Hydrothermal Deposits - Metals and Society: An Introduction to Economic Geology - page 90

Ore Shell
py 1% Cu 1-3%
Mo .003%

Propylitic
Chl-Epi-Carb

Pyrite Shell
Py 10% Cu .01-3%

Phyllic Argillic

Qtz-Kaol-Alun
Chl

Low-
Pyrite
Shell
Py 2%

Qtz
Ser
Py

Potassic
Qtz-K-Feld
-Bt + - Anh

Low
Grade
Core
Cu-Py-Mo

Qtz-
Ser-
Chl-
K-Feld

Mag
Py

Mag
Py

Mag
Py

Chl-Ser-
Epi-Mag

Chl - chlorite ; Epi - epidote ; Carb - carbonate ; Qtz - quartz ; Ser - sericite ; K-Feld - Potassium
Feldspar ; Bt - biotite; Anh - anhydrite ; Py - pyrite ; Kaol - kaolinite ; Alun - alunite ;
Mag - magnetite ; Cu - copper ; Mo - molybdenite

Fig. 4.8 Distribution of alteration zones ( left ) and types of sulfide mineralization in a porphyry
copper deposit (Modified from Lowell and Gilbert 1970 )

variable. Radial fracture zones outside the pyrite halo may contain lead-zinc veins
with significant gold and silver contents.
A supergene enrichment zone developed extensively in upper parts of some
deposits. This zone is divided into the oxidized subzone containing unusual
minerals such as chrysocolla, atacamite, antlerite, brochantite, and tenorite with
lesser amounts of malachite and azurite, and the sulfurized subzone of chalcocite,
covellite, native copper and cuprite. These minerals, a series of hydrated Cu
silicates, carbonates, sulfates and oxides, have beautiful green or blue colours and
are prized by mineral collectors.
The composition of the intrusion exerts a fundamental control on the metal
content of the deposit. Low-silica, mafic and relatively primitive plutons, ranging
from calc-alkaline diorite and granodiorite to alkalic monzonite in composition, are
associated with porphyry Cu-Au deposits; intermediate to felsic, calc-alkaline
granodiorites and granites are associated with Cu-Mo deposits; and felsic, high
silica, strongly differentiated granites are associated with Mo, W, and Sn deposits.
The oxidation state, reflected by accessory minerals such as magnetite, ilmenite,
pyrite, pyrrhotite, and anhydrite, also influences metal contents: most deposits are
related to oxidized, magnetite-series plutons, but some Sn and Mo deposits are
related to reduced, ilmenite-series plutons.
Distribution and age : Porphyry deposits are predominantly associated with
Mesozoic to Cenozoic orogenic belts in western North and South America, around
the western margin of the Pacific Basin, and in the Tethyan orogenic belt in eastern
Europe and southern Asia. Major deposits also occur within Paleozoic orogens in
Central Asia and eastern North America and, to a lesser extent, within Precambrian
terranes. Porphyry Cu deposits typically occur in the root zones of andesitic
stratovolcanoes

in subduction-related, continental and island-arc settings.