Ginger Mango Sparkling Punch

For ginger syrup:
1 1/2 cups water
1 cup sugar
1 cup thinly sliced unpeeled fresh ginger (1/4 lb; from two 5-inch-long pieces)

For punch:
3 cups unsweetened mango juice, chilled
1/4 cup fresh lemon juice (3 to 5 lemons)
1/4 cup fresh lime juice (4 to 6 limes)
Seltzer

Make ginger syrup:

Bring water, sugar, and ginger to a boil in a 1- to 1 1/2-quart saucepan, stirring until sugar is dissolved. Simmer, uncovered, stirring occasionally, 10 minutes, then remove from heat and steep, uncovered, 15 minutes. Pour syrup through a sieve into a bowl, discarding ginger. Chill, covered, until cold, about 2 hours.

Make punch:
Stir together ginger syrup, fruit juices, and seltzer in a pitcher.