418 FERGUSON. 



outcrops show rich ore are those in which manganese oxide is not pro- 

 minent, si;ch as the Gold Dollar vein on Mount Aroroy, the Eed Oxide 

 vein on Mount Bagadilla, the El Sol vein on Mount Kalakbao, the small 

 veins on Boston Hill, and others. Evidence from development work, 

 though very meager, seems to show that the values in these veins do 

 not increase downward in such a satisfactory manner as in those bearing 

 manganese, which are as a rule barren, or nearly so, at the outcrop. The 

 values in some at least of the non-manganiferous veins, seem to decrease 

 greatly a few meters from the surface. 



In this connection a recent paper by W. H. Emmons," on the agency of 

 manganese in the superficial alteration and enrichment of gold deposits, is of great 

 interest. Roughly stated, the hypothesis presented is that the presence of 

 manganese in the upper portion of a gold-bearing vein aids greatly in the solution 

 and downward transportation of the gold by the action of surface waters carrying 

 chlorine, the gold being redeposited nearer the sulphide zone. Such being the 

 case, non-manganiferous veins would show higher values on their outcrops, due to 

 the transportation of valueless constituents, and, conversely, lodes carrying 

 manganese will be poor at the outcrop and increase in value in depth, owing to 

 the downward transportation of gold in solution. Similarly, placers would be 

 developed from non-manganiferous lodes, but not as a rule from those carrying 

 manganese. Much evidence derived from chemical experiments and from a study 

 of the ore deposits of the United States is adduced in support of this view. 



It is interesting to note, with regard to the veins of tlie Aroroy dis- 

 trict, how closely the facts observed agToe with this hypothesis. Except 

 for the placers, where the evidence is inconclusive owing to the occurrence 

 close together of manganiferous and non-manganiferous veins, the facts- 

 in regard to values in the veins themselves bear out the hypothesis 

 comj^letely. The fact that the district is bounded on the north and 

 west by salt-water is also in favor of this hypothesis. Hence, the pro- 

 portion of chlorine in the mine waters should be above the average and 

 the solution of gold facilitated. However, one point weakens this evi- 

 dence. This is that in the best developed veins, that of the Colorado 

 Mine on Mount Bagadilla and the Nancy veins on Mount Kalakbao, 

 the ore is much shattered and banded, (Plate II) affording open chan- 

 nels for the descending waters to transport the fine gold mechanically. 

 Further development work in this district will be watched with especial 

 interest, as, with the conditions existing here, of veins carrying varying 

 amounts of pyrolusite and others free from this mineral occurring close 

 together and apparently the result of the same conditions, important evi- 

 dence may be found bearing upon Emmons's hypothesis. Another point 

 upon which the evidence of development work will be awaited with in- 

 terest is the effect of the calcite (which in most of the veins seems to 

 be present in increasing amounts away from the surface) upon the values. 



The close jirxtaposition of manganiferous and non-manganiferous veins 



"Bull. Amer. Inst. Min. Eng. (1910), No. 46, 767-838. 



