SOME PETROGRAPHIC DETAILS 461 



Contact ejects. — In a small gulch, just east of Gold Butte Post-Office, 

 on the west side of Middle Butte, some contact effects of poorly exposed 

 small dikes or sills have given rise in former years to a little prospecting 

 by pits and a shaft. The intrusives have penetrated the Colorado shales ; 

 have changed them to hornfels, with accompanying silicifieation, and 

 liave impregnated the rock thus affected with pyrite. A small prospect- 

 ing shaft has been sunk, and from the dump instructive specimens can be 

 collected. The igneous dike which has effected the mineralization is a 

 hornblende-bearing minette. It contains relatively large biotites and 

 hornblendes in a groundmass of fine feldspar rods, predominantly ortho- 

 clase. Much magnetite and coarse apatite are scattered throughout the 

 slide. The igneous rock is frozen tightly to the shale, and obviously 

 following its entrance and consolidation, after-effects, apparently by hot 

 waters or. vapors, were produced in both dike and shales. The contact is 

 no longer sharply discernible, even mider the microscope, being obscured 

 by the silicifieation of both rocks. In the dike, while the large biotites 

 among the phenocrysts and the small feldspar rods of the groundmass 

 are only slightly changed, the hornblende phenocrysts have become chlo- 

 ritized, and then in instances have been replaced with pyrite. Around 

 the pyrite the chlorite residues and secondary calcite appear. Through- 

 out the groundmass are small, irregular bits of pyrite, which have prob- 

 ably taken the place of earlier magnetites. These changes are closely 

 akin to those which have marked the course of mineralization at Butte, 

 Montana ; but at Gold Butte there is no appreciable sericitization, whereas 

 at Butte this change is one of the mpst widespread features.^^ 



In 'the shales silicifieation is marked for a foot or a few feet from the 

 intrusive. It has spread along the favorable layers and has developed 

 both chalcedony and quartz, sometimes destroying, sometimes preserving, 

 the old fragmental texture. Pyrite entered with the solutions and some- 

 times forms small, stocky rings around quartz crystals or grains. In 

 less altered shale the pyrite has spread outwardly along favorable layers 

 and has formed minute lenses, whicli are in shape and relations extremely 

 reminiscent of the large bodies of pyrite in slates and schists, which even 

 in English we describe as "kieslager." Some silica accompanied the 

 pyrite. No assays have been made by us of the pyritized rock, bu|; down 

 the small drainage system into which this particular gulch discharges 

 are the old placer diggings which gave rise to the name Gold Butte. 



The most extensive contact effects have been developed on East Butte, 

 along the northern edge of the laccoliths E. 1 and E. 2. It is bere that 



^6 Charles T. Kirk : Oonditious of miueralizatioii in tlie copper veius at Butte, Mon- 

 tana. Economic Geology, vol. 7, 1912, pp. 35-82. 



