REE and pyroxene compositional variation across the Niquelandia layered intrusion, Brazil: Petrological and metallogenetic implications

The Niquelandia Complex is a major Mesoproterozoic layered intrusion in cen tral Brazil. It is part of a 350 km long linear array of layered intrusions interpreted to be a major continental rift. The intrusion comprises a Lowe r Layered Series (LS) and an Upper Layered Series (US). The LS is divided i nto a lower unit (UMU), dominated by interlayered dunite and pyroxenite, an d an intermediate unit (CMU), dominated by gabbronorite. The US is formed o f interlayered anorthositic and gabbroic rocks. A progressive trend of Fe enrichment in pyroxene compositions from the lowe r UMU to the top of the CMU is compatible with fractionation under low oxyg en-fugacity conditions. The pyroxene trend for these units is matched by th e progressive increase in incompatible trace elements. Calculated liquid co mpositions for the lower UMU and the CMU indicate a significant enrichment in incompatible trace-element contents of the fractionating liquid. This co mpositional variation can be explained by use of an open-system model that involves both crystal fractionation and crustal contamination. Both pyroxene compositions and incompatible trace-element contents of the r ocks show a major reversal at the US. The upper unit is interpreted as resu lting from a new influx of primitive magma with a distinct composition. Pyr oxene compositions lack a strong Fe enrichment trend, which, together with the presence of interlayered Fe-Ti oxide layers and the more abundant prese nce of magmatic amphiboles, supports the crystallization of the US under hi gher oxygen-fugacity conditions than the lower units. The presence of quart z diorites and felsic intrusions, as well as xenoliths of crustal rocks, in the upper part of the CMU, i.e. at the contact between the LS and the US, suggests that the new influx of magma followed the complete, or almost comp lete, solidification of the lower units. Exploration for magmatic deposits in the Niquelandia Complex should conside r the composite nature of the intrusion. The stratigraphy and petrological features of the LS closely resemble those of PGE-mineralized layered intrus ions. Therefore, magmatic sulphide deposits are more likely to be associate d with the LS.