C-O-H-N Fluid Inclusions Associated with Gold-stibnite Mineralization in Low-grade Metamorphic Rocks, Mari Rosa Mine, Caceres, Spain

L. Ortega*, C. Beny and E. Vindel
* Dpto. Cristalografía y Mineralogía, Universidad Complutense, 28040 Madrid, Spain
GIS-BRGM CNRS, 45071 Orleáns Cedex-2, France

Abstract: The Mari Rosa mine lies within a low-grade Precambrian alternating series of black shales and metagreywackes in the Spanish Hercynian massif. There are two generations of mineralized veins: V2, gold-(stibnite)-bearing quartz veins, parallel to the main cleavage, and V3, stibnite-bearing quartz veins which postdate the main deformation event.

Four main types of inclusions have been identified. Type I, II and IV are aqueous-carbonaceous inclusions, with variable degrees of filling, while type III are non-aqueous and typically single-phase at room temperature. Except for type I (absent in V3), similar inclusions have been observed in both V2 and V3 veins. Gas compositions are always characterised by CH4-N2-CO2 assemblages, ranging from CO2-rich mixtures in the earliest inclusions (type I), to N2-rich mixtures in the latest inclusions (type IV).

Gold precipitation in V2 veins can be related to type I inclusions at T > 380 °C (TH = 300–380 °C). A subsequent drop in XCO2 and cooling are recorded in type II and III inclusions, interpreted to be the result of unmixing of a previously homogeneous fluid derived from type I. This boiling would provoke the precipitation of stibnite at 300 °C and 1 kbar. The type IV inclusions, which are the richest in H2O, represent a late fluid circulation at lower temperatures (TH = 190–280 °C).

Keywords: fluid inclusions • gold • stibnite • Caceres • Spain

Mineralogical Magazine; June 1991 v. 55; no. 379; p. 235-247; DOI: 10.1180/minmag.1991.055.379.12
© 1991, The Mineralogical Society
Mineralogical Society (www.minersoc.org)