Zonacion Lateral En Porfidos Cupriferos

ECONOMIC

GEOLOGY
AN'D

BULLETIN

Vo..

OF

THE

65

THE

SOCIETY

OF

ECONOMIC

GEOLOGISTS

JuNE-JuLY, 1970

Lateral and Vertical Alteration-Mineralization
Zoning
in PorphyryOre Deposits
j. DAWDLOWELL
AND JOHN M. GUmBERT
Abstract

The geologichistory of the San Manuel-Kalamazoodeposithas provided an opportunity for the examination of vertical and horizontalzoning relationships in a porphyry coppersystem. Precambrian Oracle "granite," a Laramide monzoniteporphyry,
and a Laramide dacite porphyry are hosts to zones of potassic,phyllic, argillic, and
propylitic assemblages
shown to be coaxially arranged outward from a potassiccore
through phyllic, argillic, and propylitic zones. Alteration zones at depth comprise an
outerchlorite-sericite-epidote-magnetiteassemblageyielding to an inner zone of
quartz-K4eldspar-sericite-chlorite. Mineralization zones are conformable to the

alterationzones,the ore zone (with a 0.5% Cu cutoff) overlapping
the potassic
and
phyllic zones. Occurrenceof sulfideschangesupward and outward from dissemination
at the low-grade core of the deposit through microveinlet to veinlet and finally veinoccurrenceindicating the progressivelyincreasing effect of structural control.
Several aspectsof San Manuel-Kalamazoo geology suggestthat it is exemplary of
the porphyry copper depositgroup. To test that idea and to evolve three-dimensional
aspectsof these deposits, table of geologiccharacteristics 27 major porphyry dea
of
posits is presented. Considerationof the table indicatesthat the "typical" porphyrycopper deposit is eraplaced in late Cretaceous sediments and metasedimentsand
is associated
with a Laramide (65 m.y.) quartz monzonitestock. Its host intrusive

rock is elongate-irregular,
4,000X 6,000 feet in outcrop,and is progressively
differentiated from quartz diorite to quartz monzonite in composition. The host is more like

a stockthan a dike and is controlled regional-scale
byfaulting. The orebodyis oval
to pipelike, with dimensions 3,500 X 6,000 feet and gradational boundaries.
of
Seventy
percentof the 140 million tons of ore occursin the igneoushost rocks, 30 percent in
preore rocks. Metal values include0.45% hypogeneCu with 0.35% supergene
Cu,

and 0.015% Mo. Alterationis zonedfrom potassic the core (and earliest) outward
at
through phyllic(quartz-sericite-pyrite), argillic (quartz-kaolin-montmorillonite),
and
propylitic (epidote--calcite-chlorite), propylitic zone extending2,500 feet beyond
the
the copperore zone. Over the sameinterval, sulfidespecies
vary from chalcopyritemolybdenite-pyrite
throughsuccessive
assemblages an assemblage galena-sphalerite
to
of
with minor gold and silver values in solid solution, as metals, and assulfosalts.
Occurrence
characteristics
shift from disseminations
through respective
zonesof microveinlets (crackle fillings), veinlets,veins, and finally to individualstructures the
on
peripherywhich may containhigh-grademineralization. Breccia pipes with attendant
crackle

zones are common.

Expressionof zoning is affectedby exposure,structural and compositional
homogeneity, and postorefaulting or intrusive activity. Vertical dimensionscan reach

10,000feet, with the upperreaches the porphyryenvironment
of
perhaps
only at subvolcanicdepthsof a few thousand
feet. The vertical and lateral zoning described
is
repeated
with sufficient
constancy
that depthsof exposure many deposits
at
can be cited
against the model of San Manuel-Kalamazoo.
373

No.

4

374.t. D. LOWELL AND J. M. GUILBERT
Severallines of evidence
suggest
relativelyshallowdepthsof formationand significant variations in water contentin the porphyry environment. Shallow emplacement
is consistent
with the appearance breccia
of
pipesassociated
with ring and radial diking
and with vertically telescoped
zoning. Models of the sourceof altering-mineralizing
fluids are considered....