Apatite fission track dating of fluorite ore veins from Santa Catarina state, Brazil: a complex hydrothermal evolution

The fluorite of Santa Catarina that occurs in veins cutting Precambrian gra nitic rocks of coastal Brazil has been difficult to date by Rb/Sr, K/Ar and Sm/Nd methods. New fission track dating of apatite in granites next to the veins yields ages of 144-76 Ma, which are related to the opening of the So uth Atlantic Ocean. Four groups of fission track ages were identified: the ca 145 Ma group is a hydrothermal event that preceded fluorite mineralizati on; the second group of ages, 131-107 Ma, records the first hydrothermal mi neralizing event; the third group, 98-93 Ma, represents the second hydrothe rmal mineralizing event; and the fourth group, 89-76 Ma, dates the last hyd rothermal mineralizing event. As shown by previous studies, the temperature s of these events varied from 170 degrees to 70 degrees C, but the last hyd rothermal event occurred during a gradual cooling. The smaller lengths of t he confined fission tracks from the fourth event support this interpretatio n. These results are based on sixteen carefully selected samples from four veins ranging from 1 to 4 m in thickness. The ages of these samples were es tablished using the standard methods of fission track dating. Our study cle arly demonstrates the value of apatite fission track dating for deposits wh ose mineralization occurred over a long time span at a wide range of temper atures. (C) 1999 Elsevier Science Ltd. All rights reserved.