MODERN SILICEOUS HOT SPRING PRECIPITATES: POSSIBLE ANALOGS FOR ANCIENT CHERTS

Chert deposits are fairly common in the rock record; nevertheless relatively little is known about modern geothermal analogs. As a result, three diverse suites of Yellowstone's siliceous hot spring precipitates were chosen for detailed petrographic investigation: 1) modern precipitates from Cistern Spring, an active opal-A precipitating system in the Norris Geyser Basin of Yellowstone, 2) older siliceous sinter from a largely inactive deposit at Pork Chop Geyser, and 3) successively older siliceous precipitates from an ~11m long core that penetrates a relict hot spring deposit in the Lower Geyser Basin. Detailed petrographic study of these modern siliceous hot spring precipitates may further facilitate the recognition of ancient, siliceous hydrothermal deposits.

Eight generic depositional facies have been identified in Yellowstone's siliceous hot springs and each exhibits distinctive petrographic fabrics: 1) vent, 2) proximal vent, 3) pool, 4) pool margin, 5) pool eddy, 6) discharge channel/flowpath, 7) debris apron, and 8) geyser. Siliceous hot spring precipitates form a natural progression between those with little biotic influence on the architecture and fabric (e.g., high temperature vent chalcedony) and those with high biotic influence (e.g., "shrubby" opaline precipitates from the spring flowpath). The abiotic end-member consists of translucent, hemispherical aggregates of length-fast chalcedony with no readily apparent evidence of preserved microbial body fossils. In contrast, siliceous shrubs are cm-scale spinose precipitates of opal-A that contain a diverse suite of preserved microbial remains (e.g., filaments, rods, etc.) and represent the biotic end-member. Thus, precipitates from the eight cumulative siliceous hot spring facies form a natural progression between those that are abiotic to those that have a strong passive(?) and/or active(?) biotic influence on the microfabric.