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* About 20% of Japan's total refining capacity shut (Adds details, analyst comments)

By Randy Fabi and Francis Kan

SINGAPORE, March 11 (Reuters) - Japan's earthquake forced port closures and shutdowns of oil refineries and metal plants in the world's third-biggest economy on Friday, rattling commodity and energy markets as participants weighed up how quickly activity could return to normal.

The magnitude 8.9 earthquake, the biggest to hit Japan in 140 years, struck the northeast coast, triggering a 10-metre tsunami that cleared everything in its path from houses to cars and set farm buildings on fire.

"This natural disaster could result in another sharp rise in risk aversion on markets and a continuation of yesterday's correction on commodity markets," Commerzbank said in a report. "The demand for oil could be lower, at least temporarily, because of the earthquake."

TOCOM gold rallied about 0.4 percent soon after the quake, but was down 0.7 percent at 3,473 yen per gram by 1000 GMT. Tokyo rubber fell 3.6 percent to 384.1 yen.

Metals firm Mitsui Mining said operations at its Hachinohe zinc smelter, with a capacity of 112,000 tonnes a year, were halted by the earthquake, and employees evacuated.

All Japanese ports have closed, with discharging operations stopped, shippers said. Top refiner JX Nippon Oil & Energy Corp halted operations at three plants, while��fire engulfed a storage tank at a unit of Cosmo Oil Co . The two make up about 20 percent of the country's total refining capacity.

"It's a big mess. All discharge operations are suspended in the area," said one shipbroker.

TV footage showed at least one large panamax vessel, which typically could carry upto 80,000 tonnes of coal, iron ore or grain, grounded in northern Japan by the tsunami, a second Tokyo-based shipbroker said.

"Most or all coal stocks will be washed out at many of the coal-fired power plants," he said. "Ports will be closed at least for a short period until damage assessments can take place."

JX Nippon Oil has halted operations at Sendai, Kashima and Negishi plants, Jiji News said, while��fire struck a storage tank at Cosmo Oil Co's refinery in Chiba, east of Tokyo.

The four refineries have a processing capacity of nearly 900,000 barrels per day.

This could be bearish for crude oil demand in Asia, traders said, but the impact would depend on how fast the plants could be restarted.

Mitsubishi Chemical halted operations at two naphtha crackers at Kashima after a power outage. The units produce 828,000 tonnes per year of ethylene, or around 11 percent of Japan's overall capacity.

"In the naphtha market, Japan plays the demand side," a North Asian naphtha trader said, adding that if plants were down, demand would fall.

Hokuriku Electric Co said all three reactors at its Onagawa nuclear plant in northern Japan shut down automatically.

Electric Power Development (J-Power) has also halted operations of its Isogo thermal plant in Yokohama, Jiji reported.

FUEL OIL

Asian fuel oil's front-month timespreads climbed $1.00 a tonne on Friday after the earthquake in northeast Japan forced nuclear plants to shut down, possibly boosting fuel demand, traders said.

The prompt April/May timespread rose $1.25 to be traded at $6.00 a tonne, while May/June gained $1.00 to $5.75 a tonne by 0800 GMT, according to brokers.

At least 315,000 tonnes of April/May were traded, 135,000 tonnes of May/June and 135,000 tonnes of June/July, they said.

"That's definitely the Japan Power factor kicking in on the fuel oil," said Tom James, the director and co-founder of Singapore-based energy consultancy Navitas Resources.

Japan is expected to seek more supplies of low-sulphur fuel oil (LSFO) for power generation due to the shutdown of several nuclear plants, traders said.

"I would expect that to happen and soon, and they would probably have to pay up if the demand is for prompt-arrival cargoes and in large volumes, as overall supply in Asia is quite small and quite niche," a Singapore-based LSFO trader said.

Most of the region's supply of 100,000-150,000 tonnes a month comes from Brazil's Petrobras, which produces the residual fuel from its refineries. Supplies also come from traders such as Vitol, Trafigura and Westport, who blend the cargoes, and Indonesia's Pertamina, which exports about 2 million barrels of low-sulphur waxy residue (LSWR) a month.

Middle distillates also strengthened after the quake, with gas oil's April/May timespread flipping into positive territory on the prospect of supply disruptions out of Japan, traders said.

Bids/offers for gas oil's April/May contract were seen at a backwardation of 5/10 cents by 0826 GMT, versus a contango of 9 cents in the previous session.

Jet fuel's premium to gas oil, or the regrade, traded at $1.25 a barrel, up 25 cents.

BASE METALS

Base metals reacted negatively to the quake, with copper and aluminium extending earlier losses. ������ London Metal Exchange copper dropped by around $200, or more than 2 percent, after the quake, to touch a three-month low of $9,045. Aluminium shed more than 2 percent.������ ������ Japan was expected to produce 1.6 million tonnes of refined copper in 2011, about 7.6 percent of world output. ������ Most of Japan's eight copper refineries were probably far enough away from the quake epicentre to escape serious damage. ������ But two, the 250,000-tonne-per-year Onahama plant and the 217,000 tonne Hitachi operation, both owned by Pan Pacific, were close enough to the coast for analysts to describe them as at risk. ������ Japan produces around 670,000 tonnes of zinc, or 5 percent of world output. (Additional reporting by Florence Tan, Yaw Yan Chong and Nick Trevethan in SINGAPORE; Writing by Manash Goswami; Editing by Clarence Fernandez)