Correction. The largest sulfate deposit is around 80N latitude and 240E longitude, whereas the locations for the volcanoes in the abstracts I mentioned, while at around 80N latitude, were closer to 300E longitude.
However, this Icarus article shows there are volcanic landforms at several different longitudes at around 80N, some closer to the 240E longitude of the densest sulfate deposit:

North Polar Region Craterforms on Mars: Geometric Characteristics
from the Mars Orbiter Laser Altimeter.
Icarus 144, 329–352 (2000).
http://ssed.gsfc.nasa.gov/tharsis/garvin_icarus.pdf

This report to the 7th International Conference on Mars says the leading candidate landing sites for Phoenix lie in the area referred to as Region D, at 65N to 72N latitude and 230E to 250E longitude:

OVERVIEW OF MARS EXPLORATION PROGRAM 2007 PHOENIX MISSION LANDING SITE
SELECTION.
Seventh International Conference on Mars.
http://www.lpi.usra.edu/meetings/7thmars2007/pdf/3204.pdf

And the "REMOTE SENSING OBSERVATIONS AND IMPLICATIONS OF MARTIAN NORTH POLAR SULFATE report shows in Fig. 3 that a lower concentration sulfate deposit lies at about 70N, 230E, which is within Region D.
So it is possible at least to put Phoenix close to a sulfate deposit, if not also close to a volcano.

Bob Clark