Lindgren 1935

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ECONOMIC GEOLOGYVo.. XXX JUNE--JULY,. 1955 No. 4

THE SILVER MINE OF COLQUIJIRCA, PERU.

WALDEMAR LINDGREN.

Introduction.--The Colquijirca mine, which at times has beenthe most productive silver mine of North and South America,is situated 8 kilometers south of Cerro de Pasco in the foothills

of the Junin pampa, at an elevation of about 4,ooo meters. Itis a small mine; the greatest depth attained, as far as I know,is about 7o0 feet. The folded ore beds are said to extend forabout 2,ooo feet along he strike. Figure x shows he workingsand the several ore beds. Worked with unsatisfactory resultsat the outcrops n Colonial imes, it was long abandoned ntilopened y A. Rizo-Patron about x9xo by a tunnel from the levelof the pampa. The work was continued ater by the presentowner, Mr. E. E. Fernandini. The ore proved remarkably ich.I am informed by Professor W. S. Hutchinson of the Massa-chusetts nstitute of Technology hat the production rom x91xto x9x 9 inclusive amounted o nearly 13,ooo,ooo unces of silverwith some ead and copper. Dr. Ahlfeld states hat in the twoyears x927 and 1928 the silver production ggregated 2,ooo,oooounces. The total silver production has not been made public,but is probably ot less han 4o million ounces. For comparison,it may be recalled hat the Premier mine, British Columbia, pro-duced 27,ooo,ooo ounces of silver from 1918 to 193o inclusive.The Tintic Standard Mine produced 4,ooo,ooo ounces n 1925.The whole silver production of the Tintic district from 1869 to1929 inclusive s estimated at 217,ooo, oo ounces. The Cobaltdistrict, Ontario, from 19o 3 to 193o yielded 365,ooo,ooo ounces.

33

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33 PV.4LDEM.4R LINDGREN.

Potosi, Bolivia, s believed o have yielded 96o,ooo,ooo uncessince

The Colquijirca res are rich; first class ore averaging 0ounces er ton, second lass, 0 ounces, nd poor ores; 2o ouncesor less. The poor ores contain about 15 per cent lead. Forvarious easons he production as declined aterially ince 93o.

FIG. I. Vertical E.-W. section across he Colquijirca mine. After F.Ahlfeld.

I believe all the ores are now smelted.by the Cerro de PascoCompany.

Some of the ores have been described y various authors.Dr. Ahlfeld's excellent rticle contains description f the geo-logical features as well as a description of the ores. I haveborrowed from him the data on the general geology of the mine.

In 927, I received fine suite of specimens rom the mine forthe purpose f a report. From this report have selected ertainphases elating to the mineralogy and analyses, which did notseem o be of special economic nterest. I owe several analysesto the well known skill of Mr. E. V. Shannon, and three photo-graphs of ores o the kindness of Prof. W. H. Newhouse..

On the west, in the footwall, lie sandy shales and sandstones Oreel, J., et Rivera Plaza, Gil: tude microscopique de quelques minerais mdtal-

liques du Perou. Bull. Soc. Franaise de Min., 52, 9I-O7, I92O. McKinstry,H. E.: Interpretation of Concentric Textures at Colquijirca, Peru. Amer. Miner.,4, 43o-433, x929. Lindgren, W.: Pseudo-eutectic Textures. EcoN. GV. OL., 25,-I3, x93o. Ahlfeld, F.: Die Silberlagerstiitte Colquijirca, Peru. Zeit. prakt. Geol.,4 8-87, I932.

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THE SILVER MINE OF COLQUHIRC.4, PERU. 333

believed o be of Carboniferous Age. These are unconformablycovered by a series of limy and argillaceous strata 2oo meters

thick, beginning with a quartzite conglomerate. This wholeseries s markedly folded and held to be of late Triassic age.Jurassic imestone forms the hills on the east side and coversthese beds conformably. Two smaller ntrusive masses f daciteporphyry ie to the south and west of the mine.

The mineralized part of the series has a thickness f from 3 to 50 meters. Three or four beds (mantos) are mineralized andseparated by sandstone or sandy shale. In places the severalmantos merge, yielding an ore zone which may be 2 metersthick. The upper manto, reaching 5 meters n thickness, ontainsthe rich silver-copper res. Underneath his is the lead manto,which appears n the west side and as well in the Chocayoc artof the field. Underneath his, but separated rom it by a sand-stone ayer, s the owest manto of pyritic copper re with enargite,not everywhere ully mineralized.

The relations of these different mantos or mineralized

beds have naturally an important bearing on the genesis, ut lackof acquaintance ith the details of ore occurrence revents me

from entering on this subject. Undoubtedly hey indicate con-siderable ariations n the temperature f deposition. The in-fluence of the country rock is also important and it is believedthat the more calcareous eds as in Chocayoc) avor the deposi-tion of galena and pyrite and a less marked silver mineralization.

The silver appears n force with the barite-tennantite-stromey-erite mineralization. The enargite comes n chiefly n the pyriticmanto. Speaking generally, he deposit s distinctly epithermaland was formed at temperatures which, never high, declined

markedly owards he end of the epoch. High-temperature min-erals are absent.The Country Rock and Its .4lteration.--The most common

country rock in the mantos is a gray, brown, or black chertconsisting f fine-grained halcedonic uartz, he individual grainsshowing fibrous extinction. Fibers showing cell structure areabundant, indicating silicified wood. These rocks are calcareous

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334 WALDEMAR LINDGREN.

shales eplaced y silica by colloidal rocesses. ccasionally n-altered clay shales with small clastic quartz grains are noted.The cherts commonly ontain specks f pyrite or chalcopyrite.Later veinlets of lighter chalcedony ntersect he chert. Unal-tered gray or brown imestone ccurs n the Chocayoc ection,commonly ontaining little pyrite, sphalerite, nd galena, aswell as veinlets f ankerite r calcite. The contracting nd easilyfractured gel, hardening o chert, offered excellent aths or themineralization nd is infilled with barite and sulphides r re-placed by these minerals. In places he chert contains reas orveinlets of coarser quartz.

As some of the shales were rich in alumina, kaolin resultedduring this silicification. The kaolin forms minor white orgreenish soft masses. Some is amorphous pholerite. Otherspecimens rom Mercedes show a crystalline kaolinite with n---1.565. Small pyrite grains replace he kaolinite. Chalcedony,quartz and kaolin were the result of the first attack of the hotmineralizing water on the rocks. They were practically ormedbefore any of the following phases of mineralization, hough alittle chalcedony, aolin, and calcite may have accompanied hedeposition of the ore minerals.

Other Gan#ue Minerals.--Barite is almost the only importantlater gangue mineral. Some of the barite plates formed in openspaces attain a length and width of 5 or 6 centimeters nd athickness f 5 to 7 millimeters. Barite is replaced y any of thesulphide minerals, but never replaces hem. A little calcite, orankerite, is in places deposited on galena. The limestone romChocayoc s partly replaced y dolomitic and ankeritic carbonates.A specimen rom the lower manto of the Mercedes hows ankerite

or siderite with later galena and still later coatings of kaolin.Pyrite.--Pyrite in minute crystals s common n the chert and

other country rocks. Massive pyrite occurs generally with thelead ores; in places, t assumes colloform habit. This pyrite,as well as the marcasite, appears o contain very little or no silver.

Marcasite.--In some Chocayoc res arge spear-shaped rystalsof marcasite replace the pyrite. Most abundant is marcasite

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THE SILVER MINE OF COLQUHIRCA, PERU. 335

in the very late marcasite-specularite ineralization, ccurringhere as excellent mall crystals on the sphalerite.

Sphalerite s present n varying quantities, eing most abundantin the lead ores. It is generally of light color and may formdruses overed y dolomite or halloysite. Some of the late sphal-erite is of dark color and beautifully colloform; it contains nosilver and no bismuth.

Galena.--The galena of the manto de plomo and Chocayocis massive nd granular; sometimes n crystals of octahedron ndcube. When pure t appears o contain rom 4 to 26 ounces f.silver, which s present n small grains of argentite and tennantite,probably also in solid solution. The galena always follows thesphalerite nd may replace t along the cleavage, iving rise todendritic texture. Where chalcopyrite s present in the leadores t is always ater, replacing galena along its periphery. Aslittle galena is present n the rich silver ores, its parageneticposition s not so easily determined here.

Enart7ite.--This mineral is sparingly present as crystallinemasses n the pyritic manto; it replaces yrite and barite and isreplaced y all later sulphides. Luzonite, aid O be older hanenargite, was identified by Ahlfeld.

Tetrahedrite.--The presence f tetrahedrite s suspected n oneor two specimens, ut on the whole he ores re remarkably oorin antimony.

Tennantite.--Tennantite is one of the most abundant minerals

of the older series; t replaces hert, barite, pyrite and sphalerite.It is black and has a dark reddish brown streak; greenish grayin polished section; generally massive, but also in small, sharptetrahedrons. It apparently always contains ilver, but in vary-ing amounts; zinc is also variable, some samples containingpractically none, others up to 2 per cent. Bismuth, when pres-ent, always indicates eplacing bismuth minerals.

Specimen 0, which is simply labelled filling of ore in brec-ciated silica is a breccia f brown chert containing little pyritein minute grains; it is cemented y barite plates between whichis abundant lack ennantite n minute etrahedrons.. For analysis

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the chert was separated y heavy iquid and the residue, ontainingonly barite and the sulphide mineral, was analyzed. There areno other sulphides resent. After deducting 9.. per cent barite,the following result was obtained.

ANALYSISNO. 6O.--.CoLQuIJIRCA, ERU.

E. V. Shannon, Analyst.Ratio

Copper ......................... 34.87 0.549Silver .......................... 4.27 0.039Lead ........... ............ ..... 23 o.ooZinc ............................ 2.o7 o.85

Iron ............ ............ ... x.5 0.020Bismuth ........................ none

Arsenic ......................... 20.60 0.275Antimony ....................... traceSulphur ........................ 26.64 0.833

99.83

The presence f large amounts f zinc and silver s noteworthy.There seems o be no escape rom the conclusion hat these metalsare integral parts of the tennantite. Also noteworthy s the ab-sence f bismuth, howing hat the tennantite nd silver minerali-zation was not normally ccompanied y a bismuth mineralization.This gives:

Cu (Zn, Pb, Fe, Ag) .......... 0.794 or approximately 6 X 35 = 8xoAs ............... ............. 0.275 .... 2 X 35 = 27 S ............................. 0.833 6 X 35 8o

which eads o the Prior and Spencer ormula of Cu0 As2 S,;.Specimen 2, from Mercedes, hows arite plates on and in

brown chert. In and between barite is tennantite with laterchalcopyrite. ennantite ccurs n part as small etrahedrons.In polished ection ome reas re uniform, others ontain muchchalcopyrite. With the tennantite ccur the following aterminerals:

I. Bluish gray stromeyerite.2. A soft greenish ray mineral n pseudo-eutectic ntergrowth

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THE SILVER MINE OF COLQUIJIRCI, PERU. 337

with tennantite, or as veins in it. Slightly attacked by HNO,

fumes do not tarnish; KCN attacks slightly, rubs clean; maypossibly e emplectite.3. Light gray soft aggregates n tennantite, rowned by HNO,

quickly blackened y KCN; darkens with FeCI; in places nti-mately intergrown with tennantite n feathery aggregates. Thesethree later minerals evidently contain bismuth. A part of thesection, containing only tennantite, chalcopyrite nd mineral No.2, was analyzed:

ANALYSIS NO. 62.--CoLQu1JIRCA, PERU.

E. V. Shannon, Analyst.

Recalculated Ratio

Barite ................................... 26.16Silver .......................... , ........ 4.08 5.56 0.052Copper .................................. 29.68 40.44 0.636Lead .................................... 0.90 1.22 0.006Iron .................................... 1.52 2.07 0.037Zinc .................................... 0.06 0.09 o.oolArsenic ............... ......... - ........ lO.82 x4.74 o.197Antimony ............................... o.12 o.16 - o.oolBismuth ................................. 8.20 11.17 0.054Sulphur ................................ 18.o 24.55 0.765

99.56 lOO.OO

An attempt was made o calculate his analysis n the assump-tion that emplecite Cu2Bi2S4) s present.

Bi Pb Zn Cu Fe S--

Galena .................... 0.006 0.006Chalcopyrite .............. 0.037 0.037 0.074Sphalerite .................. o.ool o.ooi

Emplectite ............... .. 0.054 0.054 O. O8--

o.o91 o.189

There remains after these amounts are subtracted'

Cu (Ag) ............... ............. 0.597As (Sb) ............................ o.i98$ .............. ............... ...... o.576

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338 PVALDEMAR LINDGREN.

which corresponds ery well with the Prior and Spencer ormulaCu6As2S6. t is, therefore, easonably ertain that mineral No.2 is emplectite nd that the principal mineral s tennantite.

Galena.--Galena s not common n the rich upper mantos ofPrincipal and Mercedes, but appears ather abundantly n theunderlying manto de plomo and n Chocayoc. It has a normalgranular exture but in places ppears n crystals of combinedcube and. octahedron. It is not rich in silver, assays iving from8 to 26 ounces er ton; much of this probably omes rom smallinclusions f tennantite nd argentite. Most of the ores contain

a trace of gold. The galena always follows the sphalerite ndreplaces t abundantly, ommonly long the cleavages, iving riseto dendritic texture.

Chalcopyrite.--The main part of the subordinate halcopyriteis associated ith tennantite and replaces t. Rounded replace-ment remnants of pyrite occur n it; chalcopyrite may rim andsurround ennantite; here is a very little bomite, formed byreplacement r unmixing. There is also a later chalcopyriteappearing with the marcasite-specularite ineralization.

I4/ittichenite CusBiSs).--Some of the ores are rich in bismuth.The principal bismuth mineral is the rare mineral wittichenite.This was first found in the lower manto of the Mercedes. As far

as I know, it is the first definite dentification of the mineral fromthe Americas. The bismuth minerals apparently always replacetennantite, ut bismuth also appears, robably n chemical om-bination, in the stromeyerite.

Specimen o. 2 4 shows black chalcedonic hert (with somewood fibers) covered by replacing barite; the sulphides mostlyfill spaces etween arite plates. The fine-grained halcedony sshattered nd nfilled by lighter veins of the same substance. Thewittichenite occurs n steel-gray massive aggregates, arnishingmarkedly pale brown on the surface. Streak black; mild orslightly brittle; contains esiduals f pyrite, also a little chalco-pyrite. The reactions re as follows: HNO browns apidly,rubs blue or brown, fumes tarnish brown; attacked onger, turnsalmost lack. HC1 negative, cid tums yellowish. KCN etches

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THE SILVER MINE OF COLQUIJIRC.4, PERU. 339

irregularly blue'or brownish. With HgC12, FeCla and KOH,faintly brown.

The mineral was carefully hand-picked ut the sample stillcontained ome barite and pyrite and a very small amount ofchalcopyrite, ess than one per cent.

AI,/ALYSlS No. 24.--COLQUIJIRCA, PERU.

E. r. Shannon, Analyst.

Insoluble

Barite and chert .......................Silver ................................

Copper ...............................Iron .................................Lead .................................Zinc .................................Arsenic ...............................Antimony ...........................Bismuth ..............................Sulphur ..............................

Per cent.

9.12O.0I

30.565.IO

.68trace

0.240.20

33.4o20.88

lOO.X9

Ratio--

o.481o.o91

0.003

0.003o.oo3o.16o

o.651

Recalculated

0.0I

38.20

0.85

0.300.25

41.75I8.85

100.21

Ni and Co are absent. If the impurities f 9. 2 per cent gangueand 5.I Fe q-5.8 S = IO. 9 pyrite be subtracted, he analysismay be recalculated s shown in the last column. Witticheniterequires 38.4 copper, 42.I bismuth and I9.5o sulphur. Thecorrespondence s very close. The ratios give the correct for-mula 3Cu2S.Bi2So: i(As, Sb) o. 66; Cu(Pb) 0.484; S o.469.Ahlfeld also reports he presence f klaprothite n intimate nter-growth with wittichenite.

Other Silver Minerals.--Argentite occurs sparingly. In myspecimens found no other minerals of this class except stromey-erite and native silver. Orcel reports proustite, and Ahlfeldfound sternbergite nd pearcite, he latter replacing tromeyeriteand sternbergite.

Stromeyerite.--Stromeyerite s the most valuable silver sul-phide at Colquijirca; I consider t to be of hypogene origin (inagreement with Ahlfeld). It replaces ennantite and chalco-pyrite, rims them or forms pseudo-eutectic ntergrowths, particu-

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34o I/V.dLDEM.dR LINDGREN.

larly with the tennantite. A specimen rom the lower manto nthe Mercedes hows pyrite, tennantite, halcopyrite nd stromey-erite in brown chert, also a little galena but no sphalerite.Native silver appears n some sections. In part the mineralsform separate masses. Enough of the pure stromeyerite wasisolated or analysis.

ANALYSIS NO. X6.--COLQUIJXRCA, PERU.

E. V. Shannon, Analyst.

Insoluble ..............

Chalcopyrite ..........Silver .................Copper ...............Iron ..................Bismuth ..............Arsenic ...............

Antimony .............Zinc ..................Lead .................Sulphur ...............

2.39

45.5832.37

.482.12

trace

trace

trace

.46t7.tx

xoo.5t

Recalculated

2.39

x.4845.583.87

2.12

trace

trace

trace

.46t6.6t

too.5x

Recalculated topure material

47.x632.97

2.19trace

trace

trace

.4717.21

IOO.OO

Ratio

0.437o.5t9

o.ox5

0.0023

0.538

The material analyzed ontained little chalcopyrite separatedin the above analysis) and possibly he lead s present s galena.The place occupied y the bismuth s doubtful. No definitebismuth minerals were observed.

As shown by the ratios, the material s almost pure massivestromeyerite, n which, however, ppears o be a little admixedchalcocite, hown as small bluish areas in polished section. Ittherefore contains a little less silver than required by the usual

formula, Ag2S.Cu_S. The color of stromeyerite s bluish-pur-plish gray. It is strongly anisotropic. The mineral can nolonger be considered are, as it .has lately been dentified rommany deposits n North and South America.

McKinstry appears o have shown hat some of the stromey-erite is earlier than galena, sphalerite and chalcopyrite, ut ]believe his means hat the latter three minerals n the specimens

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THE SILIZER MINE OF COLQUIJIRC4, PERU. 341

Fro. 2. Photograph of polished section. Late marcasite mineralization.Black; chalcedony. Gray, older sphalerite. White, galena. Strands andsheaves of marcasite replacing older sphalerite. X 55-

Fro. 3- Photograph of polished section, showing late specularite-mar-casite mineralization. Light gray, sphalerite replaced by marcasite; darkgray, chalcedony. Crystal aggregate specularite. X 38.

Fro. 4. Photograph of polished section. Late sphalerite mineraliza-

tion. Dark gray, chalcedony; ight gray elongated reas, older sphaleritesurrounded nd partly replaced by specularite n small crystals. X 36.

studied by him belong o the latest phase of ascending minerali-zation described elow. Stromeyerite appears o be most abun-dant in the upper tennantite manto of Principal and Mercedes.Chalcocite is not abundant in the rich ore. Ahlfeld states that

chalcocite ccurs n the pyritic manto which carries enargite; chal-

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cocite eplaces he latter and is said by Ahlfeld to be of hypogeneorigin.

The Late Specularite-Marcasite hase.--A very late phase ofprobably hypogene mineralization s recognized y the presenceof specularite n well developed hough small crystals of steeprhombohedrons. Hematite is said to be widespread n themine, though not in large masses, but occurring n various

mantos so that it is not confined o any particular older min-

Fig. 5. Drawing of contact on Fig. 4 between sphalerite (S) andchalcedony C) showing crystals of specularite black) replacing oldersphalerite and chalcedony. X 5o. (Drawn by W. H. Newhouse.)

eralization. Specimens ere noted from the pyritic manto, Prin-cipal, and from a lower manto with galena n Mercedes. It wasfirst observed hat veinlets of chalcedony ontained mall crystalsof specularite. The accompanying inerals were later found tobe marcasite in delicate, sometimes curved strands, uniting insheaves nd arger prismatic aggregates; urther, chalcopyrite ndgalena occur n veinlets hrough the marcasite. This associationoccurs n solution cavities n the older sphalerite, ater filled withchalcedony. Specularite orms on the walls (Fig. 3), and themarcasite preads y. eplacement n the older sphalerite Fig. 4),and probably also in the older galena. Crystallized marcasitewas observed n some of the older sphalerite. This mineraliza-tion contains no recognizable ilver minerals nor any bismuth

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THE SILVER MINE OF COLQUIJIRCA, PERU. 343

minerals. It was at first considered upergene, ut I now holdthat t is the atest ypogene hase. The prese,nce f marcasiteindicates low temperature. If this phase esulted rom descend-ing water, it is difficult o understand hy it should not be richin silver.

It is known of course hat hematite and even crystallized pecu-larite may develop n oxidized ores and in laterite, but its pres-ence n the ater hypogene hases f epithermal eposits s certainlyvery unusual. Burbank mentions t as forming part of theearliest mineralization at Bonanza, Colorado. Here it occurs nsilicified olcanic ocks with pyrite and kaolin; he' presence fmarcasite s not mentioned. Specularite s found in places nmesothermal eposits; ts occurrence n early phases f the Camp-Bird mine, Colorado, is believed to be associated with Eoceneintrusives, on the supposition hat the vein represents n epi-thermal phase superimposed n a mesothermal mineralization.It is not believed hat we have full information as to the stabilityfield of specularite.

Super#ene Native Silver.--If Dr. Ahlfeld and myself are cor-rect in holding he stromeyerite ypogene, he supergene mineralsare narrowed down to native silver (probably also some chalcociteand pearceite). Silver is very abundant n the upper mantos ofPrincipal and Mercedes and results chiefly from the breakingdown of stromeyerite. It occurs as veinlets, masses nd knobsin stromeyerite, as illustrated in my paper on pseudo-eutecticintergrowths; or it forms sheets eplacing ennantite-stromeyeriteveinlets, or as wires and hooks in other ores. Rarely it appearsin the lead ores. Naturally some of these ores contain severalthousand ounces per ton.

The zone of oxidation appears o be poor and shallow; Ahlfeldstates he depth as 5 meters. On the other hand the zone ofnative silver is deep and extensive, lthough principally confinedto the upper ennantite-stromeyerite anto n Principal and Mer-cedes.

General Genetic Consideration.--According o Ahlfeld, the U.S. Geological Survey, Prof. Paper 69, 932, p. 68 and P1. IX.

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history began by silicification, dolomitization and kaolinizationof the country rock followed by deposition of barite and laterpyrite, poor in silver. Then followed the enargite-pyrite oreof the lowest manto, and finally the latest silver-rich tennantite,stromeyerite nd the lead ores. He interprets he differences fmineralization as primary depth zones. Looking at the sec-tion, it seems hat all the variations n the hypogene re confinedwithin a stratigraphic hickness f 3 to 5o meters, and that itwould be difficult to assume hat the vertical depth can have hadany great influence. I am willing to admit Ahlfeld's generalsuccession nd that the pyritic manto was probably'the oldestdevelopment t a fairly high temperature, ut incline o ascribethe differences argely o a change f composition f the solution.The influence f country ock seems o be limited to the prefer-ential development f lead in the lime-rich sections.

The distribution of the silver is of interest: There is little or

no silver n the early pyrite; little, comparatively peaking, n thepyrite-enargite re and in the lead ore; an abundance f silverin the tennantite manto; finally no silver minerals in the latestspecularite-sphalerite arcasite ineralization. here s furtherto be considered he sudden appearance f bismuth n spots nthe tennantite ore and its absence n the latest phase ust referredto; I believe hat much of these sudden hanges f mineralizationare caused y changes n the composition f the solutions. Thereis finally he question f ducts. It seems robable hat the daciteporphyry ppearing n the surface ess han a mile to the southis genetically elated o the ore, and t also seems robable hatby proper evelopment uch ucts r feeders might be discoveredby which he mineralizing olutions eached he beds which wereto constitute the main loci of the ores. The textures of the orerepresent combination f crustification nd eplacement. mongthe later minerals pseudo-eutectic eplacement extures are verycommon, nd have been mentioned nd figured by various au-thors. Most prominent re the replacements f tennantite y

a Lindgren, W.: Pseudo-eutectic extures. ECON. GEo,., 5, -x3, 93o. Ahl~feld, F.: Op. cit. Oreel, J., and Rivera Plaza, Gil: Op.

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THE SILVER MINE OF COLQUHIRC.4, PERU. 345

stromeyerite, ut there are also similar ntergrowths f tennantite-chalcopyrite, ennantite-galena, halcopyrite-sphalerite, nd chal-copyrite-proustite. t may be recalled hat Burbank has publishedsimilar ine textures of stromeyerite nd galena and of tennantiteand stromeyerite rom Bonanza, Colorado. All these are con-sidered o be hypogene.

Summary.--In recent ears, Colquijirca, six miles distant romCerro de Pasco, has been one of the most productive ilver minesof the world. The deposit s contained n folded Mesozoic cal-careous hale and limestone. Small intrusions f dacite porphyryoccur n the vicinity. The folding preceded he mineralization,which is distinctly of epithermal ype and was probably effectedat temperatures radually sinking rom 20o or 25 o C. to some-thing like oo C. The shale has been replaced y chalcedonicsilica, forming a brown chert. Other early gangue minerals arekaolin, dolomite, and ankerite, and their deposition continuedduring the mineralization o less extent. Barite is a later andabundant gangue, particularly n the copper-silver res. Pyrite,poor in silver, is the earliest metallic mineral and is followed bysphalerite, enargite, tennantite, galena, and chalcopyrite. Bis-muth minerals follow the tennantite. Then follows hypogenestromeyerite, which, with supergene ative silver, is the principalmineral in the rich argentiferous ores. Argentite and chalcociteare not abundant. Within a stratigraphic distance of 5 metersare several mineralized beds or mantos. The lowest is the

pyritic manto, poor in silver; above his are beds n which leadand zinc appear n large amount; he highest manto s the richest,with tennantite and stromeyerite. Analyses of wittichenite,stromeyerite nd tennantite are presented.

It is held that the association f mantos with differing miner-alization within so small a stratigraphic distance cannot beexplained by telescoping, or by differences n depth, but thatthey are caused y sinking emperature nd simultaneous hangesin the composition f the solutions. An interesting eature isthe latest hypogene mineralization arrying chalcedony, pecularite,marcasite, chalcopyrite, nd galena. Not much appears o be

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346 W,4LDEM,4R LINDGREN.

known about the uppermost one of oxidation, except that itseems o be poor and shallow, as shown by the early colonialmining operations n the outcrops; robably t has been subjectedto long-continued eaching. But when the descending urfacewaters reached he stromeyerite one, hey effected rapid break-down of this mineral, resulting n the development f native silveron a large scale. For the silver zone does not simply representa setting free of the silver in tennantite and stromeyerite, utthe accumulation f a great deal of this metal carried by thedescending olutions.

LABORATORY F ECONOMIC GEOLOGY,MASS. INSTITUTE OF TECHNOLOGY,June I2, I934.

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Lindgren 1935