Vale Exploration, which was funding the work, decided to terminate its option agreement it signed on March 2, 2009.
Vertical hole FM_1001, which was drilled on the Fourmile prospect, was testing a circular magnetic feature ringed by a strong conductive zone found by a airborne geophysical survey.
The hole was pre-collared to a depth of 453 metres using a reverse circulation drill, the company said. Core drilling proceeded through tertiary volcanic rock cover.
Millrock said drill conditions were very poor and the hole was lost at a depth of 1,231 metres. The hole was terminated before reaching target depth.
The target at the Galiuro property is a large porphyry copper plus minus gold deposit hidden beneath alluvium and tertiary volcanic cover, similar to the Resolution deposit, which is being developed by Rio Tinto and BHP Billiton.
The property is situated at the intersection of the Resolution and San Manuel/Kalamazoo porphyry copper trends. Because of the Tertiary volcanic cover, other companies have previously done little exploration work.
Through careful geological mapping in combination with airborne magnetic and geophysical surveys, Millrock identified numerous targets that may represent porphyry deposits.
With the withdrawal of Vale from the project, Millrock is seeking new partners to test the remaining targets.
The Galiuro project is made up of 366 federal mining claims and 18 Arizona State prospective permits covering about 6,800-hectares, and is located near the San Manuel and copper district in southeastern Arizona.
Millrock currently has 12 active exploration projects, eight gold-copper properties in Alaska, and four porphyry copper prospects in Arizona.