Hey nice gun Dawgy. I know a little bit about guns and I can share some info for you.
In 1879 the Argentine army adopted the Remington Rolling rifle in .43 caliber as standard issue. Their next standard rifle was the M1891 Argentine Mauser, Made in Berlin, 7.65 x 53mm cartridge, 5 round mag, Weight is 8.8 Lbs., Argentine crest on top of the receiver. The Mauser was one of the best built rifles ever produced. Most modern rifles are patterned after the Mauser in one way or another. If you shoot it make sure it is clean and you have the right ammo, or have a gunsmith look at it.
Cleaning the interior of the gun does not take away the value. You could perhaps clean it and get any corrosive build up that may be in there and save the gun from any furthur damage. You can also wipe down the outside with some light oil as some protection. Just don't do anything harsh like sanding or scraping something off. You need to protect the patina. If the patina is gone, then so is the value. I have been doing this for 40 years and I know a little about it. Happy shooting!!!
The value varies considerable from $150 to $800 depending on condition. Yours looks like it falls in the middle, but it is hard to say.