AN A320 FLC ENCOUNTERED UNFORECAST WX AND DECLARED A FUEL EMER FOR LNDG AT LAS.

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DECLARED 'EMER FUEL' DURING UNPLANNED DIVERSION TO LAS. FLT REQUIRED TO DIVERT TO LAS DUE TO UNFORECAST DUST STORM RAPIDLY AND DRAMATICALLY REDUCING VISIBILITY AT PHX. FLT RELEASED TO PHX WITH SCHEDULED FUEL LOAD OF 11000 LBS; INCLUDING STANDARD RESERVE OF 4000 LBS PLUS 1200 LBS OF 'EXTRA FUEL.' CURRENT AND FORECAST WX FOR PHX WAS FOR VFR CONDITIONS WITH NO HAZARDS NOTED. OBTAINED ATIS WX AT TOP OF DSCNT. WX WAS CALM WINDS; 10 SM VISIBILITY; CB; 150 SCT; 250 SCT. AS DSCNT CONTINUED; APCH CTL BEGAN ISSUING AIRSPD ADJUSTMENTS AND DELAY VECTORS DUE TO DUST STORM N OF ARPT. DISCUSSED SIT WITH FLT DISPATCHER ON COMPANY RADIO. APPROX 5 MINS LATER THE SIT HAD DETERIORATED. WE WERE ADVISED THAT WE WERE #12 IN THE APCH SEQUENCE AND THAT PHX HAD BECOME IFR FOR BLOWING DUST. AS A NEW CAPT ON THE A320 MY MINIMUMS WERE 1 MI OR RVR 4500. VISIBILITY WAS DROPPING RAPIDLY. THE DISPATCHER AND I AGREED A DIVERT BACK TO LAS WAS IN ORDER. UPON REACHING CRUISE ALT; I FIRST DECLARED 'MINIMUM FUEL;' THEN TO ENSURE PRIORITY HANDLING; I DECLARED 'EMER FUEL.' OUR ONBOARD FMGS COMPUTED A LNDG FUEL OF 1200 LBS AT LAS. AN UNEVENTFUL APCH AND LNDG WAS MADE AT LAS. SHUTDOWN AT GATE WITH 1060 LBS.