| Abstract |
The Jaraña hot springs are located in a highland ( more than 4570 m), backarc environment in the southern Peruvian Andes, 90 km east of the present day volcanic front of the Central Andes. The Jaraña geothermal system is controlled by local structures and is hosted within silicified Miocene andesite lava that surrounds the valley containing hot springs and their precipitates. Hot springs occur as thermal pools of varied depths (5cm to 2m) with orange and red precipitates visible below the water surface. Previous geologic studies in the region have identified Cu-mineralization in drill core, suggestive of Cu-ore bodies at depth. In order to assess the potential interactions between the modern geothermal system and potential Miocene mineralization, this study examined the mineralogy and geochemistry of the subaqueous hot-spring precipitates. The subaqueous precipitates are banded and display nodular and colloform morphologies. The layers contain scattered domical shapes built by convex laminations. Furthermore, the laminations show markedly different colours (black, orange, white) and luster (metallic, vitreous), clearly suggestive of mineralogical and compositional diversity. X-ray diffraction patterns indicate a mixture of calcite and Fe-oxides. Petrographic analysis indicates calcite crystals, polymictic rock fragments, microbial filaments and diatoms. Cathodoluminescence was used to identify different stages of carbonate deposition, horizons of crystallization, and carbonate fragments. Four-acid digestion results show an inverse relationship between Fe and Ca concentration, consistent with varying proportions of Fe-oxide and carbonate material. Furthermore, in situ chemical analysis of individual horizons by laser-ablation ICPMS contributed to identifying variations in silica in vitreous horizons, abundances of Fe, Zn, Pb and Mo and the relation between them in carbonate layers. In addition, the banded horizons contained high levels of As ranging from 174 ppm to 7.12 wt%. Also, samples have variable concentrations of Cu (0.2 – 268 ppm) and show a positive correlation with base metals such as Zn. However, in samples with high Fe concentrations, Cu is uncorrelated to Fe. This research demonstrates that the Peruvian iron–carbonate subaqueous precipitates contain metal concentration in distinct compositionally laminated horizons. Finally, trace element and metal enrichments in the iron–carbonates may indicate interaction with deeper Cu ore bodies. |