Hi Gonzalo,
While I agree that most of the trepanation tools were obsidian, the Andeans worked true bronze and arsenic bronze, as well as copper. I think they started working copper alloys around 1000 CE (
link to article). The Inka certainly were using metallic maces when the Conquistadors arrived, and they also used bronze in securing their stonework.
I agree that a gold tumi is primarily a symbolic tool, but if they made bronze tumis, those would be very different. Regardless, the major weapons of the Andes were probably slings, bolos, stone headed and wooden clubs, Amazonian style arrows and spears. I haven't seen any reference to suggest that bronze weapons were widespread in the Inkan army, and as far as I can tell, metal weapons were for the wealthy.
Final note: there's a book on Incan martial arts out now, for those interested (
link to Amazon). It contains a short chapter on weapons.
my 0.002 centavos
F