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Application of lead isotopes and trace elements to mapping black shales around a base metal sulphide deposit
Journal of Geochemical Exploration, 8 (1977) 85--103
85 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
APPLICATION OF LEAD ISOTOPES AND TRACE ELEMENTS TO MAPPING BLACK SHALES AROUND A BASE METAL SULPHIDE DEPOSIT
BRIAN L. GULSON*
Division of Mineralogy, Commonwealth Scientific and Industrial Research Organization, North Ryde, N.S.W. (Australia) (Revised version received February 23, 1977)
ABSTRACT Gulson, B.L., 1977. Application of lead isotopes and trace elements to mapping black shales around a base metal sulphide deposit. J. Geochem. Explor., 8: 85--103. Pb isotopic analyses have been used in southeastern New South Wales, Australia, to distinguish Ordovician black shales, which have no associated mineralization, from Silurian black shales in which mineralization is known to occur. The more radiogenic nature of the Ordovician Pb, as shown by analysis of the sulphide, whole rock, acid leach or residue, reflects a higher U/Pb environment compared with the Silurian which is due to the absence of volcanism in the Ordovician. V concentrations and Co/Ni ratios in sulphides may prove to be an important indicator to distinguish Silurian and Ordovician black shales but lithology, mineralogy, whole rock major and trace elements, and C and S isotopes give ambiguous answers. Co/Ni ratios in the Silurian black shale sulphides average - 0.26 compared with 0.04 in the Ordovician sulphides. On the basis of Pb isotope and V concentration data, an unconformity between Middle-Upper Ordovician and Middle-Upper Silurian rocks and overturning of the strata is proposed for a drill hole previously mapped on palaeontological evidence as entirely Ordovician. No correlation between organic C and U concentration was observed in the whole rocks. An approximate negative correlation exists between ~08 pb/206 Pb and V and Ni contents; with higher Ni and V concentrations the sulphides have lower 2o8 pb/206 Pb ratios but higher 206 pb/~0, Pb ratios. Shales distant from mineralization appear to have been wholly influenced by marine processes in comparison with samples close to the mineralization which are affected by volcanic conditions.
INTRODUCTION
In s o u t h e a s t e r n N . S . W . , A u s t r a l i a , P b - Z n - C u m i n e r a l i z a t i o n s u c h as a t W o o d l a w n a n d C a p t a i n s F l a t is r e s t r i c t e d t o M i d d l e - U p p e r S i l u r i a n v o l c a n i c s * Also: Visiting Fellow, Research School of Earth Sciences, Australi:m National University, Canberra, A.C.T., Australia.
86
B,L. GULSON
which are intimately associated with black and dark grey shales. It is these volcano-sedimentary sequences which are the "target" for exploration. The Middle-Upper Ordovician sedimentary sequence, which has no known associated mineralization b u t also contaias blac k shales, unconformably underlies the Silurian rocks and because of the similarity of the black shales in hand specimen, difficulties have been encountered in distinguishing between them, particularly during core logging. A paucity of fossils (Felton and Sherwin, 1974) c o m p o u n d s the problems of mapping. Attempts to distinguish the shales other than by lithological means have proved inconclusive. McKay (1973) first tried major and trace element whole rock data on a limited number of samples and later, Petersen and Lambert (in preparation) have carried out a more extensive chemical survey on rocks which yields no greater discrimination. X-ray diffraction data (this study) are ambiguous, the mineralogy for both types being quartz (dominant), chlorite, muscovite and traces of feldspar, sulphide and calcite. Similar S and C isotopic values (see later section) are obtained for the Silurian and Ordovician rocks. During the course of a detailed Pb isotopic investigation around Woodlawn, initial analyses of one Ordovician and one Silurian pyrite showed marked differences in Pb isotopic ratios. Consequently an extended study was undertaken in this area b u t with no prior knowledge of the stratigraphy of the .samples. This study presents additional data following on the earlier preliminary work (Gulson, 1976), and includes data for a proposed unconformity in a drill hole logged solely as Ordovician. It also presents data for localities in central western N.S.W. chosen as a check on the regional extent of this Pb isotopic distinction between the Ordovician and Silurian. A summary of trace element data on the sulphides is also given. Pb isotopic investigations make use of variations in the daughter isotopes 2o6 Pb, 2o7 Pb and 2o8 Pb produced by radioactive decay from their respective parents 238 U, 23s U and 232 Th. The variations are usually expressed as a ratio of the particular daughter isotope to the orphan isotope 204 Pb (i.e. 204 Pb has no known parent). GEOLOGICAL SETTING OF THE WOODLAWN DEPOSIT The Woodlawn polymetallic sulphide deposit occurs approximately 200 km southwest of S y d n e y (Fig. 1). It is located in a sequence of acid volcanics and sediments of Middle to Upper Silurian age which form part of the Lower Palaeozoic succession in the Lachlan Fold Belt. The geological setting is similar to that found at Captains Flat, some 70 km south of Woodlawn. In the immediate vicinity of Woodlawn, sedimentary rocks are poorly represented in the lower part of t,he Silurian succession and a sequence (called the Woodlawn v o l c a n i c s ) o f acidic tuffs, rhyolites, ignimbrite agglomerates and acid volcanoclastics is developed (Felton, 1974). The immediate
APPLICATION OF LEAD ISOTOPES TO MAPPING
87
r
145
150
30
NEW
SOUTH WALES
Central
145
I
Wosdlav, n area
I
0
100
J
200
300
kilometres
Fig. 1. Locality map of the Woodlawn area and central western N.S.W., Australia, from where samples were selected. host rocks to the massive sulphides consist of rhyolitic tuffs and flows, black shales, chloritic schists and rare chert-like rocks. The Silurian succession is unconformably underlain by Middle to Upper Ordovician sediments and is overlain by the Currawang Basalt. The whole sequence has been regionally m e t a m o r p h o s e d to produce highly deformed lower greenschist facies rocks and is intruded by Upper Silurian dolerite and Devonian granite (Malone et al., 1975). The main massive sulphide lens is a polymetallic sulphide b o d y consisting of pyrite as the main constituent and variable amounts of sphalerite, galena and chalcopyrite. Silicate gangue minerals are chlorite, quartz, talc, sericite and feldspar. Stringers of sulphides are c o m m o n l y found throughout the volcanic succession. EXPERIMENTAL PROCEDURES
Sulphides. Pyrite and pyrrhotite were separated from the shales and volcanics by crushing, heavy liquid separation using tetrabromoethane and methylene iodine, sieving and then magnetic separation of the minus 100, plus 200mesh fraction. The samples (usually > 90--95% pure) were washed in 6N HC1 for 15 minutes (% 2 -3 minutes for pyrrhotite), rinsed with water and boiled in water for a b o u t 15 minutes (all reagents were prepared using subboiling distillation methods in silica glass or Teflon stills).
88
B,L. GULSON
Pyrite was dissolved in 7N HNO3/6N HCI (3:1) with a few drops of bromine water, whilst pyrrhotite was dissolved in 6N HC1. For the shales, an insoluble residue of carbonaceous material was usually present which amounted to less than 3% by weight of the sample. The solutions were ion exchanged on a 2-cm 3 anion column in bromide form and further cleaned on a 0.5-cm 3 anion column in chloride form.
Rocks. Minus 200-mesh samples, prepared under clean conditions, were " d e c o m p o s e d " in Teflon bombs using HF-HNO3 but, in all cases, even with nitric acid/sodium chlorate (E. Kiss, personal communication, 1975) and more HF treatment, some residue of carbonaceous matter, quartz and chloritic material was always present. Acid leaches were effected with 7N HNO3/6N HC1 (5:2) to dissolve the ore component, mainly pyrite. The residue was then dissolved in HF as for rocks. The ion exchange procedure for the rock analyses was the same as for the sulphides. Mass spectrometry. Samples were analyzed on the high ion beam mass spectrometer at the Research School of Earth Sciences, Australian National University. The "within-run" percent standard deviation ( l o ) of a set of 12--20 ratios ranged from 0.06 to 0.7%, with the average usually 0.1--0.2%. Runs with standard deviations greater than 0.3% are indicated in Tables ! and II. Duplicate dissolution data are given in the tables. All analyses were performed with a mixed 207 pb/204 pb_2as U double spike to control mass fractionation. DISCUSSION OF RESULTS
Lead isotopes from the Woodlawn area Analytical data for sulphides and whole rocks are given in Tables I and II and plotted in Figs. 2, 3 and 4. Most o f the d a t a in Table IA are from Gulson (1976). Minor differences in values for 6 samples, particularly for 2°TPbf°4Pb ratios, in Table IA and GuIson (1976) are due to recalculation of the fractionation correction. For clarity, only the field of data for the Silurian sulphides is outlined. The preponderance of Silurian over Ordovician data is purely a reflection of drilling for " t a r g e t " beds and this limits the availability of Ordovician samples, particularly away from mineralization. The variability and radiogenic nature, n o t only of the Ordovician sutphides, but also the rocks, compared with the Silurian black shale data are the obvious features of the results:
Ordovician Silurian
206pb/2O4Pb
2o~pb/2O4Pb
=ospb/2O4Pb
20.4--38.6 17.9--19.3
15.8--16.7 15.5--15.7
38.8--43.6 37.7--40.7
A P P L I C A T I O N OF L E A D ISOTOPES TO M A P P I N G
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When subdivided, mainly on the basis o f 2°6 pbp°4 pb ratios, into possible Silurian and Ordovician samples, the data agreed in every case with the fossil and stratigraphic controls. The isotopic values for the Silurian samples close to mineralization (< 2 km) are similar to those for Pb in sulphides from the ore horizon (pyrite,
90
B.L GULSON
galena, chalcopyrite, pyrite) and pyrite from the immediate ore host volcanics (the average value of the ore Pb is given by a solid cross in Figs. 2, 3 and
4). Similar isotopic ratios and U and Pb concentrations were obtained for coexisting pyrite and pyrrhotite. In most cases, the transformation of pyrite to pyrrhotite could be traced to the proximity of the shales to intrusive dolerites. The variability in isotopic ratios and elemental concentrations over short distances in either Silurian or Ordovician sulphides is marked, e,g. in diam o n d drill hole (DDH) CB-1, W-43, WE-1. This variability not only applies to Pb isotopes and trace elements (Tables I and III) but also to S and C isotopes and organic C contents (Table IV). U and Pb concentrations in sulphides from the Ordovician and Silurian overlap and, of course, are highly variable. Although the data are limited, particularly for the Ordovician, the Ordovician sulphides do appear to have, on the whole, higher U and lower Pb concentrations than the Silurian sulphides. On the 207 pb/204 Pb vs. 506 pb/204 Pb plot in Fig. 2, the solid line is that for the whole rock Silurian acid volcanics from around Woodlawn (Gulson, 1977a). Most of the high-quality data for the black shales lie to within experimental error of this line. Its slope is equivalent to an apparent age of about 350 m.y. compared with a stratigraphic age of about 430 m.y.; however, in this age range small variations in slope result in disproportionately
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large variations in age, so the apparent age deficiency cannot be regarded as significant. The scatter of the data is greater, particularly for the Ordovician samples, on a 2o8 pb/204 Pb vs. 2o6 pb/204 Pb plot (Fig. 3, cf. Fig. 2). However, a number of important features, besides the more radiogenic nature of the Ordovician Pb, may be noted: (1) The low 20s pb/20a Pb ratio of the Ordovician shales which is equivalent to a Th/U ratio of 0.4--0.9 compared with about 4.4 for the Silurian volcanics. (2) The acid leach-rock-residue data for the Silurian shales lie closer to the volcanic line than to the Ordovician shale line. A single, comparatively radiogenic pyrite (50051) also lies close to the volcanic line and is similar to radiogenic sulphides separated from the votcanics (Gulson, in preparation). (3) Some sulphide data for Silurian black shales (e.g. 50044, 50048, 46192) distant (> 10 km) from the Woodlawn mineralization are comparatively radiogenic and plot on the Ordovician black shale line. The distinction can be more clearly seen in Fig. 4, a plot of 207 pb/:06 Pb vs. 208 pb/:06 Pb, the advantage of which is to overcome problems of measurement of the small 204 Pb isotope. Correction of the Silurian sulphides for Pb derived by radioactive decay from U since about 430 m.y. results in less scatter than the uncorrected data, but this correction may be meaningless because of changes in the U/Pb ratio due to post-depositional processes or even during mineral separation where the ratio of sulphide to carbonaceous material may be important (Peltola, 1968). Furthermore, the enormous scatter on a 238 U/204 Pb vs. 206 pb/204 Pb plot indicates either recent m o v e m e n t or partitioning during mineral separation of U.
Lead isotopes indicative o f an unconformity and overturned sequence at Woodlawn The first analysis of pyrite and pyrrhotite from DDH W-43 at 183.8 m (39290; Table IA, Ordovician) exhibited Ordovician-type ratios (and also Ni and V contents) which agreed with the presence of Ordovician graptolites found in the upper part of the hole. A new sample at 195 m (46187; Table IA, Silurian) contained only pyrite and the isotopic data and trace elements are more typical of Silurian shales. As the possibility existed that this sample had been mixed with another during either sampling or processing, a further sample was analyzed from 185.6 m (49018; Table IA, Silurian), Although it was more radiogenic than the pyrite from 195 m, the pyrite (no pyrrhotite) also gave isotopic and trace element data of the Silurian type. At this stage, it became apparent that there was possibly an u n c o n f o r m i t y and overturned succession in t h e hole, so a further two (whole rock) samples higher in the core were analyzed. Both of these, from about 182 (52010; Table II) and 183 m (52011; Table Ii), gave values typical of those found in other
APPLICATION OF LEAD ISOTOPES TO MAPPING
97
O r d o v i c i a n samples, s u p p o r t i n g the inferred u n c o n f o r m i t y a n d o v e r t u r n i n g . T h e p a u c i t y o f fossils in this area and d i f f i c u l t y o f finding t h e m in drill core makes palaeontological confirmation remote.
Whole rock and acid-leaching experiments Because o f the time involved in s e p a r a t i o n o f the sulphides and their absence in some cases due t o o x i d a t i o n , w h o l e r o c k samples were investigated as a possibly simpler m e t h o d o f distinguishing the O r d o v i c i a n a n d Silurian black shales. T h e data, given in Table II, c o n f i r m the radiogenic n a t u r e o f the O r d o v i c i a n samples and thus the use o f w h o l e r o c k samples. The d a t a for the acid-leaching e x p e r i m e n t s (Gulson, 1 9 7 7 b ) on b o t h Silurian and O r d o v i c i a n w h o l e r o c k s s h o w t h a t the acid leaches and residues are c o n s i s t e n t l y d i f f e r e n t as in the sulphides and w h o l e rocks.
Carbon and sulphur isotopes C and S i s o t o p i c analyses were p e r f o r m e d b y J. S m i t h a n d M. Burns in a search for alternative and less expensive m e t h o d s , besides the elusive fossils, t o s u b s t a n t i a t e the Pb i s o t o p e data. T h e limited isotopic data, given in Table IV, f o r the f o u r Silurian samples f r o m the one drill bole, s h o w e d slight varia t i o n in 5 ~3 C values ( - - 2 7 . 8 t o --29.00/00 ) b u t large d i f f e r e n c e s in 5 34 S values (--0.1 t o +13.5°/00 ). TABLE IV C and S isotopic data and organic C and U concentrations for rocks and sulphides from selected samples Sample No.
Locality
Rocks
Sulphides
5'3C(°/0o )
org. C (%)
U (ppm)
534 S(°/,,,, )
--28.4 --29.0 --28.3 --27.8 --
0.68 1.26 0.81 0.78 --
4.5 8.2 13.6 11.1 --
-- 0.1 + 12.1 + 13.5 + 5.3 + 30.3
--
--
+ 11.5
0.31 0.72 --
15.1 8.2 --
+ 12.9 + 4.4 + 1.6
Silurian 46188 46189 46190 46191 Baryte
CB-1 72.1m CB-1 86.2m CB-1 87.8m CB-1 91.1m Clare Vale
Co-existing pyrite
--
Ordovician 51482 46185 39290(po)
WE-1 70.1m W-35 82.3m W-43 183.8m
--29.7 --28.6 --
Analysts: M. Burns and J. Smith.
98
B.I,. GULSON
There is no significant difference between the Ordovician and Silurian isotopic C values and the S isotope variations within samples of the same age are so large that interpretation is difficult. Relatively uniform values of 534S of a b o u t +80/00 have been obtained for sulphides from the Woodlawn ore hori~ zon and host volcanics (Ayres, Smith and Burns, in preparation). Even though the few C and S isotope values do n o t differentiate the Ordovician and Silurian shales unequivocally, the data may possibly be used to indicate their environment of deposition. The Silurian (and Ordovician) samples are distant from the mineralization (> 10 km) and so the higher values of ~ a4 S in 46189 and 46190 may be attributed to Silurian marine influence; the S isotopic value of seawater at this time is given by the baryte (+30.30/00). If sulphides from Silurian samples close to mineralization were analyzed, these would probably have 534S values closer to those of the sulphides from the volcanics, reflecting a greater volcanic than marine influence. No trends in organic C and U concentration were discernible in the Woodlawn data {Table IV) in contrast to results for other shales obtained by Wampler and Kulp {1964) and Baturin {1973). Trace element concentrations in sulphides A summary of semi-quantitative data on certain trace elements in sulphides is given in Table III. Only V and possibly Ni, Mo and Pb seem to distinguish between the Ordovician and Silurian shales close to mineralization (< 2 km). Samples from central western N.S. W. Results for pyrite and oxidized sulphide separated from shales, sandstone, and marls of Ordovician and Silurian age in central western N.S.W. (Fig. 1) are given in Table IC. In contrast t o the Woodlawn area, differences in Pb isotopic ratios were not observed and this precludes the use of Pb isotopes as a mapping tool in central western N.S.W. The similarity in Pb isotopes may be attributed to the extensive volcanic activity with limited associated mineralization {mainly Cu) which occurred during both the Ordovician and Silurian in the central west (Sherwin, 1973) with resulting low U/Pb ratios. The data, except for the low-precision analysis 48324, lie within the field of the Woodlawn Silurian black shale data. Trace element contents of the sulphides from central western N.S.W., in keeping with the isotopic data, show no differences between Ordovician and Silurian but they are t o o few in number to be useful. Co/Ni ra tios Loftus-Hills and Solomon (1967) suggested that pyrite formed by sedi-
APPLICATION OF LEAD ISOTOPES TO MAPPING
99
mentary or diagenetic process had Co/Ni < 1 in contrast to those of volcanic origin unaccompanied by Pb and Zn minerals, which had Co/Ni > 1. At Woodlawn, Co/Ni ratios of sulphides from Silurian black shales vary from 0.29 (range 0.05--5) close to mineralization (< 2 km), to 0.23 (range 0.06-6) distant from mineralization. However, the Ordovician black shale sulphides have an average Co/Ni ratio of 0.04 (range 0.01--0.07) reflecting the larger contributions of Ni in these samples. These results are in contrast with the ratio of > 3 for pyrite from volcanoclastics in the Woodlawn area (Gulson, in preparation) and substantiate the syngenetic as against volcanogenic nature of the sulphides in the black shales.
Possible relationship of nickel and vanadium and lead isotopic ratios Because of the occasional high V and Ni values in Silurian sulphides distant from mineralization and in the Ordovician shales, these data were plotted against 20s pb/:06 Pb (Fig. 5). Although few in number, the data indicate a negative correlation which is well defined for Ni but less so for V; that is, as 208 pb/~06 Pb decreases (or 2o6 pb/204 Pb increases) there is an increase in Ni and V. The poor correlation for V may be partly one of sampling -- a different split is used for the Ni and V determinations and the sulphide used in the Pb analysis is initially washed with 6N HC1. Furthermore, the emission analysis is for the whole sample (i.e. sulphide plus insoluble impurity) whereas the isotopic analysis is only of the acid-soluble sulphide. Goldschmidt (1954) suggested t h a t the V c o n t e n t in marine shales is higher than in corresponding fresh-water deposits whereas Degens et al. (1957) maintained that this is only valid for the carbon fractions of the deposits and for a limited areal extent. When considered with the other data and the marine environment in which the Ordovician shales formed, the iso150
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Fig. 5. 2o~pb/:O6 Pb vs. Ni and V concentration.
1.9
21
100
B.L. GULSON
topic and V and Ni information suggests that the Silurian shales distant from mineralization were influenced largely by marine conditions, in contrast to the dominance of volcanism near the mineralization and volcanic centre(s), which overrides the influence of the marine conditions.
Black shales and metallogenesis The major aim of this work was to investigate the use of Pb isotopes in the Woodlawn and other areas in distinguishing Ordovician from Silurian carbonaceous shales, because of the economic potential for base metal mineralization in Silurian rocks. However, because of the ubiquitous association of carbonaceous shale with most mineralization, whether it be Cu-Pb-Zn, Au, or U, it is worthwhile briefly considering certain aspects of the problems of black shales, sulphides and metals (particularly U) and h o w they relate to the present study. Wampler and Kulp (1964) carried out an extensive investigation of pyrite from black shales of varying ages on a world-wide basis. They suggested that the Pb in sedimentary pyrite was representative of the Pb in the environment in which it formed, provided that the isotopic composition was not changed by radioactive decay or by recrystallization of the sulphides long after deposition. They suggested that U occurs in surficial positions and much of it could be removed by thorough leaching with h o t HC1. The surficial radiogenic Pb derived from U may be lost from the sample or it may remain on or near the surface of the pyrite. They further suggested that there may be a correlation between the a m o u n t of carbonaceous material and U observed. In contrast, the data given in Table IV indicate that little or no correlation exists between organic C and U concentration for the Woodlawn area. Pyrites from the Woodlawn volcanics and chloritic "schists" show a homogeneous distribution of U throughout the grains (Gulson, unpublished data). U distribution studies on pyrite from black shales from a number of Australian localities are in progress. Saxby (1976) suggests that the " r a n k " of carbonaceous matter in the black shales is critical to any discussion of bonding to metals. For low-rank organic material, metals can be attached to the carbonaceous matter through carboxyl groups. At higher grades, such as at Woodlawn where partial graphitization has occurred, Saxby considers it unlikely that metal ions (e.g. V, Ni, U) bonded to carboxyl or other groups would survive. However, metals could be retained in the sediment if they reacted with some sulphur (or oxygen), forming a sulphide (or oxide), which remained in close association with high-rank organic matter. Peltola (1968) has described a protective coating of organic C surrounding pentlandite and uraninite in the O u t o k u m p u black slates. Such a coating around sulphides or oxides would inhibit metal ion extraction that might occur in nature or during laboratory dissolution of the pyrite for analysis. This protection hypothesis is important for trace element and isotopic analy-
APPLICATION OF LEAD ISOTOPES TO MAPPING
101
ses because of the possibility that some of the carbonaceous material might be dissolved during sample dissolution. Many sulphide concentrates analyzed in this study and other studies in the literature on black shales (e.g. Wampler and Kulp, 1964) contained varying amounts of carbonaceous material~ some of which occurs as inclusions in the sulphide. Because of the affinity of U for the carbonaceous matter, it was of some concern that some of the C plus U (and its radiogenic Pb) could be dissolved during the dissolution stage and that this may account for the radiogenic Pb in the Ordovician samples. Although such concern was discounted by the whole rock analyses, the possibility was further studied. As the residue after the acid t r e a t m e n t was almost impossible to get into solution for U analysis by isotope dilution, an approximation of the a m o u n t of organic C dissolved, if any, could be gauged from the V and Ni concentrations. V is concentrated with organic material and Ni in the sulphide fractions (Peltola, 1968; Kholodov and Gavrilov, 1974). Partitioning of V into the hydrocarbon phase is well known in the petroleum industry. The analyses of the insoluble residues (Table III) for sample 39292 and a composite of 46188-91 show complete partitioning of V into the residues and about 1% of the Ni. In view of the negligible amounts of V and high concentrations of Ni in the Silurian black shale sulphides, these data suggest that little carbonaceous material (and thus U) is lost during the dissolution stage. As part of his trace element investigation of the black schists and inherent sulphides of the O u t o k u m p u area (Finland), Peltola (1968) analyzed whole rock, sulphide and carbon fractions for 5 samples. U averaged 10.5 ppm in the rocks and 34 ppm in the carbon fraction and the ratio of U in carbonaceous matter to U in sulphide was about 2:1. He suggests the U occurs as uraninite grains enveloped by a thick carbonaceous shell, and because of the shell, U and, partly, radiogenic Pb are thus enriched in the carbon fraction during mineral separation. His observation of discrete uraninite grains would appear to differ from the observations of Wampler and Kulp (1964). If the uraninite was coated with a thick protective carbon shell, it is highly unlikely that much of the U would be extracted with only an HC1 wash as indicated by Wampler and Kulp (1964). The statement by Peltola (1960) that Th has n o t been found in the Outokumpu black schists would indicate that they had low Th/U ratios. The Pb isotopic data for the Woodlawn Ordovician rocks and sulphides suggest they have low Th/U ratios of 0.4--0.9. Petersen and Lambert (in preparation) have found that Ordovician black shales usually have low Th/U ratios of < 1 and certainly ~ 2. In contrast, Silurian black shales near mineralization usually have Th/U ratios > 3 and often up to 10 or 20 whereas those distant from mineralization have Th/U ratios ~ 1. The very high Th/U ratios and often high concentrations of Th (> 30 ppm) in black shales near mineralization compared with relatively normal values distant from it indicate substantial mobility of Th in this environment. In contrast, the Th/U ratios of 3--5 in many of the tuffs, tuffaceous shales and shales are " n o r m a l " . As
102
B.L. GULSON
mentioned previously, various data suggest that the Ordovician and Silurian black shales distant from mineralization are dominantly marine in environ. m e n t and this is further supported by the low Th/U ratios in the shales. Perhaps low Th/U ratios were typical of seawater during the Ordovician and Silurian periods as they are at the present time. The low 20s pb/20~ Pb ratios for the Woodlawn shales compare with similar ratios found in 3tack Sea sediments (Cooper et al., 1974) but contrast with higher values found in the Baltic Sea and extreme ratios from the North American lakes and Hudson Bay (Chow and Johnstone, 1963, 1964; Hart and Tilton, 1966). The North American sediments have average 20~ pb/204 Pb and 206 pb/204 Pb values which plot close to the upper limit of the Woodlawn acid volcanic line. CONCLUSION
Pb isotopic studies of either whole rocks or sulphides appear to give unequivocal answers as to the Ordovician or Silurian age of black shales in the fossil-poor Woodlawn area. The V concentration and Co/Ni ratios of sulphides also appear to be possible means of characterization b u t further studies are necessary. Although P~) isotopic analyses are expensive, so is drilling. Unravelling of the stratigraphy, structures and thus ore reserves and future drilling programmes may be assisted in this case by sophisticated techniques. ACKNOWLEDGEMENTS
The author is grateful to Dr. W. Compston who generously provided access to the mass spectrometer at the Australian National University which he and Dr. Clement developed and maintained. The "on-line" data processing system was designed and developed by Dr. P.A. Arriens. I would like to thank S. Mudie for technical assistance, N. Morgan for emission spectrographic measurements on the sulphides, J. Smith and M. Burns for the carbon and sulphur isotope data, and G.F. Taylor and his group for the X-ray diffraction traces. The staff of J o d o d e x Aust. Pry. Ltd. generously provided samples and useful information and many fruitful discussions were held with Messrs. E. Malone, T. Nicholas and W. McKay. REFERENCES Ayres, D.E., Burns, M.S. and Smith, J.W., in preparation. A sulphur isotope study of the Woodlawn massive sulphide orebody (for submission to J. Geol. Soc. Aust. )~ Baturin, G.N., 1973. Uranium in the modern sedimentary cycle. Geochem. Int.,9: 1031-1041. Chow, T.J. and Johnstone, M.S~, 1964. Lead isotopes in the sediments of three Canadian Baltic Sea. Trans. Am. Geophys. Union, 4 4 : 8 9 0 " 8 9 1 (abstract). Chow, T.J. and Johnstone, M.S., 1964. Lead isotopes in the sediements of three Canadian shield lakes. Trans. Am. Geophysl Union, 45:110--111 (abstract). Cooper, J.A., Dasch, E.J. and Kaye, M., 1974. Isotopic and elemental geochemistry of Black Sea sediments. Am. Assoc. Pet. Geol., Mere., 20: 554--565.
APPLICATION OF LEAD ISOTOPES TO MAPPING
103
Degens, E.T., Williams, E.G. and Keith, M.L., 1957. Environmental studies of carboniferous sediments, I. Geochemical criteria for differentiating marine and fresh-water shales. Bull. Am. Assoc. Pet. Geol., 41: 2427--2455. Felton, E.A., 1974. Stratigraphic revisions in the Tarago-Woodlawn-Mount Fairy area. Q. Notes, Geol. Surv. N.S.W., 17: 7--12. Felton, E.A. and Sherwin, L., 1974. A new Ordovician graptolite locality east of Tarago, New South Wales. Q. Notes, Geol. Surv. N.S.W., 17: 1--2. Goldschmidt, V.M., 1954. Geochemistry. Oxford University Press, Oxford, 730 pp. Gulson, B.L., 1976. Exploration and mapping around a base metal sulphide deposit using trace lead isotopes. Miner. Deposita, 11: 1--5. Gulson, B.L., 1977a. Isotopic and chemical studies on crustal effects in the genesis of the Woodlawn Pb-Zn-Cu deposit. Contrib. Mineral. Petrol. (in press.) Gulson, B.L., 1977b. Lead isotope results of acid leaching experiments on acid volcanics and black shales in an ore environment (submitted to Chem. Geol.) Gulson, B.L., in preparation. A lead isotopic study of the Woodlawn Pb-Zn-Cu deposit (for submission to J. Geol. Soc. Aust.). Hart, S.R. and Tilton, G.R., 1966. The isotope geochemistry of strontium and lead in Lake Superior sediments and water. In: J.S. Steinhart and T.J. Smith (Editors), The Earth Beneath the Continents. Am. Geophys. Union, Geophys. Monogr., 1 0 : 1 2 7 -137. Loftus-Hills, G. and Solomon, M., 1967. Cobalt, nickel and selenium in sulphides as indicators of ore genesis. Miner. Deposita, 2: 228--242. Kholodov, V.N. and Gavrilov, Yu.O., 1974. Distribution patterns of trace elements in the Chokrak-Karagan rocks of the Yaryk-Su river (eastern Ciscaucasia). Lithol. Miner. Resour. (USSR)., 6: 726--739. Malone, E., Olgers, F., Cucchi, F.G., Nicholas, T. and McKay, W., 1975. Woodlawn copper-lead-zinc deposit. In: C.L. Knight (Editor), Economic Geology of Australia and Papua-New Guinea, 1. Metals. Australasian Institute of Mining and Metallurgy, Meb bourne, Vic., pp. 701 -710. McKay, W., 1973. Major-trace elements of Woodlawn black shales. Jododex Aust. Pty. Ltd., Intern. Rep., 12 pp (unpublished). Peltola, E., 1960. On the black schists in the Outokumpu region in Eastern Finland. Bull. Comm. Geol. Finl., 192. Peltola, E., 1968. On some geochemical features in the black schists of the Outokumpu area, Finland. Bull. Geol. Soc. Finl., 40: 39--50. Petersen, M.D. and Lambert, I.B., in preparation. Implications of petrographic mineralogical and geochemical investigations of rocks around the Woodlawn copper-lead-zinc orebody, southeastern New South Wales (for submission to J. Geol. Soc. Aust.). Saxby, J.D., 1976. The significance of organic matter in ore genesis. In: K.H. Wolf (Editor), Handbook of Stratabound and Stratiform Ore Deposits. Part 1: Principles and General Studies, 2. Geochemical Studies. Elsevier, Amsterdam, pp. 111--133. Sherwin, L., 1973. Stratigraphy of the Forbes-Bogan Gate district. Rec., Geol. Surv. N.S.W., 15: 47- 101. Wampler, J.M. and Kulp, J.L., 1964. An isotopic study of lead in sedimentary pyrite. Geochim. Cosmochim. Acta, 28: 1419--1458.
85 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
APPLICATION OF LEAD ISOTOPES AND TRACE ELEMENTS TO MAPPING BLACK SHALES AROUND A BASE METAL SULPHIDE DEPOSIT
BRIAN L. GULSON*
Division of Mineralogy, Commonwealth Scientific and Industrial Research Organization, North Ryde, N.S.W. (Australia) (Revised version received February 23, 1977)
ABSTRACT Gulson, B.L., 1977. Application of lead isotopes and trace elements to mapping black shales around a base metal sulphide deposit. J. Geochem. Explor., 8: 85--103. Pb isotopic analyses have been used in southeastern New South Wales, Australia, to distinguish Ordovician black shales, which have no associated mineralization, from Silurian black shales in which mineralization is known to occur. The more radiogenic nature of the Ordovician Pb, as shown by analysis of the sulphide, whole rock, acid leach or residue, reflects a higher U/Pb environment compared with the Silurian which is due to the absence of volcanism in the Ordovician. V concentrations and Co/Ni ratios in sulphides may prove to be an important indicator to distinguish Silurian and Ordovician black shales but lithology, mineralogy, whole rock major and trace elements, and C and S isotopes give ambiguous answers. Co/Ni ratios in the Silurian black shale sulphides average - 0.26 compared with 0.04 in the Ordovician sulphides. On the basis of Pb isotope and V concentration data, an unconformity between Middle-Upper Ordovician and Middle-Upper Silurian rocks and overturning of the strata is proposed for a drill hole previously mapped on palaeontological evidence as entirely Ordovician. No correlation between organic C and U concentration was observed in the whole rocks. An approximate negative correlation exists between ~08 pb/206 Pb and V and Ni contents; with higher Ni and V concentrations the sulphides have lower 2o8 pb/206 Pb ratios but higher 206 pb/~0, Pb ratios. Shales distant from mineralization appear to have been wholly influenced by marine processes in comparison with samples close to the mineralization which are affected by volcanic conditions.
INTRODUCTION
In s o u t h e a s t e r n N . S . W . , A u s t r a l i a , P b - Z n - C u m i n e r a l i z a t i o n s u c h as a t W o o d l a w n a n d C a p t a i n s F l a t is r e s t r i c t e d t o M i d d l e - U p p e r S i l u r i a n v o l c a n i c s * Also: Visiting Fellow, Research School of Earth Sciences, Australi:m National University, Canberra, A.C.T., Australia.
86
B,L. GULSON
which are intimately associated with black and dark grey shales. It is these volcano-sedimentary sequences which are the "target" for exploration. The Middle-Upper Ordovician sedimentary sequence, which has no known associated mineralization b u t also contaias blac k shales, unconformably underlies the Silurian rocks and because of the similarity of the black shales in hand specimen, difficulties have been encountered in distinguishing between them, particularly during core logging. A paucity of fossils (Felton and Sherwin, 1974) c o m p o u n d s the problems of mapping. Attempts to distinguish the shales other than by lithological means have proved inconclusive. McKay (1973) first tried major and trace element whole rock data on a limited number of samples and later, Petersen and Lambert (in preparation) have carried out a more extensive chemical survey on rocks which yields no greater discrimination. X-ray diffraction data (this study) are ambiguous, the mineralogy for both types being quartz (dominant), chlorite, muscovite and traces of feldspar, sulphide and calcite. Similar S and C isotopic values (see later section) are obtained for the Silurian and Ordovician rocks. During the course of a detailed Pb isotopic investigation around Woodlawn, initial analyses of one Ordovician and one Silurian pyrite showed marked differences in Pb isotopic ratios. Consequently an extended study was undertaken in this area b u t with no prior knowledge of the stratigraphy of the .samples. This study presents additional data following on the earlier preliminary work (Gulson, 1976), and includes data for a proposed unconformity in a drill hole logged solely as Ordovician. It also presents data for localities in central western N.S.W. chosen as a check on the regional extent of this Pb isotopic distinction between the Ordovician and Silurian. A summary of trace element data on the sulphides is also given. Pb isotopic investigations make use of variations in the daughter isotopes 2o6 Pb, 2o7 Pb and 2o8 Pb produced by radioactive decay from their respective parents 238 U, 23s U and 232 Th. The variations are usually expressed as a ratio of the particular daughter isotope to the orphan isotope 204 Pb (i.e. 204 Pb has no known parent). GEOLOGICAL SETTING OF THE WOODLAWN DEPOSIT The Woodlawn polymetallic sulphide deposit occurs approximately 200 km southwest of S y d n e y (Fig. 1). It is located in a sequence of acid volcanics and sediments of Middle to Upper Silurian age which form part of the Lower Palaeozoic succession in the Lachlan Fold Belt. The geological setting is similar to that found at Captains Flat, some 70 km south of Woodlawn. In the immediate vicinity of Woodlawn, sedimentary rocks are poorly represented in the lower part of t,he Silurian succession and a sequence (called the Woodlawn v o l c a n i c s ) o f acidic tuffs, rhyolites, ignimbrite agglomerates and acid volcanoclastics is developed (Felton, 1974). The immediate
APPLICATION OF LEAD ISOTOPES TO MAPPING
87
r
145
150
30
NEW
SOUTH WALES
Central
145
I
Wosdlav, n area
I
0
100
J
200
300
kilometres
Fig. 1. Locality map of the Woodlawn area and central western N.S.W., Australia, from where samples were selected. host rocks to the massive sulphides consist of rhyolitic tuffs and flows, black shales, chloritic schists and rare chert-like rocks. The Silurian succession is unconformably underlain by Middle to Upper Ordovician sediments and is overlain by the Currawang Basalt. The whole sequence has been regionally m e t a m o r p h o s e d to produce highly deformed lower greenschist facies rocks and is intruded by Upper Silurian dolerite and Devonian granite (Malone et al., 1975). The main massive sulphide lens is a polymetallic sulphide b o d y consisting of pyrite as the main constituent and variable amounts of sphalerite, galena and chalcopyrite. Silicate gangue minerals are chlorite, quartz, talc, sericite and feldspar. Stringers of sulphides are c o m m o n l y found throughout the volcanic succession. EXPERIMENTAL PROCEDURES
Sulphides. Pyrite and pyrrhotite were separated from the shales and volcanics by crushing, heavy liquid separation using tetrabromoethane and methylene iodine, sieving and then magnetic separation of the minus 100, plus 200mesh fraction. The samples (usually > 90--95% pure) were washed in 6N HC1 for 15 minutes (% 2 -3 minutes for pyrrhotite), rinsed with water and boiled in water for a b o u t 15 minutes (all reagents were prepared using subboiling distillation methods in silica glass or Teflon stills).
88
B,L. GULSON
Pyrite was dissolved in 7N HNO3/6N HCI (3:1) with a few drops of bromine water, whilst pyrrhotite was dissolved in 6N HC1. For the shales, an insoluble residue of carbonaceous material was usually present which amounted to less than 3% by weight of the sample. The solutions were ion exchanged on a 2-cm 3 anion column in bromide form and further cleaned on a 0.5-cm 3 anion column in chloride form.
Rocks. Minus 200-mesh samples, prepared under clean conditions, were " d e c o m p o s e d " in Teflon bombs using HF-HNO3 but, in all cases, even with nitric acid/sodium chlorate (E. Kiss, personal communication, 1975) and more HF treatment, some residue of carbonaceous matter, quartz and chloritic material was always present. Acid leaches were effected with 7N HNO3/6N HC1 (5:2) to dissolve the ore component, mainly pyrite. The residue was then dissolved in HF as for rocks. The ion exchange procedure for the rock analyses was the same as for the sulphides. Mass spectrometry. Samples were analyzed on the high ion beam mass spectrometer at the Research School of Earth Sciences, Australian National University. The "within-run" percent standard deviation ( l o ) of a set of 12--20 ratios ranged from 0.06 to 0.7%, with the average usually 0.1--0.2%. Runs with standard deviations greater than 0.3% are indicated in Tables ! and II. Duplicate dissolution data are given in the tables. All analyses were performed with a mixed 207 pb/204 pb_2as U double spike to control mass fractionation. DISCUSSION OF RESULTS
Lead isotopes from the Woodlawn area Analytical data for sulphides and whole rocks are given in Tables I and II and plotted in Figs. 2, 3 and 4. Most o f the d a t a in Table IA are from Gulson (1976). Minor differences in values for 6 samples, particularly for 2°TPbf°4Pb ratios, in Table IA and GuIson (1976) are due to recalculation of the fractionation correction. For clarity, only the field of data for the Silurian sulphides is outlined. The preponderance of Silurian over Ordovician data is purely a reflection of drilling for " t a r g e t " beds and this limits the availability of Ordovician samples, particularly away from mineralization. The variability and radiogenic nature, n o t only of the Ordovician sutphides, but also the rocks, compared with the Silurian black shale data are the obvious features of the results:
Ordovician Silurian
206pb/2O4Pb
2o~pb/2O4Pb
=ospb/2O4Pb
20.4--38.6 17.9--19.3
15.8--16.7 15.5--15.7
38.8--43.6 37.7--40.7
A P P L I C A T I O N OF L E A D ISOTOPES TO M A P P I N G
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When subdivided, mainly on the basis o f 2°6 pbp°4 pb ratios, into possible Silurian and Ordovician samples, the data agreed in every case with the fossil and stratigraphic controls. The isotopic values for the Silurian samples close to mineralization (< 2 km) are similar to those for Pb in sulphides from the ore horizon (pyrite,
90
B.L GULSON
galena, chalcopyrite, pyrite) and pyrite from the immediate ore host volcanics (the average value of the ore Pb is given by a solid cross in Figs. 2, 3 and
4). Similar isotopic ratios and U and Pb concentrations were obtained for coexisting pyrite and pyrrhotite. In most cases, the transformation of pyrite to pyrrhotite could be traced to the proximity of the shales to intrusive dolerites. The variability in isotopic ratios and elemental concentrations over short distances in either Silurian or Ordovician sulphides is marked, e,g. in diam o n d drill hole (DDH) CB-1, W-43, WE-1. This variability not only applies to Pb isotopes and trace elements (Tables I and III) but also to S and C isotopes and organic C contents (Table IV). U and Pb concentrations in sulphides from the Ordovician and Silurian overlap and, of course, are highly variable. Although the data are limited, particularly for the Ordovician, the Ordovician sulphides do appear to have, on the whole, higher U and lower Pb concentrations than the Silurian sulphides. On the 207 pb/204 Pb vs. 506 pb/204 Pb plot in Fig. 2, the solid line is that for the whole rock Silurian acid volcanics from around Woodlawn (Gulson, 1977a). Most of the high-quality data for the black shales lie to within experimental error of this line. Its slope is equivalent to an apparent age of about 350 m.y. compared with a stratigraphic age of about 430 m.y.; however, in this age range small variations in slope result in disproportionately
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B.!,, GULSON
large variations in age, so the apparent age deficiency cannot be regarded as significant. The scatter of the data is greater, particularly for the Ordovician samples, on a 2o8 pb/204 Pb vs. 2o6 pb/204 Pb plot (Fig. 3, cf. Fig. 2). However, a number of important features, besides the more radiogenic nature of the Ordovician Pb, may be noted: (1) The low 20s pb/20a Pb ratio of the Ordovician shales which is equivalent to a Th/U ratio of 0.4--0.9 compared with about 4.4 for the Silurian volcanics. (2) The acid leach-rock-residue data for the Silurian shales lie closer to the volcanic line than to the Ordovician shale line. A single, comparatively radiogenic pyrite (50051) also lies close to the volcanic line and is similar to radiogenic sulphides separated from the votcanics (Gulson, in preparation). (3) Some sulphide data for Silurian black shales (e.g. 50044, 50048, 46192) distant (> 10 km) from the Woodlawn mineralization are comparatively radiogenic and plot on the Ordovician black shale line. The distinction can be more clearly seen in Fig. 4, a plot of 207 pb/:06 Pb vs. 208 pb/:06 Pb, the advantage of which is to overcome problems of measurement of the small 204 Pb isotope. Correction of the Silurian sulphides for Pb derived by radioactive decay from U since about 430 m.y. results in less scatter than the uncorrected data, but this correction may be meaningless because of changes in the U/Pb ratio due to post-depositional processes or even during mineral separation where the ratio of sulphide to carbonaceous material may be important (Peltola, 1968). Furthermore, the enormous scatter on a 238 U/204 Pb vs. 206 pb/204 Pb plot indicates either recent m o v e m e n t or partitioning during mineral separation of U.
Lead isotopes indicative o f an unconformity and overturned sequence at Woodlawn The first analysis of pyrite and pyrrhotite from DDH W-43 at 183.8 m (39290; Table IA, Ordovician) exhibited Ordovician-type ratios (and also Ni and V contents) which agreed with the presence of Ordovician graptolites found in the upper part of the hole. A new sample at 195 m (46187; Table IA, Silurian) contained only pyrite and the isotopic data and trace elements are more typical of Silurian shales. As the possibility existed that this sample had been mixed with another during either sampling or processing, a further sample was analyzed from 185.6 m (49018; Table IA, Silurian), Although it was more radiogenic than the pyrite from 195 m, the pyrite (no pyrrhotite) also gave isotopic and trace element data of the Silurian type. At this stage, it became apparent that there was possibly an u n c o n f o r m i t y and overturned succession in t h e hole, so a further two (whole rock) samples higher in the core were analyzed. Both of these, from about 182 (52010; Table II) and 183 m (52011; Table Ii), gave values typical of those found in other
APPLICATION OF LEAD ISOTOPES TO MAPPING
97
O r d o v i c i a n samples, s u p p o r t i n g the inferred u n c o n f o r m i t y a n d o v e r t u r n i n g . T h e p a u c i t y o f fossils in this area and d i f f i c u l t y o f finding t h e m in drill core makes palaeontological confirmation remote.
Whole rock and acid-leaching experiments Because o f the time involved in s e p a r a t i o n o f the sulphides and their absence in some cases due t o o x i d a t i o n , w h o l e r o c k samples were investigated as a possibly simpler m e t h o d o f distinguishing the O r d o v i c i a n a n d Silurian black shales. T h e data, given in Table II, c o n f i r m the radiogenic n a t u r e o f the O r d o v i c i a n samples and thus the use o f w h o l e r o c k samples. The d a t a for the acid-leaching e x p e r i m e n t s (Gulson, 1 9 7 7 b ) on b o t h Silurian and O r d o v i c i a n w h o l e r o c k s s h o w t h a t the acid leaches and residues are c o n s i s t e n t l y d i f f e r e n t as in the sulphides and w h o l e rocks.
Carbon and sulphur isotopes C and S i s o t o p i c analyses were p e r f o r m e d b y J. S m i t h a n d M. Burns in a search for alternative and less expensive m e t h o d s , besides the elusive fossils, t o s u b s t a n t i a t e the Pb i s o t o p e data. T h e limited isotopic data, given in Table IV, f o r the f o u r Silurian samples f r o m the one drill bole, s h o w e d slight varia t i o n in 5 ~3 C values ( - - 2 7 . 8 t o --29.00/00 ) b u t large d i f f e r e n c e s in 5 34 S values (--0.1 t o +13.5°/00 ). TABLE IV C and S isotopic data and organic C and U concentrations for rocks and sulphides from selected samples Sample No.
Locality
Rocks
Sulphides
5'3C(°/0o )
org. C (%)
U (ppm)
534 S(°/,,,, )
--28.4 --29.0 --28.3 --27.8 --
0.68 1.26 0.81 0.78 --
4.5 8.2 13.6 11.1 --
-- 0.1 + 12.1 + 13.5 + 5.3 + 30.3
--
--
+ 11.5
0.31 0.72 --
15.1 8.2 --
+ 12.9 + 4.4 + 1.6
Silurian 46188 46189 46190 46191 Baryte
CB-1 72.1m CB-1 86.2m CB-1 87.8m CB-1 91.1m Clare Vale
Co-existing pyrite
--
Ordovician 51482 46185 39290(po)
WE-1 70.1m W-35 82.3m W-43 183.8m
--29.7 --28.6 --
Analysts: M. Burns and J. Smith.
98
B.I,. GULSON
There is no significant difference between the Ordovician and Silurian isotopic C values and the S isotope variations within samples of the same age are so large that interpretation is difficult. Relatively uniform values of 534S of a b o u t +80/00 have been obtained for sulphides from the Woodlawn ore hori~ zon and host volcanics (Ayres, Smith and Burns, in preparation). Even though the few C and S isotope values do n o t differentiate the Ordovician and Silurian shales unequivocally, the data may possibly be used to indicate their environment of deposition. The Silurian (and Ordovician) samples are distant from the mineralization (> 10 km) and so the higher values of ~ a4 S in 46189 and 46190 may be attributed to Silurian marine influence; the S isotopic value of seawater at this time is given by the baryte (+30.30/00). If sulphides from Silurian samples close to mineralization were analyzed, these would probably have 534S values closer to those of the sulphides from the volcanics, reflecting a greater volcanic than marine influence. No trends in organic C and U concentration were discernible in the Woodlawn data {Table IV) in contrast to results for other shales obtained by Wampler and Kulp {1964) and Baturin {1973). Trace element concentrations in sulphides A summary of semi-quantitative data on certain trace elements in sulphides is given in Table III. Only V and possibly Ni, Mo and Pb seem to distinguish between the Ordovician and Silurian shales close to mineralization (< 2 km). Samples from central western N.S. W. Results for pyrite and oxidized sulphide separated from shales, sandstone, and marls of Ordovician and Silurian age in central western N.S.W. (Fig. 1) are given in Table IC. In contrast t o the Woodlawn area, differences in Pb isotopic ratios were not observed and this precludes the use of Pb isotopes as a mapping tool in central western N.S.W. The similarity in Pb isotopes may be attributed to the extensive volcanic activity with limited associated mineralization {mainly Cu) which occurred during both the Ordovician and Silurian in the central west (Sherwin, 1973) with resulting low U/Pb ratios. The data, except for the low-precision analysis 48324, lie within the field of the Woodlawn Silurian black shale data. Trace element contents of the sulphides from central western N.S.W., in keeping with the isotopic data, show no differences between Ordovician and Silurian but they are t o o few in number to be useful. Co/Ni ra tios Loftus-Hills and Solomon (1967) suggested that pyrite formed by sedi-
APPLICATION OF LEAD ISOTOPES TO MAPPING
99
mentary or diagenetic process had Co/Ni < 1 in contrast to those of volcanic origin unaccompanied by Pb and Zn minerals, which had Co/Ni > 1. At Woodlawn, Co/Ni ratios of sulphides from Silurian black shales vary from 0.29 (range 0.05--5) close to mineralization (< 2 km), to 0.23 (range 0.06-6) distant from mineralization. However, the Ordovician black shale sulphides have an average Co/Ni ratio of 0.04 (range 0.01--0.07) reflecting the larger contributions of Ni in these samples. These results are in contrast with the ratio of > 3 for pyrite from volcanoclastics in the Woodlawn area (Gulson, in preparation) and substantiate the syngenetic as against volcanogenic nature of the sulphides in the black shales.
Possible relationship of nickel and vanadium and lead isotopic ratios Because of the occasional high V and Ni values in Silurian sulphides distant from mineralization and in the Ordovician shales, these data were plotted against 20s pb/:06 Pb (Fig. 5). Although few in number, the data indicate a negative correlation which is well defined for Ni but less so for V; that is, as 208 pb/~06 Pb decreases (or 2o6 pb/204 Pb increases) there is an increase in Ni and V. The poor correlation for V may be partly one of sampling -- a different split is used for the Ni and V determinations and the sulphide used in the Pb analysis is initially washed with 6N HC1. Furthermore, the emission analysis is for the whole sample (i.e. sulphide plus insoluble impurity) whereas the isotopic analysis is only of the acid-soluble sulphide. Goldschmidt (1954) suggested t h a t the V c o n t e n t in marine shales is higher than in corresponding fresh-water deposits whereas Degens et al. (1957) maintained that this is only valid for the carbon fractions of the deposits and for a limited areal extent. When considered with the other data and the marine environment in which the Ordovician shales formed, the iso150
700:,
5:11
1(]0
!000 5o
t
: :
r::-~ Slluri3"7 '~/ ~ SiIbrlaa
i
13
chose to ore Olstanl frOr" ore
i
l 5 208pb//
i
1. ?°6Pb
Fig. 5. 2o~pb/:O6 Pb vs. Ni and V concentration.
1.9
21
100
B.L. GULSON
topic and V and Ni information suggests that the Silurian shales distant from mineralization were influenced largely by marine conditions, in contrast to the dominance of volcanism near the mineralization and volcanic centre(s), which overrides the influence of the marine conditions.
Black shales and metallogenesis The major aim of this work was to investigate the use of Pb isotopes in the Woodlawn and other areas in distinguishing Ordovician from Silurian carbonaceous shales, because of the economic potential for base metal mineralization in Silurian rocks. However, because of the ubiquitous association of carbonaceous shale with most mineralization, whether it be Cu-Pb-Zn, Au, or U, it is worthwhile briefly considering certain aspects of the problems of black shales, sulphides and metals (particularly U) and h o w they relate to the present study. Wampler and Kulp (1964) carried out an extensive investigation of pyrite from black shales of varying ages on a world-wide basis. They suggested that the Pb in sedimentary pyrite was representative of the Pb in the environment in which it formed, provided that the isotopic composition was not changed by radioactive decay or by recrystallization of the sulphides long after deposition. They suggested that U occurs in surficial positions and much of it could be removed by thorough leaching with h o t HC1. The surficial radiogenic Pb derived from U may be lost from the sample or it may remain on or near the surface of the pyrite. They further suggested that there may be a correlation between the a m o u n t of carbonaceous material and U observed. In contrast, the data given in Table IV indicate that little or no correlation exists between organic C and U concentration for the Woodlawn area. Pyrites from the Woodlawn volcanics and chloritic "schists" show a homogeneous distribution of U throughout the grains (Gulson, unpublished data). U distribution studies on pyrite from black shales from a number of Australian localities are in progress. Saxby (1976) suggests that the " r a n k " of carbonaceous matter in the black shales is critical to any discussion of bonding to metals. For low-rank organic material, metals can be attached to the carbonaceous matter through carboxyl groups. At higher grades, such as at Woodlawn where partial graphitization has occurred, Saxby considers it unlikely that metal ions (e.g. V, Ni, U) bonded to carboxyl or other groups would survive. However, metals could be retained in the sediment if they reacted with some sulphur (or oxygen), forming a sulphide (or oxide), which remained in close association with high-rank organic matter. Peltola (1968) has described a protective coating of organic C surrounding pentlandite and uraninite in the O u t o k u m p u black slates. Such a coating around sulphides or oxides would inhibit metal ion extraction that might occur in nature or during laboratory dissolution of the pyrite for analysis. This protection hypothesis is important for trace element and isotopic analy-
APPLICATION OF LEAD ISOTOPES TO MAPPING
101
ses because of the possibility that some of the carbonaceous material might be dissolved during sample dissolution. Many sulphide concentrates analyzed in this study and other studies in the literature on black shales (e.g. Wampler and Kulp, 1964) contained varying amounts of carbonaceous material~ some of which occurs as inclusions in the sulphide. Because of the affinity of U for the carbonaceous matter, it was of some concern that some of the C plus U (and its radiogenic Pb) could be dissolved during the dissolution stage and that this may account for the radiogenic Pb in the Ordovician samples. Although such concern was discounted by the whole rock analyses, the possibility was further studied. As the residue after the acid t r e a t m e n t was almost impossible to get into solution for U analysis by isotope dilution, an approximation of the a m o u n t of organic C dissolved, if any, could be gauged from the V and Ni concentrations. V is concentrated with organic material and Ni in the sulphide fractions (Peltola, 1968; Kholodov and Gavrilov, 1974). Partitioning of V into the hydrocarbon phase is well known in the petroleum industry. The analyses of the insoluble residues (Table III) for sample 39292 and a composite of 46188-91 show complete partitioning of V into the residues and about 1% of the Ni. In view of the negligible amounts of V and high concentrations of Ni in the Silurian black shale sulphides, these data suggest that little carbonaceous material (and thus U) is lost during the dissolution stage. As part of his trace element investigation of the black schists and inherent sulphides of the O u t o k u m p u area (Finland), Peltola (1968) analyzed whole rock, sulphide and carbon fractions for 5 samples. U averaged 10.5 ppm in the rocks and 34 ppm in the carbon fraction and the ratio of U in carbonaceous matter to U in sulphide was about 2:1. He suggests the U occurs as uraninite grains enveloped by a thick carbonaceous shell, and because of the shell, U and, partly, radiogenic Pb are thus enriched in the carbon fraction during mineral separation. His observation of discrete uraninite grains would appear to differ from the observations of Wampler and Kulp (1964). If the uraninite was coated with a thick protective carbon shell, it is highly unlikely that much of the U would be extracted with only an HC1 wash as indicated by Wampler and Kulp (1964). The statement by Peltola (1960) that Th has n o t been found in the Outokumpu black schists would indicate that they had low Th/U ratios. The Pb isotopic data for the Woodlawn Ordovician rocks and sulphides suggest they have low Th/U ratios of 0.4--0.9. Petersen and Lambert (in preparation) have found that Ordovician black shales usually have low Th/U ratios of < 1 and certainly ~ 2. In contrast, Silurian black shales near mineralization usually have Th/U ratios > 3 and often up to 10 or 20 whereas those distant from mineralization have Th/U ratios ~ 1. The very high Th/U ratios and often high concentrations of Th (> 30 ppm) in black shales near mineralization compared with relatively normal values distant from it indicate substantial mobility of Th in this environment. In contrast, the Th/U ratios of 3--5 in many of the tuffs, tuffaceous shales and shales are " n o r m a l " . As
102
B.L. GULSON
mentioned previously, various data suggest that the Ordovician and Silurian black shales distant from mineralization are dominantly marine in environ. m e n t and this is further supported by the low Th/U ratios in the shales. Perhaps low Th/U ratios were typical of seawater during the Ordovician and Silurian periods as they are at the present time. The low 20s pb/20~ Pb ratios for the Woodlawn shales compare with similar ratios found in 3tack Sea sediments (Cooper et al., 1974) but contrast with higher values found in the Baltic Sea and extreme ratios from the North American lakes and Hudson Bay (Chow and Johnstone, 1963, 1964; Hart and Tilton, 1966). The North American sediments have average 20~ pb/204 Pb and 206 pb/204 Pb values which plot close to the upper limit of the Woodlawn acid volcanic line. CONCLUSION
Pb isotopic studies of either whole rocks or sulphides appear to give unequivocal answers as to the Ordovician or Silurian age of black shales in the fossil-poor Woodlawn area. The V concentration and Co/Ni ratios of sulphides also appear to be possible means of characterization b u t further studies are necessary. Although P~) isotopic analyses are expensive, so is drilling. Unravelling of the stratigraphy, structures and thus ore reserves and future drilling programmes may be assisted in this case by sophisticated techniques. ACKNOWLEDGEMENTS
The author is grateful to Dr. W. Compston who generously provided access to the mass spectrometer at the Australian National University which he and Dr. Clement developed and maintained. The "on-line" data processing system was designed and developed by Dr. P.A. Arriens. I would like to thank S. Mudie for technical assistance, N. Morgan for emission spectrographic measurements on the sulphides, J. Smith and M. Burns for the carbon and sulphur isotope data, and G.F. Taylor and his group for the X-ray diffraction traces. The staff of J o d o d e x Aust. Pry. Ltd. generously provided samples and useful information and many fruitful discussions were held with Messrs. E. Malone, T. Nicholas and W. McKay. REFERENCES Ayres, D.E., Burns, M.S. and Smith, J.W., in preparation. A sulphur isotope study of the Woodlawn massive sulphide orebody (for submission to J. Geol. Soc. Aust. )~ Baturin, G.N., 1973. Uranium in the modern sedimentary cycle. Geochem. Int.,9: 1031-1041. Chow, T.J. and Johnstone, M.S~, 1964. Lead isotopes in the sediments of three Canadian Baltic Sea. Trans. Am. Geophys. Union, 4 4 : 8 9 0 " 8 9 1 (abstract). Chow, T.J. and Johnstone, M.S., 1964. Lead isotopes in the sediements of three Canadian shield lakes. Trans. Am. Geophysl Union, 45:110--111 (abstract). Cooper, J.A., Dasch, E.J. and Kaye, M., 1974. Isotopic and elemental geochemistry of Black Sea sediments. Am. Assoc. Pet. Geol., Mere., 20: 554--565.
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Degens, E.T., Williams, E.G. and Keith, M.L., 1957. Environmental studies of carboniferous sediments, I. Geochemical criteria for differentiating marine and fresh-water shales. Bull. Am. Assoc. Pet. Geol., 41: 2427--2455. Felton, E.A., 1974. Stratigraphic revisions in the Tarago-Woodlawn-Mount Fairy area. Q. Notes, Geol. Surv. N.S.W., 17: 7--12. Felton, E.A. and Sherwin, L., 1974. A new Ordovician graptolite locality east of Tarago, New South Wales. Q. Notes, Geol. Surv. N.S.W., 17: 1--2. Goldschmidt, V.M., 1954. Geochemistry. Oxford University Press, Oxford, 730 pp. Gulson, B.L., 1976. Exploration and mapping around a base metal sulphide deposit using trace lead isotopes. Miner. Deposita, 11: 1--5. Gulson, B.L., 1977a. Isotopic and chemical studies on crustal effects in the genesis of the Woodlawn Pb-Zn-Cu deposit. Contrib. Mineral. Petrol. (in press.) Gulson, B.L., 1977b. Lead isotope results of acid leaching experiments on acid volcanics and black shales in an ore environment (submitted to Chem. Geol.) Gulson, B.L., in preparation. A lead isotopic study of the Woodlawn Pb-Zn-Cu deposit (for submission to J. Geol. Soc. Aust.). Hart, S.R. and Tilton, G.R., 1966. The isotope geochemistry of strontium and lead in Lake Superior sediments and water. In: J.S. Steinhart and T.J. Smith (Editors), The Earth Beneath the Continents. Am. Geophys. Union, Geophys. Monogr., 1 0 : 1 2 7 -137. Loftus-Hills, G. and Solomon, M., 1967. Cobalt, nickel and selenium in sulphides as indicators of ore genesis. Miner. Deposita, 2: 228--242. Kholodov, V.N. and Gavrilov, Yu.O., 1974. Distribution patterns of trace elements in the Chokrak-Karagan rocks of the Yaryk-Su river (eastern Ciscaucasia). Lithol. Miner. Resour. (USSR)., 6: 726--739. Malone, E., Olgers, F., Cucchi, F.G., Nicholas, T. and McKay, W., 1975. Woodlawn copper-lead-zinc deposit. In: C.L. Knight (Editor), Economic Geology of Australia and Papua-New Guinea, 1. Metals. Australasian Institute of Mining and Metallurgy, Meb bourne, Vic., pp. 701 -710. McKay, W., 1973. Major-trace elements of Woodlawn black shales. Jododex Aust. Pty. Ltd., Intern. Rep., 12 pp (unpublished). Peltola, E., 1960. On the black schists in the Outokumpu region in Eastern Finland. Bull. Comm. Geol. Finl., 192. Peltola, E., 1968. On some geochemical features in the black schists of the Outokumpu area, Finland. Bull. Geol. Soc. Finl., 40: 39--50. Petersen, M.D. and Lambert, I.B., in preparation. Implications of petrographic mineralogical and geochemical investigations of rocks around the Woodlawn copper-lead-zinc orebody, southeastern New South Wales (for submission to J. Geol. Soc. Aust.). Saxby, J.D., 1976. The significance of organic matter in ore genesis. In: K.H. Wolf (Editor), Handbook of Stratabound and Stratiform Ore Deposits. Part 1: Principles and General Studies, 2. Geochemical Studies. Elsevier, Amsterdam, pp. 111--133. Sherwin, L., 1973. Stratigraphy of the Forbes-Bogan Gate district. Rec., Geol. Surv. N.S.W., 15: 47- 101. Wampler, J.M. and Kulp, J.L., 1964. An isotopic study of lead in sedimentary pyrite. Geochim. Cosmochim. Acta, 28: 1419--1458.