Show lei W W W 1 i geological GENESIS OF TONOPAH GOLD J E spurr in bulletin no V S geological survey lot lei of lei lei wa tonopah To nevada is situated in a volcanic region during a long period of time this region has been the seat of there was not continuous eruption however again and again the volcanoes became apparently extinct and the wearing down of the surface carved out hills and valleys but after each period of rest the forces again broke loose new volcanoes were formea and new streams of lava and showers of ash were thrown out although this volcanic activity lasted over a long period of years many thousand yet nearly the whole is comprised in one of the comparatively recent divisions of geologic time the tertiary the eruptions began in the early tertiary of these earliest volcanoes no trace has been recognized although the lavas which they poured out are found the latest eruptions were post tertiary and reached well down toward the present day the latest lava erupted in the tonopah district was basalt and at silver peak a basalt cone belonging to the same period is so perfect in form that it is probably not more than a few hundred years old it is thanks to this volcanic action that the ores have been formed the geologic history is somewhat as follows in the early tertiary period before the appearance of man on the earth the country consisted of ancient lime stones and granite and had been for millions of years free from the convulsions of nature began and great masses of hornblende andesite were poured out this is the early andesite or lode porphyry after the eruption the hot waters which usually follow volcanic outbreaks rose through the lava following fractures the waters contained silica and the precious metals which they had derived from the heated region below they deposited these along the fractures and the present quartz veins were for formed nied at the same time the andesite was decomposed and altered to a remarkable extent by the action of the waters and their contained gases A long period of volcanic rest followed the hornblende andesite was deeply worn being stripped in most cases down to the rich portion of its contained veins this period of rest was brought to a close by another andesite eruption two closely successive flows were poured out they were of pyroxene andesite and andesite respectively but they are hard to distinguish from each other without the microscope and may be grouped under the name of the later andesite the later andesite was not unlike the early andesite in tic 4 agali i 7 ie partial view of Tono showing mt butler composition and many varieties of the two can be distinguished only with difficulty moreover the later andesite has also been considerably altered in many places in general however the later andesite is somewhat coarser than the early andesite and presents a more mottled appearance appe app arnce earnee its atys crystals of mica are frequently large and it is frequently but by no means always colored a characteristic purple at the outcrop which may give way to blue or green or even white below As the early andesite is called tho the lode porphyry the later andesite may be conveniently called the purple porphyry the eruption of the later andesite was followed by another period of volcanic rest est r probably not quite so long as the previous one which occurred between the appearance of the two ande sites then the pent up energy again burst forth and many small volcanoes were formed near what Is now tonopah To most of the low mountains and high hills surrounding the camp are the roots or necks of volcanoes formed at this period curiously enough the lavas which appeared from the different vents were not exactly the same though having a family resemblance from the vents represented by parts of mount oddie and mount brougher great floods of glassy breccia c a were poured out containing fragments or earlier formed together with great boulders of the later andesite and even of the early andesite and its quartz veins some of the fragments ware moreover over are of rocks not known in the immediate district they comprise limestone diorite gneiss pegmatite peg malite etc attesting what a variety of deep seated rocks were pierced and broken and carried up with the columns of rising lava after this the same volcanoes burst into lakes probably extensive formed around the volcanoes the climate at this period was therefore moist the only volcanic activity was manifested by occasional light showers of pumice and ash in the waters water s of the lake lived millions of the microscopic animals called whose shells have accumulated to form exclusively elusively ex some of the lake beds the material worn from the rhyolite the and the blo tite andesite which had not been entirely covered by the flows accumulated to form layers of sand mud and pebbles which formed the present white tuffs 11 the movements mo along the fractures in the rock which had probably begun before the formation fot mation of the veins in the lode porphyry were renewed with violence at about this time and the region was separated into blocks by numerous faults the displacement of these different blocks gave rise to a complicated structure which adds to the difficulty of comprehending the district as it is today after this the same volcanoes burst into activity again the volcanoes reawakened with a violent explosion with showers of ash and pumice followed by a thick pasty lava which was in turn followed by the eruption of a considerable sheet of lava at about the same time rose in narrow dikes and then spread out as thin and not extensive sheets following this eruption hot waters rose along the contact of the lavas and along rock fractures depositing chalcedony chalcedonic chalce donle and jas ja peric quartz siderite iron carbonate calcite etc with small amounts of the precious metals so far as the immediate vicinity of tonopah is concerned since the close of the rhyolite outbreaks erosion the wearing away of the land has progressed rapidly and the rocks have been stripped off to a greater or less extent in favorably situated up faulted blocks all the rocks overlying the lode porphyry have been removed and the veins of the early series exposed at the surface on the other hand portions of the lode porphyry have remained deeply covered at no very remote period comparatively speaking there were flows of basalt in the vi vicinity of tonopah To though not in the camp itself the writer judges these basalt eruptions far nearer to the present day than they were to the last rhyolite outbreak from the foregoing sketch it may be seen what the chief problems of the district are since the productive veins are confined to the lode porphyry overlying formations must be sunk through before there is a chance of success except in the favored but very limited region where this porphyry outcrops the variable dimensions of the different lava flows the strip pings of the different periods of erosion and of volcanic rest and the complicated faulting make the figures obtained from one shaft not safely applicable to another even though they be close by the work now being carried on by the geological survey will throw a certain amount of light on the structure As to the chances of success it appears that within the mineralized zone of the lode porphyry the amount of vein deposition has been very great the alteration of the lode porphyry by mineralizing mineral izing waters is like what has been studied in similar districts as for example on the comstock the intensity of alteration observed here however has not been equaled in any camp studied this is indicative of an extraordinary activity of vein formation although it does not necessarily follow that the richness of the veins in the precious metals shall correspond it has been determined by microscopic study that the rich ores of the bonanzas are chiefly primary and not secondary that is they were deposited by the original rising hot waters and are not due to any great extent to subsequent concentration by surface waters this is a favorable indication di holding out the hope that rich ores may be encountered to a considerable depth these rich ores usually form shoots in the veins and between the shoots the ore ord will frequently be found to be low grade |