Isotopic Exchange During Tectonic Veining: Example from Absaroka Sheet in Wyoming Overthrust Belt

Joyce M. Budai, David V. Wiltschko

Multiple stages of tectonic veining are preserved in the Mississippian Madison Group within the Absaroka thrust sheet. Three groups of veins have been defined on the basis of structural relationships and stable isotopic character. Group I veins consist of mineralized stylolite and joint surfaces which are oriented normal to both bedding and tectonic transport. Group II veins are filled fractures oriented normal to bedding and parallel to transport. Group III veins occur only in thin-bedded limestones of the lower Madison Group and are mineralized bedding-parallel slip surfaces. Calcite twinning strains from each group indicate that, with rare exceptions, all veins formed pretectonically or syntectonically.

Group I veins have highly depleted oxygen compositions and moderately depleted carbon relative to average Madison Group values (^dgr¹⁸O = -16.5^pmil PBD; ^dgr¹³C = 1.0^pmil). In group II veins, ^dgr¹⁸O is depleted and carbon varies systematically from that in the surrounding host rock to more depleted values, depending on stratigraphic position (^dgr¹⁸O = -14.5^pmil, ^dgr¹³C = -4.0 to 6.0^pmil). Group III veins have oxygen and carbon values similar to the host rock composition (^dgr¹⁸O = -5.0^pmil, ^dgr¹³C = 4.0 to 6.0^pmil). In each vein group, the host rock immediately adjacent to vein margins has undergone some amount of exchange with vein fluids; however, the extent of exchange varies between groups. The ost extensive exchange has occurred adjacent to group III veins where the host rock is actually isotopically lighter than calcite in the veins. The patterns of isotopic alteration, when paired with three distinct vein compositions, suggest that fluid flow was highly channelized during thrusting of the Absaroka sheet and water-rock ratios were different for each stage of veining. Furthermore, stratigraphic position exerted a control on distribution of vein type as well as degree of water-rock interaction.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.