Dr. Thomas Egleston, of the Columbia   School of Mines, New
  York, carried on an incomplete series of   experiments some years
  ago (1879) to determine the solubility of gold in natural waters
  under various conditions assumed to be comparable to conditions
  existing in nature. His results appear to have justified the
  conclusion that solution and reprecipitation take place under
  favourable conditions.  The postulate of solution  in  this way may
  be admitted, but just how the precipitation is effected is not so
  clear; probably it is brought about by several different and com-
  plex agencies.    Manganese oxides are frequently present in
  alluvial gravels, especially in older more or less consolidated
  deposits, and in the absence of alkaline solutions such manganese
  salts interact with chlorides liberating chlorine- a very active
  solvent of gold.  Reducing and precipitating agents for gold in
  nature are numerous.  Organic matter of almost any kind is
  rarely or never absent from   soils, so that gold in solution is
  unlikely to be carried long distances through such alluvials
  without being precipitated, provided the circulation of the
  aqueous solutions is slow and sluggish.
      Years ago the writer was impressed by personal observations          
  and experience  in the tropics with the force of some of these
  conclusions and discussed the subject at length.  It was not,
  however, until he had the opportunity of reading Harrison's
  report on the goldfields of British Guiana that he became
  impressed with the soundness of this theory.  Harrison drew
  upon a number of observations and tests made both by himself
  and others and showed that in one instance the ashes of certain
  trees grown on auriferous laterite of the Omai district contained
   from 2 to 10 grains of gold per ton and that the ash from the               
   upper part of the tree trunks, near the branches, yielded as much           
   as 28 grains per ton.  It was also observed that the ash of the
   bark contained only 1 grain per ton, whereas  the wood itself
   produced from 7 to 10 grains.  Gold was also found on the           
   steel valves of the pumps used  in washing this laterite, but       
   Harrison thought it not unlikely  that this gold may have been
   in suspension in the water.  It was not possible, however,for                                      
   gold in that state to have obtained access  to the wood of the 

  the interior of the tree. It might be mentioned in this connection  
   that Liversidge found as much as 22 grains per ton of gold in
   the ashes of sea kelp.
                                                           
       It has been frequently noticed that old tailings from mine      
   and mill dumps, especially when overgrown with  vegetation, 
   show evidences of secondary or renewed enrichment due sup-    
   posedly to underground transport, solution, and reprecipitation
   of gold.  It has also been observed that rocks and soils relatively
   free from carbonates acquire gold more readily than others,
   owing, presumably, to the neutralisation of the  humic acids by
   carbonates.                                                 
       As the result of laboratory experiments some investigators
   have discussed the probable effect of colloidal  enrichment and 
   deposition, but, as pointed out by Blanchard, colloidal solutions         
   of gold are very unstable and it seems unlikely that they are            
   present in ore deposits in sufficient concentration to be of much      
   importance as a factor in the deposition of gold.   Blanchard             
   further observes that in the New Guinea gold deposits the               
   enrichment appears to be greatest in the highly manganiferous             
   portions, close to air exposures, and present in less than normal           
   amount in similar but highly limonitic situations. Certainly
   nascent chlorine in mangniferous ground-water will dissolve
   gold, while ferrous sulphate is an active precipitant of that
   metal.  Since manganese oxide will reduce ferric sulphate to
   ferrous, it follows that gold may be dissolved and reprecipitated
   in alluvial areas that are strongly manganese-stained and
   through which water circulates freely.  Bastin  has shown that
   a gold sol may develop by the action of sulphides on gold chloride
   solutions.

       According to Boydell the presence of small amounts of
   gold is not uncommon in limonitic and manganiferous deposits
   its presence being consistent both with the well-recognised          
   adsorptive powers of both these substances and with the adsorp-
   tion consequent on the colloidal development of surface.  He
   says-
                                                             
               The grinding and comminution resulting from the
          erosion of outcropping lodes must produce particles cap-
          able of colloidal dispersion....  Coagulation of extremely
          dilute sols, thus produced, in locations where stagnant
          conditions prevail, would account for the "flour gold"
          common in many placers. Growth of the larger of these                
          particles at the expense of the smaller would lead either
          to the formation of nuggets or to the continued growth
          of pre-existing ones.   Nugget  growth after this manner
          could, of course, only take place under favourable condi-
          tions - i.e., in stagnant or semi-stagnant waters. 

                    
       W. H.  Emmons refers to the paucity of experimental data
   relating to the solubility of gold under conditions of weathering.
   He mentions those obtained by Freise, which are discussed later
   in this article, and refers to the fact that statements of much
   gold being dissolved and reprecipitated in placers have frequently

  been made, but have not been generally accepted, in North
  America.
         F. L. Ress  refers to the gold deposited in the sedimentaries                     
  near the Homestead lode as coming from that source and as
  containing less  silver than the lode ore itself, "as might be
  expected in a placer derived from it."

         While it is generally conceded that placer gold is purer  than
  lode gold - i.e., less argentiferous - the rule has exceptions.  Thus
  Rose states that this is not the case in the Ural districts of
  Russia, where the relative amounts of gold and silver are quite
  variable both in the placers and in the lode ores.  This is pro-
  bably the general experience everywhere, but there can be no
  question that in travelling or migrating from the original lode
  to the detrital the gold is purified by the removal of part of its
  original silver.
         Although organic matter and organic acids are regarded as
  precipitants of gold, others are known to be solvents.  Gold may
  also be thrown out of solution by many sulphides,   especially
  pyrite and galena.  According to Skeyl one part of pyrite will
  precipitate eight parts of gold.  It would seem that although the
  natural solvents of gold are rather  numerous it  is also true it
  cannot be carried far where there is an abundance of organic
  matter. It has been recorded by Freise that in  Brazil the
  lower portions of old placers (old in the sense of having once
  been supposedly exhausted) have been profitably reworked after
  years under conditions that precluded mechanical enrichment.
  There seems to be no doubt that under the conditions described
  by Freise gold was transported from one part of a mineral
  deposit, to another and redeposited.  Freise further remarks-
               All milling companies or individual miners who have
         worked alluvial deposits in Brazil realise that gold placers
         once supposed thoroughly exhausted  may after a period
         of years once more be panned and yield a profitable
         amount of newly-accumalated gold. The native gold
         digger maintains that every gold placer within ten years
         is rich enough to be worked once more and that.payable
         gold reappears the sooner if the exhausted gold field has
         been hidden from the sun by vegetation or other means.

      In such instances it is affirmed that new gold is quite
  different from the original gold in colour, coarseness, and affinity
  to mercury.
      The role of the so-called flood-gold in the enrichment of
  placers is an important, but also a somewhat difficult, factor to
  evaluate in considering the origin and growth of the gold                      
  particles in such alluvials.  By the term flood-gold is meant the
  gold which is deposited during flood periods on the sand and
  gravel bars of the curves and eddies in large streams.  Accord-
  ing to Purington such gold usually accumulates  at  points from
  30 to 70 miles from the original source.  It is important to bear
  this fact in mind when testing, plotting, and evaluating gold-
  bearing alluvials, since flood-gold is transitory and  is of little
  importance when estimating the ultimate value of the deposits.
  Flood-gold is mostly flaky, whereas that on bedrock is usually
  granular.  Between bedrock and the surface of the stream-bed
  any shape and form of gold particles may be expected.

      As has been pointed out by Posepny the concentration
  of gold on bedrock cannot be, assumed to be simply a settling
  process on a large scale, since the metal generally shows no
  sorting or classification in placers.       In addition the gravels

      seldom exhibit evidence of classification, being generally com-  
     posed  throughout of particles varying greatly in size and weight.  
      At the same time it  is not safe to assume that in a cross-section  
      of a stream or river valley the so-called pay-gravel and bed-rock
     are  necessarily the richest part of the deposit   there is always     
     some horizontal variation of value and barren intervals in           
     different pay-streaks.  The mechanical distribution of gold in                                    
      alluvials is a wholly different problem- or group of problems-
      from that of the migration by solution.  Obviously, however,
     they are not independent but complementary, the gold in solut-         
      tion being precipitated on solid particles of gold or other metallic
      material associated mechanically with the alluvial sands and        
      gravels. The mechanical dispersion and concentration of gold     
      is evidently the resultant of a complex combination of conditions    
      -geological, topographical, and climatic - as well as being      
      affected by the character of the detrital material itself.              t

         It seems more than likely (and it is the general opinion
      of those who have studied the subject) that although by far the
      larger part of detrital gold has been received directly from the
      older sources, chiefly lodes, by mechanical erosion an appre-
      ciable proportion was probably derived by precipitation from

      aqueow solutions. If the secondary or sedimentary origin of the
      Transvaal gold-bearing conglomerates be conceded, detrital gold
      deposits have always been, and doubtless will long continue to
      be, the most important source of the world's gold supply.  While
      it is evident that gold  is widely if sparsely distributed through
      the crystalline rocks, it is certain that its accumulation in
      metallic veins or lodes is relatively rare, especially when of
      sufficient richness to pay for mining.. There seems to be little
      doubt that the greater part of' the gold in placer or alluvial
      depbsits was derived not so much from large workable veins or
      lodes as from the innumerable small veins and quartz stringers
      which sometimes occur as a distinct feature in areas of pre-
      Cambridn rocks, as well as in some of the older sedimentary
      rocks associated with igneous intrusions.

         Many, possibly most, geologists and mining engineers will
      agree that the future gold supply of the world is limited, also
      that  many known deposits have been greatly depleted and some
      wholly exhausted.  This is true of those in the United States
      and Europe.  It seems unlikely that any major deposits com-
      parable to those of South Africa will again be discovered.  With
      the possible exception of Siberia there are now few areas of the
      earth's surface remaining to be explored, and with the advent of
      the aeroplane there will soon be few nooks and corners of the
      land surface which have not come under observation.  As a
      by-product or associate with other metals and ores the output of
      gold is likely to continue large, but at best this quantity is
      small as compared with that of the Transvaal and the combined
      production of gold-lode mines in other parts of the world.  It
      is difficult, perhaps, for the public to understand that the once
      famous alluvial gold deposits of California, Australia, and
     Alaska  were Nature's hoards, treasure-trove whose gold was
      eroded from neighbouring rocks within the hills and mountains
      of the district through periods of thousands of years and concen-
     trated in surface accumulations easily accessible to man's simple
      tools, chiefly the pick and shovel. These diggings, as they were
     known, became quickly exhausted.

         In Roman times gold was washed from the beds and bars
      of streams of Central Europe, especially the Rhine and some of
     the smaller streams of France. With the growth of civilisation
      and increase of population these deposits were rapidly exhausted.
     Alhough contemporary replacement is a practically negligible
      factor there are or have often been in relatively large streams,
     as for instance in northern California, small quantities of gold
      widely spread throughout their beds which can often be profit-
      ably recovered with modern dredges.  Under favourable condi-
      ions such machines or boats will handle from 100 to 200 cubic
      yards of unconsolidated alluvial material  per hour of operation
      and obtain a profit from ground containing as little as 4 to 5
      cents of gold per cubic yards.  The cost of these operations
      obviously must vary widely, depending upon a variety of factors

  or conditions which are probably seldom quite the same in any
  two localities.  It may be as low as 3 cents per cubic yard or as
  high as 35 to 40 cents where the ground has to be thawed, as in
  Alaska, the Yukon, and Siberia.  On the whole this form of gold
  mining has frequently proved very profitable where conditions
  have been favourable.  By skilful prospect-drilling, sampling,
  and mapping the ore reserves, so to speak, can be established
  with fair accuracy and the life and potential profits of the opera-
  tion safely forecast and estimated to a degree seldom equalled
  in hard rock goldmining.
      The writer is well aware that the theory of gold solution and
  redeposition in alluvials has never received much support among
  North American geologists and mining engineers, probably
  because of the absence of tropical conditions in areas with which
  they are best acquainted and a preconceived notion on their part
  that such a postulate is fantastic.  It might seem, however,
  that there is reason to doubt the denial of the claim.  It is
  certain that in tropical countries, where conditions are favour-
  able, there is no gainsaying that such solution, migration, and
  redeposition of gold frequently does take place.
      Since the foregoing was written the author's attention has
  been called to the experiments of W. G. Fetzer at the University
  of Minnesota relating to the transport of gold by organic solu-
  tions. These investigations appear to throw some doubt upon
  the results of the work carried out by Freise in Brazil.  Fetzer
  states-
              The reaction between humic acid and gold may be
         regarded as oxidation when one considers that a gold
         atom would have to lose either one or three valence elec-
         trons in order to combine as a humate.  It is significant
         that by  its position in the electro-chemical series gold
         tends to retain its normal number of electrons tenaciously.
         Therefore, it is difficult to understand this paradox that
         humic acid, a reducing agent could oxidize gold.

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