Behaviour of rare earth elements in geothermal systems of New Zealand

Abstract

Rare earth element (REE) patterns of hydrothermally altered rhyolite from geothermal systems located in the Taupo Volcanic Zone in the North Island of New Zealand provide evidence of REE mobility. REE trends of unaltered rhyolites are characterised by moderate LREE enrichment ((La/Lu) cn = 3.84 to 5.62) and pronounced negative Eu anomalies. In contrast, REE patterns of hydrothermally altered rhyolites commonly exhibit different signatures and may be placed into four chemically and petrographically distinct categories. Rocks with clay + quartz + feldspar + calcite (±zeolites, epidote, sphene, chlorite, opaque minerals) assemblages typically display patterns subparallel to fresh rock, whereas, samples which contain quartz + chlorite, or quartz + clay + zeolite assemblages have flat patterns without Eu anomalies, and highly silicified samples are characterised by depleted, bowed REE trends. These patterns may be produced by interaction with alkaline or acid fluids. A fourth group of very intensely altered samples, affected by interaction with acid fluids, exhibits unusual REE trends with highly enriched HREE and depleted LREE, or depleted HREE. These results indicate that some of the REE released by the breakdown of primary phases during alteration are transported away in the fluid. In addition, the degree of depletion is positively correlated with alteration intensity and the fluid/rock ratio. The similarity of REE patterns resulting from alteration by alkaline and acid fluids suggests that the shape of the REE trends is controlled principally by fluid/rock ratios and secondarily by mineralogy. The REE are retained in rocks with a diverse alteration mineralogy, whereas in samples with only one dominant alteration phase (e.g. quartz) it is more probable that not all REE liberated during alteration can be accommodated in the altered rock. Eu commonly behaves differently from the other REE, possibly due to the dominance of Eu 2+.