KD Dijkstra suggested this is Cacoides latro. I couldn't find any photos of that species, but it seems to fit the description in Garrison e.o. The location doesn't fit with the distribution map in Garrison e.o. at all. Distribution is described as following the atlantic slope, 1000s of kilometers away from Alta Floresta.

23/01/2008
Today I have visited the Natural History Museum Naturalis to check some literature about neotropical odonates. Among them all references from Garrison/Ellenrieder/Louton about Cacoides Latro. I am now near-certain that the dragonfly on these photos is Cacoides latro (unless it is an unknown, very similar sister taxon).
Literature checked:
Belle, 1986 - New Woirld Lindeniinae, with Melanocacus interioris gen. nov. et spec. nov. (Odonata: Gomphidae). Entomologische Berichten 46:97-102
Moore and Machado, 1992 - A note on Cacoides latro (Erichson), a territorial lacustrine gomphid (Anisoptera: Gomphidae). Odonatologica 21 (4): 499-503

Among other things, Belle reveiled that the related Melanocacus have a different thoracic pattern, with more dark. The wing venation also fits Cacoides latro. In this species RP2 is concave and approaches RP1 proximal to the pterostigma. In similar species (Melanocacus) RP1 and RP2 are more or less parallel. The concavity of RP2 is clearly visible on the first photo. I will add some photos of details later.

Moore and Machado contains a black and white photograph of Cacoides latro showing a gomphid, that is extremely similar (I see no obvious differences) to the gomphid in my photographs. In the text they describe the very obvious white spot on the base of the abdomen underside.

'Adults perch on tips of snags facing waters of stagnant ponds' (Belle, 1986 in G/E/L). The gomphids in Alta Floresta showed the same behaviour at the stagnant pools in Alta Floresta, where they were always present. Unlike the descriptions in literature, I found them to be fairly approachable, when approached very slowly and quietly. The photos are the judge of this. I did not try to catch them though.

During my stay in Cristalino Jungle Lodge I regularly observed very large gomphids flying above the Cristalino and Teles Pires rivers. At the time I considered them to be the same species as the ones at Alta Floresta, but I never had prolonged views and only once saw one perched (for a short time). In retrospect, combining the knowledge I have now with the very different behaviour of these gomphids, it doesn't seem logical that these gomphids were the same species. I would like to have another look at them though.