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 Sr-87/Sr-86 values in carbonate gangue of the Late Juras sic Castellanos SEDEX Zn-Pb deposit vary both laterally and, to a less er extent, vertically. At the vent-distal site Sr, La and Eu/Eu, dete rmined by laser ablation microprobe analyses of individual grains, inc rease slightly from the base of the deposit upward. The La/Lu ratio on ly increases significantly in the upper part of the section. Both Sr/( Sr + Ca) and La/(La + Ca) in total carbonate, determined by ICP spectr ometry, remain relatively constant or increase slightly from the base upward at the vent-distal location. Values of Sr/(Sr;Ca) correlate rea sonably well with La/(La + Ca) throughout most of the vent-distal sect ion. The Sr content and Eu/Eu are higher and La much lower at the ven t-proximal location and all three exhibit very large within-sample var iations. La does not correlate with Sr at the vent-proximal site. The Sr-87/Sr-86 values at both distal and proximal sites lie between that for Jurassic (Oxfordian) seawater and that of a hydrothermal fluid wit h a Sr-87/Sr-86 value estimated from Sr-87/Sr-86 in the carbonate gang ue of Cu mineralization in a nearby feeder channel beneath a small Zn- Pb deposit similar to Castellanos, Mixing of the REE-and Sr-enriched h ydrothermal 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-sca le heterogeneity of REE distribution at the vent-proximal site suggest extensive leaching, redistribution and removal of REEs from early-sta ge carbonates by late-stage hydrothermal fluids at that location. (C) 1998 Elsevier Science B.V.