Application of Hydrogeochemistry to Uranium Exploration in the Pine Creek Geosyncline, Northern Territory, Australia

Journal of Geochemical Exploration, 19 (1983) 33—55 Elsevier Science Publishers B.V., Amsterdam — Printed in The Netherlands

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APPLICATION OF HYDROGEOCHEMISTRY TO URANIUM EXPLORATION IN THE PINE CREEK GEOSYNCLINE, NORTHERN TERRITORY, AUSTRALIA A.M. GIBLIN and A.A. SNELLING CSIRO Division of Mineralogy, PO Box 136, North Ryde, N.S.W. 2113 (Australia) Denison Australia Pty Limited, PO Box 41390, Casuarina, N.T., 6792 (Australia) (Received September 23, 1982; accepted for publication January 28, 1983)

ABSTRACT Giblin, A.M. and Snelling, A.A., 1983. Application of hydrogeochemistry to uranium exploration in the Pine Creek Geosyncline, Northern Territory, Australia. In: G.R. Parslow (Editor), Geochemical Exploration 1982. J. Geochem. Explor., 19: 33— 55. In the Pine Creek Geosyncline, fast moving, annually recharged, low-salinity ground waters dissolve uranium- and magnesium-enriched gangue minerals from mineralized aquifer rocks. The level of dissolved uranium depends on prevailing pH, Eh, salinity and degree of adsorption, which limits its effectiveness as an exploration indicator. Near each known deposit, leaching of magnesium-enriched gangue minerals produces ground waters with very similar major-element concentration plots, the shape of which constitutes a mineralized aquifer "signature". Gangue minerals also supply high levels of Mg2 + (expressed as NMg = [Mg2 + ]/[Ca 2+ + Mg2+ + Na+ + K + ] in milliequivalents per litre) to contained ground waters, NMg > 0.8 being common in ground waters from mineralized aquifers at each Pine Creek Geosyncline deposit. Data from Ranger One No. 3 ore body illustrates how progressive mixing of waters from mineralized and unmineralized aquifers causes graded reductions in NMg, which, when plotted onto a ground plan, delineate a hydrogeochemical aureole. High NMg (> 0.8) coincides with high uranium concentration (> 20 Mg/1 of U) in ground waters near Nabarlek and Ranger. Because pH-Eh conditions in aquifers at Jabiluka depress uranium solution, < 10 Mg/1 of U is present, although NMg values are generally > 0.8. To date NMg has always been < 0.8 in nonmineralized aquifer waters, whereas uranium may be > 50 Mg/1 in ground waters from felsic igneous aquifers, which can be identified as uneconomic by low (< 0.4) NMg, and by a fixed relationship between uranium and co-leached species such as F" and soluble salts. Measurements of pH, Eh, salinity, Fe(II), Ca, Mg, Na, K, Cl, S0 4 , total carbonate, phosphate, F", Cu, Pb, Zn and U in waters from 48 percussion holes in and near the Koongarra ore bodies have been related to mineralogy recorded in drill logs. The composition of waters from 20 holes near and along strike from known mineralization, fitted the mineralized aquifer "signature", had NMg > 0.8 and uranium up to 4100 Mg/1. These data confirm the use in this region of NMg as a hydrogeochemical indicator of uranium mineralization; they also indicate additional zones of possible mineralization.

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© 1983 Elsevier Science Publishers B.V.

34 HYDROGEOCHEMISTRY IN URANIUM EXPLORATION

Ground waters provide an effective prospecting medium as they reflect potentially large sample areas and deeper sub-surface zones. They have particular value in uranium exploration because aqueous solutions are involved in both the formation of uranium ores and their subsequent alteration. Because ground waters can be sampled from natural springs, agricultural bores and wells, and exploration drill holes, their use is a low-cost addition to present exploration procedures and is of special importance in the search for concealed mineralization that is not evident radio metrically. It might be assumed that the level of dissolved uranium is the only ground-water parameter useful in hydrogeochemical exploration for uranium mineralization. This ignores the restraints that ground-water chemistry imposes on uranium solubility and mobility and assumes contact between present-day ground waters and potentially mineralized zones. If prevailing chemical conditions cast ground-water chemistry into the pH-Eh zone where common uranium oxide minerals are effectively insoluble (see Langmuir, 1978), then the levels of dissolved uranium in a ground water draining a major uranium deposit may be low. Another factor involved in uranium mobility is the effect of pH variation on clay adsorption of soluble uranium species (Giblin, 1980). The greatest adsorption is in the pH range 6.0—8.0, so it is evident that clays and hydrous metal oxides (e.g. iron and manganese) in the path of a uraniferous ground water need to be considered in interpretation of ground-water uranium levels. These adsorption effects should also be considered in interpretation of soil geochemistry data; low uranium in soils may not indicate that an area is unprospective if the ground-water chemistry is such that uranium is partitioned in favour of the solution phase. Computer-calculated mineral solubility index (SI) maps take account of pH, Eh and other solution parameters in identifying whether a ground water is in contact with uranium mineralization. However, these calculations do not allow for the diminished levels of soluble uranium which can result from adsorption onto clays or hydrous metal oxides such as iron or manganese. In any geochemical study, levels of uranium in a ground water that are anomalous, and therefore expressions of uranium mineralization, can only be identified after an estimate has been made of the normal or background uranium value in that area. For example ground waters that drain felsic igneous rocks commonly have uranium levels up to 50 Mg/l. If such waters are ponded underground, evaporative concentration could produce apparently anomalous uranium values, as in some of our recent studies in which we have found values as high as 16 000 μg/l. Conversely, true uranium anomalies can be overlooked if the effect of local surface or rain water dilution is not recognized. Uranium in ground waters derived from granites or other felsic igneous

35

rocks, while not derived from an economic uranium deposit, still has significance in an exploration program. These waters could be the uranium source of a presently forming ore deposit. Because ground-water uranium values can be so difficult to interpret it may be more useful to use the relationship between the major element composition of a ground water and the geochemistry of aquifer rocks, to identify the formation to which those rocks belong. Current exploration targets in an identified uranium province are often the specific units or formations that host the presently identified uranium deposits. A regional ground-water study can indicate where target formations occur within economic range of the surface. This paper describes a detailed hydrogeochemical study in the vicinity of the Koongarra deposit in the Pine Creek Geosyncline, Northern Territory, Australia, to demonstrate the application of various hydrogeochemical exploration techniques in this region. GEOLOGY OF PINE CREEK GEOSYNCLINE URANIUM DEPOSITS

The major uranium deposits in the Pine Creek Geosyncline — Jabiluka, Ranger, Koongarra and Nabarlek — occur in the Alligator Rivers Province in the east of the Geosyncline (Fig. 1). They are all found in chloritized Lower Proterozoic metasediments of the Cahill Formation. The geology of the Province has been described in detail by Needham and Stuart-Smith (1980). Basal to the metasediments are domes of Archean granitic gneiss of the Nanambu Complex, but some of the lowermost Lower Proterozoic metasediments were accreted to these domes during regional metamorphism in the late Lower Proterozoic. The Jabiluka, Ranger and Koongarra deposits are all marginal to these Nanambu Complex domes, while the Nabarlek deposit is adjacent to the migmatized sediments of the Nimbuwah Complex (equivalent to the Lower Proterozoic sediments accreted to the Nanambu Complex). •ALLIGATOR RIVERS PROVINCE

DARWIN

PINE CREEK GEOSYNCLINE

_J Fig. 1. Location map.

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The host metasediments are schists of the Cahill Formation. Needham and Stuart-Smith (1976) described in detail the stratigraphy, petrology and structure of the Cahill Formation metasediments which they subdivided into two members. The lower member is dominated by a thick basal Mg-enriched dolomite unit which near the Ranger deposit rests directly on Nanambu gneisses in the footwall to the ore horizon. The uranium mineralization is associated with graphitic horizons within the overlying quartz-mica (± feldspar ± garnet) schists that have been transformed by intense chloritization to quartz-chlorite schists. Thus a strong stratigraphic control has been suggested for the mineralization. Following regional metamorphism and isoclinal folding of the host sediments, erosion produced a new land surface upon which the thick sandstones of the Kombolgie Formation were deposited. Each of the four major uranium deposits is located near the unconformity between the host schists and the Kombolgie sandstone. Some workers feel that this spatial relationship between mineralization and the unconformity was significant in ore genesis (Hegge et al., 1980). There are several other features common to all four uranium deposits (Hegge et al., 1980). These include structural controls, alteration chemistry, and ore mineralogy. At all deposits the main localizing feature appears to be a zone (or zones) of faulting and brecciation, grossly conformable to the host rock schistosity, that consisted once of open spaces, now filled by alteration and ore minerals. Intense and pervasive chloritization, essentially magnesium metasomatism, and lesser sericite alteration of the mineralized and surrounding lithologies, including the Kombolgie sandstone, is also a consistent feature of these deposits. Pitchblende is the main ore mineral, while disseminated sulphides occur in varying amounts (more prevalent in graphitic horizons), but gold is the only associated metal that may be concentrated in economic quantities. The Koongarra deposit displays all the above features (Foy and Pedersen, 1975), being within Cahill "mine series" schists near Cahill dolomites and the Kombolgie unconformity. The Nanambu Complex, while relatively close to Koongarra, does not outcrop nearby. Figure 2 is a typical crosssection through the No. 1 (main) ore body, depicting not only the geology, but the distribution of the various uranium minerals and the paths of ground water circulation at the present time (Snelling, 1980a). Within the primary (unweathered) zone, the ore consists of coalescing lenses containing pitchblende veins conformable within the steeply dipping host Cahill quartz-chlorite schists. The ore wedges out at about 100 m from the surface. A 100-m-long dispersion fan of secondary mineralization occurs in the 25—30 m thick zone of weathered rock above this primary zone. The primary ore zone averages a true width of 30 m at the base of weathering with the strongest mineralization and highest grades over a thickness of only several metres stratigraphically just below a thin, graphitic quartz-chlorite schist, hanging-wall unit. Scattered mineralization of vary-

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Fig. 2. Simplified cross section through the Koongarra No. 1 ore body. The distribution of the various uranium minerals and the paths of ground-water circulation at the present time are shown schematically (after Snelling, 1980a).

ing grades and mineralogy persists down through the schist sequence to the footwall breccia, formed by a reverse fault contact with the unconformably overlying Kombolgie sandstone. Oxidizing conditions have penetrated deep into the primary ore zone as evidenced by the hematitic alteration (largely of chlorites) along the footwall reverse fault breccia, and in the schists and ore above. This is also reflected in the ore mineralogy (see Fig. 2), pitchblendes having been oxidized and replaced by uranyl silicates. These data are consistent with the pattern of ground-water flow through the deposit (Fig. 2). GROUND WATERS AS PROSPECTING MEDIA IN THE PINE CREEK GEOSYNCLINE

In the Pine Creek Geosyncline high annual rainfall (1400—1800 mm) recharges ground waters, supplying regular inflows of aerated waters that should be highly corrosive of uranium deposits in their path. As a result, ground-water uranium levels should constitute an exploration indicator for deposits with which present day waters have been in contact. All the known deposits except Jabiluka 2 are in contact with present-day ground waters which carry uranium at the tens to hundreds of μg/l level (Queensland Mines, 1977; Noranda, 1978; Giblin, unpublished report), but ex-

38

perience has shown that high ground-water uranium values in isolation do not distinguish between mineralized and unmineralized aquifers. More deeply buried deposits such as Jabiluka 2 may be currently shielded physically and/or chemically from modern ground waters. Exploration strategy for such deposits often involves geophysical and stratigraphic drilling targets intended to locate Lower Proterozoic metasediments adjacent to Archean gneissic granitoids, the combination which characterizes previously discovered deposits. Ground waters may be useful in locating these specific rock types in poorly exposed regions, as the major element (Ca2+, Mg2+, N a \ K*, Cl", SOS" and total carbonate) composition of a ground water reflects locally dominant lithologies. This reflection is emphasized in Pine Creek Geosyncline ground waters because the high seasonal rainfall causes rapid ground-water flow rates and short aquifer retention times. Ground waters consequently have low salinity levels (< 300 mg/1), and are undersaturated with respect to aquifer minerals. The major element composition then preferentially reflects the more easily soluble aquifer constituents (e.g. carbonates and connate waters trapped during sediment lithification). In particular, the magnesium enrichment associated with intense and pervasive chlorite alteration that accompanies the four known deposits is reflected in the major element composition of ground waters from the vicinity of each known deposit. Figure 3 (Queensland Mines 1977; Noranda, 1978; Deutscher et al., 1980; Giblin, unpublished report) illustrates the ground water from each area by a major element or Schoeller plot (Schoeller, 1935), in each of which Mg2+ is very much in excess of Ca2+ and Na+ + K+. Graphically this is depicted by a sharp angle in the plot at the Mg2+ concentration point, giving a pattern similarity to the groundwater plots for these examples from the Alligator Rivers Province. If ground waters from other areas in the Pine Creek Geosyncline have Schoeller plots of this same pattern, it is suggested that such waters are in contact with rock units that have undergone a similar magnesium metasomatism, and thus a possible uranium enrichment process, as in the Alligator Rivers Province. To further elucidate the significance of Mg2+, a parameter, normalized magnesium (NMg), has been defined as the ratio of [Mg2+] to [Ca2+ + Mg2+ + Na+ + K + ] , using concentrations expressed in milliequivalents per litre, to allow valid comparison between concentrations of mono and divalent ions. Data used for Fig. 3 indicate that ground waters from the vicinity of each known deposit in the Alligator Rivers Province have NMg values > 0.8, in contrast to waters from nonmineralized aquifers which have a wide range of NMg levels; they are, however, always < 0.8. This suggests that NMg can be used as a hydrogeochemical exploration indicator for uranium deposits of the Alligator Rivers type. Because so much ground-water movement occurs in the Pine Creek Geosyncline, progressive mixing of waters from mineralized aquifers with waters

39

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from nonmineralized aquifers causes graded reductions in NMg with distance of sampling sites from the ore zone. Gradations in normalized magnesium levels may then constitute a hydrogeochemical aureole around uranium mineralization. For example, Fig. 4 illustrates the drill hole plan of Ranger One No. 3 ore body divided (with a degree of imagination) into groups A to E according to decreasing levels of NMg in contained ground waters (Giblin, unpubl. report). Differences in the NMg levels between ground waters from mineralized and nonmineralized aquifers conform well with the levels of magnesium in chloritized and unchloritized mine series rocks from the Ranger area (Eupene et al., 1975). Magnesium, expressed as a percentage of the total major soluble cations (i.e. Ca2+ + Mg2+ + Na+ + K*), is 87% in chloritized mine series rocks, but only 25% in unchloritized mine series rocks. Because Mg in ground waters at Ranger so closely reflects the rock Mg content, and thus the level of chloritization, the chlorite that accompanies uranium mineralization is evidently appreciably soluble at low temperatures, and may indeed be the dehydration product of the fluids which introduced mineralization, as suggested by Eupene et al. (1975). While all chlorites in the Cahill Formation in the Pine Creek Geosyncline are not associated with uranium mineralization, those which are appear to be sub-

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Fig. 4. Ranger One, No. 3 drill holes contoured A to E according to decreasing values of NMg. A = N M g > 0.85;B = 0.7 < NMg < 0.85; C = 0.5 < NMg < 0.7;D = 0 . 3 5 < NMg < 0.5;£ = NMg< 0.35.

ject to cycles of leaching and redeposition. For example, Snelling (1980b) correlated the latest generation of chlorites in the Koongarra deposit with the formation of secondary uranium mineralization, which he showed to be clearly related to the present ground-water flow patterns. Weathering processes at Koongarra have also been shown to deplete both U and Mg from the mine series rocks (Noranda, 1978). This suggests that waters draining Alligator Rivers uranium deposits should be enriched in both U and NMg. HYDROGEOCHEMICAL STUDY AT KOONGARRA

Samples (500 ml) of ground waters from 48 vertical percussion holes (air drilled in 1971—72) within the Koongarra Project area (Fig. 5), were collected from the bottom of each hole, using a flow-through drill-hole sampler (a short polycarbonate tube with rubber bilge pump valves at either end). pH, Eh, conductivity, temperature, and reduced iron were measured immediately at the sampling sites. Analyses for fluoride were made within 48 h of sample collection, and subsequent laboratory analyses were made for calcium, magnesium, sodium, potassium, chloride, sulphate, total car-

41 ORE BODY N°1

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Fig. 5. Ground-water sampling sites in vicinity of Koongarra ore bodies. Shaded areas show distribution of Group A waters in which NMg exceeds 0.8. (PH deleted from sample numbers to simplify figure.)

bonate, total phosphate, uranium, copper, lead and zinc. All analytical results are listed in Table I. pH and Eh pH and Eh values for each ground water were plotted (Fig. 6) onto a diagram which shows presently accepted uranium mineral stability fields, and pH boundaries of significant uranium adsorption (pH 5.5—pH 8). Both these factors influence the levels of uranium found in ground waters that contact uranium minerals, and should be considered when interpreting ground-water uranium values. Figure 6 clearly illustrates the equivocal nature of ground-water uranium values as an exploration parameter in isolation from other ground-water data. For example, if the waters which plot into the pH-Eh field of soluble uranium species were in equilibrium with uranium minerals, then these waters should contain uranium in excess of 240 μg/l. However, all samples in this category plot between the pH boundaries of significant clay adsorption of soluble uranium in low-salinity waters. Conductivity Conductivity gives a direct indication of ground-water salinity, approximate levels of which can be calculated from the conductivity measurements (Table I).

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TABLE I Analytical results for Koongarra ground waters Sample Depth to Sample Temp. pH water ta- depth (°C) (m) ble (m)

Eh Fe 2 + Conduc(mV)(mg/l) tivity (MS/cm)

Mg Na Ca (mg/1) (mg/1) (mg/1)

4.2 PH7 PHI 4 3.3 PH15 3.3 PH27 2.1 PH39 3.7 PH43 3.3 PH49 5 PH55 3.3 PH56 3.3 PH58 3.3 PH73 3.3 PH78 6.6 PH80 2.5 PH84 2.5 PH88 2.5 PH93 2.5 PH94 3.3 PH95 2.9 PH96 2.9 PH97 2.5 PH110 4.1 PH113 2.9 PH118 6.6 PH132 9.1 PH135 8.3 PH137 — PH139 — PH146 3.7 PH147 3.7 PH148 2.5 PH149 0 PHI 50 2.1 PH162 3.3 PH166 5.4 PH173 1.2 PH175 1.7 PH177 3.3 4.2 KD1 3.3 KD2 3.7 KD3 3.7 KI01 5.4 KI02 4.1 KOA 3.3 KPS1 KTD1 7.4 KTD2 6.6 KTD3 8.7 KTD4 2.5

343 100 156 340 210 180 307 162 134 169 140 157 192 229 210 122 289 104 237 186 130 194 207 228 383 449 169 124 184 46 129 129 156 414 331 194 224 207 234 138 341 30 244 284 93 314 -52 164

2.0 18.0 5.8 5.6 4.6 6.7 6.4 3.6 5.0 3.7 1.2 10.0 1.6 2.9 1.2 1.7 0.9 3.0 1.7 0.6 8.3 1.3 8.3 22.0 12.2 29.2 4.2 2.8 0.9 37.2 1.6 7.9 1.2 2.7 1.2 13.0 12.4 4.0 3.2 6.4 21.6 25.2 5.4 9.7 55.2 36.0 64.0 26.6

9 27.3 42.2 35.2 53.8 24.8 86 96 76.1 96 48.8 8.3 22.3 25.6 43 39.7 82.7 19.8 43.8 15.3 61.2 16.5 29.8 105 14

—

105 94.3 21.5 15.3 42.2 105 10.8 8.7 35.6 42.2 105 105 105 105 105 22.3 22.3 74.4 49.6 45.5 51.7 50.4

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—

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6.71 7.22 7.35 7.41 6.41 5.53 6.85 6.85 7.38 6.91 6.94 6.82 7.05 7.28 7.07 6.70 7.23 7.30 6.78 6.56 2.97 5.45 7.20 6.71 6.41 6.47 7.27 6.49 6.00 6.18 6.00 6.28 5.92 5.59 6.00 5.37 6.78 6.20 7.55 6.46 7.35 7.14 6.16 7.41 7.5 7.80 8.72 7.09

0 3.0 0.5 0 3.5 5 0.8 0.5 0.2 0.8 3.5 0 1.0 0.1 0.1 2 0 1.8 0.5 1.5 2.5 25 0 0 5 0

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0.1 25 0 0 0 0.1 8 0.2 5 0 0 2 0.5 2.3 0.1 0 0.3

212 219 222 147 131 175 144 162 151 169 121 140 144 142 101 106 68 95 69 48 96 79 192 255 42 62 134 183 62 182 112 175 89 72 118 170 183 123 148 177 185 177 55 143 454 334 398 297

25.6 30.4 32.8 13.2 17.2 20.0 26.4 23.6 20.0 21.8 17.0 8.0 19.2 17.3 13.4 12.7 7.2 7.7 7.8 0.7 5.8 2.4 18.9 26.0 1.3 7.4 14.9 26.0 4.2 5.8 16.6 24.0 1.6 10.1 17.4 12.8 19.6 10.2 20.0 20.0 14.0 15.6 4.0 10.4 35.2 20.0 9.9 22.8

3.6 1.9 1.8 10.5 1.3 1.2 2.8 2.0 1.5 2.8 1.6 2.2 1.9 1.9 2.0 3.0 3.6 3.0 2.7 4.0 2.3 1.7 2.0 2.4 3.7 1.3 4.4 1.9 4.3 16.2 1.7 1.4 2.0 1.7 1.2 8.7 2.8 1.6 3.0 2.0 4.1 2.5 2.0 6.2 15.7 22.4 22.4 20.0

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44 1

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Fig. 6. pH and Eh of Koongarra ground-water samples at time of collection. (PH deleted from sample numbers to simplify figure.)

All samples had low salinities (generally < 200 mg/1) reflecting extensive annual ground-water recharge from the high wet season rainfall. These salinities are sufficiently low to allow significant adsorption of soluble trace metals, such as uranium, onto clays or hydrous metal oxides. Reduced iron (Fe2+) Field measurements of reduced iron were made to provide a direct indication of redox conditions among ground-water constituents. When coupled with acidic pH levels, high levels of reduced iron can also provide evidence for local leaching of heavy metal sulphides. Reduced iron values in waters in this study were either below detection, or very low. This is in keeping with the generally near neutral pH and the moderately oxidizing conditions found in most samples. Also the absence of any samples with acidic pH indicates that none of the ground waters sampled is leaching heavy metal sulphides. Major-element chemistry Schoeller plots (Schoeller, 1935), which illustrate the Ca2+, Mg2+, Na+ + K , Cl~, SO2" and HCOJ contents, have been made for each water sample. +

45

The shape of each plot is a "signature" for that water, and "signature" similarities have been used to identify waters which are genetically related, but have undergone varying degrees of evaporation or dilution. On the basis of similarities in the cation part of each plot, samples have been sorted into 5 groups, A to E, as illustrated in Fig. 7. The low salinity of all samples implies undersaturation of these ground waters with respect to aquifer minerals. As a consequence, solutes from the more easily soluble minerals will dominate the composition of ground-water solutes. Cation relativities in the ground waters will be a function of cation relativities in aquifer rocks, modified by the relative solubility of minerals in contact with the ground waters, the flow rate of the ground water and relative distances of each mineral exposure from the point of sampling. 3 2 - PH7 E

0

O _l

-| >

-2

"Λ-\/ V 1

X

PH15

PH14

1

1

1

J

1

I

I

I

I

I

I

O

X

4-

<-> O

I

3r 2h I

O

O

O

Ο Σ

+

Ο

θ

Ο

O

Fig. 7. Examples of the five groups (A—E) of major element composition plots of Koongarra ground-water samples.

46

Samples in group A have cation relativities which closely resemble those in ground waters from uranium mineralized aquifers at each of the four known deposits in the Alligator Rivers Province (Fig. 3). This reflects the predominance of Mg chlorites in and around the Koongarra ore zone (Snelling, 1980b) in common with deposits at Nabarlek, Jabiluka and Ranger One. The sampled area covered by this group is hatched in Fig. 5 and includes drill holes in and near both ore bodies, and drill holes both up and down strike of known mineralization. Cation relativities in samples in groups B, C and D can be interpreted as the result of mixing of waters from chloritized, uranium-mineralized aquifers with waters from nonmineralized aquifers, the proportion of the former decreasing from group B to group D. Waters plotted in group E have no apparent contribution from a mineralized aquifer. The mean NMg in each group ranges from 0.85 (Group A), through 0.72 (Group B), 0.53 (Group C), 0.46 (Group D) to 0.19 (Group E). The high NMg in waters in Group A implies that Mg is leached from chloritized aquifers. This is confirmed by differences observed in Mg between primary and weathered examples of chloritized, uranium-mineralized mine series rocks from the Koongarra area (Noranda, 1978). In primary chloritized mine series rocks, Mg expressed as a percentage of the total major soluble cations (Mg2+, Ca2+, Na + , K+) comprises 82%, dropping to 56% in the weathered material. Another major element which is present in variable amounts in the ground waters sampled is sulphur as SO4". These variations reflect the limited and random occurrence of base-metal sulphides. The equivalence of Na+ + K+ with Cl" exhibited by some of the ground waters may indicate a common origin for these ions (e.g. sea water trapped during sedimentation or contributed by present-day oceanic aerosols). Ground waters containing Na+ + K+ in excess of Cl" contain contributions from aquifer rocks from which Na+ and/or K+ have been leached from silicate minerals. Carbonate is uniformly high in all samples and mostly comprises > 90% of the major anions (CF, SO4" and total carbonate). High carbonate in ground waters accords with the regional association of Alligator Rivers uranium ores with carbonate-bearing strata (Hegge et al., 1980). Uranium Table I lists uranium values ranging from < 0.6 Mg/1 to 4100 μg/l in ground waters in Koongarra drill holes. There is no simple relationship between uranium levels in ground waters and the presence of uranium mineralization in aquifer rocks because, regardless of the solid uranium phase present, its solubility is controlled by prevailing pH and Eh, and the level of soluble uranium can be limited by adsorption onto water system solids. Koongarra ground-water uranium values are, therefore, equivocal

47

as indicators of the ore bodies. Considered in isolation, Koongarra uranium values are log-normally distributed about 2.5 μg/l, with a threshold of 63 μg/l. The only waters which have uranium in excess of 63 μg/l are from six drill holes directly within No. 1 ore body (and from KTD1 4 km to the south of No. 1 ore body), so no ground-water uranium halo is in evidence about either ore body. A further complication in the use of ground-water uranium values in the Pine Creek Geosyncline arises because uranium can be just as easily leached from certain felsic igneous rocks such as the granitoids in the Nanambu complex. These are enriched in uranium (4—16 ppm) compared to the world average abundance (4.8 ppm) for rocks of this type (Ferguson et al., 1980). Ground waters from aquifers in Nanambu rocks near Ranger One No. 3 ore body have up to 30 μg/l of U (Giblin, unpubl. report). To distinguish ground-water uranium leached from non-economic dispersed uranium occurrences from that leached from discrete concentrations of uranium mineralization, use may be made of observed associations between uranium and other species leached simultaneously from aquifer rocks. Solution levels of co-leached species should exhibit a consistent trend with uranium in a series of ground-water samples from the same aquifer, although variations in pH and Eh in different parts of the aquifer may perturb the trend. Moreover, the short residence time of ground waters in the Pine Creek Geosyncline causes undersaturation of aquifer minerals which may also limit the usefulness of this approach. Measured parameters which may be useful in this method include total dissolved salts and fluoride levels. The latter is useful since uranium derived from leaching felsic igneous rocks is usually accompanied by fluoride in a particular U:F~ ratio. The level of total dissolved salts gives a measure of total material leached by the water from aquifer rocks, and uranium would be derived from the same aquifer rocks in a fixed proportion of the total solutes. For example, there are curvilinearly increasing relationships between uranium and salinity, and uranium and fluoride for ground waters in Nanambu Complex aquifers near the Ranger ore bodies (Giblin, unpub. report). While these trends are not particularly well defined, uranium values which deviate significantly should be identifiable. Positive deviation would imply that a ground water has encountered a uranium source carrying higher levels of uranium than the Nanambu rocks. This source might be part of an economic deposit. Negative deviation would suggest uranium deposition from the ground water at that sample site, particularly if pH-Eh conditions favour uranium in the solid phase. This could also be the site of economic deposition. The uranium:salinity relationship for waters from Koongarra exhibits an approximately linearly increasing trend, except for waters from drill holes PH15, PH49, PH94, PH56, PH88, PH58, PH7 and PH14 that clearly have higher uranium contents than the majority uranium:salinity ratios would predict. This is explained by the proximity of these drill hole sites to the No. 1 ore body.

48

There is a complete absence of a trend between uranium and fluoride in Koongarra waters. In contrast, waters from other potentially uraniferous provinces in Australia have uranium concentrations in the tens to hundreds of μg/l which exhibit a clear linear trend with fluoride levels in the waters (Giblin, unpubl. reports). Such a linear trend indicates that in these areas apparently anomalous ground-water uranium values are derived from leaching local basement or bedrock, and are not associated with local uranium concentrations. The absence of a U:F" trend at Koongarra clearly accords with uranium derived from the ore bodies. Uranium in ground waters from mineralized aquifers may exhibit a consistent association with a species leached from minerals identified as wall rock or regional alteration products formed during the uranium mineralizing event (e.g. ground-water levels of Mg2+ leached from the high Mg chlorite which accompanies all known uranium deposits in the Alligator Rivers Province). Ground waters from the vicinity of Nabarlek, Ranger and Koongarra exhibit coincidence between uranium and NMg. For example, Fig. 8 illustrates U and NMg across Ranger One No. 3 (Giblin, unpubl. report) and Fig. 9 the same parameters across the Koongarra deposit. At Jabiluka, pH-Eh conditions in ground-water aquifers inhibit uranium solubility, so high NMg (> 0.8) values are not accompanied by high ground-water uranium content (Deutscher et al., 1980). Figure 10 shows uranium plotted against NMg in waters from the immediate vicinity of the Koongarra ore •

J 100

Λ

' * ·' \\ /

10 0-8 06 0-4

. i -

0-2

·

\/V

1

^

1 1

·

9600

* \

\U *

1

9400

3

10 E

\ X\ °! \

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9200

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9800

10000 10200

\ « 10 400 m East

^ > ^ Hanging wall I Upper S e q u e n c e ^ — ^ mine ^ "^. \ ^ ^ . ^series ^U308>0-2% ^^Lower mine Footwall ^sequence sequence x Fig. 8. Uranium and NMg in ground water in drill holes on Ranger One, No. 3 between 11 600 mN and 11 700 mN.

49

Fig. 9. Uranium and NMg in ground waters in drill holes across Koongarra No. 1 ore body.

1000

N?1 ORE BODY

100

• 132

• 94

10 • 27 •78 • 137 10 ► 135

• 175 ,

02

0-3

KD1S . 118· * \ K D 3 ν · \

-pH-Eh conditions depress Usolubility • 80 \ ν·73ν KD2 A ••Us

-I

04

110 162 \ KOA ^«• Φ A 1 0-5

113 147 mm I

0-6

95 ,

m—I

96

m

166 \ 9 3

*->-tf>—5

NMg

Fig. 10. Uranium in ground waters from drill holes in the vicinity of Koongarra ore bodies plotted against NMg levels in those waters.

50

bodies. While there is no clear cut trend there is a certain degree of correspondence between high uranium values and high NMg. Samples KD3, PH150, PH43, PH93, PH146, PH149 which have high NMg (> 0.8) but low uranium were all sampled from drill holes in which pH-Eh conditions depressed uranium solubility. In considering the exploration value of hydrogeochemical techniques for deposits like Koongarra, it must be remembered that Koongarra carries a very distinct and readily observed radiometric anomaly. This anomaly may derive from shallow uraniferous ground waters depositing uranium in surface rocks and soils. If this is the case a hydrogeochemical uranium anomaly is merely another facet of the readily detected radiometric anomaly. Copper, lead and zinc Copper and lead values are either at or below detection (0.01 and 0.05 mg/1, respectively). Zinc is generally low except for PH97, KD2 and KPS1 which have higher levels, around 1 mg/1. In the absence of information regarding the zinc content of drill hole linings these higher values can be given no significance. These base metal data imply that aquifer rocks contain negligible amounts of base metal sulphides, which accords with analysis of primary Koongarra ore (i.e. Cu—160 mg kg"1, Pb—640 mg kg"1 and Zn—65 mg kg"1 (Noranda, 1978)). Thus base metal levels in ground waters cannot be used as exploration indicators of the Koongarra ore deposits. In contrast, high levels of Cu (0.2—0.5 mg/1) are found in the vicinity of Ranger ore bodies (Giblin, unpubl. report), and near the ore bodies at Jabiluka ground waters from drill holes and the adjacent flood plain have locally enhanced levels of Cu, Pb and Zn (Deutscher et al., 1980). Because these higher concentrations of base metals at Jabiluka are invariably associated with low pH and often with high Fe(II) concentrations, their source is ground-water leaching of base-metal sulphides associated with the ore bodies, and Cu, Pb and Zn content of ground waters can be used as exploration indicators for ore deposits in the Jabiluka area. Phosphate Phosphate values in Koongarra ground waters generally range from the detection limit (0.005 mg/1) up to the mean value of 0.04 mg/1. KTD4 and PHI5 which both contain « 0.08 mg/1 are exceptions. Primary Koongarra ore has 0.140% ΡΟΓ, and the weathered ore 0.099% ΡΟΓ (Noranda, 1978), so there is apparently minimal ground-water transport of PO4" from the ore bodies. Snelling (1980a) suggested that the concentration of phosphate in local ground water at Koongarra (initially derived from apatite leaching) causes uranium leached out of the primary ore to be locally reprecipitated as uranyl phosphates. This would account for the observed phosphate

51

conservation as primary ore is converted to weathered ore. A similar situation can be observed at Ranger where mining of No. 1 ore body initiates new ground-water flow paths, which terminate at the ground surface in clusters of saleeite crystals (Sherrington et al., 1983). Clearly ground-water phosphate is significant in Alligator Rivers Province uranium deposits, but its actual exploration significance is still unclear, apart from its obvious application in the production of mineral solubility index maps. HYDROGEOCHEMICAL DATA RELATED TO AQUIFER ROCKS AT KOONGARRA In Koongarra ground waters, the clearest indication of the ore bodies comes from the ready solubility of magnesian chlorite which accompanies the ore. Deviation in certain waters from a U:salinity trend, and the absence of any U:F" trend in the sample group also suggest local uranium deposition. Values of strategic parameters NMg and U result from a solution reaction between each ground water and its aquifer rocks, and also from various degrees of mixing of waters from different aquifers. Koongarra drill hole logs from each sample site illustrate the source of NMg and U in each water, how closely these parameters relate to known mineralization, and how this application of hydrogeochemistry indicates the direction for further exploration drilling. The Koongarra ground-water groups A to E are used to summarize relationships between composition of ground waters, and the mineralogy of aquifer rocks at each sample site. Group A Holes PH7, PH14, PH15, PH49, PH55, PH56, PH58 all intersect the uranium mineralization of the No. 1 ore body which is contained in Mgrich chloritized schists. Both the aquifer characteristics are reflected in these waters: 80—4100 μg/\ of U and NMg > 0.7. Holes KD2, PH73, PH80, PH84, PH88 with 4 - 4 6 Mg/1 of U and NMg > 0.8 all reflect the down-flow hydrogeochemical dispersion from the No. 1 ore body. Lower uranium values probably relate to dilution by surface waters which does not lower NMg. Holes KD3 and PH39 with NMg « 0.8 and 1.5-4 /zg/1 of U reflect dispersion from the deeper, radiometrically blind, smaller, lower grade No. 2 ore body which is also contained within Mg-rich chloritized schists. Holes PH43, PH146, PH149, PH150, PH166, PH173 all lie along strike of the No. 1 and No. 2 ore bodies and exhibit the same aquifer "signature" characteristic of the mineralized zones. Traces of mineralization were encountered in hole PH150, so the areas,particularly upflow, around these holes warrant detailed follow-up exploration. All holes, except PH166, are in favour-

52

able schists. Hole PH166 is in Kombolgie sandstone which suggests a "shallow" mineralized schist source further upflow under the sandstone escarpment, a distinct possibility based on current geological interpretations in the area. Group B Holes PH93, PH94, PH95, PH96, PH139 ( 1 - 1 8 ßg/\ of U and NMg = 0.63—0.75) are all in schists down-flow of the No. 1 ore body and thus reflect mixing of the waters from the mineralized zone with other waters, diluting both the U content and NMg in the process. Comparing these waters with those from holes KD1, PH118, PH132, PH177, it could be concluded that these latter waters also reflect mixing of waters from a mineralized zone, with other waters. Water in PH132 contains 21'pg/\ of U and with hole 118 is in a group of holes (along strike) indicative of a mineralized aquifer. Hole PH177 is similarly situated in a significant cluster. These results are again consistent with the suggestion that a relatively shallow mineralized schist source exists up-flow under the Kombolgie sandstone escarpment, similar to Jabiluka 2. In this regard the water in hole KD1 (which intersected Kombolgie sandstone only) also fits this pattern, so if, as some think, the No. 1 ore body is on the southern limb of an anticline, the crest of which has been truncated by erosion and bisected by a reverse fault, then KD1 water may reflect mineralization on the northern limb which now lies under the sandstone. Group C Hole PH27 is directly above the No. 2 ore body so its water (4 μg/l of U and NMg = 0.57) represents mixing/dilution of waters, some of which drain the mineralized zone. Hole PH175 is down-flow of hole PH173 in which the water gave the signature of a mineralized aquifer. So mixing has occurred during the passage of waters from hole PH173 to PH175. Hole PH147 is similarly down-flow of hole PH146 (group A water). Holes PH113 and KPS1 are both in dolomites with thin schist bands and this is reflected in their waters. Group D In this group are holes PH110, KI01 and KI02 which are also in dolomites with thin schist bands. In these waters the Ca concentration is (as expected) roughly equal to the Mg concentration. Hole KOA is close to the dolomites, which site investigations have shown to provide some seasonal recharge to the schists, so hole KOA is in a zone of mixing of waters from several aquifer rock types. Hole PH78 is down-flow of KD1 and still in barren sandstone behind the No. 1 ore body (the mineralized portion of

53

the aquifer further down-flow) so it is to be expected that its waters should reflect mixing and dilution of the "signature" pertaining to hole KD1 waters. Group E Of the remaining holes, PH135 and PH137 are in quartz-mica schists with some amphibolite and this is reflected in the relatively high Ca concentration of their waters, although the absolute concentration is not high. These two waters are in marked contrast to the waters in holes along strike on either side of them, but this may partly be due to pumping of hole PH137's water to supply the Project's camp, causing faster inflow and therefore more mixing. Geophysical investigations suggest a highly transmissive zone so the greater volume of water in this zone may also result in dilution. Hole PH148 is on the edge of the belt of dolomites and these dolomites have mica schist bands. Furthermore the dolomites are down-flow of the surrounding schists so the major element composition of the water in hole PH148 reflects dominance of mica schists as aquifer rocks. Holes PH162 and PH97 gave results which do not seem to fit either their geological setting or the results from surrounding holes. One possible explanation could be the relatively shallow depths from which the samples came. But these waters also have very low salinities, a result perhaps of these holes penetrating highly transmissive zones (fractures?) which induce higher volume flows and therefore dilution (shorter retention time). In the case of hole PH97 this highly transmissive fracture appears to cut through from the Kombolgie sandstone, thus possibly giving the water an apparent sandstone "signature". Holes KTD1—4 are discussed here as a group even though their waters exhibit a variety of characteristics. These holes, however, intersect a different ground-water flow regime where there is limited recharge through a lateritic and weathered schist aquifer overlying the fractured, relatively fresh, mica schist and amphibolite aquifer, and aquifer retention times are longer. The net result is waters with higher salinities than the waters to the north around the Koongarra ore bodies. Even though the U contents range from 2 μg/l to 91 μg/l, NMg is always < 0.5, in itself indicative of non-mineralized aquifers. Holes KTD1 and 4 intersected quartz-mica schists, while both KTD 2 and 3 intersected both quartz-mica schists and amphibolite. It is concluded that these waters do not drain from a mineralized zone of the Alligator Rivers type. CONCLUSIONS

Pine Creek Geosyncline uranium deposits are all found in chloritized Lower Proterozoic metasediments of the Cahill Formation in the Alligator

54

Rivers Province, where high rainfall causes rapid ground-water flow and brief aquifer retention, with consequent low salinity (< 300 mg/1) ground waters. Uranium concentrations in ground waters from uranium mineralized aquifers are modified by certain combinations of the ground-water parameters pH, Eh and salinity. Consequently, ground waters from uranium mineralized aquifers may under some conditions have low uranium concentrations, and uranium in isolation is a hydrogeochemical indicator of uranium mineralization which is difficult to interpret. Ground waters from mineralized aquifers at the Jabiluka, Ranger, Nabarlek and Koongarra uranium deposits in the Alligator Rivers Province of the Pine Creek Geosyncline have very similar major element relativities, which constitute a mineralized aquifer "signature" when expressed as major element concentration plots. In particular, relative magnesium concentration (normalized magnesium: NMg = milliequivalents Mg per litre/milliequivalents (Na + K + Ca + Mg) per litre) is the most effective hydrogeochemical indicator of each of the four deposits. Exceeding 0.8 in ground waters from mineralized aquifers, NMg derives from ground-water solution of the magnesium-enriched chlorite which accompanies uranium mineralization in this province. Progressive mixing with waters from nonmineralized aquifers causes graded reductions of NMg, so that contours of ground-water NMg values define a hydrogeochemical aureole for a uranium deposit of the Alligator Rivers type. Ground waters in aquifers in nonmineralized felsic igneous rocks such as the Nanambu Complex may under some conditions contain apparently anomalous uranium (tens to hundreds of Mg/1). Linear relationship of uranium in these waters with salinity or fluoride, and low ground-water NMg values indicate the non-economic nature of the uranium source. A hydrogeochemical study in and around the Koongarra ore bodies, using the principles discussed in this paper, successfully indicated the mineralized zones, and by analogy suggested three other potentially mineralized zones, which merit follow-up exploration drilling. Waters interpreted as derived from mineralized aquifers had NMg > 0.8, and uranium varying from 0.6—4100 Mg/l. Uranium in waters from aquifers within No. 1 ore body varied from 80—4100 μg/l. Although base-metal sulphides occur at Koongarra, levels of Cu, Pb and Zn in ground waters were not indicative of sulphide mineralization. Phosphate in ground waters at Koongarra is clearly important in the retention within the ore zone of uranium leached by oxidizing ground waters. However, measured levels of phosphate did not demonstrate immediate exploration significance. ACKNOWLEDGEMENTS

Gratitude is expressed to Geopeko, Denison Australia and E.R.A. Ltd for permission to sample from their drill holes, for access to data which

55

represents many years of work, and for valuable discussions with their staff. CSIRO research on hydrogeochemical exploration for uranium is supported by the Australian Mineral Industries Research Association Limited. REFERENCES Deutscher, R.L., Mann, A.W. and Giblin, A.M., 1980. Ground-water geochemistry in the vicinity of the Jabiluka deposits. In: J. Ferguson and A.B. Goleby (Editors), Uranium in the Pine Creek Geosyncline. IAEA, Vienna, pp. 477—486. Eupene, G.S., Fee, P.H. and Colvile, R.G., 1975. Ranger 1 uranium deposits. In: C.L. Knight (Editor), Economic Geology of Australia and Papua New Guinea. 1. Metals. Australas. Inst. Min. Metall., Monogr. Ser., 5: 308—317. Ferguson, J., Chappell, B.W. and Goleby, A.B., 1980. Granitoids in the Pine Creek Geosyncline. In: J. Ferguson and A.B. Goleby (Editors), Uranium in the Pine Creek Geosyncline. IAEA, Vienna, pp. 73—90. Foy, M.F. and Pedersen, C.P., 1975. Koongarra uranium deposit. In: C.L. Knight (Editor), Economic Geology of Australia and Papua New Guinea. 1. Metals. Australas. Inst. Min. Metall., Monogr. Ser., 5: 317—321. Giblin, A.M., 1980. The role of clay adsorption in the genesis of uranium ores. In: J. Ferguson and A.B. Goleby (Editors), Uranium in the Pine Creek Geosyncline. IAEA, Vienna, pp. 521—529. Hegge, M.R., Mosher, D.V., Eupene, G.S. and Anthony, P.J., 1980. Geologic setting of the East Alligator uranium deposits and prospects. In: J. Ferguson and A.B. Goleby (Editors), Uranium in the Pine Creek Geosyncline. IAEA, Vienna, pp. 259—272. Langmuir, D., 1978. Uranium solution-mineral equilibria at low temperatures with applications to sedimentary ore deposits. Geochim. Cosmochim. Acta, 42: 547—569. Needham, R.S. and Stuart-Smith, P.G., 1976. The Cahill Formation — host to uranium deposits in the Alligator Rivers Uranium Field, Australia. BMR J. Aust. Geol. Geophys., 1: 3 2 1 - 3 3 3 . Needham, R.S. and Stuart Smith, P.G., 1980. Geology of the Alligator Rivers Uranium Field. In: J. Ferguson and A.B. Goleby (Editors), Uranium in the Pine Creek Geosyncline. IAEA, Vienna, pp. 233—257. Noranda Aust. Ltd., 1978. Koongarra Project. Draft Environmental Impact Statement. Queensland Mines Ltd., 1977. Draft Environmental Impact Statement. Nabarlek Project. Schoeller, H., 1935. Utilite de la notion des exchanges de bases pour l e comparison des eaux souterraines. Fr. Soc. Geol. Co. R. sommaire et Bull. Ser. S., 5: 651—657. Sherrington, G.H., Browne, A.L., Gatehouse, S.G., Duffin, R.H. and Danielson, M.J., 1983. Number three orebody, Ranger One, Australia. A case history. J. Geochem. Explor., 19: 7—9. Snelling, A.A., 1980a. Uraninite and its alteration products, Koongarra uranium deposit. In: J. Ferguson and A.B. Goleby (Editors), Uranium in the Pine Creek Geosyncline. IAEA, Vienna, pp. 487—498. Snelling, A.A., 1980b. A geochemical study of the Koongarra uranium deposit, Northern Territory, Australia. Ph.D. thesis. University of Sydney (unpubl.).