* Spot iron ore has fallen almost 40 pct from 2017 peak
* Traders say further drops to come
* Dalian iron ore futures trading volume highest in 13 months
* Open interest climbs to most since Nov 2015
MANILA, May 31 (Reuters) - A stubborn glut and selling by Chinese speculators have slashed iron ore prices this year, with traders and industry officials predicting further declines as memories fade of the steelmaking commodity's stunning recovery in 2016.
Prolonged price weakness would make business tougher again for top miners like Vale, Rio Tinto and BHP , whose earnings were roiled as iron ore markets tumbled 70 percent in the three years through 2015.
Weaker prices could also hit marginal suppliers such as Iran, potentially forcing that country out of the market again after boosting shipments to top buyer China earlier this year.
Iron ore prices <.IO62-CNO=MB> have plunged nearly 40 percent from this year's peak, trading at just below $60 a tonne this week, with stockpiles at Chinese ports swelling to their largest in 13 years.
In May alone, iron ore has fallen 15 percent, on course for its steepest monthly drop in a year. In 2016, the commodity rallied over 80 percent.
"Demand is still healthy but there's way too much supply," said a trader at a global trading firm in Singapore that is looking for Chinese buyers for about 1.5 million tonnes of iron ore in coming weeks. He asked not to be identified.
"If you're an iron ore buyer and you see it's an oversupplied market, and supply will only increase from here, would you have any incentive to buy more today?"
Imported iron ore at China's ports reached 136.6 million tonnes on May 26 <SH-TOT-IRONINV>, the highest since SteelHome consultancy began tracking the data in 2004. That is enough to build the Eiffel Tower in Paris more than 13,000 times over.
Global iron ore output is forecast to expand an average of 1 percent annually from this year to 3.3 billion tonnes by 2021, the same average growth during 2012-2016, BMI Research said.
ARMY OF SPECULATORS
Declining prices have been amplified by China's army of speculators, the ones behind the wild swings in iron ore futures traded on the Dalian Commodity Exchange (DCE).
Dalian futures, launched in October 2013, gave many Chinese investors their first real chance to play in the iron ore futures market, and have largely influenced physical pricing.
"DCE is the primary driver and it's always been the primary driver. That's not going to change because iron ore is a China market," said Kelly Teoh, broker at Clarksons Platou Futures.
The volume of iron ore futures traded on the DCE so far in May reached nearly 2.8 billion tonnes, about twice annual global seaborne trade and the largest since April 2016, according to exchange data.
As the most-traded contract dropped 32 percent from this year's high to mid-May, open interest - or open contracts - grew to 2.45 million lots, the highest since November 2015.
"There is clear evidence of speculation in China's iron ore and steel futures," said Julius Baer analyst Carsten Menke.
"Speculation always amplifies trends, to the upside and to the downside."
Australia's Fortescue Metals Group, the world's No. 4 iron ore miner, said it was generating strong margins even at current prices.
Fortescue CEO Nev Power expects China's steel output to rise "around 5 percent compared to this time last year" with demand backed by the nation's continuing "industrialisation and urbanization."
(Reporting by Manolo Serapio Jr.; Editing by Joseph Radford)