EFFECTS OF HYDROTHERMAL ALTERATION ON THE MAGNETIZATION OF THE OLIGOCENE CARPENTER RIDGE TUFF, BACHELOR CALDERA, SAN-JUAN MOUNTAINS, COLORADO

Intracaldera Oligocene Carpenter Ridge Tuff fills the Bachelor caldera in the central San Juan caldera complex and hosts mineral deposits of the Creede mineral district. The Carpenter Ridge Tuff and unaltered p ortions of its intracaldera Bachelor Mountain Member, have strong, hig h-coercivity, reverse magnetizations with average magnetic susceptibil ity (MS) and natural remanent magnetization (NRM) of 6 x 10(-3) volume SI and 8 A/m, respectively. Oxide phenocrysts in these rocks are tita nomagnetite and ilmenite; however, magnetization appears to be control led by microcrystic titanomaghemite based on thermal demagnetization u nblocking temperatures and Curie temperatures between 580-degrees-C an d 620-degrees-C. Much of the intracaldera tuff was affected by potassi c metasomatism, a type of hydrothermal alteration characterized by add ition of K and loss of Ca and Na, between 27.3 Ma and 25.1 Ma. Potassi c metasomatism resulted in the replacement of original feldspars by po tassium feldspar and quartz, oxidation of the original oxide phenocrys ts to hematite and rutile, and consequent suppression of MS and NRM by a factor of 5. Also present, however, are metasomatized rocks that ha ve high magnetizations even though their original oxide phenocrysts we re destroyed; values of MS and NRM are similar to those of outflow tuf f. Such rocks are suspected of containing secondary magnetite on the b asis of (1) Curie and thermal unblocking temperatures at and below 580 -degrees-C, indicating that magnetization is carried by magnetite but that primary microcrysts of maghemite were destroyed, and (2) observat ion in one sample of magnetite rimming cores of hematite+rutile. The p resence of secondary magnetite in metasomatized rocks requires a local shift to more reducing fluid chemistry, although we are unable to det ermine whether this shift occurred during potassic metasomatism or dur ing later alteration or ore deposition. These altered rocks may record a change from a pervasive alteration by alkaline, oxidizing fluids to a vein-controlled alteration dominated by more reduced fluids in whic h magnetite was stable.