For commodity markets, the third iteration of U.S. central bank stimulus may play like many movie sequels -- a familiar plot, a predictable rise, yet less potent than the original.

　　But the Federal Reserve has introduced a new twist on its latest round of quantitative easing (QE), one that is giving some traders even more reason to be cautious rather than optimistic: this time around, nobody knows when it will end.

　　Chairman Ben Bernanke pledged on Thursday to inject as much money as needed into the U.S. economy to repair a battered job market, by buying $40 billion worth of bonds a month until he gets results. Under the QE2, it was a flat $600 billion. If unemployment falls, the Fed may just reduce its allocation.

　　"It was not the full-blown, giant, lump-sum initiative market participants had envisioned," Jason Schenker at Prestige Economics in Austin, Texas. "The positive impact on financial markets and commodity prices may be more gradual."

　　In the previous two rounds, raw material prices typically leapt in the run-up to the Fed announcement, with gains slowing to a trickle in the following weeks. It took months, or as much as a year, for more substantive gains to build up. And the percentage of gains after each stimulus has been diminishing.

　　"The critical question facing investors now is how long this stimulus round will last," said Adam Sarhan, founder of New York's Sarhan Capital.

　　"If they think it's going to go on for a while and the Fed will have to do even more as time progresses, then it's right to raise the call now on oil, gold and metals. But most of these markets have already had epic price moves, so I think people will be more cautiously optimistic moving forward."

　　Oil prices rose nearly a third in over just 10 weeks before the Fed's decision to widen its bond-buying program. With so much gain in such a short time, investors may be averse to continue pushing the market at such a pace, analysts say.

　　Oil's benchmark Brent crude in London settled at$116.90 a barrel, up 1 percent on the day and 29 percent higher from a June 22 low of $88.49.

　　London copper futures rallied to above $8,200 a metric tons (1.1023 tons) in after-hours business, up about 14 percent from 10 weeks ago. U.S. gold futures rose about 13 percent in the same period.

　　The bellwether 19-commodity Thomson Reuters-Jefferies CRB index .CRB rose for a sixth straight session, the longest such streak since February. The index is up nearly 20 percent from its June low, nearing bull market territory.

　　MARKET SLOW TO SHOW GAINS

　　After the previous two QE rounds in 2008 and 2010, commodity markets showed sizeable gains after months, not weeks.

　　Brent, for instance, saw a 12 percent price decline in the first three months after QE1, launched in the aftermath of the Lehman Brothers collapse in 2008 which triggered the global financial meltdown. Six months later, it was up 20 percent and a year later, it showed a 56 percent gain.

　　When the QE1 was expanded in 2009, Brent did better, gaining nearly 50 percent in three months and 71 percent a year on. But that stronger performance was in line with the sharp rebound in many assets depressed by the worst of the financial crisis.

　　When Fed Chairman Ben Bernanke hinted at QE2 in the third quarter of 2010, Brent rose by a meager 3 percent a month later. Three months after the QE2 implemented, Brent was up 13 percent. Six months on, it showed a 38 percent rise and a year later, it had pared those gains to 27 percent.

　　The one clear winner seems to be gold, which surged 2 percent on Thursday, approaching its high for the year.

　　"They (the Fed) are emphasizing the growth mandate, and that means they don't care about inflation other than giving lip service to it," said Axel Merk, chief investment officer at Merk Funds, which has $600 million in currency mutual-fund assets.

　　"The price of gold will do very well in the years to come," Merk said.

　　OUTSIDE FORCES MAY DAMPEN IMPACT

　　In the end, QE3 may have less of an impact simply because there are now more compelling stories to tell, and because the condition of the moribund U.S. economy has yielded to the euro zone crisis, Chinese stimulus, Middle East violence, devastating drought and other factors not present in 2008 or 2010.

　　"I see more of a downside risk for investors in the QE3 than an upside opportunity," says Charles Gradante, co-founder of the New York-based Hennessy Group, which invests in hedge funds.

　　"With the possible exception of gold, commodities, particularly oil and metals, just do not have the right economic outlook to continue with this kind of price growth."

　　Oil has been pushed higher lately by fresh unrest in the Middle East and Africa over a film produced in the United States that demonstrators consider blasphemous to Islam, as well as fears that Israel could launch an attack on Iran to deter its nuclear program.

　　"Our impression is that hedge fund managers in the commodity space generally see further episodes of QE acting as a tailwind for commodity markets but not being the main driver of price action," said Osvaldo Canavosio, head of commodities research at Man Investments' fund of funds division.

　　"The main driver for most commodity markets will continue to be the idiosyncratic fundamental factors affecting its supply and demand." 　　對(duì)大宗商品市場(chǎng)來(lái)說(shuō),美聯(lián)儲(chǔ)推出的第三輪量化寬松(QE3)可能像許多電影的續(xù)集那樣--情節(jié)相似,劇情高潮可期,就是影響沒(méi)有第一集那麼震撼.

　　但美聯(lián)儲(chǔ)最新一輪QE有了新的花樣,讓一些交易商有更多理由保持謹(jǐn)慎,而不是變得樂(lè)觀:這回沒(méi)有人知道QE會(huì)何時(shí)結(jié)束.

　　美聯(lián)儲(chǔ)主席伯南克周四承諾,將通過(guò)每月購(gòu)買(mǎi)400億美元債券,向美國(guó)經(jīng)濟(jì)注入其需要的所有資金,修復(fù)就業(yè)市場(chǎng),直到就業(yè)明顯改善.之前QE2的規(guī)模明確為6,000億美元.如果失業(yè)率下降,美聯(lián)儲(chǔ)可能只會(huì)減少Q(mào)E3的購(gòu)債規(guī)模.

　　"市場(chǎng)參與人士原本預(yù)期會(huì)出臺(tái)一個(gè)成熟全面、規(guī)模巨大、一次性的購(gòu)債計(jì)劃,"Prestige Economics的分析師Jason Schenker說(shuō)."對(duì)金融市場(chǎng)和大宗商品價(jià)格的積極影響可能會(huì)逐步體現(xiàn)."

　　"投資者現(xiàn)在面臨的關(guān)鍵問(wèn)題是,這輪刺激措施會(huì)持續(xù)多久,"Sarhan Capital創(chuàng)始人Adam Sarhan說(shuō).

　　"如果他們認(rèn)為將持續(xù)一段時(shí)間,而且美聯(lián)儲(chǔ)未來(lái)將必須采取更多行動(dòng),那現(xiàn)在看漲原油、黃金和金屬就對(duì)了.但這些市場(chǎng)中的大多數(shù)已經(jīng)出現(xiàn)過(guò)大幅價(jià)格波動(dòng),所以我認(rèn)為未來(lái)人們更多是保持謹(jǐn)慎樂(lè)觀."

　　2008年和2010年美聯(lián)儲(chǔ)推出前兩輪QE後,大宗商品市場(chǎng)是在幾個(gè)月後才出現(xiàn)大幅漲幅,而不是幾周後.

　　最終QE3的影響可能較小,因?yàn)槟壳捌渌�一些事件的影響更�?比起歐債危機(jī)、中國(guó)刺激措施、中東局勢(shì)*、嚴(yán)重旱災(zāi)和其它2008年或2010年所沒(méi)有的因素,美國(guó)經(jīng)濟(jì)不振的狀況顯得相對(duì)次要.

　　"QE3出臺(tái)期間,我認(rèn)為投資者面臨更多下檔風(fēng)險(xiǎn),而不是上檔機(jī)遇,"Hennessy Group聯(lián)合創(chuàng)始人Charles Gradante說(shuō).

　　"可能除了黃金之外,大宗商品沒(méi)有理想的經(jīng)濟(jì)前景作為價(jià)格漲勢(shì)的支撐,尤其是原油和金屬."

　　Man Investments組合式基金業(yè)務(wù)的商品研究主管Osvaldo Canavosio稱(chēng),"我們的印象是,專(zhuān)注于商品領(lǐng)域的對(duì)沖基金公司基本認(rèn)為,更多QE將有利於商品市場(chǎng),但不會(huì)成為推動(dòng)商品價(jià)格的主要?jiǎng)恿?"

　　"推動(dòng)多數(shù)商品市場(chǎng)的主要因素將繼續(xù)是那些影響供需的獨(dú)特基本面因素."