Preheat oven to 325 degree F. Place roast, rib side down, in a shallow roasting pan. In a small bowl, combine the 1 teaspoon dry mustard, the 1 teaspoon marjoram, and the 1/2 teaspoon salt. Sprinkle mustard mixture evenly over pork; rub in with your fingers. Insert an oven-going meat thermometer in center of meat, making sure bulb does not touch bone. Roast, uncovered, for 1-1/2 hours.
In a small bowl, stir together the peel, juice, and brown sugar; spoon over the meat. Roast about 30 minutes more or until thermometer registers 155 degree F, spooning pan juices over meat once or twice. Transfer meat to platter. Cover and let stand for 15 minutes before slicing. (The temperature of the meat will rise about 5 degree F during standing.)
Meanwhile, strain pan juices. Skim off fat. Measure juices; add enough broth to juices to equal 3/4 cup total liquid. Pour the liquid into a medium saucepan. In a screw-top jar, combine the 2/3 cup broth and the flour; shake well. Add to saucepan along with the 1/8 teaspoon dry mustard and the 1/8 teaspoon marjoram. Cook and stir until thickened and bubbly; cook and stir for 1 minute more. Season to taste with additional salt and pepper. Stir in the tangerines or oranges; heat through. Serve with pork. Makes 10 servings.