ee 
SECTIONAL TRANSACTIONS.—C. 383 
The general lines of classification of mineral deposits follow naturally from these 
connections, as do the chief subdivisions to be distinguished in each group. 
It is, however, evident that any given deposit may be associated with more than one 
major genetic process. Thus, processes of the magmatic cycle may be responsible for 
the primary concentration and metamorphism for subsequent alterations, while 
processes in the zone of weathering (cementation) have produced the features which 
make the occurrence workable. The same applies to rocks also, and in no way makes 
classification impossible. The process which has given the main mass its characteristic 
habit will also determine its position in the classification. 
The genetic description of a mineral deposit must take three points into consideration : 
1. The process which produced in loco the concentration of elements which are 
characteristic for the mineral deposit and may determine its economic value. This 
is the primary geochemical concentration process. 
2. The character of the mineral paragenesis to which this primary geochemical 
process gives rise. 
3. Subsequent alterations within the geological body which may be accompanied 
by the appearance of newly-formed minerals and secondary displacements in con- 
centration. 
a. Alterations during the chief process of formation, inseparable from the latter 
and belonging to the same major geological process (successions, crystallisation- 
sequences, type-transitions, auto-metamorphic formations, reaction series, etc.). 
b. Specifically later and secondary mineral alterations, migrations of matter 
and changes in fabric in which the atmosphere takes no part and which are inde- 
pendent of weathering processes at the surface. This is metamorphism (sensw strictw). 
c. Alterations, migrations of matter, new mineral formation in connection with 
weathering processes (in the zones of oxydation and cementation of ore deposits, etc.). 
By investigating the questions arising from 1] an attempt is made to determine the 
primary geochemical type of the deposit. A definite conclusion is not always possible. 
Coal, for instance, represents a concentration of carbon due to enrichment of organic 
matter at the surface, but later reactions and changes after covering-in had taken 
place resulted in a loss of oxygen, 7.e., a further concentration of carbon. Garnierite 
veins are another example of concentration in two stages. If the mineral composition 
is chiefly determined by 2 and 3a, the mineralogical type to which the deposit may 
be counted coincides with the primary geochemical type. If the typical character 
of the deposits is mainly defined by 36 or 3c, the present significance of the deposit 
must be considered a result of secondary processes. The primary concentration of 
matter and the present features are due to separate processes. 
V. Aconsiderable measure of agreement has been reached as regards theimportance 
of the collective treatment of the ore deposits belonging to groups A and B, and the 
various systems of classification proposed differ from one another chiefly in 
more or less insignificant questions of detail. It is, therefore, proposed to deal here 
with metamorphic ore deposits and to shew that, once again, a full understanding of 
the phenomena involved requires the application of the results obtained by the investi- 
gation of rocks and, in this case, of rock-metamorphism. 
The considerations which point to the existence of metamorphic ore deposits are 
quite similar to those which lead to the separation of the metamorphic rocks as an 
independent class. It has long been known that certain ore-occurrences shew excep- 
tional mineralogical and structural developments which make classification difficult 
and seem to contradict the derivation from purely magmatic or sedimentary processes. 
A profusion of rare minerals and such which are peculiar to the localities is often a typical 
feature of such deposits. ‘Three examples may be quoted to support this statement. 
Each year brings a series of new minerals from the Langban mine. In 1923 
Flink quoted over 200 species, of which rare manganese silicates, lead silicates, lead 
arsenates and magnesium, magnesium-manganese, calcium-magnesium-beryllium 
silicates are examples. It can hardly be doubted that many of these minerals were 
formed during a period of metamorphism and originated from solutions of unusual 
composition, subjected to physical conditions not usually encountered by ore solutions. 
Nils H. Magnussen supposes that a deposit of iron and manganese ore was first subjected 
to contact metamorphism which produced a body termed by him a reaction skarn 
(i.e., a skarn originating from already existing material through thermometamorphism). 
In a later period folding took place in the area, which was subjected to a slight dis- 
location metamorphism (skél-forming process) accompanied by exchange of matter 
between leptite layers and the ore and skarn masses. Fissures were opened, some 
