Geochemical constraints on the origin of calcite veins and associated limestone alteration, Ordovician Viola Group, Arbuckle Mountains, Oklahoma, U.S.A.

Chemical Geology, 98 (1992) 257-269

257

Elsevier Science Publishers B.V., Amsterdam [41

Geochemical constraints on the origin of calcite veins and associated limestone alteration, Ordovician Viola Group, Arbuckle Mountains, Oklahoma, U.S.A. Guoqiu Gao a'~, R.D. Elmore b and L.S. Land a "Department of Geological Sciences, The University of T2,xas at Austin, Austin, TX 78713, USA bSchool of Geology and Geophysics. The University of Oklahoma, Norman. OK 73019, USA (Received June 21, 1991 ; revised and accepted December 13, 1991 )

ABSTRACT Gao, G.. Elmore, R.D. and Land, kS., 1992. Geochemical constraints on the origin of calcite veins and associated limestone alteration, Ordovician Viola Group, Arbuckle Mountains, Oklahoma, U.S.A. Chem. Geol., 98: 257-269. Isotopic and elemental analyses constrain the origins of two types of calcite veins and associated limestone alteration in the Ordovician Viola Group, Arbuckle Mountains, Oklahoma, U.S.A. Calcite from small, millimeter-wide veins has similar STSr/86Sr ratios (0.70781-0.70792) and O13C-values ( - 1.4 to +1.6°/oo, PDB) to host Viola limestone (87Sr/86Sr: 0.70779-0.70794; 3~3C: - 2 . 2 to + 1.7%o, PDB), indicating that the fluids from which the vein calcite precipitated were chemically buffered by the host limestone. The low ~ ~80-values ( - 8.2 to - 5.5%0, PDB ) of the vein calcite, relative to the host limestone ( - 5 . 0 to -3.1%o, PDB), suggest that the vein calcite probably precipitated from ~80-depleted meteoric water. Calcite from large veins (5-50 cm wide) is characterized by radiogenic 87Sr/86Sr ratios (0.70887-0.70915 ) and low 3~3C-values ( - 8 . 0 to -2.5%0, PDB ), relative to host Viola limestone. Such isotopic compositions demonstrate that the fluids responsible for the formation of the large veins were enriched in 87Sr and depleted in 1~C, relative to the host limestone. The ~80-values ( - 4 . 8 to -3.7°/00, PDB ), of calcite from the large veins, coupled with the presence of hot (5090 C ) and saline fluid inclusions, constrain the formation of the vein calcite to ~80-enriched (Or sO > + 2%0, SMOW) fluids. On this basis, calcite from the large veins is believed to have precipitated from basin-derived fluids. Adjacent to the large veins, alteration zones up to 0.5 m wide have developed in host limestone. Relative to other limestones, the altered limestones display radiogenic 87Sr/86Sr ratios (0.70792-0.70880), low o m3C-values ( - 8 . 2 to -2.0°/00, PDB), and low Sr concentrations. The similar isotopic compositions of the altered limestone to those of the large veins suggest that alteration of limestones was also caused by basinal fluids.

1. Introduction

The occurrence of calcite as fracture-fillings is a common geological phenomenon. The origin of fracture-filling calcite, or vein calcite, can provide important information about the source and composition of the fluids which Correspondence to: G. G a o ' , D e p a r t m e n t of Geological Sciences, The University of Texas at Austin, Austin, TX 78713, USA. 'Initially from: Department of Geology, Central-South University of Technology, Changsha, Hunan, People's Republic of China.

0009-2541/92/$05.00

migrated through/into fractures in a rock body (Dietrich et al., 1983; Barker and Halley, 1986; Rye and Bradbury, 1988; Avigour et al., 1990; Szabo and Kyser, 1990; Stuckless et al., 1991 ). In addition, the formation of vein calcite is commonly associated with other geological processes such as tectonic deformation, hydrocarbon migration, base-metal mineralization, carbonate diagenesis and remagnetization (Kessen et al., 1981; Burruss et al., 1985; Mimran, 1985; Morrison and Parry, 1986; Jensenius, 1987; Clauer et al., 1989; Egeberg and Saigal, 1991; Elmore and McCabe, 1991;

© 1992 Elsevier Science Publishers B.V. All rights reserved.

258

G. GAO

OKLAHOMA

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I

34o 3O, N

8 km 97°15'W

ET AL

deposited, the Viola was exposed and underwent meteoric diagenesis (Grammer, 1983). The Viola Group was deformed (folded and faulted) and uplifted during late Pennsylvanian and Permian time accompanying dismemberment of the Southern Oklahoma Au(Johnson et al., 1988). At the Sooner Rock and Sand (SRS) quarry (Fig. 2), the folded Viola hosts two types of calcite veins. Small veins, occurring mainly on the west side of the quarry, consist of calcite crystals which fill millimeter-wide fractures (Fig. 3 ). Small veins of this type are up to 1 m long. The veins show no orientation relative to bedding or fold axes, and the small veins are not associated with any mineralization. No vein-related alteration features were observed

1-35

I W B e d d i n g Plane

Fig. 1. Location map of the study area• Samples for this study were collected from the Sooner Rock and Sand (SRS) quarry (A) and a roadcut on U.S. Interstate Highway 35 (B).

and many others ). Investigations of the origin of vein calcite can thus constrain related geological processes. This study focuses on calcite veins from the Ordovician Viola Group, Arbuckle Mountains, south-central Oklahoma, U.S.A., where two types of calcite vein occur and where one of them is associated with limestone alteration (Fig. 1 ). We present isotopic (Sr, C and O) and elemental analyses of the calcite veins and associated limestone, and address the possible origin of the calcite veins and associated limestone alteration.

l.mge veins j

Fig. 2. Sketch cross-section of the SRS quarry, showing the folded and faulted natt~re of Viola limestone (for clarity, only two bedding planes are drawn ).

\

Calcite Veins

2. Geological background The Viola Group is a lime mud-dominant limestone sequence that was deposited during late Ordovician time on a carbonate ramp within and peripheral to the Southern Oklahoma Aulacogen (Galvin, 1982). Before the overlying upper Ordovician Sylvan Shale was

1"i ,

2 cm

~.~.~__~../

Fig. 3. A hand specimen sketch of small veins•

ORIGIN OF CALCITE VEINS AND ASSOCIATED LIMESTONE ALTERATION, VIOLA GROUP, ARBUCKLE MOUNTAINS

petrographically in limestones which host the small veins. On the east side of the quarry, large fractures (5-50 cm wide), parallel to the axial plane of a fold, were developed as a result of late Paleozoic deformation (Fig. 2; Johnson et al., 1988 ). These fractures are now filled with calcite crystals, forming large veins. The large veins are tens of meters long at the quarry and extend into the subsurface. Most veins of this type consist of earlier basal calcite crystals growing on the host limestone and later overgrowth rhombs growing on the basal calcite crystals. Two types of large vein have been distinguished: one associated with asphalt (type A) and another without asphalt (type B). Although these two types of large veins are closely associated, no cross-cutting relationships have been found. The two types of large veins have different mineral associations: veins associated with asphalt have sphalerite and pyrite druses on calcite surfaces, whereas veins without asphalt have marcasite and goethite inclusions along growth surfaces. Associated with the large calcite veins, alteration zones 0.1-0.5 m wide are developed in limestones on both sides of the calcite-filled fractures. In the field, these alteration zones are composed of grey to reddish limestone. Some vein calcite crystals apparently grow into the altered limestone. The alteration around the large calcite veins obscures identification of the small calcite veins. As a result, the relationship between the small veins and the large veins at the SRS quarry is not clear. In the altered limestone, fine-grained sulfide and oxide minerals, similar to phases associated with the large veins, are found through scanning electron microscopy (SEM) examination. Petrographic examination also reveals that altered limestones are more coarsely crystalline than unaltered limestones from the same beds, indicating that limestones in the alterations zones underwent textural alteration (recrystallization ).

259

3. Methods Calcite vein samples were collected at the Sooner Rock and Sand (SRS) quarry (Fig. 1 ). Limestone was sampled at both the SRS quarry and the roadcut on U.S. Interstate Highway 35 where the Viola limestone is well exposed (Fig. 1). For chemical analysis, vein calcite was crushed to ~ 1 mm, then clean, fresh samples were hand-picked and powdered. For limestone samples, lime m u d matrix was sampled and visible cements and fossils were avoided. Limestone sample powders were obtained from fresh slabs by using a bench-top drill press with dental drill bits. All sample powders were Xrayed to ensure mineralogical purity prior to chemical analysis. All analyses were carried out by the senior author at the Department of Geological Sciences, University of Texas at Austin. Elemental concentrations of samples were analyzed by inductively coupled (argon) plasma atomic emission spectroscopy (ICPAES). Samples for ICP analyses were dissolved in 0.6 N H C I and sample solutions were filtered through 0.22-pro filters. Accuracy of the ICP analysis was monitored by repeated analyses of standard NBS 88b, and replicate analyses of the standard and unknowns indicate a precision of better than + 5%. Sample powders for Sr isotopic analyses were washed in 0.2 N reagent-grade a m m o n i u m acetate three times and then rinsed in ultrapure water (D3 ® ) three times. The cleaned samples were dissolved in 1.4 N acetic acid and insoluble residues were removed by centrifugation. Sr for isotopic analysis was isolated and concentrated through cation-exchange columns by using 2 N HC1. Sr isotopic analysis was performed on a MAT 261 ® mass spectrometer. Fractionation in the mass spectrometer was corrected using 86Sr/88Sr=0.1194. Standard NBS 987 was routinely analyzed and yielded 0.71022+0.00003 (la, n = 1 0 ) during the course of this study. Replicate analyses of unknowns demonstrate a precision within

Sample ID

V-I V-3 duplicate V-5 V-7 V-9 V-IO C-2 C-1 D-5 D-4 A B-1 G-1 H-I H-0 H-3 H-5 V-2 G F2 air. F2 B-5 B-2 red B-2 gray B-4 Aalt. E-5 FIA Vio.1 Vio.2 Vio.3 Vio.4 H-2v

Lithology

Limestone, 1-35 roadcut

Limestone, SRS quarry

Altered limestone

Small vein

99.25 99.38 99.40 99.50 99.01

99.98 99.98 99.94 99.57 99.95 99.94 99.68 99.86 99.99 99.98 -

0.009 b.d. 0.026 0.027 0.003 0.028

0.36 0.70 0.03 1.25 1.15 0.47

0.73 0.46 0.50 0.36 0.91

b.d. 0.016 0.020 0.055 0.031

0.007 0.001 b.d. 0.025 0.030 0.043 0.015 b.d. b.d. 0.013 -

0.004

0.32

b.d. b.d. 0.04 0.31 b.d. b.d. 0.21 0.09 b.d. b.d. -

b.d.

0.078 0.016 0.002 0.036

b.d. 0.006

FeCO3 (mole%)

0.65

1.24 0.98 0.80 0.83

98.33 98.93 99.08 98.91 99.31 99.62 99.44 99.21 99.84 98.54 98.73 99.37

1.18 0.83

MgCO3 (mole%)

98.68 98.95

CaCO 3 (mole%)

b.d. b.d. 0.008 b.d. 0.005

b.d. b.d. b.d. 0.045 b.d. b.d. 0.034 0.004 b.d. b.d.

b.d. b.d.

0.004 0.024 0.049 0.013

0.004

b.d.

0.001 0.024 0.001 b.d.

0.006 b.d.

MnCO 3 (mole%)

The elemental and isotopic compositions of Viola limestone and vein calcites

TABLE 1

147.0 1,197.0 587.0 806.0 328.0

78.6 170.6 226.9 475.0 174.9 516.0 417.0 83.7 37.6 -

351.0 432.0 1,666.0 563.0 480.0 1,482.0 975.0 1,097.0

3,030.0 480.0 1,005.0 1,979.0

1,145.0 1,912.0

Sr (ppm)

0.707805_+0.000013 0.707900_+0.000013 0.707784_+0.000013 0.707921 +- 0.000010 0.707842_+0.000015

0.708644_+0.000013 0.708804_+0.000013 0.707920_+0.000016

0.708673+-0.000013 0.708299-+0.000013 0.708110+-0.000013 0.707945+_0.000015 0.708363+_0.000012 0.708408+_0.000015 0.707987+0.000012

0.707841 _+0.000017 0.707764+0.000012 0.707953+-0.000015 0.707801 +_0.000012 0.707789-+0.000009 0.707788+_0.000012 0.707816_+0.000014

0.707818_+0.000015 0.707885+-0.000013 0.707935_+0.000010

0.707928_+0.000013 0.707943 + 0.000012 0.707914+_0.000012 0.707845+_0.000013 0.707870_+0.000012 0.707794+_0.000013 0.707816_+0.000012

STSr/S~Sr ± 2c~

-0.42 +0.64 + 1.60 - - 1.35 +0.18

-6.68 -5.57 -5.78 -2.35 -5.57 -5.66 -1.97 -2.45 -6.30 -8.22 -2.75

+0.34 -0.03 - 1.29 -0.68 +0.15 -0.04 -1.08 -0.24 +1.37 - 1.07

+1.16 + 1.59 + 1.63 +0.90 -0.84 +0.10 + 1.72

~ ' 3C (°/0ors. PDB)

-5.48 -6.57 -7.38 -8.17 -6.98

-4.75 -4.62 -4.56 -4.21 -4.61 -4.73 -4.96 -5.01 -4.69 -4.33 -4.59

-3.88 -4.83 -4.72 -4.98 -4.75 -3.68 -4.72 -3.59 -3.62 -4.25

-3.06 -3.61 -3.51 -4.12 -3.61 -3.78 -4.58

c~~sO (%o vs. PDB)

o "~ r" >

99.99 99.98 99.08 99.50 99.87 99.77 99.91 99.71 99.10 99.21 99.28 99.28 99.48 99.34 99.37 99.19

DIO EOO FIAO duplicate G10 V20 duplicate V30

99.31 99.61 99.87 99.60 99.46

A-O AAO.F2AO F2BO V10 D1B EOB BIB BOB F1AB GIB V2B V3B

99.61 99.59 99.58 99.89 99.99 99.57

A-b duplicate AAB F2AB F2BB VIB

b.d. = below detection limit; - = no data.

Large vein, type B

Large vein. type A

0.002 0.020 0.180 0.097 0.075 0.100 b.d. 0.220 0.263 0.527 0.469 0.470 0.301 0.371 0.358 0.517

0.41 0.08 0.05 0.06 b.d. 0.05 0.05 0.07

0.092 0.009 0.036 0.199 0.069

b.d. b.d. b.d. 0.040 b.d. 0.004

b.d. b.d. 0.39 0.34 b.d. b.d. 0.09 b.d.

0.42 0.35 0.05 b.d. 0.39

0.39 0.41 0.42 0.07 b.d. 0.42

Basal calcite: 9.8 0.709005_+0.000009 b.d. 10.1 0.709042+-0.000016 b.d. 41.5 0.708887+_0.000012 0.354 21.0 0.708951 _+0.000016 0.053 9.4 0.708868 + 0.000011 0.052 16.1 0.708926+-0.000013 0.124 14.5 0.708967+-0.000015 b.d. 19.7 0.709088+-0.000013 0.068 Overgrowth rhomb: 35.8 0.709102+0.000011 0.231 48.1 0.709132+_0.000012 0.173 42.3 0.709104_+0.000015 0.198 40.3 0.709076_+0.000012 0.191 32.5 0.708995_+0.000011 0.220 73.7 0.708892+_0.000014 0.231 71.9 0.229 43.5 0.709108_+0.000015 0.219

•Basal calcite: 17.6 0.708987+_0.000014 b.d. 17.3 0.708957+-0.000014 b.d. 14.8 0.708932_+0.000012 b.d. 9.2 0.708891 +-0.000010 b.d. 7.5 0.708931+0.000018 b.d. 17.7 0.708903_+0.000015 0.006 Overgrowth rhomb: 19.5 0.708932+_0.000012 0.174 17.6 0.708989_+0.000018 0.037 11.4 0.708924+-0.000016 0.046 21.9 0.709150_+0.000011 0.202 21.8 0.708963_+0.000014 0.081

4.20

-4.23 -4.31

-

-3.84 -4.54 -4.22 -4.39 -4.01 -4.16 -2.50 -3.12 -3.22 -3.44 -6.50 -6.45 -3.01

-6.46 -7.04 -6.65 -4.96 -6.02

-4.20 -4.19 -3.71 -4.01 -4.21 -4.11 -4.48 -4.21

-3.94 -4.37 -4.45 -3.89 -4.22

-7.72 -7.73 -3.32 -6.07 - 6.48 -5.56 -7.98 -5.76

-4.10 -4.20 -4.23 -4.64 -4.42 -4.82

-7.68 -7.76 -8.04 -7.61 -7.14 -7.41

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_+0.00003. The 875r/86Sr ratios of samples reported in this study were normalized relative to NBS 987=0.71014. Samples for C and O isotopic analysis were dissolved in anhydrous H 3 P O 4 at 25°C, following a standard technique (McCrea, 1950). Analysis was conducted on a Nuclide ® mass spectrometer, and both standards NBS 19 and 20 were routinely analyzed. Reproducibility of ~ 3C and ~ 80 for unknowns is within _+0.1%o and +_0.2%< respectively.

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Limestone [ AlteredLimestone SmallVein LargeVein

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Fe (mole %) Fig. 5. Fe vs. Mn plot, note that the large veins have high Fe and Mn concentrations relative to Viola limestones and the small veins.

4. Results

4.1. Element concentrations Table 1 lists the elemental and isotopic compositions of vein calcite and Viola limestone. In general, most vein calcite and limestone samples show similar ranges of Ca and Mg contents. The Viola limestones have high Sr concentrations (350-3030 ppm; Fig. 4). High Sr concentrations are also found in the small 0 • • +

0.7095-

Limestone AlteredLimestone SmallVein LmgeVein

calcite veins, but the large veins and the limestones from the associated alteration zones all show much lower Sr contents ( < 516 ppm) than the Viola limestone samples (Fig. 4 ). The large veins show higher and more variable Fe and Mn contents, when compared with the small veins and the host limestones (Fig. 5 ). The differences in the trace-element concentrations suggest that the two kinds of vein originated from different fluids. 4.2. Strontium isotopes

0.7091 o,3 0,0

O'3

,1-+

0.7087 0.7083

Seawater 87Sr/86Sr Range

0.707o

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0.7075

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Sr (ppm) Fig. 4. Sr vs. 875r/a6Sr plot, illustrating that: ( 1 ) altered limestones have lower Sr concentration and higher 87Sr/ 86Sr ratio than other samples of Viola limestones; (2) the 87Sr/86Sr ratio of the altered limestones increases with decreasing Sr concentration; ( 3 ) the small veins have higher Sr content and lower 878r/86Sr ratio than the large veins; and (4) the 87Sr/86Sr ratios of Sr-rich limestones and the small veins are within those of late Ordovician seawater (Burke et al., 1982), whereas altered limestones and the large veins are radiogenic.

Sr isotopic analysis of six Sr-rich limestone samples from the roadcut on 1-35, which covered the entire 200-m-thick Viola sequence, yields 87Sr/86Sr ratios ranging from 0.70779 to 0.70794 (Table 1 ). These ratios are consistent with the world-wide s e a w a t e r 875r/86Sr ratios of late Ordovician time (Burke et al., 1982), and thus closely represent the 8 7 5 r / 8 6 S r ratios of local seawater from which the Viola was deposited. The Sr-rich Viola limestones and the small calcite veins from the SRS quarry have similar 875r/86Sr ratios, and these ratios fall within the 875r/86Sr range of the original Viola limestone as identified using the samples from roadcut 135 (Table 1; Fig. 4). The similarity in 87Sr/ 86Sr ratios between the small veins and the host

ORIGIN

OF CALCITE VEINS AND ASSOCIATED

LIMESTONE

ALTERATION,

VIOLA GROUP,

limestones suggests that the 87Sr/86Sr ratios of fluids responsible for the formation of the small veins were similar to, or were buffered by the host limestones. In contrast, the large calcite veins and the Sr-depleted limestones from the alteration zones have higher (radiogenic) 875r/ S6Sr ratios (0.70792-0.70915, Table 1; Fig. 4). These radiogenic 87Sr/86Sr ratios indicate that the formation of the large veins and of associated alteration zones was caused by fluids which were enriched in 87Sr relative to the original Viola limestones.

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Fe (moi %)

0.7091

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--~ 0.7090

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0.7092

Viola limestones display a t~I3C range of - 2 . 2 to + 1.7%o, PDB (Table I; Fig. 6). These ~3C-values are similar to the primary ~13Cvalues of upper Ordovician carbonates documented by Lindh (1984), Poppet al. (1986), and Veizer et al. (1986), and thus probably represent the unaltered ~13C-values of original Viola sediments. The ~3C-values (from - 1.4 to + 1.6°/o0, PDB ) of the small calcite veins fall within the ~t3C range of Viola limestones (Fig. 6). This suggests that the carbon isotopic composition

-4

o • o •

0.3 @

263

MOUNTAINS

0.4

4.3. Carbon isotopes

2"

ARBUCKLE

-2

Fig. 7. Chemical variations within the large veins.

PDB)

Fig. 6. OtsC vs. ~ 8 0 plot, highlighting: ( 1 altered limestones have lower d~3C- but similar ~80-values to other limestones: (2) the small veins have higher ~3C- and lower ~ ~80-values than the large veins; and (3) the O13Cvalues of the small veins are similar to "unaltered" Viola limestones, whereas the ~JSC-values of the large veins are consistent with those of altered limestones.

of the (small) vein calcite-precipitating fluids was similar to, or was buffered by host limestones. However, the ~ t 3C_values (from - 8.0 to -2.5%o, PDB) of calcite from the large veins are much lower (Fig. 6 ). This indicates that calcite crystals from the large veins must

G.GAOETAL.

264

ences probably reflect different origins of these two kinds of calcite veins. Precipitation of the large veins from hot fluids is apparent because base-metal minerals and hot (50-90 ° C), saline (10-20 wt.% salts) primary fluid inclusions are present in the large veins (London et al., 1990). The 31SO-values ( - 4 . 8 to - 3.7%0, PDB ) of the large veins, coupled with the presence of hot fluid inclusions, require that the large veins originate from hot, 180-enriched fluids (with a g180> +2%0, SMOW), according to (Friedman and O'Neil, 1977):

have formed from fluids depleted in t3C relative to Viola limestones. Limestones from alteration zones also have low ~13C-values ( - 8.2 to -2.0%0, PDB), suggesting that these limestones were modified by 13C-depleted fluids.

4.4. Oxygen isotopes The 61SO-values of Viola limestones range from - 5 . 0 to -3.1%o, PDB (Table 1; Fig. 6). The heaviest value ( - 3.1 o/00) is close to the proposed original g 1SO_value for unaltered Ordovician carbonate sediments (Ludvigson et al., 1990: Marshall and Middleton, 1990). Most Viola limestone samples are thus depleted in 180 relative to primary sediments, which is probably the result of early meteoric alteration (Grammer, 1983 ). Low 6180-values (from - 8 . 2 to -5.500o, PDB) characterize calcite crystals from the small veins, relative to the g180-values (from - 4.8 to - 3.7%0, PDB ) of calcite crystals from the large veins (Fig. 6). These g180 differ18 O 0

103

In

O/calcite_wate r =

2.78" 106 T - 2 ( K ) - 2 . 8 9

Unlike the large veins, the small veins are commonly free of fluid inclusions. Without temperature and salinity data which would be available from fluid inclusion analyses, the low ~180-values of the small veins suggest that the veins may have precipitated from any of three types of fluids: ( 1 ) 1SO-depleted water o f nor-

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07087-,00

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87Sr/ 86Sr ~

0.7081- 200 0.7079-100 0

calcite vein

20 cm

Fig. 8. Sr, 87Sr/S6Sr and O~3C variation of limestone near a large fracture filled by calcite. The lower Sr concentrations, higher SVSr/S6Srratios, and lower 8~3C-values of the limestone closer to the fracture represent more intensive alteration by vein calcite-precipitating fluids.

ORIGIN OF CALCITE VEINS AND ASSOCIATED LIMESTONE ALTERATION, VIOLA GROUP, 4RBUCKLE MOUNTAINS

mal salinity (Sass et al., 1991), (2) 180-depleted meteoric water of low salinity, or (3) hot water. Sass et al. (1991) suggested that pore water of normal salinity in organic-rich sediments could evolve to be depleted in 180 as a result of sulfate reduction by bacteria. They proposed this mechanisms to explain the low c~180_values of diagenetic calcite in upper Cretaceous organic-rich carbonates in Israel. However, carbonate formed by this mechanism should also be typically depleted in 13C due to release of 13C-depleted bicarbonate from organic matter (Land, 1985 ). Apparently, this mechanism cannot explain the low ~80-values of the small veins because the small veins are not depleted in 13C relative to host limestone. Thus, the low ~80-values of the small veins should have originated either from ISOdepleted meteoric water or from hot fluids. A meteoric origin seems more likely to explain the low ~lSO-values of the small veins as will be discussed below.

4.5. Geochemical variation Of the five analyzed small vein samples, only Sr concentrations show significant variation (Table 1 ). In contrast, both elemental and isotopic compositions are variable among the large vein samples (Fig. 7). Limestones from the alteration zones associated with the large veins display systematic chemical variation relative to the position of the large veins. Specifically, the limestone samples closer to the large veins have lower Sr concentrations, higher 87Sr/86Sr ratios, and lower ~13C-values (Fig. 8). 5. Discussion

5.1. The origin of calcite veins The small calcite veins have 87Sr/86Sr ratios and d~3C-values similar to Sr-rich Viola limestones (Figs. 4 and 6 ). These similarities suggest that the strontium and carbon isotopic

265

compositions of the (small) vein calcite-precipitating fluids may coincidentally be the same as the host Viola limestones. Alternatively, these similarities probably indicate that the isotopic compositions of the fluids were buffered by dissolution of the host Viola limestones. High Sr concentrations of the small veins (Fig. 4 ) imply that either the fluids which formed the small veins were initially rich in Sr (or of high Sr/Ca ratio), or the fluids derived high Sr concentrations (or high Sr/Ca ratio) through dissolution of the Sr-rich Viola limestones. The low g l gO-values of the small veins suggest that the small veins could have precipitated either from hot fluids or from 180-depleted meteoric water. Clearly, formation of the small veins by hot fluids is possible if the fluids coincidentally had (or had achieved) strontium and carbon isotopic compositions similar to those of Viola limestone. However, 180-depleted meteoric water is believed more likely to be the fluid responsible for the formation of the small veins based on two lines of evidence: ( 1 ) the low ~180-values of the small veins are most easily explained by a meteoric origin; (2) and more importantly, dilute meteoric water could gain high Sr concentrations (or high Sr/ Ca ratio) and similar isotopic (Sr and C) compositions to Sr-rich Viola limestones when it dissolved the Viola limestones, thus explaining the Sr-enriched nature of the small veins and the similarity in isotopic (Sr and C) compositions between the small veins and the host Viola limestones. Similar cases were documented by Avigour et al. (1990) in a Sr isotopic study of some vein and cave calcites from southern Israel and by Egeberg and Saigal ( 1991 ) in a petrological and geochemical study of fracture-filling calcite from the North Sea chalk. The timing of the formation of the small veins is difficult to assess because there is no clear orientation of the small veins relative to the folds and large fractures present in the Viola. The small veins may have formed either during late Ordovician time (prior to deposi-

266 tion of Sylvan Shale) when the Viola was exposed, or during the Permian when the Viola was uplifted and exposed. In contrast to the small veins and Viola limestones, the radiogenic 87Sr/86Sr ratios (Fig. 4) and depleted ~13C-values (Fig. 6) of the large veins indicate that the fluids, from which the calcite crystals of the large veins precipitated, were not buffered significantly by host Viola limestones. The oxygen isotopic composition of the large veins, coupled with hot, saline fluid inclusions, further constrain the formation of the veins to 180-enriched ( > + 2%< SMOW) fluids as discussed previously. In addition, high Fe and Mn concentrations of the large veins (Fig. 5 ) suggest that the fluids were probably enriched in Fe and Mn. So, the chemical compositions demonstrate that the large veins originated from fluids enriched in 87Sr, 180, 12C, Fe and Mn. It appears that basin-derived fluids were the most likely fluids, based on four lines of evidence: ( 1 ) The presence of base-metal minerals and high-salinity inclusions in the large veins (London et al., 1990) indicates the formation of the large veins by basinal fluids. ( 2 ) Basinal fluids are usually enriched in 87Sr and 180, as well as Fe and Mn, as a result of water-silicate interactions (Land and Prezbindowski, 1981; Suchecki and Land, 1983; Stueber et al., 1984; Chaudhuri et al., 1987; Morton and Land, 1987; Mack, 1990; and many others). For example, a late generation of dolomite formed by basinal fluids under shallow burial in the Cambro-Ordovician Arbuckle Group, SW Oklahoma, has been documented to be characterized by radiogenic 878r/ 86Sr, heavy ~180-values, and high Fe and Mn contents (Gao, 1990). (3) The basinal fluids could be depleted in 13C due to contribution of 13C_depleted carbon from thermal reactions involving organic matter (Hudson, 1977 ). (4) The Arbuckle Mountains are adjacent to several clastic-rich late Paleozoic basins such as the Anadarko, Ardmore and Arkoma basins

G. GAO ET AL.

which could have provided a source of fluids (Johnson et al., 1988). The large veins appear to have formed in the late Pennsylvanian and Permian, based on three lines of evidence: ( 1 ) Only during the late Paleozoic did the Ouachita Orogeny produce the geometry (i.e. elevation difference between the Arbuckle Mountains and surrounding basins) and conduits (such as large-scale faults) for basinal fluids to migrate upward (Johnson et al., 1988). (2) The large, calcite-filled fractures in the Viola are oriented parallel to the axes of folds, and both the fractures and folds are believed to have formed during the late Paleozoic deformation event (Johnson et al., 1988). (3) Late Paleozoic remagnetization related to basinal fluids has been documented in (limestone) alteration zones adjacent to the large veins (Elmore et al., 1991 ). Although all calcite crystals within the large veins precipitated from basinal fluids, there exist chemical variations between type-A and B veins and between earlier basal calcite and later overgrowth rhombs (Fig. 7). The variations in chemical compositions, coupled with the fact that these two types of large veins have different mineral associations, suggest compositional changes of basinal fluids with time, or fluids from different basinal sources. However, these variations cannot be satisfactorily explained at the present time.

5.2. Alteration of limestone Viola limestones have Sr concentrations up to 3030 ppm (Fig. 4). This indicates the presence of aragonite in the original Viola sediments because calcitic sediments should not have more than 1400 ppm Sr (Turekian and Kulp, 1956; Kinsman, 1969). Compared to other Viola limestones, limestones from alteration zones associated with the large veins have much lower Sr concentrations (Fig. 4 ). The Srdepleted nature, together with its radiogenic

ORIGIN OF CALCITE VEINS AND ASSOCIATED LIMESTONE ALTERATION, VIOLA GROUP, ARBUCKLE MOUNTAINS

87Sr/86Sr ratios (Fig. 4 ), low ~ 13C-values (Fig. 6), and altered (recrystallized) textures, demonstrates that extensive chemical and textural alteration occurred in the limestones from the alteration zones. The similar ~t3C-values of limestones from alteration zones to calcite crystals from the large veins (Fig. 6), coupled with the restriction of the alteration zones to the proximity of the large veins, indicate that alteration of the limestones was caused by the basinal fluids from which the calcite of the large veins was precipitated. Although depleted 613C-values could also be related to 13C-depleted meteoric water derived from soil zones (Hudson, 1977 ), the radiogenic 87Sr/86Sr ratios of the altered limestones relative to other Viola limestones confirm that 87Sr-enriched basinal fluids were responsible for the formation of the alteration zones. The presence in altered limestones of sulfide and oxide minerals similar to those associated with the large veins also supports the alteration of the limestones by basinal fluids. Since progressive alteration of carbonates commonly results in continued Sr loss (Veizer, 1983; Gao and Land, 1991 ), simultaneous S7Sr increase and Sr decrease in the altered limestone (Fig. 4) reflect that limestone samples with lower Sr concentrations and higher 87Sr/ 86Sr ratios must have undergone more intensive alteration. In particular, limestones closest to the fractures, which are characterized by the highest 87Sr/S6Sr ratios and the lowest ~3Cvalues and Sr concentrations, underwent the most significant alteration (Fig. 8 ). Clauer et al. ( 1989 ) documented similar alteration patterns in limestone associated with calcite veins in the Cambrian Pilgrim Formation, Montana, U.S.A. At this time, it is not possible to conclusively relate the fracture-controlled alteration of Viola limestone to the formation of either earlier basal calcite crystals or later overgrowth rhombs in the large veins. However, it is worth noting that the ~3C-values of altered limestone are generally closer to the ~13C-value of

267

the basal calcite than the overgrowth rhombs (Table 1 ). This suggests that the alteration may have occurred during the formation of the basal calcite. In addition, the precipitation of the basal calcite may have sealed the wall rock from later fluids from which the overgrowth rhombs were precipitated. It is coincidental that the c~180-values of the limestones from alteration zones are similar to other Viola limestones (Table 1; Fig. 6). This was probably due to the nature ofbasinal fluids. Because basinal fluids are hot, limestone modified by basinal fluids should be depleted in 180. On the other hand, basinal fluids are commonly enriched in 180 (Land and Prezbindowski, 1981; Suchecki and Land, 1983), and thus they should reset limestone to heavy ~180values. The net balance then depends on both the temperature and 6t80 of the basinal fluids, and the fluid/rock ratio. It is believed that vein calcite-precipitating basinal fluids reached (oxygen) isotopic equilibrium with the limestone from the alteration zones, based on two lines of evidence: ( 1 ) the altered limestones have similar 6180-values to the large veins (Fig. 6); and (2) the d~3C-values of the altered limestone were reset, which requires much more intensive water-rock interaction than the 6180 resetting of limestone (see discussion in Banner and Hanson, 1990). 6. Conclusions

Different isotopic and elemental compositions of two kinds of calcite veins in the Viola Group, Oklahoma, U.S.A., originated from different processes. Calcite crystals from small veins of millimeter scale, with low oxygen isotopic composition and similar strontium and carbon isotopic compositions to host Viola limestone, probably precipitated from Viola limestone-buffered meteoric water. Calcite crystals from the large veins have radiogenic 87Sr/86Sr ratios and low ~ 13C_values relative to Viola limestone. Such isotopic compositions indicate that the calcite crystals from the large

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veins were precipitated from very different fluids. Constrained by the oxygen isotopic composition of and the presence of base-metal minerals and saline, hot fluid inclusions in the large veins, 180-, 87Sr- and ~2C-enriched basinal fluids are proposed to have been responsible for precipitation of calcite crystals in the large veins. Basinal fluids also resulted in local fracture-controlled alteration of Viola limestone, which is now characterized by radiogenic 87Sr/S6Sr ratios, low ~3C-values, and low Sr concentrations.

Acknowledgments This project was partially supported by NSF EAR-8917181 to R.D.E. and D. London at the University of Oklahoma. For help and discussion in the field and laboratory, we thank D. London and S.E. Thieben. Constructive review was provided by R.L. Folk, M.J. Bickle, and two anonymous journal reviewers. The Geology Foundation, University of Texas at Austin, defrayed certain publication costs.

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