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

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 (T-h) decreased from 380 degrees C to 200 degrees C (average of 284 degrees C). As temperature decreased further, the aqueous-carbonic fluid became dominated by CH4 (Stage II; (average T-h = 262 degrees C)). The final stage III corresponds to lower temperature mineralizing aqueous fluid (average T-h = 218 degrees C). In addition, salinity gradually decreased from 4.8 wt.% to 1.12 wt.%. The delta O-18(Fluid) values calculated for quartz-water and wolframite-water fractionation fall within the calculated magmatic water range. The increment (quartz-scheelite) fractionation occurred at about 350-400 degrees C. The increment (chlorite-water) fractionation factor calculated is about +0.05 parts per thousand for 330 degrees C, dropping to -0.68 parts per thousand and -1.26 parts per thousand at 380 degrees C and 450 degrees C, respectively. Estimated crystallizing temperatures based on semi-empirical chlorite geothermometers range from 373 degrees C to 458 degrees C and 435 degrees C to 519 degrees C. A narrower temperature range of 375 degrees C to 410 degrees C was estimated for Fe,Mn-chlorite crystallization.

Automated summary

Scheelitization of Mn-bearing wolframite, scheelite, quartz, and Fe,Mn-chlorite veins was observed in the W, (Cu,Mo) ore deposits of Borralha. The formation of scheelite was mainly influenced by a low salinity aqueous-carbonic fluid dominated by CO2 during stage I, then dominated by CH4 during stage II, and finally a lower temperature mineralizing aqueous fluid during stage III. The salinity gradually decreased throughout the stages. The analysis of quartz-water and wolframite-water fractionation indicated that they fell within the magmatic water range. The fractionation of chlorite-water decreased with increasing temperature.