Silt fraction heavy-mineral distributions in a lateritic environment: the rivers and insular shelf of north-central Puerto Rico

• Published in: Sedimentary geology Vol. 95; no. 3; pp. 251 - 268
• Main Authors: Poppe, L.J., Commeau, J.A., Luepke, Gretchen
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.03.1995
• ISSN: 0037-0738; 1879-0968
• DOI: 10.1016/0037-0738(94)00114-A

Mineralogical studies of sediments from the rivers and insular shelf in north-central Puerto Rico examine the effects of lateritic weathering and assess the silt fraction for economically important heavy minerals. This fraction, which is enriched in heavy minerals relative to the sand fraction, is mainly detrital but contains a strong authigenic component. The detrital silt heavy-mineral fraction in the rivers is dominated by an amphibole-garnet-pyroxene-epidote assemblage. Amphiboles are more abundant in the Rio Cibuco; pyroxenes are more abundant in the Rio de la Plata; and epidote and ilmenite are more abundant in the Rio Grande de Manati. The authigenic silt heavy-mineral fraction is largely a product of the lateritic weathering and dominated by iron oxides and alterites. Grains of bladed rutile and leached ilmenite are common. Spatial variability in silt-fraction mineralogy is considerable. Within the Rio Cibuco system variability is related to compositional differences in rapidly eroding source rocks. On the shelf, silt heavy-mineral abundances are greatest at the river mouths and decrease seaward. Variability in the shelf samples is controlled primarily by source rivers and shelf sorting processes. Compositional differences exist between the silt heavy-mineral assemblages in the rivers and the shelf, and between the heavy-mineral assemblages in the silt- and sand-sized fractions from these areas. Minerals of economic importance that occur in the silt fraction within the study area include cerargyrite, chromite, gold, magnetite, and rutile.