Thermochronologic constraints on the timing of Mississippi Valley-type ore formation from apatite fission track analysis

Apatite fission track studies have been initiated in three major Mississippi Valley-type (MVT) Pb-Zn ore districts. Data from Precambrian basement and Cambrian sedimentary rocks near the Viburnum Trend of southeast Missouri indicates that the mid-continent region has been heated in the Mesozoic to temperatures similar to those estimated for ore formation, followed by final cooling in the Late Cretaceous. The Pine Point district, Canada, has been heated to temperatures comparable to those inferred for sulphide precipitation for an appreciable time only once since the Devonian, with cooling from these temperatures at some time after 100 Ma. The Lennard Shelf district, Australia, has probably experienced two major phases of regional heating following Devonian sedimentation. Peak temperatures estimated from well sequences for Late Paleozoic/Early Mesozoic burial are generally less than those estimated for ore formation. However, an earlier phase of heating to higher temperatures is predicted for the Late Devonian/Early carboniferous. These studies indicate that major MVT ore districts have been regionally heated in the past to temperatures similar to those estimated for ore formation and suggest that MVT ore emplacement may be related to broad regional heating rather than to discrete fluid pulses. (author)