UPDATE 3-Bitcoin drops after dramatic gains ahead of futures launch

(Updates prices, adds graphic, analyst comment) By Jemima Kelly and Gertrude Chavez-Dreyfuss LONDON/NEW YORK, Dec 8 (Reuters) - Bitcoin lost almost a fifth of its value in 10 hours on Friday, having surged more than 40 percent in the preceding 48 hours, sparking fears the market may be heading for a price collapse. In a hectic day on Thursday, bitcoin leapt from below $16,000 to $19,500 in less than an hour on the U.S.-based GDAX, one of the biggest exchanges globally, while it was