Show the A B C of mining geology geol agy ogy WRITTEN FOR THE MINING REVIEW BY CHAS A PORTER the appearance in 1893 of pose anys famous paper on the tafe genesis of ore deposits the science of mining geology has taken on a new garb that paper proved to be the starting point of a discussion which was invaluable to mining geology facts were contributed from over the whole mining world and these correlated with Pose pays distinction between deep seated and atmospheric waters laid down the foundation of modern mining geology and our subject for the first time had become a real science roughly it may be said that this stage was reached about seven years after the appearance of paper when the general scheme of modern mining geology by the promulgation of the theory of secondary enri enrichment climent had reached a fair degree of completeness since this later development many facts have been added especially through the work of the state and national geological surveys the grand result being that mining in geology has been removed from a region of speculation to a world of fact the rapid growth and substantial value of this science is as yet practically unknown to the average miner while this condition is partly due to the misinformation strewn about by the num numberless berle ss pretenders in the geological field abnot another ler cause is the scarcity of literature written in such a iiA manner anner as to be comprehensible to the general mining man this latter consideration is what has prompted the writer to present this outline it is elementary but it is comprehensive and should furnish a key by which the many detached though excellent articles should become at least partly intelligible to the reader and above all it is hoped that it may illuminate some of the dark spots which so often occur to the practical man in his efforts to solve the riddle of dame natures hidden wealth since it is our alm aim to treat this subject in an elementary manner we shall begin with a brief resume of the main division of rocks that is the distinction between the igneous and sedimentary families As is indicated by thenamae the name the sediment aries are the result of sedimentation or the settling of suspended matter from the waters of lakes seas and oceans As a matter of course rocks formed by this process must have been in a horizontal position at the time of their formation but aday we find them at all angles even to standing on edge F formations of this character are familar to every miner in fact it represents the usual condition under which he finds ore the miner is also quite familiar with ari arf other class of rock namely porphyry this th is on igneous rock and is commonly present in the neighborhood of ore deposits deposit igneous rocks are sharply distinguished distinguish from the sediment aries in that they are ai derived from a magma or molten mass they the include besides porphyries ries the lavas lava granite syenite etc technically porphyry is descriptive of a form of structure it means any igneous rock in which coarse crystals or are contained in a fine ground mass As a general rule the U in a I 1 S u afa c ce e B e 0 0 0 00 0 oooo fa ra i more deep seated a rock is the more pro pr bounced the crystallization will be in the U lavas where the rock is formed at the sur su face the crystallization is usually so weak wea that the individual crystals cannot be de d by the unaided eye on the other oth hand granite which solidifies deep down in i the earth and cools slowly and under pressure is entirely and obviously cryst allne A typical example occurs in the case of granite gr anite and a rhyll rhyolite rhy te in the form of lava lav the chemical composition of these rod rocks is the same the pressure and rate of cooling is different however with a vast va resulting difference in the appearance and an structure of the two rocks the importance of igneous rocks in the formation of ore bodies may be realized 0 from the following quo quotation it is not too much to say that there is not a mining district among the sixty five that I 1 have tabulated in which igneous rocks do not occur in close association with ore deposits T A ricard ore dept a discussion p 58 these igneous or intrusive rocks exclusive of the lavas form a starting point for our story of ore deposits whether they are a cause or whether they happen merely by reason of the weakness which made the ore bodies possible will form no part of this article the fact of their common association is all that is of importance here the leading geologists of all schools have agreed that lode deposits of copper lead zinc goldana gold and silver such as are common in the western united states are natural concentrations brought about through the agency of circulating waters these authorities however do not agree as to the ultimate source of the waters one school maintains that they were originally of atmospheric origin and had slowly sunk to great depths beneath the surface their contention being that the water gradually percolates downward picking up the disseminated semina ted particles of valuable mineral which are known to exist in most country rocks this water when it comes into the region of heated tends to rise through cracks and fissures the whole operation era tion to use the words of one of the disputants resembles the hot water circulation in domestic use the other school contends that the min eral izing solutions are brought up by and are a component part of the magma and that they are pressed out during the cooling and crystallization that finally results in the completed rock while we are not concerned here with the discussion of this question yet it may be said that the advocates of atmospheric waters have been dealt some lusty blows and the next generation in the writers opinion will see the theory of deep seated wa waters fully acknowledged and established as a working principal this question however cannot be decided by the recognized leaders of the science today they have been advocates of one or the other theory and are naturally open to suspicion of bias to recapitulate the essential conditions so far brought out disturbance accompanied by faulting deep seated igneous rocks other than lava circulation of hot solutions the last factor of these three conditions is usually not active at the present time its affects are generally evident however in that peculiar indefinable alteration which accompanies all ore deposits of the class we are considering this condition has been aptly called the thermal metamorphism in becks nature of ore deposits Deposit 5 and means a change of form due to the action of heated waters before leaving the subject of heated mn eral bearing waters another factor must be brought to the readers attention it is that a solution charged with valuable mineral must drop some of its burden as it posses a given point or no deposit will be formed this explains why ore deposits so often vary with changes of the wall rock we ave have numerous examples in utah of deposits in limestone as the copper ores re 1 lacing placing this rock at bingham lead and silver in the cottonwoods Cotton woods alta park city tintic gintic etc and gold and silver at mercur Aler cur this shows the selective influence of the limestone A striking example in a different class of rock occurs at butte montana where a basic grante carries ore to the exclusion of a granite containing ten per cent more silica A picture has now been drawn of flat sedimentary rocks being elevated tilted faulted and folded we have seen them penetrated by molten rocks from below altered by hot solutions as they seep through cracks and fissures depositing their burden of valuable mineral at favorable points but our story is not finished we have so for accounted only for what is known as primary deposits these deposits are destined to undergo many changes through the action of surface waters percolating downward through them the action of surface waters is a factor usually of the greatest economic importance and frequently determines whether a deposit is of commercial value or not while most miners are more or less familiar with the results of erosion it will be well to adduce some examples of what has been accomplished by the forces which are so active in breaking down the surface A clear conception of this will be necessary in order to appreciate what will be said under the heads of superficial alteration te ration and secondary enrichment erosion and superficial alteration when mountain ranges are first formed they abound in steep precipices angular blocks sharp outlines and jagged points but atmospheric elements come at once to work and piece by piece these sharp outlines are planed down great upheavals leveled and in time thousands of feet are worn worn off in order to impress upon the reader the magnitude of the work done by erosion some examples will be cited the wasatch range furnishes a striking instance over five miles of rock have been removed from these mountains leconte says feet the uintah range in eastern utah has lost feet and the appalachians are lower by from to feet along with this barren rock the tops of such mineral bearing veins that may have reached this primitive surface are barred car red away often entirely removing them or leaving but a mere root odthe of the original d deposit A clear idea of this condition may be obtained by supposing a range of mountains recently formed and not yet aff affected ested by erosion imagine to yourself a perpendicular vein reaching to a point four thousand feet beneath the surface the same range ages later when three thousand nine hundred feet have been eroded will leave an ore deposit of but one hundred feet in depth when one reflects that every foot of the earths surface which is unprotected by a sheet of water is subject to the forces of erosion and that over all thousands of feet have been removed it can readily be seen that no ore deposit has been exempted from this action and that erosion must be accepted as a factor in the study of every every ore body one of the practical uses of a knowledge of this force is an antidote for the too common belief that ore depo deposits tits necessarily increase with depth change in temperature with moisture are the prime factors in erosion the actual wearing away of the surface is preceded by water seeping into the still firm rocks usually to the depth of several hundred feet these waters attack the rock constituents slowly preparing them for their final dissolution this acton as a general rule is more active in mineral veins such elements as sulphur arsenic tellurium and copper being in easily soluble are rapidly removed leaving the vein in a porous condition and still more exposed to attack in a vein composed of sulphur iron and gold practically all the sulphur and a large part fart of the iron will be removed the gold however being ordinarily able to resist the solvent action of the waters remains in the vein this results in what is known as superficial alteration as by the removal of the worthless sulphur and iron the grade of the ore is increased and the gold content cont elit per ton is doubled or trebled the foregoing clearly shows why so many gold mines fail with depth the original primary ore with its full load of worthless constituents is reached and the values form a smaller portion of t the h e ore by weight per ton an ore which contains twenty dollars per ton in the oxidized zone will when it reaches the primary sulphides sulp hides usually drop to ten or five dollars per ton in addition to which the cie character of the mineral will be changed usually necessitating an entire change in treatment secondary enrichment allied to superficial alteration is that of s secondary andary enrichment gobi is ordinarily insoluble and as pointed out remains M n the oxidized zore zone however at 04 times if the gold will be dissolved and carried carrie 1 aown own 4 lo 10 a point near the permanent water level u usel id here becomes ad this action however is not marked in the case of gold we will therefore consider the subject in its connection with copper deposits he ere it is of paramount importance as a maa majot ity of the copper deposits when the lean n sulphides sulp hides beneath the action of the surface murfa 08 waters are penetrated cease to contain nain ml eral in paying quantities in order to realize the full importance of secondary enrichment it will be well to re call the examples of erosion c brought out ear lier in this article these examples how ever are extreme cases and show erosion above the average we will therefore take as an illustration a simple fissure vein which has been eroded for five thousand feet of its depth figure 1 without considering the shaded portion marked A B C D ea E F shows immediately after the deposition of the mary ore at point A the vein is seen we will say several centuries later the surface waters have seeped into the top of this vein dissolved out the copper carried it down and deposited it at point A forming the first phase of secondary enrichment the period has been too short to materially teri ally erode the surface as yet hut but in time this is accomplished and as the surface ab a preaches pro aches the point A both the primary and secondary ore at this point goes into solution and is carried gradually to the he lower point B where it is again preci precipitated pl on coming in contact with the primary ore at B thus we have concentrated at B all the primary copper at point B plus all the copper which i formerly existed above point B this operation t Ts is constantly repeated as the surface becomes lower the copper being carried down by successive solutions and precipe lations tat ions the mineral occupies in order the points C D E until practically the whole of it becomes accumulated at 7 F just below the present surface the condition just drawn is an ideal one and does not often occur in nature in such a perfect manger manner the modifications are many and cannot be explained here as aass their consideration would divest this article of its simplicity two related features should ho however weTer be considered namely the porosity of the vein matter and the ratio of L erosion to leaching it is obvious that in order to form a secondary enrichment the surface waters must penetrate the vein and dissolve the valuable mineral before the surface is worn bearing 1 away porosity of the mineral rock aids this operation if the erosion Is imper pt rapid and the rocks comparatively we are apt to find sulphides sulp hides 0 out at the surface in such a c case ase the cropping sulphides sulphides sulp hides hides are certain 1 to crumble cr alere he off and be carried down the hill second seco of we would expect to find our zone ary enrichment both weak and sha shallona sh allowa e a copper deposit of but I 1 limited m ed vertical tent |