Dating of U-rich heterogenite: New insights into U deposit genesis and U cycling in the Katanga Copperbelt

Precambrian Research 241 (2014) 17–28

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Dating of U-rich heterogenite: New insights into U deposit genesis and U cycling in the Katanga Copperbelt Sophie Decree a,∗ , Étienne Deloule b , Thierry De Putter a , Stijn Dewaele a , Florias Mees a , Jean-Marc Baele c , Christian Marignac d a

Royal Museum for Central Africa, 13 Leuvensesteenweg, B-3080 Tervuren, Belgium Centre de Recherches Pétrographiques et Géochimiques (CNRS), BP 20-54501 Nancy, France c Université de Mons, B-7000 Mons, Belgium d UMR-G2R CREGU & Ecole des Mines de Nancy, F-54042 Nancy, France b

a r t i c l e

i n f o

Article history: Received 24 July 2013 Received in revised form 12 November 2013 Accepted 23 November 2013 Available online 4 December 2013 Keywords: Katanga Copperbelt Uranium deposition Shinkolobwe U–Pb geochronology Heterogenite

a b s t r a c t The Katanga Copperbelt region of the Democratic Republic of Congo hosts world-class cobalt deposits accounting for ∼50% of the world reserves. Heterogenite (CoOOH) is the most abundant Co-bearing secondary mineral in the region. Its occurrence is the result of oxidation of Cu–Co-sulfides and associated Co reprecipitation in the uppermost part of the deposits, during the Pliocene. In addition to sediment-hosted copper and cobalt ore deposits, the Katanga Copperbelt also hosts numerous uraniferous mineral occurrences and deposits, which can be associated with heterogenite. Within these deposits, heterogenite can have high concentrations of U (up to 3.5%) and Pb (up to ∼4%). In situ SIMS U–Pb ages were obtained for heterogenite samples from the U deposits of Shinkolobwe, Kalongwe and Kambove. These analyses yield distinct Neoproterozoic ages, at ∼876 Ma, ∼823 Ma and in the ∼720 to ∼670 Ma age range. As the geological context prevailing at those times was not favorable for heterogenite formation, these ages most probably record geological events that are not the formation of the mineral itself. For instance, the heterogenites could have inherited the U–Pb signature of a U-rich mineral, most likely uraninite, formed and/or yet reworked at ∼876 Ma, ∼823 Ma and in the ∼720 to ∼670 Ma time interval and spatially associated with primary Co-sulfides. In this hypothesis, the ages obtained in this paper are significant for understanding the cycling and re-deposition of U at given moments in the regional geological history. In such context, the ∼876 Ma and the ∼823 Ma age are consistent with syn-early diagenetic concentration of uranium in sediments of the Katanga basin. The ∼720 to ∼670 Ma ages are interpreted as a phase of U remobilization related to hydrothermal fluid circulation induced by late Nguba proto-oceanic rifting or by early stages of Congo-Kalahari craton convergence, and associated hydrothermal circulation. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The Katanga Copperbelt of the southeastern Democratic Republic of Congo, which is part of the Lufilian fold-and-thrust belt, hosts ∼50% of the world’s known reserves of minable cobalt, and it accounts for about 3% of the world’s copper reserves (USGS, 2012). Besides these base metals, the Copperbelt also hosts numerous uraniferous mineral occurrences, including the well-known

∗ Corresponding author. Tel.: +32 27695438. E-mail addresses: [email protected] (S. Decree), [email protected] (É. Deloule), [email protected] (T. De Putter), [email protected] (S. Dewaele), fl[email protected] (F. Mees), [email protected] (J.-M. Baele), [email protected] (C. Marignac). 0301-9268/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.precamres.2013.11.009

Shinkolobwe deposit, whence came the metal used in the Manhattan Project in WWII. Interestingly, this deposit is altogether well-known and poorly studied: its mineralogy has been investigated (Deliens et al., 1981; Derriks and Vaes, 1956; Thoreau and du Trieu de Terdonck, 1933) and U–Pb ages are progressively gathered (Decree et al., 2011; this study) but the size of the deposit and its mode of formation are yet to be more studied and determined. Most of the known U occurrences of the Katanga Copperbelt are located in the outermost tectonic unit of the Lufilian fold-andthrust belt (Fig. 1), along a structural lineament that extends from Kalongwe to Luiswishi (Derriks and Oosterbosch, 1958; Ngongo, 1975), and in anticlines and thrust sheets in the Kambove and Kolwezi areas (Cahen et al., 1971) (Fig. 2A). In these deposits, U is mostly hosted in the lower part of the Neoproterozoic Roan Group, corresponding to the Mines Subgroup (Fig. 2B). As its distribution is controlled by lithostratigraphy,

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S. Decree et al. / Precambrian Research 241 (2014) 17–28

Fig. 1. Location of the Lufilian Belt, between the Congo and Kalahari cratons (after Cailteux et al., 2005b), and sub-division in tectonic units (after Porada and Berhorst, 2000; Unrug, 1988); the Outer Unit corresponds to the “External fold-and-thrust Belt”, the Middle Unit to the Zambian “Dome Region”, and the Inner Unit includes the “Synclinorial Belt” and the “Katanga High”. The white square delineates the area presented in Fig. 2A.

syngenetic or syn- to (dia-)genetic concentration of uranium is commonly assumed (Cailteux, 1983, 1997; Craig and Vaughan, 1979; Franc¸ois, 1974; Loris, 1996; Meneghel, 1981; Ngongo, 1975). This early concentration of uranium is related to the establishment of a redox front (Cailteux, 1983, 1994; Loris et al., 2002), with (i) U mobilization from heavy minerals occurring in the basal Roan formation (Cailteux et al., 2005a), in oxidizing conditions (Cailteux, 1997; Loris et al., 2002), and (ii) U re-deposition in the overlying Mine Series sediments, in reducing conditions in a confined evaporitic setting with high organic matter concentrations (Loris et al.,

2003). Besides heavy minerals in the basal Roan deposits, other possible U sources are Mines Series sediment components derived from the neighboring Congo Craton or the Kibara Belt (Audeoud, 1982; Jedwab, 1997), and fluids originating from the pre-Katangan basement (see Cailteux, 1994; Sweeney and Binda, 1994). In addition to its presence in the Mines Subgroup, uranium also occurs in tectonic breccias within these sediments (Cailteux, 1997; Lefebvre and Tshauka, 1986), as well as along strike-slip faults cross-cutting the Mines Subgroup (Loris et al., 1997).

Fig. 2. Geological structure of the central part of the Katanga Copperbelt. (A) Geological sketch map (after Cailteux et al., 2005b; Franc¸ois, 1974; Meneghel, 1981); (B) Summarized lithostratigraphy of the Katangan Supergroup in the DR Congo (after Kampunzu et al., 2009).

S. Decree et al. / Precambrian Research 241 (2014) 17–28

U–Pb ages for uraninite from Katanga indicate two (re)mobilization episodes (Decree et al., 2011). The first one occurred at ∼652 Ma, coeval with the regional Nguba protooceanic episode, which has been related to the Rodinia break-up (Kampunzu et al., 1991). The 652 Ma event could relate either to thermally-driven fluid circulation, associated with deep-burial diagenesis of Lower Roan sediments or to the percolation of meteoric fluids along border faults of uplifted blocks (Decree et al., 2011). However, new REE and Y analyses suggest the involvement of hot fluids (200–300 ◦ C) (Eglinger et al., 2012, 2013), which is in agreement with fluid inclusion data (Audeoud, 1982), hence favoring the thermal hypothesis. The second U (re)mobilization episode took place at ∼530 Ma and has been interpreted to result from metamorphic fluid circulation related to the Pan-African (Lufilian) orogeny (Decree et al., 2010). In uranium deposits of Katanga, uranium is often closely associated with heterogenite (CoOOH), the most abundant mineral in oxidized Co ores in this region. Although heterogenite formation is generally related to a regional Pliocene weathering episode (De Putter et al., 2010; Decree et al., 2010; Ngongo, 1975), an age of 632 ± 20 Ma has been reported for U-rich heterogenite from Shinkolobwe (U–Pb ages; Deliens, 1974). Although it is an unusual approach, U–Pb dating of heterogenite using SIMS (Decree et al., 2010) has proven to be suitable for the detailed investigation of U deposition formation and cycling in the Katanga Copperbelt, which is explored in the present study.

2. Geological context The Katanga Copperbelt (or folded Lufilian Arc) is set in a complex geotectonic framework. It is surrounded by older and contemporaneous/younger geological units; the older units can be listed as possible source for the uranium in the Katanga basin (Fig. 1). (1) The Dome region in Zambia toward the south, comprises an assemblage of schists, granitoids and gneisses, dated between 1994 and 1873 Ma (Rainaud et al., 2005b) that is intruded by a late granite generation (883 ± 10 Ma; Nchanga granite, Armstrong et al., 2005). These metamorphic and igneous rocks form the basement of the Zambian and the southern part of the Katanga Copperbelt. This region probably constituted a topographic “high” (threshold) in the South, from the Neoproterozoic onward (Bull et al., 2011). (2) In the West, Neoarchaean granites and granulites of the Congo Craton, dated at 2523–2538 Ma (Key et al., 2001), and pelites and schists of the Kibaran Belt occur. The latter are crosscut by granitoids, emplaced at ∼1381 Ma (Kokonyangi et al., 2004). These Kibaran metasedimentary rocks have been deformed during the Kibaran tectonic stage, at ∼1079 Ma (Kokonyangi et al., 2004) and were intruded at ca. 1.0–0.95 Ga, by tin granites (Kokonyangi et al., 2006). (3) The Kundelungu Aulacogen occurs to the northeast and comprises sandstones and shales that were deposited between <635 Ma and 530 Ma (Kampunzu et al., 2000, 2009). (4) The Bangweulu Block to the east is formed by Palaeoproterozoic granitoids and associated volcanic rocks, between 1.88 and 1.83 Ga (Brewer et al., 1979; Schandelmeier, 1981). It is overlain by late Palaeoproterozoic undeformed quartzo-pelitic sediments at ∼1.8 Ga (see De Waele, 2005). Alaskites occur in the Kapulo area (André, 1976) and around Lubumba (∼1.8 Ga; Kabengele et al., 1987). Southeast of the Bangweulu Block, the Irumide Belt includes basement and supracrustal units yielding ages that are similar to those obtained for the Bangweulu Block (De Waele et al., 2006). They have been intruded by granitoid rocks between 1.66–1.55 Ga and ∼1 Ga (De Waele, 2005; De Waele et al., 2009). In the present state of knowledge, which is far from complete, the Cu–Co–U deposits of the Katanga Copperbelt are predominantly hosted in the lower part of the Neoproterozoic Katanga

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Supergroup (Roan Group, 880–730 Ma; Kampunzu et al., 2009) (Fig. 2), specifically within the Mines Subgroup (or Mines Series, R2, ∼150 m thick; Franc¸ois, 2006). Deposition of the basal RAT Subgroup (R1) constitutes the first known transgressive sequence over the basement, before the onset of Mines Subgroup sedimentation in reducing conditions, which is characterized by several major transgressive–regressive cycles (see Bull et al., 2011; Cailteux, 1994). The Lower Roan sediments were deposited in a continental rift environment (Cailteux et al., 2005b; Selley et al., 2005). Earlydiagenetic deposition of Cu and Co sulfides occurred during or shortly after this extension phase (Muchez et al., 2007), as suggested by the 816 ± 62 Ma Re–Os age obtained for primary sulfides from deposits in Zambia (Barra et al., 2004 in Selley et al., 2005). Sulfide deposition was related to circulation and evaporation of saline seawater (11.3–20.9 wt% NaCl equiv; El-Desouky et al., 2010) that was heated during its migration through the deeper subsurface (115–220◦ ) and was enriched in metals by its interaction with basement rocks (El-Desouky et al., 2010). Syn-diagenetic concentration of U could have taken place at the same time (Cailteux, 1983, 1997). The overlying rocks of the Nguba Group (∼730 to ∼635 Ma; Kampunzu et al., 2009) and the Kundelungu Group (∼635 to <573 Ma; Kampunzu et al., 2009) display basal diamictites, correlated with the Sturtian and Marinoan glaciations (Hoffmann et al., 2004; Kampunzu et al., 2009). The sedimentary Nguba series was emplaced in a proto-oceanic rift setting (Kampunzu et al., 1991), which was related to the Rodinia breakup (Kampunzu et al., 2009). Mafic magmatism was associated to this extensional context (at ∼765 and ∼735 Ma; Key et al., 2001). A major U mineralization or remobilization episode is dated at ∼652 Ma (Decree et al., 2011). The Co–Ni–(Cu) sulfides that are spatially associated with uraninite could have been remobilized at the same time as uranium (Decree et al., 2011). The overlying rocks of the Nguba series and the Kundelungu Group (∼635 to <573 Ma; Kampunzu et al., 2009) display basal diamictites, correlated with the Sturtian and Marinoan glaciations (Hoffmann et al., 2004; Kampunzu et al., 2009). The main event affecting Roan to Kundelungu sedimentation was the Pan-African Lufilian orogeny. Recent dating of metamorphic minerals shows that the Lufilian orogeny spans an intense compression on about 60 m.y. The oldest metamorphic ages are given by U–Pb monazite (592 ± 22 Ma) and 40Ar–39Ar biotite (585.8 ± 0.8 Ma) data from greenschist facies rocks in the Zambian Copperbelt (Rainaud et al., 2005a), while a maximum compression phase took place between 560 and 530 Ma. This is based on U–Pb zircon dating of syn- to post-tectonic granites and rhyolites in the Katangan high (Hanson et al., 1993). These data are supported by a peak metamorphic age of ∼530 Ma from a U–Pb monazite dating of whiteschist facies rocks in the central and western part of the Domes region (John et al., 2004). This ∼530 Ma age would correspond to the collision of the Congo and Kalahari cratons. 40Ar–39Ar biotite and Rb–Sr muscovite and biotite ages of 510–465 Ma probably represent postorogenic cooling (John et al., 2004; Rainaud et al., 2005a). The Lufilian orogen led to several metal mineralization or remobilization events (Barra et al., 2004; Cailteux et al., 2005b; Decree et al., 2011; Dewaele et al., 2006; El-Desouky et al., 2010; Haest et al., 2009, 2010; Kampunzu et al., 2009; Schneider et al., 2007; Torrealday et al., 2000). It also gave rise to U remobilization at the southern edge of the Katanga Copperbelt and in the Kolwezi thrust sheet (Decree et al., 2011). During the post-orogenic period, the Lufilian fold-and-thrust belt underwent weathering, erosion and peneplanation at various stages. A major Pliocene weathering stage was responsible for metal enrichment in the uppermost part of weathering profiles in Katanga, including Co-enrichment in

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S. Decree et al. / Precambrian Research 241 (2014) 17–28

Table 1 Concentration of selected relevant elements in heterogenite samples used for U–Pb dating. Co (wt%)

Ni (wt%)

Cu (wt%)

Fe (wt%)

Mn (wt%)

Zn (wt%)

Pb (wt%)

V (wt%)

Th (wt%)

RGM 6450 (Shinkolobwe mine): Massive heterogenite, associated with metatorbernite (type 2) 42.22 4.43 1.93 1.84 3.20 3.82 0.04 Ht 26 Ht 27 36.55 5.26 1.88 0.34 6.15 3.26 0.00 Ht 28 43.32 5.06 1.39 0.24 3.85 3.42 0.04 39.56 5.91 1.68 1.15 5.24 3.62 0.05 Ht 29 RGM 10788 (Shinkolobwe mine): Botryoidal crust on silicified substrate (type 1) 46.80 7.86 0.35
heterogenite-dominated cobalt caps (De Putter et al., 2010; Decree et al., 2010). 3. Material and methods Nine heterogenite samples from the Katanga Copperbelt that are part of the Royal Museum for Central Africa (RMCA) mineral collection were selected for this study. Seven of these samples are from the Shinkolobwe mine (samples RGM 10788, RGM 10816, RGM 10799, RGM 11433.1, RGM 6450, RGM 11525, HtSh), one from the Kambove mine (RGM 13017), and one from the Kalongwe mine (sample RGM 8950). The composition and texture of the samples were studied using (1) scanning electron microscopy, at the “Service de Microscopie Electronique” facility of Namur University (FUNDP), Belgium, and in the “Laboratoire de Géochimie et Minéralogie Appliquée” of Brussels University (ULB), Belgium, and (2) electron microprobe analyses Cameca SX50, at the “Service Commun de Microanalyse” facility of the Henri Poincaré-Nancy1 University, France, and at Geology and Applied Geology Dept. of Mons University (UMons), Belgium. U–Pb isotopic compositions were determined using a Cameca IMS1270 ion microprobe (CRPG-CNRS, Nancy), with analytical setting optimized for U–Pb measurement on Zircon (Deloule et al., 2002). A 0− primary beam with 23 kV incident energy (13 kV primary, 10 kV secondary) and 6 nA intensity was used in defocused aperture illumination mode, producing a ca. 15 ␮m spot. Positive

U (wt%)

U–Pb age (this study) 677.6 ± 9.2 Ma

2.56 2.23 2.18 2.41 694.7 ± 9.8 Ma 0.32 0.56 723 ± 11 Ma 2.24 1.46 2.29 1.39 1.84 1.73 2.29 670.5 ± 6.3 Ma 1.49 2.35 2.21 1.67 2.68 695.8 ± 3.8 Ma 1.49 1.48 876.2 ± 7.8 Ma 2.91 2.05 2.80 3.06 1.97 823 ± 5.8 Ma 0.51 0.68 823 ± 5.8 Ma 0.15 0.15 694.7 ± 9.8 Ma 3.20 2.37 0.06

secondary ions were measured with a mass resolution of ∼6000 to separate isobaric interferences from Pb. The field aperture was set to 3000 ␮m, and the transfer optic magnification adjusted to 300. Rectangular lenses were activated in the secondary ion optics to increase the transmission at high mass resolution. The energy window was opened at 20 eV, and an energy offset of 50 eV was used to minimize the matrix effect. A single collector was used in ion-counting mode, and the spectrum scanned by peak jumping. Each analysis consisted of 30 successive cycles, with measurement of the mass 203.5 for background, 204 Pb, 206 Pb, 207 Pb, 208 Pb, 238 U, 248 ThO, and 238 UO, with measurement times of 10, 10, 6, 20, 6, 6, 4 and 4 s, respectively (waiting time of 1 s). The centering of the sample spot image in the field aperture, the mass and energy calibration were checked before each measurement, after a 2-min presputtering completed in rastering the primary beam over a 50–50-␮m area. A U–Pb concordant uraninite reference mineral, dated at 540 Ma (Holliger, 1988), was used to determine the U–Pb relative useful yield, as a function of the UO/U ratio. The U–Pb isotopes punctual analyses were performed on nine separate grains, with at least three analyses per sample. To calculate the discordia lines presented in Fig. 4, their lower intercepts were fixed at 0 ± 10 Ma, because this significantly increases the precision on the age deduced from the upper intercept. The chosen 0–10 Ma time interval is consistent with the Pliocene event that affected the Katanga Co–Cu–U deposits and induced the observed discordance (Decree et al., 2010, 2011).

S. Decree et al. / Precambrian Research 241 (2014) 17–28

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Fig. 3. Heterogenite morphology. (A) Finely banded heterogenite (sample RGM 108816, Shinkolobwe; reflected light microscopy); (B) Botryoidal heterogenite, including layers with Mn–Fe(–Co) oxide admixture (RGM 10788, Shinkolobwe; backscattered electron image); (C) Massive heterogenite, intergrown with metatorbernite, on silicified substrate (HtSh, Shinkolobwe), (D) Heterogenite in association with metatorbernite and tenorite, as infillings of voids in silicified host-rock (RGM 8950, Kalongwe). Ht heterogenite, Mt metatorbernite, Tn tenorite.

4. Results 4.1. Heterogenite typology Two different types of heterogenite were investigated during this study (Table 1). The first type is characterized by a botryoidal aspect and occurs as millimeter- to centimeter-thick crusts, which are generally finely laminated (pluri-␮metric to millimetric thick laminae; Fig. 3A) and commonly include intercalations of a Mn–Fe(–Co) oxide admixture (Fig. 3B). The second heterogenite type is massive, without any layering. This second type is associated with metatorbernite (Cu(UO2 )2 (PO4 )2 ·8H2 O) (Fig. 3C and D) and occasionally with tenorite (CuO) (Fig. 3D), and it typically occurs on a silicified rock substrate in which quartz is associated with primary phyllosilicates and monazite. The two heterogenite types do not coexist in single samples. For the type 1 heterogenite, four samples from Shinkolobwe were analyzed (RGM 10788, RGM 10799, RGM 10816 and RGM 11433.1) and one sample from Kambove (RGM 13017). In addition, for the type 2 heterogenite, we analyzed three samples from Shinkolobwe (RGM 6450, RGM 11525 and HtSh), and one sample from Kalongwe (RGM 8950). The studied heterogenite samples are characterized by a Co content ranging from 36.55 to 48.68% (Table 1). They also contain variable, though sometimes high, content of Ni: up to 7.91% in the samples from the Shinkolobwe mine, Cu: up to 10.17%, Pb: up to 4.57%, Mn: up to 6.15%, U: from 0.15 to 3.2%; Pb in all samples includes no common lead, as demonstrated by ion microprobe

analysis Other elements are Fe (up to 1.84%), Zn (up to 0.21%), and V (up to 0.08%); Th concentrations were below the detection limit for all samples. 4.2. Pb–Pb and U–Pb ages The 207 Pb/206 Pb age density diagram of the studied heterogenites (Fig. 4A) allows discriminating two age populations. The first one is comprised between ∼780 and ∼880 Ma (samples RGM 11525, RGM 13017, and HtSh), and the second is between ∼630 and ∼740 Ma (samples RGM 6450, 8950, 10788, 10799, 10816, 11433.1). The corresponding calibrated U–Pb isotopic ratios were plotted in concordia diagrams. Thirteen U–Pb measurements (Table 2) performed on one heterogenite sample from Shinkolobwe (RGM 11525, heterogenite type 2) provide a discordia line whose upper intercept with the concordia curve is at 876.2 ± 7.8 Ma (Fig. 4B). The data are inversely discordant (at about 90% inverse discordance). A discordia line defined by 15 measurements on another sample from Shinkolobwe (HtSh, heterogenite type 2) and on the sample from Kambove (RGM 13017, heterogenite type 1) (Fig. 4B) has an upper intercept with the concordia line that yields an age of 823 ± 5.8 Ma. The data vary from about 80% discordance to ∼25% inverse discordance. Ten measurements performed on each of three other heterogenite samples from Shinkolobwe (Table 3; RGM 6450 of type 2 heterogenite, RGM 11433.1 and RGM 10799 of type 1 heterogenite) define discordia lines with data varying from ∼90 to ∼95% inverse discordance, and with upper intercepts with the concordia line at

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Table 2 U–Pb data for heterogenite from the Katanga Copperbelt, for samples giving a discordia line with an upper intercept at ∼823 and ∼876 Ma. Sample

Measurement

Measured ratiosa 207 206

a b c

+/−

204 206

Pb/ Pb

+/−

Pb/U

+/−

UO/U

+/−

206 238

0.0685 0.0684 0.0684 0.0684 0.0688 0.0687 0.0685 0.0686 0.0682 0.0681 0.0681 0.0681 0.0684 0.0665 0.0665 0.0666 0.0667 0.0666 0.0665 0.0664 0.0666 0.0665 0.0665 0.0663

0.0000 0.0001 0.0001 0.0001 0.0001 0.0001 0.0000 0.0001 0.0001 0.0000 0.0001 0.0001 0.0001 0.0001 0.0001 0.0000 0.0001 0.0001 0.0001 0.0000 0.0000 0.0001 0.0000 0.0001

0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000

6.79E−08 2.30E−08 8.28E−08 1.20E−07 5.64E−09 6.92E−09 9.19E−09 4.30E−08 7.02E−09 7.64E−09 4.53E−08 8.62E−09 5.00E−08 1.22E−07 1.27E−07 1.36E−07 8.16E−08 2.19E−07 1.37E−07 9.54E−08 1.37E−07 1.49E−07 7.78E−08 1.82E−07

0.0961 0.0966 0.0944 0.0948

0.0012 0.0009 0.0009 0.0012

0.0020 0.0022 0.0023 0.0022

0.0001 0.0002 0.0002 0.0002

Uncorrected (207 Pb/206 Pb, 204 Pb/206 Pb). Corrected by calibration against a uraninite standard. Calculated from the calibrated Pb/U ratios.

206

Pb/ U

+/−

Agesc (Ma) 207 235

Pb/ U

+/−

Cor.

206 238

Pb/ U

+/−

207 235

Pb/ U

+/−

207 206

Pb/ Pb

+/−

3.882 7.018 5.153 5.881 6.014 6.357 5.731 5.215 5.193 5.471 5.33 6.253 5.267 14.734 12.832 14.328 18.842 12.784 16.873 13.645 20.879 13.479 17.834 14.108

0.062 0.090 0.077 0.022 0.025 0.038 0.055 0.040 0.065 0.020 0.047 0.037 0.040 0.189 0.111 0.393 0.284 0.237 0.161 0.045 0.672 0.126 0.227 0.228

4.613 5.511 5.693 5.86 6.279 6.41 5.947 6.03 5.952 6.028 6.089 6.048 5.956 10.818 10.281 10.416 10.365 11.656 10.896 9.835 10.661 10.389 11.021 10.698

0.010 0.018 0.024 0.021 0.010 0.015 0.022 0.026 0.035 0.010 0.026 0.031 0.029 0.029 0.060 0.063 0.023 0.189 0.059 0.028 0.040 0.058 0.030 0.037

1.2297 1.6299 1.1353 1.2375 1.1372 1.1651 1.1783 1.0493 1.0663 1.1014 1.0564 1.2524 1.0804 0.1286 0.1132 0.1261 0.1659 0.1098 0.1471 0.1214 0.1828 0.1187 0.1551 0.1234

0.0198 0.0211 0.0171 0.0053 0.0052 0.0074 0.0116 0.0082 0.0134 0.0046 0.0095 0.0079 0.0085 0.0011 0.0010 0.0011 0.0014 0.0009 0.0012 0.0010 0.0016 0.0010 0.0013 0.0010

11.584 15.336 10.692 11.642 10.760 11.013 11.102 9.902 10.005 10.327 9.905 11.727 10.170 1.179 1.038 1.158 1.526 1.009 1.349 1.112 1.679 1.088 1.421 1.129

0.187 0.199 0.161 0.050 0.050 0.071 0.109 0.078 0.127 0.043 0.090 0.075 0.082 0.010 0.009 0.010 0.013 0.009 0.012 0.009 0.014 0.009 0.012 0.010

0.929 0.927 0.929 0.914 0.914 0.917 0.929 0.921 0.927 0.924 0.924 0.916 0.914 0.996 0.993 0.997 0.995 0.976 0.993 0.996 0.996 0.993 0.998 0.983

5169 6233 4890 5192 4896 4980 5019 4625 4679 4787 4648 5234 4722 780 691 765 990 672 885 739 1082 723 929 750

57.1 51.5 51.3 15.1 15.6 22.1 34.1 25.8 41.8 14.0 29.7 22.4 26.4 6.2 5.6 6.1 7.8 5.4 7.0 5.9 8.4 5.8 7.3 6.0

2571 2836 2497 2576 2503 2524 2532 2426 2435 2465 2426 2583 2450 791 723 781 941 708 867 759 1001 748 898 767

15.0 12.3 13.9 4.0 4.3 6.0 9.1 7.3 11.6 3.9 8.3 5.9 7.4 4.7 4.4 4.6 5.2 4.4 5.0 4.5 5.4 4.5 5.1 4.6

881 877 879 877 888 886 880 884 872 870 871 867 879 822 822 826 828 826 824 820 826 822 822 816

2.9 4.5 3.3 3.3 3.5 4.4 2.3 4.6 4.0 2.0 4.3 4.6 6.0 1.7 2.1 1.3 1.8 3.9 2.2 1.5 1.5 2.1 1.0 3.3

0.018 0.013 0.011 0.011

0.000 0.000 0.000 0.000

1.265 1.240 1.180 1.232

0.002 0.004 0.002 0.002

0.0348 0.0275 0.0295 0.0248

0.0036 0.0048 0.0059 0.0040

0.324 0.243 0.251 0.217

0.036 0.045 0.052 0.037

0.923 0.958 0.961 0.960

221 175 188 158

22.2 30.3 36.9 25.3

285 221 228 200

27.3 36.0 41.6 30.2

852 752 663 724

86.7 107.9 119.1 97.7

S. Decree et al. / Precambrian Research 241 (2014) 17–28

Shinkolobwe mine RGM 11525 Shinko525@1 Shinko525@2 Shinko525@3 Shinko525@4 Shinko525@5 Shinko525@6 Shinko525@7 Shinko525@8 Shinko525@9 Shinko525@10 Shinko525@11 Shinko525@12 Shinko525@13 HTSH HtSh HTSH@1 HTSH@2 HTSH@3 HTSH@4 HTSH@5 HTSH@6 HTSH@7 HTSH-B HTSH-B@1 HTSH-B@2 Kambove mine RGM 13017 Co13017 Co13017@1 Co13017@2 Co13017@3

Pb/ Pb

Calibrated ratiosb

Table 3 U–Pb data for heterogenite from the Katanga Copperbelt, for samples giving a discordia line with an upper intercept between ∼670 and ∼723 Ma. Sample

Measurement

Measured ratiosa 207 206

0.0624 0.0622 0.0621 0.0622 0.0623 0.0623 0.0623 0.0623 0.0621 0.0623 0.0681 0.0667 0.0661 0.0637 0.0636 0.0641 0.0644 0.0637 0.0642 0.0638 0.0626 0.0635 0.0627 0.0624 0.0622 0.0624 0.0625 0.0622 0.0624 0.0621 0.0622 0.0633 0.0627 0.0625 0.0630 0.0633 0.0631 0.0627 0.0633 0.0624 0.0621 0.0628 0.0627 0.0626 0.0623 0.0627 0.0625 0.0628 0.0621

+/−

206

0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0000 0.0001 0.0000 0.0001 0.0008 0.0007 0.0009 0.0001 0.0000 0.0001 0.0001 0.0001 0.0001 0.0001 0.0000 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0002 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0002 0.0002 0.0002 0.0001

Pb/ Pb

0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0004 0.0005 0.0003 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000

+/−

206

Pb/U

+/−

UO/U

+/−

206 238

1.2E−07 8.9E−08 8.8E−08 2.0E−08 3.3E−08 1.5E−08 1.5E−07 2.7E−09 9.8E−08 8.9E−08 5.1E−05 5.5E−05 5.9E−05 1.8E−09 1.3E−07 5.6E−09 1.3E−07 1.6E−07 4.0E−09 5.3E−09 1.6E−09 8.6E−09 1.8E−09 5.7E−07 6.2E−07 6.5E−07 5.9E−07 7.4E−07 7.9E−07 5.6E−07 7.6E−07 3.6E−06 1.8E−06 2.2E−06 2.0E−06 1.1E−06 1.9E−06 1.9E−06 1.7E−06 1.1E−06 1.1E−06 1.6E−06 1.8E−06 2.9E−06 2.3E−06 9.6E−07 5.4E−06 2.6E−06 2.1E−06

1.590 3.151 2.700 2.898 2.889 2.896 3.026 2.854 2.993 3.706 0.007 0.006 0.006 4.945 5.269 6.322 6.863 5.343 6.496 6.013 3.933 4.464 4.467 0.550 0.606 0.605 0.644 0.528 3.705 2.197 1.724 3.442 4.633 4.233 3.422 2.479 2.441 4.369 3.707 3.091 3.271 3.054 3.734 3.035 3.276 2.854 3.816 3.507 3.656

0.027 0.052 0.040 0.030 0.028 0.037 0.027 0.036 0.029 0.075 0.000 0.000 0.000 0.014 0.030 0.097 0.097 0.033 0.068 0.019 0.037 0.009 0.049 0.002 0.002 0.003 0.003 0.003 0.250 0.084 0.068 0.200 0.156 0.173 0.123 0.093 0.064 0.064 0.073 0.068 0.093 0.074 0.058 0.061 0.097 0.053 0.111 0.094 0.117

3.23 4.67 4.64 4.74 4.73 4.61 4.54 4.53 4.76 4.49 1.454 1.452 1.405 5.67 5.93 6.51 6.99 5.96 6.69 6.43 5.41 5.67 5.53 1.183 1.192 1.217 1.214 1.063 4.150 4.366 3.780 4.073 5.158 4.802 4.590 3.417 3.343 4.606 4.427 4.119 4.265 3.798 4.044 3.744 3.939 3.669 4.211 3.959 4.218

0.037 0.026 0.024 0.023 0.014 0.027 0.024 0.015 0.017 0.026 0.009 0.009 0.007 0.022 0.037 0.078 0.077 0.043 0.060 0.039 0.005 0.008 0.012 0.005 0.005 0.005 0.007 0.009 0.157 0.081 0.148 0.120 0.088 0.102 0.048 0.049 0.033 0.033 0.023 0.031 0.051 0.035 0.019 0.029 0.051 0.024 0.052 0.053 0.060

Pb/ U

1.1483 0.9775 0.8453 0.8734 0.8751 0.9181 0.9865 0.9359 0.8948 1.2377 0.0084 0.0081 0.0082 1.0979 1.0873 1.1334 1.1068 1.0939 1.1180 1.0977 0.9411 0.9914 1.0328 1.5008 1.5958 1.4501 1.5590 2.7911 0.7764 0.4314 0.4082 1.3761 1.3836 1.3784 1.1778 1.2463 1.2639 1.4976 1.3348 1.2184 1.2337 1.3357 1.5065 1.3523 1.3670 1.3056 1.4628 1.4541 1.3987

+/−

Agesc (Ma) 207 235

0.0173 0.0167 0.0148 0.0105 0.0098 0.0130 0.0092 0.0129 0.0098 0.0205 0.0006 0.0006 0.0011 0.0037 0.0065 0.0175 0.0158 0.0070 0.0119 0.0041 0.0091 0.0027 0.0116 0.0127 0.0135 0.0123 0.0132 0.0237 0.0066 0.0037 0.0035 0.0590 0.0351 0.0420 0.0374 0.0386 0.0279 0.0175 0.0220 0.0241 0.0300 0.0262 0.0182 0.0222 0.0312 0.0209 0.0308 0.0284 0.0335

Pb/ U

9.856 8.365 7.223 7.471 7.499 7.864 8.453 8.021 7.651 10.610 0.072 0.067 0.069 9.616 9.508 9.993 9.808 9.584 9.870 9.640 8.103 8.660 8.910 12.857 13.640 12.421 13.381 23.856 6.657 3.681 3.486 11.697 11.650 11.576 9.982 10.609 10.740 12.641 11.361 10.227 10.307 11.294 12.699 11.394 11.450 11.027 12.285 12.281 11.698

+/−

Cor.

206 238

0.017 0.017 0.015 0.011 0.010 0.013 0.009 0.013 0.010 0.020 0.005 0.005 0.010 0.004 0.006 0.016 0.014 0.007 0.011 0.004 0.010 0.003 0.011 0.110 0.117 0.107 0.115 0.205 0.058 0.033 0.031 0.059 0.035 0.042 0.037 0.039 0.028 0.018 0.022 0.024 0.030 0.026 0.018 0.022 0.031 0.021 0.031 0.029 0.034

0.998 0.998 0.997 0.996 0.995 0.997 0.999 0.997 0.998 0.999 0.965 0.966 0.988 0.918 0.995 0.996 0.993 0.983 0.995 0.917 0.997 0.930 0.997 0.994 0.991 0.988 0.990 0.987 0.975 0.952 0.962 0.999 0.999 0.999 0.999 0.998 0.997 0.992 0.997 0.998 0.997 0.998 0.995 0.994 0.998 0.990 0.992 0.993 0.998

Pb/ U

4929 4396 3950 4047 4053 4199 4425 4258 4120 5192 54 52 52 4776 4744 4885 4804 4764 4838 4776 4276 4440 4573 5909 6149 5777 6057 8591 3704 2312 2207 5579 5599 5585 5017 5217 5267 5901 5466 5136 5181 5469 5924 5514 5554 5385 5810 5787 5640

+/−

207 235

59.3 52.8 43.5 31.5 29.5 39.9 29.5 39.9 29.6 72.5 3.8 3.8 7.2 11.5 20.0 52.7 48.2 21.7 36.2 12.7 30.1 8.9 36.6 32.7 33.5 32.3 33.2 40.1 23.9 16.5 15.8 216.6 130.1 155.1 129.0 136.7 99.6 67.2 80.5 84.7 105.9 95.9 70.2 81.8 115.0 76.0 116.9 107.5 124.7

Pb/ U

2421 2271 2139 2169 2173 2216 2281 2233 2191 2490 71 66 68 2399 2388 2434 2417 2396 2423 2401 2243 2303 2329 2669 2725 2637 2707 3263 2067 1567 1524 2580 2577 2571 2433 2490 2501 2653 2553 2455 2463 2548 2658 2556 2560 2525 2626 2626 2581

+/−

207 206

15.8 15.0 13.1 9.4 8.8 11.7 8.4 11.6 8.7 18.8 5.0 4.8 9.1 3.4 5.5 14.2 13.2 6.0 9.8 3.8 8.7 2.7 10.2 8.0 8.1 8.0 8.1 8.3 7.7 7.1 6.9 53.8 32.4 38.6 34.0 35.3 25.7 16.4 20.4 22.1 27.5 24.2 17.1 20.7 28.8 19.5 28.8 26.5 30.9

Pb/ Pb

685 678 675 677 681 680 684 681 676 681 681 603 663 726 723 740 751 726 743 732 690 720 694 679 674 678 683 674 680 670 672 662 642 636 656 665 661 647 665 635 625 651 644 643 630 648 636 648 627

+/−

5.2 4.8 4.5 4.3 4.3 4.0 1.9 4.4 2.9 4.8 40.3 41.2 45.6 6.2 2.4 5.5 6.9 5.1 4.5 6.9 3.2 4.6 3.6 2.1 2.5 2.8 2.6 2.9 4.1 5.8 5.1 4.4 3.4 3.7 2.6 4.8 4.5 4.5 3.1 3.2 5.0 3.3 3.7 4.8 4.2 6.1 7.3 6.5 4.0

S. Decree et al. / Precambrian Research 241 (2014) 17–28

Shinkolobwe mine Shinko6450@1 RGM 6450 Shinko6450@2 Shinko6450@3 Shinko6450@4 Shinko6450@5 Shinko6450@6 Shinko6450@7 Shinko6450@8 Shinko6450@9 Shinko6450@10 Co10788 RGM 10788 Co10788@1 Co10788@2 Shinko-799@1 RGM 10799 Shinko-799@2 Shinko-799@3 Shinko-799@4 Shinko-799@5 Shinko-799@6 Shinko-799@7 Shinko-799@8 Shinko-799@9 Shinko-799@10 Co10816 RGM 10816 Co10816@1 Co10816@2 Co10816@3 Co10816@4 Co10816b Co10816b@1 Co10816b@2 Shinko Shinko@00 Shinko@01 Shinko@02 Shinko@03 Shinko@04 Shinko@05 Shinko@06 Shinko b@1 Shinko b@2 Shinko b@3 Shinko b@4 Shinko b@5 Shinko b@6 Shinko b@7 Shinko b@8 Shinko b@9 Shinko b@10

Pb/ Pb

Calibrated ratiosb 204

23

24

Table 3 (Continued) Sample

Measurement

Measured ratiosa 207 206

RGM 11433.1

Kalongwe mine RGM 8950

c

+/−

204 206

Pb/ Pb

+/−

206

Pb/U

+/−

UO/U

+/−

206 238

Pb/ U

+/−

Agesc (Ma) 207 235

Pb/ U

+/−

Cor.

206 238

Pb/ U

+/−

207 235

Pb/ U

+/−

207 206

Pb/ Pb

+/−

Shinko433@1 Shinko433@2 Shinko433@3 Shinko433@4 Shinko433@5 Shinko433@6 Shinko433@7 Shinko433@8 Shinko433@9 Shinko433@10

0.0624 0.0629 0.0626 0.0628 0.0629 0.0626 0.0626 0.0628 0.0629 0.0624

0.0001 0.0001 0.0001 0.0000 0.0001 0.0001 0.0000 0.0000 0.0001 0.0001

0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000

5.0E−07 3.5E−07 6.9E−07 7.3E−07 4.1E−07 4.5E−07 6.3E−07 3.4E−07 3.9E−07 3.8E−07

6.729 7.245 6.933 7.590 7.504 7.596 6.257 6.928 7.333 3.064

0.106 0.090 0.014 0.031 0.068 0.104 0.062 0.039 0.036 0.007

4.8 4.92 5.01 5.06 5.11 5.14 4.3 4.32 4.54 2.75

0.030 0.031 0.021 0.009 0.027 0.038 0.008 0.011 0.010 0.003

1.9799 2.0388 1.8951 2.0348 1.9805 1.9827 2.2732 2.4961 2.3907 3.9964

0.0315 0.0256 0.0054 0.0091 0.0185 0.0274 0.0231 0.0148 0.0128 0.0122

17.000 17.655 16.332 17.592 17.139 17.079 19.581 21.584 20.696 34.320

0.016 0.013 0.003 0.005 0.009 0.014 0.010 0.006 0.005 0.004

0.998 7039 0.995 7165 0.878 6853 0.991 7156 0.991 7040 0.996 7045 0.999 7644 0.993 8069 0.983 7871 0.858 10,370

67.7 54.1 12.0 19.4 39.8 59.0 45.3 27.3 24.3 15.8

2935 2971 2896 2968 2943 2939 3071 3165 3124 3619

15.2 12.1 3.1 4.3 9.0 13.2 9.8 5.8 5.3 3.5

690 708 698 709 706 695 694 703 707 694

4.5 5.4 6.5 2.3 5.4 5.2 1.7 2.9 4.2 7.8

8950 8950@1 8950@2 8950@3

0.0625 0.0629 0.0621 0.0948

0.0001 0.0000 0.0001 0.0012

0.0000 0.0000 0.0000 0.0022

4.6E−07 1.7E−06 1.9E−07 1.6E−04

0.607 0.358 0.793 0.011

0.006 0.011 0.012 0.000

7.394 8.433 7.221 1.232

0.142 0.066 0.030 0.002

0.1807 0.0899 0.2435 0.0248

0.0015 0.0008 0.0021 0.0040

1.558 0.778 2.083 0.217

0.014 0.007 0.018 0.037

0.971 1071 0.995 555 0.979 1405 0.960 158

8.4 954 4.5 585 10.7 1143 25.3 200

5.4 3.8 5.9 30.2

692 701 676 724

4.4 1.8 3.8 97.7

Uncorrected (207 Pb/206 Pb, 204 Pb/206 Pb). Corrected by calibration against a uraninite standard. Calculated from the calibrated Pb/U ratios.

Fig. 4. U–Pb dating results. (A) Cumulative probability Gaussian curve for obtained heterogenite 207 Pb/206 Pb ages, together with published results for Katanga uraninite (Decree et al., 2011); (B) Concordia plots of U–Pb data with upper intercepts at ∼823 and ∼876 Ma, with data point error ellipses at 1 (sample numbers and ages indicated on graphs); (C) Concordia plots of U–Pb data with upper intercepts comprised between ∼670 and ∼723 Ma, with data point error ellipses at 1 (sample numbers and ages indicated on graphs).

S. Decree et al. / Precambrian Research 241 (2014) 17–28

a b

Pb/ Pb

Calibrated ratiosb

S. Decree et al. / Precambrian Research 241 (2014) 17–28

677.6 ± 9.2 Ma, 694.5 ± 8.1 Ma and 723 ± 11 Ma (Fig. 4C). An age of 670.5 ± 6.3 Ma is similarly deduced from the upper intercept of a discordia line representing 26 measurements performed on sample RGM 10816 from Shinkolobwe (type 1 heterogenite, Fig. 4C). These data are also characterized by significant inverse discordance of ∼75 to 95%. A last discordia line, displaying an upper intercept corresponding to 694.7 ± 9.8 Ma (Fig. 4C), is provided by seven measurements on heterogenite samples from Shinkolobwe (RGM 10788) and Kalongwe (RGM 8950). The data are discordant (∼15 to ∼50%) or inversely discordant (at ∼20% and ∼90%). No obvious correlations are found between the heterogenite type (type 1 – botryoidal and type 2 – massive, associated with metatorbernite) and the U–Pb age.

5. Discussion 5.1. Significance of the ages obtained on heterogenite The ion microprobe U–Pb analyses of U-rich heterogenite presented in this paper yield distinct Neoproterozoic ages, at ∼876 Ma, ∼823 Ma and in the ∼720 to ∼670 Ma time interval. At the present state of knowledge, the geological processes that occured around these times, which include sedimentation and burial of Lower Roan sediments at ∼876 Ma and ∼823 Ma, and hot fluid circulation around 650–700 Ma (200–300 ◦ C, Eglinger et al., 2012; 270–385 ◦ C and 35–45.5 wt% NaCl equiv salinity, El-Desouky et al., 2009), were not favorable for heterogenite formation or its long-term preservation. Indeed, heterogenite formation requires highly oxidizing conditions (Rose, 1989) and its stability is limited to a lowtemperature range (mostly below 100 ◦ C; Deliens, 1974; Figlarz et al., 1974; Pauporte et al., 2005; even though its stability below 250 ◦ C was demonstrated by Deliens and Goethals, 1973 and Yang et al., 2010). As a consequence, the Neoproterozoic ages deduced from U–Pb measurements on heterogenite obviously record geological events that might be/are different from the formation of the mineral itself. In the Katanga Copperbelt, heterogenite formation results from Pliocene oxidation of Co–(Cu)-sulfides (Decree et al., 2010) even though the possibility of formation during older oxidation event(s) cannot be ruled out. The sulfides supposedly formed at ∼816 Ma (Barra et al., 2004 in Selley et al., 2005) and during the peak (Barra et al., 2004; Torrealday et al., 2000) and post-peak metamorphism at ∼450 Ma (Schneider et al., 2007; Torrealday et al., 2000) (Fig. 5). In U deposits, these Co–(Cu)-sulfides are often spatially associated with uraninite (Decree et al., 2011). During the Pliocene weathering episode, coeval oxidation of uraninite and Co-sulfides could have resulted in the formation of U–(Pb)-rich heterogenite. This mineral possibly trapped relict uraninite nanoparticles or adsorbed U and Pb, with no significant isotopic fractionation. Adsorption of U and Pb by heterogenite is a likely process (Makabu et al., 1990), because this mineral is commonly poorly crystallized and possesses a high specific surface (Burlet et al., 2012). Whatever the form in which U occurs in the analyzed heterogenite samples, the high degrees of discordance observed in the concordia diagrams indicate extreme U losses and/or Pb enrichments (up to ∼90%) at some stage(s) between the formation of an U-bearing precursor and the development of U-rich heterogenite in its present state, without any significant input of common lead. As a consequence, the Neoproterozoic events dated here are interpreted as relating to the precipitation of a U-bearing precursor mineral, such as uraninite, with which Co-sulfides were spatially associated in the early phase of the mineralization. Later on, Co experienced oxidation phases and eventually formed an oxidized, secondary, ore (heterogenite), as late as in the Pliocene. It however appears that the Co system evolution has allowed for U and

25

Pb losses/enrichment, but however kept isotopic ratios representatives of the major geological cycling events in the common Co + U system. 5.2. Early U mineralizing events at ∼876 Ma and ∼823 Ma In the study area, the ∼876 and 823 Ma age should correspond to the period where Lower Roan sediments were deposited or underwent shallow burial, in early diagenesis and within a continental rift environment (Fig. 5). Interestingly, it seems to predate the formation of primary Cu–Co sulfides, which are commonly regarded as early diagenetic precipitates at ∼816 Ma (Barra et al., 2004 in Selley et al., 2005; Fig. 5). The ∼876 Ma and ∼823 Ma ages should be the age of synearly diagenetic concentration of uranium, most probably in the form of uraninite (Cailteux et al., 2005b; Loris et al., 2002). As shown by concordia diagrams for uraninite (Decree et al., 2011), this phase remained unaltered, from the time of its formation until Pliocene late events (Decree et al., 2010). As stated above, uraninite and associated Co-sulfides were altered during the Pliocene, resulting in the formation of U-rich heterogenite, partly associated with secondary U-bearing minerals such as metatorbernite (RGM 6450, 8950, 11525 and HtSh, which are type 2 heterogenites). 5.3. U (re)mobilization in the ∼720 to ∼670 Ma time frame The ages ranging from ∼720 to ∼670 Ma in the studied material are overall comparable to the age obtained for uraninite from the same deposits (Shinkolobwe, Kalongwe; Decree et al., 2011). This overlap between uraninite and heterogenite ages, revealed by 207 Pb/206 Pb age density diagrams (Fig. 4A), suggests that both ages are indicative of the same event, related to U cycling. The proto-oceanic rifting occurring during the Nguba period (∼730 to ∼635 Ma; Kampunzu et al., 1991, 2009; Key et al., 2001) and/or the development of transcurrent shear zones and faults linked to an early stage of the Congo-Kalahari craton convergence (Kampunzu and Cailteux, 1999) (Fig. 5), could account for the progressive increase in geothermal gradient and the subsequent circulation of hot fluids. These fluids may have led to re-mobilization and deposition of uraninite and sulfides in the Copperbelt (El-Desouky et al., 2010). As suggested by the Concordia diagrams, the uraninitesulfide assemblage remained unaffected from that time until the Pliocene. 5.4. Origin of the U mineralization The 876 and 823 Ma age obtained in this study are consistent with a syn-early diagenetic concentration of uranium in the Katanga Copperbelt, as proposed earlier by several authors (Cailteux, 1983, 1997; Franc¸ois, 1974; Loris, 1996; Meneghel, 1981; Ngongo, 1975). These ages, together with more recent geodynamic concepts and geological data concerning the study area, allow an improved assessment of potential U sources in Katanga. Different potential sources have been proposed for U in the synearly diagenetic mineralization. U input to the Katanga depocenter could find its origin in the leaching of the underlying basement (see Sweeney and Binda, 1994; Cailteux, 1994), most probably by rising oxidizing fluids associated with an early (at ∼876 Ma) and late rifting phase (at ∼823 Ma). In such a context, mobilized uranium could be trapped in the Mines Subgroup strata, dominated by fine-grained deposits with high organic matter content, thus representing environments with reducing conditions (Loris et al., 2003). However, because Mines Subgroup sedimentation was characterized by a series of transgressive–regressive cycles (Bull et al., 2011; Cailteux, 1994), another possibility is that U was trapped by processes

26

S. Decree et al. / Precambrian Research 241 (2014) 17–28

Fig. 5. Schematic diagram comparing the isotopic ages obtained on magmatic rocks and sulfides in the Lufilian Arc and the new heterogenite U–Pb ages. All these ages are considered together and reinterpreted in the regional geodynamic context.

creating reducing conditions (redox front) at the basin-scale, as previously suggested for Zambian U deposits (Meneghel, 1981). The Zambian pre-Katangan basement is the most likely source of metals contained in Lower Roan orebodies (see Cailteux, 1994; Cailteux et al., 2005b; El-Desouky et al., 2010; Sweeney and Binda, 1994). The proximity of most of the Katanga U deposits, located in the southern part of the Katanga Copperbelt, to the outcrop area of the pre-Katangan basement in Zambia is consistent with

this hypothesis. This interpretation is further supported by high background radioactivity in the Dome zone basement complex (Meneghel, 1981), which itself hosts several important uranium deposits, and by the overlying position of granitic masses that formed paleo-highs to the South of the Katanga Basin during Lower Roan sedimentation (Bull et al., 2011; Garlick and Fleischer, 1972). Contribution of other basement units, located to the West and East of the sedimentary basin or underlying this basin cannot

S. Decree et al. / Precambrian Research 241 (2014) 17–28

be ruled out. For example, the south-western extension of the Bangweulu block could constitute the basement of the Katanga depocenter (De Waele et al., 2006), a basement that includes 1.8 Ga alaskites as potential U sources (André, 1976; Kabengele et al., 1987). A similar link has been recognized to the Southwest of the Damara-Lufilian Belt, for the Rössing U deposits in Namibia (Basson and Greenway, 2004). An additional potential source of U in the Katanga Basin is the components derived from geological units surrounding the Basin, particularly heavy minerals. The identification of basement units or paleo-highs as source areas during Lower Roan sedimentation is confirmed by ages of detrital zircons contained in Mines Subgroup deposits (Master et al., 2005). These are mostly comprised between 2081 and 1790 Ma, consistent with the Zambian and Bangweulu basement ages (1.88–1.83 Ga; Brewer et al., 1979; Schandelmeier, 1981). However, a minor fraction of younger zircons (∼1.3 to 0.9 Ga; Master et al., 2005) points to a contribution from Zambian and (late) Kibaran magmatic intrusions. Mobilization of uranium in the heavy minerals that were eroded from the source units would be linked to the establishment of a redox front, together with the circulation of hot saline brines, as those leading to sulfide deposition at that time (115–220 ◦ C and 11.3–20.9 wt% NaCl equiv; El-Desouky et al., 2010). 6. Conclusions In situ ion microprobe U–Pb measurements on heterogenite provide consistent and precise U–Pb and Pb–Pb ages. This mineral is therefore an appropriate material to investigate U cycling in the Katanga Copperbelt and to extent our understanding of this major Co–Cu–(U) province. The ∼876 Ma and the ∼823 Ma age confirms the syn-early diagenetic concentration of uranium in the Katanga Basin sediments. The source of the metal is to be found in the basement units. The basin geometry is far from very well constrained at the time. However, recent work (Bull et al., 2011) suggests that the Katanga depocenter was either deeper or surrounded by paleo-highs, notably to the South (where are the U-rich Zambian granites). This “deep” setting of the Katanga Basin was favorable for early U trapping, as the basal sediments were enriched in organic matter and hence intrinsically reducing. Moreover, the basin infilling took place in a series of major transgressive/regressive pulses that probably contributed to basin-wide migration of redox fronts. The ∼720 to ∼670 Ma age range is interpreted as a phase of U remobilization relating to hydrothermal fluid circulation, induced either by late Nguba proto-oceanic rifting or by shear zone development linked to an early stage of the Congo-Kalahari craton convergence. Acknowledgements This study is a contribution to the GECO project, funded by the Belgian Federal Public Service for Foreign Affairs, and has benefited from a Europlanet grant. The ion microprobe team of the “Groupe Sonde Ionique du CRPG” is thanked for isotopic data acquisitions. The authors wish to thank Jacques Cailteux for his helpful and constructive comments, and Guochun Zhao for his editorial handling of the paper. References André, A., 1976. Étude aéromorphologique et pétrographique du Katanguien de la mosaique contrôlée de Kapulo au Shaba. Unpublished MSc Dissertation, Université Libre de Bruxelles, Belgium, 107 pp. Armstrong, R.A., Master, S., Robb, L.J., 2005. Geochronology of the Nchanga Granite, and constraints on the maximum age of the Katanga Supergroup, Zambian Copperbelt. Journal of African Earth Sciences 42, 32–40.

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