The first house was built in August 1877 after two
prospectors, Dan Dugan and Dave Stitzel, working in the area of the future Opportunity Mine, collected loose pieces of rock (float is the geologic term) assayed at $160 per ton in gold and
silver. Soon,
ore was discovered at the nearby Rattlesnake
vein (geology) by Dugan and Frank Pitcher, and a
placer deposit of gold was found in November at the Rattlesnake and Wicks
gulches. Ore was pulverized with arrastres located in Hillsboro, including a 10-
stamp mill built in 1878. Total production prior to 1904 was about $6,750,000. Between 1911 and 1931, total production of gold ore was 836 tons, gold-copper-silver ore was 5,470 tons, and
copper ore was 200 tons. In his report, Harley wrote, "The ore deposits of the Hillsboro or Las Animas mining district are of four general types as follows: (1)
fissure veins in
andesite flows, (2) disseminated deposits in
monzonite porphyry, (3) replacement deposits in
limestone, and (4) placer gold deposits." The Copper Flat
volcano to the northeast of Hillsboro is the source of the gold in the area. The 4 mile wide volcano is characterized by a
Cretaceous quartz monzonite stock within a topographic depression, surrounded by surface
andesite flows at least 2,300 feet thick, and radial
quartz latite or
rhyolite dikes. The stock generated the
porphyry copper deposits, gold-bearing veins, and gold placers. There are about 26 major veins forming a radial pattern from the stock. The Golddust Camp placer deposits are found in the
alluvium of the dry
gulches radiating off Copper Flat and the andesite flows. The fissure veins produced 51,000 ounces of gold, while the placer deposits produced 110,000 ounces. The Sternberg Mine at Copper Flat produced 200 tons of copper ore between 1911 and 1934. By 1959, Bear Creek Mining Company had discovered a
mineralized breccia pipe within the stock. Quintana Minerals Corporation started
open-pit mining in 1980, averaging 0.45 per cent copper. Ore minerals include
pyrite,
chalcopyrite,
sphalerite and
galena. ==Climate==