Which of these contributed most directly to the sand creek massacre

The Sand Creek Massacre occurred in 1864, when peaceful Cheyennes and Arapahoe were assassinated by a band formed by Colonel John Chivington at Sand Creek Colorado. The main reason of this massacre was the long conflict to control the Great Plains of Eastern Colorado. The 1851 Fort Laramie Treaty established that the area from the North of the Arkansas river to the border of Nebraska belonged to the Cheyenne and Arapahoe but by the 1860´s Euro-American miners went across the region in search of gold in the Colorado Rocky mountains and this situation destabilize the zone into a new deal with the Cheyennes and Arapahoe which never came to a good term, producing the Sand Creek Massacre.