ORES OF COPPER 89 
latterly for the manufacture of sulphuric acid. The pyritic 
mass contained occasional intergrowths of primary bornite 
and chalcocite. Most of the pyrites was massive; but in 
some surfaces in the open-cut the structure of the replaced 
Schist could be seen by a sheen like a watermark. The 
North Mt. Lyell Mine has a richer quartz-ore containing 
AN average of 6 per cent. of copper in pyrites, bornite, and 
chalcocite ; it is also along the Mt. Lyell Fault (Fig. 30); 
the upper part is a pipe-lode and in places lies between schist 
And conglomerate : this pipe rises from a replacement de- 
POsit, 100 feet thick and 1500 feet long, which has replaced 
both rocks. In September, 1925, the ore reserves of the 
North Mt, Lyell Mine were a little over a million tons con- 
“aining 6 per cent. copper, 1-33 oz. of silver, and ‘015 oz. of 
cold per ton. 
There are four chief theories as to the origin of these pyritic 
masses. The first regarded them as sediments deposited 
at the same time as the adjacent rocks (von Roemer, 1873- 
76; von Groddeck, 1879; Klockmann, 1894, 1902 ; Ber- 
g€at, 1906). Dr. E. D. Peters (1893) adopted this view for 
Mt. Lyell, regarding the ore as a lake deposit, and it is 
fetained by B. E. Crump in his recent work Copper (1925, 
P. 154). According to the second theory they are fissure- 
odes, due to lateral secretion (Collins, 1885), or to ascending 
solutiong (Gonzalo y Tarin, 1888, De Launay, 1889, and 
Vogt, Z. prakt, G., 1899, pp. 241-54, who regarded them as 
Peumatolytic after-effects of the porphyry intrusions). A 
third view explained the ores as contact deposits and was 
adopted for Mt, Lyell by Daly (Tr. I.M.M., ix, 1901, p. 86) 
and T. A. Allan, once manager of the Tharsis Mine, but is 
Inconsistent with the occurrence of some of the ore bodies 
apart from any igneous rock. The alternative theories 
"OW held are either that the ores are igneous intrusions 
(Broughton Edge) or, as suggested by the author in 1904, are 
due to hydrothermal replacement of rock which had been 
completely shattered by earth-movements, and saturated 
°y sulphate solutions. Faulting near the contact of quartz- 
Porphyry and shale produced fissures in the quartz-porphyry, 
while the shale was rendered impermeable by compression. 
Hence at Rio Tinto the ore occurs mainly as a replacement