Mineralogy, fluid inclusions, and oxygen isotope geochemistry signature of wolframite to scheelite and fe,mn chlorite veins from the w, (Cu,mo) ore deposit of borralha, portugal

Abstract

Scheelitization of Mn‐bearing wolframite, scheelite, quartz, and Fe,Mn‐chlorite veins was identified in the W, (Cu,Mo) ore deposits of Borralha, by optical microscopy, electron‐microprobe analysis, and stable isotope geochemistry. Fluid inclusions derived scheelite crystallization temperature was compared with the oxygen isotope temperature estimated. Scheelite was formed mainly during stage I from a low salinity aqueous‐carbonic fluid dominated by CO2, where the homogenization temperature (Th) decreased from 380 °C to 200 °C (average of 284 °C). As temperature decreased further, the aqueous‐carbonic fluid became dominated by CH4 (Stage II; (average Th = 262 °C)). The final stage III corresponds to lower temperature mineralizing aqueous fluid (average Th = 218 °C). In addition, salinity gradually decreased from 4.8 wt.% to 1.12 wt.%. The δ18 OFluid values calculated for quartz‐water and wolframite‐water fractionation fall within the calculated magmatic water range. The ∆quartz‐scheelite fractionation occurred at about 350–400 °C. The ∆chlorite‐water fractionation factor calculated is about +0.05‰ for 330 °C, dropping to −0.68‰ and −1.26‰ at 380 °C and 450 °C, respectively. Estimated crystallizing temperatures based on semiempirical chlorite geothermometers range from 373 °C to 458 °C and 435 °C to 519 °C. A narrower temperature range of 375 °C to 410 °C was estimated for Fe,Mn‐chlorite crystallization.