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Sulfide Deposits Associated with Cl Metasomatism along Thrusts in Black Shales, Tsodilo, Botswana / JOAN CARLES MELGAREJO (1,*), LISARD TORRÓ ABAT (2), ELOI ANDREU TÁRREGA (1), MARC CAMPENY CREGO (1), MICHIEL C.J. DE WITT (3), LESEGO P. KGOTHANG (3), JOAQUIN A. PROENZA FERNÁNDEZ (1), JAMES M. BRUCHS (3), JOHN F. LEWIS (4) (1) Departament de Cristal·lografia, Mineralogia i Dipòsits Minerals. Fac. Geologia. Univ. Barcelona. C/ Martí i Franquès s/n. 08028 Barcelona (España) (2) Departament d’Enginyeria Minera i Recursos Naturals. Universitat Politècnica de Catalunya. Av. Bases de Manresa 61-63. 08242 Manresa (España) (3) Tsodilo Resources Limited. Canada Trust Tower. BCE Place. 161 Bay Street, P.O. Box 508 Toronto, ON M5J 2S1 (Canada) (4) George Washington University (USA)

INTRODUCTION. The aim of this contribution is to provide SULFIDE MINERALIZATIONS IN information on the syntectonic sulfide SYNTECTONIC VEINS. The Tsodilo prospect area is found at the veins. NW part of Botswana, close to the These mineralized veins occur in thrust Okavango delta, about 100 km NW of GEOLOGY. surfaces and can be traced along tens of the Maun town (Fig. 1). kilometers. Veins are very irregular, and The area consists of an Archaean consist largely of quartz and pyrrhotite gneissic basement unconformably (which explain the magnetic anomalies overlain by sedimentary series of associated with thrusts), with minor PanAfrican age. These Proterozoic series amounts of other minerals. Veins are of have strong lateral and vertical changes type “pinch-and-swell”; maximal of facies and thickness. Sedimentary thickness does not overpass 10 cm. domains can be established on the basis Veins are often arranged as subparallel of the dominant lithotypes in the sets, producing mineralized bands stratigraphic profiles. In some areas whose are several meters thick. carbonate platform facies dominate, whereas other domains are made up by Alteration in the vein selvages may thick sequences of black shales and consist of albitization, scapolitization, quartzites, with minor metabasites and and biotitization. Albite may occur inside conglomerates. These sedimentary the vein or along the selvages. It domains are limited by the above develops euhedral short prismatic mentioned N-S trending thrusts. crystals, in the range between several Therefore, this type of geometrical millimeters and a centimeter. Twinning relationships between tectonics and on {010} is always present; these stratigraphy can be explained by crystals do not have zoning and are PanAfrican tectonic inversion affecting a unaltered. Scapolitization is very Proterozoic riftogenic basin. The common, and produces subhedral ensemble was affected by regional prismatic crystals. Scapolite crystals fig 1. Situation of the study area. metamorphism in amphibolite facies. may replace albite (Fig. 2).

Most of the studied area is covered by Intrusives in the area consist of granitic thick series of Kalahari continental rocks ranging in composition from sediments and calcretes, and outcrops diorite to leucogranite. These granitoids of mineral deposits are scarce. However, produce the development of skarns regional geophysical exploration surveys when are found intruding limestones. in NW Botswana applied by Tsodilo Intrusion-related veins with potassic Resources Ltd allowed the discovery of alteration, sometimes accompanied by important magnetic anomalies, coupled tourmalinization, contain Cu-Pb-Mo ores. in many cases with conductivity Sedex sulfide deposits are found in the anomalies. These anomalies are aligned black shales; the limestones and defining trends approximately N-S. quartzites may be replaced by IOCG. Systematic drilling along these anomalies allowed defining several Some serpentinized ultramafic bodies styles of mineralization: sulfide veins also occur, but their textural patterns fig 2. Scapolite (scap) in a mineralizaed vein associated with thrusts, sulfide veins in and their setting suggest that they replacing albite across the twins (ab). The ensemble granites, skarns, sedex and IOCG. correspond to ofiolites. is replaced by pyrrhotite (po). Biotite (bt) is found AT the exocontact.

palabras clave: Sulfuros, Telururos, Oro, Cabalgamientos, Escapolita. key words: Sulfides, Tellurides, Gold, Thrusts, Scapolite.

resumen SEM 2011 * corresponding author: [email protected]

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The scapolite is intermediate between The fourth stage (S4) was characterized This type of deposit has been interpreted marialite and meionite, without a by tellurides (melonite, altaite, bismuth as produced by the action of

silvialite component (Fig. 3). tellurides and hessite), probably hydrothermal fluids connected with acid indicating a very low S activity, intrusives (Lindblom et al., 1996), and accompanied by native gold. All these similarities and a certain grade of

minerals are extremely fine grained, and consanguinity or overprinting with IOGC occur rimming grain borders and infilling deposits have also been suggested small fractures of other minerals. This (Wanhainen, 2005). The regional stage contains gold and precious metals occurrence of scapolite alteration has

and it should be studied in more detail been interpreted as the product of in additional samples. Probably the regional circulation of highly saline fluids mineral association at this stage was (with possible evaporitic influence)

more complex than indicated above. across regional rocks at high metamorphic temperatures, thus The fifth stage (S5) was probably favouring the development of scapolite triggered by an increase in the sulfur (Weihed et al., 2008). Although some activity, because pyrrhotite was replaced deposits are hosted by igneous rocks, by pyrite. This process was accompanied most of them have been found hosted by the deposition of several sulfides of by black schists. In fact, the mineral Pb, Zn, Ni. These sulfides are galena, a association is interpreted using this fact,

Cd-rich sphalerite and millerite. Pyrite because the carbonaceous reducing fig 3. Composition of the scapolite crystals was the only mineral spread out in this rocks can be an effective trap for many associated with the syntectonic sulfide veins. Note stage, and the other minerals are found redox sensitive elements (Eilu & the high content of marialite component. only in trace amounts Martinsson, 2007). This reducing trap

can explain the association of the Biotite may be extensively produced in A late generation of chalcopyrite was containing minerals with U-Ni-Co-U-Th- the exocontact of the veins and has a probably produced at this stage. The Mo-Fe-S-Te-Ag-Cu-Bi-As. Gold is likely to eastonitic composition. However, the sixth stage (S6) resulted in the be transported in the form of chloride most remarkable aspect on its deposition of calcite and ankerite in complexes, and the precipitation of composition is the Cl content, which small cracks, accompanied by lesser scapolite would reduce the chlorine may achieve up to 0,17wt% in the amounts of REE minerals. Many of these activity in the fluids, and hence, the vicinity of the veins. minerals contain U and Th, and hence destabilization of these complexes and

they may account for the radioactive the precipitation of gold. Therefore, the These biotite enrichments are level that may help in the exploration of occurrence of scapolite, coupled with accompanied by a strong enrichment in deposits of this type. the existence of reductive traps as thick fine-grained REE-Th-U minerals such as units of black shales, is a good criterion allanite-(Ce), monazite-(Ce), xenotime- DISCUSSION. for exploration of these types of deposits (Y), ancylite-(Ce), cancioancylite-(Ce), worldwide. bastnäsite-(Ce), thorianite and uraninite The available data suggest that the

deposit consists of an assemblage of REFERENCES. The mineral sequence has 6 stages. A sulfide-quartz-carbonate-scapolite-albite first stage (S1) produced the alteration syntectonic to late-tectonic veinlets Eilu, P., Weihed, P. (2005): Fennoscandian of the host rocks and a first generation distributed through highly Shield-Orogenic Gold deposits. Ore Geol. of gangue minerals, such as albite, metasomatized anoxic turbiditic Rev., 27, 326–327. tremolite, biotite and quartz, in some metasediments. The mineralizing fluids _., Martinsson, O. (2007): Epigenetic Au and cases accompanied by tourmaline. Cu-Au deposits. Metallogeny and tectonic seem to be alkaline, owing to the Calcite is common at this stage, and evolution of the Northern Fennoscandian extensive development of albite and its may have been accompanied by small Shield: Field trip guidebook. Geol. Surv. replacement by scapolite. amounts of apatite and REE Finland, Guide 54, 22-25. phosphates. A second stage (S2) was Lindblom, S., Broman, C., Martinsson, O. Very similar deposits are very common (1996): Magmatic-hydrothermal fluids in characterized by the replacement of the in the Proterozoic series in the Pahtohavare Cu-Au deposit in above minerals by quartz and late Fennoscandia (Eilu & Weihed, 2005). As greenstone at Kiruna, Sweden. Miner. pyrrhotite. This stage was volumetrically in the case of Botswana, all these Deposita, 31(4), 307-318. the most important in the assemblage Wanhainen, Ch. (2005): On the origin and deposits occur as veinlets hosted by of the veins. evolution of the palaeoproterozoic Aitik Cu- black cherts, and they have a typical Au-Ag deposit, northern Sweden: a regional hydrothermal alteration, The third stage (S3) was also important. porphyry copper-gold ore, modified by consisting in the development of, Chalcopyrite replaced pyrrhotite along multistage metamorphic-deformational, biotitization, albitization, silicification, magmatichydrothermal, and IOCG- grain borders and small veinlets scapolitization, carbonatization, mineralizing events. Ph.D. Thesis, Luleå accompanied by lesser amounts of sulfiditization and, in some cases, University of Technology. chlorite and molybdenite platelets. tourmalinization (Eilu & Martinsson, Weihed, P., Eilu, P., Larsen, R.B., Stendal, H., Chalcopyrite is abundant enough to be Tontti, M. (2008): Metallic mineral deposits 2007). Scapolite is the dominant considered as a possible valuable in the Nordic countries. Episodes, 31(1), mineral and can be regionally spread. It mineral, depending on the total content 125-132. is generally rich in the marialite of this mineral in the ores. component, hence, in Na-Cl.