A reservoir with very little water located in the Star Ridge area of the Owyhee herd management area.
However, the muster was postponed when four horses were found dead the following day from complications related to dehydration. Another three were euthanised after failing to respond to treatment. Another suffered a broken leg in holding pens and was euthanised.
In all, 12 have died from the 228 initially brought in off the range.
The bureau said there appeared to be a lack of drinking water in the herd area and it called a halt to operations to reassess the operation.
However, US District Court Judge Larry Hicks, sitting in Nevada, issued a restraining order on Wednesday preventing the gathering of horses from the herd areas until further order of the court.
Nevada writer and artist Laura Leight had filed her bid for a temporary restraining order on Tuesday, having filed a lawsuit last Friday against the muster.
At an initial hearing that day, the bureau explained that the gather had been postponed to, at the earliest, this Sunday.
On that basis, the court scheduled a hearing on Leigh's motion for 9.30am today (NZ time).
However, the court was informed mid-week that the bureau director, Bob Abbey, had authorised an emergency gather of horses to begin before the scheduled hearing.
Judge Hicks said: "Based on this change in the bureau's position, the court finds it necessary to grant an immediate injunction preventing the Tuscarora gathering of wild horses until further order by the court."
The Tuscarora roundup, in which 1200 horses are targeted, has been condemned by wild horse advocates.
The Cloud Foundation had warned of the dangers of mustering in the Nevada summer heat, and argued the sharp volcanic terrain was too rugged to be safely running foals over.