36 
ECONOMIC GEOLOGY 
it (e.g. K. E. Andrée, Geol. Meeresbodens, ii, 1920, p. 582). 
Such gold as there may be in sea-water is probably secondary 
and does not explain the source of gold in lodes. 
GoLp Lopes anp IgnEous Rocks—Most of the chief 
goldfields of the world have been found by their alluvial 
deposits, in which the gold is conspicuous owing to its bright 
colour and the beating of small grains into large flakes. 
Primary gold is generally found in quartz-veins in the older 
rocks, though younger lodes are found in areas of great 
earth-movements and volcanic activity. The gold-quartz 
veins were at first regarded as igneous, owing to their fre- 
quent association with igneous rocks and their resemblance 
to dykes by their branching and cutting across bedding 
planes. Some quartz-veins contain felspar and have been 
regarded as pegmatites in which nearly all the felspar has 
been deposited in some lower position, leaving only molten 
quartz to solidify at the upper end of the intrusion. Secondary 
felspars, however, occur in limestones, and the absence of 
the typical igneous minerals from the quartz-veins and of 
contact metamorphism along the edges and the fact that 
the quartz in the lodes is the low temperature variety 
(i.e. B-quartz), show that the veins were formed under 
aqueous conditions at a moderate temperature, and were 
introduced in solution and not as a molten intrusion. 
The vein-quartz often passes indefinitely into the country 
rock. The white quartz passes gradually into dark quartz, 
and through silicified into normal slate. In quartz-veins 
in granodiorite all stages can be seen between silicified frag- 
ments of that rock and pure quartz. Quartz-veins often 
include fragments of the country rock that have resisted 
silicification ; and as such blocks are often in their original 
position the quartz around them grew by replacement, 
Most primary gold-quartz lodes are due to deep-seated 
hydrothermal “action. Pneumatolytic conditions are in 
places shown by axinite, tourmaline, and kaolinite, The 
deep-seated origin of some lode gold is indicated by its 
association with tellurides, as at Kalgoorlie and Cripple 
Creek. In other cases the action was propylitic, for super- 
heated steam and carbonic acid altered the felspars into a 
mosaic of secondary quartz and felspar, and the ferromag- 
nesian minerals into chlorite, epidote, and zoisite.