A map by OMEGA of the north polar sulfate deposits is copied below, taken from Fig. 3 of the report:

REMOTE SENSING OBSERVATIONS AND IMPLICATIONS OF MARTIAN NORTH POLAR SULFATE DEPOSITS.
B. H. N. Horgan1, J. F. Bell III1, E. Z. Noe Dobrea2, and P. C. Thomas1, 1Cornell University, Department of Astronomy (****@astro.cornell.edu), 2Malin Space Science Systems (now at Jet Propulsion Laboratory).
Seventh International Conference on Mars.
http://www.lpi.usra.edu/meetings/7thmars2007/pdf/3241.pdf

I drew in an arrow in red indicating the location of the lower concentration sulfate deposit at about 70N latitude, 230E longitude, within the possible Phoenix landing area Region D.

Bob Clark

Figure 3: Distribution of Ca-bearing sulfate, based on
1.92 μm band depth, ranging from 0 to 25% band depth below
the continuum. Grayscale regions are residual water ice.