GEOLOGY OF THE LOWER GILA REGION, ARIZONA. 



197 



younger fill was deposited. A'olcanism of 

 minor extent occurred during this epoch. 

 Near Bouse, Yuma County, volcanic ash oc- 

 curs in the fill not far from the present sur- 

 face. This deposit is probably comparatively 

 recent. Several of the lava flows may be of 

 corresponding age. 



In comparatively recent time erosion of the 

 younger fill has commenced, as is shown by 

 terraces cut in it. The present flood plains 

 of the streams lie between the lowest of these 

 terraces. Along both Colorado and Gila rivers 

 other terraces can be discerned above these, 

 but they are discontinuous and apparently not 

 significant. 



At the present time both rivers are aggrading 

 in their lower courses. Their channels are 

 gradually being filled by the deposition of fine 

 silts. Both rivers are well known for the large 

 quantities of silt carried by their waters during 

 floods. At all times they are remarkably 

 muddy. 



MINERAL DEPOSITS. 



This part of Arizona has been extensively 

 prospected. Mineral deposits are now known 

 to occur in every mountain range and in many 

 of the groups of hills within the region. The 

 only hills in which mineral deposits have not 

 been, and in all probability will not be found, 

 are those composed exclusively of Quaternary 

 basalt. 



The types of deposits and the minerals 

 found are many and diverse. Mining has been 

 carried on in this region for gold, silver, copper, 

 lead, zinc, mercury, iron, and manganese. 

 There has been some prospecting for tungsten, 

 but no mining for this metal has been done. 

 Fluorite occurs in the Castle Dome district and 

 possibly elsewhere but so far as known has 

 not been developed. Gypsum occurs in some 

 places in the region, but no deposits of com- 

 mercial importance are known. 



Mining is in progress in several of the moun- 

 tain ranges in this region, but there are no 

 large mines operating at present. In the past 

 the Vulture mine, in the range of the same 

 name; the mines about Kofa, in the S. H. 

 Mountains; the Harqua Hala or Bonanza mine, 

 in the Little Harquahala Mountains; and other 

 less well-known properties have shipped con- 

 siderable gold ore. Silver and lead were mined 

 for some years in the Castle Dome Mountains. 



Gold placers were formerly worked along Colo- 

 rado River near La Paz and Elirenberg, in and 

 near the Dome Rock and Plomosa Mountains, 

 and at Gila City on Gila River, at the site of the 

 present town of Dome. Some placer mining 

 is still in progress in the Plomosa and Dome 

 Rock mountains, but elsewhere activity of this 

 sort has almost entirely ceased. The lack of 

 water appears to be the principal obstacle to 

 the successful development of the placers. 

 In the old vein mines the richer and more 

 accessible portions of the ore bodies have been 

 worked out, and lack of transportation facili- 

 ties and of capital have prevented further devel- 

 opment. The Harqua Hala mine is now being 

 reopened for copper. It is possible that many 

 of the mines now abandoned could again be 

 made profitable producers by extending the 

 workings deeper. 



At present (1918) there is considerable activ- 

 ity in the small copper mines in the vicinity of 

 Cunningham Pass, in the Harcuvar Mountains. 

 Mining for copper and other metals is being 

 carried on in the Buckskin and Plomosa moun- 

 tains and to a small extent elsewhere. More or 

 less desultory prospecting is in progress in all 

 the mountain ranges. In 1918 plans were 

 being discussed for a reopening of some of the 

 mines in the vicinity of Kofa. 



A detailed discussion of the geologic features 

 of the ore deposits in the region is beyond the 

 scope of this paper, but descriptions of some 

 of the mines will be found in the final report '' 

 now being prepared. Bancroft '"' has described 

 the deposits in the northern part of the region, 

 and the deposits noted in the southern part 

 appear to be in general similar to the types he 

 describes. According to him there were tlu'ee 

 periods of mineralization — pre-Cambrian, Meso- 

 zoic, and Tertiary. He describes numerous 

 types of deposits belonging to these periods, 

 also the placers in the vicinity of Quartzsite. 

 The reports of the governor of the Territory 

 of Arizona contain many references by W. P. 

 Blake, Territorial geologist, to deposits in this 

 area. Jones has described deposits in the 

 Dome Rock Mountains " and near Kofa, in the 

 S. H. Mountains." 



'3 Ross, C. P., The lower Gila region, Ariz.: U. S. Geol. Survey Water- 

 Supply Paper — (in preparation). 



^* Bancroft, Howland, op. cit. 



^1 Jones, E. L.,jr., Gold deposits near Quartzsite, Ariz.: U. S. deol. 

 Survey Bull. 620, pp. 45-.57, 1916. 



*2 .Tones, E. L.,jr., A reconnaissance in the Kofa Mountains. Ariz.: 

 U. S. Geol. Survey BuU. 620, pp. 151-164, 1916. 



