Since I posted my query, I got some more info. Zelick Rosenbaum's mother was Tillie Geltman. His father had died, and she remarried to Moishe Cohen. Moishe Cohen's brother Chaim Paysah Cohen had already settled in Colorado. Moishe, Tillie and three children arrived in Denver, and in 1907, Zelick (but not his wife Ruchel Grynblatt) and his half-brother Israel immigrated.
Do any of these names ring a bell?