RE: I wonder what highly silicified"Highly silicified" simply means that the rocks have undergone a high degree of hydrothermal alteration; in this case, a large amount of silica has been added. In these types of systems, the hydrothermal fluids that carry and deposit the gold (and other metals) also change the chemical makeup of the host rocks as they pass through them - they remove some elements, add others. So evidence of strong alteration is a positive sign (but not necessarily a guarantee of mineralization nearby).
At those grades, there is no guarantee of visible gold, so I wouldn't read too much into that. The gold could be realtively fine and evenly distributed through the rock and therefore not easily visible.
gm2