I have seen platinum and palladium with native silver and gold naturally occurring in carbonate (mostly calcite) veins, associated with selenium.
My recent tests have shown large amounts of CO2 in out gassing when the prepared mesh is heated above ~250°C. This can only come from the decomposition of the CaCO3, leaving CaO in its place. If the Calcite is an active catalyst in the loading of hydrogen into the Ni substrate, its conversion to the oxide may actually inhibit the desired result. This experiment gets trickier the more we learn.