TORONTO, Dec 15 (Reuters) - Rusoro Mining (RML.V) has made an unsolicited all-stock offer for Gold Reserve (GRZ.TO) to take over its Brisas gold and copper deposit in Venezuela, the Canadian-based company said on Monday.

Rusoro is offering three of its shares for each Gold Reserve share, which values the target at C$1.08 a share, or C$61 million ($49 million), a premium of 140 percent, based on Friday's closing prices.

Gold Reserve trades on the Toronto Stock Exchange and Rusoro trades on the TSX Venture Exchange.

Shares of Rusoro, which is based in Vancouver, British Columbia, but controlled by Russian Vladimir Agapov, fell 26 percent on the news, while Gold Reserve rose 51 percent.

Gold Reserve said in a statement it will wait until it receives a formal offer before it decides how to proceed.

Rusoro is already part of a joint venture with the Venezuelan government and the company appears set to benefit from the country's desire to increase control of its mining sector and boost ties with Russia.

Venezuela's mining minister, Rodolfo Sanz, told a Russian government delegation in early November it would sign an accord with Rusoro to operate the Brisas and Las Cristinas projects, the country's largest gold deposits.

Canada's Crystallex (KRY.TO) operates the Las Cristinas project under contract, and the company's shares have plunged as its efforts to get final approval to mine the deposit have dragged on.

Shares of Rusoro were down 10 Canadian cents at 26 Canadian cents, valuing the offer at 78 Canadian cents a share. Gold Reserve was up 23 Canadian cents at 68 Canadian cents.

Brisas is expected to yield nearly 500,000 ounces of gold and 60 million pounds of copper annually once it reaches production, the company has said.

($1=$1.24 Canadian) (Reporting by Cameron French; editing by Rob Wilson)