A Metallographic Study of the Copper Ores of Maryland

A METALLOGRAPHIC STUDY OF THE COPPER ORES OF MARYLAND.

ROBERT M•. OVERBECK.

INTRODUCTORY.

Copperhas beenmined in Maryland sincepre-Revolutionary days. Before the discoveryof the large westerndeposits and the consequentdrop in the price. of copper, copper-miningwas an industry of some importancein the State. Eight mines in all have been worked at one time or another: six of these were aeoandonedat leasttwenty-five years ago; and one is still worked, but only occasionally. Although no figures are availablegiving the total outputof copperfrom the state,a productionof $I,75o,- ooo from one of the mines over a period of twenty-five years will givea generalidea of the formerimportance of coppermining in Maryland. An investigationof these depositswas undertakenwith the hope that a microscopicexamination of the ores might reveal somethingof the genesisof the deposits,and also add something to the generalknowledge of the genesisof copperore. In par- ticular the object of the work was to study the structural rela- tionshipsexisting among.the copper minerals in the ores, especi- ally with regard to the questionof the primary or secondary occurrencesof bornite,chalcopyrite, and chalcocite. Certain diffi- cultiesconnected with the work preventedits beingas completeas could be desired, the chief of which were the impossibilityof underground.observation and the scarcityof good material. A closestudy of ,thinsections of country and of vein rock, however, and a very careful searchof the mine dumpselimina.ted some of these difficulties. The descriptionof the depositswill be taken up in the follow- i$i •52 ROBERT M. OVERBECK. ing order: (I) The Bare Hills mine; (2) the Eastern Carroll County mines, comprisingthe Patapscomine, the Mineral Hill mine, and the Springfieldmine; (3) 'theFrederick Countymines, comprisingthe Liberty Mine, and the New London,or Linganore mine.

DETAILED DESCRIPTIOBI OF MINES. Baltimore County Mine, Bare Hills Mine. The Bare Hills coppermine is locatedabout 4 milesnorth of Baltimore on the northern outskirts of the suburb of Mt. Wash- ington. A smallmine dumpis all that remainsto mark the site of the old mine, last worked about thirty years ago. The deposit,according to old descriptions,was in a vein which carriedchalcopyrite, bornite, and magnetite. Unfortunatelythe material on the mine dump has so succumbedto attacksof the weatherthat ore and gangueminerals sufficiently fresh for micro- scopicstudy couldnot be obtained. , The depositoccurs in a generalregion of hornblendeschist, probablyan alteredgabbro. A few hundredyards to the east lies a smallserpentine mass that hasbeen intruded into the schist. The ore is apparentlyin a vein conformingin strike to the schistosityof the rocks; and, as far as can be judged from general relationshipsand from .thekind of material found on the dump, the depositis similarin all respectsto the copperoccurrences of eastern Carroll County, which will be taken up in detail in the following pages.

The Eastern Carroll County Mines: the Finksbur#- SykesvilleBelt. If a line be drawnon the map from Finksburg,Carroll County, in a directionS. 35ø W., most of the coppermines and prospect holesgiving evidence of copperwill lie veryclose to thisline. At the northern end is the Patapscomine at Finksburg;4½ miles southwestof Finksburgis theMineral Hill mine,between the two is oneprospect hole showing copper; I mile southwestof Mineral Hill an openingshows the presenceof chalcopyrite,and a half COPPER ORES OF MARYLAND. x53 mile farther on at the point wherethe line crossesthe stateroad, copperstains were noticed when the roadwas being built; a little fartheron are a numberof prospectholes, still on the sameline. To includethe southernmostdeposits (those in theneighborhood of Sykesville),the line .mustbe allowedto swingslightly to the south. If thisis done,the Carrollmine and the Springfieldmine, togetherwith the interveningopenings, fall into place. Several foœmeriron mines are alsofound in HowardCounty on the southern extension of this belt. An interestingfeature of this ,beltof depositsis t•herelation- shipbetween the iron and the copper ores. In generaliron oreis present in all the mines. On the southernmostextension of the belt in Howard Countyiron was mined,but no copper. The Springfieldmine was openedas an iron mine; at a depthof between6o and •oo feetenough chalcopyrite .came in to change the characterof the ore to a copperore. The iron in the out- crop was in the form of specularite,but with depth it seems to have beenlargely magnetite. At Mineral Hill both iron and copperwere found,but copperwas muchnearer the surfacethan was the caseat Springfield. Magnetitepredominates here. At thePatapsco mine copper ore occurs at thesurface, together with much magnetite. PatapscoMine. Location.--The Patapscomine, the mostnorthern of the eastern Carroll Countygroup of mines,lies at the easternend of the villageof Finksburg,a few yardsnorth o.f the Westminsterturn- pike. Thenearest railroad station is Falls,on the Western Mary- land Railroad,2 3 milesfrom Baltimore.A smallmine dump and a depressionover the old shaft are all that remain to mark the site of the mine. Geolo#y.--Allthe coppermines of Marylandlie in that part of the stateknown geologically as the PiedmontRegion. The characteristicfeatures of Piedmontgeology are so well known that a discussionof themhere would be outof place. The rocks, in•eneral, consist of schistosesedimentary andigneous rocks; and of unmetamorphosedigneous rocks intruded into the schists. ROBERT M. OVERBECK.

The prevailingrock of the generalregion in whichthe Patapsco minelies is a quartzmica schist which, in the immediateneighborhood of the mine, has most of the characteristicsof a phyllite. A thin sectionof this rock showsit to consistlargely of fine.- grainedanhedral quartz with much biotite, sericite,garnet, and magnetite. The natureof the mineralspresent and their relation- shipssuggest at oncean originalsedimentary origin for the schist. _Anexamination of the rockmaking up the minedump, however, showsit to be of an entirely different characterfrom that of the countryrock; for here are exposedamphibole and biotite schists. The contactbetween these two kinds of rock is effectuallycon- cealed,although the occurrenceof weatheredamphi'bole schist on the road-sid,e about •oo yards east of the mine showsthe existence of this rock for the distance mentioned at least. _Amicroscopic study of the rocksfound on the minedump was undertakenin order to determine,if possible,their naturebefore metamorphism. _Athin sectionof the biotite schistshowed it to consistentirely .of flakesof greenishbiotite in an unalteredcon- dition. There are severaltypes of hornblendeschist, one of which consistsalmost entirely of commonhornblende with small amountsof magnetiteand biotite; and another of which is made up of common'horn•blende, zoisite, some plagioclasefeldspar, quartz, and much epidotewhich replacesthe hornblendeand is evidentlya productof the decompositionof feldsparand hornblende. The rock appearsto be derivedfrom a basicigneous rock, and is probablya metagabbro. Aside from the schists,a smallamount of pegmatiticmaterial was found,which from its unalterednature and from its relationto the schistsevidently had its origin after the schistswere formed. It has a miariolitic texture, and crevices which intersect it are lined with zeolites. About all that can be said on the geologicalstructure of the region aroundthe mine is that the schistshave a generalstrike N. 3ø0 E. with a nearlyvertical dip that variesfrom eastto west. Type of Deposit.--The copperore, accordingto Tyson1 who x First Report of Philip T. 'Tyson, State Agricultural Chemist, to the Houseof Delegatesof Maryland,Doc. E, February•2, x86o. COPPER ORES OF M,zIRYL.4ND. I55 waswriting at the time the depositwas being worked, was found in a "true vein" that "became thinner, or pinchedoff" with depth. The ore minedconsisted of chalcopyrite,bornite, chalcocite, and carrolli'te,together with their oxidationproducts, and much magnetite. Descriptionof the Ore.--The descriptionof the ore will be consideredunder two heads: first, the relation of the ore minerals to gangueminerals as shownin thin sections;and, second,the relationsthat existamong the opaqueminerals as revealedby the study of polishedsections. The copper-bearingminerals--chalcopyrite, bornite, and carrollite--were found both in the biotite and the hornblende schists, and in the pegmati'ticmaterial: chalcopyritebeing found in both schistand pegmatite;bornite being confinedlargely to the peg-. matite rock; and carrollite2 to the schist near the contact with pe•rnatite. Magnetite is present in the schistin large amounts closelyassociated with the copperminerals. An examinationof the thin sectionsshows that, with the possibleexception of some magnetite,all the opaqueminerals are later than the transparent mineralspresent. The magnetiteappears to be of two ages: part of it, occurring'as sharp crystalsand as grains in hornblende, being at least as old as the hornblende;and part of it being de- cidedly later than the hornblende. Fig. t, shows chalcopyrite and .bornitereplacing hornblende, chlorite, and calcite; Fig. 2, is an excellentillustration of the replacementof epidoteand feldspar by bornite. A microscopicexamination of polished sectionsof the ore brings out the rel'ationshipsthat exist among the ore minera,ls, and betweenore minerals and gangue. The opaqueminerals will be treatedhere in the approximateorder in which t:heycame out of solution. Ma#netite.--Magnetite,the earliestof the opaqueminerals to crystallize,occurs in closelypacked grains and in sharpor slightly roundedcrystals. Although, as pointedout above,the magnetite is of two distinct agesof formation, by far the greater part is •' Carrollite---CuCo2S,, a very rare cobalt mineral first described from the Patapsco mine. 156 ROBERT M. OVERBECK.

EXPLANATION OF PLATE VIII.

Fro. •. The Mineral Hill Mine. (X 47.) Thin section showing chalcopyrite and bornite (both black) replacing hornblende (the mass of the slide) and chlorite (large light flake near upper left-hand corner). Fro. 2. The Patapsco Mine. (X 47.) Thin section showing replacement of epidote (rough surface) and feldspar (upper part of picture) 1)y bornite (black). Fro. 3. The Mineral Hill Mine. (X 60.) Polished section. Inclusions of bornire (b) and chalcocpyrite(cp) in a large magnetite(mr) crystal. The presenceof both bornite and chalcopyritein the same inclusion in the magnetite crystal shows their contemporaneity. Note the parallel or perpendicu- lar arrangement of the rod-like inclusions. (Cf. Fig. •2.) Fro. 4. The PatapscoMine. (X 32o.) Polishedsection. Chalcopyrite(cp) is shown here in contact with a magnetite (mr) crystal. The corrosion and the rounding of the corners of the magnetite crystal indicate that the mag- netitecrystallized before the copperminerals came out of solution. Fro.5. ThePatapsco Mine. (X 375.) Polishedsection. This section shows a triangular inclusionin sphalerite(se), of bornite (b) and chalcopyrite (cp). Graton and Murdoch call attention to such triangular inclusions of chalcopyrite in sphalerite and suggest that they are sphenoidal crystals of chalcopyrite. The inclusion in this case is probably not a crystal. Corellite and chalcocite(cc) are seen replacing the sphalerite. FI6. 6. The Mineral Hill Mine. (X 70.) Polished section. Typical ore consistingof chalcopyrite (cp), bornite (b), magnetite (mr), and secondary chalcocite which replaces bornite and chalcopyrite along crevices. The chalcopyrite and bornite show typical primary relationships. E Viii. ECONOMIC GEOLOGY. VOL. XI.

FIG. •. Fro. 2.

FIG. 3- FIG. 4-

•. Co.

Fro. 5. FIG. 6. COPPER ORES OF MARYLAND. 157 later thanth,e schist in whichit is found. A typicalsample of the copperore in whichmagnetite is presentshows the following arrangementof minerals'first, a mixture of magnetiteand schist in which the magnetiteis both earlier and later than the ganflue minerals;then a bandof closelypacked rounded grains of magnetite; next, a band of large well-formed magnetite crystals whichare surrounded,or partly surrounded,by borniteand chalcopyrite,and whichinclude numerous grains of the copper-bearing minerals. The facts,then, concerningthe occurrenceof the magnetite warrant the conclusionthat it was the first of the opaque mineralsto crystallize,and that someof it at least crystallized out of copper-bearingsolutions. Fig. 3, showsinclusions of chalcopyriteand •bornitein magnetite. The roundingof several of the crystalsthat are in contactwith the copperminerals sug- gestsa slightchange in physicalchemical conditions just after the crystalwas formed. Fig. 4, is at a contactbetween chalcopyrite and magnetite,showing rounding and corrosionof the magnetic crystal. Specularite.--Specularite,occurring in a narrow band, was noted in only two places,both of which were in the pegrnatitic ore. In one caseit distinctlyreplaces quartz; in the other case,in which it is surroundedby copperminerals, it is greatly corroded. Its relationsto magnetitecould not be determined,although it is evidently earlier than the copper-bearingminerals. Carrollite.--Carrollite (CuCo2S4)occurs in suchsmall amount and in suchunfavorable surroundings that little that is definite can be said about it. It is evidentlylater than magnetite,but its relations with bornite and chalcopyriteare somewhatuncertain. The fact that it has roughly cubicaloutline, and that in one case chalcopyriteseems to be includedin it would indicatethat it is somewhatearlier than the other copperminerals present. The crystalsare muchfractured; chalcocitization has taken place along the fractures. Sphalerite.--Sincesphalerite occurs here in only very minor amounts,its discussionwill be reservedfor the Mineral Hill ores, where its relations to the other minerals can be more clearly observed. In the Patapscoore it seemsto be restrictedto the peg- ROBERT M. OVERBECK. marltic material, and occursas irregular massesthat have been alteredlargely to chalcociteand corellite. Fig. 5, showssphalerite altering to chalcociteand corellite, and includingchalco- pyrite and bornire. The sphaleriteis earlier than the chalcopyrite and bornire. Uhalcopyriteand Bornite.--Chalcopyriteand bornire are for the most part contemporaneousin origin. They show a typical primary intergrowth, both forming irregular massesthat are inti- mately related. (See Fig. 6.) No secondarybornire was observed,although a little secondarychalcopyrite is present. The occurrenceof secondarychalcopyrite developing along cleavage lines of bornire has been describedand illustratedby Graton and Murdoch.a Fig. 7, and Fig. 8, show this type of alteration. In Fig. 7, the gash-likestructures are chalcopyritesurrounded by narrow bands of .chalcocite;the main body of the slide, aside from gangue, is 'bornire. The .thin dark line from which the gashesseem to start is a .smallcrevice in the ore. In Fig. 8, the alterationbands narrow where they crossone another and swel'l out in the intervening space. The points of interest .aboutthe occurrenceof this type of alterationare its limitation to contacts and to crevicesin the ore, or to their immediateneighborhood, and its closeassociation with chalcocite. The chemistryof this alteration has not been worked ou't. Uovellite.--Covelliteis relativelywidespread through the slides, where it is found accompanyingchalcocite. Its usual appearance is in bunchesor in featherycrystals within the chalcociteor at its contactwith someother mineral. It is particularlyabundant as a replacementmineral of sphalerite,its smallcrystals being so orientedas to conformin large part to the cleavagedirections o.f the sphalerite. Fig. 5, showsa .massof sphaleritealmost com- pletelyaltered to corelliteand to chalcocite.It showsalso that the sphaleriteis muchmore susceptible to alterationthan. is the includedchalcopyrite. Uhalcocite.--All the chalcocite observed from this mine is clearlysecondary, since it is foundreplacing all the opaquemin- a Graton and Murdoch, "The SulphideOres of Copper,"Trans..4met. Inst. Min. En#., Vol. XLV., x914, pp. 44, 49. COPPER ORES OF MARYLAND. I59

eraIspresent except magnetite. Replacement has gone on largely in the vicinity of crevicesin the ore, and 'at the contactbetween ore mineralsand gangue,although, in the caseof sphalerite,practically the whole mass has been altered. The con.tact between chalcociteand the alteredmineral may be sharpor it may be fuzzy,the fuzzinessbeing due in somecases to the presenceof small featherycorellite crystal•. The boundarylines between massesof ore mineralsappear to be i.mportantplaces fo.r the de- velopmentor limitationof chalcocite;for often a crevicebordered by chalcocitewill stopshort at .acontact, and sometimes, especi- ally in passingfrom chalcopyriteto bornite or to sphalerite,the bandwill widenout. Fig. 9, •bringsout thesepoints very strik- ingly. Fig •o, showsthe wideningof a band of chalcociteon passingfrom chalcopyriteto ,bornite. It is interestingto note the unalteredgrain of chalcopyritewit'hin the cl•alcocite'band. Under the conditionof chalcocitizationsof thesedeposits, then, it is evident•hat sphaleriteis the mos.teasily altered, that bornireis lessso, and that chalcopyriteis l:heleast susceptible to alteration. The paragenesisof the opaqu.e minerals of the Patapscomine, then, seemsto be magnetite(the oldest), carrollite,sphalerite, chalcopyriteand bornite,secondary chalcopyrite, corellite, and chalcocite. Of thesemagnetite, carrollite, sphalerite, and bornite are in all casesprimary; chalcopyrite is largelyprimary, but also occursas a secondarymineral; covellite and chalcociteare in all cases secondary. The opaque minerals are all later than the gangue,with the possibleexception of a little magnetitewhich is earlier.

Mineral ttill Mine. I•ocation.--Fourand one half miles southwest of Finks. burg on .thenorth side of MorganRun, onehalf mileeast of the state roadcrossing, lies the Mineral Hill mine. A ratherlarge mine dumpand a numberof shallowpits showthe locationof the mine,which was last worked about 2 5 yearsago. Geology.--Geologicallythisoccurrence of copper ore is similar to thatat thePatapsco mine. Thecountry rock to theeast and I6O ROBERT M. OVERBECK. to .thewest consistslargely of hard gray quartzoseschists, which sometimesbecome rather micaceous,and then usuallycarry gar- nets; and sometimesvery quartzose,consi,sting chiefly of large bands of quartz, 4 or 5 inchesthick, separatedby narrow mi- caseousstringers. These schistsstrike uniformly between N. 35ø E. and N. 45 ø E., and have almostvertical dips. Pegrnatite was not found with' this o•e as at the Patapscomine; but just to the east of the mine the quartz schistis injected with pegrnatitic material. A thin.section of the country rock shows it to be similar to that of the Pat.apscomine. Here again,the materialon the mine dumpand from the out- cropsin the immediatevicinity of .the•mine are seento be of a different characterfrom that of the countryrock. Biotite, amphibole, chlorite, and talc sc'histscomprise most of the rocks found on the du,mp;and chlorite and talc schistsmake up the actual Outcropsat the mine. The biotiteschists were seenunder the microscopeto consistalmost entirely of biotite; the amphibole schistsof commonhornblende (altering in many casesto chlorite, talc, and epidote), zoisite,some quartz, and magnetite. A common rock noted at this mine was talc schist .containingmany beautiful little crystalsof actinolitewhich under the microscope cut sharplyacross the platesof talc and undergoalteration to talc only at the endsor at placeswhere a crystal had beenbent and broken. This rock was not seenat the Pa.tapscomine, but it occurs to the southat the Springfieldmine. Epidote is presentin large amountsin all the slides,and appearsto have been one of the latest minerals to form. Type of Deposit.--Since .the undergroundworkings of the Mineral Hill mine are no longer accessible,it is necessaryto de- pendon old reportsfor informationconcerning conditions underground. The ore is describedas being in a vein 2 to 6 feet wide, whichconformed in strikeand dip with the schist. The northern end of the vein ran into the pre-Revolutionaryworkings, and apparently was not followed; the southern extension was cut out by talcoseschist. The walls of the vein were of talcoseand mica schist;the ganguewas chlorite. Both copperore and iron COPPER ORES OF MARYLAND. x6x

oreoccurred, the copper ore occupying the hanging wall side of the vein, the iron ore the foot wall side. The valueswere not distribu.tedevenly, but occurred,in shoots. Descriptionof Ore.--An examinationof thin sectionsshowed •hatthe relationsbetween the rockminerals and between ore and gangueis in all respectssimilar to. the relationsthat exist at the Patapscomine. The chief differencesnoted were the absenceof pegrnatitematerial at the MineralHill mine,and thegreater abundance .there of chloriteand talc schists. Mag- netiteis again present in largeamounts, part of it beingprimary and part being introducedaf,ter the rock had formed. Bornite andchalcopyrite everywhere replace all therock-forming mineralspresent with the possible exception of tal'c. Epidoteis in mostcases seen replacing hornblende and feldspar, and being re- placedin turnby copperminerals. Zoisite appears to be one of theearly constituents of the country rock, and probably wa• formedunder the action of regionalmetamorphosing forces. Fig.t t, showsbornitc replacing epidote, and surrounding quartz and feldspar. Polishedsections of ore from theMineral Hill mine shoxvthat theore minerals (and their relations to oneanother) are similar to thosedescribed from the Patapsco mine, and, consequently, littlecan be said other .than that already stated. Magnetite.--Magnetiteis again primary and secondary(in the sensethat it is later than the schist),and was the earliestof theopaque minerals to crystallize.Fig. 3, hasalready been used to illustratethe characterof inclusionsin magnetite.Fig. t2, gives a more highly magnifiedview of someof the inclusions. The largeone near the center of '•hepicture shows a grainof chal- copyritewith secondarymagnetite within it, and within the secondarymagnetite a stillsmaller grain of chalcopyrite. Carrollite.--Carrolliteshows roughly cubical outline, and con- tains someinclusions o.f chalcopyrite.It is apparentlyearlier than the chalcopyrite. Sphcderite.--Asat the Patapscomine, sphalerite is later than I62 ROBERT M. OVERBECK.

EXPLANATION OF PLATE IX.

Fro. 7. The PatapscoMine. (X 365.) Polished section. This sectionshows the alteration of bornite (b) to chalcocite (cc) along crevices and at the contact with gangue (G), (feldspar) and to chalcopyrite (cp) along cleavage lines. (Cf. Fig. 8.) Fro. 8. The Patapsco Mine. (ca. X 600.) Polished section. This section shows the alteration of bornite (b) to chalcopyrite (cp) along cleavagelines. A narrow band of chalcocite (cc) intervenes between the two minerals. It is interesting to note that the gash-like replacement or alteration bands be- come narrow where two bands cross, instead of broadening out as should be expected, if ordinary replacement were taking place. Fro. 9. The Patapsco Mine. (X87.) Polished section. Here chalcocite (cc) and corellite (cv) replace bornite (b), chalcopyrite (cp), and sphalerite (se). The section shows the extent to which alteration and replacement have taken place in the different minerals. The sphalerite is almost completely altered to corellite and chalcocite; bornite has altered along crevices to rather wide bands of these minerals whereas the chalcopyrite shows only slight replacement along these same lines. (Cf. Fig. •o.) Fro. •o. The Patapsco Mine. (X 380.) Polished section. This shows the widening of an alteration band of chalcocite (cc) on passing from chalcopyrite (cp) to bornite (b). Note the unaltered bit of chalcopyriteincluded within the band of altered bornite. Fro. •. The Mineral Hill Mine. (X47.) Thin section showing bornite (black) surrounding and replacing epidote (mass of slide). The light areas ar.e plagioclase feldspar. Fro. •2. The Mineral Hill Mine. (X32o.) 'Polished section. This is a more highly magnified picture of Fig. 3, showing inclusions of bornite (b) and chalcopyrite (cp) in magnetite (rnt). IX. ECONOMIC GEOLOGY. VOL. Xl.

Fro. 7. FiG.

Fro. 9- I-IG. IO.

FIG. 12. COPPER ORES OF MARYLAND. I63 magnetiteand earlier than chalcopyriteand ,bornite. Inclusions of chalcopyriteand borniteare exceedinglynumerous, and are of two kinds:relatively large masses o.f the copperminerals, and small particlesof chalcopyriteand bornite, which under the highestpower of themicroscope are seento be definitegeomet- rical figures--rectangles,trapezoids, and trianglespredominat- ing. Graton and Murd,och 4 call attention to the. existenceof inclusionsof chalcopyritepossessing triangular outline, and suggest that they are probablysphenoidal crystals. Such a triangular inclusionis seenin Fig. 5. The inclusionhere showncon- sistsof a mixtureof borniteand chalcopyrite,and appearsnot to bea crystal. Fig. I3, showssphalerite altering to chalcocitealong cleavagelines; the smallwhite spotsscattered through the slide are inclusionsin the sphalerite. Chalcopyriteand Bornite.--Chalcopyriteand bornite show typical primary intergrowth,and are contemporaneous.Both are alteredto chalcocitealong the numerousfractures in the ore. Fig. t4, showsthe character of alteration. Chalcocite.--Chalcocite,as has just been said, is presentin ratherlarge amount, and is alwayssecondary. It differsfrom the chalcocitepresent in the Patapscoore in that it doesnot show a fuzzy coneactwith .thealtered mineral. Corellite was not observed. The paragenesisof theseminerals, is the sameas that for the Patapscominerals: namely, magnetite (the oldest),carrollite, sphalerite,chalcopyri'te and b.ornite,and chalcocite. Betweenthe Mineral Hill mine and the Springfieldmine, which lies about four and three fourths miles a little west of south of Mineral Hill, there are indications of the existence of a min- eralizedbelt. About one-halfmile southof Morgan Run on the stateroad there is exposedin the side of the road the weath- eredoutcrop of a band.of amphibo.leschist (about t5 ¸ feet wide at thispoint), from which 'a littlecopper was taken where pros- pec'tholes were 'opened in the woodssouthwest of thispoint. No more exposurescan be seen until the Carroll mine, three miles to 40p. cit. p. 43. 164 ROBERT M. OVERBECK.

EXPLANATION OF PLATE X.

Fro. •3. The Mineral Hill Mine. (X 77.) Polished section. This section showssphalerite (se) replacedby chalcocite(co) along cleavagecracks. Note the numerous inclusions of chalcopyrite (cp) and bornire (b). F•. •4. The Mineral Hill Mine. (X 60.) Polished section. Typical section showing alteration of chalcopyrite to chalcocite (co) in the Mineral Hill ore. Fro. I$. The Liberty Mine. (X37o.) Polished section. Intergrowth of bornite (b) and chalcocite(co). An etchedsection of this piece of ore shows that the fingers of bornire are parallel to the cleavage lines in the chalcocite crystal, a fact which suggeststhat the bornire is younger than the chalcocite, or at least contemporaneouswith it. Fx•. I6. The Liberty Mine. (X4oo.) Polished section. Graphic intergrowth of bornire (b) and chalcocite(cc). The cleavagelines in the chalcocite show the relatively large size of the crystal. FI•. •7. The Liberty Mine. (X32o.) Polished section. Alteration of bornire to chalcocite along crevices and at the contact with gangue (G) (barite). Note the fuzzy contact. This is due probablyto the formation of minute covellite crystals. Fro. •8. The New London Mine. (X 47.) Thin section of the "schistose ore." Chalcocite (black) surroundingand replacing calcite grains. Note the large crystals of calcite showingbent twinning lamella•. The low index mineral (light) surrounded by chalcocite is quartz. FIG. 14.

FIG. 15. F•O. 16.

F•o. x7. COPPER ORES OF MARYLAND. I6 5

the southwest,as reached. The Carroll mine is .on the north side of Piney Run aboutone fourth mile westof the stateroad, and consistsof several small openingsexposing a banded quartz- speculariterock. This mine was never of importanceas a copper mine. One 'half mile a little west of southof the Carroll mine lies the Springfield deposit,one of the most important in the days of coppermining in Maryland. Since, however,this mine does not afford material for study under the mctallo- graphic microscope,a discussionof the ore occurrencewill be omitted.

Genesisof the Carroll County Deposits. Genecis.--Noneof the early writers who have describedthe eastern Carroll County deposi,tshave discussedtheir genesis. The simi'larityof the depositsand their confinementto a narrow belt, however,were early recognized. Ansted* in z854 mentions thesefacts, and Weedø in z9zz speaksof the depositsas occurring along "a copper-bearingfissure vein . . . [that] is dis- tinctly traceableacross Carroll County." The first questionto be settled,then, concernsthe nature and extent of this •bel,tin whichthe copperdeposits are found. The following facts about it are known: •. The copperdeposits are found along a line whosestrike is approximatelythat of the schistosity.of the countryrock. 2. The characterof the rock foundon the variousmine dumps and in ou,tcrops at the minesand in the state road southof Mor- gan Run,where the line.was projected to cross,differs markedly from the prevai}ingcountry rock. 3. A dike of highly metamorphosedgabbro is found along a projectedcontinuation of the line to the northof Finksburg, and can be traced for several miles. 4. Copperstains are reportedfrom Blue Mount, sixteenmiles northeastof Finks'burgstill on the strikeof the line,and finally, 5Aristed, D. T., "On SomeCopper Lodes near Sykesvillein Maryland," Quart. Iour. Geol.Soc., London, Vol. XIII., pp. •42-•45, z857. 6Weed, W. H., "CopperDeposits of the AppalachianStates," Bull. 455, U.S. Geol. Survey, x9xx,pp. 62-65. 166 ROBERT M. OVERBECK.

5. Copperores are not kr}ow,n anywherein easternCarroll Countyexcept along the line described. It ,appears,then, that this is a line of structuralweakness along which intrusion.could take placeor ore-bearingsolutions travel. That suchlines exist is shownby the occurrence,farther west, of a Triassic d.iabasedike, which can be traced practically without break across the state. As hasbeen pointed out above,the rocksin the vicinityof the mines,are probably altered basic intrusives whi•ch were injected along this line of weaknessbefore or during the time intense dynamo-metamorphicchanges were goingon. Sometime after regionalmetamorphism ceased, pegrnatitic material was injected into the cou=tryrock, and, at the Patapscomine, into the altered intrusives. That the pegrnatiteis later than the schistosityis clearlyshown by its relationto the schistsas well as by its rel- ativelyfred and unsqueezedcondition as revealedby the microscope. After the pegmatiticinjection came extensive epidotiza- tion followed'by the depositio.nof the ore minerals. It is proper, now, to see what evidencethe mineralsasso- ciatedwith the ore depositshave to offer concerningthe conditions under which ore depositiontook place. The chief rock- forming mineralspresent--biotite, hornblende, zoisite, and epidote•may al'l be the product of regional metamorphism;and hencewould be expectedto add nothing to a knowledgeof the conditionsunder which the ore formed. Epido.te,however, although probably part of it is due to regional metamorphism,is found replacingfeldspar in the pegmarl.tic material, and is, there- fore, later than the schistosityof the rocks. Aside from being a productof regionalmetamorphism, epido.te, according to Em- mons,7 may be present in contactmetamorphic deposits, in the weathered zone, and in depositsof the deep-vein zone. There is no evidencethat these depositsare contactmetamorphic deposits;and it hasbeen shown that the epidoteis earlierthan the ore minerals,and gives no evidenceof being due to weathering. The conclusion,then, is that the epidote is the product of the deep-veinzone. 7 Eco•v. Gv,ox,., Vol. III., I9O8, p. 6x9. COPPER ORES OF M,,,IRYL,,'IND. I67

Of the copper-'bearingminerals and their accessories(sphalerite, magnetite,and spcularite),bornite, chalcopyrite, and sphal- eritemay occur in depositsof moderatedepths, as well as in those of great depth;tbut magnetite and specularireare foundonly in veins of the deepzone. The absenceof fissure-fillingand of crustificationin the vein materialand the presenceof extensivereplacement phenomena give furtherevidence of a deep-seatedorigin for theseores. The sourceof the ore-bearingsolutions can only be surmised, but in view of the fact that pegrnatitesare commonthroughout the region,and that largegranite masses occur east and southof the belt, it seemsprobable that the ore-bearingsolutions had their origin in a coolinggranite magma. Sincethe graniticintrusives in Maryland are probablyof two or more periodswhose ages have never been determineddefinitely, it is impossibleto say in what geologicperiod mineralizationtook place. Some of the intrusivesare clearlypre-Cambrian, while otherssh'ow no influ- ence of t.he metamorphicac.tion which produced the general schistosityof the region. Sincerocks as young as the Ordovician are schistose,the final granitic intrusionsmay be Devonian or younger,and it is with theselate intrusionsthat theseore de- Ix)sitsseem to be connected. The ores,then, of the .eaSternCarroll Countybelt occurin re- placementveins which were formed at great depth, and whose time of formationwas after the countryhad undergoneintense regionalmetamorphism. The ore-bearingsolutions .were ascending solutions,and probablyhad their origin in the graniticmagma which underl.aythe regionto the east.

The Frederick County Mines. In the easternpart of FrederickCounty, about twenty miles west of the Carroll Countycopper belt just describbed,is another region in w•hichcopper mining was carried on. Two mines ,in this dis.trict--theLiberty mine and the New Londonmine--were onceof considerableimportance in the state •/s copperproducers. Two otherdeposits--the Dolly Hyde and the New Windsor de- I68 ROBERT M. OVERBECK. posits--wereconsidered promising when they were opened,but later work did not fulfill their early promise. One lead mine-- the MountainView Lead Mine--was opened,but was soonaban- doned. There is a marked difference in the mode of occurrence between the Frederick County ores and those of easternCarroll County. Some of the characteristicsof the Frederick County occurrences are as follows: z. The Frederick County copperores are scatteredwidely fiaroughoutthe regionof easternFrederick County. 2. Many limestoneoutcrops occur showingcopper stain, or thin stringersof copper.minerals. 3- The oresare apparentlyconfined to the vicinityof a contact betweenthese limestones and the adjacentschists. 4. The copperores occur as pocketsor stringersin the limestones. 5- The ore minerals are bornire and chalcocitewith minor amountsof chalcopyrite. 6. The gangueis calcite,quartz, and barite. Some miles to the west o.f the Frederick County mines,the narrow 'belt of Triassic rocks that extends southward from New York cutsacross the state. Tracesof copperhave been reported in theserocks from a numberof places,but noneof the deposits ever lookedtempting enough to be developed. Still farther wes.t, on the eastern side of South Mountain, copperagain occurs accompanyingthe pre-Cambrianlava flowsthat form the eastern part of the mountain. No work has beendone in Maryland on thesedeposits, although a numberof mineshave beenopened at onetime or anotherjust north of the stateline in AdamsCounty, Pennsylvania. 'LibertyMine. Location.--The Liberty mine is situatedabout five and three fourths miles southwestof Union Bridge--a town on the West- ern Maryland Railroad, forty-five milesnorthwest of Baltimore-- on the CopperMine road, one mile west o.f where it crossesthe Liberty-Union Bridge turnpike. COPPER ORES OF M,'IRYLAND. I69

A comparativelylarge mine dumpand an old mill partly dis- mantiedand in a completestate oœ decay shows •he locationoœ the mine property. Undergroundwork has been.carried on at two placeson the property; one placeknown as the Old Workings, and the other as the New Workings. The Old Workings have completelycaved, and are henceno longeraccessible; the New Workings (last worked about 4 years ago) are only partly accessible. Geolog•y.--TheWestern Division oœ the Piedmont,in which the FrederickCounty deposits lie, is characterizedby quartzites, schists,and limestones. s The quartzitesare undoubtedly0/5 sedi- mentaryorigin, consistingas they do oœwater-worn pebbles (varying in size œrom.x inch to .3 inch in diameter). The schistsare o/5both sedimentary and 0/5igneous origin. The sedi- mentaryschists are gray and sandy,.or so/St, dark phyllites;the igneousschists are metamorphicequivalents 0/5 'acid and basic tuffs and lavas, showingvery often an amygdaloidalstructure. The basicrocks alter to greenstone,and seemto be more common in the southernpart 0/5the district. The limestones'are found in long narrowbands in the schists;or they outcropin smallirregular patches .whose geological significance is difficult to explain. Many theoriesconcerning the structure0/5 this por- tion o/5 the Piedmont have been advanced, but none has been generally accepted. The Libertydeposit occurs at the top 0/5'a broadflat hill, and as a consequencethere are no exposureso/5 the country rock other than •hose seen in the mine. A' œewhundred yards to the west 0/5the mine conglomeratefloat was found in the field; at the mine limestoneis exposed;and a œewhi•ndred yards to the east o/5 the mi.ne, schist occurs along the road. Here, as at mosto/5 the other localitiesalrea. dy described,the mine dumphas to be dependedon to show the ch:aractero/5 the rock in the immediatevicinity 0/5•he deposit. It is composedlargely of dol.o- mitic limestone,which is sometimesa pure carbonate,and at other times a mixture 0/5 limestoneand schist. Purple, green, 8 ', The Limestones o• Maryland," E. B. Mathews and J. S; Grasty, M•I. Geol. Survey, Vol. VIII., pp. 35o et seq. t7o ROBERT M. OVERBECK.

and gray schistsare also found. The purple schistis a soft, thinly-beddedrock that showsamygd. aloi,dal structure, and under the microscopeis. seen to be an altered effusiverock probablyof acidictype. The soft light-greenschist, apparently, is derived from the impuritiesin the limestonewith which it is closelyassociated. The gray schistis amygdaloidal,and is a metamor- phosedeffusive similar to the purple schistjust described. The limestoneis a fine-graineddolomite cut by numerousstringers of coarse-grainedcalcite. Description of Ore.--The copper-bearingminerals found at the mine are bovnite, ch.alcocite,chalcopyrite, and mal'achite. Bornite and chalcociteare presentin aboutequal amounts;chalcopyrite is rare; and malachite although widespread is of no imp.ormncecommercially. Thin sectionsof the .ore mow the copperminerals to be the latest mineralspresent, since they replace the dolomite,and enter along cleavagecracks of the large calcite and barite crystalspresent. Quartz replacescalcite and dolomite, but is earlier than the copper minerals. The ore minerals present in the polishedsections are chalcopyrite, b.ornite,and chalco.cite.All occuras primary minerals. Chalcopyriteand chalcociteare also secondary. Primary9 Chcdcocite.--Primarychalcocite seems to have been •he earliest of the ore minerals to crystallize. Etching brings out the fact that it consistsof large crystalswhich includebor- nite. This bornite also seemsto replacethe chalcociteto some extent. Fig. •5, showsbornite with stringersarranged parallel to the cleavagelines (brought out by etching, but not shown in plate). Bornite is seen,too, in stringersbetween the chalcocite crystals. At times, however, the relation between bornite and chalcociteshows that they are, in somecases, probably contemporaneous; for some excellent graphi.cintergrowths were noted (Fig. •6). Bornite and Chalcopyrite.--Borniteand chalcopyriteare present in typicalprimary intergrowths. Later chalcopyriteis pres- 9 Primary is used here and below with the connotation of deposition by heated ascendingsolutions; secondary,below, of deposition by cold descending solutions. COPPER ORES OF MARYLAND. I7I

ent to a smallextent, occurring along cleavage lines of the bornitc. The relations•between bornitc and .chalcopyrite are similarto those already descri,bedunder the Patapscoores. SecondaryChalcocite.--Secondary ch, alcocite is found re- . placingboth bornitc and chalcopyritealong cracks,and at contacts with •he gangueminerals. This, too, h'asbeen described above. In the sectionscontaining the primary chalcociteand the primary bornitc,there is a form of chalcocitepresent that is of a deeperblue than the ordinary chalcocite. Graton and Murdochxø mentionthe occu.rrence of this mineral and makethe suggestion that it is a mineral disti.nct from chalcocite. In the section ex- amined,.the mineral was seenin large amountintimately mixed with primary ehalcocœte.Its exact nature, however,could not be determined. • GangueMinerals.---Barite, calcite, and quartz occur as gangue. The bariteand calcite are in well t•ormedcrystals; the quartzis in grainsintimately mixed with the ore. The barite and calciteare earlier than the ore minerals, but the intimate relations between quartzand ore minerals make' it appearas if someof thesilica came in with the ore-bearingsol. utions, and was depositedas quartz at much the same time as those miner.als.

Nee. London Mine. Location.--TheNew London,or Linganore,mine i's about four and one half mile east of southof the Dolly Hyde mine, and aboutthree and onehalf milesnorth of Monrovia, a stationon the mainline of the Baltimoreand OhioRailroad, fifty mileswest of xoOp. clt., pp. 47, 48. xxCf. "The Sulphidesof Copper,"by E. Posnjak,E. T. Allen, and H. E. Merwin, ECONOMmG.vx•oa¾, Vol. X., No. 6, September-October,I915. The authorsof this article call attentionto the frequent occurrenceof solid solutions of cupric sulphidein cuproussulphide associated with chalcocite. These solid solutionsare characterizedby a blue color. The blue mineral in the section studied may be such a solution. Cf., also, "Observations on Certain Typesof Chalcociteand Their Characteristic Etch Patterns," by C. F. Tolman, Jr., Bulletinof the AmericanInstitute of MiningEngineers, February, •9•6. The "bluechalcocit•" is thoughtby Tolmanto representa transitiontype in the replacementof bornitc by chalcocite. ROBERT M. OVERBECK.

Baltimore. This mine is still worked, althoughvery spasmodi- cally. Geolo#y.--T.heNew Londondeposk is in the samegeneral geologicprovince as the Libertydeposit. The rocktypes are the same,although greenstones are much more prominentin t.he neighborhoodof the New Londonmine. The depositis found in an impurelimestone, which outcrops on the sideof a rather steephill. The hill is cappedby a heavyquartzite, and the slopes are madeup of grayschist, phyllite, greenstones, .and impure limestone,all of whichappear to dip to the west and henceunder the quartzite. The strikeof the bedsnear the mineis roughlya little eastof north. Rockexposures are not frequent,and are difficult to interpret. _Atleast one well-marked fault canbe seena short distancenort.h of the mine, and there are facts that suggestother faulting. In themine itself a fault cutsout the orebody entirely, and frequentoffsetting of the vein showsstill .othermovements. Thin sections were made of some of the rocks found in the vicinityof themine. The greenstonewas seen to consistlargely of fine-grainedepidote with numerousinclusions of specularite, cutby smallstringers of calciteaccompanied by plagioclase.The originalstructure o,f the rocksis obscured;but its chemicalcom- position,as revealedby the mineralspresent, and its similarity to rocks from the regionto the west, where their nature is defi- nitely known,•' leavelittle dou'b.tthat the greenstonesare altered basiceffusives. A specimenof schistfrom the field north of the mine is seenunder the microscopeto be a fine-grainedamygda- loidal rock .showingmarked flow structure. The groundmassof the rock consistsof a very fine-grained aggregate of minerals which are too small to be determinedwith certainty, but which, on accountof their generallow index and doublerefr.actio.n, suggest quartz and feldspar. The rock correspondsclosely to some of the aporhyolitesfrom the South Mountain region described by F. Bascom. The impure limestone from the mine itself is a contortedschist with bands•of limestoneand soft gray phyllite. Under the microscopethe bands .of limestoneare seen to •2 Bascom, F., Bull. U.S. Geol. Survey, No. x36. COPPER ORES OF MARYLAND. I73 consistof long narrow crystalsof calcite,without crystalbound- ary, ranged side by side with their long axes parallel; the phyllite bandsare made up in large part of sericitewith numerous inclusi'onsof specularite. Type of Deposit.--The New London depositbelongs in the samegeneral class with the Liberty deposit,although it occursin somewhatdifferent form. At the Liberty mine, it wil'l be re- membered,t. he ore was found in .stringersor bunchesin rather ' pure limestone. At the New London mine the ore is in a fissure cutting acrossan impure limestonewhich has been mineralized. As a consequenceof the impuritiesin the limestone,the ore is of two generaltypes: (•) a massiveore called"typical 'vein' ore" by Butler and McCaskey•a and (2) a schistoseore (Fig. •8). The massiveore occurswhere ,the limestoneis relatively pure, and consistsof massesof chalcocitewith' ganguecalcite and quartz in the limestone. The schistoseore occurswhere the li.me- stone consistslargely of phyllite, with narrow bands of pure limestone,and representsthe replacementof someof thesesmall limestonebands (by chalcocite. Description.oœ Ore.-The ore mineralsobserved in the polished sectionswere chalcociteand bo.rnite. Chalcopyriteis reported from this localityby Weed, but none.of it was observed. Chal- cocitepredominates, bornite being present only in small amounts. An etchedsample of the ore showsthe chalcociteto be in rather large individual crystals. In some casesit appears to be c6n- temporaneouswith the bornite, but in other cases the bornite seemsto have .comeout of soluti'onlater than the chalcocite,since its dispositionat theseplaces seems to be entirely dependenton the previousexistence' .o f the chalcocitecrystals. This condition is similarto that alreadydescribed under the oresof eheLiberty mine. No chalcocitethat is undoubtedlysecondary was seen. Alth'oughthe .thinsections show that somequartz is earlier than the ore minerals,the intimate relationsexisting between chalcocite,bornite, and quartz, as .shownby the polishedsections, xa,, Copper Ores of the New London Mine," Bull../1met. Inst. Min. Eng., July, I914. x 74 ROBERT M. OVERBECK.

makeit very probablethat someof the quartzwas contempora- neouswieh the ore-bearingminerals.

Genesixof the FrederickCounty Deposits. Genesis.--Thefirst hypothesis as to the,origin of the Frederick Countycopper deposits is oneproposed •by Weed TM in x9xx. He says:

"The copperoccurs in or alongsideof massesof dolomiticlimestone and marble included in micaceous schist .... In most of the area from New London northeastward belts of marble . . . lie east of the greenstone. The ores appear to be confinedto thesemarbles and to the schists alongsideof them, occurring either along the contact or in the fractured rock itself, as, for example, in the recent workings of the Liberty mine. "The meager facts cited would not warrant any generalizationswere it not •or the frequent presenceof copper in the greenstonebelt from Pennsylvania to the Carolinas. It is believed that this rock furnished the copper for the solutions and that the limestone and its graphitic matter suppliedthe precipitant for the copper."

Weed does not say whether he considersthe copperto have beenconcentrated by coldatmospheric waters, or by warm waters, when the greenstonewas first pouredout as a 1.avaand was still in a heated condition. The igneousrocks at the Liberty mine, as ,a matter of fact, are practicallyall acid intrusivessimilar in mostrespects to the aporhyolitesdescribed from the South Moun- tain region. Greenstoneswere not noted in the immediate neighborhoodof the deposit. The ore minerals,bornire and chalcopyrite,and bornite and chalcocite,are seenin the polishedsections to form typical primary intergrowthsas illustrated•by Graton and Murdock in their excellentpaper on "The SulphideOres of Copper." The instability of the bornite is shown,in the sectionsfrom the Liberty mine, 'by its alteration to sec'ondarychalcocite and secondary chalcopyrite. The examplesof graphic intergrowthof borniteand chalcociteseem to leaveno doubtas to the primary x• Bull. U.S. Geol. Survey, No. 455, x9xx, p. 62. COPPER ORES OF MARYLAND. 175 natureof theseores. Emmonsx• saysthe followinga•bout graphic intergrowthof bornite and chalcocite:•.0 "It appearspossible from theserelations that undercertain conditions chalcociteis depositedas a primary mineral by hot ascendingwaters. The intergrow.th of the copper sulphidein graphic-like pattern is a feature that is difficultto explain on the hypothesisof an origin from descending sulphat.e waters.... It appears l•robable that under conditionõ where the concentrati'onof iron is low in the solutions chalcocite may form as a primary mineral instead of chalcopyrite,which is the com- moner primary ore."

The presenceof calcite, quartz, and particularly barite a• gangue,together with •he absenceof magnetite,specularite, and other high-temperatureminerals, indi'cates(on the assumption that the oreand gangueare dueto hot ascendingsolut!ons) that the depositswere formed at moderate depth. The locationof many of thesedeposits at contactsis probably due simplyto the fact that the contactoffered a path along which the solutionscould move more readily, and also to the fact that the limestone furnished the agent for precipitatingthe copper mineral.s. The depositat New London is not locatedat a contact, but along•a fissureor a fault which cutsacross a limestone band. The derivationof theseore-bearing solutions is not certain,but it seemsprobable that the depositsbelong to the samemetallo- :t• Bull. U.S. Geol. Survey, No. :•59, P. lO7. x6Cf., however,"The SecondarySulphide Enrichment of Copper Ores with Special Reference to Microscopic Study," by A. F. Rogers, Min. and Scien. Pre**, October 3I, x9x4, pp. 68o-686. Rogers considersthe so-called graphic intergrowths of bornitc and chalcociteto be a criterion of upward chalcocite enrichment. Cf., also, "The Origin and Occurrence of Certain Crystallo- graphic Intergrowths," by J. Segall, Ecoa. G•.o•_..,Vol. X., No.' 5. Segall, referring to some so-called graphic intergrowths of bornite and chalcocite, described and illustrated in the above article, says, "it is inferred that intergrowths of sulphides may be formed by secondary processes. These inter- growthsmay havea closeresemblance to what are known as contemporaneous intergrowths." Segall's explanation of the formation of these intergrowths would hardly apply in the case illustrated in Fig. I6, since here the chalcocite on etching develops cleavage lines in one direction only (orthorhombic cleavage), and does not show the intersectingcleavage (isometric cleavage) that seems to be the foundation upon which his argument rests. I76 ROBERT M. OVERBECK. geneticepoch as do the Carroll Countyores. Both are associated areally,,but not genetically,with greenstones.The type of ore- bearingsolutions in bothdistricts was apparentlysimilar, although dispositiontook place under somewhat different conditions-- deep-seatedin Carroll County, and moderateto shallowin Fred- erick. Finally, mineralizationin both casesis shown to have occurredafter the devel'opmentof schistosity--/. e., in post-Or- dovician times. T.hin sectionsof the ore show that mineralizationtook place after the major dynamic movementsthat rendered the rock •chistose. It has been shown that Ordovician rocks are included in the schistsof the Piedmont region: hencethe date of mineralization can be put in post-Ordoviciantimes.

SUM MARY. Copperhas beenmined in Maryland from before •76o to the present. The industry,though small in production,was of considerableimportance prior to the middle of the nineteenthcen- tury, but at presentit is insignificantand the productionnegli- gible. All the commerciallyutilized depositsoccur in the metamor- phosedcomplex of the Piedmont,where the countryrocks include metamorphosedsediments and early eruptives with later less metamorphosederuptives. On the east, associatedwith the Bal- timore County and easternCarroll County deposits,deep-seated igneousmasses predominate; while on the west, associatedwith the FrederickCounty deposits, metamorphosed acid and basic lavas and tuffs are a,bundant. T•he eastern Carroll County deposits,comprising the Finks- burg, the Mineral Hill, and the Springfielddeposits, lie alonga definitestructural line of weaknessthat can be tracedfor many miles acrossthe state. The countryrock of the regionis phyllite and quartz mica schist. A large granite intrusive lies near the southernend of the copperbelt, and pegmatitedikes are commonthroughout the region. Altered basic intrusives are found along the line of weakness,and it is with thesethat the COPPER ORES OF MARYLAND. 177

copperores are really associated. The oreoccurs in replacement 0 veinsroughly conformable in strikeand dip with the country rock. The oreminerals are chalcopyrite, bornite, carrollite, chalcocite,and corellite,and associatedwith theseare sphalerite, magnetite,and' specul.arite. Chalcopyrite,bornite, sphalerite, magnetite,and specularite form primary intergrowths; chalcocite and corelliteare alwayssecondary, replacing all the minerals presentexcept magnetite and specularite.The gangueminerals presentare hornblende, tbiotite, epidote, and feldspar,all of which are partlyreplaced by the ore minerals. The paragenesisof the ore mineralsand associatedminerals is magnetite,hornblende, biotite,epidote, quartz, feldspar, magnetite, carrollite, sphalerite, chalcopyriteand bornite,secondary chalcopyrite, corellite, chalcocite--magnetitebeing the oldest. The varietiesand relationshipsof theseminerals present in the depositsindicate that depositiontook place from hot ascending solutions,and at considerabledepth. Secondaryalteration to chalcocitedue t'o descending solutions has taken place along crevicesin the ore,but is of little importance. Depositionwas shown to have taken place after the period of metamorphismof the region,probably in post-Ordoviciantime. That the sourceof the ore-bearingsolution was probablythe graniticmagma from which the pegmatiticmaterial came, is indicatedby the presence of a large graniticmass near the copperbelt, and by the general agreementin age of the ore depositionand the intrusionof the granite. The Frederick County deposits,comprising the Liberty and the New London .deposits,are confinedto limestonesat or near their contactwith other rocks. The country rock of the region consistsof metamovphosedsedimentary rocks, and altered acid and basic extrusives. The ores occu.ras pocketsand stringers in limestone,or as replacementsof calcareousbands in impure limestone. The ore minerals are chalcopyrite,bornite, chalcocite. Quartz, calcite, and barite form the !gangueminerals. Chalcopyriteand .bornite show primary relationships;bornite and chalcociteoccur in graphic intergrowths,and are in part prob- 178 ROBERT M. OVERBECK. ably contemporaneousin origin. Secondaryalteration to chalcociteis prevalent,but is not important. The paragenesisof the minerals(in the order of deposition)would be calcite(?), barite (?), quartz,.chalcocite, chalcopyrite and bornite,secondary chalcopyrite,secondary chalcocite. Somequartz is proba.bly contemporaneouswith the depositionof the ore minerals. The characterof the mineralsand their associationsthus point to depositionfrom hot ascendingwaters at moderate depth. Mineralizationapparently took placeafter the metamorphismof the region. The sourceof the ore-bearingsolutions is not certain, but it seems probable that the mineralization was contemporaneouswith that in Carroll County, and that the ore- bearingsolutions probably have a commonsource. There is no evidencethat the solutionswere derived from the greenstones of the regionwith which the deposit.sare frequentlyassociated.