The explorers mentioned the finding of ''eleven mine prospects, all having great veins of silver'' because the silver was greatly valued in that period and it was one of the most demanded things on the market, as well as for the internal usage of the country that has it. So mentioning that there's lots of silver was with the purpose so that they will be sponsored again by the king to go on another expedition because he will think that there's lots of treasure for getting and will not spare money to get it.