Geological Survey of Denmark and Greenland Bulletin 33, 2015, 81-84 81 Composition of ilmenite and provenance of zircon in northern Brazil Christian Knudsen, Tonny B. Thomsen, Feiko Kalsbeek, Jeppe A. Kristensen, Helenice Vital and Roger K. McLimans Th e mineral ilmenite (FeTiO3) is an important component of heavy-mineral placer deposits and constitutes the largest volume of valuable mineral in such deposits. Th e minerals zircon (ZrSiO4) and rutile (TiO2), which occur in lower con- centrations than ilmenite in the deposits, have a greater value per ton – c. 1100 and 900 $/ton respectively – compared to ilmenite that ranges from 100 to 200 $/ton depending on its composition. Other minerals such as staurolite, silliman- ite, amphibole and garnet are generally also present in placer deposits, but are of minor or no commercial value and, e.g. amphibole needs to be separated from the valuable heavy minerals which adds to the production cost. Ilmenite is more valuable as a raw material in titanium dioxide manufacture if the titanium content is enhanced by natural leaching of the iron component. When exploring for potentially economic heavy-mineral placer deposits, both the variation in composition and distribution of ilmenite are of interest. Accordingly, it is also important to understand not only the concentration of heavy minerals in the ground but also the abundance and composition of the individual minerals. Th e source (provenance), route and mechanism of trans- port from source to potential reservoir sandstones are of in- terest when attempting to understand petroleum systems in sedimentary basins. Heavy minerals in reservoir sandstones contain a wealth of information about their formation that characterises their source. Th us a database with the charac- teristics of possible sediment sources is a key tool to investi- gate the distribution, composition and other characteristics of the heavy minerals in a given area. Both heavy-mineral exploration and locating off shore petroleum reservoir sand sources are relevant in northern Brazil. In June 2011, the Ge- ological Survey of Denmark and Greenland in cooperation with Universidade Federal do Rio Grande do Norte, Natal, RN, Brazil set up a project to sample and analyse in situ Cre- taceous sandstones, sands in river beds and sands from the coastal sediments. Th e results were entered into a database of heavy-mineral compositions and properties. Th e varia- tion in the distribution of the heavy minerals as well as their composition were determined for 34 samples from northern Brazil using computer-controlled scanning electron micros- copy (CCSEM; Keulen et al. 2012). U-Pb ages of detrital zir- cons were determined by laser ablation inductively coupled plasma mass spectrometry (for analytical procedures see Frei et al. 2006). Four samples for the U-Pb age determinations came from Cretaceous sandstones, two from river beds and two from beaches. Alteration of the heavy minerals When subjected to alteration in the sedimentary environ- ment, heavy minerals react diff erently depending on the local physical and chemical conditions. In hot and humid conditions minerals such as olivine, pyroxene, amphibole © 2015 GEUS. Geological Survey of Denmark and Greenland Bulletin 33, 81–84. Open access: www.geus.dk/publications/bull Ilmenite Leucoxene Rutile Zircon Staurolite Silimanite-kyanite Garnet Epidote Amphibole Baía de Marajó 45°W 3°S Rio Piranhas-Açu Amazon Basin 200 km Brazil São Luís Parnaíba Acaraú !! ! !! ! ! !!! ! 40°W45°W 3°S 200 km 62–65 60–62 58–60 56–58 54–56 52–54 TiO2 (%) Quaternary Neogene–Quaternary Tertiary–Quaternary Tertiary Cretaceous Jurassic Triassic Carboniferous–Permian Cambrian–Devonian Neoproterozoic–Palaeozoic Precambrian Litho-chronology Belem São Luís Fortaleza Teresina Tianguá São Luís-Grajaû Basin Parnaíba Acaraú Camelá sub-Basin Rio Capim Baía de Marajó Fig. 1. Modal composition of the heavy-mineral fraction in sand samples from northern Brazil. Fig. 2. Average composition of ilmenite and altered ilmenite in samples from beaches, from river beds and from outcrop of Cretaceous sandstone. For location see Fig. 1. 8282 and epidote are unstable and gradually disappear from the heavy-mineral assemblage (Morton & Hallsworth 1999). Th e heavy-mineral assemblages in the fi ve easternmost beach sand samples from the northern Brazilian coast (Fig. 1) all contain abundant amphibole. Th e source of those heavy minerals is the Precambrian basement in the hinterland (pink, Fig. 2). A similar, diverse, heavy-mineral assemblage is described by da Silva & Vital (2000) in samples from the Rio Piranhas-Açu, north-eastern Brazil. Th e low degree of al- teration of the heavy minerals is probably due to the climate which is arid in this part of Brazil. Th e heavy-mineral assem- blages of the fi ve eastern samples change westwards with in- creasing contents of alumina-silicates such as staurolite, silli- manite and kyanite; minerals which must be abundant in the hinterland, and which can fi ngerprint the sediment source for the eastern samples. To the west of Parnaíba (Fig. 1), the heavy-mineral assemblages are dominated by ilmenite, leucoxene, rutile, staurolite and zircon, which are very stable minerals (Morton & Hallsworth 1999). Th e precipitation, humidity and temperature increase towards the west and towards the Amazon Basin. Th is may account for the min- eralogical change indicative of intense alteration where even fairly stable heavy minerals like garnet have disappeared. Th e Cretaceous sandstone in the interior of northern Brazil is altered by intense kaolinisation. Mendes & Truck- enbrodt (2009) describe a mature heavy-mineral assemblage from Albian sandstones (Itapecuru Group) in the São Luís- Grajaû Basin and Góes et al. (2007) described similar assem- blages from the Campanian–Maastrichtian (Ipixuna For- mation) in the Camelá sub-basin to the west. Only robust heavy minerals such as ilmenite, zircon, rutile and staurolite are present in the Cretaceous samples inland south of São Luís and Baia de Marajá (Fig. 1) whereas less stable heavy minerals are lacking. Th e higher degree of alteration found in the heavy-mineral assemblage in the coastal beach samples in the western section of the coast could accordingly also be an eff ect of re-deposition of Cretaceous sandstones from the hinterland. Ilmenite composition Th e titanium content of ilmenite changes when it is sub- jected to chemical weathering; iron is leached and the rela- tive content of titanium increases (Fig. 3A). Ultimately the mineral leucoxene, which mainly consists of TiO2, is formed (Bailey et al. 1956). A gradual increase of TiO2 in ilmenite in beach sand is evidenced towards the west (Fig. 2). As dis- cussed above that may be an eff ect of increasing humidity, but it may also be caused by an infl ux of reworked sediment from altered Cretaceous sandstones in the hinterland. Th e TiO2 content of ilmenite in the Albian sandstones (Itape- TiO 2 (%) 50 60 70 80 90 100 IlmeniteTitano- magnetite Leucoxene RutileB TiO 2 (%) 50 60 70 80 90 100 50 40 30 20 10 0 F e 2 O 3 ( % ) Ilmenite Leucoxene Rutile A a b c 50 µm Fig. 3. CCSEM analysis of titanium minerals from a Cretaceous outcrop along Rio Capim (sample GGU 538118). A: The distribution of TiO2 ver- sus Fe2O3 in the ilmenite, leucoxene and rutile show an inverse relation- ship between these two components, where Fe2O3 decreases with increas- ing TiO2 – and with the degree of weathering. B: Histogram showing the content of TiO2 in the titanium mineral grains. Fig. 4. Scanning electron microscope (SEM) backscatter image of an il- menite grain leached from the rim. In the light grey central part of the grain (a), the ilmenite is un-leached with preserved white hematite lamel- lae. Surrounding the un-leached core there is a zone (b), where the hema- tite is leached away forming pores (black) and where the ilmenite shows partial leaching (dark grey patches). In the grey rim (c), the ilmenite is highly leached and almost all iron is removed leading to the formation of leucoxene. 83 curu Group; eastern line of samples in Fig. 2) is high and the TiO2 content is even higher in the Campanian–Maastrich- tian (Ipixuna Formation) located in the more humid climate to the west. Th e heavy-mineral assemblage from a Cretaceous out- crop along Rio Capim is dominated by ilmenite. Figure 3B shows that the content of TiO2 in the titanium minerals has a bimodal distribution with almost no unaltered ilmenite (50% TiO2). Th e bimodal distribution may indicate that the weathering is a two stage process. An example of chemical weathering of an ilmenite grain is shown in Fig. 4. Th e grain has a central core that is largely unaltered with hematite lamellae preserved, an intermediate zone where the hematite is leached away giving rise to poros- ity, and an outer margin that is completely transformed to leucoxene. Zircon provenance Th e crystalline basement complexes underlying the investi- gated sedimentary rocks in the eastern part of the area is the Borborema Province (de Almeida et al. 1981) that is com- posed of a complex assemblage of Palaeoproterozoic grani- toid and metasedimentary gneisses, locally with outcrops of Archaean rocks and with numerous plutons of Neoprote- rozoic granite. Neoproterozoic sedimentary successions are also present. Palaeoproterozoic granitoid rocks are the most common, forming some 70% of the basement complex. Th e São Luís Craton in the west is dominated by the Trans Ama- zonian Orogeny with ages ranging from 2000 to 2200 Ma (Klein et al. 2005). Th e U-Pb age distribution patterns for eight sam- ples from north-eastern Brazil (Fig. 5) show the ages of zircon sand grains. Four of these are from Cretaceous sandstone, two are riverbed sand, and two are coastal sand. Th e age distributions (Fig. 5) are complex. Th e largest component in all samples is formed by Palaeo- Fig. 5. U/Pb age distributions of detrital zircons from samples from northern Brazil. The two green stars to the east represent sandstone samples from the Albian Itapecuru Group and the two green stars to the west represent sandstone samples from the Campanian–Maastrichtian Ipixuna Formation. For location see Fig. 1. Beach sand River bed sand Cretaceous sandstone 45°W 3°S 100 km GGU 538254, n = 76/114 GGU 538111, n = 94/140 GGU 538118, n = 86/119 Age (Ga) 210 3 50 40 30 20 10 0N u m b e r o f a n a ly se s GGU 538205, n = 103/125 Age (Ga) 210 3 50 40 30 20 10 0N u m b e r o f a n a ly se s GGU 538219, n = 96/119 Age (Ga) 210 3 50 40 30 20 10 0N u m b e r o f a n a ly se s GGU 538144, n = 75/117 210 3 50 40 30 20 10 0N u m b e r o f a n a ly se s GGU 538125, n = 62/105 Age (Ga)210 3 50 40 30 20 10 0N u m b e r o f a n a ly se s 30 20 10 0 N u m b e r o f a n a ly se s Age (Ga)210 3 40 30 20 10 0 Age (Ga)210 3 Age (Ga)210 3 50 30 20 10 0N u m b e r o f a n a ly se s 8484 Authors’ addresses C.K., T.B.T. F.K. & J.A.K, Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark; E-mail: ckn@geus.dk H.V., Universidade Federal do Rio Grande do Norte, Natal, R N, Brazil. R.K.McL DuPont Titanium Technologies, Wilmington, DE, USA. proterozoic zircons, 1800–2300 Ma, comprising 50–70% of the zircon populations. Archaean zircons (2500–3500 Ma) constitute 10–25% of the population and Neoproterozoic zircons (500–700 Ma) constitute 10–20% of the popula- tion. Only some 15% of the zircons have ages outside these age groups. Th ere are no obvious diff erences in age for the zircon populations in the Cretaceous sediments, the riverbed sands and the coastal sands. Zircons with ages in the range 2000 to 2200 Ma, equiv- alent to the Trans-Amazonian Orogeny (green columns on Fig. 5) are common in all samples, in good accordance with observations made by Klein et al. (2005). Th e content of zircons in the age range 1800–2000 varies considerably. Neoproterozoic ages (blue columns on Fig. 5) equivalent to Braziliano or Pan-African Orogeny are less frequent in the analysed sands as compared to what is previously described from the Borborema Province (Nascimento et al. 2007). Discussion and conclusion Th e heavy-mineral assemblages in the east are less mature than assemblages in the west. Th at may refl ect a lower degree of alteration of the heavy minerals caused by a drier climate. In the hot and humid area to the west in the Amazon Basin, the heavy-mineral assemblages are very mature refl ecting the intense chemical attack and removal of heavy minerals such as pyroxene, amphibole and garnet. Indications of more in- tense chemical alteration of the heavy minerals are also seen from the composition of ilmenite that shows decreasing iron content and accordingly increasing content of TiO2 towards the west. Cretaceous sandstones in the area are kaolinised and the heavy minerals are also strongly altered, most in- tensely in the hot and humid area in the Amazon Basin. Th e high degree of alteration found in river and beach sediments in the western area could also be caused by reworking of pre- viously altered Cretaceous sandstones. Apparently, nearly all zircons in the investigated sedi- ments may originate from the underlying crystalline base- ment, suggesting mainly local source areas. Th e zircon-age spectra are fairly uniform suggesting either that the geology in the source area is rather uniform or that the zircons were homogenised in the sedimentary environment and that the sand in the rivers and on the beach at least partly represent reworked Cretaceous sandstones. Acknowledgements DuPont Titanium Technologies, Wilmington, Delaware, USA is thanked for fi nancial support. 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