* Thermal coal futures sink 5 percent, threaten months-long rally
* More than a million lots of January contracts traded
* Meteoric gains in thermal coal prices not sustainable -analysts (Updating with details, comment throughout)
BEIJING, Oct 27 China's Zhengzhou thermal coal futures sank almost 5 percent on Thursday and were on track for their biggest daily drop as fee hikes aimed at curbing speculation triggered an exodus of cash, threatening a months-long price rally.
On Monday night, the exchange introduced the latest in a series of measures targeting speculative cash that has helped fuel this year's 85 percent surge in thermal coal prices, stoking worries in Beijing about soaring electricity bills during peak demand over the winter months.
The most-active January coal contract closed down about 5 percent at 610.2 yuan ($90.03) per tonne after hitting an all-time high on Wednesday. Earlier in the session, prices had fallen limit down to 609.4...