Geochemical associations between the lateritic crusts and carbonatitic rocks of the Carbonatitic Complex at Morro dos Seis Lagos, AM, Brazil

Abstract

The carbonatite rocks present at the Seis Lagos Carbonatite Complex (SLCC) are siderite carbonatites, and constitute the source of the titano-niobian mineralization hosted in the lateritic crusts that cover the complex. This study is based on the geochemical data from 6 drill holes conducted by the Geological Survey of Brazil (CPRM) in the 1980s plus the mineralogical descriptions and analyzes performed on these cores. The quality of the geochemical data was evaluated in order to select the most reliable elements for statistical treatment. Given the compositional character of the geochemical data, these were transformed into Centered Log Ratio (CLR) and submitted to principal component analysis. These data enabled definition of geochemical association of 4 types of carbonatite rock: (i) C1, siderite carbonatite; (ii) C2, friable siderite carbonatite; (iii) C3, light siderite carbonatite; and (iv) B1, carbonatite breccia. B1 is more phosphatic and enriched in Al, P, La, Ce, Ba, Sr, Zr, S, Be, U and REE minerals like bastnaesite-(Ce), monazite-(Ce), as well as those belonging to the plumbogummite group (such as florencite-(Ce)) — which were formed during a later carbohydrothermal stage. C3 is also enriched in the same suite of metals as a result of hydrothermal alteration after C1. The transition from C1 to its friable equivalent, C2, is more abrupt, suggesting that the weathering took place with well-defined limits in depth and was possibly delimited by structures. C2 is characterized by its enrichment in Th, Zn, Sn, Co, and Sb, with some influence from both Mn and Fe, and the leaching of Ce, Ba, La, and Sr. C2 contains goethite, rutile, brookite and gibbsite, evidencing the impact of weathering. Lateritic crusts are distinguished into 5 types: (i) L1, a fragmented cavernous crust; (ii) L2, a reddish-brown crust; (iii) L3, a manganese crust; (iv) L4, a gray crust; and (v) L5, a compact, grayish crust. The L1 crusts are richer in Al, P, Zr, La and U due to the residual enrichment of their immobile elements and to the formation of secondary aluminum phosphates, mainly florencite-(Ce). The L3 manganese crusts are characterized by a suite of metals, composed of Mn, Ba, Mo, Co and Ce. The contrasting compositions between segments of the upper and lower crusts it is more likely due to compositional differences in the siderite carbonatite, rather than reflecting the depth and hence intensity of weathering. A specific type of crust to which the NbTi mineralization might be associated with was not identified. No specific geochemical association was identified for the deeper crusts, such as L4 and L5; however, the occurrence of Ce-rich pyrochlore, cerianite and carbonates in them, coupled with their absence in the upper crusts, suggests that the aforementioned ones are less evolved and therefore, closer to the carbonatite protolith. Our results indicate that despite intense weathering the composition of the primary siderite carbonatite exerts an influence on the composition of the lateritic crusts. However, it was not possible to establish a direct, lithodependent relationship between the crusts and the siderite carbonatites.