374 Ball — Pre- Cambrian Rocks of Georgetown, Col. 



Pre-Cambrian Rocks. 

 The Idaho Springs Formation. 



Name. — The name Idaho Springs formation is applied to a 

 series of interbedded, metamorphic, crystalline rocks, presum- 

 ably of sedimentary origin, which are typically exposed in the 

 hills surrounding Idaho Springs. 



Distribution. — The formation is widely distributed over the 

 quadrangle and there is scarcely a square mile in which it does 

 not occur in areas of some size injected by igneous rocks or as 

 shattered fragments included in them. 



Petrography. — The Idaho Springs formation includes four 

 intensely metamorphosed crystalline members, three of which, 

 the biotite-sillimanite schist, the biotite-schist, and the quartz- 

 gneiss, are interbedded with and grade into one another, while 

 the fourth, the lime-silicate rocks, although interbedded with 

 the others, appears only to grade into the quartz gneiss. 



The biotite-sillimanite-schist is a foliated and often intensely 

 crennlated, normally tine-grained black rock which on weath- 

 ered outcrops has a rusty appearance. Biotite, quartz, feld- 

 spar and sillimanite are always recognizable megascopically, 

 and muscovite, garnet, tourmaline and corundum are some- 

 times prominent. Sillimanite occurs in single rods and bundles 

 of rods elongated in the plane of schistosity and these are cut 

 by transverse fractures whose interstices are filled by biotite 

 flakes. The biotite-sillimanite-schist is injected lit-par-lit by 

 an ancient pegmatitic rock and it also often contains "eyes" 

 of a more modern pegmatite, which will be referred to later. 



Microscopic examination shows the texture to be that of a 

 typical schist, the constituents all having a common parallel 

 alignment, that of the biotite and muscovite plates and the 

 sillimanite rods and bundles being very pronounced. The 

 feldspars prove to be orthoclase and microcline with some 

 plagioclase (albite or an acid oligoclase, rarely andesine) whose 

 twinning is notably uneven. Quartz and orthoclase are often 

 micropegmatically intergrown and in a rock which has suf- 

 fered such profound metamorphism it seems probable that this 

 texture originated during recrystallization. Sillimanite was 

 produced largely at the expense of feldspar, and to a less extent 

 of biotite. Sillimanite, in turn, sometimes alters to a kaolin- 

 itic material. Zircon, apatite and magnetite, rather constant 

 accessory minerals, are only of interest from the fact that the 

 last two often show a distinct orientation parallel to the plane 

 of schistosity. Corundum, andalnsite, and in part garnet, 

 muscovite and tourmaline are clearly later than the major 

 recrystallization of the rock, and from field relatious are con- 

 sidered products of contact metamorphism. 



