
ORE-FORMATIONS 249 
native metals. These last, owing to their superior weight 
and insolubility, are not washed away with the lighter and 
more soluble constituents, and thus tend to become concen- 
trated in the gossan. This is the reason why the gossanous 
parts of auriferous and argentiferous lodes are usually richer 
than the underlying, unweathered portions. The richness 
of a gossan, therefore, is apt to deceive the unwary as to the 
value of the subjacent deposit—rich gossans having sometimes 
been found capping lodes which were too poor to work. 
When a gossan yields valuable metals or ores, it certainly 
indicates the presence of these in the lode below; but 
whether the latter is rich enough to be advantageously 
worked cannot be determined until the undecomposed material 
below the gossan has been carefully examined. The effect of 
percolating water can frequently be traced to a considerable 
depth below the “iron-hat”—or true gossan—the general 
result being a concentration of secondary products, consisting 
partly of oxides and partly of sulphides. This secondary 
enrichment of a lode sometimes extends to a depth of 600 
feet or even 750 feet from the surface. 
Association of Ores in Lodes.—The minerals in lodes 
often show paragenetic relations—that is to say, certain 
minerals are frequently found associated. For example, 
manganese- and iron-ores often occur together, and the same 
is true of galena (PbS) and zinc-blende (ZnS), of cobalt- and 
bismuth-ores, and of cobalt- and nickel-ores. In like manner 
the copper-sulphides (bornite and chalcopyrite) not infre- 
quently are accompanied by iron-pyrite. Again, when 
bismuth glance (Bi,S,) is present, chalcopyrite is seldom or 
never absent. Similarly, pyrrhotite and chalcopyrite are con- 
stant associates. Once more, it is most usual to find fluor-spar, 
topaz, molybdenite (MoS,), wolframite [(Fe,Mn)WOQ,], and 
cassiterite (SnO,) occurring together in the same ore-formation. 
Succession of Mineral Deposits in Fissures, etc.—It is not hard to 
understand why ore-deposits should often seem to have preferred one 
rock to another. It is obvious, for example, that relatively hard, porous, 
and highly fissured rocks would be more readily traversed by solutions, 
than soft impervious masses, in which joints and faults are apt to be close, 
and even approximately water-tight. Again, some rocks, particularly 
limestone, are more or less readily dissolved by acidulated water, and 
thus, in time, yield ample space for the deposition of such ores as 

