Five stream-sediment and heavy-mineral concentrate geochemical anomalies have been identified in an area of the lower Back Rio Grande, 10 km south-southwest of Port Antonio, in Portland Parish. They were discovered by a detailed follow-up study of a regional stream sediment and heavy-mineral concentrate anomaly in gold, arsenic, tellurium, antimony, copper and barium, identified during a reconnaissance geochemical survey of Jamaica's Cretaceous Inliers and the Early Tertiary Wagwater Trough, in 1986. Participants in the current studied included employees of the Jamaican Ministry of Production, Mining and Commerce (Geological Survey Division) and Canadian staff members of CIDA Project 504/12713-142061 (Metallic Minerals Survey -- Phase II) and of Canadian Executing Agency Le Groupe Minier SIDAM (1992) Inc.
All the geochemical anomalies are in areas underlain by the Upper Cretaceous Bellevue Volcanics group, a strongly-faulted and locally intensively-altered unit of porphyritic andesite flows and volcaniclastic and closely-related calcareous rocks. Four of the anomalies are characterized by anomalous gold, or of the pathfinder elements arsenic and antimony; one exception is a zinc-cadmium-lead anomaly, with subordinate precious-metal values, located in the northwest of the study area. Within the bounds of one of these anomalies, a sample of sheared and silicified andesite, veined with quartz, was collected from an outcrop at the site of a stream sediment sample. This sample has returned a gold analysis of 1884 ppb (0.06 oz/ton). The site has been named the 6X occurrence, and the associated Little Root South geochemical anomaly is considered the most promising in the study area.
No intrusive source for the alteration, or the metals, has been mapped in the study area. However, geophysical and geochemical data suggest that felsic-intermediate intrusions may underlie the volcanics, and one sample of highly-deformed rock has been identified in thin section as a granodiorite.
The study has successfully enhanced the status of what was previously classed as a Priority 2 anomaly. This has important implications for similarly-classified anomalies elsewhere in Jamaica, and for the island's metallic-minerals potential as a whole.