The Origin of the Rammelsberg Ore Deposit

THE ORIGIN OF THE RAMMELSBERG ORE DEPOSIT. WALDEMAR LINDGREN AND JOHN DUER IRVING. INTRODUCTION. in the courseof a brief visit to the great copperdeposit of Rammelsbergon the northern slopesof the Harz Mountains the authors had an opportunity to study the occurrenceand miner- alogy of its ores. The resultsof sucha short investigationare, of course,entirely insufficientas a base for a detailed descrip- tion.' Nevertheless,it appearedto them as if certain relations not previouslyemphasized might help to elucidatethe difficultand much discussedquestion of the genesisof that ore deposit. For kind permissionto visit the mine the writers are indebtedto Ober- berghauptmanKriimmer in Clausthaiand to BergrathWolff, of Goslar. For our guidancein the mine we are under obligations to ObersteigerVoigt, who directsthe exploitationof the deposit. The Rammelsbergdeposit, as shownon theaccompanying map, Fig. 47, is situatedon the northernslope of the Harz Mountains in closevicinity to the town of Goslar. As well known, it has beenworked for copperores since ancient times, the first records datingback to the tenth century. Its geologicalstructure has beeninvestigated by a numberof authors,but its completeand detaileddescription is as yet a problemof the future. The most diverseexplanations have been offered as to its modeof origin. By some,perhaps by a majority,including such well- knowngeologists as Bergeatand Klockmann,it has beencon- sideredas a sedimentarydeposit contemporaneous with the sur- roundingsedimentary rocks. Others,like ProfessorVogt, fol- lowingFreiesleben and Lossen,explain it as a depositfrom solutionsimmediately derived from igneousmagmas. Still others,like ProfessorBeck, occupy a non-committalstandpoint. 3o3 304 WALDEMAR LINDGREN AND JOHN DUER IRVING. Geologyand Structural Features.--The depositis enclosed, apparentlyconformably, in Devonianrocks, which at Goslarap- pear as an overturnedanticlinal and dip towardsthe north. It lies in the so-called Goslar slates of the Middle Devonian; these slatesbeing again overlain by a thickseries of LowerDevonian Spirifersandstone, which makes up the summit of theRammels- bergMountain, at the footof whichthe mine is located.The followingcut (Fig. 48) after Klockmannwill showthese relations. The slates have suffered considerablecontortion and the ore Fro. 47. Sketch map shorting location of Rammelsberg. body apparentlyfollows these contortions more or lessclearly. The undergrounddevelopments extend over a horizontaldistance of about 2,ooo metersand have attained a vertical depth from the surfaceof 380 meters from the level of the Richtschacht. It will be seenfrom this that mininghas not yet penetratedto great depthsin spite of the fact that the deposithas been worked for nearly a thousandyears. The ore body is divided horizontally into two parts; respectivelyreferred to as the old and the new ORIGIN OF RAMMELSBERG ORE DEPOSIT. 305 •r beds;they are connectedby a narrowand contortedseam showing, however,beyond doubt that the two are reallyparts of one deposit. The followingsketch, after Klockmannshows the relationof thesetwo ore-massesas seenin plan. Fro. 48. Cross-section of Rammelsberg ore deposit showing overturned anticline and manner in which the deposit is believed by earlier writers to be conformable to the lamination of •/he enclosing slates. (After Klockmann>) The thicknessof the ore bed, as it is generally referred to, variescongiderably; in placesit swellsto dimensionsof as much as 3ø meters,but this is rather due to folding and local enlarge- ment. In mostplaces where we observedit in the workingsthe thicknessdid not extend over 2 or 3 meters and was sometimes ,IOO •00 .300 400 5OOm. I I I I I Old o•t.• ••..- NewD* ,•;t,.'• only o.5 to x meter. The dip is vertical uniformly 45 ø to the southeast. In depththe ore in generalappears to becomea little richer in zinc and barite. At presentthe oresare sortedby hand at the surfaceand transportedby wagonsto the railroad, about 306 WALDEMAR LINDGREN AND JOHN DUER IRVING. one mile distant. The older methodsof working are now being replaced by more modern ways of exploitation. One of the main difficultiesin the treatmentof the ore lies in the fine grained intergrowth of chalcopyrite,zinc-blende and galena, which is eminentlycharacteristic of the deposit. It is stated, even in the modern descriptionsof the Ramreels- berg, suchas that of ProfessorBeck, • in the third editionof his book on ore deposits,that the beddingor schistosityabsolutely conforms with the outline of the ore and with its banded structure. The ore is said to take part in all the folding and contortions of the slate. This idea of a massof ore conformableto the schistosityof the slates,which has for so long been acceptedin regard to this ore body, is shownin the well-knowncross-section (Fig. 48) of the older of the two ore lenses. The slaty lamination is here showncurving around the irregular offshootor apophysiswhich is figured in this section. In view of the widely circulatedopinion of this conformity the writers were surprisedto find that, while true in places,it is certainly not a general characteristicof the deposit. In part, as was first noted,and calledto their attentionby Bergrath Wolff, it is absolutelyunconformable to the stratificationof the slate. The ore bandingfollows in all caseswith great faithfulness the outlinesof the sulphidemass whether thesebe long smooth bordersor smalldetailed irregularities of outline. For muchof the distanceabove the third level the edge of the new ore body, and consequentlythe bandingof the ore also, is indeedparallel to the lamination of the enclosingrock. On the third level, however, the ore mass flattens out and crossesthe lamination at an angleof only a few degree,to again turn down parallelto it after intersectingthe laminae for a very considerabledistance. In this portion of the massthe banding of the ore follows the edge of the sulphidemass and makes therefore the same angle with the lamination of the slates as the outline of the ore mass itself. • Beck,"Lehre yon denErzlagerstiitten," 3d ed.,Vol. 2, pp. •34-•38. ORIGIN OF RAMMELSBERG ORE DEPOSIT. 307 The relationsare shown in the sketchesI and II of Fig. 5ø below: Where the ore crosses the lamination of the slates it is about five feet in width and shows no differences in either material or structure from that portion which is parallel to the structure of the enclosingrock. The outlineof this ore body is not a plane but a warped surfacewhich showsbroad gentle undulationsof large radius. Occasionallya sharpinequality is presentand the ore veersoff in a projectionor apophysisonly a few inchesin II Fig. 5o. Sketch of new ore body, 3d level, Rammelsberg. I shows manner in which the ore body, which is conformablewith the lamination of the enclosingslates crosses this laminationat the point ,4. II is an enlarged sketch of the transition zone ,4 showing how the banding of the ore truncates the lamination of the schists and how this lamination terminates abruptly at the ore and continuesin the same direction on the other side. cross section for a distance of 8 to •o inches into the slate. One of these is shown in Fig. 5 •. In this casethe gneissoidbanding of the ore still follows the outlineof the peripheryand runs into the apophysiswith a series of embaym.entshaped lines, which exactly parallel in their arrangement the lines drawn on a map to indicate bodies of water which follow faithfully the outlineof the shoreabsolutely irrespectiveof the structure or nature of the material of which the land may be composed. 308 WALDEMAR LINDGREN AND JOHN DUER IRVING. In another placeon the third level the new bed is folded into the form of an S as shownin Fig. 52. Here also the gneissoid Fro. 5r. Small embayme.ntsabout eight incheswide, showinghow banding of ore follo'ws smaller inequalities in the enclosing rock. bandingfollows •vith perfect 'faithfulness the peripheryof the masswhile the schistosityis abruptly truncatedby it. Fro. 52. Sketch of S-shaped fold in new ore body, 3d level, Rammelsberg mine. Shows how banding of ore truncates the lamination of the enclosing rock. In all caseswhere the ore bodiesare set at an angle to the bedding the laminae of the slatesextend up to and terminate abruptly againstthe sulphidemass and thus continuein the same ORIGIN OF R.4MMELSBERG ORE DEPOSIT. 309. directionon the oppositeside. They do not enter it nor are they in the least distorted against it. The Oves.--A number of varieties of ores are recognized. The principal minerals are zinc-blende, chalcopyrite, galena, pyrite and arsenopyrite; the quantity is approximately propor- tional to the place of the minerals as enumerated. The gangue is almost entirely barite, but rarely appearsin large quantities, Fro. 53- Lenticular augen,Rammelsberg Ore. Showsalso fine bandingand intimate intergrowth of minerals. and often is entirely inconspicuous.Masses and veinlets of calcite are often found in the surrounding slate, but rarely contain ore. On the whole the limits of the "ore bed" are sharply defined and the ores themselvesare entirely or pre- dominantlycomposed of sulphides. An alteration of the enclosing slates is rarely observed. At most there is a slight ira- 3 I0 W.dLDEM.dR LINDOREN .dND JOHN DUER IRVING. pregnationof pyrite. The so-calledkupferkniest, which adjoins the ore in certain places, is a siliceousargillite which contains disseminatedpyrite and chalcopyrite. It has often been con- ORIGIN OF R.4MMELSBERG ORE DEPOSIT. 31 1 sideredas an impregnation,later than the ore itself,

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