Descriptive Mineralogy of Pugh Quarry, Northwestern Ohio: Sphalerite1

DESCRIPTIVE MINERALOGY OF PUGH QUARRY, NORTHWESTERN OHIO: SPHALERITE1

DAVID F. PARR2 and LUKE L. Y. CHANG, Department of Geology, Miami University, Oxford, OH 45056 Abstract. The Devonian rocks at Pugh Quarry have three distinct types of sphalerite (banded massive, spheroidal, and tiny euhedral). Occurrence of the banded massive sphalerite is restricted to the mineral zone, predominantly as blebs in marcasite. The color of banded sphalerite ranged from nearly colorless to various hues of yellow. The replacement of banded sphalerite by marcasite was observed. The spheroidal sphalerite occurred in association with marcasite of euhedral habit. The spherules were small, the largest no greater than 1 mm across. Where present in great profusion, the sphalerite spherules merged together forming botryoidal surfaces. The euhedral sphalerite occurred in the voids of the sponge-like and stromatolitic dolostone below the mineral zone, and in a layer of soft laminated mud associated with the dolostone. The euhedral sphalerite was predominantly red-brown, and no crystals larger than 1 mm in maximum dimension were observed. OHIO J. SCI. 78(5): 272, 1978

Sphalerite was found in cavities in all RELATION TO HOST ROCK parts of Pugh Quarry. Along with mar- Where sphalerite occurred between casite (Parr and Chang 1977), it com- the bands of marcasite and the dolostone prised nearly all of the sulfide mineraliza- host rock, replacement relationships could tion. Sphalerite occurred in three dis- be observed between the two. Dolomite tinct types. The banded sphalreite par- rhombs were commonly observed to be tially replaced by marcasite was the most engulfed in the banded sphalerite. Many unusual occurrence of this mineral at of these rhombs showed no feature indi- Pugh Quarry. Spheroidal aggregates of cating replacement by sphalerite, but fine, radially grown crystals of sphalerite others clearly showed dolomite corrosion on dolostone matrix constituted the illustrating the replacement relationship second type. Tiny euhedral crystals between the two minerals (fig. 1). Banded were the third type of sphalerite oc- sphalerite was also found in the host currence. dolostone. For the most part, the mineral appeared to be interstitial between BANDED SPHALERITE oolites and was always in association with Sphalreite of this type was found only marcasite (fig. 6, Parr and Chang 1977). in the mineral zone (Parr and Chang 1977), primarily in the northeastern por- COLOR AND SIZE tion of the quarry. An additional re- The color of banded sphalerite ranges striction on the occurrence was that in from nearly colorless to various hues of every case, this type of sphalerite was yellow, even approaching orange. The intimately associated with banded mar- banding characteristic of this type of casite. All specimens of banded mar- sphalerite is emphasized in part by color casite examined (Parr and Chang 1977) variations. Thin bands of darker colored contained fragments of at least a small sphalerite, approaching red-brown, were quantity of this type of sphalerite. observed in a few specimens. The variations in color observed in sphalerite are Manuscript received June 6, 1977 and in re- commonly attributed to the presence of vised form November 9, 1977 (#77-46). 2Present Address: Department of Geology, trace elements (Deer et al 1962). Under Wisconsin State University, Superior, WI 54880. short and long wave ultraviolet light, the 272 Ohio J. Sci. SPHALERITE FROM PUGH QUARRY 273

FIGURE 1. Thin section, sphalerite and dolomite. Corrosion of some of the dolomite rhombs is apparent. X60. banded sphalerite fluoresces a dull red to reddish yellow. Variations in the fluors- cence colors correspond to the bands observed in the sphalerite under normal light. The darker bands fluoresce a deep dull red whereas the light bands fluoresce reddish yellow. Banded sphalerite at Pugh Quarry oc- Figure 2. Thin section, one nicol. Blebs of curred predominantly as blebs in mar- banded sphalerite in marcarsite. The elon- casite. The majority of the blebs were gated bleb shape, color banding, parallel character of bands, continuity of band sequence roughly 2 to 3 mm long by one mm from one bleb to another, and bleb horizon are or less wide (fig. 2). The blebs tended visible. Note the banding appears to be cen- to be considerably longer than they were tered on a nucleation point near the base of the wide; the elongation being roughly par- bleb. X28. allel to the direction of elongation of the marcasite crystals (fig. 3). This direction was approximately perpendicular to taining many small pits (fig. 4). A third the sphalerite banding and the free sur- type of concentric banding consisted of face of the marcasite crust. Commonly continuous narrow zones of larger pits or the sphalerite blebs were teardrop-shaped other open spaces (fig. 5). Apparently masses with the small ends pointing in extremely small sphalerite crystal faces the convex direction of the banding were developed on the surfaces of banded (%• 2). sphalerite bounding these zones of open space, producing a drusy texture. All HABIT AND FORM banding observed in the blebs of sphaler- The banding was represented by sev- ite was concentric to an apparent nuclea- eral physical characteristics of the banded tion point. sphalerite masses. Color variation within Hemispherical mounds of sphalerite the blebs was the most apparent mani- commonly were scattered among the festation of the banding (fig. 2). Exami- marcasite terminations on the surfaces of nation of polished sections revealed bands the crusts of marcasite (fig. 6). These of lower reflectivity (darkest bands) con- mounds were the tops of blebs of sphaler- 274 D. F. PARR AND L. L. Y. CHANG Vol. 78

FIGURE 3. Polished section, crossed nicols. Blebs of banded sphalerite in mareasite. Note horizons of small banded sphalerite blebs, twinned character of the mareasite crystals, paral- lelism of the direction of elongation of both sphalerite blebs and mareasite crystals, and continuity of mareasite crystals through the sphalerite bleb horizon. X32. FIGURE 4. Polished section, one nicol. Banded sphalerite bleb. Bands of low reflectivity (arrow) caused by numerous small pits in the sphalerite are visible. X32. FIGURE 5. Polished section, one nicol. Banded sphalerite belbs in mareasite. Note the band of large pits in banded sphalerite and continuity of pit bands in the two separate blebs. X 32. ite which could be seen to extend down stated, "marcasite is often precipitated as into the marcasite crust a short distance. an incrustration upon such mineral spe- The surface of these sphalerite mounds cies which themselves are not involved in was composed of numerous extremely these replacement phenomena, e.g., on small crystal terminations, which gave sphalerite." Our interpretation runs con- the surface a delicate velvety appearance. trary to what is apparently the general RELATIONSHIP BETWEEN BANDED SPHALERITE thought to be the paragenetic relationship AND MARCASITE between these two minerals. The evi- The occurrence of banded sphalerite in dence we obtained is presented below: the study area was obviously not unique. 1. Banded sphalerite was almost com- The paragenetic relationship between it pletely replaced by marcasite in all speci- and marcasite (Parr and Chang 1977), mens examined. The sphalerite not re- replacement of the former by the latter, placed occurred as blebs scattered was unusual. To our knowledge, previous throughout the marcasite crusts. Com- investigators have not made reference to monly, certain horizons within the mar- the paragenetic situation in which marca- casite contained a greater number of site replaces sphalerite. Ramdohr (1969) blebs than the remainder of the marcasite Ohio J. Sci. SPHALERITE FROM PUGH QUARRY 275 crusts. The horizons along which the puzzle-type fit between the different blebs sphalerite blebs were concentrated were in a horizon. not flat, rather they closely followed the 3. Nearly complete spherules and contours of the surface of the replacing hemispherules of banded sphalerite ex- marcasite. tended from the host rock a short dis- No variations in band color, polishing tance up into the marcasite crust. In- characteristics, curvature, or thickness variably, portions of these structures were were observable at or near the marcasite- truncated by the enclosing marcasite sphalerite interface. If the relationship (fig. 8). Several of these spherulites between the two minerals in question was clearly showed selective replacement of the result of contemporaneous precipita- certain sphalerite bands by marcasite. tion, variations in the character of the Bands which have been selectively re- bands in the sphalerite could be expected placed may correspond to areas of num- along the interface between the two erous pits. The replacement marcasite minerals. Slight differences in the solu- found in this relation followed the curva- tions precipitating the two minerals ture of the remaining sphalerite bands. would very likely cause the bands to show 4. Zimmerman and Amstutz (1973) some variation in one or more of the described a paragenetic relationship be- characteristics noted above in the vi- tween banded sphalerite and marcasite cinity of the interface. No features in- in which distinct crystal faces of mar- dicating the impingement of marcasite casite could be seen projected up into on sphalerite during growth were ob- sphalerite. They interpreted this rela- served. The bands simply end, appar- tionship as indicating that the banded ently cut off by the marcasite (fig. 2, 4, sphalerite was later than the marcasite. and 5). The polished sections and thin sections 2. The arcuate features described prepared for this study were specifically here as banding were not random phe- examined for this relationship. Isolated nomena, different in different blebs in a examples showing marcasite crystal faces given specimen. Sesquences of bands projecting into the concave, lower side of with some arrangement of wide, narrow, banded sphalerite blebs were observed. dark, light, high polish, low polish, were The vast majority of the blebs, however, observed in one bleb, and can be observed showed no indication of the presence of in similar sequence in other blebs in the marcasite crystal faces intruding into same horizon. In many specimens a them from any direction. The sphalerite- particular band or series of bands could marcasite interface was characteristically be traced across a prepared section in a a nongradational irregular surface show- manner similar to that used in correlating ing embayments of one mineral into the tree rings and varved sediments (figs. 2 other. This line of evidence while not and 5). totally conclusive, indicates, for the most Commonly blebs were close enough to part, that the sphalerite is not overgrown one another in the prepared sections that on the marcasite, but rather that it is several could be examined simultaneously being replaced by it. under low magnification. In addition to 5. Polished sections of marcasite con- the obvious correlation of band sequences taining sphalerite blebs, examined be- in the different blebs, it was apparent tween crossed nicols, showed the varia- from the curvature of the bands in adja- tions in the orientation of the individual cent blebs that they were at one time marcasite crystals composing the speci- joined together, forming a continuous men. Most of the marcasite crystals band across the specimen (figs. 2, 5, and were oriented with their direction of 7). The points at which the bands form- elongation parallel to the direction of erly joined were now occupied by mar- elongation of the sphalerite blebs. This casite replacement. Examination of fig- direction was perpendicular to the termi- ures 2, 5, and 7 dispels the alternate ex- nation surface of the marcasite crust. planation that the marcasite in some way Twin laminae within the individual physically displaced the blebs from one crystal units were also clearly apparent another. There was no hint of a jigsaw in the polished sections. 276 D. F. PARR AND L. L. Y. CHANG Vol. 78

FIGURE 6. Marcasite and remnant bodies of banded sphalerite. Note the encroachment of marcasite into the spherical and hemispherical bodies of banded sphalerite. X4. FIGURE 7. Thin section, one nicol. Banded sphalerite blebs enclosed in marcasite. Note the banded features and the continuity of band sequences of bands in different blebs. X25. FIGURE 8. Thin section, one nicol. Banded sphalerite spherule partially enclosed in marcasite. Note selective replacement (arrow) of banded sphalerite by marcasite along certain bands. X60. FIGURE 9. Marcasite replacing banded sphalerite. Note the veinlets of marcasite cutting large spherulite of banded sphalerite and the continuation of bands in sphalerite across marcassite veinlets. X4. FIGURE 10. Polished section, one nicol. Shredded island texture, marcasite replacing banded sphalerite. X185. Ohio J. Sci. SPHALERITE FROM PUGH QUARRY 277 On the basis of continuity of inter- ference colors and the continuity of twin laminae, it was possible to trace individual crystal units of marcasite through their entire length. Many crystals of marcasite could be traced without interruption through the entire thickness of the crust from the dolostone-marcasite interface up to the termination surface of the crust. If the horizon of sphalerite blebs mentioned above were the result of alternating precipitation of marcasite and sphalerite, a general interruption in the continuity of the marcasite crystals should have been found at the horizons of sphalerite blebs. No indication of any interruption of the marcasite crystals at the bleb horizon was observed, clearly indicating that the formation of the marcasite was not interrupted by periodic deposition of sphalerite. 6. Several textural relationships diag- nostic of the replacement of one mineral by another were observed in the specimens prepared for microscopic study. These include: a. a large spherulite of sphalerite approximately 6 mm in diam- eter cut by veinlets of marcasite (fig. 9), [The banded characteristics described above are present in this sample,] b. shredded island texture (Ramdohr 1969, p. 206) (fig. 10), c. skeletal growth of marcasite in sphalerite (fig. 11), d. feathery intergrowth of marcasite and sphalerite (fig. 12), and e. gradational surface between marcasite and sphalerite. The evidence presented above, when con- sidered collectively, strongly indicates the replacement of sphalerite by marcasite. FIGURE 11. Polished section, crossed nicols. Skeletal growth of marcasite in banded sphale- SPHEROIDAL SPHALERITE rite. Marcasite replacing sphalerite. Note Specimens with sphalerite occurring as the twinning in marcasite. X350. crystal aggregates in the form of sphe- FIGURE 12. Polished section, crossed nicols. Feathery intergrowth of marcasite replacing roidal bodies were found in the mineral banded sphalerite. X32. zone in the northeastern area of the quarry. The sphalerite spherules were replacement of dolostone by sphalerite associated in every case with marcasite was noted. Thin sections showed sharp crystals of the euhedral habit. Spheroidal euhedral rhombs of dolostone to be in- sphalerite aggregates were not found in corporated into the basal portion of the any other part of the quarry or in any sphalerite spherules, but these rhombs other stratigraphic horizon. did not show any corrosion features indi- cative of a replacement origin. RELATION TO HOST ROCK All examples of this sphalerite show COLOR AND SIZE the spheroidal bodies directly on the The color of the sphalerite spherules dolostone. No evidence indicating the was predominantly a drab yellow, the 278 D. F. PARR AND L. L. Y. CHANG Vol. 78 color being subdued by the influence of HABIT AND FORM brown tint. Generally the surface of the The crystal aggregates included in this spherules, being the result of many tiny group had a spherical to hemispherical crystal terminations, lacked the strong form regardless of the size of the body resinous luster characteristic to sphalerite. (fig. 13). Fibrous to accicular sphalerite The sphalerite was translusent but def- crystals radiated from a central point near initely not transparent. When illumi- the contact point between the sphalerite nated by long wave ultraviolet light, the and host rock. Apparent on some ag- sphalerite weakly fluoresced a dull red. gregates of this type are concentric shells No phosphorescence was observed. The of dark and light sphalerite. spherules examined were small, the larg- Extremely fine sphalerite crystal termi- est no greater than 1 mm across. Most nations bound the surfaces of these bodies of the spherules were considerably smaller (fig. 13). The minute size of the crystals than this, being just visible without a and their terminations, which compose microscope. the spheroidal aggregates, prohibited the

FIGURE 13. Sphalerite spherules and marcasite on dolostone. Note the very fine sphalerite crystal terminations on the hemispherical bodies of sphalerite. X2.2. FIGURE 14. Thin section, one nicol. Euhedral marcasite and euhedral sphalerite crystals on stromatolitic dolostone matrix. Note complex nature and shape outline of the sphalerite crystals (arrow) and the general lack of included dolomite rhombs. X35. FIGURE 15. Euhedral sphalerite crystals washed from the mud layer. Note complex nature of these crystals. X10. FIGURE 16. Thin section, crossed nicols. Euhedral sphalerite crystals in mud layer. Note distorted mud laminae around sphalerite crystals. X25. Ohio J. Sci. SPHALERITE FROM PUGH QUARRY 279 determination of the crystal forms. in the particle size of the dolomite Where present in great profusion, the rhombs. Also included in the mud were sphalerite spherules merged together, calcite crystal aggregates greater than 1 forming botryoidal surfaces. mm. EUHEDRAL SPHALERITE COLOR AND SIZE Crystals representing this type were The color observed on the euhedral observed only in the strata lower than the sphalerite crystals was predominantly a mineral zone in the northeastern area of red-brown with some crystals approach- the quarry. The rock in which the ing a golden brown. Most crystals were crystals were found was an extremely nearly transparent and some were com- porous gray brown stromatolitic dolo- pletely so. One characteristic of the stone (fig. 2, Parr and Chang 1977). sphalerite crystals was their uniformity Associated with this rock type was a of size. No single crystals were observed layer of soft laminated mud about 20 larger than 1 mm in maximum dimension, mm thick, which contained large numbers most were considerably smaller. of euhedral sphalerite and marcasite crystals no larger than 1 mm. HABIT AND FORM Faces of the dodecahedral form domi- RELATION TO HOST ROCK nated the crystals and served to control Sphalerite crystals occurring in the the basic habit of most of the crystals. voids of the sponge-like and stromato- In addition to the dodecahedral form, litic host rock did not show evidence in- many vicinal faces, curved faces, and dicating replacement formation (fig. 14). conical forms were observed on the Some of the crystals had engulfed micro- crystals. The conical form was pro- scopic dolomite rhombs. These rhombs duced by rounding together of the {113} did not show embayments, rounded and {011} forms (Palache et al 1944). crystal edges, or other replacement fea- Twining, common in sphalerite, has tures. Most commonly the crystals were given many of these crystals a complex found disseminated randomly throughout appearance. the vugs in the rocks. High concentra- Acknowledgments. The authors are indebted tions of sphalerite crystals were observed to Drs. James Bever and Charles Kahle for their on some of the stromatolitic laminae. assistance and advice. Special thanks go to the Crystals from the laminated mud layer Pugh Quarry Company for its cooperation. showed no evidence of attachment to any solid object (fig. 15), and appeared LITERATURE CITED Deer, W. A., R. A. Howie and M. A. Zussman to have grown free in the mud. Thin 1962 Rock-Forming Minerals, Vol. 5, Non- sections of the mud and its contained Silicates. Longmans, London, 351 pp. sphalerite crystals support the conclusion Palache, C, H. Berman and C. Frondel 1944 that the mineral is authigenic. Distor- System of Mineralogy, 7th edition, John Wiley, New York, 1124 pp. tion of mud laminae could be seen near Parr, D.F. and L.L.Y. Chang 1977 Descrip- the crystal, suggesting crystal growth in tive mineralogy of Pugh Quarry, northwest- place (fig. 16). ern Ohio: Marcasite and pyrite. Ohio J. Sci. The composition of the mud layer 77" 213—222 Ramdohr, P. 1969 The Ore Minerals and which contained the sphalerite crystals is Their Intergrowths. Pergamon Press, Lon- interesting. It is predominantly com- don, 1169 pp. posed of small dolomite rhombs with Zimmerman, R. A. and G. C. Amstutz 1973 much interstitial hydrocarbon material. Intergrowth and crystallization features in the Cambrian mud volcanoes of Decaturville, The laminae were defined more by the Missouri, U.S.A. p. 339-350. In: Amstutz, variance in the concentration of this G. C. and A. J. Bernard (eds), Ores in Sedi- organic material than by the variation ments. Springer-Verlag, New York.