The Union Mine was located by Clint Duffey, Wash Tice, Bill Burdette and Fred Huntington in 1884. It was among the first to be discovered. The mine was worked from 1884 until 1941 with only three years of shutdown, from 1927 to 1930.

It is situated in the Cornucopia Mines Group in Section 28, Township 6 South, Range 45 East, on the steep west slope of Pine Creek, Twelve miles north of Halfway.

The Union Mine was from the beginning the richest of the mines in the Cornucopia area.

Records show the Union Mill was on of the area's earliest large mills in operation.
Work began on the Union Vein in 1888.

A twenty stamp mill was built on the Union Mine in 1892, by the Cornucopia Mines Company of Oregon, a company that went into a receivership about 1907-08. Robert Marion Betts, representing a group of millionaires in New York, came to the mines and the Cornucopia Mines Company of New York was formed. Betts stayed as manager for the company and the company built him a mansion to live in at Cornucopia.

The old Union Mill, which was at the portal of the Clark Tunnel, was rebuilt into a slime cyanide plant in 1912 and 1913 by the New York owners.

It was in 1898 that Andrew Brown earned his place in mining history by wheeling the first wheelbarrow of ore out of the Clark Tunnel.

The Union and Companion Mines were combined early mining activity. The development honey combing the inside of the mountain criss-crosses and consists of three main adits, following the Union-Companion and Last Chance ledges over three thousand feet vertically. The deeper the gold veins were, the richer they were, with the lowest adit being about the level of Pine Creek.

Last Chance ore was worked in the Union-Companion Mill for three years. The Union-Companion Mill ceased to operate in 1936 when Coulter Tunnel reached Union workings. The Cornucopia Mill at the mouth of the Coulter Tunnel then began its operation.

When the Union-Companion Mill closed down, some equipment, including flotation units, were moved to the new Cornucopia Mill.

Durning the early 1930's, a tube type ball mill was used at the Union-Companion mill in addition to te 20-stamp mill equipment.

To save on the cost of steel balls, volcanic pebbles were brought in to grind the ore from Lookout Mountain, near Richland.

The Union mill used stamps for grinding ore longer that any other mill.