When doing a ICP assay, you can choose between 2 or 4 Acid Digestion.  Here is the basic difference:

2-acid digestion is good for sulfide and carbonate material but does not digest the more resistant minerals such as unaltered silicates and some oxides. The 4-acid digestion is a near “total” digestion and will digest almost everything except for the very resistant minerals such as barite and chromite. Unless you have any of the resistant minerals in your sample or you have a zinc silicate, the 2-acid should be sufficient.