Geochunica e’f Co.wnochimrca Acfa Vol. 5 I, pp. 1323-1334 B Pergamon Journals Ltd. 1987. Pnnted in U.S.A.

0016.7037/87/$3.00 + .oO

Evidence for Late-Paleozoic brine migration in Cambrian carbonate rocks of the central and southern Appalachians: Implications for

Mississippi Valley-type sulfide mineralization

PAUL P. HEARN, JR., JOHN F. SUTTER and HARVEY E. BELKIN

U.S. Geological Survey, Reston, VA 22092, U.S.A.

(Received October 6, 1986: accepted in revised,form February 26, 1987)

Abstract-Many Lower Paleozoic limestones and dolostones in the Valley and Ridge province of the central and southern Appalachians contain 10 to 25 weight percent authigenic potassium feldspar. This was considered to be a product of early diagenesis, however, 4oAr/39Ar analyses of overgrowths on detrital K-feldspar in Cambrian carbonate rocks from Pennsylvania, Maryland, Virginia, and Tennessee yield Late Carboniferous- Early Permian ages (278-322 Ma). Simple mass balance calculations suggest that the feldspar could not have formed isochemicahy, but required the flux of multiple pore volumes of fluid through the rocks. reflecting regional fluid migration events during the Late-Paleozoic Alleghanian orogeny.

Microthermometric measurements of fluid inclusions in overgrowths on detrital K-feldspar and quartz grains from unmineralized rocks throughout the study area indicate homogenization temperatures from 100” to 200°C and freezing point depressions of - 14” to - 18.5”C ( 18-2 I wt.% NaCl equiv). The apparent similarity of these fluids to fluid inclusions in ore and gangue minerals of nearby Mississippi Valley-type (MVT) deposits suggests that the regional occurrences of authigenic K-feldspar and MVT mineralization may be genetically related. This hypothesis is supported by the discovery of authigenic K-feldspar intergrown with sphalerite in several mines of the Mascot-Jefferson City District. E. Tennessee. Regional potassic alteration in unmineralized carbonate rocks and localized occurrences of MVT mineralization are both explainable by a gravity-driven flow model, in which deep brines migrate towards the basin margin under a hydraulic gradient established during the Alleghanian orogeny. The authigenic K-feldspar may reflect the loss of K during disequilibrium cooling of the ascending brines. MVT deposits are probably localized man- ifestations of the same migrating fluids, occurring where the necessary physical and chemical traps are present.

INTRODUCTION

MANY OCCURRENCES of authigenic K-feldspar have been reported in Lower Paleozoic rocks of the mid- continent area and the Appalachian Basin during the past 60 years (DALY, 1912; TESTER and ATWATER, 1934; WOODARD, 1972; BWCE and FRIEDMAN, 1975). Far from being a geologic rarity, authigenic K-feldspar is now known to be a common constituent in Cam- brian carbonate rocks throughout the Appalachians (BUYCE and FRIEDMAN, 1975; HEARN and SUTTER, 1985; HEARN et al., 1985). This phenomenon was once thought to be the product of early diagenesis involving potassium-rich brines (MAZULLO, 1975); however, re- cent 40Ar/3gAr analyses of authigenic K-feldspar in Cambrian rocks of the Valley and Ridge Province of the central and southern Appalachians have yielded Late Carboniferous ages (HEARN and SUTTER, 1985; HEARN et al., 1985). This paper will present evidence in support of the hypothesis that the authigenic K- feldspar is genetically related to nearby occurrences of MVT mineralization, and thus may allow constraints to be placed on both the timing and the chemistry of ore formation.

EXPERIMENTAL METHODS

“OAr/‘“Ar age-spectrum analysis

Pure (>95%, the remainder being quartz) mineral separates of detrital K-feldspar grains with authigenic overgrowths were

obtained by removing carbonates in dilute HCI and sieving to the 63- 150 pm size fraction, followed by gravitv separation in a mixture of bromoform and dimethyl formamide (specific gravity = 2.60). The ages of the authigenic overgrowths in these samples were determined by the 40Ar/39Ar age-spectrum technique (LANPHERE and DALRYMPLE, 1971; DALRYMPLE and LANPHERE, 1974). Two age-spectra were generated for each sample: one from a split of detrital K-feldspar with au- thigenic overgrowths, and a second from a split of the same sample whose overgrowths had been removed in a solution of 5 percent hydrofluoric acid. After the HF treatment, detrital cores were examined optically to ensure that the authigenic overgrowths had been completely removed. The ages of the authigenic overgrowths were determined using a method em- ployed by HEARN and SLITTER (1985), which is based on the assumption that the samples represent physical mixtures of phases with different ages but having similar argon-diffusion systematics. For a sample of igneous K-feldspar cores with authigenic K-feldspar overgrowths, the age of the authigenic overgrowths is expressed by the equation:

where:

A, - M,A, A,=-

M0 (1)

A, = apparent age of total sample A, = age of core A, = age of overgrowth MC = mass fraction of core M, = mass fraction of overgrowth.

The mass fraction of cores and overgrowths were determined in the following manner. A representative split of each sample was imbedded in epoxy and then ground and polished to ex-

1323

I324 P f’ l-learn. Jr.. J t Sutter and H. I. Belkm

pose grain interiors. These samples were then placed tn an ETEC Autoscan* scanning electron microscope at low mag- nification (30-50X), so that the field of view contained several hundred grains. The total area of K-feldspar grains (cores + overgrowths) was first measured using a backscattered elec- tron detector and particle analysis software on an EDAX 9 100 analyser: this procedure was then repeated on a cathodolu- minescence image of the same field. The relative mass fraction of cores (luminescing) and overgrowths (non-luminescing) were obtained from the average ratto of three pairs of these measurements. As no independent measurement of the mass fraction of cores and overgrowths was available, the error of this value was assumed to be equal to the analytical precision which ranged from 3 to J?

The ages of authigenic overgrowths were determmed by applying Eqn. (1) to the stepwise-release data in two ways. In the first method, a “synthetic” age spectrum was obtained by fitting smooth curves through the step-wise spectra, and then computing an apparent age for each increment of 5 percent jYArk released. The resulting synthetic curve is an approxt- mation of the age spectrum that would be obtained if a discrete sample of authigenic overgrowths could be analyzed.

In the second method, overgrowth ages were computed from Eqn. ( I ) in a single step by using total-gas ages: a total-gas age is essentially the integral of the age-spectrum and represents the age that would have been obtained if ali the argon had been extracted from the sample in one step.

Fluid inclusions in K-feldspar and quartz separates were examined by microthermometric techniques using a modified Chaixmeca heating and freezing stage (CIXNINGHAM and COROLLO, 1980). Because the samples were monomineralic separates consisting of individual grains - 100 pm in diameter. special sample preparation techniques were used.

Grinding and polishing samples was not considered feasible because the inclusions of interest, if present. would be con- tained in the small overgrowth volume. ‘The method used involved sandwiching the sample grains between two covet glasses held together with epoxy. First. a thicker cover glass (e.g. no. 1. -0.15 mm) was cleaned and a drop of mixed epoxy (Devcon “Two-ton” clear epoxy) placed in the middle. A number of grains were sprinkled on the epoxy drop so that the final product would contain a layer only one grain thick A thinner cover glass (no. 0. -0. I mm) was placed on top and then moved gently about to produce the desired grain distribution. The cured epoxy provides a suitable medium (rr - 1.53) for the examination of quartz and K-feldspar grains and is sufficiently stable and free from darkening to allow heating up to -220°C.

The estimated instrumental uncertainty ol the vapor and liquid homogenization temperature (Th) is +7.O”C. The ob- servation of bubble disappearance was enhanced by rapid bubble motion as sample temperature approached Th. The estimated instrumental uncertainty in the freezing mode is ?0.2”C. However, observing the melting point of ice (Tm- ice) was difficult because of the very small size ofthe mclusions. After Tm-ice was observed. the temperature was rapidly re. duced (- 15°C in 10 seconds) to check for the presence 01 any hidden ice. If ice is present in the inclusion, a rapid cooling till precipitate more ice and the crystal will grow to observable size. By using this technique and making multiple runs the limitatton of small inclusion size was considerably reduced. We estimate the actual freezing mode uncertainty to be +-O..?“C (t -0.5 weight% NaCl equivalent).

_____-.. .._-.. ._ ~_...

* Trade names are used for descriptive purposes only and do not constitute endorsement by the ITS. Geological Survey

GEOLOGIC AND TECI’ONIC SF’I’TI~G

Mississippi Valley-type sulfide mineralization tn thr ccntrai and southern Appalachians is hosted exclusively by? (‘ambrian- Ordovician limestones and dolostones of the Valle? and Ridgx: province. These rocks represent the thickest pal-t r>f the vas! accumulation of platform carbonate sediments that fringed the eastern North American craton in Lower Paleozoic trmz. extending from Newfoundland to Alabama. Depositional cii- vironments ranged from shallow subtidal to suprdtidal. ax CL tdenced by a wealth ofdelicately preserved sedimentary strut- tures. Characteristic features include flat and wavy lamination. thin bedding. mudcracks. evaporite casts and molds. and algai stromatolites (REINHARDT and H~RDIE, IO7h: DF k?!: ( i) am: MITCHELL, 1982).

The Piedmont and Blue Ridge provmces ofthi ;cntra! dnc! southern Appalachians experienced a long and cornpIe metamorphic and deformation history during the Paleozoic. However. the bulk of the structural development and subse quent uplift of the central and southern Valley dnd Ridge province took place during the Alleghanian orogcny. which apparently began in Late Mississippian time and culminated in Late Pennsylvanian to Early Permian time (WI)OI)M 4t<J) 1957: \'AY I)FR v(X), 1977)

DISTINGUISHING FEATURES OF AU’I’IIIGENIC’ K-FELDSPAR

Authigenic K-feldspars are distmguished from &nitai \rC neous) varieties both by their luminescence behavior and bl. their chemical purity. Since they precipitate under relatr%zi; low temperatures (t200”C). authigenic feldspar5 gcneraib contain much lower concentrations of impurity element\ which substitute more readily in an igneous envmmmrnt Consequently, authigenic K-feldspar; and albites arc the closest natural representatives of end-member composition in the alkali-feldspar series; authigenic K-feldspar typically contains more than 99 mole percent KAISi,08. The high chemical purity of authigenic K-feldspar is most pmbabiy responsible for its lack of luminescence when exposed to ionizing radiation (KASTNER and SIEVER, 1979); this property is perhaps the most diagnostic in dti authjgmic from igneous K- feldspar. which luminesces brightly (KASTNER, 1971).

RESULTS AND DLSCUSSION

High concentrations of authtgenic K-feldspar t i I) 25 wt.%) were found in close proximity to dress \>I

rinc, lead. and barite mineralization throughout the Valley and Ridge province in the central and southern .4ppalachians (Fig. I). The greatest volume of the au-- thigenic K-feldspar occurs in elastic-rich i.‘ambrian dolostones and limestones stratigraphically below mineralized horizons: it is aiso present a\ .t major component of siliciclastic residues which occm in smaii quantities in some mineralized zones.

Cambrian carbonate rocks containing abundant au- thigenic K-feldspar include the Gatesburg and Allen- town Limestones in Pennsylvania. the Conococheaguc Limestone in Maryland and Virginia, the Shadt, Do- lomite in Virginia, and the Maynardsville Limestorx in Tennessee and Georgia. These rocks arc ail char- acterized by alternating dolostone and limestone and typically contain substantial amounts of srliciclastrc detritus.

Brine in Cambrian carbonate rocks 1325

I Occurrence of abundant

authigenic K-feldspar

SdO Ar/?Ar Age Determination

0 Lead

l Zmc

A Barate

*.. Small mine or pi*spect

0,) Medium mine or district

0mA Large mine or distract

FIG. I I Map showing sampled occurrences of authigenic K-feldspar and locations of deposits and occurrences of zinc, lead, and barium mineralization in Lower Paleozoic sedimentary rocks of the central and southern Appalachians. Locations of ore deposits and occurrences were taken from CLARK (in press).

Authigenic K-feldspar occurs most conspicuously in these rocks as overgrowths on grains of detrital or- thoclase and microcline, but it is also present in large quantities in the form of a silt to clay-sized matrix (Fig. 2). X-ray diffraction analyses indicate that both the fine-grained matrix K-feldspar and the K-feldspar overgrowths are predominantly monoclinic, whereas the detrital igneous K-feldspar is a mixture of mono- clinic and triclinic phases.

The two modes of occurrence for authigenic K-feld- spar appear to reflect grain-size variations in the initial depositional environments, and typically parallel pri- mary sedimental structures. The matrix-forming au- thigenic K-feldspar is typically found in fine-grained dolostone interbeds; its characteristic occurrence in what were apparently original mud layers suggests that it may represent an alteration product of detrital clay minerals. This conclusion is supported by X-ray dif- fraction analyses; samples confining significant con- centrations of fine-grained authigenic K-feldspar are

usually depleted in clay minerals. In contrast, samples with little or no authigenic K-feldspar typically contain some combination of mixed-layer clays, vermiculite, chlorite, or illite. This suggests that the fine-grained matrix K-feldspar represents the alteration of original clay minerafs by K-rich fluids.

K/Rb ratios of K-feldspathized carbonate rocks

The fact that authigenic K-feldspar is markedly de- pleted in Rb relative to igneous K-feldspar makes the K/Rb ratio another useful indicator in distinguishing the two types (BASKIN, 1956). Ratios higher than 440 are considered anomalous for igneous feldspars (HEIER and TAYLOR, 1959). In contrast, K/Rb ratios for au- thigenic K-feldspars are almost always greater than this value. Whole-rock concentrations of potassium in a suite of 43 Cambrian dolostones from the Valley and Ridge range from 0.5 to more than 6 weight percent; rubidium concentrations range from less than 2 ppm

13% P P Hearn. Jr.. J. I’. Sutter and ti. E. Belkm

FIG. 2. Scanning electron micrographs of feldspathized do- lostone from the Maynardsville Formation, near Jefferson City. Tennessee. Ail micrographs arc of the zamv area. Scale

K (wt

FIG. 3. Plot of whole-rock potassmm and i-un~~l~urn <on centrations for samples of Cambrian limestones and titrlononc~ from the Valley and Ridge province of the L~entral and \outhrr:-b Appalachians.

to as much as 90 ppm. Optical and X-ral drifiiac?w analyses of these samples indicate that k-Wdspar !i the major potassium containing phase and [hat illit< and muscovite are either absent or present iI1 onlv rni

nor amounts. The plot of potassium \clr,\ci’> !.ubidun:

shows most samples to fall along a line dt%ned b? :: K/Rb ratio of approximately 1300 (Fig. 3). 7%~ marked depletion in Rb indicated by these data suggests tha: a significant proportion of the K-feldspar in thcsc rcrcki is authigenic-a conclusion consistent \I i t h petr cd- graphic observations.

Authigenic K-feldspar has been identrtied ds LL pr!- mary component of jasperoid gangue in three minixhS of the Mascot-Jefferson City district in eastern 1 c’n nessee (New Market. Young, and Immel mines) Ai! ofthese occurrences are in the Beekmantown dolomrtc. the major host formation for MVT minerahration I!:

the Appalachians. The jasperoids are thinit band<l.i. dark gray, shaley to cherty zones which arc often ii! timately associated with ore. These zones arc believed to have formed during silicification events in the earl\ part of the paragenetic sequence-prior to the dcposltion of sphalerite (MCKORMICK tit (ti., 197 I). huthigenic K-feldspar in the jasperoids is typically intergrown with silica and dolomite, and occurs as broad Lanes 01’ c+ alescing overgrowths on grains of detrital K-feldspar-

(Fig. 4). In all the cases observed. the naturr: ot’gram contacts between authigenic K-feldspar and sphalerltc

bar = 100 micrometers. A) Backscattered electron tmagz showing interface between coarse-grained limestone mterhed and fine-grained dolostone interbed. Note intergrowths ofdo- lomite and authigenic K-feldspar in dolostone layer Key. k: feldspar (Kf): quartz (Q); dolomite (D); calcite (C“I. pyrite (P! B) Cathodoluminescence image showing detrital (luminescingi K-feldspar grains. C) Potassium X-ray map showing distrl- hution ofauthigenic and detrital K-feldspar. X-m! dIffraction analysis of the ~63 pm fraction of the insoluble rcs~duc :9 this sample showed K-feldspar and quartz ((1 he tbc primer,. phases present: no clay minerals \\ere found

Brine in Cambrian carbonate rocks I321

FIG. 4. Scanning electron micrographs of jasperoid from the New Market Mine showing nature of contact between sphalerite and authigenic K-feldspar. All micrographs are of

tend to support the contention that the K-feldspar is

pre-ore. While inclusions of K-feldspar (both authigenic and detrital) are found in sphalerite, no inclusions of

sphalerite are found in the authigenic K-feldspar. Nonetheless, the apparent lack of any replacement of K-feldspar by sphalerite suggests that the ore-fluid was not markedly undersaturated with respect to K-feld- spar.

Mass balance considerations

While the presence of MVT deposits are believed by most geologists to reflect the migration of fluids through the host rocks, the mass of authigenic K-feld-

spar in nearby unmineralized dolostones suggests that fluids passed through these rocks as well. The average potassium content of the 43 unmineralized samples analyzed in this study is 2.4 weight percent as K. Based

on the relative proportions of luminescing and non- luminescing K-feldspar, at least 50 percent of this po- tassium is estimated to be authigenic. Assuming a rock density of 2.6 gm/cc, this corresponds to approximately 3 1 kilograms of authigenic potassium per cubic meter. A cubic meter of unlithified sediment with a porosity of 70 percent and pore water equivalent to present- day seawater in composition (400 mg/l K) would con- tain only 0.3 kg of dissolved potassium. Potassium concentrations for sedimentary brines seldom exceed 6000 mg/l (HANOR, 1979; CARPENTER et al., 1974). Even using this extreme value, the pore water in a cubic meter of sediment would still only contain some 4 kg of potassium. Using a porosity value more typical of buried and compacted sediment, the estimated potas- sium content would be even lower. It is difficult to explain how the remaining mass of authigenic potas- sium could be emplaced by an isochemical process. It seems much more likely that the observed quantities of authigenic K-feldspar derived from an external source, and therefore must have involved the migration

of fluids through these rocks.

Results of 4UAr/39Ar age-spectrum analyses

Ages were determined for authigenic overgrowths on detrital K-feldspar grains separated from unminer- alized Valley and Ridge carbonate rocks of central Pennsylvania, western Maryland, southwest Virginia, and east Tennessee. Sample locations and descriptions are given in Table 1. It was not possible to obtain sep- arates of discrete K-feldspar grains with overgrowths from the mineralized jasperoids as the detrital and au- thigenic K-feldspar in these rocks are typically bound together in multi-grain aggregates by K-feldspar or

the same area. Scale bar = 100 micrometers. A) Backxattered- electron image. Key: K-feldspar (Kf); silica (Sil); sphalerite (Sp). B) Cathodoluminescence image showing detrital (lu- minescing) K-feldspar grains. C) Potassium X-ray map show- ing distribution of authigenic and detrital K-feldspar.

132x t’ P. l-learn, Jr., J. b-. Sutter and H. E. Relkin

Table 1. Locations and descriptions of samples used for AI/AI

qeochronoloqy of authiqenic K-feldspar overqrowths.

FoLmatlon iAqri Locat I i>ii Description

Gatesburq \‘ t4e.31 Tyioi!l~. Pd. Sandy dolomite

Conococheaqw t i Near Clear Sprinq. Md. Ribbon laminated dolomitic limestone

Shady / Near Austlnvlile. Va. Ribbon laminated dolomitic limestone

Maynardsvillv : t : Near Jeftcrson City. Ribbon laminated Tt=nn.

quartz cement (see Fig. 4). For this reason, only K- feldspar separates from unmineralized dolostones were

analyzed. Ages were computed for each sample using both 01

the methods described above: in every case the syn- thetic age spectrum shows a distinct plateau region. indicating the age of formation of the authigenic over- growths (Fig. 5). The discrepancy between ages ob- tained by each computational technique is less than three percent for each sample. The values obtained using both techniques range from 278 to 322 Ma. and yield an average value of 300 Ma.

The range of ages computed by both techniques probably reflects analytical error more than real dif- ferences. The error in the age calculation is largely a function of the uncertainty of the mass fraction mea- surement and the magnitude of the age difference be- tween the authigenic and the detrital K-feldspar. Thus the analytical error is greatest when the age of the de- t&al core is much greater than that of the overgrowth or when the mass fraction of overgrowth is relatively small.

The similarity in the estimated ages of authigenic K-feldspar in rocks extending through some 750 km ofthe Valley and Ridge province. and the large masses of authigenic K-feldspar found in these rocks is com- pelling evidence for a fluid migration event of regional scale. The agreement between the Late Carboniferous- Early Permian age suggested for the authigenic K-feld- spar in this study and the age proposed for Alleghanian folding in the central Appalachians (VAN DER Voo. 1977) suggests that this tectonic event provided the driving force for the fluid migration.

Two-phase (liquid and vapor) liquid-rrch aqueous fluid inclusions were found in authigenic K-feldspar and quartz overgrowths on detrital grains in carbonate rocks from various locations throughout the study area. All of the measured inclusions appear to be primary or pseudosecondary; however, the small grain size and poor optical quality of some samples makes this clas- sification equivocal. The measured inclusions ranged in size from 2 to 12 pm (average = 5 pm). The inclu- sions had spherical, ellipsoidal or irregular shapes (c>.~

dolomitic limestonr

Fig. 6a). In two cases in K-feldspar, the inclusions could be related to crystal growth or entrapment phenomena (Fig. 6b). The number and distribution of inclusions

amenable for microthermometry does not reflect thr population of inclusions that can be observed by high- power oil immersion microscopy (1200X). Fluid in-

chtsions of a size too small to be easily distinguished from trapped solid phases could be seen because of the presence of a rapidly moving bubble (observed as a moving black dot). No daughter crystals were observed in any inclusion.

Fifty-one inclusions were heated and homogenized

in the liquid phase to determine Th (Fig. .~a) and twenty-one inclusions were frozen and then warmed to determine the temperature of the eutectic (Te) am! Tm-ice (Fig. 7b). Th ranged from 100” to 200°C (av.- erage - 140°C) and Tm-ice ranged from 14 I<:

- 18.5”C (average - - 16°C). This Tm-ice range rep-

resents a salinity range of 18 to 21.5 weight percent (NaCl equivalent). Te was difficult to observe on most inclusions; however, when observed Te ranged from -50” to -45°C. This indicates the presence of a least

CL” in the fluid; in addition, fluids trapped in K-fcld-

spar must contain some K+.

C‘omparison offluids trapped 111 quart-_ un~i A.

.jeldspar overgrowths with MC’T-deposit /hi\

Mississippi Valley-type ore deposits have been Ed-

tensively studied by fluid inclusion techniques (KOEII- DER, 1976, 1984). The overall picture of fluid com-

position emerging from these studies is surpnsingly uniform, despite the wide variety of MVT deposits,. ROEDDER (1976, 1984) has synthesized the MVT fluid-

inclusion data and concludes that the fluids were low temperature (100” to 150°C). and high salinity (usually > I5 weight percent, frequently >20 weight percent NaCl equivalent); he aslo points out that Te data and the usual lack of NaCl daughter crystals imply that appreciable amounts of cations other than Na* arc present. Figure 8 shows various fields of Th and Tm- ice for fluid inclusions from various representative MVT deposits. Although the data set from this study is limited compared to those shown for the various MVT deposits, the present data indicate clear similar- ities between fluids in K-feldspar and quartz over.

Brine in Cambrian carbonate rocks I329

900 TY- 1

0

3gArK Released f% 1 100 0

3gArK Released (%)

100

Gatesburg Limestone Conococheague Limestone near Tyrone, Pa. 60% authigenic near Clear Spring, Md. 55% authigenic

Plateau Age=284 Ma Plateau Age=317 Ma Total-gas Age=288 Ma Total-gas Age=314 Ma

1000 ( 1

t

SH/AU .l

3gArK Released f% 1

Shady Dolomite

near Austinville, Va. 92% authigenic

Plateau age=287 Ma Total-gas Age =278 Ma

288

278

1000

900 - sv-4

I I I I I I I I 0 100

3gArK Released (% 1

Maynardsville Limestone near Jefferson City, Tenn. 56% authigenic

Plateau Age=322 Ma Total-gas Age =315 Ma

EXPLANATION

. . . . . . . . . . . . . . . . . Cores + overgrowths

____--_ Cores alone

315

“Synthetic” overgrowth spectrum

FIG. 5. 40Ar/39Ar age spectra of K-feldspar separates.

growths and fluids in MVT deposits, most notably the Appalachian deposits in Pennsylvania (Friedensville), Virginia (Timberville and Austinville-Ivanhoe) and east Tennessee (Mascot-Jefferson City).

Thermochemical considerations

In a theoretical study of hydrothermal alteration, GICCENBACH (1984) predicted that secondary mineral assemblages in or close to major fluid upflow zones (i.e. formed by cooling fluids) will be characterized by primarily potassic alteration. This phenomenon is il-

lustrated in Fig. 9, slightly modified from GIGGENBACH

(1984) which shows a hypothetical cooling trajectory for an ascending hydrothermal fluid and the relative changes in sodium and potassium activities that would occur upon isothermal re-equilibration with respect to both alkali feldspars in a “fluid dominated” system. A fluid at point “A”, with a Na/K activity ratio of 10 (Log Na/K = 1) at 300°C is in equilibrium with both feldspars. During the initial stages of cooling, the fluid travels along the Na-K-feldspar phase boundary. The re-equilibration of the fluid lags behind as cooling con- tinues, however, and the Na/K ratio moves away from

1330 I’ P. tlearn. Jr., J. l: Sutter and H. I:. Belkin

FIG. ha. Photomicrograph of a pnmary. two-phase (liquid + vapor) fluid inclusion in a quartz overgrowlh on a detrital quartz grain from the Shady Dolomite. Austinville. Va. (Sh; .Au-2). The picture was taken at O.O”C to stop the huhble movement occurring at room temperature i-73°C). -Ih

137°C: Tm-ice = - 15°C. 6b. Photomlcrograph of a prl- marl, two-phase (liquid + vapor) fluid inclusion in a K-feld- spar overgrowth on a grain of detrital K-feldspar from tllc Shady Dolomite, Austinville. Va. (Sh/Au-2). The picture was taken at -0.O”C to stop the bubble (B) movement occurring at room temperature (-23°C). The inclusion appears to have been trapped during the overgrowth process that also trapped two adjacent mineral grains (X) Th i 3 -7 “C‘. Tm-lci

17v(-‘

the equilibrium line into the K-feldspar stability field. Isothermal re-equilibration of the fluid at point “H” on the cooling curve would require the precipitation of more than 80% of the potassium in solution, hut only a 3% increase in sodium. As shown in the shad4 area of Fig, 9. the range of temperatures and Na/K ratios for fluid inclusions in MVT ore minerals suggests

that these fluids followed a similar cooling trajectoc and were trapped before they were able to re-establish equilibrium. The general lack of siliciclastic material in horizons hosting Mississippi Valley-type mineraf- ization suggests that the fluids trapped in ore and gangue minerals may have remained in a disequilih- rium state for kinetic reasons. However. if similar fluids were to migrate through carbonate rocks containing detrital K-feldspar and clay minerals, one would expccr a significant mass of potassium to be precipitated--a prediction which appears to be supported by the oh- servations of this studc

There appears to be fairly common agreement that MVT deposits are emplaced by hot fluids with salinities greater than seawater, which migrate out of nearb! sedimentary basins-the so-called “basinal brine” model of OHLE (1980). The migration of basinal fluids to the sites of ore deposition has been explained by ;i number of mechanisms. One theory involves the movement of fluids as a result of compaction during basin evolution (JACKSON and BEAMS. 1967: Don,.

1970). A second calls for episodic dewatering events, whereby excess pore pressures created by compaction are released suddenly and force pulses of fluids towards basin margins (SHARP, 1978; CATHLES and SMITH.

1983). A third theory, which was first suggested in the nineteenth century and has been recently revived, sug- gests that heated brines were expelled from deep strata

by gravity-driven groundwater flow. due w !~~drauirc gradients established by topographic differences across the basins (DAUBREE, 1887; (‘ox. 19 I I ; Civic! I x md FREEZE, 1984a,b: BETHK~. 1985 I. Ot‘ these rhree !h~- ories. the gravity-flow model has rrccivcci !tw mcxt

support recently-based both on geologic ~.~:rdenct

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i sv-4 -T---T -dL.. i-r

10 15 2 0 T m b 2 e ; c

k-r<;. 7. a) Histograms of the temperature of homogemzahon i’l‘h) and b) the final melting point of ice (‘I’m-lcri data f&r fluid inclusions in quartz and K-feldspar overgrowths on dc- trital grains from the Gatesburg Limestone near ‘Tyrone. Pa (‘Ty-I j, the Conococheague Limestone near Timberviiie. Va. (Tb-3). the Shady Dolomite near Austinville, Va. (Sh/Au-Al. and the Maynardsville Limestone near Jefferson (‘itv, ‘T’cnn r&4).

Brine in Cambrian carbonate rocks 1331

-30

-25

8

100 150 200 Th’C

FIG. 8. Range of Th versus Tm-ice for primary inclusions in major MVT deposits compared to data for fluid inclusions in this study (Fig. 7). Data for representative deposits were taken from ROEDDER (1976; table IV, 92-93): area I-East Tennessee (Zn); area 2-Laisvall, Sweden (Pb); area 3-North Pennines, England (Pb-Zn-F); area 4-New Lead Belt, Mis- souri (Pb); area S-Hansonburg. N. Mex. (Pb-Ba-Fh area 6- Timber&e, Va. (Zn); line 7 (?m-ice da& only):Friedens- ville, Pa. (Zn); line 8 (Th data only)-Austinville-Ivanhoe, Va. (Zn), from FOLEY (1980).

(LEACH and ROWAN, 1986) and also on the results of numerical modeling studies (GARVEN, 1985; BETHKE,

1986). This mechanism provides a geologically con- sistent explanation for Late-Paleozoic fluid migration in the central and southern Appalachians. GARVEN and FREEZE ( 1984b) determined that gravity-driven flow could produce temperatures of 100” to 150°C along the margins of a basin several kilometers in maximum thickness and several hundred kilometers in lateral extent. These dimensions are consistent with the known geometry of the central and southern Ap- palachian basin in the Late Paleozoic. Also, the timing of fluid migration suggested by the ages of authigenic K-feldspar agrees well with the known age of the Al- leghanian orogeny. Throughout the central and south- ern Appalachians, patterns of alluvial deposition in- dicated by Pennsylvanian sandstones provide ample evidence for elevated recharge areas to the east of the Valley and Ridge province (MECKEL, 1970; HARRIS

et al., 1978). These observations are consistent with

the hypothesis that both the basin-wide occurrences of

authigenic K-feldspar and MVT ore deposits were em-

placed by gravity-driven flow systems established dur- ing Late-Paleozoic deformation. In this scenario, the widespread occurrences of authigenic K-feldspar reflect the disequilibrium cooling ofascending basinal brines; areas of sulfide mineralization reflect the movement of the same fluids, but represent either locations where the chemical and structural traps necessary for ore for-

mation were present, or local differences in the ore- carrying ability of the brines.

Arguments for and against a Late-Paleozoic age.ftir ore formation

While theories regarding the timing of MVT ore

emplacement in the southern Appalachians have evolved considerably during the last century, perhaps the most vigorous debate has been generated between researchers favoring a Late-Paleozoic (Alleghanian) age and those who favor an Early-Paleozoic (Ordovician) origin. Some early workers cited apparent structural controls on ore emplacement as evidence that min- eralization developed during Late-Paleozoic tectonic activity (CURRIER, 1935; BROKAW and JONES, 1946). More recent studies have suggested a Late-Paleozoic age for mineralization based on sphalerite-deformation textures (TAYLOR et al., 1983) and paleomagnetic data (BACHTADSE et al., 1985).

CHURNET (1985) summarized several lines of evi-

dence which have been frequently cited in arguing for an Early Paleozic age of ore formation. These include: 1) the presence of detrital sphalerite grains in the lam- inae of some breccia sand bodies which parallel the regional tilt, indicating that mineralization preceded tilting (KENDALL, 1960); 2) the cross-cutting of ore by faults of Alleghanian age (HILL, 1969); 3) the restriction of ore deposits to the Knox Group, and the general absence of ore in overlying (post Middle Ordovician) carbonate rocks (CRAWFORD and HOAGLAND, 1968). In fact, none of these observations are inconsistent with the Late Paleozoic age which we have suggested in this

paper. As pointed out by TAYLOR et a/. (1985), the only real restriction that observations 1) and 2) place on the timing of mineralization is that some ore must have been emplaced prior to the faulting and folding stages of the Alleghanian orogeny. This constraint does not rule out the movement of ore fluids in the early stages of orogenesis, nor does it preclude later ore for- mation in rocks not exhibiting these structural features. While it has often been suggested that decollements between thrust sheets could serve as conduits for fluid flow, it is not unreasonable to assume that fluid mi- gration began prior to folding and thrusting. In the gravity-driven flow model, deformation and uplift along the distal margin of a foreland basin forces fluids to the surface along the proximal edge of the basin (GARVEN and FREEZE, 1984a). It is not necessary for folding and thrusting to occur along the proximal basin

131’ P I’ Hearn, Jr., J. F. Sutter and H. E. Belkin

Temperature (OCI

FAG. 9. Relative proportions ofalkali ions produced (t) or consumed ( --) during isothcrmnl oqulllbr;it!i,;m of a solution with a given Na/K activity ratio with respect to a full equilibrium assemblage containing bc?!t, Na- and K-feldspars (activity coefficients for both species are assumed to be equal); The cur\ed arr~ represents a hypothetical cooling trajectory for an ascending hydrothermal fluid (modified slightly ii-~; GIGGENBACH, 1984; see this reference for a detailed treatment of the derivation of this ligurc). The shadnj area represents range of reported trapping temperatures and Na/K ratios for fluid inclusions in Mississipp: Valley-type ore deposits from the data of VIEWS (‘/ L, rf (19841. and ROEDDER (19721

margin for fluids to rise towards the surface in thestb

areas. At the Pine Point deposit, which has been at- tributed to emplacement by gravity-flow. the ore is hosted in rocks which are structurally undefonned (GARVEN, 1985).

The lack of extensive mineralization in sediments

younger than Middle Ordovician is not convincing ev- idence that ore emplacement did not occur significantly later than this time. The collapse structures which ap- pear to be necessary for ore formation are largely absent in carbonate rocks above the Middle Ordovician un- conformity. Furthermore, numerous minor OCCUI- rences of lead and zinc mineralization have been cited in rocks as young as Carboniferous in ‘Tennessee, Ken- tucky, and Pennsylvania (.IEwE~_I_, 1947: KYI .E. 1976: HOWE 1981).

CONCLUSIONS

1) 4oAr/39Ar age-spectrum analyses of authigenic K- feldspar in Cambrian carbonate rocks of the Valley and Ridge province of the central and southern up-

palachians yield Late Carbonif’erous-Early Permiatl

ages (278-322 Ma). Simple mass-balance calculation> suggest that the K-feldspar did not form isochumic&>. but involved the flux of multiple pore-volumes oftiu~c! through these rocks. These observations arc believed to reflect the regional migration of basinal brines ir: response to tectonic activity during the Late-Paleozoic.

Aileghanian Orogeny. 2) Microthermometric measurements of iiuld in-

&sions in overgrowths on K-feldspar and quarti grams in unmineralized carbonate rocks indicate that these phases precipitated from a hot i, lOO”-200°C) aud saline ( 18-3 I weight percent NaCl equivalent) fluid. The ap- parent similarity of these fluids to fluid inclusions in

ore and gangue minerals of nearby MVT deposits sug- gests that both the mineralization and the regional (~2 currences of authigenic K-feldspar were tsmplaceci during basin-wide fluid migration events.

3) The suggestion that authigenic K-feldspar in un- mineralized rocks and MVT ore deposits were emu placed by similar fluids is also supported by the dis- covery of authigenic K-feldspar intergrown wrth

Brine in Cambrian carbonate rocks 1333

sphalerite in several mines of the Mascot-Jefferson City CRAWFORD J. and HOAGLAND A. D. (1968) The Mascot-

District in eastern Tennessee. Textural observations Jefferson City Zinc District, Tennessee. In Ore Deposits of

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and formed during a silicification event in the early (ed. J. D. RIDGE), pp. 242-256. Amer. Inst. Mining Metall.

part of the paragenetic sequence. Petroleum Engineers, Maple Press.

COX G. H. (191 I) The origin of the lead and zinc ores of the 4) The genesis of MVT deposits in the central and Upper Mississippi Valley district. Econ. Geol. 6, 427-448,

southern Appalachians as well as regional occurrences 582-603.

of authigenic K-feldspar in unmineralized rocks can CUNNINGHAM C. G. JR. and COROLLO C. (1980) Modification

of a fluid-inclusion heatina/freezina staae. Econ. Geol. 75. be explained by the migration of basinal brines in a

-. _ I 335-337.

gravity-driven flow system established by Alleghanian CURRIER L. W. (1935) Structural relations of southern Ap- tectonism. The presence of authigenic K-feldspar in palachian zinc deposits. Econ. Geol. 30, 260-286.

Valley and Ridge dolostones is believed to reflect the DALRYMPLE G. B. and LANPHERE M. A. (1974) 40Ar/39 age

loss of potassium from ascending basinal fluids as they spectra of some undisturbed terrestrial samples. Geochim. Cosmochim. Acta 38, 7 15-738.

cooled and attempted to move towards equilibrium in DALY R. A. (1912) Stratigraphy and structure of the Clark the K-feldspar-albite system. We believe that MVT de- Range: in America Cordillera-Forty-ninth Parallel. Geol.

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