Geology of Colorado Ore Deposits by Arthur Lakes

Author:Arthur Lakes
Language: eng
Format: epub
Publisher: News printing company
Published: 1888-03-25T05:00:00+00:00

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cii GEOLOGY OF COLORADO ORE DEPOSITS.

granite with the stratified gneiss, along and between stratification planes of schist, and along the joint planes of granite."

He attributes the veins to percolating alkaline waters dissolving metalliferous material and veinstone from the surrounding rocks. Alkaline springs, he observes, still exist in the neighborhood, as they do in the mining district of Idaho Springs. " The veins occur where the foldings are abrupt, and the direction of the veins is parallel to the strike of the stratification. As,a rule, the veins in Boulder County are hot of great extent; a single vein can rarely be traced on the surface or beneath it for more than 600 feet Before that distance is reached the vein spurs off into another vein, follows it for a while, and spurs off again into another.

" Where veins cross at a small angle or where a spur branches off from the main vein, accumulation and enrichment of ore takes place. There are two courses of veins, one East and West, the other North-East by South-West. The former system appears to be the oldest, as the latter faults it.

" The ore occurs in chimneys or pockets, with a good deal of nearly barren ground between. Small veins run parallel with each other for some distance, the interval filled with granite or pegmatite. Sometimes a vein pinches out entirely. The ore streak is from i to 20 inches wide, containing more horn-quartz than the country rock. Some of the veins interlace like arteries in a body. Minute particles of pyrites (marcasite) often produce a dark stain in the telluride quartz. By moistening the stone the telluride minerals and pyrites appear distinctly."

GILPIN COUNTY,

The geology of the mines and veins of Gilpin County, which congregate around the vicinity of Central City and Black Hawk, resembles that of Boulder. The region consists of Archaean granite and granite-gneiss, penetrated by felsite and quartz porphyry dykes. The veins are here also only alterations of the country rock along certain planes, but do not occupy a once wide, gaping fissure. In some mines the vein material is a porphyry dyke. The vein of the Minnie mine is a felsite porphyry;

GEOLOGY OF COLORADO ORE DEPOSITS. ciii

of the Cyclops mine, a quartz porphyry. Dykes of porphyry occur near the lodes or in contact with them.

The veins have been traced to a considerable depth, over i,ooQ feet, and in length over 3,cxxd feet. The direction of veins is between North and South and North-East and South-West The dip is nearly vertical.

The ores are a mixture of iron and copper pyrites, very little galena, and some zincblende. All carry more or less gold.

There is also a silver district in the northern portion. The diameter of the gold district, which is quite distinct, is not more than i^ miles.

In the gold veins the

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