pH of Sulphide ore

    Hi,
I work in a silver/ lead/ zinc mine.  Usually our ore runs into the mill at
a pH or around 9, but lately our acid consumption has gone through the roof!
Testing the pH of the raw ore (just soaking a crushed up sample of high
grade ore in distilled water), the ph is aound 12.  I cannot explain it- can
anyone out there help??
There is no cement backfill or backfill of any sort where we are mining, so
in theory it should be mineralogically related.  Any adive would be really
apprieciated!