TECHNICAL REPORT N. 11 Brasília, Brazil, May 2017 ISSN: 2448-2242 DOI: Occurrence of chromiferous horizon in the Trincheira Complex, municipality of Corumbiara, southeast of the State of Rondônia, Brazil Guilherme F. da Silva (guilherme.ferreira@cprm.gov.br) Thyago de J. Ribeiro (thyago.ribeiro@cprm.gov.br) Lívio W. C. Corrêa (livio.correa@cprm.gov.br) Dalton R. V. da Silva (dalton.rosemberg@cprm.gov.br) Carlos E. S. de Oliveira (carlos.oliveira@cprm.gov.br) CPRM – Geological Survey of Brazil, REPO – Porto Velho Office Reference: CPRM – Geological Survey of Brazil website: www.cprm.gov.br / Geologia em Evidência/RECUR- SOS MINERAIS/Informe Técnico – CPRM/Edição Atual: Informe 11 (05/2017): Ocorrência de horizonte cromí- fero no Complexo Trincheira, município de Corumbiara, sudeste de Rondônia. Abstract The Trincheira Mafic-ultramafic Complex comprises a Mesoproterozoic layered intrusion hosted in me- tasedimentary rocks of the southwestern portion of the Amazonian Craton. This intrusion has an outcropping area of approximately 35 km2, however, it has been associated to a larger ellipsoidal magnetic anomaly with N-S elongated shape. The Trincheira Complex is com-posed of partially metamorphosed pyroxenites, webste- rites, gabbros and anorthosites. The complex hosts Cr-rich layers associated to relevant concentrations of Ni. High concentrations of Cu, Co and V have also been detected. Preliminary analysis performed with portable X-ray diffraction indicates up to 13% Cr and occurrence of cochromite, a special variety of chromite enriched in cobalt, nickel and iron, whereas ICP-MS analyses returned concentrations of up to ~3600 ppm Cr. Keywords: Mafic-ultramafic intrusions, Cumulate rocks, Amazonian Craton. INTRODUCTION Considering that the occurrences are hosted in the Trincheira Complex, which has similarities with the Ni-bearing ultramafic bodies described in Morro New mineral occurrences were identified in ul- sem Boné and Morro do Leme, it is expected that the tramafic rocks of the Trincheira Complex, southeast presented data can draw the attention to new areas of the State of Rondônia, Brazil, located at Fazenda to investigate the potential of mineral deposits rela- Patuá (or Andreazza), sited to the margins of the sta- ted to mafic-ultramafic bodies. te highway RO-370, "Estrada do Boi", municipality of Corumbiara, near the limit with the State of Mato Grosso and the Brazil-Bolivia border (Figure 1). GEOLOGICAL SETTING In this report, we present petrographic infor- mation, X-ray diffraction and fluorescence analyses, The Trincheira Complex was defined by Roma- lithogeochemical and magnetometric data. nini (2001) as an association of mafic-ultramafic rocks The analysis of the mafic-ultramafic rocks intrusive in the metavolcano-sedimentary Nova Brasi- shows one sample with relevant contents of lândia Sequence, and consisting of gabbros, gabbrono- chromium (3,624 ppm), nickel (1,829 ppm) and rites, tremolitites, websterites and bronzitites, partially cobalt (651 ppm), in addition to copper (470 ppm) metamorphosed under upper greenschist facies, ha- and vanadium (318 ppm). ving an outcropping surface of approximately 35 km2. 1 CPRM – Geological Survey of Brazil Figure 1: A) Location of the project area between the states of Rondônia and Mato Grosso, near the border with Bolivia; B) Simplified geology of the Southeast Rondônia Project (Silva et al., 2016) and main mineral occurrences of nickel, chromium and gold; C) Detailed geological map of the outcropping portion of the Trincheira Complex, including the location of Cr-Ni (Co-V) mineral occurrences and stations with chemical and pXRF (portable X-ray fluorescence, Olympus DELTA Professional model) analyses. 2 CPRM – Geological Survey of Brazil Silva et al. (2016) revisited the Trincheira Complex by adding new field, petrographic and whole-rock geochemi- cal information. Accordingly, the Trin- cheira Complex is formed by layered and tabular bodies, roughly divided from north to south into ultramafic component, which grades to mafic ro- cks. These rocks are little deformed and locally transformed in tremolitites. The wall rocks comprise a sequence of calc- -silicate rocks, para-amphibolites and metavolcanic rocks metamorphosed under upper amphibolite facies, which occasionally occur as xenoliths within the complex. Comparatively, the body in the Andreazza Farm resembles the Ni-mi- neralized ultramafic bodies of Morro sem Boné and Morro do Leme (Nunes, 2000), which are located in the muni- cipality of Comodoro (State of Mato Figure 2: X-ray diffractogram with the main mineral phases Grosso, Brazil), 100 km southeast of detected: cochromite, magnetite, enstatite, diopside and Mg3V2O6 the occurrences discussed here, in the (magnesiocoulsonite?). The analyses were done with pXRD (portable boundary region between the Rio Ne- X-ray diffractometer, Olympus TERRA Mobile model). gro-Juruena Province and the Alto Gua- poré Belt, both occurring in the same The cumulate nature of both wall and mine- geological setting (Rizzotto et al., 2000). ralized rocks was confirmed by petrography, where mineralized samples show cumulate crystals of chro- OCCURRENCE DESCRIPTION mite and magnetite dispersed in a partially serpenti- nized matrix (Figure 3). Four samples were identified with relevant concentrations of Cr and Ni, cropping out as suban- CHEMICAL ANALYSIS gular to tabular blocks within tremolitites and pyro- xenites layers (Table 1). Three of the occurrences are Sixteen rock samples were collected along the aligned parallel to the igneous bedding, indicating Trincheira Complex (Table 2). The chemical analyses that they are part of the same horizon. were done in the SGS Geosol laboratories. The con- In outcrop, mineralized rocks have an altera- centrations of MgO, TiO , CaO and FeO were obtai- tion cover of iron oxides and hydroxides, which give 2 tned by X-ray fluorescence in samples opened with a reddish color to the rock and that can be easily lithium metaborate. The contents of S, Cr, Ni, Co, V confused with laterite. When fresh, the rocks are and Cu were obtained using ICP-MS after multiacid black, with sub-centimetric crystals of minerals with digestion. Pt values were obtained by ICP-AES after metallic brightness, some of which strongly magne- fusion by fire assay. tic, immersed in a matrix of silicates visible to the In addition, in situ analyses were carried out naked eye. in three samples, using a portable X-ray fluorescen- The X-ray diffraction analysis of sample TJ- ce (pXRF). The results returned high concentrations 190 (Figure 2) indicates the presence pyroxenes in of chromium and nickel, and one sample also sho- addition to magnetite, magnesiocoulsonite (?) and ws relevant concentrations of cobalt and vanadium cochromite (chromite variety enriched in Ni, Co (Table 3). and Fe). The analyses were compiled in order to simu- late a possible intrusion stratigraphy (Figure 4). Con- Table 1: Location of occurrences sidering that the layered body is not inverted, the ultramafic (basal) portion lies to the north, and the upper mafic units to the south. Thus, it is possible to Outcrop *UTM-E *UTM-N observe the anomalous values of the chromitites of GS-085 687411 8581659 the sample GS-87 in FeOt, Cr, Ni, Co and Cu compa- GS-087 656913 8582233 red to the average contents of their host rocks. There is a negative correlation between sulphur and nickel TJ-190 684319 8581724 concentrations, suggesting that the latter may also TJ-200 685428 8581181 be present in a mineral phase other than pentlandi- *Datum: SIRGAS 2000, zone 20 South. te, as previously described by Romanini (2001). 3 CPRM – Geological Survey of Brazil Figure 3: A) Blocks of massif chromitite occurring within partially tremolitized pyroxenites; sample TJ-190; B) Cumulate chromitite with coarse texture, partially lateritized; sample GS-087; C) Photomicrography (crossed polarizer) of websterite, composed of enstatite accumulations with augite intercumulus; sample TJ-030; D) Photomicrography (crossed polarizer) of gabbronorite composed of accumulations of enstatite and augite, dispersed in plagioclase matrix; sample GS-092; E) Photomicrography in crossed polarizer showing ultramafic protolith metamorphic rock composed of tremolite, serpentine and opaque minerals, without well-developed planar fabric; sample GS-086; F) Photomicrography under reflected light showing predominantly cumulate chromite and magnetite, with lamellae of ilmenite exsolution, dispersed in serpentine matrix; polished section of sample TJ-190. Abbreviations: En = Enstatite; Aug = Augite; Pl = Plagioclase; Srp = Serpentine; Tr = Tremolite; Chr = Chromite; Mt = Magnetite; gg = Gangue. 4 CPRM – Geological Survey of Brazil Table 2: Results of chemical analysis in rock samples of the Trincheira Complex MgO TiO CaO FeO S Cr Ni Co V Cu Pt Sample UTM_E UTM_N 2 t % % % % ppm ppm ppm ppm ppm ppm ppb GS-086 686390 8581731 27.1 0.31 1.4 12.1 < 0.01 770 488 46.3 126 147.8 < 5 GS-087 685913 8582233 3.53 0.91 0.06 42.4 0.02 3624 1829 651.2 318 470.8 < 5 GS-139 683871 8581343 26.3 0.35 0.93 10.6 < 0.01 803 395 51.6 113 98.7 < 5 GS-140 683968 8580941 7.67 0.96 10.6 12.0 0.03 76 56.3 44.2 215 36.9 < 5 GS-141A 683917 8580614 13.6 0.61 11 10.7 0.11 309 230 52.8 176 100.6 5 GS-141B 683917 8580614 13.6 0.67 14 9.7 0.09 313 185 49.7 199 137.7 10 GS-142 684000 8580279 23.9 0.56 2.85 11.6 0.01 549 614 54.8 124 195.8 < 5 GS-148 684979 8581489 17.9 0.27 3.84 18.5 0.11 370 530 66 150 34.2 < 5 GS-154 685837 8581416 30.1 0.19 1.24 11.3 0.01 580 1286 75.3 68 26.8 < 5 GS-156 686081 8581070 28.0 0.28 0.68 10.7 < 0.01 763 500 66.1 103 14.6 < 5 GS-157 686232 8581023 28.3 0.19 2.32 9.9 0.03 525 641 73.4 77 71.7 N.A. GS-159 686638 8580941 24.4 0.23 1.21 15.7 0.02 432 690 79.8 108 12 7 GS-163 682828 8581068 6.89 1.43 10.9 15.0 0.04 39 46.8 40.4 210 42.1 < 5 GS-166 682991 8580284 9.39 0.34 10.9 8.2 0.12 86 131 43.5 113 43.4 < 5 GS-178 685679 8582924 13.9 0.22 10.9 8.2 0.15 143 216 48.7 116 43.1 < 5 TJ-193 684203 8581081 6.08 1.65 11 13.4 0.14 14 9.9 37.8 258 36.3 N.A. N.A. = not analysed Table 3: Chemical results for mineral occurrences analyzed by pXRF MgO TiO FeO Co Ni Cr V Amostra UTM_E UTM_N 2 % % % ppm ppm ppm ppm GS-086 686390 8581731 27.1 0.31 12.1 770 488 46.3 126 GS-087 685913 8582233 3.53 0.91 42.4 3624 1829 651.2 318 GS-139 683871 8581343 26.3 0.35 10.6 803 395 51.6 113 FINAL REMARKS Figure 5 suggests that the magnetic anomaly attributed to the Trincheira Complex occurs in a much larger area than that of the outcrop. If this magnetic The Trincheira Mafic-Ultramafic Complex anomaly actually represents buried portions of the shows to be a new prospective target with Cr, Fe, complex below the calc-silicate wall rocks, the Trin- Ni, Co, V and PGE chemical association from the cheira Mafic-Ultramafic Complex will have its pros- recognition and characterization of a probable pective potential significantly extended. cumulate chromite horizon within the layered body Detailed studies are recommended to recogni- (more horizons might be present). ze other mineralized horizons, seeking to verify the Our data are insufficient to make any inferen- lateral and depth extension of these mafic-ultramafic ce about the accumulation of sulfide layers. So far, it bodies. We also suggest detailed ground-based gra- appears that the Ni-Co concentrations may be in the vity and magnetometry because of the density and oxides, possibly in a cobaltiferous chromite, the co- magnetic contrasts between the mineralized and chromite, as the main mineral. Previous descriptions other rocks, as observed in hand sample. Induced reported the presence of pentlandite as the sulfide polarization (IP) survey can be done to identify pos- phase in the complex (Romanini, 2001). It was ob- sible layers enriched in PGE and of massive sulfide. served that calcium enriched rocks had the highest Microprobe studies could provide more reliable in- concentrations of sulfur (gabbros, gabbronorites and formation on the mineral phases that contain the Fe- anorthosites). -Ni-Co and Cr-Ti associations. 5 CPRM – Geological Survey of Brazil Figure 4: A) Chemostratigraphy of the Trincheira Complex from the projection of chemical analyses along an NE-SW-trending intermediate axis, which crosses the lithofacies (see Fig. 1). The color associated with the samples in the horizontal bar shows the lithotype as indicated in the legend. B) Ni, Cr and Co (ppm) versus MgO (%) diagrams. There is a strong positive linear correlation with the gabbros, pyroxenites and tremolitites samples in the three diagrams. Samples GS-154 and GS-087 show concentrations above this trend in Ni and Ni, Cr and Co respectively. The GS-087 sample shows the highest concentrations of the three elements, but has a low concentration of magnesium. This can be explained by the fact that this rock is composed of oxidized cumulate minerals and smaller amounts of magnesium rich silicate phases (serpentine, pyroxenes and tremolite). 6 CPRM – Geological Survey of Brazil Figure 5: Total Gradient Magnetic Map with indication of the outcropping portion of the Trincheira Complex and its probable sub-surface extension, based on the magnetic anomaly. REFERENCES ROMANINI, S.; 2001 Aspectos geológicos, geoquí- micos e metalogenéticos do magmatismo básico/ ultrabásico do estado de Rondônia e área adjacen- NUNES, N. S. V.; 2000. Geologia e resultados pros- te. CPRM - Companhia de Pesquisa de Recursos Mi- pectivos das áreas Morro do Leme e Morro Sem nerais, 2001. Boné – Mato Grosso. Goiânia, GO: CPRM - Compa- nhia de Pesquisa de Recursos Minerais, 2000. SILVA, G. F.; RIBEIRO, T. J.; OLIVEIRA, C. E. S.; CORRÊA, L. W. C.; ADÔRNO, R. R.; SILVA, D. R. V.; 2016. Mapa RIZZOTTO, G. J. et al. 2013. The Mesoproterozoic Geológico Preliminar Integrado Novas Fronteiras Guaporé suture in the SW Amazonian Craton: Geo- Sudeste de Rondônia. Escala 1:100.000, apresenta- tectonic implications based on field geology, zircon do em escala 1:200.000. Projeto Avaliação do Po- geochronology and Nd-Sr isotope geochemistry. tencial Mineral do Sudeste de Rondônia. Porto Ve- Journal of South American Earth Sciences, v. 48, p. lho, RO. CPRM - Companhia de Pesquisa de Recursos 271–295, 2013. Minerais, 2016. 7 CPRM – Geological Survey of Brazil PUBLICATION COMMITTEE Director of Geology and Mineral Resources TECHNICAL REPORT N. 11 José Carlos Ferreira Garcia Brasília,Brazil, May 2017 Editorial Board Evandro Luiz Klein (Editor) Serial online publication by CPRM – Geological Survey of Brazil João Henrique Larizzatti Luiz Gustavo Rodrigues Pinto Available at www.cprm.gov.br Edilton José dos Santos Reviewers Geysson A. Lages CPRM - Geological Survey of Brazil Wilson Wildner SBN – Quadra 02 – Bloco H, Ed. Central Brasília, 1st floor Brasília - DF - Brazil Bibliographic normalization CEP: 70040-904 Nelma Fabrícia da P. Ribeiro Botelho Phone: +55 (61) 2108-8400 Translation Contacts: bibliotecarj@cprm.gov.br Paulo Henrique Macedo Varão and Evandro Luiz Klein evandro.klein@cprm.gov.br Desktop Publishing Marcelo Henrique Borges Leão Nelma Fabrícia da P. Ribeiro Botelho 8