Everheard of the maroon people? It has happened before
There are maroons (quilombolas in Portuguese) in Brazil (and pretty much everywhere where there was slavery), not very much in the Amazon region, though, as we have at least 1,000 km between the Amazon and the main plantation areas. You could at least check wikipedia to know a thing or two about simple geography. It's mildly offensive to Brazilians to claim that most of Brazil is just a big jungle, but I won't take any offense.
? many of the black slaves where already muslim
Imagine how hard it is to keep some sort of organized religion when you don't even have the power over your own workforce. Beliefs simply syncretize and eventually lose their original meaning.