Epithermal paleosurfaces

Many active volcanic-hydrothermal and geothermal systems are characterized by distinctive surface and near-surface landforms and products, which are generated during discharge of a spectrum of fluid types under varied conditions. Remnants of most of these products are preserved in some of their less-eroded, extinct equivalents: epithermal deposits of high-sulfidation (HS), intermediate-sulfidation (IS), and low-sulfidation (LS) types. Steam-heated alteration occupying vadose zones and any underlying silicified horizons formed at paleogroundwater tables characterize HS, IS, and LS deposits as do hydrothermal eruption craters and their subaerial or shallow sub-lacustrine breccia aprons and laminated infill. Although rarely recognized, HS, IS, and LS systems can also contain finely laminated, amorphous silica sediments that accumulated in acidic lakes and mud pots and, exclusive to HS systems, in hyperacidic crater lakes. In contrast, silica sinter and more distal carbonate travertine are hot spring discharge products confined mainly to LS and IS settings, as both form from near-neutral-pH liquids. Hydrothermal chert deposition and sediment silicification can take place in shallow, lacustrine rift settings, also largely restricted to LS and IS deposits. These surface and near-surface hydrothermal products are typically metal deficient, although mercury concentrations are relatively commonplace and were formerly exploited in places. Nonetheless, sinters, hydrothermal eruption craters, and silicified lacustrine sediments may contain anomalously high precious metal values; indeed, the last of these locally constitutes low-grade, bulk-tonnage orebodies. The dynamic nature of epithermal paleosurfaces, caused by either syn-hydrothermal aggradation or degradation, can profoundly affect deposit evolution, leading to either eventual burial or telescoping of shallower over deeper alteration +/- precious metal mineralization. Formational age, tectonic and climatic regime, hydrothermal silica content and texture, and post-mineralization burial history combine to determine the preservation potential of paleosurface products. Proper identification and interpretation of paleosurface products can facilitate epithermal precious metal exploration. Proximal sinters and hydrothermal eruption craters may mark sites of concealed epithermal mineralization, whereas paleogroundwater table silicification and steam-heated blankets can be more widely developed and, hence, less diagnostic. Epithermal precious metal deposits may immediately underlie paleosurface features but are commonly separated from them by up to several hundred vertical meters, especially in the case of IS deposits. Furthermore, the tops of concealed, particularly IS epithermal orebodies in any particular district, irrespective of whether or not paleosurface features are preserved, can also vary by several hundred vertical meters, thereby imposing an additional exploration challenge. Precious metal contents of paleosurface products are unreliable but nonetheless potentially useful guides to concealed deposits. However, sub-paleosurface geochemical anomalism, particularly for arsenic and antimony, may indicate proximity to subjacent ore.