Lithocaps - characteristics, origins and significance for porphyry and epithermal exploration

Lithocaps are subsurface, broadly stratabound alteration domains that are laterally and vertically extensive. They form when acidic magmatic-hydrothermal fluids react with wallrocks during ascent towards the paleosurface. Although lithocaps typically have steeply-dipping structural roots, there is a significant component of lateral fluid flow involved in lithocap formation, either through permeable aquifers and/or a well-developed fracture mesh. Lithocaps can have lateral dimensions greater than 10 km and thicknesses of more than 1 km. In ancient settings, partially eroded lithocaps are typically exposed as silicified ridges and cliffs. These features do not mark the original paleosurface – instead they are erosional remnants of what was once an extensive subsurface alteration domain that may have been capped by low temperature argillic- and/or propylitic-altered rocks. High sulfidation state mineralisation typically occurs in silicicaltered rocks within lithocaps, either as stratabound replacements, veins and/or breccia cement. The quartz-rich mineralized domains can produce a resistivity high. Pyrite is ubiquitous in lithocaps prior to weathering, and can yield complicated chargeability responses, some of which may be associated with mineralization. The alteration assemblages are invariably magnetite-destructive, and can obscure the magnetic signature of an underlying porphyry deposit. Combining SWIR and whole rock geochemistry can provide effective vectoring tools within lithocaps.