Put the sugar and 100ml water in a pan and bring slowly to a simmer, making sure all the sugar has dissolved. Bubble for 2 minutes, then cool a bit before adding the alcohol and lime juice. Purée the mango in a blender and stir in the alcohol mixture.
Pour the lot into an ice-cream machine, set it to freeze and churn. Churn for 15 minutes or until the frozen margarita is the consistency you want.