Temporal Variation in Stream-Sediment Geochemical Data from Jamaica
Robert G. Garrett
Applied Geochemistry Subdivision
Geological Survey of Canada, Ottawa
Stephen D. Amor
Waterdown, Ontario


In 1986 a joint CIDA-Government of Jamaica project, the Metallic Minerals Survey of Jamaica (Phase I), undertook a regional stream-sediment and heavy-mineral survey of selected parts of the island. Subsequently, in 1988, Hurricane Gilbert devastated many of the areas surveyed. That event, coupled with a requirement to know how reproducible stream-sediment data were in an actively-eroding mountainous tropical environment, led to a temporal variation study being undertaken between 1990 and 1992, during Phase II of the project. The study consisted of two parts: firstly, the resampling of 24 1986 sites in October and November 1990, within three areas (to study the effects of Hurricane Gilbert); and secondly, the repeated monthly sampling of six streams in the same three areas, from October or November 1990, for periods of 12-24 months, to study seasonal variations. After all fieldwork was complete, the sequence of the samples was randomized so that they were not analyzed in the order in which they were collected, and a broad suite of trace elements were determined. In this report, the data for Au and Cu are presented as they are of particular relevance to mineral exploration; in addition, Au is believed to be predominantly clastically dispersed, whereas Cu is both clastically and chemically dispersed.

The pre- and post-Gilbert comparison indicates no consistent enrichment or depletion in either Au or Cu, at any of the three areas, as a result of that catastrophic event; 1986/1990 ratios both greater than, and less than, unity are equally represented and range from 0.6 to 2.5, for Cu, and from 0.1 to 10.7 for Au. Furthermore, both 1986 and 1990 values fall within measured seasonal ranges for Au at all of the six sites, and for Cu at three.

The study of seasonal variations involved the triplicate sampling of nearby stream pairs, originally perceived as "anomalous" and "background", in three areas. The results indicate that for Cu, compositional variations from month to month exceed the natural inhomogeneity of the sample medium, though this is not the case for Au. Each of the six sites displays certain unique characteristics; at four, there is evidence of seasonal cyclicity in copper content, though at the other two, which were not both in the same area, two days of torrential rain in May 1991 were followed by a lasting change in compositional characteristics, much greater than the observed effects of Hurricane Gilbert, including an increase in Cu classification from "background" to "anomalous".

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