Douglas Island news., 1903-11-04
Mining on Douglas Island

Aiming on D0uglaS ISIdIlu we'll Group of Mines, Douglas Island, Alaska, by Robert A. Kinsie. Assistant Superinten- dent, Treadwell City, AlaslSa.l
 GEoLoGY-Before taking up the sub ject of the underground mining in de- tail, it would be we'll to give brief de- scription of the geological features that bear on the economic extraction of the ore. While the geological conditions are the same in the Treadwell, Mexican and Ready Bullion mines, there are conditions due to faulting, shape and the position of bodies with regard to salt water, that have great influence on the methods and costs of mining.
 The deposit lies at a slight angle. to Gastineau channel. The hanging wall of the Treadwell deposit is at dis tance of 1000 feet from the present shore-line, while that of the Ready Bullion lies for the most part under tidal water.
 According to Prof. Becker, who un- doubtedly had an excellent opportunity of studying the formation and char acter of the ore. bodies, this deposit represents the upper portion or 'feath- CT edge" of an intrusion of sodium sye- nite, or, more logically, an albite dior ite, which, owing to peculiar conditions, has been decomposed and silicifed by solfataric orhydrotheTmal or action, caus- ing concentration of gold in the na- tive state and in the pyrite sufficient to make it profitable mining. The coun, try-rock in this locality is a carbonace- ous slate, in which the original lines of bedding and principal cleavage coin eide. It can be traced for miles both to the north and south. Its strike IS N. 90 deg. w., being practically the same as the ore. bodies and of Gas tineau channel, and coinciding with one of the two Assure systems which affect nearly the whole of the Alexan- der archipelago.
 Following the disposition of the slate came the intrusion of the syenite. This dike is extremely irregular in shape. In the Treadwell mine it has width of 42O feet; going southeast it narrows down to mere stringer until the Mexi- can mine is reached, where it again swells out to width of 150 feet. From the Mexican to the Ready Bullion, distance of 2,5OO feet, it is traced with difficulty on account of faulting and the broken character of the country. At the Ready Bullion it again attains width of 300 feet.
 The next intrusion, that of the gab- bro, was of fundamental importance in the formation of the ore. Accompany ing and following this intrusion came the naturalization of the syenite and the disposition of the ore. The dike of gabbro forms the hanging-wall of the deposit, and varies in width from 8 feet in the Mexican to several hundred at the Ready Bullion. In places the gab- bro is badly shattered and contains nu- merous stringers of quartz; but these are mostly barren, and nowhere is the gabbro converted into ore. This dike can be traced the full length of Doug las Island.
 The third and last of the intrusion was what Becker has called an analcite- basalt. This strikes N. 20 degrees w., cutting the slate, syenite and gabbro, and dips to the west at an angle of S6 degrees. Preceding and during this eruption, the country-rock was evi- dently badly shattered, and the Assures thus formed filled with auriferous ma terial. In no place does the basalt dike contain ore. In the upper levels it is quite continuous, but on the lower lev els each dike has split into several smaller ones, which in places enclose fragments of the syenite.
 In the Treadwell, 700-Foot and Ready Bullion mines, and in the west end of the Mexican, the ore consists of the mineralized syenite. This ore is readily devisible into two distinct varieties. The one consists of stringers of quartz and calcite, occupying the spaces formed by the crushing of the syenite, the syenite itself being somewhat min eralized along the planes of fracture. The other consists of syenite which has been crushed and broken by dynamic agencies, and the fragments thus form ed have been saturated by the mineral bearing solutions.
 .h. a..4 .-,a of 4hn Mnxinnn nina