A review of the origins of metal-rich Pennsylvanian black shales, central U.S.A., with an inferred role for basinal brines

Applied Geochemistry,Vol. 4, pp. 347-367, 1989

0883-2927/89 $3.00 + .00 Maxwell Pergamon Macmillan plc

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A review of the origins of metal-rich Pennsylvanian black shales, central U.S.A., with an inferred role for basinal brines RAYMOND M. COVENEY, Jr Department of Geosciences, University of Missouri-Kansas City, Kansas City, MO 64110-2499. U.S.A.

and MICHAEL D. GLASCOCK University of Missouri Research Reactor, Research Park, Columbia, MO 65211, U.S.A.

(Received 13 August 1987; accepted in revised form 20 February 1989") Abstract---Compared to most shales, even most black shales, numerous thin Pennsylvanian marine black shales of the U.S. Midwest are very enriched in organic C (>5-40 wt %) and heavy elements. For example, instrumental neutron activation analysis of 74 samples of mostly black and dark gray Pennsylvanian shales of the U.S. Midwest average 1300ppm Zn, 85 ppm U, 655 ppm Mo, 130 ppm Se and 55 ppm Cd--amounts sufficient to raise concerns about heavy element pollution. Direct precipitation of sulfides and fixation by abundant organic matter during sedimentation and early diagenesis may account for heavy metals contained by shales in most cases, Metal supply calculations, however, indicate that a special source would be needed to supply metals for a strictly syngenetic origin. Possible sources for such metals include submarine hot springs and Pennsylvanian seas that may have been more enriched in metals than modern ocean water. At least at some localities, additional metal values may have been added later from basinal brines (e.g. Zn) or from modern groundwaters (e.g, Mo, Se, U). The actions of modern supergene processes may fix metals, mitigating detrimental effects from black shales.

INTRODUCTION THROUGHOUTthe midwestern U.S.A. (Fig. 1), thin black Middle and Upper Pennsylvanian (-291-302 Ma) shales are anomalously enriched in Zn, V, U, Mo and other metals. RUNNELSetal. (1953), OSTROM et al. (1955), SWANSON (1960; 1961), HYDEN and DANILCHIK (1962), WOODLAND (1963), TOURTELOT (1963, 1979), ZANGERL and RICHARDSON (1963), ANGINO (1964), VINE and TOURTELOT(1970), JAMES (1970), Cualrr (1979), COVENEY (1979), MARTIN (1982), COVENEYand MARTIN (1983), ALLEN (1986) and COVENEYet al. (1987b) have all contributed data concerning enriched values of metals in such shales. Like many of these earlier works, this paper reports results for the Middle Pennsylvanian Mecca shale member and its stratigraphic equivalents. However, new data are also included for several other Pennsylvanian beds including the Excello, Holland, Heebner, Hushpuckney, Stark and Muncie Creek shales and unnamed black shales in the Minnelusa Formation. Many of these beds are as enriched in heavy elements as the Mecca Quarry shale member long known to be metal-rich (e.g. WOODLAND,1963). In view of their widespread occurrence metal-rich

*This paper was originally submitted for the Penrose Conference special issue of Applied Geochemistry (Vol. 2, Nos 5/6). On the wishes of the Executive Editor it was expanded into a major review,

black Pennsylvanian shales may act as sources of heavy elements for groundwater pollution. However, it is possible that some black shale beds may absorb Mo and other noxious elements that tend to occur in anionic complexes in oxygenated surface waters (e.g. Se, V and U) as discussed below. Several workers including ZANGERL and RICHARDSON (1963), VINE (1966), HOLLAND (1979), COVENEV (1979) and COVENEY and MARTIN (1983) have suggested that metal-rich Pennsylvanian shales were mineralized during sedimentation and early diagenesis by direct precipitation of sulfide minerals or by interactions between metals dissolved in seawater and organic matter. This concept, involving essentially syngenetic interactions among shales, seawater and diagenetic pore fluids may explain many geochemical features peculiar to the shales and is favored here provided that a special source of metals was available to provide higher concentrations of metals to Pennsylvanian seas than exist in modern oceans. COVENEYet al. (1987b) suggested the further possibilities that metals were added by epigenetic and supergene agents. If this is true metal-rich Pennsylvanian shales may be viewed as products of - 3 0 0 Ma of interaction with fluids including seawater, basinal brines and groundwaters. Such long term interactions would have significance to the origins and evolution of metal-rich brines, the genesis of Mississippi Valley-type (MVT) ore deposits of the midwestern U.S.A. as well as to environmental geochemistry.

347

348

Raymond M. Covcney, Jr and Michacl D. (ilascock

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29 ILL Sh,

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R Sh.

5m

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HUSHPUCKNEY SHALE

3h. ~ ICKNEY Sh. ~

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QUARRY ALE

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coal

C. FIG. I. (A) Distribution of metal-rich black Pennsylvanian shales in central ti,S. Areal and subsurface extents of 17 beds--Mecca Quarry through Queen Hill shale members--generalized from WANLESSand WRIGHT (1978). The 17 major shales between the Mecca (Verdigris cyclothem) and the Queen Hill (Deer Creek cyclothem) were probably deposited over a time interval of 11 Ma (Ross and Ross, 1987)• However, as Ross and Ross (1987) note, precisions for Pennsylvanian dates are --~6-12 Ma. so the time interval could have been considerably longer. Each of the 17 major beds covers at least 20,000 km z and most can be correlated between the Western Interior Basin and the Illinois Basin. but some are only local. For example, the Holland and Logan Quarry shales probably occur only in Parke Co.. Indiana. (B and C) Local stratigraphy surrounding Mecca and Hushpuckney shales• These beds exemplify: (C) black shales in Middle Pennsylvanian cyclothems and (B) Upper Pennsylvanian cyclothems. Notice that Middle Pennsylvanian cyclothems (C) have lesser amounts of limestone and more elastic beds and coal than those of the Upper Pennsylvanian (B).

METHODS Pennsylvanian shales have been analyzed for major elements and heavy metals by a variety of analytical techniques but values reported here were determined chiefly by INAA (instrumental neutron activation analysis) at the University of Missouri Research Reactor (details of procedures are in COVENEV et al., 1987b). Inductively coupled plasma and atomic absorption spectrography were used for major elements and for Pb and Cu. Some samples were analyzed by XRF (X-ray fluorescence) on a Philips four-position universal spectrograph. Sulfur was determined mainly by LECO analyses. Loss on ignition (LOI) values are based on wt % losses at 6IX)°C. Organic matter has previously been characterized by Rock-Eval techniques and pyrolysis gas chrom-

atography (COWNEV ('t ,1.. 1987b}. Host sites for metals were determined by a combination of microscopy and SEM (scanning electron microscopy) using both energy dispersive and wave-length dispersive techniques (the latter method was particularly important in determining the residence site for Mo, whose L,, emission line overlaps with the K , line for S). DISTRIBUTIONS OF METAL-RICH PENNSYLVANIAN SHALES Black metal-rich shales arc ubiquitous a n d distinctive m e m b e r s of the cyclic assemblages of strata, k n o w n as cyclothems, which occur in the Pennsyl-

Review of origins of metal-rich Pennsylvanian black shales vanian of the midwestern U.S. craton. Up to 40 separate Pennsylvanian black shales occur in the Midwest (SWANSON, 1960). About half of these have been investigated for metals by various workers and all that have been studied are enriched in metals relative to normal shales by the criteria of VtNE and TOURTELOX (1970). In contrast to many dark gray to black shales, metal-rich beds of the Pennsylvanian (i.e. those with ~>1000 ppm Zn o r M o ) are universally thin (<~0.5-1.0 m). Some thin Pennsylvanian black shales can be correlated across the midwestern U.S. (WANLESS and WELLER, 1932; see Fig. 1). At least 17 have subsurface distributions exceeding 20,000 km 2 and some (e.g. Excello Shale) underlie >100,000 km 2. The Mecca shale member of Indiana is one of the most widespread of these shales. For brevity it will be referred to as Mecca Shale, but it is known officially in Illinois as the Mecca Quarry shale member as originally named by ZAN6ERL and RICHARDSON (1963) from Mecca, Indiana. It is known unofficially by that name in Kentucky. Stratigraphic equivalents are the Oakley shale member (or Swede Hollow Shale) in Iowa, the Verdigris Shale in Missouri, the unofficial "V"

349

shale in the Cabannis Formation of Kansas and an unnamed member of the Senora Formation of Oklahoma. Besides those of the U.S. Midwest listed above, there are similar Pennsylvanian shales in the Minnelusa Formation of South Dakota. CHARACTERISTICS

General Major constituents of Pennsylvanian black shales include organic matter, quartz, clays (mixed-layer illite with - 1 0 % smectite, illite, kaolinite and traces of chlorite) (TOURTELOT,1963; COVENEY, 1979; CUBI~, 1979; MARTIN, 1982; ALLEN, 1986). Calcite and dolomite are commonly present in minor amounts, particularly where phosphatic nodules are present. Apatite is the main phosphate mineral but some samples contain other phosphates (possibly of secondary origin), such as vivianite, diadochite, ferrostrunzite and leucophosphite (CoVENEY et al., 1984). Pyrite, marcasite, sphalerite, chalcopyrite, barite and clausthalite are ubiquitous minor or trace constituents.

FI6.2. Stark Shale, Unity Village, Jackson County, Missouri. The hard black metalliferous portion of the shale bed (behind hammer) is approximately 0.4 m thick. The upper half of the unit is gray shale which is relatively free of heavy metals. Note the prominent joints spaced at about 0.3 m. Analyses for this exposure are given in Tables 1 and 3. The Stark member overlies the Galesburg (gray) Shale and is succeeded by the Winterset Limestone member.

35(I

Raymond M. Coveney, Jr and Michael D. Glascock Table 1. Major constituents in Pennsylvanian shales (wt % )

Bed sampled

Thickness (cm)

SiO,

AI~O~ Fe203

CaO

MgO

Na,O

KzO

TiO,

P205

LOI

Upper Pennsylvanian (Missourian) shales Heebner shale member, Oread Fm., Leawood, Johnson Co., Kansas (from CusrrJ, 1979) Grab sample ? 61.6 17.(I 5.3 4.2 3.0 3.4

.....

M uncie ('reek shale member, Iola Fro., Leawood, Johnson Co., Kansas (from CUBm. 197% Grab sample ? 43.9 12. I 3.4 18.2 13;' -28 Stark shale member, Dennis Fro., Unity Village, Jackson Co., Missouri Black bed 41 41.1 12.4 5.7 3.9 2.2

0.8

2.7

11.O

-

0.6

4.8

-

1.5

27.1

Stark shale member, Dennis Fm., Leawood, Johnson Co., Kansas (from CuBm, 1979) Black bed ? 69.9 12.9 5.2 (I. 1 1.1 -2.2

--

Hushpuckney shale member, Unity Village, Jackson Co., Missouri Black bed 45 42.6 12.7 5.{1 7.4

2.5

0.9

2.7

0.6

2.1

33.(I

Average Upper Pennsylvanian Black bed --

2.1

{I,8

2.8

0.6

1.8

20.3

Mecca equivalent (unnamed shale over Verdigris Limestone) Cabannis Fro., Ft. Scott, Bourbon Co., Kansas* A-9-10 (gray) 25 42.3 13.1 4.6 9.9 2.6 0.5 3.11 0.5 <2 A-8 8 38.4 11.9 4.0 8.2 2.4 (1.5 2.7 0.4 <2 A-7 I(I 34.4 1(1.3 5.2 9.3 2.3 0.4 2.3 0.3 8.5 A-6 8 45.(I 13.(I 5.0 6.5 2.11 (1.5 2.6 0.4 <2 A-3-5 (gray) 25 41 .(~ 12.2 4.5 6.9 2, 1 0.7 28 0.4 3,5 1 l.(3 7.6 5.2 2.2 0.6 2.5 0.5 <2 A-1-2 (gray) 15 42.8

-23.3 25.11 21,8 --14.t~

Mecca shale member, Linton Fen., Hesler Farm, Mecca, Parke Bed A 9 29.1 10.7 5.4 Bed B 17 33.9 11.2 5.3 Bed C (dk gray) 9 46.6 15.0 6.4 Bed D 4 32.7 10.8 7.0

Co., Indiana 1.2 1,7 0.6 1.5 2.6 1.8 0.6 1.2

0.6 (1.7 0.3 0.3

2. i 2. I 2.4 2.4

I).5 0.5 0,5 11.5

(1.2 11.2 0.6 0.1

47.7 38.5 19.8 44.7

Mecca Quarry shale member, Vermillion Riv., Lowell, LaSalle Bed A 18 30.6 9.3 5.0 Bed B 19 25.3 7.3 3.5 Bed C (dk gray) 10 37.3 10.7 4.5 Bed D 6 30.3 8.3 4.{1

Co. ,lllinois 3.6 1.1 2.0 1.1 3.3 1.5 1.6 1.1

0,6 11.5 0.4 0.7

2.0 1.6 2.9 2.3

().4 0.3 0,5 (L4

0.9 0.6 1.7 0.9

45.2 56.6 34.9 48.2

Average Middle Pennsylvanian Black bed --

3.7

(1.5

2.2

0.4

1.3

39.0

51.8

13.4

4.8

6.7

--

25.1i

Middle Pennsylvanian (Desmoinesian) shales

33.3

10.3

4.9

1.6

- n o data. *Loss on ignition at ~600°C. Data from CUBtTr(1979) are by emission spectrography. Data for samples of shale over the Verdigris Limestone of Kansas by INAA. Remainder of analyses are by inductively coupled plasma spectrography from a commercial laboratory. Note: all Fe is reported as FezO >

Metal-rich Pennsylvanian black shales are universally crosscut by p e r p e n d i c u l a r l y - o r i e n t e d joints set normal to b e d d i n g planes and spaced at - 0 . 3 - 1 . 0 m intervals (Fig. 2). Because they are platy in the shallow subsurface, the shales are often logged by drillers as slates. R a t h e r than behaving as aquitards, like most shales, thin metal-rich Pennsylvanian beds are aquifers. For e x a m p l e , near Kansas City, wells in the H u s h p u c k n e y and Stark shale m e m b e r s yield up to 10 l/min of water, which has b e e n used domestically and for livestock despite marginal quality ( O ' C o n NOR, 1971). T h e r e are two distinct types of Pennsylvanian metal-rich shales: Mecca-type and H e e b n e r - t y p e , which probably intergrade with each o t h e r (CovENEY, 1985). Mecca-type shales contain a b u n d a n t large fish fossils (ZANGERL and RICHARDSON, 19631. They are

typically d e p l e t e d in p h o s p h a t e ( ~ 0 . 4 wt % P205) and contain a b u n d a n t Mo (>1000 p p m ) . Organic C c o n t e n t s can exceed 15-40 wt % , reflecting the abundances of the p r e d o m i n a n t terrestrial-type organic m a t t e r (CoVENEV et al., 1987b) and deposition in very shallow n e a r - s h o r e waters (ZANGERL and RICHARDSON, 1963). Besides the Mecca Shale of western Indiana, examples include the Holland, Logan Quarry and Excello shales of Indiana and adjacent states. H e e b n e r - t y p e shales are richer in major e l e m e n t s such as Ca and Mg (Table 1) and less enriched in Mo than Mecca-type shales. They seem to have b e e n d e p o s i t e d offshore (HECKEL, 1977). They contain lesser a m o u n t s of organic m a t t e r (--5-30 wt % ) , chiefly of marine origin. Besides the p r o t o t y p e H e e b n e r shale m e m b e r of Kansas, e x a m p l e s include

Review of origins of metal-rich Pennsylvanian black shales the Upper Pennsylvanian Hushpuckney, Muncie Creek and Stark members and Kansas equivalents of the Excello and Mecca shales.

Middle Pennsylvanian shales of the midwestern United States Certain black shales from the Desmoinesian Series of the Middle Pennsylvanian, such as Excello and Mecca (CovENEY et al., 1987b), can be traced from Indiana to Oklahoma and from Kentucky to Iowa (Fig. 3; WANLESSand W R I G H T , 1978). Eastern exposures typify Mecca-type shales in that they are closely associated with underlying coal seams. For example, in Indiana and Kentucky, the Mecca Shale overlies the Colchester coal member which, in turn, rests on a leached underclay (Fig. 1). Similarly, in Indiana, the Holland, Logan Quarry and Excello shales overlie coals, although in the case of the Holland the black

351

metal-rich part of the bed is removed by several meters from the underlying coal (Fig. 1C). In the Illinois Basin, the Mecca and Excello contain about 30-60 wt % organic matter. Distinguishing characteristics of Mecca-type shales include large total organic matter contents, abundant terrestrial-type organic matter and low phosphate. In addition, Mecca-type shales are commonly enriched in kaolinite and relatively free of apatite, calcite and dolomite, except for large ( - 0 . 1 - 1 m) lithographic carbonate concretions which occur at some localities ( Z A N G E R L and RICHARDSON, 1 9 6 3 ; M A P L E S , 1988). To the west, where they are consistently separated from underlying coals by several meters of gray to dark gray shale, the Excello and Mecca shales contain lesser amounts of organic matter and more closely resemble Heebner-type shales of the Upper Pennsylvanian (e.g. less Mo). In addition at all locations, including the type locality for Mecca Shale in Indiana,

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FXG.3. Distribution of Mo in Mecca Shale and equivalent beds. Average Mo contents are illustrated for 22 full sections of Mecca Shale of Indiana (IN), four sections of the Mecca Shale of Kentucky (KY), nine sections of the Mecca Shale of Illinois (IL), four sections of the Oakley Shale of Iowa (IA), five sections of an unnamed shale in the Verdigris Limestone of Missouri (MO), six sections of an unnamed shale in the Cabannis Fm. of Kansas (KS) and one section of an unnamed shale below the Verdigris limestone member in the Ardmore Fm., Oklahoma (OK). Limit of black shale facies is taken from WANLESSand Wra6HT (1978). Bed designations (A-D) and ancient shoreline position from ZANGERLand RICHARDSON(1963). Molybdenum values are highest in Indiana and Kentucky near the ancient shoreline and lowest to the west in Kansas and Oklahoma near what was open ocean. Generally correlating with declining Mo values is a decrease in terrestrial-type organics and an increase of phosphate nodules.

R a y m o n d M. Coveney, Jr and Michael D. Glascock Table 2. Minor element contents of Middle Pennsylvanian shales Thickness (cm)

< Mo

ppm (g/t) tI Se

Cu

ppb Au

--

3.1

2.7

52.0

U n n a m e d shale over Verdigris Ls., Cabannis Fro., (Mecca equiv.) Ft. Scott, Bourbon Co., Kansas A-9-10 (gray 25 1 99 405 I 15 12 55 --A-8 8 13 275 290 13 25 32 95 65 120 A-7 10 45 334 310 9 54 73 515 95 105 A-6 8 58 4(13 265 23 39 78 1860 ~5 85 A-3-5 (gra~) 25 26 428 245 2 37 73 132 --A-I-2 (gray) 15 22 187 4411 <1 18 57 65 1311 60

1.2 2.0 2.0 1.9 1.6 1.7

1.7 1.2 2.4 1.9 2.0 4.8

-23.3 25.0 21.8 -14.9

Mecca Quarry shale m e m b e r , Deckoven Quadr., Bed A 5.7 800 1340 Bed B 15.2 114t) 1730 Bed C (dk gray) 7.0 711t 820 Bed D 3.2 2135 1950

4.2 2.6 2.2 3.7

4.5 3.6 2.8 4.0

31.5

144 116 42 70

2.4 2.0 2.4 1.3

3.2 3.() 3.2 4.4

47.7 38.5 19.8 44.7

Mecca Quarry shale m e m b e r , Carbondale Fro., Vermillion River, Lowell, LaSalle ( o . , Illinois BedA 18 214 1820 125 190 72 178 638IP 46 66 Bed B 19 52(I 3190 115 390 105 272 l 1,300" 44 16(1 Bed C (dk gray) 10 5411 30211 21t11 158 102 177 2920* 65 14(1 Bed D 6 660 3660 165 132 76 263 21/50" 37 123

3.2 2.3 1.8 3.4

2.8 2.6 2.2 2.8

45.2 56.6 34.9 48.2

Mecca equivalent, Verdigris Fro., Randolph Co., Missouri (ddh BM-2) 36 cm interval 220 680 31/I 46 66 84

33711

55

105

2.8

3.2

28.2

Mecca equivalent, Oakley shale m e m b e r , Polk Co.. Iowa Top 20 295 965 375 21 Middle 11 1035 2740 290 154 Bottoln 18 9111 2800 50(I 169

2.85 2.25 2.24

30.11 42.0 36.0

Bed sampled

V

Mn

Cd

Excello shale m e m b e r , Bethel Church, Pike Co., Indiana Fossil bed 8.2 1340 1440 1(10 8

107

145

Union Co., Kentucky 140 43 81 143 141) 42 111 159 130 l1 66 91 105 100 258 255

Zn

Pb

300

1480 1670 360 3760

Mecca Quarry shale m e m b e r , Linton Fill., Hcsler Farm. Mecca, Parkc Co., Indiana BedA 9 12511 2180 i65 117 162 190 2870 BedB 17 1350 2330 145 114 144 198 2400 Bed C (dk gray) 9 52(I 800 270 10 85 95 22(I Bed D 4 860 440 1911 38 62 130 465

-

-

_

38 24 46 3(/

m

~---wt % - - ~ S LOI

32.2 17.1 53.9

49 45 134

1(10 165 18(1

445 26011 3275

..... --

---

3.6 t.7 1.7

Logan Quarry shale, Staunton Fm.. Midway, Parke Co.. Indiana BedKb 9 210 328 13,40(/ 11t 37 Bed Eh 33 21t43 14111 120 21 125

54 2(t7

202 520

---

---

2.1 <2.6

15.6 3.2

47.0 58.0

-. . . .

6.6 <2.8

2.2 1.9

34.1/ 52.0

Holland shale, Staunton Fm., Barren Cr.. Mecca, Parke Co., Indiana Bed 5 8 2394 1600 210 330 704 208 Bed4 33 16211 2100 110 32 162 187

2690 4411

no data. * Averages of values from COVENt~ and MARXtN (1983) and COVENEVet al. (1987b).

g r a y b e d s o c c u r in M i d d l e P e n n s y l v a n i a n s h a l e s . F o r e x a m p l e , in I n d i a n a , b e d C o f t h e M e c c a S h a l e is g e n e r a l l y d a r k g r a y r a t h e r t h a n b l a c k , a n d less e n r i c h e d in m e t a l s ( T a b l e 2).

Upper Pennsylvanian shales o f the midwestern United States Upper Pennsylvanian s h a l e s , s u c h as t h e Hushpuckney and Heebner shales of Kansas and M i s s o u r i , o c c u r in c a r b o n a t e - d o m i n a t e d M i s s o u r i a n a n d V i r g i l i a n - s e r i e s c y c l o t h e m s (Fig. 1). T h e s e b e d s e x e m p l i f y H e e b n e r - t y p e s h a l e s (CoVENEY, 1985) which are characterized by prominent phosphate n o d u l e s (RUNNELS et al., 1953; HYDEN a n d DANTECHIK, 1962; HECKEL, 1977). C o m p a r e d to M e c c a - t y p e

s h a l e s , H e e b n e r - t y p e s h a l e s a r e r e l a t i v e l y rich in calcite a n d d o l o m i t e (CovENEY, 1979; MARTIN, 1982; PAYTON a n d THOMAS, 1959), b u t t h e y l a c k t h e l a r g e g r a y l i t h o g r a p h i c calcite c o n c r e t i o n s d e s c r i b e d b y MAPLES ( 1 9 8 8 ) f r o m M e c c a - t y p e s h a l e s . E x c e p t f o r r a r e 1 - 2 c m s e a m s , little coal is a s s o c i a t e d w i t h Heebner-type shales. However, mixed terrestrial and m a r i n e o r g a n i c m a t t e r (HATCH et al., 1984; HATCH a n d LEVENTHAL, 1985) f o r m a b o u t 1 5 - 3 5 % o f t h e beds. Large proportions of organic matter are r e f l e c t e d in loss o n i g n i t i o n v a l u e s ( T a b l e 1). In a d d i t i o n to t h e d i s t i n c t i v e b l a c k m e t a l l i f e r o u s b e d s , most Middle Pennsylvanian shales contain medium to d a r k g r a y l a y e r s t h a t a r e d e f i c i e n t in t r a c e m e t a l s . In m o s t c a s e s t h e s e g r a y l a y e r s , w h i c h c a n b e n e a r l y as t h i c k as t h e p r e d o m i n a n t b l a c k b e d s , o c c u r at t o p s o f s h a l e m e m b e r s ( F i g s 1 a n d 2).

Review of origins of metal-rich Pennsylvanian black shales

FIG. 4. Minnelusa outcrop, Hot Brook Canyon, South Dakota. (A) Black shales with adit along a lower metal-rich bed (L). An upper metal-rich shale (U) is found about 10 m above adit, which was probably driven in the 1950s for U exploration and which extends --10 m into the hillside. Note the solution pitting visible in sandstones and dolomites that intervene between shales and especially near upper shale. (B) Close-up of upper shale (U) from 4A. Note vugs and geodes lined by calcite and quartz crystals, giving evidence of solution activity.

353

354

Raymond M. Covency, Jr and Michael D. Glascock

Pennsylvanian shales o f the Black Hills

At least seven thin Minnelusa shales are discernable in gamma ray logs near the Black Hills (DEsMOND et al., 1984) in the Pennsylvanian part of the Minnelusa Formation in Wyoming and South Dakota. HARRIS and HAUSEL (1984) note up to 1.45 wt % U in black shale from an outcrop near Hot Springs (TROMP, 1981) and also quote unpublished work by J. D. Love (1953) reporting thin DesmoinesJan black shales with ~100-200 ppm U from the subsurface of eastern Wyoming. Two Minnelusa shales, resembling Mecca-type shales, were sampled in Hot Brook Canyon, about 3 km northwest of Hot Springs, South Dakota (Fig. 4). These beds are probably Desmoinesian but may be of Missourian age. The shales are associated with reddish to buff dolostones, massively erossbedded sandstones (former dunes?) and evaporites which imply a near shore depositional environment (DESMOND et al., 1984). Geodes lined with secondary quartz and calcite are common and provide material for fluid inclusion studies.

ORGANIC GEOCHEMISTRY

WHITE (1915) suggested that most organic matter contained by carbonaceous Pennsylvanian shales of the Midwest was washed into shallow epeiric seas from surrounding peat swamps. Subsequently ZANGERL and RICHARDSON(1963)reported that Middle Pennsylvanian (Mecca-type) shales in Indiana contain mainly terrestrial organic matter and suggested there might be a complete intergradation between dark shales and coal along the ancient shoreline in Indiana. MOORE (1929) argued that whereas it may be reasonable to infer that shales directly overlying coals derived organic matter directly from peat swamps, such an origin would make little sense for Heebner-type beds that are intercalated with marine carbonate beds. Rock-Eval data and pyrolysis gas chromatography show a predominance of terrestrial organic matter in Mecca Shale of Indiana (CoVENEYet al., 1987b). The data of HATCH et al. (1984) suggest that both Middle and Upper Pennsylvanian black shales (Heebnertype) from Iowa, Kansas, Missouri and Oklahoma contain chiefly marine-type organic matter. In regional studies both JAMES (1970) who worked on the Excello Shale and CovENEVetal. (1987b) who worked on the Mecca Shale found lesser amounts of total organic matter and smaller fractions of terrestrial organic matter in samples of Middle Pennsylvanian black shales collected from the Western interior Basin (as opposed to the Illinois Basin). In contrast with the Mecca Shale in Indiana, equivalent beds from offshore areas such as Kansas contain a predominance of marine-type organic matter. Numerous attempts have been made to assess the

hydrocarbon potential of the Pennsylvanian beds which are low-grade oil shales. For example, LAMAR et al. (1956) recovered an average --30 l/t (peak = 150 l/t) for Pennsylvanian shales from Illinois. SnAFVERet al. (1984) analyzed 126 Pennsylvanian shale samples from Indiana which yielded an average - 2 5 l/t by modified Fischer assay. Mecca Shale samples from Indiana averaged only - 2 0 I/t as might be expected given the predominance of terrestrial organic matter in the bed. Nevertheless, one Mecca Shale sample yielded the highest single value measured ( - 8 0 l/t). In Kansas SCHLINSOG'S(1982) modified Fischer assay values exceeded >35 l/t for several Pennsylvanian black shales. Heebner shale samples consistently yielded 30-40 l/t, but Desmoinesian shales (e.g. "V'" and Little Osage) were less predictable with values ranging from traces to ~50 I/t. By detailed layer-by-layer sampling in Kansas and Oklahoma, WENGER and BAKER (1986) showed that basal layers of the Middle Pennsylvanian Excello and Little Osage studies are enriched in organic matter of terrestrial origin. In both shales basal layers rich in terrestrial organic matter are succeeded by younger laminae rich in marine organic matter. WENGER and BAKER (1986) inferred that an influx of humic debris during marine transgression was followed by "high algal productivity", which yielded marine-type organic matter and that this algal episode was ultimately succeeded by normal marine conditions as the supply of nutrients diminished. Thus, as might have been inferred from the early views of WHITE (1915) and MOORE (1929), shale deposition might be divided into two stages--an initial period dominated by terrestrial organics, swept up by transgressive seas, followed by a later period with organic matter derived from marine organisms. Speaking of black shales m general, TOURTELOT (1979) has aptly noted that in many cases the "accumulation of organic material is a cause of conditions rather than an effect" (as is so often assumed to be the case).

INORGANIC GEOCHEMISTRY

General

As noted by numerous workers cited previously, both Middle and Upper Pennsylvanian black shales of the midwestern U.S. are enriched in heavy elements at concentrations that lie between those of ores and normal shales (Tables 2-4). Arithmetic means of 74 precision I N A A analyses of mostly black and dark gray Pennsylvanian shales are: 655 ppm Mo, 1400 ppm V, 55 ppm Cd, 130 ppm Se and 1300 ppm Zn (Table 4). These analyses chiefly consist of values for black and dark gray Mecca Shale equivalents in Illinois, Indiana, Iowa, Kansas, Kentucky, Missouri and Oklahoma but also include data for the Holland, Logan Quarry, Excello, l-tushpuckney, Stark, Muncie Creek and Heebner shales. Some gray inter-

Review of origins of metal-rich Pennsylvanian black shales

355

Table 3. Minor element contents of Upper Pennsylvanian shales* Pb

Cu

ppb Au

~---wt%----* S LOI¢

Heebner shale member, Oread Fm., Clinton Lake, Lawrence, Douglas Co., Kansas Black bed 100 67 925 200 56 35 118 2220 --

--

1.8

2.2

22.0

Muncie Creek shale member, Iola Fm., Leawood, Johnson Co., Kansas (from Cuarrr, 1979) Black bed ? 60 488 140 97 --910 180 58

--

--

4.8

Stark shale member, Dennis Fm., Leawood, Johnson Co., Kansas (from CuBrrr, 1979) Black bed ~ 240 2360 55 58 --695 77

--

--

25.0

Bed sampled

Thickness (cm)

~ Mo

V

Mn

Cd

ppm (g/t) U Se

Stark shale member, Dennis Fm., Unity Village, Jackson Co., Missouri Gray bed 30 30 . . . . . Black bed 41 86 1150 150 55 80 103

Zn

400 1930

Hushpuckney shale member, Swope Fro., Unity Village, Jackson Co., Missouri Gray bed 104 83 . . . . . 113 Black bed 45 79 790 130 53 64 76 1620

-129 D

118 - -

1.2

1.8

1.7

- -

0.9

63

1.7

1.4

33.0

120:t:

3.2

1.5

30.0

--

121

Hushpuckney shale member, Swope Fm., Kansas City, Jackson Co., Missouri Black bed 41 86 584 175 23 50 73 1 0 7 7 200:~

2.8 27.1

--

71

1.7

*Data from INAA except values for gray beds which are based on X-ray fluorescence spectrography and the values from CoBnx (1979) based on emission spectrography. ~Loss on ignition at -600°C. :~Parkville,Missouri.

calated beds are also included. Most metals are more concentrated in Pennsylvanian black beds than in average shales (TUREKIANand WEt)EPOHL, 1961), in average black shales (VINE and TOURTELOT,1970) or even in such well known metal-rich shales as the Chattanooga shale (exemplified by black shale standard SDO-1 of LEVENTHALet al., 1978) (see Table 4). Enrichment in Zn is nearly ubiquitous in all Pennsylvanian black shale samples. Values ~1 wt % occur in at least one case in shales which are transitional between Mecca- and Heebner-type near Lowell, Illinois (Table 2). Among the most notably enriched elements are Mo, U and Se. Average crustal abundances are - 2 ppm for Mo and U and 0.05 ppm for Se. In Pennsylvanian black shales values range from tens to thousands of clarkes (Table 4) with greatest concentrations in Mecca-type shales from Indiana, Kentucky and Illinois contrasting with substantial but somewhat lower values for Upper Pennsylvanian shales and Middle Pennsylvanianbeds from the western part of the U.S. Midcontinent (e.g. Kansas, Missouri and Oklahoma). One exception to this generalization would be the large amounts of Mo, U, Se and V found in some samples from Iowa. Lead values average 50-140 ppm, 4-11 clarkes (Tables 2 and 3; LONGand ANGINO, 1982). Although chalcopyrite is visible in nearly all polished sections, Cu contents range only from - 2 4 - 1 6 0 ppm (~<2 clarkes) (Tables 2 and 3; Cu~rrr, 1979; LONG and ANGINO, 1982). As noted by VINE and TOURTELOT (1970) many black shales show little enrichment in Pb or Cu. As in many black shales, Au occurs only in low concentrations (averaging 2.5 ppb; - 0 . 7 clarkes). Comparable with the 4--6 ppb Ir in Cretaceous-Tertiary boundary clays at Gubbio Italy (ALVAREZet al.,

1980) ~1 ppb Ir occurs in bed Kb of the Logan Quarry Shale of Indiana (CoVENEY,1986), one of four samples of Mecca-type shale to be analyzed so far for Ir (range - 0.14-1.0 ppb Ir).

Middle Pennsylvanian shales of the midwestern United States Most notable of the Middle Pennsylvanian beds is the Mecca Shale of ZANGERLand RICHARDSON(1963). From XRF and INAA data ALLEN (1986) estimated that the Mecca Shale and its stratigraphic equivalents across the Midwest contain an average of 500 ppm Mo, 1400 ppm V, 100 ppm U and 1500 ppm Zn (Table

4). There are strong geographic, lithologic and stratigraphic controls on metal contents in Middle Pennsylvanian shales. For example, across the midwestern U. S. A., the Mecca Shale and equivalent beds display a prominent zonation of Mo values. Average contents for the entire member, including the less-metalliferous C bed, in excess of 800-1100 ppm Mo are universal for Kentucky and Indiana (Fig. 3). The high average of 655 for 74 INAA shale analyses (Table 4) reflects the large proportion of samples from Meccatype shales in Indiana. Bed B is sufficiently enriched in Mo and other metals in Indiana to have prompted GLASCOCK and COVENEY (1988) to prepare a large sample of black shale powder as interlaboratory standard MQSB-1 (Table 4). Values for Mo gradually decline westward to <50 ppm in Kansas (Table 2; COVENEYet al., 1987b) and Oklahoma. Similarly, V, Se and U are all less abundant in western samples than in those from the east (Table 2). Among Middle Pennsylvanian shales,

356

Raymond M. Coveney, Jr and Michael D. Glascock Table 4. Comparative geochemical data Mo

Average shale

(TUREKIAN

V

Mn

and WEDEPOHL, 1961) 2.6 131/ 850

Average black shale (VINE and TOURrt,.Lor, 1970) I0 150 150 SDO-I (LEVENTHALel al., 1978) 122 lol

---ppm Cd

U

(g/t)

().3

4

Sc

1).6

-

('u

ppb Au

wt % S

21)

45

---

(}.2

211

7t)

7t~

-211

74

---

5.0

283O

33

130

3,0

3.3

Zn

Pb

t,J5

<300

3911

< 1

51~

MQSB-1 (GLASCOCKand COVENEY, 1988) 1655 2984 695

1(13

125

232

Midwestern averages for Mecca Quarry shale (ALLEN, 1986)* 500 1400 -I011

--

-

1500

.

.

.

.

--

.

.

.

.

Midwestern averages for INAA analyses of 74 Pennsylvanian black, dark gray and gray shale samples {from GLASCOCK, Univ. of Missouri Research Reactor) 655 1401t 445 55 85 13(I 1300 --2.511 3.7 Average Upper Pennsylvanian black or dark gray shale (based on Table 3) 90 10511 150 55 55 95

14111i

140

Averages of 16 INAA analyses of western (Iowa, Missouri, Kansas, and Oklahoma) M. Penn., black and dark gray shales (transitional Mecca- and Heebner-type) 287 1127 3(12 48 54 107 12111 Averages of 411 INAA analyses of eastern (Indiana, Kentucky and Illinois) Mecca-type M. Penn. black and dark gray shales 1141 1830 253+ 69 133 162 Average concentration in Earth's cruse (MASON and MOORE, 1983) 1.5 135 950 0.2 1.8

(}.115

1531} 70

-13

85

2. l

1.7

--

2.3

2.5

--

3.0

3.3

55

4.11

0.03

Clarke values Average Clarke values for all 74 Middle Pennsylvanian samples (INAA) 565 10 0.2 451) 71t 3200

35

Average Clarke values for Upper Pennsylvanian samples listed in Table 3 60 8 11.2 275 .~0 1900

211

--

11}.7

1211

0.5

70

Average Clarke values for 16 (western) Middle Pennsylvanian black and dark gray samples (INAA) 191 8 11.3 241/ 30 2140 t7 . . . .

11.6

83

Average Clarke values for 40 (eastern) Middle Pennsylvanian black and dark gray salnplcs (INAA) 761 14 0.3 345 74 3240 22 --

11.8

110

Ill

-,~

*ALLEN'S(1986) averages are based on INAA and XRF analyses of Mecca shale equivalents from Illinois, Indiana, Iowa, Kansas, Kentucky, Missouri, and Oklahoma. weighted for thicknesses of sampled beds. Glascock's analyses cover five Upper Pennsylvanian shale samples (one Heebner, one Muncie Creek and two Hushpuckney samples) and 69 Middle Pennsylvanian samples (one Excello, three Logan Quarry, two Holland and 63 Mecca shale equivalents). tExcluding one Logan Quarry Shale Mbr. 13,400 value: 590 ppm Mn if this is included. extremely large c o n c e n t r a t i o n s of Mo, U , V and Se (~>1000 p p m M o and ~>200 p p m U) are not confined to the Mecca Shale but also exist in the Holland, Excello and Logan Quarry shales.

Mo, U, Se and V (Table 3). To date, no Mecca-type shales have b e e n found in the U p p e r Pennsylvanian although they may exist in shore line areas.

t'ennsylvanian shales o f the Black Hills Upper Pennsylvanian shales o f the midwestern United States C o m p a r e d to most shales, black zones in H e e b n e r type beds are relatively e n r i c h e d in Z n , U and phosphate (Tables 1 and 3; Cus~rr, 1979; COVENEY, 1979; MARTIn, 1982; LONG and ANGINO, 1982). C o m p a r e d to Middle Pennsylvanian beds and to all Mecca-type shales, H e e b n e r - t y p e shales are e n r i c h e d in lithophile c o m p o n e n t s , especially Ca and Mg (Table 1), but are deficient in organic C; they are also less enriched in

South of the Black Hills near Hot Springs, South D a k o t a , extremely high values for Z n , U , Mo, V and Se occur in thin black shales in the Minnelusa Formation (Fig. 4). The shales were previously discussed by VINE (1969) who cites earlier geochemical work by U.S. Geological Survey personnel. A s n o t e d previously, HARRIS and HAUSEt (1984) r e p o r t large a m o u n t s of U in shale near H o t Springs but only 100-200 p p m in D e s m o i n e s i a n shales from the subsurface of W y o m i n g .

Review of origins of metal-rich Pennsylvanian black shales Table 5. Mo, V and U in Black Shales from Minnelusa Formation. Hot Brook Canyon, about 3 km northwest of Hot Springs, South Dakota [NW ¼, NE ¼, NW ¼, sec. 15, Tp 7 S, Rg 5 E] < ppm (g/t) , Sample Mo Zn V U Lower shale (L), south side of adit 1C 6.4 cm 3500 940 1B 7.6 6210 2140 1A 8.9 4600 8950

2090 1523 4500

610 3830 560

Lower shale (L), north side of adit 2C 6.4 cm 3700 180 2B 7.6 5510 540 2A 8.9 3420 5720

1150 2520 3240

250 200 110

Lower shale (L), at face of drift entrance) 3C 8.9 cm 1160 3B 10.2 1440 3A 6.4 1260

1050 320 490

Lower shale (L), at face of drift entrance) 4C 8.9 cm 1010 4B 10.2 880 4A 6.4 3140

(south side, - 1 0 m from 210 350 220

170 120 70

(north side, ~10 m from 510 290 430

Upper shale (U), - 3 m above adit. full 10 cm 1700" 80*

1040 690 820

180 110 200

1700"

350*

* Data from R. G. Blair and J. D. Vine. Zinc values c o m m o n l y exceed thousands of ppm, but are m o r e erratic than those in Midwestern shales (especially H e e b n e r - t y p e shales). O n e particularly metal-rich sample section contains - 1 7 0 0 ppm U and

357

--4800 ppm M o (Table 5). Such high values for roll-front elements, however, may be confined to outcrops because 10 m from the portal, at the face of the mine working, metal values average only about 150 ppm U and 1000 ppm Mo.

METAL HOSTS

Microscopy has focussed on the Mecca, Excello, Stark and Hushpuckney shales. Iron occurs chiefly in disulfide minerals, including ubiquitous framboidal pyrite. Marcasite is c o m m o n , especially in Meccatype shales from Indiana and Kentucky. In both types of shales the main heavy-metal hosts are: sphalerite for Zn, chalcopyrite for Cu and clausthalite (MARTIN, 1982) for Pb. V a n a d i u m may occur in illite or organics (COVENEYe t al., 1987b). The only other sulfide minerals identified are rare hawleyite and covellite which are probably of secondary origin. M o l y b d e n u m contents correlate well with organic C (VINE, 1966) and even better with terrestrial organic matter (COVENEYet al., 1987b). H o w e v e r , at least in Mecca Shale, much M o occurs on or near the surfaces of pyrite framboids (Fig. 5). Neither molybdenite nor jordisite have been observed by microscopy or by X-ray diffraction, although it is possible that some Mo occurs as a thin layer of MoS2 on framboids (or perhaps MoS 3 as was inferred by VOLKOV and FOMINA (1974) for Mo associated with iron sulfides in H o l o c e n e sediments of the Black Sea). Some Mo is dispersed in the groundmass and

Ft6. 5. SEM dot map for Mo, Mecca Shale, Velpen, Indiana. (Wavelength dispersive detection) dot map for Mo. Most Mo occurs near surfaces of pyrite framboids, such as the one indicated by arrow. Some seems to be dispersed in organic matter, but may be associated with submicroscopic pyrite.

358

Raymond M. Coveney, Jr and Michael D, Gtascock

there is a preferential association with dark organicrich layers (Fig. 5). Possibly, this Mo is adsorbed on or chelated within organics. However, organic matter in shale contains ubiquitous pyrite grains ranging in size down to the limit of optical resolution. Conceivably, therefore, even Mo that appears to be dispersed in organics is associated with pyrite. Resolution of remaining ambiguities awaits analyses of other shales and may require other techniques such as Auger electron spectrometry.

METAL ORIGINS

At various stages between their deposition - 3 0 0 Ma ago and today, Pennsylvanianshales have been in contact with three genetically distinct types of fluids: brackish seawater, basinal brines and modern groundwater. Each type of fluid has had the opportunity to add metals to (or remove metals from) shakes.

Syngenetic mineralization

FLUID INCLUSIONS Some sphalerite grains from the Illinois Basin are coarse enough to study their fluid inclusions. Homogenization temperatures (i.e. minimum temperatures of entrapment) of such inclusions exceed 75-80°C and melting temperatures correspond to - 1 0 - 2 3 wt % chlorides (NaC1, CaC12, etc.) for included fluids (COVENEYet al., 1987a). Calcite crystals from geodes found near metal-rich Minnelusa shales of South Dakota contain primary fluid inclusions with no vacuoles, indicating entrapment ~<45°C. Included fluids melt near -0.2°C, implying the presence of relatively fresh water during calcite deposition. Efforts to find brine inclusions in quartz and calcite from this locality were unsuccessful.

SHALE ORIGINS

ZANGERL and RICHARDSON (1963) inferred that shales near Mecca, Indiana, were deposited near shore in shallow water within an interval of only a few years (4 for Mecca Shale). As pointed out by COVENEY and MARTIN(1983) near-shore deposition fits in with exceptionally large Mo values for the Mecca and Logan Quarry shales. The concept also fits with the terrestrial nature of organic matter. Moreover, MAPLES (1988) has found evidence for oxic events within a Mecca-type shale that are most easily explained by near-shore deposition. Highly variable but generally heavy S isotope values in Mecca-type shales fit better with shallow, near-shore origins than with deep, euxinic, starved-basin conditions (CoVENEY and SHAFFER, 1988). HECKLE (1977) inferred that Upper Pennsylvanian (Heebner-type) shales in eastern Kansas were deposited slowly at maximum transgression in relatively deep water. Relatively constant values for S isotopes (CoVENEY and SHAFFER, 1988) and other factors fit with an offshore origin. It is assumed, therefore, that Mecca-type shales formed relatively quickly in near shore waters. whereas Heebner-type shales formed offshore at a slower rate.

TOURTELO~[(1963) and ZANGERLand RICHARDSON (1963) found that sphalerite in Mecca-type shales was deposited during the earliest stages of diagenesis as H2S released by decaying organisms reacted with Zn dissolved in pore fluids. In a Heebner-type shale, the Hushpuckney Member, COVENEY(1979) found that bedding laminations were deformed around sphalerite grains implying deposition of ZnS prior to compaction. So at least some sphalerite was deposited early in Pennsylvanian shales. Initial accumulation of Zn and other metals m Mecca-type shales probably took place during sedimentation or shortly after when the shoreline of the ancient sea was fringed by peat swamps. Seepage from swamps into shallow seas caused acid conditions near shore (MOORE, I929)--a situation which might be expected to prevent fixation of most metals. However, Mo is readily retained by either terrestrial or 1.0 meq./!

100

at

z '~ 0,,' x

Mo\ "50

V..

\

algae,,_ \

~.

2

4

6

8

pH

FIG. O. Rates of fixation for Mo and V by organic matter as a function of pH. Results for peat are from BERTINE(1972) and those for algae are from DISNAR(1981). Both types of orgamcs readily retain Mo and V under acid conditions. BER/~I~E(1972) found that deposition of Mo by co-precipitation with pyrite is relatively rapid (occurring within a few days). Similar rates of fixation were found by COVENEVet al. (1987b) who used powdered (pyritiferous) Mecca Shale as a substrate.

Review of origins of metal-rich Pennsylvanian black shales marine-type organic matter (Fig. 6) or co-precipitated with pyrite under acid conditions (SZILA6YI, 1967; BERTINE, 1972; BERTINEand TUREKIAN, 1973; DISNAR, 1981), which probably explains the unusual concentration of Mo along the ancient shoreline (CoVENEY and MARXtN, 1983). Some high values for Mo, U, Se and V occur in Iowa near the edge of the Mecca Shale outcrop belt which may reflect a shoreline in the northern Midcontinent. Acid conditions along shore (and in pore fluids) may also be reflected in the presence of marcasite (REYNOLDSand GOLDHABER, 1983) and the rarity of carbonate minerals in samples from near shore. MUROWCHICKand BARNES (1986) have recently shown that marcasite can form only at pH values <5.0. Such values may seem to be extremely low for seawater, but it should be recalled that the Pennsylvanian epeiric seas were very shallow and those in which Mecca-type shales were deposited were not in contact with any calcareous sediment (Fig. 1C). Moreover, in Indiana, they were probably fringed by some of the most extensive peat swamps known from the geological record. WANLESS (1964) notes that the Colchester Member which underlies Mecca Shale equivalents is the most widespread North American Pennsylvanian coal. Given the presence of such extensive nearby peat swamps and early iron sulfides there seems to be little question that organic muds of the littoral zone, now represented by Mecca-type shales, contained relatively acid pore fluids during early diagenesis. Such conditions would have been particularly favorable for initial fixation of Mo either by adsorption on (flocculating?) organic matter, co-precipitation with iron sulfides or replacement of outer zones of framboids. DISNAR(1981) notes that the conditions most favorable for simultaneous fixation of U, V and Mo would be mildly acid (pH = 6), oxidizing (Eh > 200

359

mV) fluids--the sort of conditions that might have been associated with peat or with the oxic side of a redox boundary near shore in the Pennsylvanian seas. It should be noted here that like most black shales those of the Pennsylvanian of the Midwest are mostly depleted in Mn (Tables 2-4). However, in western Indiana, high values for Mn occur occasionally in black shales. For example up to 1.3 wt % is present in bed Kb of the Logan Quarry Shale near Mecca (Table 2). As in the model of FORCE and CANNON (1988) this fits with at least sporadic existence of a redox front near shore at the edge of a black shale basin during the Pennsylvanian. Such conditions might tend to promote local acidity from oxidation of sulfides.

Metal supply calculations Calculations presented in Table 6 are modeled on those of ALLEN (1986) who estimated the feasibility of deriving metals for Mecca Shale during sedimentation from seawater. ALLEN (1986) assumed water circulation rates similar to the present Black Sea which has areal dimensions similar to those of the Mecca Shale (although it is much deeper). Fluxes into the present day Black Sea average - 188 km3/a of Mediterranean water through the Bosporus, the only channel open to the sea, and an additional 320 kma/a of river water, for a total of 508 km3/a (RICHARDS, 1971). Calculations in Table 6 assume modern-day ocean metal contents of 10 ppb for Mo, 3 ppb for Zn, 2.5 ppb for V and 3.3 ppb for U (HOLLAND, 1978; 1979). For these calculations it is assumed that metals were completely stripped from fluids contacting shales. With seawater alone (at ~ 188 km3/a) considered as a source of metals, estimates suggest that up to - 1 . 7 Ma would have been necessary to derive metals for

Table 6. Metal budget for Mecca shale member Metal contents

Metal Mo Zn V U

Average tenors for Mecca Shale* 0.05% 0.15 0.14 0.01

E metal content of Mecca S h . - t 2.8 8.4 7.8 5.6

x x x ×

108t 10s 108 107

Average for modern seawater,: 10.0mg/t 3.0 2.5 3.3

E metal content of Mecca Sea§

Water changes needed

Time (Ma) E interval to derive metals from seawater

4.0 1.2 1.0 1.3

7000 70,000 78,000 4300

0.15 1.5 1.66 0.092

× × X ×

104t 104 104 104

*Arithmetic means of values for full thicknesses of Mecca shale member and equivalent beds in the states of Illinois, Indiana, Iowa, Kansas, Kentucky, Missouri, and Oklahoma (from ALLEN,1986). ?Based on a total mass of 5.6 x 109 t for Mecca equivalents. SU from HOLLAND(1978); Mo, Zn and V from HOLLAND(1979). §Assuming metal concentrations similar to modern oceans and a 10 m water depth [which would amount to 4000 km 3(~4 × 1012t)]. Assuming a flux rate similar to that of the Black Sea, which has similar areal dimensions and which has a flux of about 188 km3/a through the Bosporus, it would take 21.3 a to completely change the water content of the Mecca Sea. Note: calculations are not sensitive to water depth (i.e. assuming a depth of 100 m rather than 10 m would mean it would take longer to change the water, but more metal would be introduced during changes). Calculations are dependent on assumptions about flux rates and metal contents of seawater which might have been affected by introduction of metals from peat swamps and underclays or expulsion of basinal brines as hydrothermal plumes into Pennsylvanian sea. Implications of these calculations depend on how long it took for the Desmoinesian, Missourian, and Virgilian Series to be deposited. Ross and Ross (1987) note that the assumed 11 Ma duration is not well constrained by dating.

360

I

Raymond M. Covcney, .lr and Michael D. Glascock

STAGE ~. , N m A " S E O I M E N T A n O N WiTH POSSl6"E INPUT FROM S U " M A R I N E S P R I N ~ ~ . , , ~ ~

.OT ~ ~.._~

, ] ......

~:~

~::~,,i,,~,o~

~,~

-'-

~

~

eg

Mecca

K

STAGE 3. SUPERGENE ENRICHMENT. / ~

F[6. 7. Genetic model for deposition of metals in Pennsylvanian shales (modified from COVENEYand MARTIN, 1983 and COVENEYet al., 1987b). (A) During Stage 1, exemplified by several shales near Mecca, Indiana, metals were deposited along with muds containing abundant organic matter derived from nearby swamps. Fixation of Mo was enhanced (Fig. 6) by the existence of extremely acid conditions caused by drainage from swamps. Relatively acid conditions for near-shore waters or pore fluids are now reflected by relative abundance of syngenetic or early diagenetic marcasite which forms only at pH values <5.0 (MuROWCHICKand BARNES,1986). Metal supply calculations suggest that ordinary seawater would not be an adequate supply of metals given the time available for sedimentation. However. metal contents of digenetic fluids or epeiric seas may have differed from the modern ocean partly because of extensive weathering on land during the Carboniferous Period or metal-charged diagenetic fluids or submarine hydrothermal venting. Possible sources of hot fluids include basinal brines formed during a massive, possibly hot-spot related, hydrothermal event that caused country rock temperatures in excess of 75-100°C in maturing sedimentary basins of midwestern U.S. (COVENEYand GOEBEL, 1983; COVENEYet al., 1987a). Rather than being of such local origin such fluids may have been expelled from thick piles of sediments which began to accumulate in the Ouachita region in Middle Pennsylvanian times (LEACHand ROWAN, 1986; OLIVER, 1986). Oxidation of fluids derived from a dispersed hydrothermal plume may have been partly responsible for the acidity of seawater near shore and for localized precipitation of Mo. Structures such as those shown on the index map (the LaSalle anticline, near Lowell, the Humboldt fault in east Kansas and Nebraska, and the Reelfoot rift of southern Illinois) may have influenced the migration paths of basinal brines. (B) During Stage 2, possibly exemplified by beds at Lowell, Illinois, metals may have been precipitated as sulfides from basinal brines, as evidenced by the presence of saline inclusions that were entrapped at temperatures ~>75°C. BETHKE (1986) suggests that epigenetic brines moved northward through the Illinois Basin to provide metals for the Upper Mississippi Valley Pb-Zn mining district. Structures including minor faults associated with the LaSalle anticline (see 7A index map) may have deflected such basinal brines into Pennsylvanian receptor beds. However, brine migration may have occurred during the Middle Pennsylvanian (Stage 1) as shales were deposited. (C) During Stage 3, best illustrated by shales in the Minnelusa Formation of the Black Hills area, Mo and U have recently been added to the shales and may still be accumulating from groundwaters such as artesian fluids that have passed through extensive cave systems in the underlying Paha Sapa Limestone. In South Dakota, association of U and Mo values with the present topographic surface overlying abundant solution cavities, suggests involvement of supergene processes in development of the metal values. Supergene enrichment could be occurring more pervasively (but less obviously) at other locations.

Review of origins of metal-rich Pennsylvanian black shales Mecca Shale (0.15 Ma for Mo; 1.5 Ma for Zn; 1.66 Ma for V; 0.092 Ma for U [Table 6]). If HOLLAND'S (1979) alternate figure of 0.4 ppb for Zn in seawater were used several million years would have been needed to derive Zn for Mecca Shale. If river waters are also considered to be significant metal sources (ALLEN, 1986) [which would fit in well with WOODLAND'S (1963) inference of a land source for metals] less time would he needed to derive metals but there still would be difficulties. ALLEN (1986) assumed the same values for dissolved metals as indicated above except for Zn where he used a concentration of 7.5 ppb. His results indicate that up to - 0 . 6 Ma would be needed to derive metals for Mecca Shale [0.055 Ma for Mo, 0.070 Ma for U, 0.22 Ma for Zn and 0.62 Ma for V]. The portion of the Pennsylvanian Period during which most metal-rich shales (Fig. 1) were deposited (mid-Desmoinesian Mecca [Verdigris] Cyclothem through Late Virgilian Queen Hill [Deer Creek] Cyclothem) lasted - 1 1 Ma (Ross and Ross, 1987). Each main cyclothem was probably deposited in <0.4 Ma (HECKLE, 1986). It is assumed that the period of time of deposition for the Mecca cyclothem was no different than for those containing other metal-rich shales and that prior to erosion all such shales had about the same amount of metals. Granting perfect efficiency for fixation, the above calculations show that even the entire interval for sedimentation of each cyclothem would not be sufficiently long to derive V or Zn for shales from normal seawater. Derivation of sufficient amounts of Zn would be possible but difficult even with river water assumed as a source. Clearly, derivation of metals from normal seawater would be out of the question if ZANGERLand RICHARDSON'S(1963) estimate of 4 a for deposition of Mecca Shale is correct. There are only four possible means of circumventing this difficulty: (1) First, assumptions about similar time intervals for deposition, similar metal contents, etc., for all of the Pennsylvanian black shales may be erroneous. One must concede this to be possible and a final consensus must await further investigations on the parts of many investigators. Ross and Ross (1987), for example, stress that potential errors of 6-12 Ma for radiometric dates on the onset and end of the Middle and Upper Pennsylvanian. However, HECKEL (1977) and others have reasoned that the basic period of repetition of cyclothems appears to be constrained by Milankovich astronomical cycles which would tend to fix the time periods for depositions of major cyclothems at ~0.235-0.4 Ma. Second, all available evidence suggests that about the same amount of metals were deposited within each shale. For example western samples of Middle and Upper Pennsylvanian black shales average 1210 and 1400 ppm Zn, respectively (Table 4), not far from ALLEN'S average for Mecca equivalents. Upper Pennsylvanian shales contain less Mo than Middle Pennsylvanian

361

shales (Table 4), but this may be attributed to the greater exposure of the Upper Pennsylvanian shales to erosion on the edges of basins where they are likely to have been more enriched in Mo, U, V and Se. (2) Waters of the Pennsylvanian epeiric seas may have differed in composition from modern-day seawater, perhaps because of inherited primordial compositions or because of extensive weathering that took place on land during the Carboniferous. Local differences are also possible. For example, coals underlying the black shales rest on underclays from which metals were leached during the Pennsylvanian (Fig. 7a). Release of metals from these extensive underclays may have caused locally elevated values for metals in epeiric seas, provided that no immediate precipitation or fixation intervened. However, given that underclays are only slightly thicker than black shales it is unlikely that they would be adequate to explain the elevated values for metals in Pennsylvanian shales. The shales display enrichments of hundreds to thousands of clarkes for some constituents (see Table 4), hence a considerably larger source of metals than these relatively thin paleosols is clearly needed to explain the bulk of the metals. Perhaps a very thick section of weathering is involved but data do not exist to test this possibility. (3) Postdepositional processes, for example epigenetic hydrothermal activity, may have affected shales after lithification. COVENEYet al. (1987b) have proposed that migrating basinal brines may have added metals long after sedimentation (Fig. 7B). Supergene processes may also have been important in some cases (Fig. 6C). (4) Hydrothermal processes may have introduced metals to newly deposited muds or to the epeiric seas during shale deposition (Fig. 7A). It is even possible that submarine springs introduced metals to the Pennsylvanian seas. Although the exact time of deposition is unknown it is widely believed that the main Mississippi Valley-type P b - Z n ores of the midwestern U.S. formed during the Late Paleozoic, perhaps as early as Pennsylvanian time. These ores may have formed in response to the OuachitaAppalachian Orogeny (LEACh and ROWAN, 1986; OLIVER, 1986) and leakage of MVT mineralizing fluids into late Paleozoic epeiric seas may have added metals now found in the Pennsylvanian shales. Hydrothermal mineralization

HATCH et al. (1976) inferred that migrating basinal brines introduced widespread epigenetic sphalerite mineralization found in coals of Illinois. CoBB (1981) subsequently found that inclusions from coal-hosted sphalerite contain highly saline fluids and were deposited at temperatures similar to those typical of MVT ore districts. BETHKE (1986) has suggested that basinal brines migrated northward through the Illinois Basin to deposit P b - Z n ores in lower Paleozoic host beds of the Upper Mississippi Valley mining district.

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Raymond M. Coveney, Jr and Michael D. Glascock

GOEBEL (1968) has noted the widespread occurrence of minor amounts of sulfide minerals in carbonates of the midwestern U.S. Where sphalerite samples were available from these and other samples, fluid inclusion data reflect what was once a virtually ubiquitous occurrence of hydrothermal (75-120°C) brines in Paleozoic strata (e.g. CoBa, 1981; LEACH and ROWAN, 1986; COVENEY et al., 1987a). These data indicate a widespread heating event reflecting country rock temperatures >75°C throughout the midwestern U.S. Moreover, as noted above, there is direct fluid inclusion evidence that warm (+75°C) highly saline (10-23 wt % equiv. NaCI) fluids bathed Pennsylvanian shales of Illinois. Anomalous concentrations of Mo in acid residues from lower Paleozoic carbonate strata of Missouri and Illinois also may reflect hydrothermal brines (ERIcKSON et al., 1981: 1987). The highest values yet observed for Zn in Pennsylvanian shales, up to 1 wt % (Table 2), are found in Mecca Shale at Lowell, Illinois. This location is on the LaSalte anticline where the shale is structurally disturbed, dipping ~20 ° to the west. At this same location, the overlying Excello Shale (Fig. 1) contains 0.7 wt % Zn. Such a close spatial association of two of the most Zn-rich shale sections found fo date in the Midwestern U.S. suggests either that epigcnetic agents have locally introduced appreciable Zn or that persistent submarine hot springs locally supplied metals to bottom waters or unconsolidated sediments. The difference between these two possibilities is one of timing and either may involve input from basinal brines. Tentatively, the possibility of syngenctic deposition of Zn in shale from metal-laden diagenetic fluids or submarine hot springs is favored given the evidence of early precipitation o f s o m e sphalerite. Even enriched values of Mo and sporadic high values for Mn (sample Kb, Table 2) at the edge of the black shale basin (FORCE and CANNON, 1988) could be reconciled with an origin involving cooling hydrothermal plumes which supplied and fed metals to epeiric seas ultimately reaching the anoxic-oxic boundary (Fig. 7A). However, attractive as this alternative may be it cannot be proved given the present limited data. Particularly lacking are radiometric dates either for mineralization in shales or for MVT Pb-Zn mining districts from which mineralizing fluids may have leaked. And when such data materialize they may indicate an epigenetic origin or could even rule out any genetic relation. Supergene mineralization

Secondary enrichment has been inferred for Ag in shales from Nevada (DESBOROtJGH, pers. comm., 1981) and for Mo, V, Se and even Zn at the Gibellini deposit in Nevada (BoHLRE et al., 1981). HATCHand LEVENTHAI, (1985) proposed the possibility of supergenc enrichment during the Pennsylvanian.

COVENEY et al. (1987b) suggested the influence of supergene processes on Pennsylvanian shales. For example, Mo occurring on and near the surfaces of framboids in Mecca Shale in Indiana may stem from modern-day supergene enrichment although Mo may have been fixed quasi-syngenetically by the same mechanism, consistent with the view of HATCH and

LEVENTHAL(1985). To this day Mecca Shale retains a capacity to fix Mo from aqueous solutions. For example, after only a few hours at room temperature, shale powders arc able to adsorb thousands of ppm Mo from dilute aqueous solntions containing as little as 0.01 M MoO~ (COVENEY et al., 1987b). The mechanism for fixation of Mo is uncertain. Direct proxying of Mo for Fe at the surfaces of framboids is possible, but so is adsorption by organic matte> Near the Black Hills. in Wyoming and South Dakota, beds in the Minnelusa Formation are enriched in U and Mo to the highest values known for Pennsylvanian black shales. The Mo values are corn-parable to those reported from the Mississippian Heath Shale in Montana by DESBOROUGHand Poot, E (1983). The Minnclusa shales are associated with crossbeddcd sandstones, dolostones, limestones and evaporites, a situation which is consistent with a shallow water sabkha-type environment during sedimentation (DESMONDet al., 1984: A. R. RENFRO, pers. comm., t9871. The possibility of syngenetic mineralization of shales in a nearshore environment is consistent with available evidence. There are other factors, however, which complicate the situation. For example, near Hot Springs, South Dakota, in one Minnelusa shale which has been prospected for U, high values for metals (even for Zn), decline substantially only 111 m into an adit heading where the shale is fresher (Table 5). This suggests that such extreme enrichments may result from supergene processes, Reinforcing such inferences is the proximity of the nearby Edgemont {.J mining district where mineralization occurs chiefly m Cretaceous beds. ()~rrT cta/. (1974) have shown that U mineralization in the district may have resulted from artesian flow of groundwater carrying up to - 17 ppb U through breccia pipes and karst features m Paleozoic beds, including the Minnelusa and the underlying Paha Sapa Limestone. Although deposits of economic interest appear to be confined to Cretaceous beds where U-laden fluids encountered reducing conditions, U precipitation probably also occurred (and may still be taking place) in Paleozoic beds where reducing conditions exist (e.g. in and near black shales) BACK el al. (1983) and BAKALOWlCZ et al. 11987) have documented recent and ongoing large scale flow of groundwater in Carboniferous beds of the Black Hills. Flow of groundwater in the immediate vicinity of strata containing metal-rich shales is reflected by the presence of numerous vugs (Fig. 4) lined with carbonate crystals. When taken as a whole, evidence suggests that the

Review of origins of metal-rich Pennsylvanian black shales extremely enriched values found in Minnelusa shales near Hot Springs result from secondary fluids (J. D. VINE, pers. comm., 1984). THE PRESENT GENETIC HYPOTHESIS SUMMARIZED

VINE and TOURTELOT (1970) inferred that organicrich shales can accumulate metals from various solutions, including seawater, connate waters and other formational fluids, such as circulating basinal brines. Considering the widespread nature of the Pennsylvanian shales and the diversity of geological environments across the midwestern U.S. it seems likely that metals contained by the beds have complex origins. Provided that a special source for metals exists, the one-stage syngenetic model of metal deposition of COVENEY and MARTIN(1983) may apply in many cases besides the vicinity of Mecca, Indiana. Deposition of Mo and other roll-front suite metals near shore in acid waters is consistent with several paleogeographic features (nearby peat swamps) and mineralogic features (early marcasite). Acid surficial fluids draining from peat swamps and an ancient ocean composition different from that of today may have contributed to mineralization but are not viewed as the principal sources of metals. Most likely Mo and other metals came chiefly from hydrothermally charged diagenetic fluids or submarine hydrothermal springs fed by basinal brines. In order to account for very abundant sphalerite at several locations, exemplified by Lowell, Illinois, a role for basinal brines seems likely. This interpretation is consistent with existing fluid inclusion data and the presence of epigenetic sphalerite veins in nearby coals (HATCH et al., 1976). As mentioned above, the impervious nature of most shales to solution activity has little relevance here because the black shale beds are sufficiently jointed to behave as aquifers (O'CoNNOR, 1971). Moreover, the extreme thinness (<0.5 m) of the beds would tend to facilitate both metal exchange by bulk material transfer and diffusion. Nevertheless, there is a distinct possibility that metals were added to shales syngenetically from hydrothermal fluids expelled from basins. Early introduction of sphalerite from such sources is a particularly likely possibility in this regard. However, even Mo and other roll-front elements may have been introduced by submarine hot springs leading to enriched values in bottom waters and associated sediments. It is likely that appreciable metal values, especially for Mo and U, were added later from meteoric waters in some cases (CovENEV et al., 1987b) as typified by the locality in South Dakota. The extreme thinness of the beds, in this case only 10-20 cm, would facilitate exchange of metals with fluids. The experimentally demonstrated ability of at least one shale (Mecca) to fix Mo in a few hours from solutions (CoVENEV et al., 1987b) suggests t~e feasibility of continued accumulation of metals for - 3 0 0 Ma to the present day. AG 4 : 4 ° B

363

Similarly, on different lines of evidence, TOURTELOT (1979) noted that some black shales might be "receptor beds" for metals carried by fluids of secondary origin. GENERAL SIGNIFICANCE OF METAL-RICH PENNSYLVANIAN SHALES

Organic-rich shales containing >300 ppm Mo are common in the rock record (HoLLANO, 1984). The extreme values for Mo in Mecca-type shales are less common. Nevertheless, high values for Mo (~>1000 ppm) and other roll front-suite elements as well as Zn are not unique to the midcontinent Pennsylvanian, but also have been reported from Mississippian shales of the western U.S.A. (DESBOROUCH and POOLE, 1983; DERKEY et al., 1985) and from the Permian Kupferschiefer (WEDEPOHL, 1964). Lesser but still substantial amounts of roll-front metals are generally recorded for the New Albany and Chattanooga shales (SHAFFERet al., 198t; LEVEr~THALet al., 1983). In the Black Sea, black muds which were formed within only 2-4 ka in the early Holocene (CALVERT et al., 1987) are Mo-rich containing up to 50 ppm Mo (PILIPCHUK and VOLKOV, 1974). Unlike the Mecca the most enriched values are restricted to the centers of abyssal basins rather than near shore. Nevertheless, such examples suggest that the proposed interactions between shales and fluids of diverse origins may pertain to other settings besides the Pennsylvanian. Perhaps thin, organic-rich beds in other areas besides the midwestern U.S. have derived metals from connate fluids, circulating basinal brines and modern groundwaters as well as from seas in which they were originally deposited. Such inferences may be of value in understanding the chemical evolution of sedimentary basins in general. To the extent that such shale fluid interactions have influenced rock compositions, complementary effects on fluids interacting with shales are likely to have been important in controlling fluid chemistry. The degree of influence would, of course, depend upon fluid-to-rock ratios applicable in each individual case, but could be substantial. In rock-dominated systems trace metal solutes are likely to have been controlled by interactions with organics or previously formed Fe-disulfides. In some cases (e.g. Mo) significant interactions causing precipitation (rather than dispersion) of metals may occur in supergene environments. Thus rather than behaving as point sources of metal pollution, some black shales may actually remove heavy metals from groundwaters. REMAINING PROBLEMS

Because this is a summary paper, it is appropriate to comment briefly on remaining problems concerning Pennsylvanian shales. More defnitive data on timing of mineralization are sorely needed--ideally

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Raymond M. Coveney, Jr and Michael D. Glascock

radiometric dates, although additional textural evidence would also be helpful. The general hypothesis of metal origins in shales presented here is testable. Most critical to the hypothesis would be more direct evidence concerning the possible role of hydrothermal vents in mineralization. Bona fide "Black Smokers" have yet to be found, although there is an immense area of poor outcrop to search. It may be noted that in western Missouri Gentile (1979) has found barite in vertical clam borings in a Virgilian limestone near the black metal-rich Eudora shale member, which may represent hydrothermal venting. Possible genetic connections between Pennsylvanian shales and MVT ores need to be resolved. Suggestions have been made that Pennsylvanian shales were source beds of Pb and Zn for MVT ores (COVENEY, 1979; LONG and ANGINO, 1982). With synthetic (90°C) chloride brines tens to hundreds of ppm Pb and Zn can be mobilized (LON(; and ANGINO, 1982). This seems to conflict with metal-supply inferences that the shales have been geochemical sinks for metals such as Zn and with data for fluid inclusions in sphalerite implying that some ZnS was deposited (rather than leached) by basinal brines. Yet, possible instances of epigenetic sphalerite, such as near Lowell. Illinois may only be local. It is also conceivable that despite long-term accumulation of Zn, under special conditions, such as those in LONG and ANGINO'S (1982) experiments, metals were released from shale source beds. Given what is known of timing it is conceivable that metals in shales were derived syngenetieally from MVT ore fluids leaking into Pennsylvanian seas. The shales were deposited in the same basins and perhaps at the same time as the formation of some of the world's largest MVT ore deposits and a broad thermal anomaly (e.g. COVENEYand GOEBEL, 1983) which may reflect the passage of the midwestern U.S. over a mantle hot spot (CovENEYetal., 1987a). The choice among these possibilities (and others) awaits better resolution of dates for formation of MVT ores and mineralization of shales. Aside from any possible connections with MVT ores, it is not known whether, or to what extent, metal distributions are affected by proximity to major structures in the midcontinent, although this is an important question bearing on resource potential. The highest Zn values known for midcontinent Pennsylvanian shales ( ~ l wt %) are, however, associated with a tectonic feature of regional significance, the north-trending LaSalle anticline belt which passes through Lowell (Fig. 7A). Major values for metals (perhaps even Cu or precious metals) might occur near the Humboldt fault (Fig. 7A) in Kansas where the Pennsylvanian is hidden beneath thick Permian beds. On the other hand the distribution of Mn on the fringes of Pennsylvanianblack shale basins warrant more comprehensive study.

The relative significance of supergene processes in the precipitation of metals is uncertain, but has practical as well as theoretical importance, particularly respecting the possible significance of shales as point sources of heavy element pollution. In at least some cases (e.g. Hot Springs, South Dakota) it is inferred that U, Mo, V and Se (all of which occur in low temperature solutions as oxycomplexes) are probably now being precipitated within receptor shale beds rather than being dispersed. This process could be occurring pervasively wherever the shales crop out and some supergene enrichment may have taken place during the Pennsylvanian (HATCH and LEVENtriAl_, 1985). If it is taking place modern day fixation may be detectable, for example by U-series dating. Assuming that fixation of metals is occurring in the shales at the present time it is possible that shales are behaving as metal sinks rather than pollution sources. Available data show that Pennsylvanian black shales yield only relatively small quantities of Rn, perhaps because the gas is mainly retained by clays or organics until it decays (HILPMANel al., 1988). The age relations between shales in the Minnelusa and the major stratigraphic series boundaries need to be clarified. The shales may be of Upper Pennsylvanian and could be the first examples of Mecca-type shales above the Desmoinesian. It remains to be established whether or not any Mecca-type shales occur in the Upper Pennsylvanian (they should if the working hypothesis is correct). The contention that all Mecca-type shales contain chiefly terrestrial organic matter relies heavily on data from the Mecca and Excello shales. Further organic geochemical studies are needed on other beds in order to test this conclusion. Additional studies of S isotopes are desirable, especially to gain additional information on Upper Pennsylwmian beds which seem to have less variable and lighter h34S values than Middle Pennsylvanian black shales. Subjects which lead beyond the confines of black shales, but which have been raised here, are the need for independent tests of the metal contents of Pennsylvanian seas and more precise information on the absolute ages and rates of accumulation of cyclothems would be helpful in refining estimates of times needed to accumulate metals from seawater. An enduring questkm concerns the economic importance of the beds. Most past efforts have been of a reconnaissance nature and, without notable success, have focused on hydrocarbon potential. The best hope for metal recovery would seem to lie in the subsurface near major structures such as the LaSatle anticline and the Humboldt fault (Fig. 7A). The likelihood of economic benefit may seem small, but there is a vast terrane to search and as TOURTELOT (1979) has noted one of the most important shaleassociated ore bodies (Rammelsberg) occupies less than 1 km 2.

Review of origins of metal-rich Pennsylvanian black shales

365

CONCLUSIONS

after sedimentation at least in some cases. COVENEYet al. (1987b) suggested that metal enrichments in

In Indiana four thin Middle Pennsylvanian black shales contain ~>1000-2000 ppm Mo, Zn and V. Similar values occur in the Mecca Shale of Kentucky and in other Mecca-type shales (shales which were deposited in near-shore, in shallow waters and which are extremely enriched in terrestrial-type organic matter and Mo). Mecca-type shales appear to be confined tO the Middle Pennsylvanian, but this is uncertain. Enriched contents of Mo seem to reflect an acidic shoreline environment in Indiana (CovENEY and MARTIN, 1983), South Dakota, Kentucky and possibly Iowa. Concentrations of U, Cd and Se in Mecca-type shales exceed 100-200 ppm with a peak value of 700 ppm U being found in the Holland Shale of Indiana. Even larger amounts of Mo and U occur in Minnelusa shales of South Dakota. At least for Mecca Shale equivalents, as lithologic characteristics change from Mecca-type to those of Heebner-type shales, values for Mo, Se, U and V decline southwestward in samples from Illinois, Missouri, Kansas and Oklahoma. Heebner-type shales predominate in the Upper Pennsylvanian, where the Hushpuckney, Stark, Muncie Creek and Heebner shales serve as examples of metal-rich phosphatic beds. Such shales typically contain <50-250 ppm Mo, much less than Mecca-type shales. Zonal relations between metal values, organic matter type and paleogeography imply syngenetic mineralization of the Mecca Shale and its equivalents. Conservative metal flux calculations suggest that up to 1.7 Ma would have been necessary to derive metals from normal seawater even though it is likely that <0.4 Ma elapsed during deposition of each host cyclo~em. These calculations are based on the assumptions that Pennsylvanian seas had compositions similar to the modern day ocean, that about the same amount of time was available for deposition of each main Pennsylvanian cyclothem (and shale), and that each shale contains about the same amount of metals. It should be stressed that the estimates of feasibility of syngenesis from seawater also depend on imprecise (+-6--12 Ma) radiometric dates for the durations of the Desmoinesian, Missourian and Virgilian when the metal-rich shales were formed. Nevertheless, calculations were done conservatively and assum~ that perfectly efficient fixation occurred. The results suggest a need for an enriched source of metals. In the absence of other viable possibilities, the shales are mainly thought to have derived metals essentially syngenetically (including during early diagenesis) from basinal brines. These may have been the :same fluids which many believe were responsible for P b - Z n ores of the Mississippi Valley. This possibility fits with evidence fJr early deposition of some sphalerite, with fluid inclusion results, and with known temporal and spatial relations. It is likely that further additions of metals occurred

Mecca Shale formed from a complex series of interactions between shale constituents (including organic matter) and successively, seawater, connate fluids, migrating basinal brines and meteoric waters. Association of Mo with pyrite in weathered shale (COvENEY et al., 1987b) suggests a possible role for supergene enrichment but the timing is ambiguous. Supergene enrichment for Mo, U, V and possibly Z n seems very likely for Minnelusa shales in South Dakota. Just as shales are thought to have picked up metals from various fluids, solutions with which they were in contact have lost heavy metals by shale-fluid interactions. Pennsylvanianshales may have acted as source beds for MVT ores, but for most of their history it is likely that they were sinks for metals and especially for Zn and U roll front constituents. Acknowledgements--This research was mainly supported

by U.S. NSF Grant EAR 82-18742, the Weldon Spring Endowment Fund of the University of Missouri and a University of Kansas City Trustees Faculty Fellowship. The study was conducted in cooperation with UNESCO's International Geological Correlations Program Project 254, entitled "Metalliferous Black Shales". James D. Vine and Rainer Zangerl were generous with time and advice in the field in the Black Hills and Indiana, respectively. Discussions with R. G. Blair, A. C. Brown, E. D. Goebel, J. R. Hatch, J. S. Leventhal, A. R. Renfro, D. R. Sangster and K. L. Shelton were extremely helpful. R. G. Blair and J. D. Vine provided some data for Minnelusa shales. A. V. Allen, J. C. Blankenship, P. Jancich and S. P. Martin assisted with other analyses. Personnel from the state geological surveys of Iowa, Illinois, Kansas, Kentucky, Missouri and Oklahoma aided with sampling. This paper benefited from readings by E. D. Goebel, C. R. Maples and W. L. Watney and from thorough formal reviews by J. S. Leventhal and D. T. Long. REFERENCES

ALLEN A. V. (1986) Geochemistry of the Mecca Quarry shale and Colchester Coal, Linton Formation (Middle Pennsylvanian), in Parke and Vermillion counties, Indiana. M.S. Thesis, University of Missouri--Kansas City. ALVAREZL. W., ALVAREZW., ASAROF. and MICHELH. M. (1980) Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science 208, 1095-1108. ANGINOE. E. (1964) Trace elements and cyclicdeposition. In Symposium on Cyclic Sedimentation (ed. D. F. MERRIAM)pp. 21-30. Kansas Geol. Surv. Bull. 169. BACKW., HANSHAWB. B., PLUMMERL. N., RAHNP. H., RIGHTMIREC. T. and RuBINM. (1983) Process and rate of dedolomitization: mass transfer and 14C dating in a regional carbonate aquifer. Geol. Soc. Am. Bull. 94, 14151429. BAKALOWICZM. J., FORDD. C., MILLERT. E., PALMERA. N. and PALMERM. V. (1987) Thermal genesis of dissolution caves in the Black Hills, South Dakota. Geol. Soc. Am. Bull. 99,729-738. BERTINE K. K. (1972) The deposition of molybdenum in anoxic waters. Mar. Chem. 1, 43-53. BERTINEK. K. and TUREKIANK. K. (1973) Molybdenum in marine deposits. Geochim. cosmochim. Acta 37, 14151434. BETHKEC. M. (1986) Hydrologic constraints on the genesis

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