Show investigation invest i gati on of low grade and complex ores in colorado BY R R HORNOR AND V H COGHILL it has been recognized that the metal mining industry in colorado has been on the wane for several years according to statistics compiled by the state board of immigration colorado year book 1920 mining now ranks fourth among the basic industries of the state being exceeded by agriculture live and manufacturing this marked decline of metal mining from its former supremacy in the state is due partly to the unusually rapid growth of the other industries and partly to the decrease in mineral production both in quantity and value the richer and more accessible ores have been largely exhausted and the remaining ores are in many instances too low grade or too complex for profitable treatment by the usual methods the future of the industry has become therefore a matter of grave concern to the mining men of the state and of deep interest to the state government and the united states bu bureau reau of mines with a view to aiding and encouraging metal minin mining in the state and in the hope that the industry might be restored to something of its former importance a fund of was made available by the state under an act approved december 19 1919 for an investigation of the low grade and complex ores of the state to be undertaken jointly by the U S bureau of mines and the colorado school of mines the object was to determine the character and approximate extent of these ores and where found in sufficient quantity to conduct concentration tests at the golden laboratories in an effort to devise a cheap and suitable treatment it was thought this might result in making available a large amount of low grade material left in the mines on the dump or in unworked deposits work was begun in february 1920 and concluded in december field work in the mining R R hornor mining engineer of the bureau of mines had charge of the field survey assisted by B C essig representing the state they visited each of the principal mining districts in the state studied mining and milling practice character of the ores being mined and the treatment of problems involved in all sixty three samples were obtained and forwarded to golden for test the low grade ores occur both as oxides and sulphides sulp hides the latter being the more abundant the sulphides sulp hides are usually of complex nature and frequently consist of an intimate mixture of galena pyrite and together with the sulphin mining alining engineer and metallurgist respectively ively of the U S bureau of mines kimonides timo and of silver in the central city and idaho springs districts in sou thein gilpin county and northern clear creek county the ores may be classi classified fled according to the predominating metals as 6 gold ol silver ores copper ores and uranium ores the gold silver ores may be grouped in three types ore with some galena with pyrite and and composite ores intermediate between the first and second types the veins are worked chiefly for gold and silver and in some deposits copper lead and zinc are the important minerals at georgetown and adjacent districts in clear creek county the ores are chiefly gold bearing ore and silver ores containing chiefly galena blende and pyrite the marked decline in mining in gilpin and clear creek country is due primarily to depletion of the richer ore bodies and impoverishment of ores with depth to some extent the complexity of the ores and difficulties in treatment with high costs of mining freight rates and smelter rates is responsible but the chief cause is lack of ore the mines offer little encouragement of finding important ore bodies be below low depths already attained two methods of procedure are suggested as a possible means of helping the industry the first is the establishment of central custom mills employing improved methods for treating the complex ores such mills might best be owned and operated on a cooperative basis the second is that careful and systematic prospecting be done along the strike and in hanging and footfalls foot walls of the veins in those zones known to have contained rich ore bodies in order to disclose deposits overlooked in early operations this work should be much more likely to disclose ore of commercial grade than any search in the zones opened by the various deep tunnels of the district which in every instance have been disappointing and have not given the results predicted by their promoters leadville still premier district L Lead cadville ead ville villem in lake county is still unquestionably tion ably the premier mining district of the state the bulk of Lead villes present production is sulphide ores chiefly pyrite carrying gold and silver which are smelted smelter direct the second largest tonnage is of mixed sul aphides of iron lead and zinc the higher grade zinc ore is smelted smelter direct but the lower grade and mixed sulphide ore is concentrated cent rated of the oxidized ores iron oxide carrying manganese and zinc oxides are the more important oxidized lead ore forms only a small part of the production these are smelted smelter direct siliceous sulphide ores carrying gold silver and copper are mined in large quantities and either smelted smelter direct or concentrated to eliminate silica the milling of Lead villes low grade sulphide ores has not been conducted locally with any degree of success as various idle mills erected attest only one mill is now in operation the leadville district mill which is treating siliceous ore at the time of this survey mining at leadville was at the lowest ebb it has reached in many years this depression may be attributed in part to high costs of operation but the main cause is the inability to mine ant and treat at a profit the low grade and complex ores both oxide and sulphides sulp hides of which large tonnages ton nages are known to exist notably the low grade oxidized silver ores on fryer hill and the low grade and mixed sulphides sulp hides of lead zinc and iron on carbonate hill iron hill and breese hill granting that a cheap milling method can be worked out cheaper methods of mining and handling the ores must be devised to do this mining on a large scale and consolidation of small units seems essential sent ial mining districts tributary to breckenridge kokomo kokomos and montezuma in summit county and alma in park county have in the past produced large quantities of the precious metals present mining is being done for the lead and zinc which since 1909 have exceeded the value of gold and silver produced indicating that mining in the future must depend on the base metals after depletion of the surface deposits carrying the precious metals in the bonanza district in saguache county the ore being mined is sulphide ores carrying iron lead zinc and copper with gold and silver tell urides and finely disseminated native silver in the nort northern bern part of the district lead zinc silver ores predominate and in the southern gold copper ores there is also a manganese belt in which manganese minerals are abundant at some of the mines notably the rawley there is a large tonnage of complex ore that would justify careful study of milling problems in the mining region surrounding aspen in pitkin county the ores mined are chiefly silver sulphides sulp hides associated with lead and zinc sulphides sulp hides the silver production has decreased to less than one tenth what it was in early years no estimate could be made of the approximate quantity of low 21 grade rade ore available but judging from the past history of the district it is presumably large neither the monarch garfield district in nor the TOM tomichi southwestern chaffee county district in gunnison county have been large producers as compared with other districts the madonna mine at monarch has been the star producer these districts contain no ores that present unusual difficulty in treatment the dreede creede district in mineral county produces chiefly oxidized silver ore with a siliceous gangue and a small amount of sulphide ore carrying lead zinc iron silver and gold the oxidized ore is smelted smelter direct the high silica content is objectionable to the smelters shelters sm elters and ore below a certain grade is unmarketable because of the high smelter charges and high freight rates the principal reason for the decline of this district is the lack of ore of shipping grade no reliable estimate can be formed of the amount of low grade or complex ores available in the san juan district with the exception of telluride the ores mined are chiefly sulphide ores carrying silver lead zinc and copper in the early days of the district the acher ores were smelted smelter direct later concentration mills were erected to treat the sec ond class ore As the mines attained still greater depths the deposits became barren or changed to ore of too low grade to work under the then existing conditions in recent years the increased costs of mining and treatment and higher freight rates has made working of the low grade ore impracticable telluride has always been a gold camp with a small production of base metals at first the ores were treated by stamp milling and with the exhaustion of the free milling ores concentration became necessary and the present practice is treatment by gravity concentration and flotation with one or two exceptions no large deposits of low grade ore are known to exist in the san juan district metallurgical work and results the metallurgical work was under the direction of W H coghill metallurgist assisted by C 0 anderson J P bonardi and max of the bureau of mines and J C williams and E V engels representing the state the first step was to make a careful physical examination of the ore followed by chemical analysis to determine the content and character of the minerals present if this showed that the valuable minerals present were so intimately associated or I 1 locked oc ked in the ore that they could not be beadily separated then methods of treatment were applied which experience had bad shown would be most likely to give the desired results in all twenty nine of the ores submitted were examined and their amenability to concentration cent ration determined insofar as time permitted the principal processes tried were flotation gravity concentration and dyani dation the most promising results were obtained by using flotation combined with gravity concentration differential flotation or flotation of one mineral in advance of another to make complete separation is not applicable except in rare instances to colorado complex ores from the complex sulphide ores galena and the precious metals were recovered as far as possible by gravity concentration to recover the finest grains of galena the chal co pyrite and the remainder of the gold and silver means had to be devised to float them in advance of the sodium sulphide was found to be a valuable reagent in this step results in the laboratory tests with sodium sulphide and with an acid solution of sodium dichromate to grade up the zinc by flotation have been so successful that they should receive further attention the principal function of the dichromate was to remove the iron sulphide from the zinc but it also removed some of the lead silver and gold the oxidized ores were tested principally by but some flotation tests were made some of the ores were readily amenable to but others proved refractory frac tory the work on account of the limited time and the large number of ores examined was necessarily of a cursory nature in general the results are encouraging and detailed examination of we the more promising ores is now justified the results of field and laboratory work are being published as bulletin 10 of the colorado bureau of mines which will ve be ready for distribution in the near future those who desire a copy should send name and address to horace E lunt state commissioner of mines state museum denver colo |