* Inquiry follows Brazilian collapse that killed hundreds
* Of 726 companies contacted by investors, 43% replied
* 166 dams around world found to have had stability issues
* Anglo boss: industry has 'moral imperative' on safety (Adds Anglo American CEO)
By Barbara Lewis and Ernest Scheyder
LONDON, Oct 31 (Reuters) - A global inquiry into how mining companies store billions of tonnes of waste in huge dams, launched after a collapse in Brazil killed hundreds, shows about a tenth of the structures have had stability issues, investors said on Thursday.
The research was led by the Church of England (CoE) and fund managers after the collapse of a Vale dam in January unleashed an avalanche of mining waste on the Brazilian town of Brumadinho, killing an estimated 300 people.
The investor review, which found at least 166 dams have had stability issues in the past, relied on companies' disclosures about their dams holding mining waste, known as tailings.
However less than half of the 726 companies contacted have responded, with most Chinese and Indian miners not providing information, leaving a significant hole in efforts to create a global picture of safety risks posed by these dams and avoid another disaster.
"Tailings dams are amongst some of the largest engineered structures in the world and we have seen the catastrophic consequences earlier this year in Brazil when they collapse," said Adam Matthews, ethics director at the CoE Pensions Board, a global investor with assets worth more than $3 billion.
"We note that many companies already operate to a very high standard as evidenced by some of the disclosures, but this is not universal across the sector and dams are continuing to fail, putting lives and the environment at risk."
Tailings dams are the most common waste disposal methods for mining companies, whether they're extracting iron ore, gold or copper. Some tower dozens of metres high and stretch for several kilometres.
There are no established global mining industry standards defining what a tailings dam is, how to build one and how to care for it after it is decommissioned.
The major investors, who manage assets worth a combined $13.5 trillion across a range of industries, wrote to mining companies in April asking for information about tailings dams to be disclosed about every mine they control.
They warned they might have to divest their shares unless they had clear information on potential risks, in what has become one of the largest shareholder mobilisations in history in reaction to a single event.
The CoE and the some other funds sold their Vale shares after the Brumadinho dam collapse, and the Brazilian miner lost a quarter of its market value immediately after the disaster.
Of the 726 companies contacted by investors, 43% responded. All the major listed miners, including Vale, were among those who replied, according to the investors, jointly led by the CoE Pensions Board and the Swedish AP Funds Council of Ethics.
Initial analysis of company disclosures found tailings dams across the globe hold more than 44 billion square metres of waste.
The disclosures so far showed 166 out of 1,635 of tailings dams have had stability issues in their history, although it was unclear how severe those issues had been and the miners said the problems had been addressed, the investors said.
The investors aim to complete a global database of risks posed by dams by the first anniversary of the Brazilian disaster on Jan. 25, and ultimately create global safety standards. Many dams will have to be forcibly closed, investors told Reuters.
John Howchin, secretary general of the Council on Ethics of the Swedish National Pension Funds, said the investors would redouble efforts to secure the missing disclosures.
"There is simply no excuse to not disclose on a material risk, that as owners of these companies, we need to urgently understand. It is clear that investors' patience with non-disclosing companies will not remain for much longer," he said.
Anglo American CEO Mark Cutifani said the sector faced a "clear ethical and moral imperative" to use new technologies to ensure the highest safety standards for tailings.
"Rather than simply scaling up mining's processes to meet demand, the industry will need to find new, more efficient and more sustainable ways of working," he told a meeting of executives in London.
While the Americas are home to most of the world's tailings dams, India and China also store vast amounts of waste in these structures, including the Weikuang dam in northern China which is about 11 km long.
The Weikuang dam is owned by Baotou Iron & Steel Group , which did not respond to the investors' request for data or a Reuters request for comment.
Coal India Ltd and Metallurgical Corp Of China Ltd , two of the world's largest coal miners, said they were not aware of the investor initiative and never received the request. Both companies are controlled by their respective governments.
The CoE said it aimed to work with Chinese and Indian miners over time to compile a truly global database.
"There is, we hope, a cumulative effect where bit by bit in personal relationship with these countries, we hope there'll be a tipping point," said David Urquhart, the CoE's Bishop of Birmingham and a former BP executive.
The eventual aim of the initiative is to set the global standards for tailings dams, together with the International Council on Mining and Metals (ICMM) industry group. The ICMM said in March it was working on new standards with the U.N. Environment Programme (UNEP) and the ethical investors. (Reporting by Barbara Lewis and Ernest Scheyder; Additional reporting by Tom Daly, Sudarshan Varadhan, Min Zhang, Moira Warburton, Helen Reid, Suzanne Barlyn and Jeff Lewis; Editing by Pravin Char)