I'll give a shot at a half-educated guess ;)

Considering the sample location and its history, it is very difficult to predict what they have found.

They are sampling in an ancient stream bed going back to the early days of Mars when organics were produced and sedimented under a thick atmosphere.

Then the Gale crater was filled with aeolian (wind blown or wind eroded) sediments well before the atmosphere was thinned.

And now aeolian erosion has excavated down to the stream bed again.

I'm going to take a guess that they may have found the 'desert varnish' suggested by previous imaging of Mars. On Earth, desert varnish is a clay coating found on exposed rock surfaces in arid environments. This varnish coating is typically about a micron thick and on earth contains iron and manganese oxides and organic matter formed by microbial processes.

Although desert varnish testing with high levels of Mn would not be direct evidence of life, it would be very strong evidence indeed.

ref: http://www.icamsr.org/..._4495-13_rock_varnish.pdf