The Rio Tinto has a Ph ranging from 1.7 to 2.5. it's unusual (for modern Earth) acid chemistry makes it a good analogue for Earth some 3 billion years ago, and the presence of highly-adapted life forms is a testament to the endurance and resilience of life.

http://www.biolbull.org/cgi/content/full/204/2/205

Andrew Knoll has specifically studied the Rio Tinto as an analogue for Mars:-
http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V61-4HG67C8-2&_user=10&_coverDate=11%2F30%2F2005&_alid=692188016&_rdoc=7&_fmt=summary&_orig=search&_cdi=5801&_sort=d&_docanchor=&view=c&_ct=10&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=ca24cc2e9f8347df8d4a79c856647dcd</> A bit of an ugly URL, but that's science for you... I'm trying to find an exact quote for the Ph values he estimates for Early Mars, but you can bet your shirt that if he felt Rio Tinto was a relevant analogue, he would have said so, and thumped the table when he did so! As it happens,another article offers a fuller quote:-

The new findings bring scientists no closer to determining whether there was ever microbial life on Mars, but they do "constrain our thinking about life on Mars," Knoll said, painting a picture of an environment that is very "forbidding" for life.

Previously, Mars mission data have revealed that liquid water at Meridiani Planum was very acidic.

"So it is doubly bad if it's acidic and salty," Knoll said.

"I'm not sure the effects are necessarily additive, but certainly there are limits to the physiological ways that microorganisms can adapt to tolerate acidity," he said. "There are also limits to ways that they can adapt to tolerate highly saline environments and those are completely different physiological systems. In a sense, it makes it harder to adapt that there are two biochemical systems that would need adapting."

So the bottom line is that nobody has PROVED that life could not survive on mars, but it certainly looks VERY difficult...