Variations in REE and Sr-isotope chemistry of carbonate gangue, Castellanos Zn–Pb deposit, Cuba

Abundances of REEs and Sr, the magnitudes of the positive Eu anomalies (Eu/Eu*) and 87Sr/86Sr values in carbonate gangue of the Late Jurassic Castellanos SEDEX Zn–Pb deposit vary both laterally and, to a lesser extent, vertically. At the vent-distal site Sr, La and Eu/Eu*, determined by laser ablation microprobe analyses of individual grains, increase slightly from the base of the deposit upward. The La/Lu ratio only increases significantly in the upper part of the section. Both Sr/(Sr+Ca) and La/(La+Ca) in total carbonate, determined by ICP spectrometry, remain relatively constant or increase slightly from the base upward at the vent-distal location. Values of Sr/(Sr+Ca) correlate reasonably well with La/(La+Ca) throughout most of the vent-distal section. The Sr content and Eu/Eu* are higher and La much lower at the vent-proximal location and all three exhibit very large within-sample variations. La does not correlate with Sr at the vent-proximal site. The 87Sr/86Sr values at both distal and proximal sites lie between that for Jurassic (Oxfordian) seawater and that of a hydrothermal fluid with a 87Sr/Sr86 value estimated from 87Sr/86Sr in the carbonate gangue of Cu mineralization in a nearby feeder channel beneath a small Zn–Pb deposit similar to Castellanos. Mixing of the REE- and Sr-enriched hydrothermal component with seawater in proportions calculated from the Sr-isotope data will explain the small vertical variations throughout most of the vent-distal section. However, the very low La abundances, the decoupling of the La from Sr and Eu/Eu* and the extreme micro-scale heterogeneity of REE distribution at the vent-proximal site suggest extensive leaching, redistribution and removal of REEs from early-stage carbonates by late-stage hydrothermal fluids at that location.