Applied Earth Science: Transactions of the Institutions of Mining and Metallurgy

Gold in Benin occurs as primary and alluvial forms in various deposits and occurrences. The mineralisation has a regional distribution covering, in particular, the Atacora area (Natitingou) and the regions of Bembèrèkè, Djougou and Dunkassa which are between the 9th and 11th parallel north. The data collected during previous work allow us to recognise the main style of primary gold as being quartz-vein hosted. This style occurs principally in quartzite, mica schist, schist and amphibolite lithologies, outcropping in the criss-crossing of the faults and the rugged relief. The mineralised veins are milky, deformed and gold is associated with sulphides and tourmaline. In these veins, gold exhibits three forms: free native metal, tellurides, and combined or included in sulphides. These gold-bearing quartz veins would proceed from hydrothermal solutions related to granitic mass. Classical characteristics available about the primary gold mineralisation in Bénin are inadequate to better clarify its geodynamic context. However, it has some similarities with deposits of the Precambrian terranes in adjacent countries (Pourra deposit in Burkina Faso and Kalana deposit in Mali) which are characterised by epigenetic gold deposits in Precambrian metamorphic terranes. Also, the vein gold of Bénin are mostly located in Precambrian metamorphosed sedimentary and volcanic formations; these could also have the epigenetic disseminated gold style like in some West African deposits (Ashanti deposit).

Geostatistics aims to spatially model multiple correlated attributes with some under-sampled variables. Missing samples can distort the data set statistics, and for the model to be consistent, it is necessary to understand the mechanisms that cause the missing data (MD). There are three mechanisms that can govern MD: Missing Completely at Random (MCAR), Missing at Random (MAR) and Missing Not At Random (MNAR). More specifically, when there is a systematic difference between measured and the missing samples, the data is subject to the MNAR mechanism, which leads to biases in the final model if not dealt with adequately. A case is presented where an under-sampled attribute, U (ppm), is modelled to infer its values over the study area through co-simulation approach and an univariate simulation performed on an artificially complete set, considering the MNAR mechanism, which shows a reduction in the relative estimation errors compared to the co-simulated model.

The Florida Canyon deposit hosted by a carbonate-evaporite sequence of the Pucará Group, Peru, has both Zn–Pb sulphide and Zn non-sulphide mineralisations. The mineralogical characterisation of samples from the deposit enables the definition of a supergene assemblage dominated by smithsonite, in addition to hemimorphite, cerussite, and goethite. The main oxidation in the study area involved the direct replacement of primary MVT stratabound sulphide ore. In comparison with other non-sulphide deposits of the Bongará District, the Florida Canyon deposit can be correlated with the Cristal deposit by means of the original sulphides preserved in the supergene textures, the similar hosting dolostone layer, lack of hydrozincite, and depth of emplacement, which are characteristics that contrast with those of the Mina Grande deposit, also inserted in the Pucará Group. The Florida Canyon supergene mineral assemblage is a paleoclimatic indicator suggesting a humid tropical climate during the late Miocene in the Peruvian Amazon region.