Preheat the oven to 300 degrees F. Combine the crust ingredients and press into the bottom, and partially up the sides, of a 10-inch spring-form cake pan, then set aside. Place the lemon and orange juice and zest in a pot. Bring to a boil and reduce to a 1/4 cup. Remove from heat and cool this concentrated mixture to room temperature.
Beat the cream cheese until smooth. Gradually beat in the sugar. Beat in the eggs one at a time, scraping the sides of the bowl after each addition. Beat in the reserved lemon/orange juice mixture. Pour the batter into the pan. Bake 70 minutes, or until the centre of the cake barely jiggles when the pan is tapped. Cool on a rack to room temperature. Refrigerate for at least 3 hours before unmoulding. Melt marmalade in a small pot with 2 Tbsp. of cold water. Arrange the lemon and orange slices on top of the cheesecake. Brush with marmalade and then refrigerate until set. Garnish wedges of cake with mint sprigs.