64 ECONOMIC GEOLOGY 



lite which appears at the 800-ft. level of the Homestake mine and 

 is a common rock in various parts of the northern Black Hills 

 (Fig. 45). 



According to J. D. Irving, the ores of the Homestake zone are 

 poorly defined masses of rock sufficiently impregnated with gold 

 to pay for working, but otherwise hardly to be distinguished from 

 the country rock in which they occur. They are singularly bar- 

 ren of the usual ore minerals. The gold occurs in so finely divided 

 a state that the particles are invisible even with a magnifying 

 glass. Leaf gold has, however, been found but without evidence 

 of crystalline structure. Pyrite and arsenopyrite are the only 

 other metallic minerals present. The former is more abundant. 



Quartz is the most abundant gangue mineral. It occurs in 

 veins or lens-shaped masses often of considerable size and of sev- 

 eral different periods of formation. Calcite and dolomite are 

 also present as gangue minerals usually of secondary origin, but 

 not universally present. 



Origin of The Homestake Ores : There is no definite evidence as 

 to the source of the gold and pyrite. Irving considers them to 

 have been leached from the rocks at some distance below the sur- 

 face by percolating waters and to have been precipitated in con- 

 tact with graphitic matter and possibly also with original pyrite, 

 present in the slates. 



A second period of mineralization came after the later intrusion 

 of the rhyolite porphyry, followed the same general channels and 

 deposited the gold and pyrite. This intrusive did not stop at the 

 Cambrian contact, but continued on through cracks and fissures 

 into the Cambrian rocks and deposited gold and pyrite abun- 

 dantly in the basal conglomerate of the Cambrian series and in 

 the calcareous terranes immediately overlying the outcrop of the 

 Homestake belt. In the conglomerate, wolframite replaces some- 

 what, the pyrite but there is no evidence of pneumatolytic action. 



The secondary enrichment of the ores by surface leaching has 

 been of relatively small importance. There is little evidence of 

 decrease in value of ore with depth. In fact the size of the ore 

 body appears to be increasing rather than decreasing with descent. 

 The ore as a whole averages between $5 and $6 per ton. 



According to J. D. Irving, the ore occurs in three distinct var- 

 ieties. (1) Banded ore: That is, ore wherein the mineralization 

 has not been accompanied by distortion of the original structures 

 of the rock. (2) Contorted ore : That is, ore where the original 



