An older source that I have used when discussing the Black Cowboy in the west is "The Black on New Spain's northern frontier : San Jose de Parral, 1631 to 1641 / by Vincent Mayer, Jr. Editor: Robert Delaney. Imprint Durango, Colo. Fort Lewis College, 1974."
The author covers Black Spanish cowboys working for about two hundred pesos a year during the time frame mentioned as well as discussing Black ore miners.
A very informative post!