US Geological Survey


Date of this Version



PART IV OF 1973 PHD THESIS, Department of Geological Sciences, Brown University, Providence, Rhode Island 02912


Massive metallic-sulfide deposits were implaced in the Leadville Limestone {Mississippian) of Central Colorado less than 70 million years ago. The thermal fluids which precipitated ore at Gilman, Colorado have regionally altered the Leadville Limestone. The sequence of alteration began with the recrystallization of limestone to a dark medium~grained dolomite containing a homogeneous distribution of Fe. This Was followed by partial recrystallization of the medium-grained dolomite to coarse-clear dolomite which contains an inhomogeneous distribution of Fe. The resulting banded rock is known as "zebra rock" . Precipitation of Si0 2 (jasperoid) occurred next. The jaspe=oid formed prior to the deposition of sulfides (O'Neil, 1951) which is the final event of the hydrothermal epoch. The alteration sequence is essentially the same everywhere. The one exception is the presence of ferroan-calcite in localities outside the Leadville-Gilman hydrothermal aureole. This calcite was deposited prior to the formation of the homogen· eous ferroan-dolomite. Regional alteration of sediment by thermal fluids is herein proposed as a fundamental diagenetic process. This process may be the s.ource of ferroan-carbonate cement which. fills fractures and vugs in numerous sedimentary sequences.