Preheat oven to 350°F. Grease and flour pan; set aside. In a medium bowl, stir together flour, baking powder and salt; set aside. In a large bowl, combine sugar and orange zest. Add butter to orange-sugar mixture. Using an electric mixer on high speed, beat until mixture is light and fluffy, about 4 minutes. Add eggs one at a time, beating well after each addition and occasionally scraping down sides of the bowl, until mixture is light and very fluffy, about 4 minutes longer. In a 2-cup measuring cup, combine buttermilk and orange juice. Reduce speed to low. Add dry ingredients in thirds, alternating with buttermilk mixture and ending with dry ingredients. Spoon batter into prepared pan. Bake 60-70 minutes, until toothpick inserted comes out clean. Cool in pan for 10 minutes. Invert cake onto rack and let cool completely. For the glaze: combine marmalade and orange juice in a small saucepan. Bring to a boil, reduce heat and simmer, stirring occasionally, until syrupy, about 3 minutes. Strain glaze into a small pitcher or bowl; discard solids in strainer. Pour glaze over cooled cake and let sit for at least 10 minutes before serving.