VIENNA — Oil, gas, banks and more, worth tens of billions of dollars to the highest bidder. Less than 10 days after signing its nuclear deal, Iran formally threw open its doors Thursday to European Union companies, eager to do business – and dozens of firms answered the call. Amir Hossein Zamania, an Iranian deputy oil minister, told the “Iran-EU Conference, Trade and Investment” forum that by 2020, his country hoped for foreign partnerships for oil and gas projects that alone were worth $185 billion (nearly 170 billion euros.) But even more than oil and gas are at stake, with Iranian officials also pitching the mining sector, Iran’s automotive industry, and the financial sector, at what they bill as the first forum of its kind since Tehran and six world powers sealed the deal. Nearly three times the size of France, Iran holds plenty of economic promise.