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A Second Find of Euhedral Bornite Crystals on Barite G

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A Second Find of Euhedral Bornite Crystals on Barite G THE AMERICAN MINERALOGIST, VOL. 45, SEPTEMBER-OCTOBER, 1S60 A SECOND FIND OF EUHEDRAL BORNITE CRYSTALS ON BARITE G. KurrrnuD ANDG. DoNN.tv,Geophysi.cal Laboratory, CarnegieInstitution of Washington,Washington, D. C. AND J. D. H. Dowuav, The Johns Hophins Uni.oers,ity,Baltimore, Md,. Assrn,{cr Bornitecrystalson barite have been found at thecoppercorp Mine in ontario,canada, thesecond locality for thisassociation. The geology resembles that of theupper Michigan copperarea. The bornite cubes, truncated by octahedronfaces, prove to betwinned edifices of two kinds,which simulate either cubic (a:21.94 A) or orthorhombic(a:b:21.90, r:10.95A) symmetry. InrnooucrroN Bornite, CusFeSa,although a common mineral in many copper_iron sulfideore deposits,has rarely been reported in singlecrystars. well de- veloped,steel blue, euhedralcrystals were Iound by one of us (G.K.) in August 1956 at the coppercorp Limited Mine of the Mamainse point copperarea, ontario, canada. The hand specimenshows bornite crystals growing on barite (Fig. 2), an associationpreviously reported only from the Cheshirelocality in England. GBxnnal Geolocy The Mamainse Point copper area is located on the east shoreof Lake superior in the district of Algoma and lies about 60 miles north of the city of Sault ste. Marie,* The Keweenawanrocks of this area form the east end of the Lake superior syncline,are the youngestin the precam- brian era, and are generally unaltered. Amygdaloidal lava flows pre- dominate and are interbanded with a boulder conglomerate.The faults in the areaare often well-definedtensional structures. Most of the copper- bearing zones are faulted and the apparent horizontal displacement variesfrom a few to more than 1000feet. The faults are consideredto be normal and generallydip in the directionopposite to the dip of the strata. since the geologyof the Mamainse Point areais similar to that of the Michigan copperdistrict, the occurrenceof chalcociteand native copper on small fracturesled to intensiveexplorations. These showed that, as to structure and mineralogy,the Mamainse copper deposits,which consist mostly of chalcociteon fissuresand fractures,are quite differentfrom the * The areal geology and individual ore deposits of the Mamainse point copper area have been described by Thomson (1954), who also presented an excellent geoiogical map and thus provided invaluable data for the present report. 1062 1063 EUI\EDIIAL BORNITE CRYSTALS ON BARITE in flow tops and Michigan copper deposits, where native copper occurs bdrrb SUPERIOR I .'1 r...i"* ra.$ ,{.ili* Poini Hibbard 8ay in circlets along highway 17 Frc. 1. Sketch map oi Mamainse Point area. Numbers rectangle shows shaft of Copper- indicate mileage from Sault Ste Marie. Biack and white corp Mine. shown that a Iocalitiesand may be secondary.chemical analyseshave are highest Iittle silver occurs with the copper minerals; the silver values wherebornite is present(Thomson, 1954) ' MINE Gnor-ocv AND MTNERALoGYoF THE CoPPERCoRPLrMrrED zone ore The Coppercorp Limited mining operations explored the C 1 mile easrof Hibbard deposit tr,lg. rl. ih" mirreshaft is situated about to the strata Bay of Laie Superior. The c zone strikes almost parallel strike (N't5'W), dips almost normal to them (55-68'E) and may be a 1064 G. KULLERUD,G. DONNAYAND J. D. H. DONNAY fault. Geologicinformation (Thomson, 1954)suggests that it is a normal fault with about 50 feet of vertical displacement.rt doesnot outcrop but hasbeen traced for 5400feet along the strike by 34 drill holes.rt has been estimated to average8 feet in width. Testing to a depth of 400feet and on a distanceoI 2275feet showedits coppercontent to average1.9 per cent. rt is a fractured and brecciated zonein interbanded amygdaloidal basalt and conglomerateof the Keweenawan series.The wall rocls, to a depth of about 600 feet along the dip, are mostly lava flows from 20 to 100 feet thick;below this horizon the zoneenters a thick conglomerateband. The zone c consistslargely of vein breccia which contains massivechunks and sometimesstringers of erratically distributed chalcocite.sometimes chal- cocitealso occursin small fracturesand amygdulesin the adjacent lava. The breccia carries fragments of wall rocks, quart z, and,calcite and is often vuggy (Thomson,1954). Bornite crystals - growing most commonly on barite (Fig.2), but also frequently on qtartz, were found in vugs of the breccia. Bornite crystars occurring on barite appear with onry few exceptions to have grown di- rectly by deposition from a solution, while the bornite found on qtrartz without exception appearsto have been formed by replacementof chalco- pyrite (Fig.3). rn somecases some of the bornit. *hi.h replaceschalco- pyrite has in turn been replaced in part by chalcocite. Because of the presence of residual chalcopyrite and replacement chalcocite, it was not possible to obtain pure bornite in amounts sufficient for chemical analy- sis. The exact bornite composition, therefore,is unfortunately not known. rt was noted on crushing bornite crystals under acetonethat fresh sur- faces are tombac colored. rrowever, on exposureto the atmosphere such surfaceschange their color immediately and will after only a few seconds have acquired a pinkish cast. on continued exposure the pinkish color changesthrough brownish purple and purple to deep prrrpL and finally an iridescentblue. Cnysrarr,ocRApHy Bornite crystals (at first presumed to be single crystals) were detached from the barite crystals found on the hand specimens.Examined under the binocular microscope they show the cube as their dominant form, modified by small faces of the octahedron. Their cube edge ranges in Iength from 0.2 to about 1.5 mm. The appearanceof theseedges consti- tutes the most striking morphological feature of the bornite crystals examined:every cube edgeis replacedby a "staircase" of indentations, which consistof alternatingoctahedral faces, as could be establishedby a detailed study on the two-circle reflecting goniometer. For instance (Fig. 4) the three edgesthat meet at the cubecorner truncated by (111) are re- ELTHEDRAL BORNITE CRYSTALS ON BARITE 1065 3(a) 3(b) Frc. 2. Bornite cube on barite crystals. Hand specimen' Frc. 3. (a) Residual chalcopyrite (white) replaced by bornite (dark gray). Polished section. (b) Chalcopyrite (white) replaced by bornite (dark gray), in turn replaced by chal- cocite (light gray). Polished section. Frc.4. Bornite "crystal" with cube truncated by octahedron. Cube edges show stair- cases due to alternation of octrahedral faces. Hand specimen. placedby an alternationof (111)faces with (111),(111), or (111),respec- tively, according as the edge is parallel to the o, 6, or c axis. A survey of the available crystals showsthat the height of the stepsin the "staircase" rangesfrom 0.01 to 0.05 mm. Thesesteps are visible on the figure, where they appear as small illuminated trapezia, between large and darker cube faces. Several chips were studied by r-ray difiraction, on the rotation, Weis- 1066 G. KLILLERUD,G. DONNAYAND I. D, H. DONNAY senberg,and precessioncameras, using MoKa and CoKa radiations.Two types of crystalline edificeswere observed. One type appearscubic, as shown by rotation patterns taken about the three crystallographicaxes. We find cell edgeo: 21.94+0.06A, and dif- fraction aspectP***, provided weak reflectionsare taken into account. The secondtypex is metricaily tetragonalwith a:D:21.90*0.06, c:10.95+0.03 A, but the symmetry of the edifice,as determinedfrom the relative intensitiesof the diffraction spots,is not higher than ortho- rhombic, with primitive lattice mode. our observationsagree with those reportedby Frueh (1950)on low-temperaturebornite from Bristol, Con- necticut. Frueh gives the following cell dimensions,o:b:Z!.94, c :10.97 A, and considershis material to be primitive orthorhombic and pseudotetragonal. He later informed us, however (priv. comm., July 28, 1958),that he had been dealingwith a "multiple crystal." euoting him: "the reasonit rvascalled orthorhombic was that the intensity of reflec- tions did not show 4-fold symmetry. From the smaller fragment it ap- pears to be truly tetragonal with a:10.97 and c:21.94 A." Sincethen, Morimoto and Kullerud (1959)have establishedbeyond doubt that the true symmetryis indeedtetragonal, with o:b:10.95, c:21.90 A, rpu.e SroupP42tc, and that the apparentsymmetry results from the twinning (by 90' rotation about an o axis) of two crystals with diferenl volumes. This tetragonalform has beenfound by Morimoto and Kullerud (1959) on synthetic samplesand alsoon an annealedspecimen from the Copper- corp iocality. Our cubic type could likewisebe explainedas a trilling of the tetragonalform, in which the three mutually perpendicularcrystals would have equal vo}umes, A powder pattern of bornite of our "cubic" type was obtained on the Philips diffractometer. With very slrong amplification one peak is ob- served,at 20:3I.9" in FeKa radiation, that can be indexed only if the cell edgeis 21.90A. rts symbol is 643.All other peakscould be indexedon the cell reported first by de Jong (1928) and later by Lundqvist and Westgren(1936) : a: 10.93A, spacegroup Fd.3m. It is possiblethat these authors did not observeany weak reflection that would have required doubling the cell edge,or elsethey were dealing with a structural type that has not been confirmed for low-temperature bornite by later in- vestigators.(A cubiccell with a:70.97 A and spacegrotp Fm3m, F432,or F43m has been reported by Frueh, 1950,for high-temperaturebornite.) For a bornite crystal from Cornwall, Tunell and Adams (1949)reported " In a preliminary report (Carnegie Institution of Washington year Book 57, 1957-58, p. 248) the limits of error on the value of o and b are misprinted as *0.006. EUHEDRAL BORNITE CRYSTALS ON BARITE 1067 4 -6 -12 5 -0 -4 -2 3 -6 5 2 -4 -2 olw!lo 94:2 x 5 47i or^ :32.8:615.47i o.t64t=3x547i Frc. 5. Schematic explanation of bornite rotation pattern (observed by-Tunell and Adams). The pro:osed intergrowth of two cubic {orms, one with a:2X5.47 A (found by Lundqvist and Westgren) and a secondone with o: 3 X 5.47 A (not yet reported in the liter- ature) accounts for the observed absence of first, fifth, seventh, and eleventh layer lines.
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