392 



UNITED STATES MINERAL RESOURCES 



tain. The depth of the water into which submarine 

 volcanics were extruded may be one governing fac- 

 tor, for below a certain depth the confining water 

 pressure is so high that included volatiles in the 

 molten lava cannot readily escape. 



Because many volcanic piles and extensive areas 

 of volcanic rock contain no concentrations of man- 

 ganese, the mere presence of volcanic rocks is no 

 indicator of potential manganese discoveries. Char- 

 acteristically, however, if one manganese deposit is 

 found in such rocks, many others will be found in 

 the same general area. Commonly, one or two de- 

 posits will be much larger and (or) higher grade 

 than the others. Although deposits are known from 

 continental volcanic rocks, the more important de- 

 posits are found in a marine or lacustrine environ- 

 ment, as in Mexico, Chile, and Cuba, and thus are 

 really sedimentary deposits, in the strict sense of 

 that term. 



Volcanogene deposits are commonly but not ex- 

 clusively composed of the oxide minerals, and the 

 mineralogy is usually somewhat more complex than 

 that found in unmetamorphosed sedimentary de- 

 posits without volcanic affiliation. Replacement of 

 the diluting clastic and pyroclastic fragments in the 

 ore horizon by manganese minerals is common ; this 

 seems to be a diagenetic feature in many places but 

 may also be the result of supergene or hypogene 

 alteration. In many such deposits, chert in small to 

 rather large lenses accompanies, underlies, or over- 

 lies the manganiferous material, which here may be 

 the hydrous manganese silicates bementite and 

 neotocite. These manganese silicates are not ore 

 minerals. 



Deposits large enough to be economically interest- 

 ing are commonly either sharply lenticular or strati- 

 form ; the larger ones are stratiform. Numerous but 

 normally very small deposits may be associated with 

 altered volcanic rocks and spilites. The volcanogene 

 deposits interfinger with tuflFaceous and (or) cal- 

 careous sediments or cherty and jaspery rocks. 

 Local unconformities and abrupt lithologic changes 

 in the host rocks are common. 



Reserves in volcanogene deposits are character- 

 istically small to medium in size. Very few deposits 

 contain more than a million tons of recoverable ore, 

 the largest probably being the Charco Redondo de- 

 posit in Cuba, which has produced about 5 million 

 tons, and the Autlan deposits in Mexico, which pro- 

 duced about 4 million tons before exhaustion. Both 

 are stratiform deposits. 



With some notable exceptions, such as the Cres- 

 cent mine in the Olympic Peninsula, most such 

 deposits are medium grade, and the ore must be 



concentrated before shipment. 



As with many other types of ore deposits, the 

 localization of volcanogene deposits is apparently 

 caused by relatively abrupt and localized changes 

 in the nature or environment of the fluids which 

 brought the manganese to the site of deposition. 

 For deposits such as those in the Olympic Penin- 

 sula, the interface between the hot lava and the sea 

 water into which it was extruded was the site of 

 an abrupt change, and the manganese was depos- 

 ited at and near that interface. For deposits such 

 as Charco Redondo, the manganese evidently was 

 transported some distance and deposited in a sedi- 

 mentary environment with limestone and water- 

 laid tuff. 



The sources of the manganese deposited in such 

 bodies may be either fluids evolved from the magma 

 at depth during volcanism, or, in small deposits such 

 as those in the Olympic Peninsula, perhaps the al- 

 teration of the rock as it reacted with the sea water. 



HYPOGENE DEPOSITS 



Myriad small and a few medium-sized deposits of 

 manganese ores are of hypogene origin — ^that is, 

 they were deposited directly from warm fluids ris- 

 ing from sources deep in the earth. Some of these 

 deposits merge into the volcanogene type, and it is 

 impossible to be sure how closely they are related 

 genetically to the volcanic rocks that enclose them. 

 They also merge into the sedimentary type in that 

 the fluids that brought the manganese into the ba- 

 sins in which sedimentary ore deposits formed by 

 the normal sedimentary processes are thought, in 

 some instances, to have been thermal waters. 



The hypogene deposits occur in the same struc- 

 tural setting as the base- and precious-metal depos- 

 its with which they are often associated. Most are 

 related to intrusive rocks, about which they are 

 zonally arranged. They may take the form of veins, 

 as at Butte, Mont., or replacement deposits in lime- 

 stone or dolomite, as at Philipsburg, Mont. Another 

 type, more abundant in the Basin and Range prov- 

 ince of the United States and Mexico, is the vein 

 deposit, predominantly in volcanic rocks. A few 

 minor deposits are in the aprons of hot springs. 



Most of the vein deposits formed at relatively low 

 temperatures. In most, the manganese is in oxide 

 form, but in a few it is in carbonate form. The few 

 known occurrences of manganese sulfide are typi- 

 cally in veins. The replacement deposits may be in 

 either oxide or carbonate form; carbonate deposits 

 have commonly been oxidized at and near the surface 

 by supergene action. 



Few vein deposits have been followed to consid- 



