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OCR for page 50
Evolution and Style of Fracture
Permeability in Intrusion-Centered
Hydrothermal Systems
SPENCER R. TITLEY
University of Arizona
ABSTRACT
Deep and tall columns of permeable crustal rocks evolve as a consequence of
fracturing during shallow emplacement and rapid cooling of water-bearing felsic
magma. Rapid cooling of the magma and sudden pressure drops result in both the
development of the characteristic porphyritic texture of rocks in these systems
and overpressures of magmatic water; sudden release of the hydraulic energy
built up in the melt is believed to have resulted in development of stockwork-style
(reticulate) fracturing of the magma and its wallrock. Early stages of cooling of
magmas considered here takes place through conduction, but subsequent cooling
is dominated by development of the stockwork and consequent convective flow;
rapid transfer of heat to wallrock is focused by the stockwork system and results
in rapid local rise of temperatures and consequently pressures of pore water,
resulting in further rock failure in the walls. The dominant character of flow
porosity that evolves is that of an intricately interconnected three-dimensional
network of planar fractures with large length-to-aperture ratios and continuities
that range from microns to hundreds of meters.
Field study and analysis of these systems reveal that they form from a complex
process characterized by episodic rock failure and a consistent and predictable
evolutionary course of hydrothermal reaction with walTrocks, usually in a thermal
regime of declining walTrock temperatures. Studies of waters suggest changing
provenances with magmatic waters dominating early hydrothermal systems,
subsequent mixing with waters of different sources including pores, and, ulti-
mately, dominance by meteoric water.
50
OCR for page 51
EVOLUTION AND STYLE OF FRACTURE PERMEABILITY
INTRODUCTION
Emplacement and cooling of magma at shallow crustal
levels are attended by thermal-mechanical stresses result-
ing in the formation of extensive interconnected networks
of closely spaced (centimeter) joints in large rock vol-
umes. These volumes of densely fractured rocks with
reticulate fracture patterns, informally referred to as stock-
works, represent secondary permeability imposed on crus-
tal rocks that underlie areas of up to hundreds of square
kilometers, centered on the locus of magma emplacement.
In most instances a volcanic center may be inferred as
such a locus from regional or areal geology. These centers
of areally small (kilometers), subvolcanic intrusions are
the object of discussion in this chapter.
The shape of the rock volume affected, which includes
the pluton complex and many times its volume of wall-
rocks, is crudely cylindrical and may have had a height
that extended from a permeable volcanic superstructure to
depths corresponding to the height of the cooled magma
column. This height is believed to have been as great as 7
km at Yerington, Nevada (Dilles, 1987), and at least 3 km
at San Manuel, Arizona, as scaled from Lowell (1968~.
The permeability imposed on these systems in the form of
joints provides a means for localized rapid cooling of
magmas by convection of fluids of magmatic and meteoric
provenances. The movement of these fluids from crustal
wallrocks and magmatic sources results in the formation
of metal-sulfur deposits and some centers of modem geo-
thermal activity. The passage of fluids through joints
centered on the igneous system also results in mineralogi-
cal modification (hydrothermal alteration) of joint walls
and filling of open space in the joint system by precipi-
tated minerals.
This chapter presents the results of detailed field and
laboratory analyses of the geometrical and mineralogical
~ ~-~ ~'~t ;:
PACIFIC
J .\ o ,lND.-AUST~ '.,
J ~\_/ REPLATE A ~'
Hi., 1~1
aNTwnTIc Pl ATE -
51
characteristics of these joints and their host rocks from a
broad sampling of sites. These studies reveal information
concerning the development of flow porosity in a specific
but widespread geological phenomenon of modern as well
as old tectonic regimes. Intrusive centers have been stud-
ied in both island arc and continental settings, where they
have been found to reveal generally consistent styles and
histories of joint evolution as well as a consistent pattern
of evolution of hydrothermal minerals; these phenomena
are, in turn, consistently interpretable in a context that
allows assessment of some aspects of geochemical and
thermal evolution of the hydrothermal systems.
This chapter addresses some relevant geological as-
pects of the occurrence of these intrusive centers, relevant
aspects of petrology, an overview of the characteristics of
fracturejoint evolution, and through a synthesis, some of
the implications of the results of these studies.
GEOLOGICAL SETTING AND ENVIRONMENTS
Volcanic and seismic activity are phenomena of mod-
ern erogenic regions. The existence of volcanic rocks and
young epizonal intrusions within erogenic belts attests to
historical volcanic activity; the presence of the eroded
tops of swarms or chains of epizonal intrusions, as well as
the presence of erosional remnants of coeval volcanic rocks,
trace and reveal the position of still older orogens. In
Figure 3.1 regions of both older (Mesozoic and Cenozoic)
erogenic belts and modern erogenic regions are shown.
The belts of current activity may be seen to transgress both
oceanic and continental lithosphere and are marked by the
lines of seismic activity and volcanism. These settings are
sites of plate convergence, where, apparently, subduction-
related processes result in generation of magma and trans-
fer of heat to the shallow crust.
Widespread geological field evidence from the circum
=~=~. fAMERICAN
~ ~ C:`,, PLATE
PLATE ~
6a
FIGURE 3.1 Map of major plates and
continental masses of the world, showing
(black circles) important centers of stud-
ied Mesozoic and younger pluton-centered
hydrothermal activity. Shaded areas trace
presently active regions of seismic and vol-
canic activity where erogenic processes
are taking place. Modified from Stanton
(1978~.
OCR for page 52
52
Pacific erogenic belts indicates that extrusive volcanic
activity and epizonal intrusion are related processes.
Magmas rise in regions where subduction-related stress
has resulted in localized sites of crust weakened by fault-
ing, folding, or thinning or where extensive magma gen-
eration at depth has resulted in deep regions of buoyant
silicate melt. Within the cores of volcanic systems, bodies
of magma are emplaced and ultimately cool.
Some important characteristics of these systems merit
comment. They occur in clusters of a few to many tens of
intrusion centers in areas of 104 to 105 km2, and many were
apparently formed at times of high subduction-related
compression in both island arcs (Titley and Heidrick, 1978)
and continental settings (Heidrick and Titley, 1982~.
Individual pluton centers or clusters give rise to chains
that trace the edges of cratons or lie on or adjacent to
island arcs. In some locations pluton centers occur within
crust that has been strongly deformed, and their settings
have been described as mobile regions. Viewed at re-
gional scale, the generally circular intrusion centers ordi-
narily range from a kilometer in diameter to systems up to,
but rarely more than, about 5 km across, but they have
been localized beneath volcanic rocks covering areas an
order of magnitude greater.
In young Tertiary systems, volcanic rocks coeval with
intrusions are commonly, although not invariably, closely
adjacent; in older Tertiary or Mesozoic systems, similarly
coeval volcanic rocks are present but distant, which is
interpreted as representing remnants after weathering of a
larger volcanic superstructure. The intrusive rocks com-
prise porphyritic phases and are ordinarily members of
felsic rock clans, ranging from quartz diorite through gran-
ite. The suites of many intrusive centers are accurately
described as complex, such as at Ajo, Arizona (Wadsworth,
1968), and Ray, Arizona (Banks et al., 1972~. Volcanic
rocks are seldom more mafic than andesite and range
through progressively more felsic rocks to rhyolite.
Whereas many different textural varieties of igneous rocks
are present in the complex rocks of a volcanic system, a
young mass of porphyritic igneous rock is invariably at the
core of the volume of fractured rock.
NATURE OF SECONDARY POROSITY IN
INTRUSION-CENTERED SYSTEMS
Stockworks associated with porphyry intrusions are
vertical columns of fractured rocks, kilometers in diameter
and height. This volume of fractured rock includes not
only rocks of the intrusion complex but also volumes of
wallrock surrounding porphyry that are many times greater
than the intrusions. Within the stockwork, the fractures
that compose the porous volume vary widely in scale and
style; they range from through-going veins with continuity
SPENCER R. TITLEY
of kilometers, through intersecting veins and veinlets of
meters to centimeters of continuity, visible to the unaided
eye (the mesoscopic veins of this chapter), to veinlets
whose scale ranges downward in size to those resolvable
only with special optics. Evidence from the field, based
on the volume and distribution of hydrothermal alteration
products, suggests that it is the fractures (joints) at mes-
oscopic scale that dominate the properties of flow porosity
(permeability) of these systems.
The style of mesoscopic joints in the stockwork is shown
in Figure 3.2. There it can be seen that the style of
openings in the stockwork is dominated by a three-dimen-
sional mesh of intersecting planar to curviplanar joints.
The development of domains of variably fractured rock
within stockworks is an important characteristic wherein,
for reasons that are uncertain and that are not obviously
related to the mechanical properties of Ethology, some
volumes of rock manifest many more fractures than con-
tiguous volumes; the flow porosity is thus anisotropic at
the scale of tens of meters.
Field studies reveal that there has been little or no
rotation or transport of rock following the fracturing pro-
cess in most porphyry-centered systems. Such a generali-
zation excludes volumetrically insignificant masses of
localized breccia that occur in some systems and clearly
Joint Zone and Jl-Joint Set
Sn~ooth- surfaced, continuous -
planor; pronounced influcace oa
local izing ~ ~ ~ arotion - m i nerolirof ion
J2 ~ Joint Set s
Roug-h -sur f oced, discontinuoua
curvi ploner; rorely ollered or
m i neroi i zed
;,l - ~; ';' `/ !
:'
J3- Joint Sets Dikes,Veins ond Fault
Ouite irrequlor, discontinuous veins ( Fv)
microtroctures ond hoirlin.
crocl`s
i1 ~ ~ ~
lI y4, ('/
i`'`l/ Aplite dike ~
J3-Joint Se1
FIGURE 3.2 Schematic block diagrams illustrating typical and
characteristic styles of fractures with stockwork systems. The
several kinds of fractures successively overprint each other and
result from episodic events. Reprinted from Heidrick and Titley
(1982) with permission, University of Arizona Press, Tucson.
OCR for page 53
EVOLUTION AND STYLE OF FRACTURE PERMEABILITY
FIGURE 3.3 Photographs of thinned sections showing habit of
microveins as used in this paper. Scale bar is 1 cm. The upper
diagram shows apparently early microveins in relationship to
thicker 0.5-cm quartz veins. In both the upper and lower dia-
grams an apparent systematic orientation or set of orientations is
shown that may be interpreted in the context of oriented stresses
that control fracture directions, even down to this scale.
excludes some systems such as Cerro Verde (Peru) and
Cananea (Mexico), where large breccia masses are major
constituents of the fractured rock volumes. Whereas the
mass of hydrothermally derived minerals that occur within
and adjacent to the joints may aggregate up to 10 percent
of the total volume of fractured rock, field evidence does
not indicate that this volume was produced instantane-
ously. The walls of both steep and flat veins of stock-
works are parallel over meters of length, regardless of vein
thickness, revealing negligible rotation. Evidence of offset
of old veins by younger fractures is extremely rare and
53
where present is at the scale of centimeters. Fracture
asperity may locally result in pinching and swelling of
vein walls, but such apparent roughness cannot be un-
equivocally related to movement. Consequently, transport
of fractured rock has been considered trivial, the dominant
motion or displacement only that of true joints, normal to
the fracture wall. The only reasonable explanation of the
relatively high volume of crack-related alteration minerals
is that fractures evolved episodically and appear to have
been filled continuously through a time of cooling and
consequent shrinking of the hot rocks.
Within the domains of mesoscopic joints, smaller
domains of microscopic fractures are present. These frac-
tures, defined here as features less than 0.1 mm in width,
are also interconnected but are discontinuous with persis-
tence ordinarily on the scale of centimeters (Figure 3.31.
Whereas they contribute significantly to the porosity of
the rocks, by virtue of their discontinuous nature and
apparent small widths, they do not appear to have contrib-
uted to the permeability of the stockwork; filled with fluid
nonetheless, they may be representative of fluid-rock reac-
tions controlled more by diffusion than flow.
ROLE OF MAGMATIC PROCESSES IN
FORMATION OF JOINTS
The occurrence of porphyry plutons associated with
volcanic centers is significant. The intricately intercon-
nected, closely spaced joints that compose the common
and typical stockwork evolve in association with rocks of
porphyritic textures; stockworks are not widely, if at all,
recognized as having evolved in the course of the forma-
tion of igneous intrusions with phaneritic textures.
The evolution of porphyritic igneous rocks and their
associated stockwork fractures appears to be integrally
related, and the evolution of porphyries continues as a
subject of fundamental petrological study. A generalized
and simplified concept concerning the origin of these rocks
explains that they represent multistage cooling of felsic
silicate melts. Interruptions in uniform cooling, which
commenced at great but unknown depths, and chilling of
melt accompanied by drops in pressure, attend the rapid
rise of magma to shallow levels of the crust, a phenome-
non consistent with the apparently rapid emplacement of
magmas in volcanic systems and the presence there of
porphyritic igneous rocks.
The characteristics of porphyry intrusions in the com-
plexes considered here, as seen in the field, attest to this
likelihood of rapid emplacement of magmas. Xenoliths of
any sort are rare to absent within, at the margins, or in the
caps of these porphyry plutons, and wallrocks are unde-
formed, the implication being that magmas were emplaced
as fluids with a minimum amount of sloping. Studies of
OCR for page 54
54
many porphyry stocks only rarely reveal the presence of
recognizable foliation, flow lines, or other evidence of
motion of magma along contacts. These characteristics of
rock texture, foliation, and the absence of xenoliths have
been ascribed to a process of "permissive" rather than
"forceful" (Mutch and McGill, 1962) emplacement of the
porphyry magmas. A "permissive" or unimpeded em-
placement of such magmas is envisioned as possible, and
even likely, in the active tectonic environment in which
they occur, with tectonic stresses resulting in deeply pene-
trating faults and consequent channelways.
The most important result of emplacement of magmas
at shallow depth is that of rapid cooling and quenching of
melt that contained crystals of earlier-formed minerals.
The porphyritic textures are thus formed in the walls and
across some great but unknown depth of the cooled magma.
Rapid cooling and crystallization of melt bring about
changes in water pressure, the effects of which have been
described by Burnham (1967, 1979~. Briefly stated, the
exclusion of water from the silicate melt by crystalliza-
tion, especially at the shallow depths (3 to 5 km) consid-
ered here, results in overpressures of exsolved water that
exceed the tensile strength of the rocks. Brittle failure
under these conditions (hydraulic fracturing) ensues and
evolves the fractures composing the stockwork.
Emplaced magmas may cool initially by conduction of
heat to their walls (Norton and Knight, 1977~. This pro-
cess is believed to result in rock failure and to complement
the highly energetic effects bringing about the hydraulic
failure described above. Water contained in the wallrocks,
chiefly in pores, becomes an agent of energy when heated;
the effects of this heating have been described by Knapp
and Knight (1977) and Knapp and Norton (19811. As
wallrocks to porphyry plutons are heated by conduction or
convection in the early stages of magma emplacement,
contained pore water passes into the supercritical region
and effective pressures exceed the tensile strength of the
rock. The result is rock failure seen now in the presence of
the large volumes of fractured wallrock surrounding por
. .
P nyry intrusions.
NATURE OF JOINTS AND FRACTURE SYSTEMS
At regional scale the intrusive centers may be seen in
some terranes to be related to, and presumably controlled
by, variations in the effects of stress in different domains
within regional fault systems. Whereas such control is
difficult to establish or to even propose in older terranes
subjected to tectonic overprinting, it may be reasonably
inferred from geological features in young terraces of is-
land arcs. As revealed in the geology of the mobile belt of
Papua New Guinea (Figure 3.4), batholiths and related
centers of porphyry complexes lie within or adjacent to
SPENCER R. TITLEY
Probable Net
Convergence Directions ({its)
Ma cK:~n` ~P1~ro t ~ant
IVe~V GlJ/4~£-A ~ L
~_'~_
CJ6f /£ ,£ 'I'm;, - -''a
set r '\`2 t%_
3L~ 1 ~290 ~4
KlLO~ETEltS
~ -
FIGURE 3.4 Modified geologic map of Papua New Guinea
showing trace of regional faults and location of principal batho-
liths and centers of pluton emplacement. Black areas are those of
mappable igneous bodies and centers. Modified from Titley and
Heidrick (1978~.
southward deflections of generally WNW-striking regional
linear faults. Interpreted right-slip in this regional fault
system result in areas of "low compression" at the south-
ward deflections of their trace. Regional stresses that
appear to have controlled positioning of these systems at
such a scale are interpreted as having even more funda-
mental importance in affecting properties of joints at kilo-
meter to meter scale.
Joint and Fracture Orientations
The orientation of mesoscopic fractures in stockworks
has been traditionally described as "random," and at cas-
ual glance in outcrop the swarms of joints appear to mani-
fest such characteristics. In detail, however, measure-
ments of fracture orientation in numerous pluton- centered
systems in cratonic and arc settings (Rehrig and Heidrick,
1976; Titley and Heidrick, 1978) reveal properties of align-
ment and dip that are consistent with inferred orientations
of the regional or local stress field, an association that is
intuitively obvious, although it is not conspicuous or promi-
nent at outcrop scale.
Detailed studies of joint orientations in large systems
reveal two extremes in the habits of orientation of mes-
oscopic fractures, from each of which may be deduced the
orientation of principal stresses. These extremes are shown
in Figure 3.5, geologic sketch maps of fracture systems in
two pluton-centered stockworks in Arizona. In Figure
3.5A fractures radial and concentric to a center of intru-
sion reveal patterns evolved at high levels in pluton sys-
tems where the maximum principal stress is vertical and
intermediate and least principal stresses are hydrostatic,
equal, and horizontal. The example of Figure 3.5A views
OCR for page 55
EVOLUTION AND STYLE OF FRACTURE PERMEABILITY
a Laramide intrusion complex very near its top with the
orientations of fractures related to stress in a shallow epi-
zonal environment. Stresses in this crustal region result in
a pattern of centrosymmetric, concentric, and radial frac-
tures. In Figure 3.5B (a map of a surface weathered into
the deeper parts of a Laramide intrusion complex in south-
e~n Arizona) a pattern of joint development is present that
was controlled by stresses in deeper parts of the crust. In
the example of Figure 3.5B, fluid inclusion data derived
from study of quartz formed in the joints are interpreted to
indicate that we are viewing the system at a depth at least
2 km beneath the original surface (Preece and Beane,
1982~. The fracture orientations of Figure 3.5B may be
further interpreted as reflecting a horizontally oriented
maximum principal stress, a roughly south to north ori-
ented minimum principal stress and a vertical intermediate
principal stress.
The analysis of geometry and distribution of joints
formed near the locus of intrusions, coupled with radio-
metric age data that show fracturing to be essentially
synchronous with intrusion, has led to interpretations of
origin and orientations of regional stress. In Arizona the
consistent, nearly monotonous ENE and NNW directions
of fracturing seen in Laramide intrusion systems (Figure
3.6) correspond to the directions of stress inferred for the
normal convergence directions of Laramide plate subduc-
tion (Rehng and Heidr~ck, 1976; Heidr~ck and Titley, 1976,
19821. Miocene stress directions acting on eastern Papua
New Guinea are seen to be consistent with the orientations
of fractures surrounding centers of felsic plutons in the
Eastern Highlands (Titley and Heidrick, 1978; Asami and
Batten, 1980~.
Field work completed in these regions, widely sepa-
rated in space and in times of igneous activity, reveals
without known exception joint geometries that are region-
ally consistent within regions with the time and inferred
\~
, *<
in.
· . N_
-
~, .
0 0.5 1
-
ki iometers
-I
/
v ,. .
, ,; -,
A
0 1 2 3
ki lometers
55
related tectonic style. A reasonable interpretation of the
genesis of these fracture networks may be drawn from
these numerous observations; the thermal and mechanical
energetics of pluton emplacement and cooling bring about
rock failure; the geometry of resulting joints is controlled
by orientation of regional and local stresses.
Alteration, Succession, and Distribution of Altered
Fractures
Three important physical and mineralogical properties
of fractures that make up the volumes of stockwork have
been studied in detail. The interrelationships of these
features establish a basis for understanding the way in
which joints and corresponding characteristics of flow
porosity evolve.
The first of these properties is that of the associated
hydrothermal alteration products. A second is that of the
succession of veins and alteration products as revealed in
cross-cutting relationships. The third is the relative abun-
dance, in space, of different sets of veins as defined by
alteration products.
Vein Alteration and Paragenesis The flow of fluids
through the joint network results in alteration of the crack
walls and deposition of hydrothermal minerals. Evidence
from the field, from the study of many slabbed specimens,
and from the results of mineralogical and fluid inclusion
analyses reveals that this process is complex, takes place
through many stages, and proceeds in the hydrothermal
environment under declining temperatures.
Viewed at outcrop scale at the surface (Figure 3.7) and
on mine bench, the mesoscopic fractures of stockworks
reveal heterogeneous characteristics of style and manner
of alteration and of weathering. The heterogeneity in
appearance stems from the style of joint that formed at a
a/ //
,,,~(,, \/
/ / ~
' /
\
FIGURE 3.5 Geologic sketch maps of
structures and joint-fault-dike orientations
in Laramide intrusion centers in Arizona.
Figure 3.5A a map of the San Juan Pluton
at Safford, Arizona, modified from Heid
rick and Titley (1982), manifests patterns
seen at high intrusive levels where-frac
tures trace radial and concentric patterns
with respect to the center of plutons. Fig
ure 3.5B shows fracture pattern orienta
tions revealed at a weathered depth of about
2 km into the Sierrita, Arizona, pluton
B system (patterned areas). Pronounced
orientation of fracture pattern believed to
represent effects of region-wide compres
sion during the Laramide.
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Representative terms from entire chapter:
economic geology
56
particular time and the contrasting mineralogy formed by
the hydrothermal solutions that passed through it. Within
systems the habits of the altered joints that occur at spe-
cific stages in fracture evolution are broadly consistent
and predictable from place to place. They differ, however,
between systems.
Distinctive assemblages of alteration minerals develop
within specific joint sets at different stages of the hydro-
thermal process. Timing of stages is revealed from the
cross-cutting relationships of joints with different altera-
tion products and styles. A typical example of this ubiqui-
tous habit and a common succession of hydrothermal
products are shown on the polished and mapped slab of
Figure 3.8, wherein veins and veinlets characterized by
specific alteration assemblages reveal consistent cross-
cutting relationships. The habit of intersection of cracks
with different vein mineralogies bespeaks a complex and
protracted process of fracture development in stockwork
.
evolution.
The development of vein alteration assemblages has
been described in reports spanning many decades of work,
with increased levels of understanding by numerous au-
thors (Schwartz, 1947; Creasey, 1966; Meyer and Hemley,
1967; Carson and Jambor, 1974; Brimhall, 1979; Rose and
Burt, 1979; Beane and Titley 1981; Einaudi et al., 1981;
Beane, 1982; Einaudi, 1982~. A widely described and
generalized succession of vein alteration assemblages
(Titley, 1982) from early to late in a quartz-2 feldspar-
mafic wallrock is as follows:
1. The assemblages quartz, quartz-biotite-orthoclase, or
quartz-biotite form in and adjacent to the thermal center
and in some instances may be of large areal extent, proba-
bly synchronous with assemblages 2 through 5.
2. The assemblage chlorite-epidote-carbonate-zeolites
that evolves at the periphery of the system.
3. The assemblage quartz-orthoclase-(biotite)-sulfides
near the thermal center.
4. The assemblage quartz-(orthoclase)-sulfides near the
thermal center.
5. The assemblage quartz-sericite-pyrite developed
within the system that overprints earlier stages.
6. Ultimate overprinting of stages 3, 4, and 5 by the
mineralogy of stage 2 that collapses on the system as its
center cools.
This common succession of alteration types in potas-
sium silicate rocks, as also shown in the slab of Figure 3.8,
reveals important characteristics of the progress of frac-
ture and chemical evolution in the systems. Because each
mineral assemblage forms in its own set of fractures at a
specific time, it must be interpreted that the development
of fractures is episodic. Detailed fluid inclusion data from
many hydrothermal systems such as these (e.g., Preece
SPENCER R. TITLEY
5 o
.,, o
Ul I= _
, =-
o~ m
Z m~
_ a:
:? · NTOUR INTERVAL
OFF . r°,
{~' ~:-4
~ N ~ 2172
~1~
s A
_ ~CONTOUR INTERVAL
/ ~\ 10-1~'o r¢-4
,b ~
EVOLUTION AND STYLE OF FRACTURE PERMEABILITY
volume of solution increases or that certain ion activity
ratios change, bringing about an effective increase in the
concentration of H+. By stage 5 the manifestation of
areally large volumes of rock, completely and pervasively
converted to masses of quartz-sericite and pyrite, suggests
profound H+ metasomatism, from which may be inferred a
considerably enhanced volume of solution flow. The shift
from anhydrous to hydrous phases also has been described
in carbonate wallrocks where calc- and magnesium-sili-
cate alteration minerals are formed (Einaudi, 1982; Johnson
and Norton, 1985~.
57
/ / \ ~
I ,l~q ki+mt
~it?-~
58
method involves measuring of fracture area in rock vol-
umes or fracture length on surfaces and then dividing that
value by, respectively, either the volume or area of the
sample. The value obtained is per centimeter. For the
fracture abundance present in the systems studied, values
were determined from length/area relationships in 2500-
cm2 circular or square sample areas. Area/volume values
were determined on drill core and selected samples of
slabbed rocks. The values obtained were used to deter-
mine both the distribution of vein types and their abun-
dance (Titley, 1978; Haynes and Titley, 1980; Titley et al.,
1986).
Recognition of the existence of vein successions as
revealed by contrasts in alteration allows an assessment of
the distribution and abundance of fractures that form at
different stages in the process. Such a study for a system
has been carried out in detail and complete scope only at
Sierrita, Arizona. Data collected from a thousand sample
sites at Sierrita, over an area of about 70 km2, have been
treated by area-averaging techniques and contoured as
shown in Figure 3.9, wherein isopleths of fracture density
values, representing all fractures, close on the porphyry
center (from Titley et al., 1986~. Separate maps showing
the distribution and abundance of five different alteration
types are integrated into separate curves of density versus
distance (Figure 3.10~.
Most of the measurements of fracture abundance at
,'" - V~/~}
/~
'\ ~ ~
't ~(
~\:
· ~A- / ~
( ~ , _ _
-\ ~ /
isopleth values
in cm~
ki lometers
FIGURE 3.9 Vein/joint abundance map of the Sierrita system,
Arizona. Isopleths show values for the total numbers of fractures
(per centimeter) in outcrop and close on the porphyry center
(patterned areas). Outer isopleth shows values nearly zero, but
rare altered joints persist outward for another kilometer. Within
the center of porphyry, fracture abundance values double and
locally exceed values of 1.5 cow. Figure modified from Titley
e! al. (1986) and data in Haynes and Titley (19801.
SPENCER R. TITLEY
O?5- .
0 15
0.10
SIERRITA
0.20 ~ CZ~ CHL-CPY-MO (ANH-~1M)
of- PY ( OR-EP- MT)
~ \\ QZ-OR (Bl)
0.05
~ )~
KM
FIGURE 3.10 Plot of fracture abundance (vertical) as a function
of vein-alteration type and distance (horizontal) from an as-
sumed center within the porphyry mass at Sierrita. Oldest-to-
youngest vein sets proceed from qz-or to the complex qz-or-chl-
cpy veins that are the youngest. The values shown in Figure 3.9
include the values of the quartz-sulfide set and the quarcz-ortho-
clase set. qz = quartz, or = orthoclase, chl = chlorite, cpy =
chalcopyrite, mo = molybdenite, anh = anhydrite, hm = hematite,
ep = epidote.
Sierrita were made on the vertical faces of benches in the
pit or walls of washes or on horizontal to subhorizontal
surfaces of adjoining areas. Inspection of vertical faces
across a pit depth of several hundred meters, as well as
inspection of vertical faces of surrounding outcrops, indi-
cates a relatively low (much less than 10 percent) propor-
tion of flat (less than 45°) fractures at these depths in the
system. Whereas flat fractures are present at higher levels
of such systems (e.g., the young systems of Papua New
Guinea; see also Knapp and Norton, 1981), at the deeper
(2 km?) level exposed at Sierrita they are uncommon. In
these circumstances the determination of length/area rela-
tionships in fractured rocks is a close approximation of the
area/volume value. Such a conversion is reasonable at
Sierrita and otherwise comparable deeply exposed intru-
sion centers.
The information shown in Figure 3.10 is revealing in
several important respects. It shows an episodic progres-
sion of fracturing that commences with an early, wide-
spread event of relatively uniformly low numbers of frac-
tures, followed by progressively more centrally restricted
episodes of fracturing. Hydrothermal fluids flowed through
fractures following each episode, and each stage of frac-
tures is altered in a unique and characteristic way with the
most dense fracturing and subsequent alteration in a stock-
work closely centered on the porphyry center. Studies of
fluid inclusions from vein quartz from the different vein
sets reveals further that fluids depositing quartz in each
EVOLUTION AND STYLE OF FRACTURE PERMEABILITY
subsequent event were cooler than those of each older
stage of quartz formation. The most dense fracturing is
the youngest, most constricted, and most centrally located
of the joint sets. And where these stockworks are hosts to
ore minerals such as chalcopyrite, molybdenite, or tin
minerals, it is this centrally restricted set that localizes the
most abundant sulfide mineralization in the porphyry-stock-
work environment. From the data in Titley et al. (1986)
and summarized and shown in Figures 3.9 and 3.10, it is
possible to make estimates of rock volumes containing
equal fracture areas for specific fracture sets and their
measured fracture densities. Such an estimate is shown in
Figure 3.1 1 where cylindrical volumes 1 km in height with
each containing 100,000 km2 of fracture area are schemati-
cally portrayed. At the low measured densities of frac-
tures containing quartz and orthoclase (Figure 3.10), the
relatively large volume shown is necessary, whereas at the
much greater fracture densities measured in the orebody
from the ore-sulfide-bearing quartz veins, a much smaller
volume is required to contain the same area of fractures.
Each of the fracture types shown in Figures 3.10 and 3.11
is overprinted in the central volume of rock by the younger
vein stages. Viewed in two dimensions the progress ap-
pears to evolve with development of early, widespread
joints, each successive fracture-forming event resulting in
more closely spaced joints progressively more closely
focused on the center of the system.
4.0km-
25 km- ~ 1
/~km ~I
\
QZ-OR
QZ- S
QZ- ClJ-MO
VOLU MES OF 100,000 kn,2 OF
FRACTURE AREA
FIGURE 3.11 Generalized diagram, from data of Figure 3.10,
showing relative cylindrical volumes of rock 1 km high, in the
Siemta system, that would contain 100,000 km2 Of fracture area.
Values were estimated by planimeter of plotted and interpolated
isopleths of fracture abundance in the data of Figure 3.10 and
conversion to area/volume relationships. Illustration of the frac-
ture abundance data in this way reveals the apparent focusing
effect of joint-evolving events, with the passage of time, progres-
sively closer to the intrusion center.
59
SOURCE OF WATER
It is appropriate to review the nature and origin of
waters that have been so critically important in the evolu-
tion of stockwork and its intrinsic secondary permeability.
The foregoing has underscored the complex character of
these systems in the context of apparent episodic develop-
ment of joints and in the context of the implied changing
patterns of chemical evolution. There is no question that
water in some form and likely from different sources has
been an important agent of change in the evolution of
these systems. Interpretations of fluid types and source
stem from the data developed from analyses of fluid inclu-
sions and from studies of oxygen and hydrogen isotopes,
generalized and summarized in Figure 3.12.
Fluid Inclusion Data
Numerous workers, cited in Roedder (1984), have stud-
ied the fluid inclusions found in the hydrothermal quartz
of porphyry-cored stockwork ore deposits. The solutions
in these inclusions reveal both very high (e.g., 60+ wt.%
NaCl equivalent) and low salinities (on the order of 2 to 10
wt.% NaCl equivalent). Although some exceptions to the
generalization have been found, the high-salinity inclu-
sions are those that are also interpreted to have been formed
at high (more than 500°C) temperatures and the low salin-
ity inclusions at lower (less than 350°C) temperatures.
From these characteristics a common and widespread in-
ference as to source has evolved that high-temperature,
high-salinity inclusions derived from magmatic water and
that the low-temperature, low-salinity solutions evolved
from a meteoric source. This inference is strengthened
from evidence of the isotopic character of water involved.
Data from Isotopes
Oxygen and hydrogen isotopes have been studied in a
few porphyry stockwork systems and are reviewed in
Sheppard et al. (1971) and Ohmoto (1986~. Viewed inde-
pendently of the data from fluid inclusions, the isotopic
character of early (high-temperature, high-salinity) solu-
tions reveals the habit of inferred magmatic solutions; the
isotopic character of solutions that formed late-stage al-
teration minerals requires a component of meteoric water.
Beyond identification of meteoric water by its isotopes,
interpretations of other provenances of hydrothermal wa-
ters in these systems, such as basin, pore waters, or meta-
morphic waters (each likely), become clouded in uncer-
tainty because of overlapping isotopic characteristics and
uncertainties resulting from mixing of waters from differ-
ent sources.
60
70~
~ 60
x
4l
`' 40
30
-
:~ 2~)
-
O 10
In
0~
-40
ED
( /00)
-80
-120
A
at, .............
i. ... .... \
I m ~
(\,,, . ji
A/ / / ~
at//,,,,)
a/ ~ ~ // / I
,1//////~1
~ll///llllll
~§l/////////}
~///~/l////
/// /~ ,/ 1/ / / /
/ // /// /lll//]
(////~ .~//~/},
vll/llll/l///
(///l/l l/ll/,
(/llll ill'
C//////~J'
i/ -
2< lo 300 4~ 560 6bo 7 50 Who
Temperature of Homogenization (°C)
B / SMaN
/ , %%
Rito
~Q`f
-20 -10
0 +10 +20
O(YOo)
FIGURE 3.12 Generalized data from fluid inclusion measure-
ments on quartz from porphyry-centered systems (Figure 3.12A),
after Reynolds and Beane (1985~. The diagram plots homogeni-
zation temperatures (Th) against salinities (wt.% equivalent NaCl
+ KCl) of fluid inclusions in hydrothermal minerals (mostly
quartz) and reveals a general separation of fluids into three types,
based on salinities and temperatures. Figure 3.12B shows the
isotopic composition of water (~80 versus SD) from po~phyry-
centered systems, as modified from Ohmoto (19861. In a general
way that corresponds to the data from fluid inclusions, there is a
gross bimodal nature of the data revealing likely magmatic char-
acter to early waters, mixing, and ultimately meteoric character
to the latest fluids.
NATURE AND EVOLUTION OF FRACTURE-
RELATED PERMEABILITY
The dominating element of flow porosity or permeabil-
ity in porphyry-pluton-centered systems is the fracture
network. Because fracture networks are shown to evolve
sequentially, however, the total number of cracks measur-
able in rocks is not a measurement of characteristics useful
in a determination of instantaneous permeability. Earlier-
formed cracks are sealed, and, as part of a succession of
formation of cracks, temporally intermediate stages of joint
formation are succeeded by still younger joints that are
successively sealed by alteration products. It is important
SPENCER R. TITLEY
to emphasize, again, that interpretations of data developed
from fluid filling temperatures reveals that in most in-
stances the early history of crack formation in wallrocks
(cracks that formed under increasing wallrock tempera-
ture) has not been recognized. Whereas it is likely that
joints formed under these conditions, it is possible, indeed
likely, that they remained closed or at least relatively
impermeable through the time of rising wallrock tempera-
tures and thermal expansion. The thermal and mechanical
effects of this early stage of pluton emplacement and
wallrock heating remain enigmatic and are an important
object of both laboratory and field search.
~ Estimation of permeability may be made by evaluation
of the expression
nip
· v~v
k = -, (3.1 )
where k is permeability (cm2), n is fracture abundance
(cm-l), and d is fracture aperture (cm) (Norton and Knapp,
1977~. Measurements of fracture abundance reported here
have been made in the units of the expression. It may be
seen, however, from the continuously varying value of
fracture density for each fracture set (Figure 3.7) that the
values of permeability at a constant value of aperture will
vary in a similar way. The additional parameter necessary
for calculation is that of joint aperture. This dimension, in
the altered systems studied here, is not directly determin-
able and has been estimated elsewhere in only a few re-
ported instances.
Inspection of the character of altered joints reveals that
their width, as manifested by their filling and wallrock
alteration selvages, is variable, most commonly between
millimeters and a few centimeters. Although it is tempting
to assign aperture values on the basis of such widths, the
textural data indicate that such assessments must be made
with caution and in most instances would be in error.
Interpretations based on textural evidence and vein habits
lead to the belief that the vein apertures were narrow (i.e.,
millimeters or less, rather than centimeters) in these sys-
tems.
Inspection of many altered joint sets from many sys-
tems reveals that crack walls are parallel over distances
commonly measured in meters. This habit is seen in three
dimensions, wherein large blocks appear suspended in the
network of parallel-walled, altered fractures. Whereas this
characteristic may not establish a rigorous basis for inter-
pretation of the evolution of narrow (millimeter-wide) veins,
the case of centimeter-wide veins requires that the vein
selvages "grow" (or apparently widen) at a rate suffi-
ciently slowly to restrict rotation or transport of the af-
fected rock masses. Inasmuch as fluid inclusion data
indicate cooling during the life of crack formation and
EVOLUTION AND STYLE OF FRACTURE PERMEABILITY
filling, it is reasonable to propose that the joints may
originate with small apertures (millimeters or less) and
maintain some degree of flow porosity and capacity to
transmit fluids as a result of continuous thermal contrac-
tion. Succinctly stated, wide (millimeters to centimeters)
alteration selvages are viewed to be a result of a process of
gradual crack widening during cooling and alteration, not
the result of the instantaneous development of numerous
wide (centimeter) joint openings. The flow porosity is not
indicated by the width of vein alteration products. Further,
the times at which a crack formed and at which it opened
to the passage of fluid may well be different.
Results of the microscopic study of textures of altera-
tion minerals adjoining and within veins reveal that re-
placement of walls is a dominating character of the joint-
altering process. Open-space filling textures, such as in-
ward and interpenetrative growth of quartz toward vein
centers, is uncommonly rare. Further, there is rarely any
indication of inward growth as might be seen in optical
properties of quartz as viewed through the polarizing
microscope. The habit of apparent replacement persists
across all of the alteration sets studied from these systems;
the process results, apparently, in nearly complete filling
of the original space available. The microscope com-
monly reveals that even in wide (centimeters) vein fill, the
site of residual porosity remains largely in the central part
of the filled-altered joint where, it is interpreted, the last
fluids passed. Beyond the evidence from textures, addi-
tional observations and inferences from the habit of joints
and joint alteration lend support to the interpretation of
small values of aperture.
Young vein sets, representing different alteration char-
acteristics from older sets, are imposed on older sets with-
out their visible offset along the older veins (see Figure
3.6~. Such nonoffsetting fracturing would be likely to
develop only in a rock mass in which the mechanical
competence is maintained. From such characteristics and
interpretation, Titley et al. (1986) proposed that the altera-
tion of time-specific joint sets results in annealing of the
rock and restoration of mechanical homogeneity.
SYNTHESIS
High pore pressures in the environment of emplace-
ment of shallow plutons are a result of localized sources of
heat acting on pore water. A consequence of this phe-
nomenon is rock failure when its tensile strength is ex-
ceeded by the pressures of contained water.
Emplacement of small felsic plutons into shallow por-
tions of the crust is a rapid process in active tectonic
regimes where plutons appear to have followed the paths
of earlier magmas of volcanic systems. The rapid em-
placement results in marked thermal contrasts between
61
cool, shallow crust and magmas; this contrast results in
episodic fracturing of porphyry and its wallrocks as cracks
form and become annealed by alteration, and magma
cooling retreats to progressively greater depths. Refer-
ence volumes of wallrock in;close proximity to the por-
phyries undergo heating and then cooling; in the process
rocks fail by a process resulting in widespread jointing
and, at shallow depths, small bodies of breccia (trans-
ported rock fragments). Joints evolve in abundance near
the centers of thermal-mechanical energy and diminish in
number outward.
Development of joints results in instantaneously im-
posed permeability characterized by a network of fractures
and fluid flow that ultimately results in cooling of the
thermal center and synchronous alteration of fracture walls.
Evidence of episodic breaking of rocks exists in the super-
position of temporally and mineralogically distinct altera-
tion assemblages in their own fracture sets in the same
rock volumes; textural evidence permits interpretation that
original joint walls continuously separate. Concomitantly,
open space is apparently filled, inhibiting transport of blocks
but maintaining minimal flow porosity from thermal con-
traction until the joint space is completely filled by altera-
tion products and deposition of hydrothermal minerals,
throttling fluid flow. As the joint space becomes restricted,
so does fluid flow, further resulting in episodic increases
in pressure above deeper but still cooling magma. This
process of hydrothermal flow and reaction brings about
constriction of gradually widening vein apertures phenom-
ena necessary to the process of intermittent but continuing
rock failure.
ACKNOWLEDGMENTS
This review has been improved by suggestions from D.
L. Norton, R. V. Kirkham, and an anonymous reviewer.
Some of the research reported here was based on work
supported by the National Science Foundation under grant
EAR 78-22897. Figures 3.2, 3.4, 3.5A, 3.6, 3.7, and 3.8
are reproduced here by permission, from Advances in
Geology of the Porphyry Copper Deposits: Southwestern
North America, edited by S. R. Titley, University of Ari-
zona Press, Tucson, 1982.
REFERENCES
Asami, N., and R. M. Britten (1980~. The porphyry copper
deposits at the Frieda River Prospect, Papua New Guinea,
Society of Mineralogists and Geologists of Japan; Geological
Special Issue 8, 1 17-139.
Banks, N. G., H. R. Cornwall, M. L. Silberman, S. C. Creasey,
and R. G. Marvin (1972~. Chronology of intrusion and ore
deposition at Ray, Arizona, Part I, K-Ar ages, Economic
Geology 67, 864-878.
62
Beane, R. E. (1982~. Hydrothermal alteration in silicate rocks, in
Advances in Geology of the Porphyry Copper Deposits, South-
western North America, S. R. Titley, ea., University of Ari-
zona Press, Tucson, pp. 117-137.
Beane, R. E., and S. R. Titley (1981~. Porphyry copper deposits,
Part II, Hydrothermal alteration and mineralization, Economic
Geology, 75th Anniversary Volume, 235-269.
Brimhall, G. H., Jr. (1979~. Lithologic determinations of mass
transfer mechanisms of multiple-stage porphyry copper miner-
alization at Butte, Montana: Vein formation by hypogene leach-
ing and enrichment of potassium-silicate protore, Economic
Geology 74, 556-589.
Burnham, C. W. (1967~. Hydrothermal fluids at the magmatic
stage, in Geochemistry of Hydrothermal Ore Deposits, H. L.
Barnes, ea., Holt, Rinehart and Winston, New York, pp. 34-
75.
Burnham, C. W. (1979~. Magmas and hydrothermal fluids, in
Geochemistry of Hydrothermal Ore Deposits, 2nd ea., H. L.
Barnes, ea., Wiley-Interscience, New York, pp. 71-136.
Carson, D. J. T., and J. L. Jambor (19741. Mineralogy, zonal
relationships and economic significance of hydrothermal al-
teration at porphyry copper deposits, Babine Lake area, British
Columbia, Canadian Institute of Mining 67, 1-24.
Creasey, S. C. (1966~. Hydrothermal alteration, in Advances in
Geology of the Porphyry Copper Deposits, Southwestern North
America, S. R. Titley, ea., University of Arizona Press, Tucson,
pp. 51-74.
Dilles, J. H. (1987~. Petrology of the Yerington batholith, Ne-
vada: Evidence for evolution of porphyry copper ore fluids,
Economic Geology 82, 1750-1789.
Einaudi, M. T. (1982~. Skarns associated with porphyry plutons:
Description of deposits, southwestern North America. II.
General features and origin, in Advances in Geology of the
Porphyry Copper Deposits, Southwestern North America, S.
R. Titley, ea., University of Arizona Press, Tucson, pp. 139-
183.
Einaudi, M. T., L. D. Meinert, and R. J. Newberry (1981~. Skarn
deposits, Economic Geology, 75th Anniversary Volume, 317-
391.
Gustafson, L. B., and J. P. Hunt (1975~. The porphyry copper
deposit at E1 Salvador, Chile, Economic Geology 70, 857-912.
Haynes, F. M., and S. R. Titley (19801. The evolution of frac-
ture-related permeability within the Ruby Star granodiorite,
Sierrita porphyry copper deposit, Pima County, Arizona,
Economic Geology 75, 673-683.
Heidrick, T. L., and S. R. Titley (1976~. Structural evolution of
southwestern North American Laramide porphyry copper
deposits and its relationship to the history of plate interactions
(abs.), International Geological Congress, 25th, Sydney, 1976,
740.
Heidrick, T. L., and S. R. Titley (1982~. Fracture and dike
patterns in Laramide plutons and their structural and tectonic
implications, in Advances in Geology of the Porphyry Copper
Deposits, Southwestern North America, S. R. Titley, ea.,
University of Arizona Press, Tucson, pp. 73-91.
Johnson, J. W., and D. L. Norton (1985~. Theoretical prediction
of hydrothermal conditions and chemical equilibria during
SPENCER R. TITLEY
skarn foImation in porphyry copper systems, Economic Geol-
ogy 80,1797-1823.
Kanbergs, K. (1980). Fracturing along the margins of a porphyry
copper system, Silver Bell district, Pima County, Arizona,
Unpublished M.S. thesis, University of Arizona, Tucson, 90
PP.
Kistner, D. J. (1984~. Fracture study of a volcanic lithocap, Red
Mountain po~phyry copper prospect, Unpublished M.S. thesis,
University of Arizona, Tucson, 75 pp.
Knapp, R. B., and J. E. Knight (19771. Differential thermal
expansion of pore fluids; fracture propagation and microearth-
quake production in hot pluton environments, Journal of
Geophysical Research 82, 2515-2522.
Knapp, R. B., and D. Norton (1981~. Preliminary numerical
analysis of processes related to magma crystallization and
stress evolution in cooling pluton environments, American
Journal of Science 281, 35-68.
Lowell, J. D. (1968~. Geology of the Kalamazoo orebody, San
Manuel district, Arizona, Economic Geology 63, 645-654.
Meyer, C., and J. J. Hemley (19671. Wall rock alteration, in
Geochemistry of Hydrothermal Ore Deposits, H. L. Barnes,
ea., Holt, Rinehart and Winston, New York, pp. 166-235.
Mutch, T. Q., and G. E. McGill (19621. Deformation in host
rocks adjacent to an epizonal pluton (the Royal Stock, Montana),
Geological Society of America Bulletin 73, 1541-1544.
Norris, J. R. (1982~. Fracturing, alteration and mineralization in
Oxide pit, Silver Bell mine, Pima County, Arizona, Unpub-
lished M.S. thesis, University of Arizona, Tucson, 72 pp.
Norton, D., and R. Knapp (1977~. Transport phenomena in
hydrothermal systems; the nature of porosity, American Jour-
nal of Science 277, 913-936.
Norton, D., and J. Knight (19771. Transport phenomena in
hydrothermal systems; Cooling plutons, American Journal of
Science 277, 937-981.
Ohmoto, H. (1986~. Stable isotope geochemistry of ore deposits,
in Stable Isotopes in High Temperature Geological Processes,
P. H. Ribbe, ea., Mineralogical Society of America, Reviews
in Mineralogy, vol. 16, Washington, D.C., pp. 491-559.
Preece, R. K., III, and R. E. Beane (1982~. Contrasting evolu-
tions of hydrothermal alteration in quartz monzonite and quartz
diorite wall rocks at the Sierrita porphyry copper deposit,
Arizona, Economic Geology 77, 1621 - 1641.
Rehrig, W. A., and G. L. Heidrick (1976~. Regional tectonic
stress during the Laramide and late Tertiary intrusive periods,
Basin and Range Province, Arizona, Tucson, Arizona Geo-
logical Society Digest X, 205-228.
Reynolds, T. J., and R. E. Beane (1985~. Evolution of hydro-
thermal fluid characteristics at the Santa Rita, New Mexico,
porphyry copper deposit, Economic Geology 80, 1328-1347.
Roedder, E. (1984~. Fluid Inclusions, Mineralogical Society of
America, Reviews in Mineralogy, vol. 12, Washington, D.C.,
644 pp.
Rose, A. W., and D. M. Burt (1979~. Hydrothermal alteration, in
Geochemistry of Hydrothermal Ore Deposits, 2nd ea., H. L.
Barnes, ea., John Wiley & Sons, New York, pp. 173-235.
Schwartz, G. M. (1947~. Hydrothermal alteration in the "por-
phyry copper" deposit, Economic Geology 42, 319-352.
EVOLUTION AND STYLE OF FRACTURE PERMEABILITY
Sheppard, S. M. F., R. L. Nielsen, and H. P. Taylor, Jr. (1971~.
Hydrogen and oxygen isotope ratios in minerals from por-
phyry copper deposits, Economic Geology 66, 515-542.
Stanton, R. L. (1978~. Mineralization in island arcs with particu-
lar reference to the south-west Pacific region, Australian Insti-
tute of Mining and Metallurgy Proceedings No. 268, 9-19.
Titley, S. R. (1978~. Geologic history, hypogene features, and
processes of secondary sulfide enrichment at the Plesyumi
copper prospect, New Britain, Papua New Guinea, Economic
Geology 73, 768-784.
Titley, S. R. (1982~. The style and progress of mineralization
and alteration in porphyry copper systems, American south-
west, in Advances in Geology of the Porphyry Copper Depos-
its, Southwestern North America, S. R. Titley, ea., University
of Arizona Press, Tucson, pp. 93-116.
63
Titley, S. R., and T. L. Heidrick (1978~. Intrusion and fracture
styles of some mineralized porphyry systems of the southwest-
ern Pacific and their relationship to plate interactions, Eco-
nomic Geology 73, 891-903.
Titley, S. R., A. W. Fleming, and T. I. Neale (1978~. Tectonic
evolution of the porphyry copper system at Yandera, Papua
New Guinea, Economic Geology 73, 810-828.
Titley, S. R., R. C. Thompson, F. M. Haynes, S. L. Manske, L. C.
Robison, and J. L. White (1986~. Evolution of fractures and
alteration in the Sierrita-Esperanza hydrothermal system, Pima
County, Arizona, Economic Geology 81,343-370.
Villas, R. N., and D. L. Norton (1977~. Irreversible mass transfer
between circulating hydrothermal fluids and the Mayflower
stock, Economic Geology 72, 1471 - 1504.
Wadsworth, W. B. (1968~. The Cornelia pluton, Ajo, Arizona,
Economic Geology 63, 101-1 15.
