Agates from the Bighorn district in Montana (USA), the so-called Dryhead
area, and their adjacent host rocks have been examined in the present
study. Analyses by XRD, polarizing microscopy, LA-ICP-MS,
cathodoluminescence (CL), SEM and of oxygen isotopes were performed to
obtain information surrounding the genesis of this agate type.
Investigations of the agate microstructure by polarizing microscopy and
CL showed that chalcedony layers and macrocrystalline quartz crystals
may have formed by crystallization from the same silica source by a
process of self-organization. High defect densities and internal
structures (e. g. sector zoning) of quartz indicate that crystallization
went rapidly under non-equilibrium conditions. Most trace-element
contents in macrocrystalline quartz are less than in chalcedony due to a
process of `self-purification', which also caused the formation of Fe
oxide inclusions and spherules.
Although the agates formed in sedimentary host rocks, analytical data
indicate participation of hydrothermal fluids during agate formation.
Trace elements (REE distribution patterns, U contents up to 70 ppm) and
CL features of agate (transient blue CL), as well as associated minerals
(fluorite, REE carbonates) point to the influence of hydrothermal
processes on the genesis of the Dryhead agates. However, formation
temperatures <120 degrees C were calculated from O-isotope compositions
between 28.9 parts per thousand (quartz) and 32.2 parts per thousand
(chalcedony).