Fe-, Fe,Mn- and Fe,Mg-chlorite: a genetic linkage to W, (Cu, Mo) mineralization in the magmatic-hydrothermal system at Borralha, northern Portugal

• Type: Article in International Scientific Journal
• Date: 2018

Authors

Bobos, I (Author) FCUP; Noronha, F (Author) FCUP; Mateus, A (Author) Other

Journal

Title: Mineralogical Magazine

Vol. 82

Pages: S259-S279

ISSN: 0026-461X

Publisher: MINERALOGICAL SOCIETY

Indexing

ISI Web of Knowledge - 8 Citations

Scopus - 9 Citations

Other information

Authenticus ID: P-00N-ZDC

DOI: 10.1180/minmag.2017.081.104

Abstract (EN): A genetic linkage between W, (Cu, Mo)-mineralization and chlorite minerals, and the discrimination of different mineralization events in the magmatic-hydrothermal system of Borralha, northern Portugal, is discussed on the basis of textural relationships, crystal chemistry and stable isotopic data obtained from chlorite. Chlorite minerals were identified in assemblages with quartz, feldspars, tungstates and sulfides. X-ray diffraction studies of selected chlorite minerals shows a trioctahedral structural type. Electron probe micro-analyses identified four different compositions and associations: (1) Fe,Mn-chlorite with scheelite I; (2) Fe-chlorite with wolframite+scheelite II +/- sulfide; (3) Fe,Mg-chlorite with molybdenite+bismuthinite; and (4) Mg,Fe-chlorite with chalcopyrite. The composition of Fe-chlorite (Al3.01Fe0.253+Fe7.952+Mn0.26Mg0.19)(11.66)(Si5.44Al2.56)(8)O-20(OH)(8) corresponds to daphnite and Fe,Mn-chlorite (Al2.69Fe0.023+Fe7.542+Mn1.08Mg0.62)(11.89)(Si5.31Al2.68)(4)O-20(OH)(8) to a mixed composition between daphnite and amesite. The Fe,Mg-chlorite (Al2.89Fe0.243+Fe6.422+Mn0.21Mg2.08)(11.84) (Si5.31Al2.79)(8)F0.31O20(OH)(8) corresponds to ripidolite and Mg,Fe-chlorite (Al2.63Fe0.373+Fe1.722+Mn0.01Mg6.40Ca0.26)(11.39)(Si6.02Al1.98)(8)O-20(OH)(8) to pychnochlorite. Chlorite geothermometry estimates a temperature for Fe,Mn-chlorite (scheelite I) from 400 degrees C to 500 degrees C, for Fe-chlorite (Mn-rich wolframite+scheelite II +/- sulfide) from 250 to 350 degrees C, for Fe,Mg-chlorite (Mo-mineralization) from 200 degrees C to 250 degrees C and for Mg,Fe-chlorite at similar to 150 degrees C. Oxygen isotopes (V-SMOW) yielded values of +3.8 (1 sigma) (Fe-chlorite), +6.91 (1 sigma) (Fe,Mn-chlorite) and +1.5 (1 sigma) (Fe,Mg-chlorite). The calculated O-18(F) of Fe- and Fe,Mg-chlorite is similar to+3.75 (1 sigma) and +1.45 (1 sigma) for the mineralizing fluid, whereas for Fe,Mn-chlorite it is +8.17 (1 sigma). The delta O-18 data obtained from quartz in W- and Mo-mineralization yielded values of +12.6 and +11.4 (1 sigma), whereas for adularia delta O-18 is about +10 (1 sigma). These estimates allow us to conclude that the Fe,Mn-chlorite crystallized from a magmatic-hydrothermal fluid, whereas the Fe- and Fe,Mg-chlorite quartz and adularia resulted from a mixed contribution between meteoric and magmatic-hydrothermal fluid.

Language: English

Type (Professor's evaluation): Scientific

No. of pages: 21