82 ALASKAN MINERAL RESOURCES IN 1904. [bull. 259. 
takes the place of the pyrite and may be readily isolated from the 
concentrates by means of a magnet. Chalcopyrite, galena, and sphal 
erite occur sporadically, and native arsenic, realgar, and orpiment 
have been noted in small quantities. Assays are said to indicate the] 
arsenical nature of much of the pyrite, though the presence of true 
arsenopyrite has not been recognized. Molybdenite is frequently 
noted, though it is irregularly distributed. 
OCCURRENCE OF GOLD. 
Visible gold has been observed in veins of coarsely crystalline cal- 
cite inclosed in the ore bodies. This occurrence is, however, rare, and 
in general even the microscope does not reveal the form in which the 
precious metal exists. I have not been able to distinguish gold in the 
thin sections studied under the microscope, but Prof. F. D. Adams, 
who examined the material collected by Dawson in 1887, observed 
gold mechanically inclosed in crystals of pyrite. It is evident that a 
considerable amount of gold must be in the metallic condition, because 
a large proportion is saved by amalgamation, the amount sometimes 
being as high as 75 per cent of the total assay value. 
The gold is perhaps mainly associated with p3 T rite, but rather coarse 
crushing is the present mill practice,® and so much of the pyrite passes 
the screens in comparatively large grains or unbroken crystals, that it 
seems open to doubt whether from 60 to 75 per cent of the gold could 
be free-milling if it were all associated with the iron sulphide. The 
nonamalgamating portion undoubtedly does occur with the pyrite, 
because the concentrates contain only pyrite and magnetite, with a 
small amount of pyrrhotite, all the molybdenite going into the tail- 
ings. Molybdenite can hardly be an important carrier of gold, 
because it seems to be somewhat limited in distribution, although its 
presence in visible quantities is said to indicate high values. 
As a rule, the values vary with the amount of interstitial vein mat- 
ter, but the position of the pyrite in the rock or in the quartz and caB 
cite seem to have no influence upon the amount of gold. In some 
places, where the ore is of average grade, all the metallic minerals 
seem to be in the rock, and careful search is necessary for the discoi 
ery of any sulphide in the quartz or calcite. Elsewhere the sulphides 
may be confined almost entirely to the veinlets. A limited amount of 
material is mined which contains practically no stringers of quartz ol 
calcite, the sulphide being disseminated through the mass of the 
rock — for instance, in the crosscut on the 440-foot level and in stope 
No. 1 of the 330-foot level in the Treadwell mine. In other placJ 
material of similar appearance, containing an equal amount of pyritJ 
yields only a veiy small amount of gold. 
a Slot screens equivalent to 18- and 20-mesh wire screens are used. 
