Spurt, J. E., Bull. U.S. Geol. Survey No. 208, p. 9I, I9O3. 4 Turner, H. W., "A Sketch of the Historical Geology of Esmeralda County, Nev.," Amer. Geologist,vol. 29, p. 265, 19o2. • Spurt, J. E., "Quartz Muscovi,teRock from Belmont, Nevada," Am. ]our. Sci., 4th Set., vol. •o, p. 35•, t9oo. 6 HENRY G. FERGUSON. breccias,and the Ordovicianlimestones on SalisburyHill just south of the lava-covered area and show intense contact metamorphism. There is, however, a small dike of coarsemuscovite granite grading into aplite, about 1,5oo feet northeastof the White Caps mine, and two small massesof a much altered apliticrock have been cut on the 8oo-footlevel of the White Caps: The age of the granodiorite is probably late Cretaceousor early Tertiary, though no conclusivedata can be obtained in this range. Although the granitic intrusion probably closely followed the mountain-buildingmovements which producedthe intense folding, there is no evidenceof any direct association. The major folds had been developedbefore the rise of the intrusive to the level of the presentsurface, for the granodiorite cuts the folded rocks at all anglesand showsno sign of any gneissicstructure or even of important shearing. There is no evidenceof doming of the sedimentsby the intrusive. The rim of intensemetamorphism surrounding the granite is narrow. The limestonesclose to the contact are epidotized and abundantbiotite is developedin the schists. The more dis- tant effects are shown in the developmentof silicate.minerals, particularlydiopside, in someof the limestonesand the presence of small knots of silicates in the Ordovician schists. Tertiary Rocks.--The Tertiary rocks lie to the north of the productivepart of the districtand consistpredominantly of rhyolite. The earliestis a brecciacomposed of angular fragmentsof the older rockswithout volcanicmaterial. Rhyolite bracciascon- taining fragmentsof the underlyingsediments form a large part of the effusive rocks. The lower brecciasare cut by irregular intrusionsof a fine-grainedrhyolite. Near the top of the Ter- tiary seriesis a considerablethickness of shale,fine-grained sand- stone, water-laid tuff, and conglomerate. The finer grained phasesof this resemblevery closelythe Siebert tuffs of the Tonopahdistrict consideredto be of late Mioceneage. Above this comesanother seriesof rhyolite breccias. Following this there was intruded a considerable mass of rather coarse andesite porphyry. The principalbody of this rock lies about2 miles to LIMESTONE ORES OF MANHATTAN, NEVADA. 7 the northeast of the White Caps mine and numeroussmaller massesand dikes occur throughoutthe region. The principal intrusivemass appears to be laccolithicin nature and has tilted and wedgedapart the tuffs and brecciaswhich it intrudes. $tructure.--The Paleozoic rocks are compressedinto close folds in part overturnedtoward the north. The axes of the major foldstrend a few degreesto westof north, at right angles to the presenttrend of the Toquima range. The principal anti- cline is cut off obliquelyon its northern side by an overthrust fault which brings the Cambrian above the Ordovician schists and limestone. The overthrustdips to the southand southwest at from 360 to 7ø0 , and was undoubtedlyformed at the time of folding of the sedimentsthrough the irregularitiesof its plane maybe in partdue to later faulting'and tilting. A normalfault alongthe southernborder of the Cambrianschists may also date from this period,since it is cut by later faults trendingin an oppositedirection. As may be seenfrom the detailedgeologic maps (Figs. 2 and 3), the productivepart of the regionis cut by a largenumber of smallnormal faults. Thesecould be mappedwith certaintyonly in the portion of the territory traversedby the Cambrianlime- stone. In oneinstance, a smalldike of alaskiteappears to occupy a fault plane. Two of the faults were traced to the northward acrossSalisbury Mountain. On the northernslope of the mountain the contactbetween the Ordovicianand the Tertiary breccia follows the strike of one of thesefaults, so it is to be presumed that faulting was of Tertiary age. The field relations,however, do not precludethe possibilitythat the brecciawas deposited against a pre-Tertiary fault scarp. The faults are roughly parallel, for the most part varying in strike from a few degrees eastof north to abouteast-northeast, and there is a tendencyfor thoseof more northerly strike to be cut off by thosetrending to the eastward. This intense normal faulting is confinedto the productiveportion of the district. Numeroussmall faults are, however,observable in the vicinity of the prospectsin the lavas on Bald Mountain,3 milesto the north. There are alsonormal 8 HENRY G. FERGUSON. LIMESTONE ORES OF MANHATTAN, NEVADA. 9

faults of considerablemagnitude which in placescut the lavas but do not appearto extend to the sedimentsof the Manhattan district. These appear to be youngerthan the intrusion of the andesiteprophyry and probablyowe their origin to the readjust- ments following this intrusion. There is likewiseevidence of a period of faulting in the lavas prior to the andesiteintrusion. Erosion.--The lava-coveredregion was eroded until an ex- tremely mature topography,approaching the peneplainstage, was reached. The topographywas fairly generalover this portion of the Great Basin region, for Ball6 mentionsthe existence of similar old topographicforms on other rangesto the south, and Meinzer7 has found a similar mature topographyon the Toyabe Range. Ball considersthis mature land surfaceto be latePliocene in age,as it is laterthan the "later rhyolite"of early P!ioceneage and older than the later tuffs and older al- luvium which mark the transition from Pliocene to Pleistocene. Hencethe major portionat leastof the mountain-buildingmovements must have been confinedto the early Pleistocene,for at the periodmarked by the Pleistocenelakes a topographyap- proachingthe presenthad beenattained. The easternface of the Toyabe Range, a few miles west of Manhattan,presents a well-markedfault scarp. Faulting has continued until recent times, for Meinzer8 reports that fault scarpscut the alluvialfans at the canyonmouths. Smallpatches of Pleistocenegravel occur here and there along the borders of Manhattan Gulch, and the deep gravels of the gulchcontain fragments of bonesof Pleistocenemammals. It is evidentthat a topographywhich at leastin its main out- lines resembledthe present,has beendeveloped in Pleistocene time. During a periodof milderclimate than the present,pre- o Ball, S. H., "A Geologic Reconnaissancein SouthwesternNevada and Eastern California," U.S. Geol. Survey Bull. 308, pp. I6-I7, 99, ZI9, •6•, •02, •907. * Meinzer, O. E., "Ground Water in Big Smoky, Clayton and Alkali Val- leys, Nev.," U.S. Geol. Survey Water Supply Paper 423, p. 2•, I9•7.. o Meinzer, O. E., op. cit., p. 22. I0 HENRY G. FERGUSON. sumably contemporaneouswith the Pleistocenelakes Bonneville and Lahontan,two lakes existedin Big Smoky Valley?

ORE DEPOSITS.

VEINS.

The ores of the Manhattan district occur in veins and as replacementdeposits in limestone. Two typesof vein depositsare present,an unimportantolder group, connectedin origin with the intrusive grandiorite massesof the region, and a younger groupwhich is regardedas havingbeen formed during the period of Tertiary volcanism,although the productivedeposits are inclosedin the Cambriansediments. The replacementdeposits in the Cambrianlimestones are allied mineralogicallywith the Ter- tiary veins, and, though the evidenceis inconclusive,they are regardedas of contemporaneousorigin. Older Veins.--Veinsof the oldergroups have not beenprofit- ably workedin the ManhattanDistrict, though elsewhere in the regiondeposits genetically connected with the granodioriteand showingmineral associationscharacteristic of a more deep- seatedorigin than the Tertiary veinshave supported workable silver mines,as at Belmont,I5 miles to the northwestof Man- hattan, and Barcelona,near the crestof the rangebetween Bel- mont and Round Mountain. In the Manhattan District the veins representingthis epochof mineralizationare characteristically narrowand of no greatcontinuity. They are composed of white glassyquartz with minoramounts of calciteand here and there containpatches of coarselycrystalline sulphides, including pyrite, galena,tetrahedrite, and chalcopyrite.The tenor in precious metalsis low and the silvercontent is commonlygreater than the gold. Tourmalineoccurs as a minorgangue mineral in a few veinsof this type. In onevein a little feldspar(orthoclase or albite) is present,thus forminga connectinglink with the quartzosesegregations of the aplitedikes. TertiaryVeins.--The volcanic rocks, including the andesite o Meinzer, O. E., op. cit., p. 30. LIMESTONE ORES OF MANHATTAN, NEVADA. I I porphyry,which is post-Siebert(Miocene) in age,contain small veins, apparentlynot continuousover long distances,which usually follow joint cracksor minor faults in the lava. The ore of theseconsists of minuteveins of combyiron-stained quartz, with minoramounts of tabularcalcite, which is largelyreplaced by quartz, and rarely fluoriteand adularia. Pyrite has been presentbut is now completelyoxidized to limonite. 3Iere and there minute specksof free gold can be seenon the surfaceof the grains of oxidizedpyrite. The veinsof Tertiary age in the Cambrianschists have been the largestproducers of the Manhattan district: Theseare confinedto the vicinity of Gold Hill, just westof the town. The oresare of relativelysimple type. At the Reilly Fraction,Big Pine and Big Four minesthe schistsare traversedby innumer- ablelittle veinlets,for the mostpart followingthe beddingand jointing. Thesestringers contain free goldin a gangueconsist- ingchiefly of combyquartz, but in placescontaining quartz pseudomorphicafter tabularcalcite together with clustersof adularia crystals. The enclosingschists are pyritizedin places,but this pyrite is saidto be practicallybarren. The zoneof workableore is fairly well definedand is about 50 feet wide and 800 feet long. In the Big Four mine the ore is stopedto a depthof 300 feet, about xoo feet abovepermanent water level, but most of the productionfrom the three mines has come from less than too feet depth. The groupof mineswest of thesummit of GoldHill, including the Stray Dog, Jumping.•ack, Union lq'o. 9, and Little Gray, showsthe same type of mineralization,but here the veins are better defined and follow faults. There has been later movement along many of the veins and also along planesintersecting the veinsat small angles. Where this has occurred,gold, probably of secondaryderivation, is found in associationwith crushed vein material and gougy manganeseoxide. The mineralogicalcharacter of the schistores, particularly the presenceof abundantadularia and lamellar calcite replaced by quartz,indicates that they belongto the classof veinsformed 12 HENRY G. FERGUSON. undernear-surface conditions. The similarityof vein fillingto thatof theunproductive veins in thelavas suggests very strongly that both setsare of contemporaryorigin.

DEPOSITS IN LIMESTONE. The ore depositsin the limestonebeds of the Cambrianshow most complexmineralization and a wide variety of minerals. Productivemineralization is almostentirely confined to a single limestonebed, the upper of the three lower limestones,locally known as the White Caps limestone. This bed has been traced from a point about a mile east of the White Caps mine through Litigation Hill to the summitof April Fool Hill, where it is cut off against the overthrust fault.' Throughout its course it is broken by closelyspaced normal faults, mostly of small throw. This faulting appearsto be mostintense and complicatedin the region betweenthe White Caps and Manhattan Consolidated shafts. A detailed study of this region was made by Messrs. McCraney and Dynan of the White Caps Companyin connec- tion with ihe White Caps-MorningGlory litigation in I917, and throughthe courtesyof the officialsof the White CapsCompany, their map, with slight changesby the writer, is here reproduced (Fig. 3). The regionwest of the Consolidatedshaft was mapped by the writer on a muchsmaller scale, which admitsof showing only the more importantof the faults. Occasionalreverse faults strike in the samegeneral direction as the limestoneand are shownby the repititionof the limestone on the 3oo-footlevel 'of the MahhattanConsolidated, and on the 6oo-foot level of the Union Amalgamated. The different mines in the limestone,although near neighbors, show sucha wide diversityin their mineralizationthat a short descriptionof each will be given before the theoreticalside of the ore depositionis discussed. V?hite Caps Mine.--The White Caps mine appearsto mark the easternlimit of profitablemineralization of the limestone. The Zanzibar,White CapsExtension and Red Capsprospects LIMESTONE ORES OF MANHATTAN, NEVADA. 13 have exploredthe limestonebed to the east, but so far without success. The White Capsmine is developedby sevenlevels, the lowest of which is 80o feet below the collar of the shaft. The limestone is here cut by threemajor faultswhich strikenortheasterly and dip southeast.These are knownas the East Fault, White Caps

Fro. 3. Geologicmap of area betweenthe White Caps and Manhattan Con- solidated mines, by O. McCraney and J. L. Dynan, with minor modifications by H. G. Ferguson.

Fault and West Fault. There are also a numberof smallnortherly faults whosehorizontal displacement in most casesdoes not exceed5 or •o feet. There has beenmore recentmovement on the three main faults than on the minor faults, sincethey cut the earlier seriesand containrounded fragments of ore. The earlier faults precededthe ore depositionand apparentlyto a large extent controlledit. They are cementedby ore and almost losetheir identity in the ore bodies. 14 HENRY G. FERGUSON.

The ore bodiesoccur as large replacementsin the limestone. It is rare that mineralizationextends completely across the bed but usuallyfollows one wall, more commonlythe footwall. The areas in which the small faults are most numerousappear to be best mineralized. It is not uncommon for the ore and associated mineralized limestone to have a horizontal section of as much as •oo by 3ø feet. Two main zonesof mineralizationhave been developed,the east ore body which has yielded ore from the

Pz...t•, o,• ,310 voo.r z.l•v•x,

Fro. 4. Section through White Caps shaft, looking north; and plan of 310- foot level. (Mineralized areas shaded.)

surface to the 8oo-foot level, and the zone containingthe West and Shaft ore bodies,which has been productivefrom near the xoo-footlevel to the 55o-foot level. The easternore body has a steepeasterly pitch, which brings it in contactwith the eastfault betweenthe 550 and 8oo-foot levels. The westernbody is in LIMESTONE ORES OF MANHATTAN, NEVADA. 15

contactwith the White Caps fault betweenthe 45o and 55o-foot levels. The gangue minerals of the White Caps mine are calcite, 'quartz, and less commonlydolomite, fluorite, and leverrierite. The metallic mineralsinclude arsenopyrite, pyrite, stibnite,realgar and orpiment. Cinnabaris reportedto have beenpresent in the ore found above the 2oo-foot level.• The arsenopyrite, realgar, and orpiment are auriferous, but visible gold is completely lacking even in the oxidized ore. Oxidized minerals, presentonly in minor amount and confinedto the two upper levels, include limonite melanterite and oxidation productsof stibnite such as valentinite and stibioconite. The followinganalysis of ore from the 3oo-footlevel is quoted from Dynan's paper

StOa. AI•02. CaO. Fe S. Sb. As. Mgo. H•O. CCh. Total. 55.8 x.8 7.2 8.9 8.2 0.7 x.5 3.2 0.8 9.2 97.3

Dynan has calculatedfrom this the following mineral com- position:

Pyrite ...... x3.8% Arsenopyrite ...... 3.3 Stibnite ...... x.o Calcite ...... x9.6 Quartz ...... 52.0 (Ca, Fe, A1) Silicates ...... 7.4 97.•

Ore from the lower levels of the easternore body showsa muchhigher percentage of' arsenic. The earliest of the minerals associated with the ore is white calcite. Huge rhombs are often observedin large irregular shrinkage cavities. The mineral is coarsely crystalline, and cleavagefaces 8 inchesacross are frequently seen. The alteration of limestoneto coarsecalcite commonly follows the bedding and joint planesof the limestone. These replacementsvary in xxDynan, J. L., "The White Caps Mine, Manhattan, Nev.," Mining and Scientific Pre&r, vol. x3, p. 885, Dec. I6. x916. x2Dynan, J. L., lac. cit., p. 884. 16 HENRY G. FERGUSON. size from small lensesan inch or two acrossto large massesex- tending nearly acrossthe limestonebed. The calcite is usually glaringwhite in color,except where the presenceof finely dividedrealgar impartsa pinkishtinge. In a few places,however, small pieceshave been obtainedapproximating Iceland sparin clearnessand transparency. As far asknown, the calcite itself is everywherebarren and is mostplentiful near the outer edgesof the ore bodies. In partsof the minesome coarse white

F•s. 5 and 6. Calcite replacing limestone, 4o0 foot level, White Caps mine. calciteoccurs unaccompanied by ore, and is also found here and there throughoutthe district, even in the dark Ordovician limestone,though it is 'bestdeveloped in the ore-bearinglimestone. The calciteis persistentin depth. The bestore of the mine consistsof a dark, fine-grainedquartz, in placesso denseas to have the appearanceof blackjasper. This occursas a replacementof the limestone and, to a less extent, of the coarse calcite. The quartz is closelyconfined to the ore bodiesand doesnot showthe wide distribution characteristic of the barren calcite. The replacementof limestoneby quartz has involveda considerableloss of volume,indicated by numerousdrusy cavities,usually elongate with the beddingof the limestone. In theseminerals of later age --dolomite, stibnite,realgar, and orpiment,have beendeposited. In placesthis reductionof volume, possiblyassisted by the ab- stractionof the arsenopyriteby later solutions,has beensufficient to permit collapseand local brecciationof the quartz, following later movementalong the major faults. LIMESTONEORES OF MANHATTAN,NEVADA. 17

The hand lens disclosesoccasional minute specksof pyrite in the quartz. Under the high power of the microscope,however, the dark coloris seento be due to the presenceof carbonaceous matter and minute crystalsof a gray metallic mineral. These range from the lower limit of visibility to a maximum of 0.05 mm., but most of them are lessthan 0.22 mm. in diameter. Many of theselittle crystals are so twinned'as to resembleminute jack- stones. Qualitative tests indicate.that the metallic mineral is arsenopyrite. Evidently it is highly auriferous for assaysex- ceeding$200 per ton have beenobtained from this type of ore. No gold canbe seenunder the microscopeand noneis obtainable by amalgamation. Palmer,xa however, found that this dark ore, concentratedby panningand treated with nitric acid, yielded minutespecks of free gold. A little muscovite was observed in some of the thin sections of the dark quartz. It. is not certain whether this is contemporaneouswith the inclosingquartz or is residualfrom the limestone,though no muscovitewas foundin the few sectionsof' the limestone examined. The replacementore depositsare later than the first periodof faulting, sinceboth quartz and calcite,but especiallythe quartz, are foundrecementing the shatteredzones where the minor faults enter the limestone. On the other hand, they are clearly older than the last movementon the larger faults for these contain rounded fragmentsof the quartz. Dolomite, stibnite, realgar and orpiment, with minor leverrierite and rarely fluorite, are clearly younger than the quartz and calcitesince they were depositedin openspaces of the cellular and brecciatedquartz. The realgar and orpiment,at least, are subsequentto the secondperiod of faulting sincesmall crystals occurin the fault gouge. Stibniteis found throughoutthe deposit,but is more prominent in the westernpart of the mine. Small crystalsare frequently found in the vugs of the dark quartz. Its most usualoccurrence xaPalmer, W. S., "Occurrence of Gold in Sulphide Ore," Eng. Min. iour., vol. Io7, pp. 923-924, May 24, I919. 18 HENRY G. FERGUSON. is in roughlyradiate crystallinemasses, in placesseveral inches in diameter, which replace the limestoneor coarsecalcite, or more rarely the dark quartz. In placessmall "sunbursts"of stibnitecrystals occur along cleavageplanes of the coarsecalcite and small needleswere found in the fault gouge of the later faults. In all thesepositions, particularly in the easternore body, stibnite may be closelyintergrown with realgar. A rare occurrenceof stibniteis in the form of delicatehair-like crystalsfound in cavitiesin the calciteand rarely in the quartz. Orpiment is found in this same habit and association,but not realgar. Al- thoughthe stibniteis practicallybarren of gold it is confinedto the ore bodies. Only rarely do smallclusters of stibnitecrystals occur ouside of the mineralized areas. Realgar occursin large amountsin the easternore body but only sparinglyin the westernpart of the mine. In the stopeabove the 665-foot level and on the 4oo-footlevel realgarwas present in such large massesthat it was smeltedas an ore of arsenic. The mode of occurrenceis very similar to that of the stibnite. It is found as a- replacementof the coarsecalcite, in crystalline masseswhose boundaries follow the cleavageplanes of the calcite, in narrow streaksfollowing the cleavages;it is also commonas a filling of the numerouscavities in the dark quartz. A stope abovethe 665-foot level showsrealgar and dark quartz in irregular bandsup to severalinches across. In placesblocks of prac- tically pure realgar a couple of feet in diameter have been obtained. The realgar is gold-bearing,though, as far as the writer is aware,does not yield suchhigh assaysas are sometimes obtainedfrom the dark quartz. Smelter shipmentsof 3o per cent. arsenic,carried $2o to the ton in gold. In the vicinity of the ore, but outsidethe ore bodies,realgar is more widely distributedthan the other nfinerals. It is the only ore mineral found outside the limestone and occurs in the foot and hangingwall slatesin small veinletsand impregnationsfor. distancesof a few feet from the limestone. Realgar, in small scatteredcrystals, is found in placesin the fault gougeof the faults of the most recentseries, particularly the White Caps LIMESTONE ORES OF MANHATTAN, NEVADA. 19 fault. In generalrealgar showsthe sameassociations and mode of occurrence as the stibnite, and the two minerals are often found closelyintergrown. In one specimenrealgar was found in a clusterof little radiating rods, as if replacingstibnite. Realgar occursin quantity throughoutthe easternore body from the 3oo-footlevel to the 665-foot level. Above and below this it is present only in minor amount. Orpiment also occursin the easternore body but only above the 4oo-foot level. It is clearly an alteration product of the realgar. All stagesof the alterationprocess can be observed, beginningwith threadsof the orpiment in the realgar, through specimensshowing cores of realgar surroundedby orpiment to pure orpiment. Only very rarely doesorpiment occur, which is not clearly the result'of alteration of realgar in place. In a few of the rugs in the coarsecalcite there are delicateclusters of hair-like orpimentcrystals. The changefrom realgar to orpimentdid not free any of the gold containedin the realgar. The orpiment is said to have about the same tenor as the realgat. Later pyrite is found, particularlyin the upper levels. This, like the stibniteand realgar, occursin drusy open spacesin the dark quartz or more rarely alongthe cleavagefaces of the white calcite. It is inconspicuousin amountcompared to the stibnite and realgar. Marcasitehas beenreported, but its presencecould not be definitelyproved. Among the later mineralsof the depositsare minor amounts of quartz and calciteof a secondgeneration, and rarely leverrierite. Small crystalsof fluorite on projectingquartz crystals of one of the drusesin the fine-grainedquartz were observedon the 2oo-foot level. Oxide minerals are nearly absent even in the upper levels. In the lower levels the waters flowing from the mineralized limestoneare depositingstalactites and sludgymasses of limonite which contains arsenic, but whether as an arsenate or oxide is not known. In someof the stibnite specimenscollected from the Ioo and2oo-foot levels there is a partialalteration to valenti- 20 HENRY G. FERGUSON.

nite and stibiconite.Microscopic crystals of a transparent mineralwith high refractiveindex which may be arsenolitewere observedin places,and pharmacoliteis probablyalso present in small amount. No free gold has beenfound in the White Capsmine. The bullion obtainedfrom the cyanideprocess contains almost no silver,having a valueof over $20 an ounce. The ratio of gold to silver is said to be about 17 to I. The gold is clearlyasso- ciatedwith the arsenicalminerals, particularly the arsenopyrite. Possiblythis explainsits abnormalpurity since arsenopyrite, realgar, and orpimentall tend to precipitategold rather than silver from solutions. Manhattan Consolidated.--The Manhattan Consolidated has beendeveloped by five levelsto a depthof 5oo feet. The IOO- foot and 5oo-foot levels were not accessibleat the time of the writer'.svisit. The shaft cutsthe overthrustfault below the 4oo- foot level. A normal fault of large throw, known as the Mud Fault, cutsthe workingsin a northeasterlydirection and dipsto the southeast. The limestonebed is repeatedon the 3oo-foot level, possibly by a reversefault nearly parallel to the strike, and small faults of nearly northerly strike and small displacementare frequent. These are older than the faults of the northeast series, such as the Mud Fault. Motion parallel to the bedding is shown in gouge streaks along the top of the limestone. The ore west of the fault is similar to that of the easternpart of the White Capsmine. The ore body is formed by the replace- ment of limestoneon both sides of a small northerly fault. Coarsewhite calciteis prominent,particularly near the edgesof the mineralized area. The best ore is a dark quartz much like that of the White Caps but containingonly a little microscopic arsenopyriteand more pyrite. S't•bniteis present in ismall amount, in drusy cavitiesin the dark quartz and in the limestone and calcite. Realgar is occasionallyfound. Small realgat crystals occur in the gouge along the Mud Fault, particularly near the surface. LIMESTONE ORES OF MANHATTAN, NEVADA.

The ore differs from that of the White Capsmine in that there is a considerableproportion of silver associatedwith the gold. Bullion from the easternore body is said to have a finenessof about 620. The ore west of the Mud Fault differsmarkedly from that of the east ore body. Insteadof a large replacementbody, mineralizationis closelyassociated with small fault fissures,giving the so-called"vein deposits." Four of thesehave been exploited; two follow northeasterlyfaults of smallthrow; another,less well defined,a fault nearly parallel to the strike of the limestone;and the fourth rather irregular mineralizationalong the top of the limestone. The mineralization does not extend for more than a few feei from the controllingfissure, and the ore shootsare small and irregular. Both coarsecalcite and fine-grainedquartz are present. These occur as replacementsalong the bedding of the limestone,thinning out away from the fissure. The quartz resemblesthat of the White Caps in texture, but is lighter in color, bluish gray rather than black. Pyrite is present, but arsenopyriteis lacking. Irregular solutionchannels, in placesfollowing the dip of the limestonefor 30o feet or more, are commonin the limestone near the fissures. These containdeposits of muddy material, apparentlychiefly limonite and manganeseoxide, in which wire gold occurs. Fluoriteis plentifulin the ore westof the fault, in the larger cavitiesin the fine-grainedquartz. Stibniteand realgar are absentin the westernpart of the mine. Other Mines.--The samelimestone bed outcropsto the westward from the ManhattanConsolidated along the crestand north sideof LitigationHill. The principaldevelopment has been done from the Bath and Earle shafts,and the Union No. 4 (Kane shaft). The deepestof theseis the Earle shaft, which has fol- lowedthe dip of the limestonefor a distanceof 7o0 feet. The mineralizationis similarto that of the westernpart of the Man- hattan Consolidatedand is dependentupon the sametypes of smallnortherly faults. The northeasterlyfaults of largerthr.ow 22 HENRY G. FERGUSON. seemto be later than the periodof mineralizationmarked by the coarsecalcite and fine-grainedquartz. The ore shootsare small and irregular, and it is said that no importantbodies of workable ore were found at a depthgreater than 3o0 feet on the incline. Coarselycrystalline white calcite,fine-grained bluish quartz, and fluorite, and a little leverrierite,are the principal gangme minerals. The fluorite and leverrierite occur in cavities of the quartz. The quartz carriesmore or lesspyrite, in grains barely visibleto the nakedeye, but apparentlyno arsenopyrite. Micro- scopicstudy revealedrare grains of adularia. The gold is con- tained in the fine-grainedquartz, presumablyin the pyrite, and also occursas minute specksof free gold in the oxidized ore. The samelimestone follows the crest of April Fool hill until it is cut off by the overthrustfault. The limestoneis extensively mineralizedand has been mined for nearly the whole length of the outcrop. The high-gradeore, which gavethe hill its fame in the early days of the camp,was apparentlya near-surfacephe- nomenon; most of the ore appearsto have been taken out at depthsof lessthan 3o feet, and no stopingextended below IOO feet. The ore occursalong small faults which offset the limestone a few feet. Coarsecalcite and fine-grainedquartz appearas re- placementsof the limestonefollowing the beddingplans irregu- larly for short distancesfrom the fissures. The valuable ore, however,seems to be containedin small veinlets following these fissures. These consistchiefly of quartz and fluorite with a little calcite. The quartz carries free gold in places,and minute pyrite grains. Most of the coarsegold was found alongthe walls of the veinletsbetween the calcite and the wall rock. In places small rhombiccrystals of adularia occurwith the quartz. On the northern side of the hill the limestone has been developedby severaltunnels, the lowestbeing about I4O feet below the highestpoint of the outcrop. Here the ore is of different character. The rich ore of the surfaceworkings is lacking,and that extractedhas seldomyielded over $20 per ton. The miner- alizationtakes the form of an irregular replacementof limestone LIMESTONE ORES OF MANHATTAN, NEVADA. 2 3 extending outward from small fissures. For the most part the ore is similar to that of the mineson Litigation Hill, 'exceptfor a higher proportion of fluorite. In places,particularly in the lowest tunnel, there are large, irregular cavernsin the limestone, whichmay be linedwith largefluorite crystals. Commonly,however, the roof and walls show large numbersof white inverted domes,up to about6 inchesin diameter. Theseconsist of alter- nate layers of fine-grained white quartz and white clayey material of sericiticappearance, the opticalproperties of which cor- respondto leverrierite,a hydrous aluminum silicate. The ]ever- rierite layersare rarely pure and commonlycontain large numbersof little grainsof quartz and chalcedonyscattered through them. More rarely crystalsof fluoriteare inclosedin the lever- tierite. A few pseudomorphsof limoniteafter pyrite alsooccur in the leverrierite. This material carriesa small amountof gold, saidto be about$4 per ton. Upper LimestoneOres.--The upper limestonebeds, near the top of the Cambrian,have beenprospected extensively but contain ore only on the Mustangclaim closeto the overthrustfault. The limestonehere shows the effects of contact metamorphism and is largely alteredto diopside. Gold has beenobtained close to the surface from little fissures,similar to the veinlets which yieldedthe rich surfaceore on April Fool Hill, and small re- placementveinlets carrying quartz and fluorite. Small flakesof barite were also found in the concentrates from this ore. Most of the production,however, has come from peculiarpipe-like ore shootswhich follow the dip of the limestone,usually along the intersectionof a beddingplane with a well-developedjoint planeor small fault. Thesepipes are nearly circularin cross section and from 8 inches to 2 feet in diameter. Around each is a rim of fine-grainedwhite quartz,a coupleof inchesthick. The interiorof the pipesis composedof a soft white material, with the samesilky luster characteristic of sericite. The optical properties,however, were found to agreemore nearly with the mineralleverrierite, a hydrot,s aluminumsilicate, with the for- 24 HENRY G. FERGUSON. mala AI•Oa.2SiO•.2•H•O. TM The following partial analysis, made by R. K. Bailey, of the U.S. GeologicalSurvey, confirmed this determination.

Per Cent. H•O -- I00 ...... '...... 4.56 H•O + IOO ...... •6.I5 K.•O ...... o.3I

Small grains of quartz occurintimately mixed with the leverrierite, and it was found impossibleto separatesufficiently pure materialto warrahta completeanalysis. Specks of limonite scatteredirregularly through the pipe representoriginal pyrite. Free gold occursthroughout the leverrierite,in sufficientamount to have yieldedover $4o,oooin a distanceof about IOO feet along the pipe. The gold is mostly very fine, but occasionallyoccurs as round pelletswhich look like small shot. As far as couldbe determinedfrom microscopic examination, the gold is in/tll cases surroundedby the leverrieriteand has apparentlyno closeassociation either with the oxidized pyrite or disseminatedquartz. The principal pipe has been followed down the dip of the limestonefor about IOO feet. Several smallerpipes branching upwardsfrom the main pipewere encountered but not developed. The richestspots were said to have beenat the intersectionsof the main pipe with its branches. The lower workingswere not accessibleat the time of the writer's visit, but it is said that at depththe pipesbecame less well definedand difficultto follow.

MINERALOGY. Distribution and Paragenesis.---Thefollowing table illustrates the principalvariations of the perplexingvariety of limestone ores encountered as one crosssesthe district from east to west, alonga distanceof about2 miles.. For comparisonthe oresof the schists have also been included: x• Larsen, E. S., and Wherry, E. T., "Leverrierite from Colorado," lour. Wash. Acad. Sci., vol. 7, PP. 208-217, I917. LIMESTONE ORES OF MANHATTAN, NEVADA. 2 5

TABLE SHOWING PRINCIPAL MINERALS PRESENT IN THE LIMESTONE ORES. Bold face and parenthesesindicate relative importanceor scarcity,respectively.

WhiteCaps {Western Easternpart. X (X) (X) X (X) X X (X) X X (X) i Manhattan.JCon. [WesternpartEasternpart. X (X) X X (X) X X X (XX) LitigationHill ...... X X X X X i X (X) i April Fool Hill ...... X X X X X X X

Sunsetprospect ...... X X X (X) X MustangSchi,tos...... IxX I (X)x, '1 !__ XX (X) X.X i(X)

The paragenesisof the principal minerals of the limestone minesappears to be as follows: (•) Coarselycrystalline white calcite,(2) fine-grainedquartz containing, in the White Caps mine, contemporaneousarsenopyrite and elsewherepyrite. In somedeposits a little adulariaappears to have crystallizedcon- temporaneouslywith the quartz. (3) Fluorite and leverrierite and rare adularia,found chiefly in the easterndeposits. In places quartzand leverrieriteare closelyintergrown. (4) Stibniteand realgarare of distinctlylater date, togetherwith later pyrite, quartzand calcite. All the oresof the first groupthough later than the first periodof faulting, and are older than the most recentmovement .along the major faults. The realgarand possibly the stibniteon the otherhand were introducedsubsequently to the secondperiod of faulting. It appearstherefore that in the White Capsand the easternpart of the Manhattan Consolidated mines,at least,there is a superpositionof two or moreperiods of mineralizationof widely separatedages. The oresshow great variance in characteralong the samebed of limestone. Leverrierite is prominenton April Fool Hill and in the higherlimestone on 'MustangHill, and is nearly lacking at the easternend of Litigation Hill; fluorite also decreasesto the eastward,though it is found in the Manhattan Consolidated. 26 HENRY G. FERGUSON. and very rarelyin the White Caps. The fine-grainedquartz, on the otherhand, is moreprominent in the easternmines. Arseno- pyrite is largelyconfined to the White Caps,though it may be presentin smallamount in the'dark quartz of the ManhattanCon- solidated. Pyrite is persistentthroughout, but is lessprominent in the White Caps than the other mines. The coarse calcite occursthroughout and indeedis not confinedto the vicinity of the ores, but seemsto occursporadically in all the limestones, particularly where fissured. The differencein the oresalong so sh.ort a distanceis probably duein large measureto changingcharacter of the solutionsalong the strike. It may be also to someextent, due to differencesin original vertical position. The larger faults have their down- throw sidesto the eastward,hence at the time of the deposition of the first seriesof mineralsthe White Caps stoodhighest, followed by the easternpart of the Consolidatedand the Litigation Hill mines,and lastly April Fool Hill, which was much lower than the White Caps at the time of the ore depositi.on,though there is no way of determininghow much of the displacementis due to post-mineral faulting. The stibnite of the limestone mines is believed to be of later date. That stibnite should be confined to the limestone and no- where presentin the neighboringschist is probablydue to the influenceof the country rock, sincethe waters passingthrough the limestonewould containbicarbonate, which would precipitate stibnite from an alkaline solution. Realgar: Occurrenceand Origin.--Realgar is found in notable amountonly in the White Capsmine, where it occursin the same general manner as stibnite. It is, however, rather more migra- tory in its distribution, sinceit is found in the schistat somedis- tance from the limestone,and small crystals of realgar have formed in the gouge accompanyingthe later seriesof faults, whereasstibnite is rarely found in thesepositions. Realgar and stibniteare in placesfound closelyintergrown, and it is believed that they are approximatelycontemporaneous, although in one specimenrealgar appearsto have replacedstibnite. Realgar is LIMESTONE ORES OF MANHATTAN, NEVADA. 27

foundonly where the darkquartz carries arsenopyrite and not in thosemines in whichpyrite is the principal metallic miner;t[ It occursin quantityin the White Capsmine, only downas far as the 665-footlevel. Belowthis the ore consistsof very cavernous dark quartz,brecciated and crumblyin places,with occasional ßpatches and scattered crystals of realgar. Dynan•5 has suggested that the realgaris a secondarymineral derived from the arsenopyrite. The fact that realgar is confinedto the White Caps mine, which is the only mine in which arsenopyriteis presentin notable amount, renders this suppositionextremely probable. Normally, under the action of oxidizing waters, scoroditeor pharmacosideriteshould be the endpoint of oxidationof arsenopyrite.•6 There appearto be numerousinstances, however, in which realgar is a secondarysulphide, •7 and LindgrenTM even says: "Realgar and orpimentare probablyalways supergenesulphides but they are not found in the secondaryzones of copperdeposits. They are rather more characteristic of the oxidized zone, and often are derived from arsenopyrite. The chemistryof their depositionis uncertain." It is possiblethat the methodof formationof supergenereal- gar from arsenopyriteis analagousto the formationof sulphur in the first stagesof the oxidationof pyrite. Stokes•ø givesthe following equation: FeS• + Fe.o(SO4) a• 3FeSO4+ 2S. The equivalentreaction in the caseof arsenopyritewould be: FeAsS-• Fe2(SO4) a = 3FeSO4-• g•sS. Stokes, however, did not obtain arsenic sulphide when arsenopyrite was treated with ferric solutions.•ø a5 Dynan, op. cit., p. 885. ;o Lindgren, W., "Mineral Deposits,"p. 898, New York, I919. ;* Emmons, W. H., "The Enrichment of Ore Deposits," Bull. U.S. G. S. No. 625, p. 466, I917. •8 Loc. cit., p. 899. •9 Stokes, H. N., "On Pyrite and Marcasite," U.S. Geol. Survey Bull. I86, p. I5, I9OI. 2oLoc. cit., p. 33. 28 HENRY G. FERGUSON.

In this case,moreover, the realgar is not confinedto the zone in which oxides occur but is found to the lowest depth yet explored,though only prominent above the 665-footlevel. The similar mode of occurrenceof the realgar and stibnite suggestsanother possible explanation. If, as appearsto be the case,the stibniteis distinctlylater tl•an the quartz and arseno- pyriteit is possiblethat the hypogenesolutions, presumably de- ficientin oxygen,which deposited the stibnitealso attacked the arsenopyrite.The arsenicthus dissolved would be depositedas realgarin the zonewhere the ascendingwaters began to be con- taminated with the bicarbonateof the supergenewaters of the limestone. The fact that sibniteis barren and the realgat gold bearing is perhapsadditional evidence of the secondaryorigin of the realgar. For if the realgarwere derivedfrom the arsenopyritethe gold presentin the arsenopyritewould tend to migratewith the arsenicto the realgar,while if both stibniteand realgar were of hypogeneorigin and contemporaneous,it would not be expected that the gold would be presentin only one of the two minerals. On the other hand realgar has often been found in hot spring deposits,and there is no .inherentreason why it shouldnot be found as a primary mineral in ore depositsformed at shallow depth. In the microscopicexamination of the White Caps ore, the writer has not been able to find any clear evidenceof alteration in place of arsenopyriteto realgar. The large amount of realgarfound bet,ween the 4ooand 66o-foot levels would require the alteration of an enormousamount of arsenopyrite,and there . is no evidenceof any marked leachingof the arsenopyritein the upperlevels. There is a smallamount of later pyrite, apparently contemporaneouswiththe realgar and stibnite but not at all com- parablein volumeto the realgar. 'It may be, however,that an iron-rich gossanhas been eroded. OxidizedMinerals.--The amountof oxidation,particularly in the deposits in the limestone, is comparatively small. In the White Capsand 'ManhattanConsolidated mines slightly oxidized stibniteoccurs in the surfaceworkings. The oxidized antimony LIMESTONE ORES OF MANHATTAN, NEVADA. 29 mineralsare inconspicuousand cannoteverywhere be certainly identified. The principalones appear to be valentiniteand stibi- conire. The latter appearsto have formed both directly from the sfibniteand as an alterationproduct of the valenfinite. Pal- mer•x recordsthe presence of kermesiteas well. Exceptfor rare minutegrains of what may be arsenolitein ore from the upper levelsof the White Capsand the presenceof arsenicin the limoniticmaterial depositedon the walls of the drifts, no oxi- dizedarsenical mineral has been recognized. In thepyrite-bear- ingores of theschists, on the other hand, the pyrite has in places beenaltered to limohireto depthsof 300 feetor more. Orpirnent:Occurrence and Origin.--Orpiment, which is found in theupper 400 feetof theeastern ore body of theWhite Caps, is clearlyderived from realgar, presumably by theaction of oxi- dizingwaters. Specimens can be coll. ected which show all stages of alteration,from the first development of threads of orpiment alonglittle cracks in therealgar through cores of realgar,surroundedby orpiment,to pureorpiment. It is difficultto under- standwhy the changefrom realgarshould be to a mineralwith highersulphur content (AsS to A%Sa),without the formation of oxidized arsenicalminerals at the sametime. In the lower levelsof themine, however, below the zone in whichthe orpi- meritoccurs, the minewaters flowing from fissuresnear the ore aredeposfing a limonific sludge on the walls of thedrifts. Quali- tative testsshow the presenceof considerablearsenic in this material.Apparently orpiment is thestable sulphide under sur- faceconditions, and the alteration takes place through oxidation of a partof thearsenic of therealgar rather than of thesulphur. Thisis soon redeposited in association with iron oxide; whether as oxideor an ironarsenate has not yet beendetermined. An analogousreaction isthat by which chalcocite changes byoxida- tion to corellite. 22 The alterationof the realgarto orpimenttakes place with 2xPalmer, W. S., "Formationof Quicklimein RoastingOres from Man- hattan,Nev.," Mining and Engineering Journal, vol. xo4, p. 525,Sept. 22, I9X7. 2• Lindgren,W., op. cit., p. 859. 30 HENRY G. FERGUSON. comparativerapidity in the sunlight. Fragmentsof realgat left on the mine dump soon lose their bright red color due to the formation of a thin coating of orpiment. A small amount of pure realgar was placedunder glassin the sunlight. Incipient alteration was noticedwithin a few days and in a few weeksthe transformationto orpimentwas well advanced. No recognizable arsenicoxides were found, thoughaccording to Dana"a realgar alters in the light to orpimentand arsenolite. To test the probabilityof the alterationof realgar to orpiment by oxidizing waters small amountsof realgar were placed in a dilute solutionof ferric sulphateand gently heated. The realgat was in part alteredto orpimentand arsenicoxide. The hair-like crystals of stibnite and orpiment, which are sometimesfound, suggest supergenesulphide deposition. C. Doelter"4 statesthat stibnite at 8oø is slightly solublein water and that recrystallizationis also perceptible. Gold.MAt least a part of the gold in the rich surface ores is due to secondarydeposition. No free gold has been found in the White Caps or easternore body of the Manhattan Consoli- dated. In the White Caps, and p•obably to someextent in the Consolidatedalso, the gold is presentin the minute arsenopyrite crystalsscattered through the dark quartz. The stibniteis barren, but the realgarand orpimentare both gold bearing,so it is ap- parent that in whateverform the gold is presentin the arsenopyrite, it has remainedwith the arsenicthrough the succcessive changesfrom arsenopyriteto realgar and thenceto orpiment. In the westernpart of the ManhattanConsolidated and in the LitigationHill mines,free gold occursas smallflakes in sandy material consistingof minute fragmentsof quartz and calcite with manganeseand iron oxidesin irregular solutionchannels in the limestone.This wasprobably derived from the pyriteof the massivequartz and depositedby supergenewaters. The rich 2aDana, E. S., "System of Mineralogy,"p. 34, •895. 24Min. pet. Mitt., vol. •, •89o,p. 3i•2,cited by F. W. Clarke,"Data of Geochemistry,"U.S. Geol. SurveyBull. No. 616,p. 689, I9•6. LIMESTONE ORES OF MANHATTAN, NEVADA. 31 surfaceores of the workingson April F0ol Hill, in which the gold occursin closeassociation with calcite,are apparentlydue to secondaryenrichment. These were very rich closeto the surface, but nearly barren at shallowdepth. Gold carried in acid surfacewaters would be quickly depositedin contactwith calcite and superficialenrichment could not extend to any appreci- able depth. Secondaryenrichment appears to have played a much more importantpart in the oresof the schiststhan in thoseof the limestones. In thesedeposits the richestore is commonlyfound at the intersectionof the principalvein wi-th small fissuresrich in manganeseoxide. Sincecalcite is lackingin thesedeposits the lower limit of enrichmentis at a greaterdepth than in the ores of the limestone,but rich ore doesnot seemto have beenmined belowabout 2oo feet. In the schistmines such as the Big Pine, where the gold occursin a multitudeof small stringersin the schist,there is little manganeseoxide and no clear evidenceof secondaryenrichment.

RELATION OF ORE DEPOSITION TO FAULTING. The productivemines of the districtall lie in a belt extending about IO,OOOfeet from east to west and less than 2,ooo feet from north to south. The overthrust fault marks the northern limit of productivemineralization. This fault is older than the numerousnormal faults which cut the ore-bearinglimestones and is undoubtedlycontemporaneous with the intensefolding which precededthe intrusion of the granodiorite. Movement on the normal faults may have begun at the time of the granodioriteintrusion for an alaskitedike seemsto follow one of the faults. The major normal faulting, however, probably did not take placeuntil late Tertiary time, subsequentto the intrusion of the andesiteporphyry. Productive ore depositsoccur only close to the overthrust fault, and along the southernor hanging wall side. Although the actual fault plane is nowheremineralized, it is believedthat 3 2 HENRY G. FERGUSON. the presenceof this major fault was the determiningfactor in the localizationof the deposits. The minesare all on the hanging-wall side of the overthrustand for the most part associated with the small normal faults of later date. It may be that the overthrust fault with its narrow zone of comparativelyim- perviousgouge acted as a dam to prevent important migration of the solutionsnorth of the fault, analthat the intersectionof the overthrustwith the minornormal faults determined the principal channelsfollowed by the ascendingsolutions. This fur- nishesa possibleexplanation for the localizationof mineralization in the White Caps limestone,to the exclusionof the two similar beds at slightly lower horizons. The limestonesat the White Capsmine and on Litigation Hill, while approximately parallelwith the overthrustin strike,have a slightlysteeper dip. Thus the White Capslimestone would be cut by the fault plane at a lower level than the two lower bedsand uprisingsolutions controlled to the north by the overthrust would tend to be divertedalong the first solublebed.

AOE OF T•E ORES. Ores of deep origin and those characterizedby minerals in- dicativeof depositionat shallowdepth are both representedin the Manhattandistrict. The former are repr½sen•dby the small glassyquartz veins found here and there throughoutthe district. These veins seemto be dearly associatedwith the granodiorite intrusion,probably most closely with the final phasewhich producedthe apllte dikes. Productivedeposits of the sameepoch have been mined within a few miles of Manhattan. The small veins of the lavas and the veins in the schists of Gold Hill, on the other hand, show certain characteristics of veins formed at shallowdepths, such as adularia and quartz pseudomorphicafter lamellar calcite. Since ores of this type occurin the post-Siebertandesite, these veins can be assignedto a period betweenthe late Miocene and the formation of the late Plioceneerosion surface. Presumablythis periodof ore deposition is geneticallyconnected with the andesiteintrusion. LIMESTONE ORES OF MANHATTAN, NEVADA. 33

The oresof thelimestone are morecomplex and lack minerals or mineralassociations clearly diagnostic of eitherof the above groups,and it is likelythat more than one period of mineralization is represented. Dynan2• corlsidersthat the ore of the White Capsmine owes its originto solutionsemanating from the intrusivegranite. Outcropsof graniticrocks occur much nearer the minesthan do the Tertiary igneousrocks. On the hill north of Litigation Hill there are two small dikesof a coarse-grainedalaskite, and severalsmall aplite dikes cut the sedimentsa short distanceto the south. Two small massesof a much sericitizedigneous rock, probablyan aplite,have beenencountered in the lowestlevel of the White Caps. The coarse calcite, usually associatedwith some valuable mineralization,occurs in placesclose to the granodioriteand Dynan mentions2ø specimens of coarsecalcite and garnet from the granodioritecontact which assayed0.0 3 oz. gold per ton. Similar coarsecalcite in the silver-leaddeposits of the Darwin district, California, has been describedby Knopf?7 There it seemsto be directly connectedwith an intrusionof granodiorite. On the otherhand, this typeof calciteis not presentin limestones of the Belmont and Barcelona districts where the occurrence of the ore is clearlyrelated to the presenceof the granodiorite. Leverierriteis presentin all of the ore depositsin the limestone. This mineralis uncommonin ore depositsbut was found by Larsenand Wherry•8 in quartzveins with manganeseoxide. Racewinitefrom the HighlandBoy mineof Bingham,Utah, describedby A. N. WinchelF9 appearsto be identicalwith lever- . rierite. 8ø Here it occurs in ores associated with a monzonite porphyry intrusion. 25Loc. cit., p. 885. 2oIdem, p. 885. 2, Knopf, A., "The Darwin Silver-Lead Mining District, California," Bull. 580, U.S. Geol. Survey, p. 7, •9•5. 28Larsen, E. S., and Wherry, E. T., loc. cit., p. •7. 20Winchell, A. N., "Racewinite, A Peculiar Mineral from Ore Deposits Utah," Ecoa. Gm•.., vol. •3, PP. 6•-6•5, •9•8. s0Butler, B. S., "Ore Depositsof Utah," Prof. Paper No. •, LI. S. Geol. Survey, p. •3, •92o. 34 HENRY G. FERGUSON.

It is believed,however, that the weightof the evidencefavors theprobability that these ore deposits were formed fairly close to the surfaceand are of Tertiary age. The depositsare abso- lutelyunlike those of Belmontand Barcelona, in whichthe primary ore consistsof sulphidessuch as galena,sphalerite, tetrahedrite,and chalcopyrite in a quartz-calcitegangue, nor is there any similaritybetween these and the smallunproductive veins of deep-seatedorigin found here and there throughout the Man- hattan district. The large amountof fluorite likewiseis con- sideredindicative of depositsof the shallowrather than of the deepvein zone. Moreover,the presence of a little adulariainter- grown with the fine-grainedquartz is believedto form a connectinglink with thequartz adularia veinlets of GoldHill which clearlybelong to the classof veinsformed at shallowdepth. Alsothe fine-graineddrusy quartz suggests deposition nearer the surfacethan the coarselycrystalline quartz and sulphidesof the ores of Belmont. The principalminerals of the limestonedeposits, quartz, fluorite, leverrierite,arsenopyrite, appear to hawebeen deposited prior to the mostrecent movement along the larger faults. The stibniteand realgar, on the other hand, are youngerthan the latestfaulting and appearto representa still later periodof Ter- tiary ore deposition.It is probablethat therewere two periods of late Tertiary mineralization,probably both subsequentto the intrusionof the andesitcporphyry. It is possible,however, that the coarselycrystalline calcite may be a representativeof the mineralizationfollowing the granodiorite intrusion.

CONCLUSIONS. The productivemines of the ManhattanDistrict occurclose to large overthrustfault and are all in the upper or overthrust block. Althoughno ore depositionoccurred in the fault plane itself, the presenceof this fault is believedto havebeen the con- trolling factor in localizingore deposition. Particularly favor- ablesituations appear to be in localitieswhere late normal faults LIMESTONE ORES OF MANHATTAN, NEVADA. 35 displacethe overthrust. Two seriesof normal faults are recognized,the first, characterizedby smallthrow, steepdip, and general directionabout north-northeast, and the other with generally larger throw, flatter dip and less accordantstrike. While the two series may be of cont,emporaneous origin, there has been more recent movementalong the faults of the secondgroup. The faults of the first group may have beeninitiated by the intrusion of the granodioritebatholith, but the more important faulting is tentativelycorrelated with the intrusionof a massof andesiteporphyry .of late Miocene or Plioceneage. The ore depositsshow a wide variety. Small quartz veins in the schists,with pyrite, chalcopyrite,tetrahedrite, and galena, have not proved workable. These are clearly of deep-seated origin and may be correlatedwith the granodioriteintrusion and are presumablylate Cretaceousor early Eocenein age. The ores of the Cambrian schists show a mineral association characteristic of shallow zone depositionand may be geneticallyconnected with the andesireporphyry. The characteristicminerals of these ores are quartz, in small comby veinlets,and pseudomorphic after tabularcalcite, adularia, pyrite and free gold. A bed of crystallinelimestone occurring in the lower part of the Cambrianis heavily mineralizedand showsa complexasso- ciationof minerals. At the easternpart of the districtthe principal gangueminerals are coarsecalcite and fine-grainedquartz, and the metallic mineralsarsenopyrite, pyrite, stibnite,realgar and orpiment. The ore occursin large replacementdeposits in the limestone closely connectedwith the faults in the earlier series. Stibnite, realgar, and orpiment,however, are later than the secondseries of faults. It is believedthat the depositionof stibnitewas due to a laterperiod of ore deposition,and that the realgaris secondaryand derivedfrom the arsenopyrite,though possiblythrough the action of hypogenesolutions. The orpiment has beenderived from the realgarthrough the oxidationof a part of the arsenic. Thecharacter of theore changes greatly to thewestward along the strikeof the limestonebed. Arsenopyriteis not presentin 36 HENRY G. FERGUSON. the quartz, and realgarand orpimentare no longer found. Stib- nite is presentonly in one other deposit. Free gold, lacking in the White Caps ore, is found farther to the westward,fluorite and leverrierite becomeincreasingly important gangueminerals, and minor adularia is present. It is possiblethat three periodsof mineralizationare representedin the limestoneores; one marked by coarselycrystalline white calcite,which may have followed the granodioriteintrusion,and if so is of late Cretaceousor early Eoceneage; the sec- ond,which introduced the quartz,arsenopyrit. e, pyrite and gold, togetherwith the fluorite and leverrierite,is of later date, but prior to the most recent faulting; and the thii'd, characterizedby the stibniteand the realgar of the easternpart of the district, is still younger. U.S. GEOLOGICAl'.SURVEY, WASaINGTON,D.C.