Although mining is banned in Puerto Rico, this study provides insight and a comparison to important economic deposits throughout the Caribbean region. These new mineralogical and geochemical constraints enable a better understanding of geogenic exposure issues and geochemical background associated with laterites in Mayagüez, Puerto Rico. Care should be taken using XRF data for Co and Ni determinations and should not be used exclusively for economic or geohealth decisions. These results provide context for ongoing environmental investigations of road sediment and potential broader health studies relating to geogenic exposure. Future work should involve local communities and health professionals to best capture community concerns, needs and comparative health data.
Initial XRF findings overestimate chromium concentrations and place it as an average of 10,677 ppm for the laterite samples. This is very different from the HR-ICP-MS results showing an average of 8,575.38 ppm Cr. The XRF results also found that most cobalt was under the detection limit. It had an average of 1,613 ppm. This contrasts largely with the HR-ICP-MS results that show an average of 2,773 ppm Co and traces in each sample. This shows the limits of X-ray fluorescence especially when it comes to measuring chromium and cobalt. The bedrock represents the laterite’s parent rock, and both are comparable to each other. After the process of laterization, vanadium within the soil increases by around 450%. More extremely, cobalt increased by 2,015%. There were also losses that occurred, the laterite lost about a quarter of its nickel content (from an average of 6259.0 ppm to 4453.8 ppm) as well as some significant cadmium. Weaver summarizes data from Heidenreich and Reynolds (1959) and states that the average concentrations are Ni (0.71 to 1.08 Wt%, mean: 0.88 Wt%; Co 0.06 to 0.11 Wt%, mean: 0.09 Wt%; Fe 12.47 to 29.45 Wt%, mean = 23.20 Wt%: and Cr 0.34 to 0.75 Wt%, mean: 0.59 Wt%) (Weaver, 1992). XRF data is generally consistent with these previous historical surveys of laterites and indicates metal concentrations of an average of 33.8 wt.% for Fe, 0.576 wt.% for Mn, 0.330 wt.% for Ni, 1.07 wt.% for Cr, and 0.161 wt.% for Co. However, there is notably less trace amounts of nickel and iron as well as much more amounts of cobalt (~2X).
There are some important differences to recognize between the XRF data and the ICP-MS data where both Co and nickel were underestimated by XRF. These inconsistencies show the limits of XRF and the importance of HR-ICP-MS in assessing both resources and potentially hazardous materials (e.g., Co). XRF may be a useful tool for exploration purposes, but care should be taken regarding using data for decisions on grade or waste.
These samples contained high levels of chromium and comparatively high levels of cobalt. Both of which may be of interest as resources, however the functional mining ban in Puerto Rico would prevent extraction and we do not advocate extraction in this case. Academically, cobalt is of particular interest and findings here may support other investigations of laterite by serving as a comparative material in general.
The cobalt, nickel, cadmium, zinc, copper and vanadium observed are likely of some environmental concern for the local population as a geogenic exposure to these metals may pose health concerns (ATSDR, 2025). The results of this investigation suggest further detailed work on the nature and variation of metal of potential environmental health concern should be conducted in the future.
ATSDR (2025) Toxicological Profiles.
Butt, C. R., Cluzel, D. (2013). Nickel laterite ore deposits: weathered serpentinites. Elements, 9(2), 123-128.
Heidenreich, W.L., and Reynolds, B.M. (1959). Nickel-cobalt-iron-bearing deposits in Puerto Rico: Bureau of Mines, Report Investigations 5532, 68 p.
Weaver, J. N.(1992) Mineral Resource Assessment of Puerto Rico. USGS Open-File 92-567.
We thank Dr. Marion Lytle for assistance with ICP-MS data acquisition. We thank J. Curtis, E. Krekeler, and H. Wudke for assistance in the field sampling. We thank Miami University Department of Geology and Environmental Earth Science for support to attend this meeting. K. Bensing was supported by a Miami University Honors College Fellowship and the Miami University Mineralogy & Petrology Career Development Fund.