Origin of the Rubian carbonate-hosted magnesite deposit Galicia, NW Spain: mineralogical, REE, fluid inclusion and isotope evidence

摘要

The Rubian magnesite deposit (West Asturian-Leonese Zone, Iberian Variscan belt) is hosted by a 100-m-thick folded and metamorphosed Lower Cambrian carbonate/siliciclastic metasedimentary sequence-the Candana Limestone Formation. It comprises upper (20-m thickness) and lower (17-m thickness) lens-shaped ore bodies separated by 55 m of slates and micaceous schists. The main (lower) magnesite ore body comprises a package of magnesite beds with dolomite-rich intercalations, sandwiched between slates and micaceous schists. In the upper ore body, the magnesite beds are thinner (centimetre scale mainly) and occur between slate beds. Mafic dolerite dykes intrude the mineralisation. The mineralisation passes eastwards into sequence of bedded dolostone (Buxan) and laminated to banded calcitic marble (Mao). These show significant Variscan exten-sional shearing or fold-related deformation, whereas neither Rubian dolomite nor magnesite show evidence of tectonic disturbance. This suggests that the dolomitisation and magnesite formation postdate the main Variscan deformation. In addition, the morphology of magnesite crystals and primary fluid inclusions indicate that magnesite is a neoformed hydrothermal mineral. Magnesite contains irregularly distributed dolomite inclusions (<50 mu m) and these are interpreted as relics of a metasomatically replaced dolostone precursor. The total rare earth element (REE) contents of magnesite are very similar to those of Buxan dolostone but are depleted in light rare earth elements (LREE); heavy rare earth element concentrations are comparable. However, magnesite REE chondrite normalised profiles lack any characteristic anomaly indicative of marine environment. Compared with Mao calcite, magnesite is distinct in terms of both REE concentrations and patterns. Fluid inclusion studies show that the mineralising fluids were MgO_2-NaCl-CaCl_2-H_2O aqueous brines exhibiting highly variable salinities (3.3 to 29.5 wt. percent salts). This may be the result of a combination of fluid mixing, migration of pulses of variable-salinity brines and/or local dissolution and replacement processes of the host dolostone. Fluid inclusion data and comparison with other N Iberian dolostone-hosted metasomatic deposits suggest that Rubian magnesite probably formed at temperatures between 160 and 200 deg C. This corresponds, at hydrostatic pressure (500 bar), to a depth of formation of ~5 km. Mineralisation-related Rubian dolomite yields delta~(l8)O values (delta~(18)O: 12.0-15.4 per thousand, mean: 14.4 + - 1.1 per thousand) depleted by around 5 per thousand compared with barren Buxan dolomite (delta~(18)O: 17.1-20.2 per thousand, mean: 19.4 + - 1.0 per thousand). This was interpreted to reflect an influx of ~(18)O -depleted waters accompanied by a temperature increase in a fluid-dominated system. Overlapping calculated delta~(18)O_(fluid) values (approx +5 per thousand at 200 deg C) for fluids in equilibrium with Rubian dolomite and magnesite show that they were formed by the same hydrothermal system at different temperatures. In terms of delta~(13)C values, Rubian dolomite (delta~(13)C: -1.4 to 1.9 per thousand, mean: 0.4 + - 1.3 per thousand) and magnesite (delta~(13)C: -2.3 to 2.4 per thousand, mean: 0.60 + - 1.0 per thousand) generally exhibit more negative delta~(13)C values compared with Buxan dolomite (delta~(13)C: -0.2 to 1.9 per thousand, mean: 0.8 + - 0.6 per thousand) and Mao calcite (delta~(13)C: -0.3 to 1.5 per thousand, mean: 0.6 + - 0.6 per thousand), indicating progressive modification to lower S C values through interaction with hydrothermal fluids. ~(87)Sr/~(86)Sr ratios, calculated at 290 Ma, vary from 0.70849 to 0.70976 for the Mao calcite and from 0.70538 to 0.70880 for the Buxan dolostone. The ~(87)Sr/~(86)Sr ratios in Rubian magnesite are more radiogenic and range from 0.71123 to 0.71494. The combined delta~(8)C -delta~(13)C and ~(87)Sr/~(86)Sr r data indicate that the magne-site-related fluids were modifi