Question: In 1940, where was the richest source of uranium?
About the article "Manhattan Project", section "Ore".

Candidate passages:
 "See main article Uranium mining in Colorado" Uranium.
A small amount of uranium ore has been mined from the Dakota Sandstone at Morrison, Colorado, where the sandstone is impregnated with petroleum.
Uranium is known to exist in roll-front type deposits in the Denver Basin, but the basin has never been a major source of uranium. 
The key raw material for the project was uranium, which was used as fuel for the reactors, as feed that was transformed into plutonium, and, in its enriched form, in the atomic bomb itself.
There were four known major deposits of uranium in 1940: in Colorado, in northern Canada, in Joachimstal in Czechoslovakia, and in the Belgian Congo.
All but Joachimstal were in allied hands.
A November 1942 survey determined that sufficient quantities of uranium were available to satisfy the project's requirements.
Nichols arranged with the State Department for export controls to be placed on uranium oxide and negotiated for the purchase of of uranium ore from the Belgian Congo that was being stored in a warehouse on Staten Island and the remaining stocks of mined ore stored in the Congo.
He negotiated with Eldorado Gold Mines for the purchase of ore from its mine in Port Hope, Ontario, and its shipment in 100-ton lots.
The Canadian government subsequently bought up the company's stock until it acquired a controlling interest.
While these purchases assured a sufficient supply to meet wartime needs, the American and British leaders concluded that it was in their countries' interest to gain control of as much of the world's uranium deposits as possible.
The richest source of ore was the Shinkolobwe mine in the Belgian Congo, but it was flooded and closed.
Nichols unsuccessfully attempted to negotiate its reopening and the sale of the entire future output to the United States with Edgar Sengier, the director of the company that owned the mine, Union Minière du Haut Katanga.
The matter was then taken up by the Combined Policy Committee.
As 30 percent of Union Minière's stock was controlled by British interests, the British took the lead in negotiations.
Sir John Anderson and Ambassador John Winant hammered out a deal with Sengier and the Belgian government in May 1944 for the mine to be reopened and of ore to be purchased at $1.45 a pound.
To avoid dependence on the British and Canadians for ore, Groves also arranged for the purchase of US Vanadium Corporation's stockpile in Uravan, Colorado.
Uranium mining in Colorado yielded about of ore. Mallinckrodt Incorporated in St. Louis, Missouri, took the raw ore and dissolved it in nitric acid to produce uranyl nitrate.
Ether was then added in a liquid–liquid extraction process to separate the impurities from the uranyl nitrate.
This was then heated to form uranium trioxide, which was reduced to highly pure uranium dioxide.
By July 1942, Mallinckrodt was producing a ton of highly pure oxide a day, but turning this into uranium metal initially proved more difficult for contractors Westinghouse and Metal Hydrides.
Production was too slow and quality was unacceptably low.
A special branch of the Metallurgical Laboratory was established at Iowa State College in Ames, Iowa, under Frank Spedding to investigate alternatives, and its Ames process became available in 1943.
Uranium sulfate (U(SO4)2) is a water soluble salt of uranium.
It is a very toxic compound and should not be ingested.
Uranium sulfate minerals commonly are widespread around uranium bearing mine sites, where they usually form during the evaporation of acid sulfate-rich mine tailings which have been leached by oxygen-bearing waters.
Uranium sulfate is a transitional compound in the production of Uranium hexafluoride.
It was also used to fuel aqueous Homogeneous Reactors. 
__NOTOC__The Lorado Mine was a uranium mine in northern Saskatchewan, Canada located around south of the community of Uranium City, Saskatchewan in the Beaverlodge Uranium District. 
The Eldorado Mine at Port Radium was a source of uranium ore for the project.