Separating a powdered mixture of Zinc Oxide and Silver Oxide
Your second suggest that you also want to save the silver oxides apart from zinc oxide. Sodium hydroxide won't help because it also reacts with silver oxides. So, you need a weak acid to selectively react with zinc oxide but neither silver oxide or silver. Hydrobromic acid will work because it reacts with zinc oxide to form zinc bromide which is highly soluble in water. leaving silver oxide and silver untouched. Silver bromide is also expected to form but hydrobromic acid is not strong enough to do that. Instead, if an alkali bromide salt was used, silver bromide would definitely form. From wikipedia article of zinc bromide:


  $\ce{ZnBr2 · 2H2O}$ is prepared by treating zinc oxide or zinc metal
  with hydrobromic acid.
  
  $$\ce{ZnO + 2 HBr + H2O → ZnBr2 · 2H2O}$$
  
  The dihydrate can be dehydrated by passing it over hot $\ce{CO2}$