Alu Group Base Replacement  

base detched how?
When you say the base was detached, do you mean just the spider part? That's a pretty simple job (either a single set screw, or a snap ring, depending on the model). However, if the antlers or tilt base were removed from the side frames, you're in for some work.

If it is the latter
I have found that heating the material helps. That is, underneathe the fabric their is a vinyl shell that when heated allows for adjustment onto the frame.
If memory serves me right I left it in the sun for a few hours on a hot day and putting it together was no problem.
Buena suerte

I haven't tried the heat method,
but I've used a block of wood about the width of the antlers to wedge the side frames apart. It helps if you have a second person. One can pry the frames apart, using the block of wood for leverage, while the other positions the antlers.
I used to ship these chairs pretty frequently. After the first time trying to wrestle one of these monsters back together, I figured out a better way:
I'd remove the screws, and slide the antlers along the frame, keeping the tension in the sling. Once the antlers cleared the mounting holes, I'd attach a scrap piece of angle iron, so that constant tension on the sling was never lost - even in shipping. Reassembling was just a matter of reversing the process.