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Proceedings of the Iowa Academy of Science

Volume 91 Number Article 7

1984

Hydrothermal Mineralization of the Mississippi Valley Type at the Martin-Marietta Quarry, Linn County, Iowa

Paul L. Garvin Cornell College

Copyright ©1984 Iowa Academy of Science, Inc. Follow this and additional works at: https://scholarworks.uni.edu/pias

Recommended Citation Garvin, Paul L. (1984) "Hydrothermal Mineralization of the Mississippi Valley Type at the Martin-Marietta Quarry, Linn County, Iowa," Proceedings of the Iowa Academy of Science, 91(2), 70-75. Available at: https://scholarworks.uni.edu/pias/vol91/iss2/7

This Research is brought to you for free and open access by the Iowa Academy of Science at UNI ScholarWorks. It has been accepted for inclusion in Proceedings of the Iowa Academy of Science by an authorized editor of UNI ScholarWorks. For more information, please contact [email protected]. Garvin: Hydrothermal Mineralization of the Mississippi Valley Type at the

Proc. IowaAcad. Sci. 91(2): 70-75, 1984

Hydrothermal Mineralization of the Mississippi Valley Type at the Martin-Marietta Quarry, Linn County, Iowa

PAUL L. GARVIN Department of Geology Cornell College Mount Vernon, Iowa

A hydrothermal sulfide mineral deposit is exposed at the Martin-Marietta Quarry (MMQ) near Cedar Rapids, Iowa. Mineralization occurs along solution-enlarged vertical joints in host rocks of the Silurian Scotch Grove and Gower formations. The hydrothermal minerals in general order of deposition are: marcasite, pyrite, sphalerite, calcite. Wall rock alteration is not extensive, and consists of solution enlargement of joints and dissemination of microscopic marcasite in the host rock. Fluid inclusion homogenization temperatures for sphalerite and calcite range from 69° to 99°C. The physical mineralogical similarities between the MMQ deposit and main-district Upper Mississippi Valley hydrothermal deposits evidence cogenesis. The origin of the MMQ deposit is considered in light of the East Central Iowa Basin model of Ludvigson, et al ( 1983a). INDEX DESCRIPTORS: Sulfide mineral deposits, Linn County.

INTRODUCTION AGE ROCK UNIT LITHOLOGY DESCRIPTION In recent years there has been considerable interest in outlying / / occurrences of hydrothermal mineral deposits of the upper Mississippi Wapsipinicon / / I I Valley type, and in their relationship to commercial deposits of the DEV. Formation I / / / main district (Heyl, 1968b; Heyl and West, 1982; Hatch, et al, / / .L ./ / Gower mounded 3finely 1976, 1983; Ludvigson, 1976; Ludvigson and McAdams, 1980; non-lamin. laminated Ludvigson and Garvin, 1981; Coveney and Goebel, 1981, 1983; Formation ~ dolomite dolomite Garvin, 1979, 1983). Since many of these minor deposits occur in / / / " rocks that are stratigraphically beneath or above host rocks of the main 4/ll/ll/ dense, cherty dolomite, Scotch Grove I I / with abundant district, they provide important information about physical and disseminated pyrite I ll I Ill chemical conditions of fluid migration and mineral deposition. Formation /fl/Ill / Knowledge of these occurrences may also provide clues for future z I / / 4 exploration (Heyl and West, 1982). <( o I I A I a: , The Martin-Marietta Cedar Rapids Quarry contains one of these :::::> _J I/ 4 I I I med.- to thick-bedded, deposits. It is located in southern Linn County, on the south side of the u; I I I I fine-orained dolomite~ Cedar River, near the junction of highways 30 and 13, and west of the Hopkinton scattered chert I 4 / / Palisades-Kepler State Park (Fig. 1). Quarrying began in 1963, but Dolomite the deposit of interest in this report was not exposed until the middle 4 I I I 1970's. I / 4 I .11 I I I / n STRATIGRAPHY AND PETROLOGY Blanding = fine-orained dolomite, n ... Formation A nodular to bedded chert The host rocks for mineralization at the Martin-Marietta Quarry are - Maquoketa .... ORD. ~ Shale

Fig. 2. Generalized stratigraphic section of the Martin-Marietta Quar­ ry, Linn County, Iowa.

of Silurian age. Silurian rocks in Linn County were first described systematically by Norton (1895). Recently, the Silurian in eastern Iowa has been redefined. Detailed discussions of Silurian stratigraphy and petrology can be found in Witzke, 198 la, 198 lb and Bunker, et al, (in review). (See Figure 2).

Scotch Grove Formation The Scotch Grove Formation is exposed in the lower part of the quarry. It consists of dense, light-brownish gray to medium-dark gray, finely crystalline fossiliferous dolostone with intercalated thin beds and nodules of tan to black chert. Contained fossils indicate deposition in an open marine sea with stable salinity (Witzke, Fig. 1. Location map of the Martin-Marietta Quarry, Linn County, 198 lb). Vugs up to a few centimeters across occur locally. The dark Iowa. coloration of the dolostone is caused by abundant disseminated pyrite

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HYDROTHERMAL MINERALIZATION 71

A localized area ofbrecciation is exposed at the southwest corner of the quarry, within the Brady facies. The breccia·consists of: 1) clasts of dolomite whose sizes range from a fraction of a centimeter co nearly a meter, cemented by subhedral to euhedral dolomite, and 2) clasts of dark-brown, fine-grained silicified rock, up co a few centimeters across, cemented by microgranular quartz. The silicified clasts exhibit apparent relict laminations, suggesting replacement of original sedi­ mentary fabric. Sparse to abundant euhedral dolomite rhombs occur in the silicified clasts, indicating incipient replacement by dolomite. The two kinds of clasts are randomly mixed in the breccia, indicating Q ,,,/.,.,....,..//'+--- ~edarRapids that silicification occurred prior to brecciation. The brecciation / -/- Quarry --::;..... \l N MMQ) appears co be localized within the Brady; no evidence of it was found in --/~6 the underlying Scotch Grove. The localized nature of the brecciation _...... _;-- suggests solution collapse, but the origin of the silicified clasts is

./ SCALE + unknown. Hydrothermal mineralization was not observed in the / 0 300011 breccia, nor was silicified material found in the areas of mineraliza­ 0 tion. "" Strike ond dip Q /""/ Mopped fault Joint set j // Foldom A' of beddinQ // oa downthrow~ Side "" overa9e strike \) Us upthrown Side ...~ FORM AND STRUCTURE OF THE MINERAL DEPOSIT

Fig. 3. Structure map of the Martin-Marietta Quarry area. Fold data Observed hydrothermal mineralization is confined essentially co after Dow and Mettler ( 1962). Fault data after Bunker; et al (in review).

(identified by x-ray analysis of acid-insoluble residue). Presumably, the dark coloration of the chert is also due to finely divided iron sulfide. Pyrite in the Scotch Grove is a regional characteristic, and is a result of either primary deposition or early diagenesis (Witzke, 198 lb). The chert is chalky in part due to dissolution and replacement by dolostone. Where chalky, it is devoid of iron sulfide.

Gower Formation The Gower Formation is exposed in the upper part of the quarry. It is represented by two facies: the Anamosa and the Brady (Witzke, 198 la). The Anamosa consists of finely laminated, very light gray to medium dark gray, finely crystalline non-fossiliferous dolostone. At the south end of the quarry it interfingers with the Brady, which consists of medium bedded, yellowish gray to medium gray, finely crystalline brachiopod-rich dolostone. In contrast to the uniform horizontal bedding of the Anamosa, Brady beds are steeply inclined (maximum dips - 40°) due to deposition upon the flanks of an adjacent biohermal mound. Contained fossils in the Gower Formation indicate deposition in a restricted marine environment from probably hypersaline waters (Witzke, 198 lb). Bedding has been interrupted locally by wedging and slump folding. Although present in the Brady, hydrothermal mineralization is much more prominent in the Anamosa.

STRUCTURAL GEOLOGY

The Martin-Marietta Quarry is located in an area of structural significance in eastern Iowa (Fig. 3). It lies on the west flank of the Silurian East-Central Iowa Basin (Bunker, et al, in review). Approxi­ mately 2 km west of the quarry is the Skvor-Hard structure described by Dow and Mettler ( 1962), which consists basically of a southwest­ plunging syncline and anticline. The Plum River fault zone passes through the Skvor-Hartl structure and just north of the quarry (Ludvigson and Bunker, 1978; Bunker, et al, in review). Within the quarry itself the only structures of importance are vertical joints. These occur in two sets, one with an average strike ofN 35° E (range N 30° - 40° E), the other with an average strike ofN 77° W (range N 60° - 90° W). Accurate strikes of the joints were difficult to obtain in Fig. 4. Solution-enlarged, mineralized joints in Anamosa facies of places because of irregularities caused by solution enlargement. The Gower Formation, southwest part of Martin-Marietta Quarry. joints are developed best in the Scotch Grove Formation and in the Note dark sulfide halos around mineralization. (Camera lens Anamosa facies of the Gower Formation. cap to right of center for scale.) https://scholarworks.uni.edu/pias/vol91/iss2/7 2 Garvin: Hydrothermal Mineralization of the Mississippi Valley Type at the

72 PROC. IOWA ACAD. SCI. 91 (1984)

the recently-exposed south end of the quarry. The mineralization is PARAGENESIS localized within vertical fissures, which resulted from solution en­ largement of joints prior tO mineral deposition (Fig. 4). The fissures The mineralization at the Martin-Marietta Quarry is hydrothermal are most readily observed on the upper faces of the quarry, where and non-hydrothermal (Fig. 12). The non-hydrothermal mineraliza­ oxidation has converted iron sulfide tO limonite. Dissolution of the tion includes Type A calcite and quartz. These minerals are wholly host dolosrone produced brecciation along the fissures. Rotated confined tO scattered vugs, primarily in the Scotch Grove Formation. breccia clasts cemented by later mineralization are especially notice­ The quartz vug linings are a regional characteristic of Silurian rocks able in the Anamosa fac ies in the southwest part of the quarry (Fig. 5). (Witzke, 198 lb). Mineralization occurs as linings and fillings of fissure openings The firs t clearly hydrothermal mineral is marcasite, which Jines the (Fig. 6). Euhedral crystal terminations indicate that some fissures walls of the fi ssures and is disseminated in the host rock. It also occurs remained open throughout the period of hydrothermal deposition. in small frac tures in Type A calcite. Pyrite formed next . Complex Individual fissure widths range from less than a centimeter tO almost interlayering · of pyrite and marcasite indicates an overlap in the half a meter, with marked changes in width over short distances depositional periods of these two minerals. Sphalerite followed pyrite. vertically and horizontally. Locally, mineralization extends from Marcasite crystals perched on sphalerite demonstrate that marcasite fi ssure openings up tO a few centimeters inro wallrock along bedding deposition continued after the cessation of sphalerite growth, or that plane fractures. Cross-cutting relations between minerali zation and there was a second generation of marcasite. Type B calcite was host rock demonstrate that these deposits are epigenetic. deposited last. Minor late marcasite and pyri te occur in growth zones within calcite. In the southwest part of the quarry calcite fi lled openings in the fissures not previously fi lled by sulfides. Prior tO calcite deposi tion, the earlier sulfides were extensively frac tured and sphalerite was partly dissolved (Fig. 8). Marcasite and sphalerite MINERALOGY crusts were detached from the walls and enclosed by later calcite (Fig. 9). In the southeast part of the quarry calcite does not fi ll the widest The mineralogy of the epigenetic deposits of the quarry is simple, parts of the fissures, whereas in the southwest part fissures are consisting of pyrite, marcasite, sphalerite, calcite, and quartz. Pyrite completely fi lled. The ratio of calcite tO rota! sulfides increases occurs as cubes up tO a half millimeter across, modified by octahedron eastward across the quarry. and pyrirohedron. It is most commonly observed upon and beneath crusts of marcasite. Minor pyrite is included in growth zones in WALL ROCK ALTERAT ION calcite. Marcasite occurs as well-formed single blades up tO a half centime­ The only types of hydrothermal alteration of the host carbonate ter in length, and as simple and complex polysynthetic twins. rocks observed at the quarry are dissolution and marcasitization. Marcasite crystals line fractures in crusts up tO 6 cm thick, and, like Hydrothermal fluids enlarged the joints creating space for later pyrite, are included in calcite. Microscopic marcasite blades are also deposition of sulfides and calcite. Marcasite was disseminated in, and present as disseminations in, and replacements of, the dolosrone host replaced, dolomite near fissure margins (Fig. 10 , 11). Marcasitization rock. is generally confined tO within a centimeter or so of mineralized Sphalerite occurs generally as massive, medium ro yellow brown fractures, with concentration gradually increasing coward fractures. anhedral grains, and locally as malformed tetrahedral crystals. Band­ Hydrothermal dolomitization, common in base metal sulfide deposits ing like that prominent in sphalerites of the main district (Heyl, et al, in carbonate hosts (e.g. Heyl , et al, 1959, p. 107), is apparently 1959, p. 85; Mclimans, 1977; Mclim;ms, et al, 1981) was not absent. Identifying characteristics of hydrothermal dolomite from observed. A few crystals are zoned with dark cores and light rims. other Mississippi Valley type deposits (color change, coarsening of Individual sphalerite crusts reach 2 cm in thickness (Fig. 7). Sphaler­ texture, zoning) were not observed at the quarry. Despite the ite is most abundant in the southwest part of the quarry. abundance of silica (chert) in the Scotch Grove Formation, evidence of Calcite is of two types: scattered vug linings and fillings , (Type A), hydrothermal silicification was not found. and fissure li nings and fillings (Type B). Type A calcite ranges in color from gray tO amber tO chocolate brown. Brown varieties fluoresce dull red tO bright red in long wave ultraviolet light; gray varieties do not fluoresce. Crystals combine dominant scalenohedron, rhombohedron, and prism. Rhombohedral faces are often curved . Locally, the gray Fig. 5. Rotated breccia clasts of Anamosa facies of Gower calcite appears ro have been deposited upon etched surfaces of the Formation cemented by sulfide and calcite minerali­ brown calcite. Type B calcite ranges in color from white ro amber. The zation. Original orientation of host rock bedding white varieties fluoresce bright pink in long wave ultraviolet light; planes approximately parallel to scale bar. the amber varieties fluoresce dull violet tO dull pink. The white calcite Fig. 6. Near vertical mineralized joint in Anamosa facies of is massive (sparry). Crystals of amber calcite are common and reach 10 Gower Formation. Minerals are marcasite and sphal­ cm in length. The dominant crystal form is scalenohedron, modified erite. by one or more rhombohedra, prism, and a second scalenohedron. Simple and polysynthetic twinning (both probably (0112)) were Fig. 7. Sphalerite mineralization of breccia. Calcite at top. observed at one locality. Zoning is frequent and is accentuated by inclusions of microscopic iron sulfide crystals. Type B crystals corres­ Fig. 8. Calcite (white) replacing sphalerite (mottled gray). Marcasite pond closely with the Type I calcite of Heyl , et al, (1959, p. 100). (black) in lower left corner. 35X. Plane light. Amber Type B calcite is observed best in the southeast part of the Fig. 9. Marcasite (tabular gray masses) detached from joint walls and quarry. enclosed by later calcite. Quartz, apart from its presence in the breccia clasts previously mentioned, is an uncommon mineral at the Martin-Marietta quarry. Fig. 10. Marcasite (black) disseminated in dolomite (light gray). Con­ It occurs as drusy linings of small, widely scattered vugs. Individual centration decreases away from mineralized fracture, which crystal lengths are less than a half centimeter. is off photo at upper left. 35X. Plane light.

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74 PROC. IOWA ACAD. SCI. 91 (1984)

448-450). 3) The fissure fillings at the MMQ are very similar to the vertical gash veins of the UMV. The pinching and swelling, solution and brecciation, and complete or incomplete filling of joint­ controlled fissures recognized in the UMV by Heyl, et al ( 1959, pp. 128-130) are readily observable at the quarry. 4) The temperatures of hydrothermal mineralization at the quarry, as determined by fluid inclusion analysis, lie within the range of temperatures of mineraliza­ tion reported for UMV deposits (ca. 200°C for early sphalerite to ca. 50°C for late calcite) Heyl, 1968, p. 451, McLimans, 1977). Since the MMQ lies at considerable distance from the center of UMV mineralization, lower temperatures are likely, as previously reported by Coveney and Goebel ( 1981). In most of its essential physical and mineralogical characteristics the MMQ deposit exemplifies UMV­ type mineralization. The close correspondence of these deposits suggests cogenesis.

COMMENTS ON THE ORIGIN OF THE DEPOSITS

Recent studies ofUMV deposits, in the main district as well as in outlying areas, demonstrate that mineral emplacement was controlled by a vertical ore-fluid plumbing system (Heyl, et al, 1959; Ludvigson and McAdams, f980; Ludvigson and Garvin, 1981; Ludvigson, et al, 1983a, 1983b; Garvin, 1983). At the MMQ the concentration of mineralization along vertical fissures, and its virtual absence along bedding plane fractures, clearly indicates that most of the mineraliz­ ing fluid migrated vertically. Ludvigson et al (1983a) have noted the coincidence of the area of commercial zinc and lead mining in the UMV with the central portion of the mid-Paleozoic East Central Iowa Basin, in contrast to the common assumption that it coincides with a basin margin (e.g. Anderson and MacQueen, 1982; Cathles and Smith, 1983). Using this model ofo re fluid generation and migration entirely within the basin, the highest temperatures and greatest concentration of hydrothermal activity would be expected near the basin center, assuming the existence of vertical fluid pathways in the Fig. 11. Marcasice (large subhedral crystals) and pyrite basin interior (Ludvigson et al, 1983a). The numerous minor hydro­ (small subspherical grains) in dolomite. lOOX. Plane light. thermal sulfide deposits which ring the district (Heyl and West, 1982), and lie along the western margins of the deeply eroded East Central Iowa Basin, are consistent with the model. The Plum River FLUID INCLUSION GEOTHERMOMETRY fault zone, which was active during basinal development (Bunker, et al, in review) may have provided one avenue for fluid migration Homogenization temperatures were obtained from primary fluid within the basin. It appears to be the locus for several of these outlying inclusions in sphalerite and calcite from the Martin-Marietta Quarry deposits, including the MMQ deposit. Vertically-ascending fluids by Coveney and Goebel in conjunction with a regional fl uid inclusion from within the basin encountered physically and chemically favor­ analysis of minor sulfide occurrences in the midwest (Coveney and able Silurian host rocks. The abundant disseminated early diagenetic Goebel, 1983). From this preliminary analysis, homogenization temperatures in sphalerite range from 85° to 99°C, and homogeniza­ tion temperatures in calcite range from 69° to 82°C. Presumably, the PRE- HYDROTHERMAL early iron sulfides were deposited at somewhat higher temperature HYDROTHERMAL than the sphalerite. Given the comparatively low temperature of Calc ite ~ hydrothermal activity indicated at the quarry, and the corresponding­ Type A Type ~ ly low predicted solubility of silica, the absence of hydrothermal silicification at the quarry is not surprising (see Holland and Malinin, Quartz 1979, pp. 465-47 1). - Fracturing x x COMPARISON OF THE MARTIN-MARIETTA QUARRY DEPOSIT WITH MAIN-DISTRICT Marca site ~ - --- UPPER MISSISSIPPI VALLEY DEPOSITS Pyrite ~ ---- The hydrothermal deposits at the Martin-Marietta Quarry (MMQ) and the deposits of the Upper Mississippi Valley Zinc-Lead District Sphalerile ~ (UMV) are similar in several significant ways: l) The major minerals of the two areas are the same, except for galena, which has not been observed at the MMQ. 2) The paragenetic sequences for the two areas Fig. 12. Paragenecic sequence of fracturing and primary mineral are quite similar (iron sulfide - sphalerite - calcite) (Heyl, 1968, pp. deposition, Marcin-Marietta Quarry.

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HYDROTHERMAL MINERALIZATION 75

pyrite in the Scotch Grove Formation may have .reacted with the . ascending mineralizing fluids, thus localizing sulfid;mineral deposi­ -----, PRATT, W. P. and MOSIER, E. L. 1983. Anomalous metal contents in Paleozoic rocks from southeast Iowa: Possible indicators tion (Lovering, 1961; White, 1968). Other occurrences of pyrite-rich of concealed Mississippi Valley-Type ore deposits: Geo!. Soc. of America, Scotch Grove rock, which has been faulted or jointed, should be Abst. with Program 15:219. regarded as possible target areas for discovery of sulfide mineral ------1968b. Minor epigenetic, diagenetic, and syngenetic sul­ accumulations. fide, fluorite, and barite occurrences in the central United States: Econ. Geo!. 63:585-594. -----·AGNEW, A. F., LYONS, E. J., and BEHRE, C.H. Jr. 1959. The geology of the Upper Mississippi Valley Zinc-Lead District: U.S. Geo!. Survey Prof. Paper 309, 3 lOp. ------, and WEST, W. S. 1982. Outlying mineral occurrences ACKNOWLEDGEMENTS related to the Upper Mississippi Valley Mineral District, Wisconsin, Iowa, Illinois, and Minnesota: Econ. Geo!. 77: 1803-1817. I am grateful to the staff of the Martin-Marietta Corporation for HOLLAND, H. D. and MALININ, S. D. 1979. The solubility and permission to enter the quarry and for the use of maps and drilling occurrence of non-ore minerals, in "Geochemistry of Hydrothermal Ore information, and to Raymond Coveney and Edwin Goebel for Deposits," 2nd ed., H. L. Barnes, ed.,John Wiley and Sons, p. 461-508. providing valuable fluid inclusion data. Greg Ludvigson, Herb LOVERING, T. S. 1961. Sulfide ores formed from sulfide-deficient solu­ Hendriks, and Gene Hinman read the manuscript and provided tions: Econ. Geo!. 56:66-99. LUDVIGSON, G. A. 1976. LANDSAT-I identified linears in northeast numerous helpful suggestions and criticisms. Their assistance is Iowa and southwest Wisconsin and their relation to the Ordovician greatly appreciated. The research was supported in part by a grant stratigraphy, structure, and sulfide mineralization of the area: U npub. from Cornell College. M.S. Thesis, Univ. of Iowa, Iowa City, IA., 196p. ------and BUNKER, B. J. 1978. Stratigraphic significance of the Plum River fault zone: 42nd Tri-State Geo!. Conf., Iowa Geo!. Survey, p. 21-26. and McADAMS, M. P. 1980. New evidence of early REFERENCES Ordovician tectonism in the upper Mississippi Valley: Iowa Geo!. Survey Tech. Inf. Series No. 9, 29p. ANDERSON, G. M. and MACQUEEN, R. U. 1982. Oredepositsmodels- ______and GARVIN, P. L. 1981. Evidences for a vertical plumb­ 6 Mississippi Valley lead-zinc deposits: Geosci. Canada 9: 108-117. ing system in the Upper Mississippi Valley Zinc-Lead District: Geo!. Soc. BUNKER, B. )., LUDVIGSON, G. A., and WITZKE, B. ). (in review) of America Abst. with Programs 13:286-287. The Plum River Fault Zone and the structural and stratigraphic frame­ -----,BUNKER, B.)., WITZKE, B. )., and GARVIN, P. L. work of eastern Iowa: Iowa Geo!. Surv. Tech. Inf. Series. 1983a. A burial diagenetic model for the emplacement of zinc-lead sulfide CATHES, L. and SMITH, A. 1983. Thermal constraints on the formation of ores in the upper Mississippi Valley, U.S.A.: in Proc. of the lnternat. Mississippi Valley-Type lead-zinc deposits and their implications for Conf. on Mississippi Valley-Type lead-zinc deposits, G. Kisvarsanyi, episodic basin dewatering and deposit genesis: Econ. Geo!. 78:983-1002. S. K. Grant, W. P. Pratt, and). W. Koenig, eds., University ofMissouri­ COVENEY, R. M. and GOEBEL, E. D. 1981. Fluid inclusion geother­ Rolla, p. 497-515. mometry of sphalerite from minor occurrences in Kansas, northern -----·BUNKER, B. )., WITZKE, B. )., and GARVIN, P. L. Missouri and eastern Nebraska: Geo!. Soc. of America, Abst. with 1983b. Burial diagenetic origin for the upper Mississippi Valley (MVT) Program, 13:274-275. zinc-lead ores: a hypothesis: Geo!. Soc. America Abst. with Programs, ------1983. New fluid inclusion homogenization temperatures for 15:219. sphalerite from minor occurrences in the mid-continent area: in Proc. of McLIMANS, R. K. 1977. Geological, fluid inclusion, and stable isotope the Int. Conf. on Mississippi Valley-Type Lead-Zinc Deposits, G. studies of the Upper Mississippi Valley Zinc-Lead District, southwest Kisvarsanyi, S. K. Grant, W. P. Pratt, and J. W. Koenig, eds., Univ. Wisconsin: unpub. PhD Dissertation, Pennsylvania State Univ., Univer­ Missouri-Rolla, p. 234-242. sity Park, PA., 175p. DOW, V. E. and METTLER, S. D. 1962. The structure and stratigraphy of -----· BARNES, H. L. and OHMOTO, H. 1980. Sphalerite the Skvor-Hartl area, southeast Linn County, Iowa: Iowa Acad. of Science stratigraphy of the Upper Mississippi Valley Zinc-Lead District, south­ Proc. 69:326-332. west Wisconsin: Econ. Geo!. 75:351-361. GARVIN, P. L. 1979. Hydrothermal alteration at Mineral Creek Mines, NORTON, W. H. 1895. Geology of Linn County: Iowa Geo!. Survey Ann. Allamakee County, Iowa: Iowa Acad. of Science Abst. of Papers, 91st Rpt. 4: 121-195. Session, p. 54. WHITE, D. E. 1968. Environments of generation of some base-metal ore ______1982. A hydrothermal mineral deposit in southern Linn deposits: Econ. Geo!. 63:301-335. County, Iowa: (Abst.) Iowa Acad. of Science Proc. 89:20. WITZKE, B. J., 198 la. Silurian stratigraphy of eastern Linn and western 1983. Sulfide mineralization at Mineral Creek Mines, Jones counties, Iowa: Geo!. Soc. of Iowa, Field Trip Guidebook No. 35, Allamakee County, Iowa: Iowa Acad. of Science Proc. 90:44-49. 36p. HATCH, J. R., AVCIN, M. J., WEDGE, W. K. and GRADY, L. L. 1976. 198 lb. Silurian stratigraphy, petrology, and depositional Sphalerites in coals from southeastern Iowa, Missouri, and southeastern environments of eastern Iowa: Unpub. PhD Dissertation, Univ. of Iowa, Kansas: U.S. Geo!. Survey Open-File Rpt. 76-96, 26p. Iowa City, IA., 540p.

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