66 GOLD AND TIN DEPOSITS OF SOUTHERN APPALACHIANS. 
deposits has recently been advocated by Weed,« and the consideration of the subject in 
this place seems to be justified. In the particular case of the Haile mine, moreover, cer- 
tain facts appear to point rather strongly to such a possibility; and in anticipation of the 
detailed description of the mine these facts will be presented, so far as they bear on this sub- 
ject. In view of the erosion which has certainly gone on since the formation of the diabase 
dikes it is unreasonable to suppose that any secondary enrichment of present importance 
took place before that event. Moreover, the facts that the diabase is known to be later 
than the deposition of the original ore, and that most of it is in a fresh condition, make it 
appear probable at first thought that secondary enrichment has not taken place to any 
important extent since the intrusion of the dikes. It is true, however, that the diabase is 
decidedly altered in its upper part, and that it has doubtless been protected from rapid 
decomposition at greater depth by the layer of clay-like, fairly impervious material pro- 
duced by decomposition on the top. It is certain that, owing probably to the presence of 
many joints, which are absent in the diabase, decomposition and oxidation have gone on 
to a greater depth in the body of the siliceous ore than in the diabase. The absence of these 
joints may also account for the absence of gold of secondary deposition in the diabase. It 
should be borne in mind that the idea of secondary enrichment is now applied practically 
only to deposits of copper. The fairly distinct zones of different mineral composition and 
strikingly different appearance occurring in enriched copper deposits have unquestionably 
aided greatly in the recognition of the presence of this phenomenon. But the much simpler 
mineralogy of gold affords no aid in this respect. 
Furthermore, the changes in the vertical distribution of values at the Haile mine are not 
so extreme or abrupt that they forcibly demand an explanation, as is the case with copper, 
for example. A little consideration will show also that if secondary enrichment of gold 
should take place, the results as regards vertical distribution of values might differ markedly 
from those commonly ascribed to this process. For example, a copper deposit which has 
experienced secondary enrichment may be divided on the basis of richness into three more 
or less distinct zones — (1) an upper impoverished zone from which most or all of the copper 
has been leached; (2) an intermediate rich zone in which the copper originally above has 
been added to that already existing in the primary deposit; and (3) a relatively low grade 
zone, of indefinite downward extent, which in value stands intermediate between 1 and 2 
and which represents the character of the whole deposit originally. Owing to the extreme 
ease with which copper minerals are decomposed and the copper taken into solution, an 
instance is rarely found in which solution and reprecipitation at a lower level have not gone 
on to a marked extent, unless the gangue and the surrounding walls have been so imper- 
vious as to restrict decomposition to the uppermost portion of the deposit. In other words, 
where secondary enrichment of copper deposits has taken place appreciably it has reached 
a rather decided degree. With gold, on the other hand, the case is different. The metal 
is so slightly soluble that currents of oxidizing surface waters may flow around it and reach 
and decompose minerals far below without appreciably changing the relative position of the 
gold. But some solution of the gold is undoubtedly taking place and, given sufficient time, 
during which the process continues, the gold near the surface must ultimately be dissolved 
and carried to a greater depth, where without any question it will be precipitated, most 
probably by sulphides or other reducing agents. The difference between the behavior of 
gold and copper, therefore, is largely one of rate. Since the process in the case of gold 
requires a very much longer time to accomplish equal results, it is natural to expect that 
intermediate stages, showing less marked results, will be more commonly encountered than 
in the case of copper. The recognition that gold is dissolved and redeposited to a slight 
extent is a recognition of an early stage in this process. This action in the case of gold is 
so exceedingly slow, however, that it must be a comparatively rare combination of condi- 
tions when degradation by erosion will proceed so slowly that concentration of gold from 
higher levels may become an important determinant of the value of the ore. 
a Weed, W. II., The enrichment of gold and silver veins: Trans. Am. lust. Min. Eng., vol. 'AO, 1900, 
pp. 423-448. 
