and most of the ore beneath is primary. The weathering of the silicated limestone gangue results in great masses of clay which are characteristic features of the oxide zones of these deposits.
Copper deposits in schists. Other copper deposits, as at Jerome, Arizona, in the Foothill and Shasta County districts of California, at Ducktown, Tennessee, etc., are irregular lenticular bodies in schists and other rocks, but all show relationship to igneous rocks. The Rio Tinto ores of Spain and Portugal, which belong in this group, have been referred to on page 108.
In the Jerome or Verde district of central Arizona, folded pre-Cambrian greenstones and sediments were invaded by masses of quartz-porphyry, and after further deformation, rendering many of the rocks schistose, were intruded by an augite-diorite.
Contact metamorphism along both the quartz-porphyry and the diorite contacts was practically lacking.
The ore bodies were formed as irregular pipe-like replacements of the schists, being localized in one case by a steeply pitching inverted trough of impervious diorite, and in other cases by shear zones which favored vigorous circulation.
A later series of small diorite or andesite dikes cut the ore bodies.
The primary ores consist of pyrite, chalcopyrite, and other sulphides, with large amounts of jaspery quartz and some calcite and dolomite.
They were clearly formed by replacement of the schists particle by particle, as shown by the frequent preservation of the schist structure in a banding of the sulphide minerals, the residual shreds of unreplaced schist material in the ores, and the usual gradual transition from unreplaced schists to those completely replaced by massive sulphides.
The localization of the most important mineralization in an inverted trough is good evidence that the solutions came from below, and the nature of the mineral associations suggests an origin through the work of hot waters associated with igneous intrusives.
The diorite, being most closely related in time and space with the ore bodies, seems the most logical source of the ore materials.