Show notes on the geology of goldfield nevada E p jennings one hesitates to add to the bulk of literature that has been published ad nauseam nasseam nau seam by the local papers on this peculiar camp but a few notes on the geology of the region may be acceptable to the readers of the mining review the camp is situated at the south end of the san antonio antonio desert twenty five miles due south of tonopah To in esmeralda county nevada the first locations were made in the summer of 1903 and the first ore of value was discovered on the combination claim in the fall of the same year physically the country is an absolute de des prominent ent by a number of of intrusive a quartz feldspar granite only remnants of this limestone remain on the hills the rest has been eroded but undoubtedly it underplays under lays the greater part of the dietrict after the intrusion of the and partial erosion of the limestone the entire country was covered by flows of rhyolite of three and possibly four different periods with longtime long time intervals between them the sheets ds as far as it is possible tu ti determine them in the field are all rhyolite no andesite or rocks of the intermediate class have been noted the older sheets are by prominent blowouts blow outs of dark reddish brown rock that project from a few to many feet above the surrounding country while there is no regularity regu arity in the strike of these outcrops the larger proportion of them have a N ES as E S W course the outcrops cons consist of rhyolite in all stages of from a 1 slight hardening in which the original form of rhyolite is preserved preserved to masses of pure jasper these masses which have roughly a dyke form are not continuous for any great di distance along their strike but disappear and reappear at short intervals softer parts of the outcrop having been eroded and covered by their own debris these outcrops E VV sao loix TOV to 00 T 3 1 l 1 st 1 t VT cv eia sa lovix ert as forbidding as any in a state noted for its aridity topographically the district may be described as an amphitheatre the floor of which is a series of low hills the trend of their longer axes being north south surrounded by somewhat higher hills on the northeast and south and by a lava capped plateau to the west the basal rock of the district is a highly limestone of unknown but probably cambrian age that has been called Jasper Jas period lod by J E spurr of the U S geological survey this rock has been brought to the surface on the hanks flanks of a series of low hills in the northern part of the district of 0 which columbia mountain is the most greatly altered and locally hardened by the subsequent introduction of silica the latest flow which covers a series of tuffs and tertiary sandstones is an unaltered rhy rhyolite olite with a dark ground mass inclosing quartz grains and of hornblende this later rock is only developed in the southern and western edges of the district and as far as known contains no mineral veins the country rock of the goldfield alphi theatre proper is the lower rhyolite the later flows having been eroded and contains all the mineral so far found we can assume that this sheet of unknown but limited thickness rests on the limestone before described the crests of most of the hills are crowned 13 carry common about 1 in gold in places a little more and in in others only a trace the carries in gold according to analyses made by the U S geological survey the peculiarities of the district are the highly character of the rocks and the universal distribution trib ution of gold in al the older the outcrops have evidently been formed by the action of silica bearing waters probably hot acting along zones of fracture in the older rhyolite the source of these waters was undoubtedly the sheets of tuff and rhyolite that resulted from a later eruption and which have been mostly eroded though remnants of these sheets remain on many of the hillsides hill sides that this is superficial and extends to no great depth is shown in the deeper workings of some of the mines the rich ore bodies that have made the camp famous in a few months occur in fissures in these blowouts blow outs or outcrops or on the contact between them and the country rock there is apparently no connection between the ore bodies and the outcrops except the latter are on a plane of weakness that determined the of the later fractures which are now the mineral veins these mineralized veins are very irregular on the outcrops but become more regular as they pass through them into the rhyolite below in the upper portions the pay streaks often consist of a mass of quartz that has been corroded into roughly rounded masses and partly cemented by quartz and various decomposition minerals as clay and iron oxides the values are almost entirely contained in this cementing material below the zone of the outcrops the vein changes somewhat in character and becomes more regular but with much material which is more or less silica bilici fled rhyolite containing abundant iron py N 5 avow co luva xo vv ta III 1 1 I 1 E I 1 I 1 I 1 7 1 I j lr L r ta r T 1 X 7 1 5 J r OX vivyl wv X X mt NV af ak alt rites with seconda secondary 7 copper minerals as burnite nite and probably however the amount of copper co p per is very small the original source of the ore was undoubtedly the rhyolite as the rock now contains from traces to 1 per ton in gold the rich veins are the results of a concentration of this widely diffused gold contained both in the earlier and later rhyolite sheets the vein filling being due to the solvent action of chemically active I 1 waters on the rhyolite walls of the later fractures in fact a modified form of lateral secretion the very ich rich ore now contained in some of the fractures is due to a secondary enrichment rich ment similar to the secondary enrichment of copper veins the erosion of the country was accompanied by the solution of the gold contained in the upper parts of the vein this gold was precipitated again in the lower portions of the fissure the precipitant probably being iron pyrites and the solvent ferric sulphate formed from the oxidation of pyrites As the erosion has been extensive the results are the concentration in the present fissures of the contents of many hundred feet of vein now eroded this process is possible in an arid country like nevada which has probably had its present nt climate since tertiary times the solution which originally deposited the gold in secondary seconda ry fractures fracture s wis was not saturated with silica but was chemically active as it shown by its corrosive action on the breccia contained in the veins veins are common in the region but many of them are cemented by quartz in this case they contain but little gold the order of the sequence of the formation is as follows first the elevation of the hills second A period of erosion third the lower rhyolite flows fourth later rhyolite flows with probably long periods of time intervening F fifth fth frac fracturing tu ring of the rhyolite sheets and the deposition of silica forming the present outcrops sixth later fracturing of the outcrops forming the veins seventh the filling of these veins in some cases by quartz and in others by decomposition minerals and gold eighth the subsequent enrichment of the veins by concentration of the upper content into the lower zone there are no known dikes that can be connected with the but later work may demonstrate their presence the future of the camp largely depends on asyet two undemonstrated propositions first do all or even a moderate proportion of the outcrops contain the ore bearing fissures or are they confined to certain belts of which the january combination is the principal one to date second will the fissures carry values in depth work alone will demonstrate both the ore s so 0 far has been very spotted very rich ore occurring with ore of moderate value the appearance of the poor ore is so similar to the good that the sorting has to be checked by many assays essays the ore in the upper levels is very low in silver but below the water level which is feet in the january shaft about one quarter ounce of silver to each ounce of gold has appeared in the sulphides sulp hides no tellurium has been found the water supply is derived fr from om springs which have their source in the tuffs and sandstones south and west of the town and amounts to about 1700 gallons per day also some water has been developed by shallow wells in the town of goldfield the supply is sufficient for domestic purposes but more will be required for milling the combination mine will obtain water for the new mill from alkali springs twel miles distant oni only two y mines the january I 1 and combination have reached the wat water level and the amount of water is insignia cant in both cases the value of the output in milling and shipping ore is said to he be about which is certainly a fine rec ord for a six months old camp the high value of the shipping ore makes the camp very attractive and its possibilities are great due to the area occupied by bk the out crops which may contain valuable ore de posits the prospector should search for zones and fissures in or near the outcrops and when found prospect them thoroughly many of the zones are barren particularly those cemented by white quartz but so far they are the best indications of rich ore the cleme element nt of luck enters very largely into the proposition together with a vigorous use of pick and shovel |