Age and sources of the ore at Tharsis and Rio Tinto, Iberian Pyrite Belt, from Re-Os isotopes

Re-Os isotopes were used to constrain the source of the ore-forming element s of the Tharsis and Rio Tinto mines of the Iberian Pyrite Belt, and the ti ming of mineralization. The pyrite from both mines has similar Os and Re co ncentrations, ranging between 0.05-0.7 and 0.6-66 ppb, respectively. Re-187 /Os-188 ratios range from about 14 to 5161. Pyrite-rich ore samples from th e massive ore of Tharsis and two samples of stockwork ore from Rio Tinto yi eld an isochron with an age of 346 +/- 26 Ma, and an initial Os-187/Os-188 ratio of about 0.69. Five samples from Tharsis yield an age of 353 +/- 44 M a with an initial Os-187/Os-188 ratio of about 0.37. A sample of massive su lfide ore from Tharsis and one from Rio Tinto lie well above both isochrons and could represent Re mobilization after mineralization. The pyrite Re-Os ages agree with the paleontological age of 350 Ma of the black shales in w hich the ores are disseminated. Our data do not permit us to determine whet her the Re-Os isochron yields the original age of ore deposition or the age of the Hercynian metamorphism that affected the ores. However, the reasona ble Re-Os age reported here indicates that the complex history of the ores that occurred after the severe metamorphic event that affected the Iberian Pyrite Belt massive sulfide deposits did not fundamentally disturb the Re-O s geochronologic system. The highly radiogenic initial Os isotopic ratio ag rees with previous Pb isotopic studies. If the initial ratio is recording t he initial and not the metamorphic conditions, then the data indicate that the source of the metals was largely crustal. The continental margin sedime nts that underlie the deposits (phyllite-quartzite group) or the Volcanic r ocks (volcanogenic-sedimentary complex) in which the ores occur are plausib le sources for the ore-forming metals and should constrain the models for t he genesis of these deposits.