NEW MAJOR DISPERSED SOURCE OF SWEETWATER AGATES

Grayish pebbles of translucent chalcedony with internal manganese dendrites are common in the Miocene Split Rock Formation, which fills basins in the Granite Mountains of Wyoming. These pebbles are called Sweetwater agates, and they are used in lapidary work. The provenance of these agates has been somewhat uncertain. J.D. Love suggested that the agates formed within the lower gravel layer of the Split Rock Formation, and that the smaller, smoother agates in the uppermost Split Rock layer may have been reworked from the lower layer after differential subsidence. Love stated that no other agate source was known.

I found extensive sources of rough gray dendritic chalcedony in many places at contacts of the Archean Granite Mountains and the adjacent Miocene basin fills. The deposits are fractured irregular layers up to perhaps 8 cm thick. The chalcedony is sometimes found bonded within Archean bedrock layers. The deposits usually show a variety of silicas, including lithified cream-colored ooze, lighter common opal and gray translucent chalcedony. Black dendrites are common. The deposits are typically found at elevations a few meters above the present-day Miocene-Archean contacts.

The Miocene beds are alkaline and contain amorphous volcanic silica. This combination would yield large concentrations of dissolved silica. Exposed Archean rocks weathering under acid conditions produce free silica. Silica would precipitate if waters from these two sources commingled, particularly with net evaporation.