Debt comprises only 12 per cent of the funding for mining companies, with bank debt accounting for only one third of that amount, research by the Reserve Bank of Australia shows.While it is well known that mining companies have had little reliance on banks as a source of capital, the RBA research casts a fresh light on the topic.In a speech to a Melbourne Institute forum in Canberra yesterday, Guy Debelle, assistant governor of the RBA, said that "analysis shows that 80 per cent of investment spending by Australian-listed resources companies, and over 90 per cent by foreign companies, has been funded through internal sources. "Operating cash flow has been the predominant source of internal funding, made possible by the historically high level of commodity prices throughout the period."Debelle's colleagues examined the financial accounts of around 900 listed Australian resources companies and 37 foreign companies."This analysis shows that since the start of the resources boom in 2003, Australian companies have raised no equity in net terms. "While they raised substantial amounts of equity in 2009, the Australian companies have been returning greater amounts of capital to shareholders since 2011. The foreign companies have been consistently returning large amounts of capital to their shareholders, and, as a group, have not resorted to external equity funding during the resources boom."The funds raised by these resources companies have been used overwhelmingly to make new physical investments, with Australian and foreign companies using around 80 per cent of their funding for this type of spending. "The rest of the funding has been used primarily to acquire existing assets from other companies, while funding for other investment purposes has been negligible.Debelle said that the "absence of borrowing from the banking sector is one of the reasons why intermediated business credit in Australia remains relatively subdued despite the around trend growth in the Australian economy in recent years. "It also means that unlike previous investment booms in Australia's history, this one has been associated with very little leverage."