The gold mining activity in Colorado is related to the mines situated in Gilpin county. Concerning the lodes, they are enclosed in a rock of one common type chiefly granitic, with gneissic varieties. The Bobtail lode was the most productive vein Colorado. The average course for 800 ft is due East to West, the dip is nearly vertical, varying from North to South; the width of the vein ranges from a few inches or a mere seam to 10 or 15 feet. The enclosing rock is gneiss, sometimes of granitic character. The walls are usually well defined. The vein matter was a quartzose and felspathic mixture, highly siliceous and carrying free quartz, but did not have the appearance of solid quartz veins. In many places it had a granitic look differing from the country rock. The gangue is soft, whitish or pale-greenish rock and the richer ore was concentrate in a seam of solid sulphides composed mainly of pyrite and chalcopyrite, with less quantity of galena, sphalerite and arsenopyrite. The gold, rarely if ever entirely without silver was associated with pyrite, most richly with fine chalcopyrite and varying in the pyrite content according to the closeness of grain. The productive part of the vein carried a scam of solid auriferous pyrite, 1 in to 3 ft wide, an average of 10 to 12 in being deemed excellent.
The Gregory lode was worked over a length of 1500 to 2000 ft. the country rock is generally similar to that of the Bobtail, but sometimes abounding in mica. The walls were not very regular and were liable in places to scale off and fall in large pieces. The vein matter was like that of the Bobtail. The value of the compact ore was somewhat less, but its development was greater. Some lumps of free gold were found. The yield of the lode can be only partly stated with regard to one portion of it. The Black Hawk mine in 1867 produced 12,200 oz of crude bullion. In the latter half of 1868, 4,000 oz. of amalgam. In the first half of 1869, 3100 oz of retorted amalgam.
The Bates lode was located nearly parallel to and several hundred feet North-West of the Gregory. It was worked to considerably depth for about 1100 ft in length. The country rock has the same feature of the other lodes. The veins were somewhat less in width, but regular, well defined and promising as to permanence. The ores in some parts were distinguished by a large proportion of argentiferous galena, blende and silver sulphides associated with pyrite, given them sometimes a high value in silver, but rendering difficult or practically impossible by any simple process known in the district the extraction of gold as a placer deposit.
The Garner lode was developed over about 800 ft in length. The vein was large and regular, rarely less than 3 or more than 10 to 12 ft wide; the walls were smooth and well defined. The ore is similar to the previous lodes, but containing considerable galena and blende, and the valuable mineral is widely diffused. About 1/2 of 2/3 of all the vein matter broken was sent through the stamps and produced an average of 6 oz of bullion per 20 tonnes of material processed.