It seems to be a spin or a spiral dive. Spiral is different to a spin but both cause sudden loss of height. With a spiral the angle of attack is low.
This aircraft fell 17,000 ft in less than a minute.
The blackbox has been recovered and they will find what caused it. Most likely its a control failure or loss of a wing part.
Icing is a possibility too (as mentioned by
@Sub Zer0) being winter in Brazil.
Note: A similar event took place on October 31, 1994 -
American Eagle Flight 4184 from Indianapolis to Chacago. The ATR 72 crashed killing all 68 on-board. It was due to icing.
Also previously, on 15 October 1987, Milano to Bonn - Italian crash ATR 42 -312 crash again Icing.
Sad incident...