Descriptive Mineralogy of Pugh Quarry, Northwestern Ohio: Sphalerite1
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Copyright © 1978 Ohio Acad. Sci. 0030-0950/78/0005-027211.50/0 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 pro- fusion, 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 min- eral 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 varia- tions 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 dolo- mite. 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 ob- served 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 char- acter 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 direc- tion 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 ,m 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 con- tinuity 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