From Lakota America: A New History of Indigenous Power, by Pekka Hämäläinen (The Lamar Series in Western History; Yale U. Press, 2019), Kindle p. 342:
In 1874 the times were bad, spectacularly so, and when news arrived that rich veins of gold had been found in the Black Hills, it galvanized the nation. Gold had helped lift the nation from the material and moral ruins of the Civil War; now it could lift it from a debilitating depression.
The news came from one of the many small expeditions that the U.S. Army had sent in to find a suitable site for a fort near the Black Hills to protect the tracklayers of the blocked Northern Pacific Railroad. The fort, Sherman thought, would allow the army to deliver a crippling blow to the seemingly invincible Lakotas whom the national media now conflated with rebellious blacks, Chinese immigrants, disaffected farmers, and labor activists as an acute threat to the fragile industrial order. Sheridan, who had orchestrated total war against Native civilians during Cheyenne, Arapaho, and Comanche campaigns in 1868–69, believed that Lakota hostility now amounted to “a general Indian war” and proposed to President Grant that military forts in the Black Hills would “make it a little hot for the [Lakota] villages and stock if these Indians attempted to raid on the settlements south.” Grant agreed, and Sheridan picked Custer, whose conduct during the Yellowstone Expedition had much enhanced his professional standing, to lead the Black Hills Expedition: one hundred covered wagons; more than nine hundred cavalry and infantry; sixty-one Arikara scouts; several guides, engineers, and “practical miners”; three Gatling machine guns; three journalists; a photographer; and a geologist, Newton H. Winchell from the University of Minnesota, all moving out of Fort Abraham Lincoln, skirting the Great Sioux Reservation and entering the hills from the north. In mid-August, after six weeks of travel, the convoy found traces of gold. Custer exaggerated the discovery, and the expedition delivered what he had geared it up for: it created a national event. Reporters dispatched excited press releases, newspapers picked up and magnified the story, and in the late fall of 1874 the Black Hills gold rush was a reality.