Show LIMONITE DEPOSITS OF THE ELY MINING DISTRICT NEVADA by S herbert williams A brief description of a limonite ore deposit without any any attempt at profound geology may be interesting to those engaged 6 in the development of mines particularly those who are engaged en I 1 aged ib in exploring similar deposits the deposit in question is found in the robinson mining district better known as ely nevada the immense porphyry deposits found in the ely district are too well known to require a description here having ino b been examined by the U S G survey professional paper no 96 A C spencer geologist briefly they consist of a porphyry intrusion having and east and west strike the copper yield of this immense deposit may be stated at one and one half per cent copper with precious metal values below one dollar per ton nearly all of the copper ore produced in this district has been confined to the porphyry but there is no question but what a large tonnage 6 of high grade or ore better known know as contact ore will be found at the contact of the lime and the porphyry it is generally b conceded that the contact ore limestone receiving its value from the intrusive porphyry will prove to be an important source of copper as deeper developments are attained and more attention given to this class of mining up to the present time little or no attention has ha been paid to anything but the porphyries ries because tonnage was the principal factor ely like many other mining districts produces varied mineral deposits including zinc and manganese and as frequently happens the ores from the different mines bear no resemblance to each other for instance the ore from what is known as the veteran mine is entirely distinct in appearance color and composition from that found in the adjoining property known as the consolidated the latter having a distinct zone of ore containing 1820 18 20 of AIO with a copper content of eight per cent in making the above statement I 1 am not referring to the porphyry deposit of the Copp ermines which adjoins the similar deposit of the nevada consolidated the above ore bodies are located in the western part of the district which wh ich is about seven miles long I 1 surface droppings Crop pings of massive gossan an examination of the surface droppings crop pings of this district show massive gossan or droppings crop pings for the entire distance of over seven miles some of which with a little surface work yield copper carbonates frequently rich enough in copper to ship direct the droppings crop pings are massive and generally lay immediately above the ore deposits in many instances there is a remarkable relation between the droppings crop pings and ore bodies immediately beneath this is particularly noticeable at the alpha group of the copper mines and so far as developments have advanced in the mitchell tunnel of the boston and ely consolidated mining company the droppings crop pings on the ely northern copper company are very dark red or black in color and are hi highly 9 aly the shading is from dark red to black baack partial analysis of droppings crop pings from a I 1 limonite d deposit ansol iron lime zinc lead gold silver 1031 gig 04 oz the prominent red to dark red droppings crop pings found over the porphyry and other sources of ore in the westerly part of the district are not found on the ely northern Norther tf copper company partial analysis of droppings crop pings over known porphyry ore bodies no ansol cao fe cu 1 none 2 1209 none unique deposit in boton boston ely property situated in the eastern part of the district on property formerly known as the ely northern copper company and now owned by the boston ely consolidated mining company is a deposit unique in its occurrence it is locally known as limonite the mass on the tunnel level has a length of feet and a width of 60 feet the limonite con southwesterly for an additional hundred and fifty feet with a width of from five to twenty feet yielding small quantities of copper the entire distance the limonite itself has from 40 to 50 per cent iron hematite and from an ochre achre yellow to a dark red and dark brown and frequently this structure is sometimes very noticeable in the ore deposits which exhibit alternate showings s of copper and limonite up to a foot in diameter cavities or bugs are frequently found up to one foot in diameter usually lined with a hard hematite several water courses have been found up to six feet wide showing recent calcareous depositions the supposition is that the whole mass at one time contained copper the same having been leached beached in keeping with other local deposits and far below the present zone of exploration limonite zone once copper bearing baring that this limonite zone once contained copper is evidenced by the fact that ore bodies of direct smelting smelling sm elting ore are found in the limonite mass consisting of azurite malachite cuprite and native copper spencer says in referring to the examination of this particular group inspection of the surface outcrops and of the old excavations suggest bt oreSt that the original sulphide body was formed by the replacement of limestone at a place where two nearly vertical fractures meet A small vein of iron pyrite was found 24 2 4 inches wide dipping 30 SW protected both above and below by a thin seam of hard hematite this pyrite vein yields 65 per cent copper it shows no sign I 1 of and only once has the latter mineral been found in this zone following is a partial analysis of ore taken from different ore bodies above and below the tunnel cu 50 fe ansol sin arn cao caa 60 18 40 40 there is no regularity in the different occurrences of 0 ore found in this limonite mass but further work may connect all or several of them in the present zone raises in ore from the tunnel demonstrate that the ore is within 95 feet of the further work may find that the ore body continues upward this particular limonite zone occurs in the hard bluer blue limestone of nevada so familiar in this district by referring to the geological map the work of professor G M W young of the university of nevada two distinct porphyry masses may be noted this porphyry bears no resemblance to the ore bearing porphyry found in the centre and the west part of the camp it is just as hard on the surface asin the mine and nowhere does it show any sign of disintegration te gration it is described by spencer as a s follows these masses have not suffered the same thorough metamorphism that has affected all the porphyry bodies farther west along the principal zone of intrusion but the rock exhibits incipient stages of alteration and contains more or less cyri pyrite t e this rock is quite distinct from anything found in the district having large crystals of pink ortho classe and a liberal quantity of hornblende horn blend the mica group is absent in no place lias has copper been found in the mass excepting at one place on the surface where copper copper is found adjacent to an iron vein leading directly to the limonite lini onite zone it is therefore doubtful if the porphyry has been responsible for the copper contents of the limonite lini onite excepting that it tilted or crush crushed edthe the limestone allowing the mineralizing mineral izing agent to deposit its contents direct this argument ment is aided by the fact that the usual lime contents of the lini limonite onite is never over four per cent with small quantities of aluminum and manganese iuan C anese as found in the foregoing analysis I 1 will take the liberty of quoting b professor young 6 in in the matter of limestone deposits contiguous to cc lawson does not specifically mention any work upon the deposits in the limestone contiguous to the from this it could be reasonably argued that the presence of the bore ore bearing porphyry is a necessity for the occurrence of copper ores in in any given area in the area examined this core ore bearing porphyry is absent or if present is only so in in small quantities and this would cause the area to be looked upon tipon favorably however the writer is of the opinion that the occurrence of ore is is not limited to the porphyry and gives as his reason the examples afforded by the clifton morenci and bisbee districts both of which present analogies to the robinson district at both of the above localities valuable copper deposits have been discovered in the limestone and metamorphic rocks close to igneous intrusions disclosures made in mine development the limonite zone above referred to is found at shaft no 1 and in a northeast direction from this point a number of small pits are located almost in a direct line all of these pits yield limonite and copper and probably indicate a fissure which may lead not only to additional zones of the same character but in all probability proba bilty is the connecting link between all of them for the purpose of further exploration a shaft no 2 was started feet north from the present zone from the surface it has yielded commercial ore to a depth of feet here a crosscut cross cut was driven east under the above mentioned line of droppings crop pings and crystallized limestone was found for a distance of eight feet since we had proven limonite in the foot wall of the shaft we had a right to expect that a crosscut cross cut under the line of fracturing to the east cast would open up the zone for which we are exploring A westerly crosscut cross cut was driven having limonite and yielding copper for a distance of seventeen feet this serves to illustrate the erratic nature of a limonite deposit and shows that this zone flattens out towards the porphyry eight hundred feet northeast of no 2 shaft we find the best example of limonite li I 1 droppings crop pings with commercial ore this occurrence is is feet above the portal of the mitchell tunnel the superficial extent has not yet been determined ter mined this was opened up by the owners of the claim at least twenty years ago long before the arrival of railroad transportation this and many other ore occurrences are undoubtedly due to radial fracturing from the big porphyry mass to what extent or depth this pene penetrates the nevada limestone is a matter of development personally I 1 shall look forward t to p the extension of the tunnel in this direction with a feeling that some day the ore from this particular zone will be delivered to the mitchell tunnel four hundred feet below I 1 have taken several photographs which may serve to illustrate to some extent the difference between the crop pings over known porphyry deposits and those over a limonite deposit it is doubtful if the photographs will convey the marked difference beewen the two the porphyry droppings crop pings often assuming tall cathedral like masses i in in contrast to the limonite droppings crop pings which conform to the limestone bedding planes it is too frequently the case that the matter of crop pings is overlooked except by the prospector to whose persistence si stence and courage ely owes its existence |