SILT FRACTION HEAVY-MINERAL DISTRIBUTIONS IN A LATERITIC ENVIRONMENT - THE RIVERS AND INSULAR SHELF OF NORTH-CENTRAL PUERTO-RICO

Mineralogical studies of sediments from the rivers and insular shelf i n north-central Puerto Rico examine the effects of lateritic weatherin g and assess the silt fraction for economically important heavy minera ls. This fraction, which is enriched in heavy minerals relative to the sand fraction, is mainly detrital but contains a strong authigenic co mponent. The detrital silt heavy-mineral fraction in the rivers is dom inated by an amphibole-garnet-pyroxene epidote assemblage. Amphiboles are more abundant in the Rio Cibuco; pyroxenes are more abundant in th e Rio de la Plata; and epidote and ilmenite are more abundant in the R io Grande de Manati. The authigenic silt heavy-mineral fraction is lar gely a product of the lateritic weathering and dominated by iron oxide s and alterites. Grains of bladed rutile and leached ilmenite are comm on. Spatial variability in silt-fraction mineralogy is considerable. W ithin the Rio Cibuco system variability is related to compositional di fferences in rapidly eroding source rocks. On the shelf, silt heavy-mi neral abundances are greatest at the river mouths and decrease seaward . Variability in the shelf samples is controlled primarily by source r ivers and shelf sorting processes. Compositional differences exist bet ween the silt heavy-mineral assemblages in the rivers and the shelf, a nd between the heavy-mineral assemblages in the silt- and sand-sized f ractions from these areas. Minerals of economic importance that occur in the silt fraction within the study area include cerargyrite, chromi te, gold, magnetite, and rutile.