Re-evaluation of lead isotopic data, southern Massif ... - Springer Link

Mineral. Deposita 28, 122-128 (1993)

lepineralium D osita

~C~Springer-Verlag 1993

Re-evaluation of lead isotopic data, southern Massif Central, France A.J. Sinclair 1, J.C. Macquar 2, and H. Rouvier 2 Department of Geological Sciences, The University of British Columbia, Vancouver, B. C. V6T 1Z4, Canada 2 Laboratoire de Geologie Appliqu6e, Universit6 Pierre et Marie Curie, 4, Place Jussieu, F-75252 Paris, France Received: December 14, 1991/Accepted: September 15, 1992

Abstract. Three independent Pb isotope homogenizing processes operating on large volumes of rock material during limited intervals in the Phanerozoic have been used to define a unique evolutionary curve for rock and ore lead isotopic compositions of the southern Massif Central, France. The model is x t = 18.641 - 9 . 5 6 ( e L l ~ t - 1) Yt -- 15.678 - 0.06934(e Lz~t - 1) z t = 38.701 - 30.8(e L3~t- 1) where xt, Yt and zt are the calculated isotopic ratios 2~176 and 2~176 respectively) at time t years ago and L1, L2 and L3 are the decay constants of 238U, 235U and 232Th respectively. The model gives the following ages for averages of the three sets of data used in its generation: age of mineralization for carbonate-hosted lead-zinc ore of Les Malines, ca. 150 Ma; emplacement of the Saint-Guiral-Liron granite, ca. 290 Ma; and an estimated average age of a group of Cambrian syngenetic deposits, ca. 520 Ma. These ages are in close agreement with ages determined by independent methods.

(2~176

During the past two decades several lead isotope studies have been carried out on rocks and ores of the Montagne Noire and Cevennes regions (Fig. 1) of the southern Massif Central, France (Brevart et al. 1982; Lancelot et al. 1971; Le Guen 1989; Le Guen and Lancelot 1989, etc.), and various arguable conclusions regarding lead evolution and/or ore genesis have emerged. Thus, we have been prompted to undertake a re-evaluation of the available Pb isotopic data from a different perspective than hitherto attempted. The early work of Lancelot et al. (1971) concluded that the 6 carbonate-hosted Zn-Pb deposits studied by them (one sample from each) were characterized by a uniform Pb isotopic composition which was interpreted to indicate a single period of mineralization for which they developed a flexible 3-stage evolutionary model. Subsequent work by

Brevart et al (1982) recognized two Pb isotope signatures, one for mostly stratiform deposits in Cambrian and Ordovician rocks and a second for post-Hercynian mineralization. They assumed two ages of mineralization, 540 Ma and 200 Ma, for the purpose of developing a 3-stage model of lead isotope evolution in the area. The first two stages of their model are not founded on data from the area and the third stage is based on limited data with considerable scatter. Le Guen (1989) and Le Guen et al. (1988, 1989) have contributed a substantial amount of local data in the particular case of Les Malines, a group of Mississippi Valley type deposits situated at the southern extremity of Les Cevennes (Fig. 1). They showed that various types of Pb-Zn mineralization at Les Malines (crosscutting bodies in Cambrian dolomites and Triassic

Fig. 1. Map showing location of the Massif Central, the Cevennes and Montagne Noire areas, the Saint-Guiral-Liron granite and the principal mineral deposits (black dots), especially Les Malines (largest Pb-Zn deposit). The approximately E W Line is the location of the cross-section of Fig. 2. Pale hached is Precambrian to Permian formations; dark hashed is Triassic to Jurassic formations; post Jurassic formations are white; the St. Guiral-Liron granite is black. Modified from Michaud (1980)

123 conglomerates; disseminations, fissures and stratabound bodies in Triassic conglomerates and shales and Bathonian dolomites) were characterized by a remarkable uniformity of ore Pb isotopic ratios. Both Charef (1982) and Marcoux (1986) have added to the available lead isotope data base, the former for Les Malines and the latter for numerous other deposits from the southern part of the Massif Central. Over the past 30 years a variety of mathematical models of lead isotopic evolution have been developed that purport to represent world averages for the crust, the mantle or specific global tectonic environments within the crust or mantle (Cumming and Richards 1975; Doe and Zartman 1979; Russell and Farquhar 1960; Stacey and Kramers 1975). These models or growth curves (and related isochrons) have been used extensively by others in the interpretation of local lead isotope data, particularly in calculation of model ages, despite caution by their originators. The use of a world average model for an age determination rests on one or other of the implicit assumptions that (1) the specific average model curve in question applies to a given data set, or (2) that isochrons derived from the model can be used to date mineralization. One of these assumptions must be demonstrated as applicable if a given model is to be used for dating mineral deposits. The second assumption (above) is not as critical for the estimation of Precambrian ages as for deposits of Phanerozoic age because the effect of variable U/Pb environments on model age calculation increases with decreasing age. Godwin and Sinclair (1982) have shown that for a large part of Western Canada, as the true known ages of shale-hosted Zn-Pb deposits become increasingly younger, the model ages calculated with some average crustal lead evolution models, become more and more incorrect. In fact, application of some crustal average models to the deposits in question gave model ages more than 300 Ma too young. Thus, Godwin and Sinclair (1982) were led to develop a new model specifically for western Canadian shale-hosted, stratiform Pb-Zn deposits. Their work showed that world average crustal growth curves, in general, cannot be applied locally; in other words, lead evolution is area-specific and local curves are particularly essential to describe Phanerozoic lead evolution. Of course, the areas and volumes of crust in question may be very large. In the Western Canadian example the area of original and recycled provenance extends from southern British Columbia to the northern Yukon, a distance of about 2000 kilometers and is at least several hundreds of kilometers in width.

Processes that homogenize Pb isotopes on a regional scale We have adopted the approach of Godwin and Sinclair (1982), that is, the development of an area-specific, lead evolution model consistent with the ages of known control points, for the southern Massif Central, France (Fig. 1). In our attempt to develop the general lead evolution model for this region we have used lead isotopic data that we believe to be representative of homogenization of lead

isotopic data by large scale processes operating at known times in the past. These processes include basin sedimentation, magma generation and ore fluid genesis from a large volume of crust.

Basin sedimentation Since the early work of Chow and Patterson (1962), among others, there has been recognition of the relative homogeneity of lead isotopic compositions of sediments in large modern sedimentary basins. This uniformity of composition has been interpreted to reflect homogenization of lead from a common provenance for the basin sediments. The implications of this work to ore genesis (Sinclair 1963) served as the philosophical basis for the work by Godwin and Sinclair (1982) in the development of a growth curve (the so-called Shale Curve) for the upper crustal evolution of lead in sedimentary basins containing continental material derived from the western part of the Canadian Shield. Their results necessitated (1) that recycling of Shield material into younger sedimentary basins retained closely the average geochemical character (in particular, with respect to U, Th and Pb) of the provenance, that is, of the Hudsonian Province of the western Canadian Shield, and (2) that the collecting mechanism of ore fluids from sediments within any one of the basins in question, extracted, transported and deposited Pb of approximately average basinal composition at the time of formation of stratiform Pb-Zn sulphide deposits formed within the basins. We have selected 8 lead isotopic analyses from Brevart et al. (1982) and Le Guen (1989) for stratiform mineral occurrences in the Montagnes Noirs, considered to be syngenetic and assigned a Cambrian age. More accurate ages for individual analyses are not available and we have assigned an average Cambrian age of about 520 Ma to the average isotopic value of the 8 samples for our purposes. If the Cambrian designation for these deposits is correct, our assumed average age must also be close to the true age.

Magma generation Michard-Vitrac et al. (1981) studied various geochemical characteristics of widely distributed Hercynian granitic bodies in the Massif Central and elsewhere. Their lead isotope results prompted them to conclude that lead was of homogeneous composition within individual granitoid bodies, and that the granitic lead represented recycled crustal material with little or no evidence of a mantle contribution. Moreover, they showed that despite a suggestion of a regional trend to the lead isotopic composition of granitoid bodies, the compositions indicated for 12 Hercynian bodies show little dispersion. Unfortunately, they do not include U and Pb analyses with their lead isotope ratios; consequently, the present day values cannot be recalculated back to their original values at the time of emplacement, Nevertheless, their present day isotopic abundances are comparable to those of Le Guen's data (4 analyses for feldspars in the Saint-Guiral-Liron

124

granite) for which she provides the prerequisite U and Pb analytical data as well. Thus, we have used Le Guen's 4 lead isotopic analyses of feldspars from this 280-290 Ma intrusive mass (Rb/Sr results, see also Vialette and Sabourdy, 1977a, 1977b) as part of our control data set.

Ore fluid channeling The work of Godwin and Sinclair (1982), is an application of the tenet that basinal fluids are responsible for the development of exhalative Pb-Zn deposits that are a common component of many sedimentary basins. A companion work (Godwin et al. 1982) demonstrates the liklihood that shale-derived, basinal fluids can be directed to marginal carbonate zones to form carbonate hosted deposits of the Mississippi Valley type (MVT) and, in some cases, to result in Pb isotopic signatures that are uniform and which plot on the average growth curve for the region. Sheppard and Charef (1990) have interpreted carbonate-hosted Pb-Zn deposits in the southern Massif Central, including Les Malines and nearby deposits, in this manner based on their structural setting (basin margin, see Fig. 2), local structural control (karst, fissures, breccias, etc) and known geochemical characteristics (S and Pb isotopes) of the deposits. Le Guen (1989) presents 62 analyses from a variety of types of Pb-Zn occurrences in carbonates of various ages CAUSSES

W

(Cambrian, Triassic and Jurassic) in a very restricted area (about 12km2), Les Malines, and concludes that they have a remarkably uniform Pb isotopic composition. We interpret this to indicate that the very large sampling of lead represented in her analyses is very likely an average of the large basin from which the ore fluids were derived, probably during a single, major period of mineralization (cf. Macquar et al. 1990). In our interpretation this period of mineralization includes at least Phase 1 and Phase 2 of the three paragentic phases of mineralization described by Macquar et al (1988), although we recognize that basin dewatering leading to ore formation may have extended intermittently over a considerable period of geological time, perhaps as much as 20 Ma based on the dispersion of Le Guen's data. In our opinion the mineralizing interval must have included the Kimmeridgian stage because Kimmeridgian strata near Les Malines are mineralized (BoisMadame deposit) and have a lead isotopic signature similar to the average for Les Malines (cf. Marcoux, 1986, annexe 2, page 41).

Pb isotopic evolution, Cevennes-Montagne Noire Area, France The data that we use in developing a Pb isotope evolution model are shown plotted on Figs. 3 and 4 and are available in references cited above. Averages and standard

CEVENNES

SUBALPIN BASIN E ,0

13 11

\

:-

1000

10

Bois Madame 2000 Ls, fine grained Ls, w, calpionella ~ _ . ~ Ls, w. chert Ls, bioclastic Ls, oolithic Ls, argiltac~ous marls and arg. Ls mads and clays ---

~ ~ ~ ~ ~ ~ ~ ~

~

secondary dol. primary doL anhydrite-gypsum halite ss and cong. ~ and shale Q13 cong. Dol. cong.

Forms of mineral deposits ~. mineralized strata and stratiform karstle 31 |racture controlled

7

3000 Les Malines

4000 m

50 km. Fig. 2. East-West cross section showing the principal stratigraphic units of the Cevennes area and the stratigraphic positions of known Pb-Zn mineral deposits in the area. See Fig. 1 for location of the section. Datum is the beginning of the Cretaceous. Numbers 1 14 represent ages of strata as follows: I Trias; 2 Hettangian;

3 Sinemurian; 4 Lotharingian; 5 Carixian; 6 Toarcian-Domerian; 7 and 8 Bajocian; 9 Bathonian; 10 and 1l Oxfordian-Callovian; 12 Oxfordian; 13 Kimmeridgian; 14 Portlandian. From J.C, Macquar, in Routhier (1980)

125 The dispersion of isotopic abundances in each of our three groups of control data (see Table 1) permits some leeway in determining the lead evolution model; we defined a model with mu = 9.56, coordinates x = 18.641 and y = 15.678 for t = 0 Ma and x = 17.757 and y = 15.626 for t = 570 Ma. The model can be expressed as follows:

15.75 15.70 ...0 13_

o tL..4

15.65 .B

n 15.60 l

f~

o

15.

55 I

M (n=61)

15.50 17.4

I

I

I

17.8

I

I

18.2 206pb/204p b

I

19.0

39.0

LM c, 38.5 0.. o o4

-~ 38.0

~/ S

K

,

.

r

C (n=8)

o4 37.5 M (n=61) 37.0 17.4

I

I

17.8

I

I

I

x t = 18.641 - 9.56 (e L t ' t -- 1)

(2)

Yt = Yo - (mu/137.88) (e L2*t - e L2*t~

(3)

y~ = 15.678 - 0.06934 (e L 2 * t -

Fig. 3. Control data and average growth curve (LM) for z~176 vs 2~176 Cevennes-Montagne Noire region. Three averages of control data sets are: Les Malines deposits (M), Saint-Guiral-Liron granite (G) and Cambrian syngenetic deposits (C), The upper growth curve (LM) is that developed in the text; the lower growth curve (SK), shown for comparison, is the StaceyKramers model with muz = 9.736 for stage 2. Points shown on the LM growth curve correspond to time scale boundaries listed in Table 2

...O rl {30 o

(1)

and

I

18.6

xt = Xo - mu*(e Ll-t - e Ll't~

I ,

18.2 18.6 206pb/204p b

I

19.0

Fig. 4. Control data and average growth curve for 2~176 vs 2~176 Cevennes-Montagne Noire region. Averages of two groups of control data are: Les Malines deposits (M) and Cambrian syngenetic deposits (C). The upper growth curve (LM) is that developed in the text; the lower growth curve (SK), shown for comparison, is the Stacey-Kramers model with parameters mu 2 = 9.736 and kappa 2 = 36.8 for stage 2. Points shown along the LM growth curve correspond to time scale boundaries listed in Table 2

deviations for the three sets of data are listed in Table 1. We first developed a U - P b - b a s e d growth curve for two control sets of data, the Saint-Guiral-Liron granitoid b o d y (ca. 290 Ma) and data for the C a m b r i a n syngenetic deposits (ca 520 Ma) with the idea that we would extend this curve to see whether or not mineralization at Les Malines, for which there is some controversy on age(s), might compare. In brief, the two extreme points of view concerning age of mineralization at Les Malines are (1) multiple stages of mineralization (Triassic, Liassic and CaUovian), and (2) a single major period of mineralization that must be at least as y o u n g as the youngest rocks mineralized.

1)

(4)

where L~ and Lz are the decay constants of 238U and 235U respectively (0.155125 x 10 - 9 yr -1 and 0.98485 x 10 . 9 y r - i respectively), Xo and yo are estimates of the present day values of the lead isotope ratios z~176 and 2~176 respectively, t is the age in years for which the lead isotope ratios x t and Yt are calculated. and, mu is the present day value of 2 3 8 U / 2 ~ taken as 9.56 in the model. This model is consistent with the following ages for our control data sets: Les Malines mineralization, ca 150 Ma; Saint-Guiral-Liron granite, ca 290 Ma; and C a m b r i a n syngenetic deposits, average of 520 Ma. The resulting growth curve for average lead isotopic evolution (U-derived) in the Cevennes-Montagne Noire area is shown in Fig. 3 for the past 700 Ma. We elect not to extent it further back in time because no control exists in the area beyond 700 Ma ago. Coordinates for the model are listed in Table 2 for a variety of ages that define the generally accepted time scale (Palmer 1983). Where appropriate (i. e. expectation that Pb isotopes represent average crustal evolution in the area), the model can be used as a quantitative dating tool either by direct application of equation 2 above, or by linear interpolation between the points listed in Table 2. F o r such applications of the model to dating other events we feel that only the model for the x-coordinate can be used to obtain realistic estimates of ages; this arises from the fact that analytical error is roughly the same magnitude as the changes expected in 2~176 values over the past few hundred million years. It is also possible to calculate a growth curve involving thorogenic lead (2~ but, because a Th decay correction term could not be applied to the rock leads, only two of the control data sets are available for this purpose (see Table 1 and Fig. 4). Accepting the ages determined above by the uranogenic lead model we can solve the equation Zl = Z2 -- K*(e L3"tl -- e L3.t2)

(5)

where z~ and z 2 are the 2~176 ratios at times t~ and t2 respectively (Table 1), L3 is the decay constant of 232Th (0.049475 x 10 - 9 y r - 1), and K (kappa) is the present day value of 232Th/2~

126 Table I. Average lead isotope values for the three groups of control data

Group Les Malines h (M) Trias host Lias host Bath host Post-Bath? Average Saint-GuiralLiron Gran.b(G) Cambrian syngenetic depositsc(C)

n

x

y

z

29 4 25 3 61

18.406(0.022) 18.444(0.007) 18.423(0.026) 18.426(0.003) 18.416(0.026)

15.668 (0.019) 15.686 (0.005) 15.672 (0.019) 15.669 (0.011) 15.671 (0.019)

38.479 (0.061)" 38.546 (0.037) 38.488 (0.064) 38.488 (0.036) 38.471 (0.061)

4

18.201(0.022)

15.660 (0.008)

8J

17.84(0.075)

15.62 (0.023)

37.90 (0.109)

a bracketed figures are standard deviations of the data; bdata of Le Guen (1989); c data of Brevart et al (1982) and Le Guen (1989); d n = 7 for the average 2~176 value

Table 2. Tabulation of points on the average Pb isotope evolution model for Cevennes-Montagne Noire area"

Time (Ma)

x (2~176

y (2~176

z (2~176

0 23.7 66.4 97.5 144 163 187 208 230 240 245 286 360 408 438 505 570 600 650 700

18.641 18.606 18.542 18.495 18.425 18.396 18.360 18.328 18.294 18.278 18.271 18.207 18.092 18.016 17.969 17.862 17.757 17.708 17.627 17.544

15.678 15.676 15.673 15.671 15.667 15.666 15.664 15.662 15.660 15.660 15.659 15.655 15.648 15.644 15.641 15.633 15.626 15.622 15.616 l 5.609

38.701 38.665 38.600 38.552 38.481 38.452 38.415 38.382 38.349 38.333 38.325 38.262 38.148 38.073 38.026 37.922 37.820 37.773 37.694 37.616

Base of b

Neogene Paleogene Late Cret. Early Cret. Late Jur. Mid Jur. Early Jur. Late Trias. Mid Trias. Early Trias. Permian Carboniferous Devonian Silurian Ordovician Cambrian

" Based on a model with mu = 9.56 and kappa = 30.8 which returns the following ages for control data: an average age of Cambrian syngenetic deposits, tc, of 519 Ma; age of SaintGiral-Liron granite, tc, of 288 Ma; and an age of mineralization at Les Malines, tM,of 150 Ma; b Time scale of Palmer (1983}

Using e q u a t i o n 5 we calculate K = 30.8. Thus, the model can be expressed as zt = 38.701 - 30.8"(e e3*t - 1)

(6)

Discussion O u r model has been developed for a relatively confined area but the u r a n o g e n i c c o m p o n e n t , especially the equation for x (2~176 may have considerably broader application. F o r example, the use of C a m b r i a n sedim e n t a r y data to establish the model implies a regional application. It is interesting to examine granitoid bodies on a more regional scale t h r o u g h o u t the French Massif Central. Lead isotopic data reported by Michard-Vitrac et al. (1981) for a n u m b e r of different intrusions are similar to data reported by Le G u e n (1989). Unfortunately, the lead isotopic data of Michard-Vitrac et al. (1981) do not

have a c c o m p a n y i n g u r a n i u m and lead analyses so present day isotopic values c a n n o t be corrected to the 290 M a age (Rb/Sr isochron method) attributed to the host intrusions. It is likely that the mu values (present day 238U/2~ ratios) will be low; hence, the corrections, although significant, will be small. The thorogenic c o m p o n e n t of our model (equation 6) may not have very wide application because it represents only two sets of control data and consequently, a single T h / U ratio. M a n y studies have shown that wide variations exist in T h / U ratios in the crust and that control data for average evolutionary models involving thorogenic crustal lead, plot with much more scatter on 2~176 vs 2~176 diagrams than on 2~176 vs Z~176 graphs. O u r single result could well be one of a family of such curves that corresp o n d to variable T h / U ratios for more local segments of the crust than that to which the uranogenic model (equation 2) applies.

127 Table 3. Parameters of several plausible growth curve models 1. 2. 3. 4. 5.

tM

tc

tc

mu

k

Th/U

x

y

z

163 150 150 146 140

290 288 290 288 288

498 513 519 520 529

10.53 9.70 9.56 9.40 9.06

33.9 31.3 30.8 30.1 29.2

3.20 3.20 3.20 3.18 3.20

18.686 18.644 18.641 18.630 18.615

15.678 15.678 15.678 15.678 15.678

38.746 38.705 38.701 38.696 38.674

tM = age of Les Malines mineralization, tC = age of Saint-Guiron-Liron granite; tc = age of Cambrian syngenetic deposits; the remaining parameters are described in the text The kappa and mu parameters of the model can be used to estimate an average T h / U ratio of 3.20 for crustal rocks represented by equations 2 and 6. A value of 3.20 is reasonable for a crustal environment but, for reasons indicated above, may not truly represent a crustal average in the area. However, Th/U ratios calculated as above are relatively insensitive to small changes in mu and/or kappa (see Table 3); for example, if mu = 9.4, kappa = 30.1, we calculate a T h / U weight ratio of 3.18. Ages in the model are slightly sensitive to changes in mu values (Table 3). For example, another model which fits the data reasonably well has the following parameters: m u = 9 . 4 , k a p p a = 3 0 . 1 , Xo= 18.630, yo=15.678, zo = 38.696, and gives the following ages for average isotopic values of the control data sets: tc = 520 Ma, t~ = 288 Ma; and t M = 146 Ma. Le Guen et al. (1989) consider mineralization of Bathonian rocks to have occurred during the Callovian. If we select 163 Ma (Collovian-Oxfordian boundary according to Palmer (1983), we calculate an age of 498 Ma for the average of the Cambrian isotopic data (Table 3). This latter age is inconsistent with the time scale of Palmer (1983) but the comparatively large dispersion of the Cambrian isotopic data does not completely rule it out as a possibility. The various models summarized in Table 3 give some idea of the error inherent in the growth curve. Our model has been weighted by group C data to slightly lower values of 2~176 than are optimal to explain average values for groups G and M. The Cambrian data may not be totally consistent with data for the other two groups because 7 of the 8 values that we used were not analyzed in the same laboratory as the rest of the data. Consequently, we look to new data to refine the model presented here. It should be noted, however, that changes in the y-coordinate alone will have no effect on the ages quoted here and based on the mean isotopic compositions of the three control groups because only equation 2, involving the x-coordinate, has been used in our calculations. Nevertheless, new lead isotopic data for Brioverian and Cambrian rocks in particular, combined with more accurate age control for such data, could provide some refinement to the model proposed here. Similarly, improvements in the accuracy of ages of mineralization at Les Malines and for the Saint-Guiral-Liron granite could also alter parameters of the model slightly.

Conclusions A single growth curve representing crustal lead isotope evolution for the Cenvennes and Montagne Noire regions

of the southern Massif Central has been estimated from lead isotopic data representing three independent crustal homogenizing processes, viz. basin sedimentation, m a g m a generation and ore fluid derivation from large volumes of crust. For the area in question these processes represent events occurring during the Cambrian, late Hercynian and mid Mesozoic times respectively and provide control points for the lead evolutionary model. The model unifies 3 independent, large scale crustal processes and provides realistic ages for each of the three events used for control; thus, the model represents a better framework upon which to base interpretation of the multitude of lead data for the area (e.g. Michard-Vitrac et al. 1981) than do so-called world average curves such as those of Stacey and Kramers (1975) and Cumming and Richards (1975). In particular, the model provides a basis for dating mineral deposits that meet the criterion of 'homogenization of source lead' as well as providing specific support for a Kimmeridgian age for mineralization at Les Malines.

Acknowledyements. AJS was supported by a Natural Sciences and Engineering Research Council of Canada operating grant. He thanks Dr, G. de Marsily for the opportunity to spend time as Visiting Professor at the Laboratoire de Geologie Appliquee, Universite P. and M. Curie, Paris, where this work was carried out, and appreciates the many courtesies and support extended him.

References Brevart, O., Dupre, B., and Allegre, C.J. (1982) Metallogenic provinces and the remobilization process studied by lead isotopes: lead-zinc ore deposits from the southern Massif Central, France. Economic Geology 77:564-575. Charef, A. (1982) La nature et le role des phases associees a la mineralisation Pb-Zn dans les formations carbonatees et leurs consequences metallogeniques. Etude des inclusions fluides et des isotopes (H, C, O, S, Pb) des gisements des Malines (France), Fedj-el-adoum et Jbel-Hallouf-Sidi Bou Aouane (Tunisie). These d'etat, Nancy, France, 291 p. Chow, T.J., Patterson, C.C. (1962) The occurrence and significance of lead isotopes in pelagic sediments. Geochimica Cosmochimica Acta. 26:263 308 Cumming, G.L., and Richards, J.R. (1975) Ore lead isotope ratios in a continuously changing earth. Earth and Planetary Science Letters, 28, 155-171 Doe, B.R., Zartman, R.E. (1979) Plumbotectonics: Chap. 2. In: Barnes, H.L. (ed.) Geochemistry of hydrothermal ore deposits, 2nd edn, Wiley, New York, pp. 22-70 Godwin, C.I., Sinclair, A.J. (1982) Average lead isotope growth curve for shale-hosted lead-zinc deposits, Canadian Cordillera. Economic Geology. 77:675-690 Godwin, C.I., Sinclair, A.J., Ryan, B.J. (1982) Lead isotope models

128 for the genesis of carbonate-hosted Zn-Ba, shale-hosted Ba-ZnPb and silver-rich deposits in the northern Canadian Cordillera. Economic Geology 77:82-94 Lancelot, J., Sarazin, G., Allegre, C.J. (1971) Compositions isotopiques du plomb el du soufre des galenes liees aux formations sedimentaires. Interpretations geologiques et geophysiques. Mineralogy Petrology 32:315-333 Le Guen, M. (1989) Les mineralisations Pb-Zn du Bathonien dans la region des Malines (gard) : cadre geodynamique, caracterisation isotopique (Pb) et implications genetiques. These de Doctorat, Universite des Sciences et Techniques du Languedoc, 231 pp. Le Guen, M., Lancelot, J.R. (1989) Origine du Pb-Zn des mineralisations du Bathonien sud-cevenol. Apport de la geochimie isotopique comparee du plomb des galenes, de leur encaissant et du socle. B.R.G.M. Chronique de la Recherche Miniere, No. 495, pp. 31-36 Le Guen, M., Orgeval, J.J., Lancelot, J.R. (1989) Lead isotope behaviour in a polyphased Pb-Zn ore deposit (The Malines, France). Unpublished manuscript as appendix in Le Guen (1989) Le Guen, M., Orgeval, J.J., Lancelot, J.R. (1988) Composition isotopique du plomb des mineralisations Pb-Zn d'un gite a evolution polyphasee: Les Malines (Gard, France), B.R.G.M, Rapport d'activites, Orleans, France Macquar, J.C., Marini, D., Rouvier, H. (1988) Metallogenie posttriasique dans le district plombo-zincifere des Malines (Gard). In: Johan, Z., Ohnenstetter, D. (eds,) Gisements metalliferes dans leur contexte geologique. Document du B.R.G.M. No. 158, Vol. I1, pp. 773 802 Macquar, J.C., Rouvier, H., Thibieroz, J. (1990) Les mineralisation Zn, Pb, Fe, Ba, F, peri-cevenoles: cadre structuro-sedimentaire et distribution spatio-temporelle. In: Mobilite et concentration des metaux de base dans les couivertures sedimentaires manifestations, mecanismes, prospection. Document du B. R. G. M., No. 183, 143-155, 1990. Marcoux, E. (1986) Isotopes du plomb et parageneses metalliques,

traceurs de l'histoire des gites mineraux. Illustration des concepts de source, d'heritage et de regionalisme dans les gites francais applications en recherche miniere. These de Doctorat d'Etat, Clermont-Ferrand, Document du B. R. G. M., No. 117, 289 p. Michard-Vitrac, A., Albarede, F., Allegre, C.J. (1981) Lead isotopic composition in Hercynian granitic K feldspars constrains continental genesis. Nature 291:460 464 Michaud, J.G. (1980) Gisements de plomb-zinc du sud du Massif Central francais (Cevennes-Montagne Noire) et caracteristiques geologiques de leur environnement. Bulletin Centre de Recherche Elf-Aquitaine 3:335-380 Palmer, A.R. (1983) The decade of North American geology, 1983, Geological time scale. Geology 11:501-564 Routhier, P. (1980) Ou sont les metaux pour l'avenir? Les Provinces metallogeniques, essai du metallognenie globale. Memoire du B. R. G. M., No. 105, Fig. 79 h.t., Orleans, France Russell, R.D., Farquhar, R.M. (1960) Lead isotopes in geology. Interscience, New York, 141 p. Sheppard, S.M.F., Charef, A. (1990) Isotopic studies (H, C, O, S, Pb) on carbonate-shale hosted Pb-Zn deposits. Document du B. R. G. M., No. 183, pp. 37-49 Sinclair, A,J. (1963) Oceanic leads and ore genesis, a discussion. Economic Geology 60:1533 1538 Stacey, J.S., Kramers, J.D. (1975) Approximation of terrestral lead isotope evolution by two stage model. Earth Planetary Sci Letters 26:207 221 Vialette, Y., Sabourdy, G. (1977a) Age et origine de granitoides du Mont Lozere dans le Massif central francais. Compte Rendu Somm. Soc. Geol. France, Fasc. 3:127-129 Vialette, Y., Sabourdy, G. (1977b) Age du granite de l'Aigoual dans le massif des Cevennes (France); Compte Rendu Somm. Soc. Geol. France, Fasc. 3:130 132 Editorial handling: D. Rickard