THE DEVELOPMENT OF FRACTURES IN THE HARRIS RANCH QUARTZ
MONZONITE RELATED TO THE SIERRITA PORPHYRY
COPPER SYSTEM, PIMA COUNTY, ARIZONA
by
John Lester White
1v he C/iad vs lCecid inff WoOnr
QEPART[v1 LN-r
uF
GEOSCI EN CES
UNIVERSITY OF ARIZONA
A Thesis Submitted to the Faculty of the
DEPARTMENT OF GEOSCIENCES
In Partial Fulfillment of the Requirements
For the Degree of
MASTER OF SCIENCE
In the Graduate College
THE UNIVERSITY OF ARIZONA
1980
STATEMENT BY AUTHOR
This thesis has been submitted in partial fulfillment of requirements for an advanced degree at The University of Arizona and is
deposited in the University Library to be made available to borrowers
under rules of the Library.
Brief quotations from this thesis are allowable without special
permission, provided that accurate acknowledgment of source is made.
Request for permission for extended quotation from or reproduction of
this manuscript in whole or in part may be granted by the head of the
major department or the Dean of the Graduate College when in his judgment the proposed use of the material is in the interests of scholarIn all other instances, however, permission must be obtained
ship.
from the author.
SIGNED:
-
APPROVAL BY THESIS DIRECTOR
This thesis has been approved on the date shown below:
1
`
S. R. Ti ey
Professor of Geosciences
-;25-
/?ed
Date
ACKNOWLEDGMENTS
I am deeply thankful to Professor S. R. Titley for the help and
advice he has given me in every phase of research and preparation of
this thesis.
Professor R. E. Beane and Professor G. Davis have also
provided useful comments and advice on all aspects of my thesis.
ing for this project has been provided by NSF Grant EAR 78- 22897.
iii
Fund-
TABLE OF CONTENTS
Page
v
LIST OF ILLUSTRATIONS
vii
ABSTRACT
INTRODUCTION
1
GENERAL GEOLOGY
5
Rock Types
5
Structural Geology
6
ALTERED FRACTURES IN THE HARRIS RANCH QUARTZ MONZONITE
8
Methods of Study
Results
Fracture -Controlled Alteration Assemblages
Fracture Densities
Fracture Orientations
Other Field Data
Fluid Inclusion Temperatures
8
11
il
13
19
20
24
DISCUSSION
28
Influence of Faults on Fracture Distribution
The Formation of Fractures
Distinction of an Early Hydrothermal Event .
Evolution of Altered Fractures
Comparison with Other Studies at Sierrita
.
.
...
.
.
.
.
28
28
29
30
32
SUMMARY AND CONCLUSIONS
35
REFERENCES
37
iv
LIST OF ILLUSTRATIONS
Figure
Page
1.
Location Map
2
2.
General location map of rock types, study area, and
open pit mines
4
Location map of fracture data stations with geology
modified from Copper (1973), eastern Sierrita
Mountains, Pima County, Arizona
in pocket
3.
Study area divided into north, center, and
south domains
10
Paragenetic sequence of fracture -controlled alteration
assemblages in the Harris Ranch Quartz Monzonite outside
of the Sierrita open pit mine
12
6.
Fracture density versus distance, north domain
14
7.
Fracture density versus distance, center domain
15
8.
Fracture density versus distance, south domain
16
9.
Fracture density versus distance, with the latter three
assemblages added together
18
4.
5.
10.
Map of contoured, averaged fracture densities of the
second fracture- controlled alteration assemblage in
the Harris Ranch Quartz Monzonite., Pima County,
Arizona
in pocket
Fracture orientations of each alteration assemblage
from the entire study area
21
Fracture orientations of assemblage B (quartz, K- feldspar,
and pyrite) by domains
22
13.
Histogram of cumulative vein thicknesses
23
14.
Histogram of fluid inclusion temperatures of all four
alteration assemblages at 3.5 kilometers from the
porphyry stock
25
11.
12.
v
vi
LIST OF ILLUSTRATIONS-- Continued
Page
Figure
15.
16.
17.
Histogram of fluid inclusion temperatures of the first
alteration assemblage (epidote, quartz, and K- feldspar)
at both 3.5 and 5.8 kilometers from the porphyry stock .
.
.
.
26
Summary of the evolution of fracture -controlled
alteration in the Harris Ranch. Quartz Monzonite
31
Correlation of alteration assemblages at different
locations in the vicinity of the Sierrita porphyry
copper deposit
33
ABSTRACT
Detailed field and laboratory observations of altered fractures
in the Harris Ranch Quartz Monzonite lead to a description of the
sequential evolution of those altered features.
Field observations in-
clude the measurement of fracture densities, orientations, and alteration assemblages at 300 stations in the area studied.
Vein -filling
temperatures are determined from laboratory study of fluid inclusions.
A sequence of four fracture -controlled alteration assemblages
result from two periods of hydrothermal activity in the Harris Ranch
Quartz Monzonite.
The earliest alteration assemblage represents an
early hydrothermal event not related to mineralization.
The existence
of this early event is recognized by the spatial distribution of altered
fractures, fluid inclusion temperatures, fracture styles, and the absence of the early assemblage in the Ruby Star Granodiorite.
The later
three assemblages are products of hydrothermal activity responsible for
mineralization at Sierrita.
As the Sierrita system matured in the Har-
ris Ranch,. the extent and abundance of fractures open to hydrothermal
fluid flow decreased, as did the temperature of fluids within those
fractures.
Regional tectonic activity, pre -existing faults, and the
concentrator intrusive all influenced the distribution and orientation
of fractures during the evolution of the Sierrita porphyry copper
system.
vii
INTRODUCTION
The fracturing of rock is vital to the development of geothermal
systems (Norton and Knapp 1977; Knapp and Knight 1977; Haynes and Titley
1980).
The flow of hydrothermal fluids through fractures permits the
convective transfer of thermal energy and chemical components (Norton
and Knight 1977).
These processes are manifested in active and fossil
geothermal systems by altered and mineralized fractures and veins
(Batzle and Simmons 1976; Titley, Fleming, and Neale 1978).
Progressive
changes in fracture -controlled alteration assemblages provide a record
of the development of hydrothermal systems through time (Batzle and
Simmons 1977; Titley 1978).
In addition, fossil geothermal systems such
as porphyry copper deposits are characterized by the localization of
base metal mineralization along fractures (Anderson 1948).
Thus, the
detailed study of fracture -controlled alteration and mineralization is
necessary to build a dynamic picture of evolution of both active and
fossil geothermal systems.
Recent studies at the Sierrita porphyry copper deposit provide
a basic methodology for describing fracture- controlled hydrothermal
alteration in terms of time, space, and temperature (Haynes and Titley
1980; Preece and Beane fin prep..)).
The Sierrita deposit is located
approximately 40 miles south of Tucson, Arizona (Figure 1).
The distri-
bution of fracture- controlled alteration assemblages through time is
1
2
ARIZONA
!00 KM
TUCSON
PIMA MINING
DISTRICT
XMISSION
X PIMA
sE°
1
TWIN
Xsu rTE
SIERRITA
ESPERANZA
5 Kit
Figure 1.
Location Map.
X =OPEN PIT NINE
3
examined in the Ruby Star Granodiorite at Sierrita by Haynes and Titley
(1980).
The area studied by Haynes and Titley is lithologically homo-
geneous, and is not disturbed by major structures.
This paper describes a continuation of these studies in a different host rock at Sierrita.
The area chosen for this study is located
west of the Sierrita open pit mine (Figure 2).
the Harris Ranch Quartz Monzonite.
The area is dominated by
Using the framework provided by pre-
vious workers, the sequential evolution of altered fractures in the
Harris Ranch Quartz Monzonite is described.
to that of the Ruby Star Granodiorite.
This evolution is compared
Further, data from this study
provide some general explanations for the origin and distribution of
altered fractures in the Harris Ranch Quartz Monzonite.
4
Mid-Tertiary Volcanic
Rocks
Biotite Quartz Monzonite
Porphyry
Outline of Study Area
Early Tertiary
Intrusive Rocks
Harris Ranch Quartz
a
Monzonite
Volcanic
Rocks
Reg Mesozoic
.
...`
,
-
..
,,.,
,:.-::.`....-,.
., .,;,
.
..`..`:...
. .
.
.,..;.,`.,;.,;:,`.,;.
.
...;..
.l+
,
.
i ',. `'.
.
,.,
,
..;.,..,..
.
,. -Ti
, ., ...,..,`
`
,
.
`
Sierrita `"'
.,`.,
-. Pi t
"
.
.1
:Esperanza
Pit
- ,-,,-.--'.-..
_1_'Í;,_,-
-,-,-,
-._,_,_,_:;
.-
\
Figure 2.
General location map of .rock types, study area, and open
pit mines -- from Cooper 1973; Aiken and West 1978.
'
GENERAL GEOLOGY
The Sierrita -Esperanza porphyry system is one of three extensive
porphyry systems in the Pima mining district.
The Sierrita deposit is
contained in a Mesozoic- Tertiary complex of extrusive and intrusive
rocks at the southern end of the Ruby Star Granodiorite batholith (refer
to Figure 2).
Various aspects of the general geology of the Sierrita
mine vicinity are described in several sources (Lacy 1959; Cooper 1960;
Lootens 1966; Cooper 1971, 1973).
Several additional papers describe
various aspects of mineralization and alteration at the Sierrita porphyry copper deposit (Lynch 1966, 1967; Smith 1975; Aiken and West 1978;
Preece and Beane fin prep.]; Haynes and Titley 1980).
A brief descrip-
tion of the general geology in the vicinity of the study area is given
below.
Rock Types
The oldest rocks in the vicinity of the study area belong to the
Triassic Ox Frame Formation (Figure 3, in pocket).
The Ox Frame con-
sists of andesític to rhyolitic flows, rhyolitic tuff, and quartzite
(Cooper 1971).
These rocks are in intrusive contact with the 190 mil-
lion year old Harris Ranch Quartz Monzonite (Cooper 1973).
Ranch is medium -grained and roughly equigranular.
The Harris
It contains approxi-
mately 60% alkali feldspar, 20% quartz, 10% plagioclase, and 10% fine grained biotite.
Younger rocks of Mesozoic age include the Jurassic
Sierrita Granite and Cretaceous intermediate to felsic volcanic rocks.
5
6
Three plutons, 67 to 53 million years old, are present in the
vicinity of the study area (Cooper 1973; Damon and Mauger 1966).
In
sequence from oldest to youngest they are fine -grained biotite quartz
diorite, Ruby Star Granodiorite, and biotite quartz monzonite porphyry.
The biotite quartz monzonite porphyry is spatially associated with copper mineralization at Sierrita.
In this paper it is assumed that the
biotite quartz monzonite porphyry is the concentrator intrusive at Sierrita.
The center of outcrop of the porphyry stock is assumed to repre-
sent the center of the hydrothermal system responsible for Sierrita
mineralization (Haynes and Titley 1980).
Structural Geology
Several high -angle faults and three breccia pipes are present
in the study area.
trend.
The breccia pipes are aligned along a northwest
They are subcircular to elliptical, and average between 50 and
75 meters in diameter.
The center breccia pipe contains clasts of Har-
ris Ranch Quartz Monzonite, Sierrita Granite, and Ox Frame Formation.
The outer breccia pipes contain only clasts of Harris Ranch Quartz Mon -
The clasts range from a few centimeters to a meter in diameter,
zonite.
and vary from angular to subrounded.
rock flour.
The matrix in all three is a fine
Quartz -sericite alteration is pervasive in all three
breccia pipes.
Two types of fault are present in the area.
The first is a
single northwest- trending fault that dips an average of 50 degrees to
the west.
This fault cuts an early Tertiary, premineralization quartz
latite dike.
The fault is in turn cut by altered and mineralized
7
fractures associated with the Sierrita hydrothermal system.
The fault
is characterized by a 20- to 60- meter -wide zone of deformational features.
The medial portion of the zone is marked by one to two meters
of slatey cleavage with myloniti.c textures.
Surrounding this slatey
cleavage is 5 to 10 meters of abundant irregular fractures and granulated rock.
Deformational features are negligible in the footwall
beyond about 10 meters from the medial portion of the fault zone.
In
the hanging wall, however, near horizontal layers of intensely sheared
and granulated rock -(5 to 50 centimeters thick) are spaced every 1/2 to
1 meter and extend 20 to 50 meters from the medial portion of the fault
zone.
Localized along the east side of the fault zone are patches of
pervasive quartz- sericite -clay alteration which extend as much as 20
meters from the fault zone.
Absolute offset of this fault is
indeterminate.
The northwest- trending fault is cut by several east- northwesttrending faults.
Features of deformation related to these faults are
commonly restricted to a 1- to 3- meter -wide gouge zone.
Offset of the
northwest fault by the east -northeast faults usually involves left
separation on the order of 10's of meters.
ALTERED FRACTURES IN THE HARRIS RANCH QUARTZ MONZONITE
Although this study involves some geologic mapping and minor
petrographic work, the largest portion of this study is based on detailed field and laboratory observations of altered fractures in the
Harris Ranch Quartz Monzonite.
Field observations are used to determine
the distribution of fracture -controlled alteration through time and
space, and to provide information on the factors which resulted in the
formation of fractures.
Laboratory measurements of fluid inclusion
temperatures are made to show how fluid temperatures varied through time
and space.
In this paper, the term "fracture" is a collective term that
refers to both small -scale faults and joints.
Methods of Study
Field work includes the measurement of fracture densities, fracture orientations, and fracture -controlled alteration assemblages at 300
stations in the study area (refer to Figure 3).
At most stations the
width of veins and the relative offset along fractures have also been
measured.
Station locations are determined largely by the availability
of good outcrop.
outlined.
At each station a 1/4 square meter circular area is
Data are collected on all altered fractures which pass com-
pletely through the circle.
Observations of fracture -controlled alter-
ation minerals and cross- cutting relationships between altered fractures
are used to determine a paragenetic sequence of four alteration assemblages.
Fracture densities are calculated by dividing the sum of the
8
9
lengths of the fractures within the circle by the area of the circle
(Snow 1970; Titley et al. 1978).
given in units of cm-1.
In this study, fracture density is
A value of 0.02 cm -1 is about equivalent to
one fracture spaced every one -half meter.
A value of fracture density
is calculated at each station for each individual alteration assemblage.
The sum of fracture densities of each fracture- controlled assemblage
at a given station represents the integrated fracture density for that
station (Haynes and Titley 1980).
Because of the inhomogeneity of rocks in the field area, the
-
field data are analyzed by domains.
The area of study is divided into
three domains: north, center, and south (Figure 4).
is structurally simple.
The north domain
The center and south domains contain the
northwest -trending fault and several east -northeast- trending faults.
Vein -filling temperatures are determined using a W3B gas flow
heating /freezing stage (Werre et al. 1979).
A flow rate of 40 SCFH is
used, at which measurements have an accuracy of ±5 °C, and a precision
of ±2 °C.
Samples for fluid inclusion study come from sites located at
3.5 and 5.8 kilometers from the center of the biotite quartz monzonite
porphyry stock (refer to Figure 3).
Samples from the 3.5 kilometer site
contain all four alteration assemblages observed in the area.
Rocks at
the 5.8 kilometer site contain only the earliest assemblages.
Homogeni-
zation temperatures are determined for both primary and secondary inclusions.
The temperatures
as presented are not pressure corrected, and
therefore are not actual temperatures of trapping.
10
NORTH
Sierrita
Pit
CENTER
f
%
rr
a. .i. 4. y
r
%
/1%r
Esperanza
Pit
KM
Figure 4.
Study area divided into north, center, and south domains -The distribution of major faults is shown. The open pit
mines and the biotite quartz monzonite porphyry stock are
located on the map for reference.
11
Results
Fracture -Controlled Alteration Assemblages
Four fracture -controlled alteration assemblages are present in
the Harris Ranch Quartz Monzonite (Figure 5).
they are:
In paragenetic sequence
(1) epidote+ quartz ±chloritetpyritettourmaline fracture- filling
with a selective epidote +K- feldspartquartztchlorite halo,
(2) quartz+K-
feldspar+pyrite±chlorite±chalcopyrite fracture -filling with a K- feldspar
halo, (3) pyrite tquartztchlorite±ch.alcopyrite fracture -filling, and
(4) epidote ±K- feldspartchloritetquartz fracture-filling with a selective
epidote ±K- feldspar±chlorite halo.
In each assemblage listed above, the
minerals are in order of decreasing abundance (for example, epidote is
the most abundant fracture -filling mineral of the first assemblage).
The styles of fracturing and fracture -filling vary among different mineral assemblages.
The early assemblage (A) occurs along both
planar and sinuous fractures.
The contacts between early veins and the
host rock are irregular and gradational.
The strike lengths of these
fractures are usually between .1 and 3 meters, and widths of fracture -
fillings may vary in a given vein between 2 and 0.1 millimeters.
The
intermediate assemblages (B and C) invariably occur in planar fractures
which have strike lengths of up to 10 or 15 meters.
Vein thicknesses of
these assemblages average about 1 millimeter, but may reach 1 centimeter
in some veins.
Contacts between the vein material and wall rock are
generally sharp and planar.
The late assemblage is usually localized
12
PARAGENESIS OF FRACTURE - CONTROLLED
ALTERATION ASSEMBLAGES
VEIN-FILLING MINERALS
A
HALO MINERALS
Epidote, Quartz,
Chlorite, Pyrite,
Epidote, K-feldspar,
Tourmaline
Quartz, Chlorite
>J
OC
W
Quartz, K-feldspar,
B
Pyrite,
K-feldspar
Chlorite, Chalcopyrite
Pyrite, Quartz,
C
Chlorite, Chalcopyrite
Epidote,
DK-feldspar,
Chlorite, Quartz
Figure 5.
Epidote,
K-feldspar,
Chlorite
Paragenetic sequence of fracture -controlled alteration
assemblages in the Harris Ranch Quartz Monzonite outside
of the Sierrita open pit mine -- Underlined minerals are
always present and serve to characterize the mineral
assemblages.
r
W
Q
J
13
along planar fractures, with veins commonly less than 1 millimeter thick
and less than 3 meters in length.
Fracture Densities
To examine the spatial distribution of altered fractures in the
Harris Ranch Quartz Monzonite, fracture densities are plotted as a function of distance from the center of the biotite quartz monzonite porphyry stock (Figures 6, 7, and 8).
As previously stated, the center of the
porphyry stock is assumed to be the center of the Sierrita hydrothermal
system.
The curves in Figures 6, 7, and 8 are based on averages of
fracture densities for each assemblage. calculated at 1/2 kilometer intervals.
tions.
Each point on a curve represents an average of several staThe number next to each point is the number of stations on which
that point is based.
On the vertical axis is fracture density in units
of inverse centimeters.
On the horizontal axis is distance in kilome-
ters from the center of the porphyry stock.
The location of the
northwest- trending fault is shown in plots of the center and south domains (Figures 7 and 8).
Figures 6, 7, and 8 serve to summarize the
fracture density data.
It should be emphasized that the curves in these diagrams represent averages of fracture densities from several outcrops.
An indivi-
dual outcrop may have a fracture density much different than that
suggested by Figures 6, 7, and 8.
In the unfaulted north domain (Figure 6), the early alteration
assemblage (A) does not follow the same pattern of distribution as the
latter three assemblages (B, C, and D).
The early assemblage achieves
14
w
4
DISTANCE, in KILOMETERS
Figure 6.
Fracture density versus distance, north domain -- Distance
is measured from the center of the biotite quartz monzonite
The number next to each point is the number
porphyry stock.
of stations on which that point is based. The letters represent fracture -controlled alteration assemblages (A = epidote,
quartz, and K- feldspar; B = quartz, K- feldspar, and pyrite;
C = pyrite and quartz; D = epidote).
15
W
E
DISTANCE, in KILOMETERS
Figure 7.
Fracture density versus distance, center domain -- Distance
is measured from the center of the biotite quartz monzonite
The number next to each point is the number
porphyry stock.
of stations on which that point is based. The letters represent fracture -controlled alteration assemblages (A = epidote,
quartz, and K- feldspar; B = quartz, K- feldspar, and pyrite;
C = pyrite and quartz; D = epidote).
16
w
4
DISTANCE, in KILOMETERS
Figure 8.
Fracture density versus distance, south domain -- Distance
is measured from the center of the biotite quartz monzonite
porphyry stock.
The number next to each point is the number
of stations on which that point is based. The letters represent fracture-controlled alteration assemblages (A = epidote,
quartz, and K- feldspar; B = quartz, K- feldspar, and pyrite;
C = pyrite and quartz; D = epidote).
17
only moderately high fracture densities but extends beyong 6 -1/2 kilo-
meters west of the porphyry stock.
The latter three assemblages have
maximum fracture densities near the center of the deposit and fall off
rapidly away from the center.
The second assemblage achieves the high-
est fracture densities of any assemblage.
Fractures of later assem-
blages progressively diminish in extent and abundance.
In the center and south domains, fractures which localize the
latter three mineral assemblages do not follow the systematic pattern
which occurs in the north domain.
In Figure 7, fracture densities of
the second assemblage (B) drop off sharply on the east side of the northwest fault, while the latter two assemblages (C and D) peak on the west
side of the fault.
In Figure 8, fractures containing the second assem-
blage also drop sharply in abundance on the east side of the fault.
both Figures 7 and 8,, the intermediate assemblages have additional minor
peaks between 4 and 5 kilometers from the porphyry stock.
The difference between the north and center domains is further
illustrated in Figure 9.
In this figure the latter three assemblages
are added together to form a single curve.
In the north domain this
curve of the latter three assemblages varies smoothly, but in the center
domain this curve is irregular.
Fracture densities immediately east of
the NW fault in the center domain are lower than predicted from the
north domain.
Fracture densities immediately west of the fault are
higher than expected.
Note that this corresponds with the greater ex-
tent of deformational features in the hanging wall of the northwest trending fault.
18
.15
10
.05
6
4
DISTANCE,
Figure 9.
2
in KILOMETERS
Fracture density versus distance, with the latter three
assemblages added together -- The upper graph is for the
north domain, the lower graph is for the center domain.
Distance is measured from the center of the porphyry
The letters represent fracture -controlled alterastock.
tion assemblages (A = epidote, quartz, and K- feldspar;
B = quartz, K- feldspar, and pyrite; C = pyrite and quartz;
D = epidote).
`Room
4e, Ntevs
!}`il. iENcES
[)EÉ f1i f i.
UNIVERSITY OF r,ìtZÚtVA
19
of
Figure 10 (in pocket) is an example of an alternative method
presenting fracture density data.
It shows contoured averages of frac-
ture densities of the second alteration assemblage.
Such a map is not
density plots and
as easily interpreted as the distance versus fracture
is time consuming to construct.
However, it has some uses
and thus is
briefly discussed below.
grid on a
The map is made by overlaying a 2 centimeter square
of 1:10,000.
map of raw fracture density values which has a scale
All
avervalues within a 2 centimeter radius of each point on the grid are
aged.
A map of averaged fracture densities is thus produced which can
be contoured to yield a map such as Figure 10.
In general the map does not serve it original purpose of showing
and
how fracture densities vary with respect to individual structures
geologic units.
The features it does show are:
(1) a sharp drop in
of the
fracture densities between 2 -1/2 to 4 kilometers from the center
prophyry stock and (2) two anomalous areas containing quartz -sulfide
filled fractures about 5 kilometers from the center of the stock.
A map of this type may be useful as an exploration tool.
It may
fractures.
be used to quantitatively identify anomalous areas of altered
useful in loIt can also show fracture density gradients which may be
cating new mineralization or extending known areas of mineralization.
Fracture Orientations
Fracture orientations are represented by contouring plots of
projecpoles to fracture planes on a southern hemisphere stereographic
tion.
A separate plot is shown for each fracture -controlled alteration
20
assemblage in the entire area (Figure 11).
Orientations of the second
assemblage are also plotted by domains (Figure 12).
Over the entire study area, the early assemblage (A) has several
dominant orientations with considerable scatter away from these major
orientations.
80E.
The most significant orientations are E -W, 70S and N14W,
The second assemblage (B) has just one dominant trend of N82E with
dips to both north and south.
Later assemblages (C and D) show an in-
crease in the number of dominant orientations.
consistent throughout the area of study.
Strikes of fractures are
No plan -view radial pattern is
present.
The plots by domain show a change in dip of altered fractures
from north to south.
This is well illustrated by the second assemblage
(refer to Figure 12).
In the north fractures commonly dip to the south.
In the center they are usually near vertical.
In the south most frac-
tures dip to the north.
Other Field Data
Figure 13 shows a histogram of cumulative vein thicknesses for
every 10 degrees of fracture trend.
A clear maximum of cumulative vein
thickness occurs in fractures which trend near N80E.
Assuming the sta-
tions are representative of fractures throughout the field area, the
cumulative vein thickness of N80E trending fractures in the entire area
is estimated to be 5 meters.
If vein thicknesses are the result of both
replacement and open space filling, 5 meters represent a maximum possible extension in the study area due to fractures alone.
This exten-
sion would be normal to a near vertical plane striking N80E.
21
Figure 11.
Fracture orientations of each alteration assemblage from the
entire study area -- The letters denote fracture -controlled
mineral assemblages (A = epidote, quartz, and K- feldspar;
B = quartz, K- feldspar, and pyrite; C = pyrite and quartz;
D = epidote). The diagrams are contoured plots of poles to
fracture planes on a southern hemisphere stereographic proThe number of data points (DP) on which each diajection.
gram is based, and the contour interval (CI) of each diagram
are as follow: A - DP =830, CI =0.8 %; B - DP =498, CI =2 %;
C - DP =172, CI =1.5 %; D - DP =61, CI =2 %.
22
NORTH
CENTER
SOUTH
Figure 12.
Fracture orientations of assemblage B Cquartz, k- feldspar,
and pyrite) by domains -- The diagrams are contoured plots
of poles to fracture planes on a southern hemisphere stereo graphic projection. The number of data points (DP) on which
each diagram is based, and the contour interval (CI) of each
diagram are as follow: North - DP =179, CI =3 %;
Center - DP =116, CI =2.5 %; South - DP =96, CI =4 %.
23
._..
'-..11
20
W
West
-7
N6OW
:0w
Nof th
N3 E
N60E
East
VEIN TRENDS
Figure 13.
Histogram of cumulative vein thicknesses -- Veins less than
0.1 mm thick are not included.
24
Nineteen relative offsets have been measured along near vertical
fractures which trend within 10 degrees of N80E.
to give 66.8 centimeters of left separation.
These offsets add up
Again assuming these meas-
urements are representative of all fractures in the study area, a cumulative left separation of 41. meters is estimated across the entire study
area.
It is interesting to note that the offsets and dominant orienta-
tions of the altered fractures is similar to that of the east -northeast
trending faults.
Fluid Inclusion Temperatures
Figure 14 shows a histogram of fluid inclusion homogenization
temperatures for each vein type at a distance of 3.5 kilometers from the
center of the porphyry stock.
Figure 15 shows histograms of homogeniza-
tion temperatures of fluid inclusions in the early assemblage (A) at
both 3.5 and 5.8 kilometers.
At 3.5 kilometers from the prophyry stock,
the second assemblage (B) has the highest temperatures of any recorded
at that distance.
310 °C.
Temperatures in later veins (C and D) do not exceed
Thus the range of temperatures between 310 °C and 360 °C in the
second assemblage is
representative of fluid temperatures during depo-
sition of that assemblage at that location.
By similar reasoning, the
third assemblage (C) has a group of maximum filling temperatures between
310 °C and 230 °C, and the fourth assemblage (D) has a range of filling
temperatures below 220 °C.
Temperatures of primary inclusions in general
fall within the respective maximum temperature ranges.
Temperatures from primary inclusions in the early assemblage
(A) at 3.5 kilometers range between 200 °C and 290 °C.
At 5.8 kilometers,
Figure 14.
Histogram of fluid inclusion temperatures of all four
alteration assemblages at 3.5 kilometers from the porphyry
stock -- The letters denote alteration assemblages as
follow:
A = epidote, quartz, K- feldspar
B = quartz, K- feldspar, pyrite
C = pyrite and quartz
D = epidote
25
15-
4 4
4S 30
A
10-
4 }
t64 t7
ta
B
.11
.1.10111
wommor
4
5-
MISINIPV
r-
Cr)
3t
C
.1.1.11
15-
4
34 tt
10-
5f-1
200
TEMPERATURE,
Figure 14. Continued
3b0
in
°G
Figure 15.
4a
st
200
-1
TEMPERATURE, in
-1
°C
1
300
ONNINENIOI
400
5.8 KM
3.5 KM
Histogram of fluid inclusion temperatures of the first alteration assemblage (epidote,
quartz, and K- feldspar) at both 3.5 and 5.8 kilometers from the porphyry stock.
10-
27
the same assemblage has a well- defined peak of temperatures ranging
between 350 °C and 400 °C.
Thus, temperatures in this early assemblage
are clearly higher at 5.8 kilometers that at 3.5 kilometers from the
center of the porphyry stock.
Note also that the low temperature peak
(140 -180 °C) present in all veins at 3.5 kilometers is not present at
5.8 kilometers.
DISCUSSION
Examination of the data presented in preceding sections permits
conclusions to be made concerning several important aspects of hydroThe following
thermal activity in the Harris Ranch Quartz Monzonite.
discussions examine the factors that influence the distribution of fractures and fracture -related alteration and then describe the evolution
through time of fracture -controlled alteration in the Harris Ranch
Quartz Monzonite outside of the Sierrita open pit mine.
This evolution
is then compared to the evolution of fracture- controlled alteration in
other parts of the Sierrita porphyry system.
Influence of Faults on Fracture Distribution
The differences in fracture distributions between the unfaulted
north domain and other domains suggest that the faults in the area,
particularly the northwest fault, influence the distribution of fractures.
Moderately high fracture densities of the second assemblage are
restricted in lateral extent by the northwest fault.
The distribution
of later assemblages also appears to be influenced by the deformational
features associated with that fault.
this is only circumstantial.
Unfortunately, the evidence for
Further study of similar structures is
necessary to understand the true nature of their influence.
The Formation of Fractures
The orientations and distribution of altered fractures in the
Harris Ranch Quartz Monzonite allows some suggestions to be made about
28
29
the phenomena responsible for such fractures.
The localization of high
fracture densities and alteration about the concentrator intrusion suggests the emplacement and crystallization of the stock has a strong
influence on the formation of fractures.
This is supported by the pat-
tern of dips of fractures in the Harris Ranch from north to south.
Such
a pattern is expected above a crystallizing pluton (Koide and
Bhattacharji 1975).
However, the dominant trend of fractures is east -
northeast, similar to many other porphyry copper deposits in southern
Arizona (Rehrig and Heidrick 1972).
This suggests that regional stresses
also play a role in the orientation of fractures in the Harris Ranch
Quartz Monzonite.
Thus, the fracture geometry appears to be a result
of the combined effects of regional tectonics and a pluton chronológically related to fracture development.
Distinction of an Early Hydrothermal Event
The spatial distribution of fractures of the intermediate and
late assemblages indicates they are a result of hydrothermal activity
associated with mineralization at Sierrita.
However, several lines of
evidence indicate the early fracture -controlled alteration assemblage
represents a separate hydrothermal event not related to Sierrita mineralization.
These lines of evidence are:
(1) a pattern of distribution
of fractures inconsistant with later assemblages, (2) a continuation of
moderately high fracture densities well beyond 6 kilometers from the
center of the concentrator intrusion, (3) higher fluid inclusion tem-
peratures in the early assemblage at 5.8 kilometers than at 3.5 kilometers from the intrusive center, and (4) a sharp contrast in the style
30
and orientation of fractures between the first and second assemblages.
It is possible that this early assemblage is a part of the Sierrita
system, but the evidence above is more compatible with a separate
hydrothermal event.
Evolution of Altered Fractures
Figure 16 summarizes the evolution of altered fractures in the
north domain of the Harris Ranch Quartz Monzonite.
This evolution be-
gins with an early hydrothermal event represented by an assemblage of
epidote, quartz, and K- feldspar.
This early assemblage has moderately
high fracture densities distributed throughout most of the area studied.
The two most prominent trends of these fractures are E -W and N14W.
Max-
imum filling temperatures of this assemblage vary from a high range of
350 °C to 400 °C at 5.8 kilometers, to a lower range between 200 °C and
290 °C at 3.5 kilometers.
Later assemblages manifest a younger hydrothermal event that is
probably responsible for mineralization at Sierrita.
These three assem-
blages allow documentation of this hydrothermal system through time.
Mineral assemblages changed with time from early quartz-K- feldsparsulfide, to intermediate sulfide- quartz, to late epidote.
Fracture
densities reached a maximum early in the system's history and progressively diminished.
Fluid temperatures also declined with time from a
maximum of 310 °C to 360 °C in the quart -K- feldspar- sulfide assemblage,
through a range of 310 °C to 230 °C in the sulfide -quartz assemblage, to
temperatures below 220 °C in the late epidote assemblage.
The dominant
trend of fractures is consistently east -northeast, but the number of
31
QUARTZ, K- FELDSPAR, SULFIDE
N!!E
.12
EPIDOTE, QUARTZ, K-FELDSPAR
Ew, NI4W
SULFIDE, QUARTZ
350-400°C
NatE
310 -360°C
200 -290 °C
230-310°C
<220°C
EPI , OTE
Ni3E, Nt w, NSW
6
4
2
DISTANCE, in KILOMETERS
Figure 16.
Summary of the evolution of fracture- controlled
alteration in the Harris Ranch. Quartz Monzonite.
32
significant orientations increase with time from l dominant trend in
the quartz -K- feldspar- sulfide assemblage to 3 or 4 in the epidote
assemblage.
Comparison with Other Studies at Sierrita
Figure 17 shows a comparison of paragenetic sequences of
fracture -controlled alteration assemblages present in the Harris Ranch
The later three assem-
Quartz Monzonite and the Ruby Star Granodiorite.
blages observed in the Harris Ranch Quartz Monzonite west of the open
pit mine all have paragenetic equivalents in the Harris Ranch inside the
mine and in the Ruby Star Granodiorite outside the mine.
Mineralogies,
fracture density distributions, orientations, and temperatures of the
three late assemblages follow the same patterns of behavior in both rock
types.
The difference in altered fractures within the two rock types is
in the early assemblage.
The early epidote -quartz -K- feldspar assemblage
is present only in the Harris Ranch Quartz Monzonite outside of the open
pit mine.
This early assemblage is absent in rocks inside the pit and
outside the pit in the Ruby Star Granodiorite.
One explanation for this
discrepancÿ is that the early assemblage represents an early stage of
the Sierrita system that varies in alteration mineralogy between the two
rock types.
However, later assemblages associated with the Sierrita
system do not vary significantly between the two rock types.
Thus, a
better explanation for the origin of the early assemblage is that it is
a result of the emplacement and cooling of an intrusive rock that is
older than the biotite quartz monzonite porphyry.
This is in agreement
Figure 17.
Correlation of alteration assemblages at different locations in the
vicinity of the Sierrita porphyry copper deposit -- The column headings
denote locations and rock type as follow:
HRWP = Harris Ranch Quartz Monzonite west of the open pit (this study).
RSNW = Ruby Star Granodiorite northwest of the open pit (Haynes and
Titley 1980).
RSNE = Ruby Star Granodiorite northeast of the open pit (Manske 1980).
HRIP = Harris Ranch Quartz Monzonite in the Sierrita open pit mine
(Preece and Beane jin prep.]).
The following mineral abbreviations are used in this figure:
Epi = Epidote, Qtz = Quartz, Ksp = K- feldspar, Mag = Magnetite,
Bio = Biotite, Hem = Hemitite, Py = Pyrite, Cpy = Chalcopyrite,
Chl = Chlorite, Sulf = Sulfides, Ser = Sericite.
Py, Qtz
Epi, Ksp
Py, Qtz
Epi, ±Ksp
Figure 17. Continued
Qtz, Py
Ksp, Qtz
RSNW
Qtz, Ksp, Py
Epl, Qtz, Ksp
HRWP
-
.
Qtz, Epi, Chi
Epi, Ksp, Qtz, Chi
Py, Cpy, Ser
Qtz, Py
Ksp, Qtz, Mag
RSNE
Q tz, Ser, Sul f
Qtz, Epl, Chl, Suif
Qtz, Epi, Chl, Sulf
Qlz, Epi, Chl, Suif
Qtz, Ksp, Bio, Hem
HRIP
-
m
Q
r
r
Q
nt
,
34_
with earlier discussions which suggest the early epidote- quartz -K-
feldspar assemblage is not associated with the Sierrita system.
Thus,
the early assemblage may represent a hydrothermal system genetically
related to the Ruby Star Granodiorite, the biotite quartz diorite, or
the Harris Ranch Quartz Monzonite, all of which crystallized before the
biotite quartz monzonite porphyry.
SUMMARY AND CONCLUSIONS
The history of the Sierrita hydrothermal system in the Harris
Ranch Quartz Monzonite is similar to that in the Ruby Star Granodiorite.
In the Harris Ranch Quartz Monzonite this history involves an early
high - temperature quartz -K- feldspar- sulfide assemblage with high fracture
densities, followed by an intermediate quartz -sulfide assemblage with
lower temperatures and fracture densities, it turn followed by a late,
low -temperature epidote assemblage with very low fracture densities.
Thus as the Sierrita porphyry system natured in the Harris Ranch Quartz
Monzonite, the extent and abundance of fractures open to hydrothermal
fluid flow decreased, as did the temperature of the fluids within those
fractures.
The spatial distribution and orientations of altered fractures
associated with the Sierrita system indicate they are a result of a
combination of regional tectonic activity and the crystallization and
cooling of a concentrator intrusive.
The distribution of these frac-
tures is also influenced by the presence of premineralization faults.
An early hydrothermal event not related to the Sierrita system
is suggested by early epidote -quartz -K- feldspar veins and veinlets in
the Harris Ranch Quartz Monzonite.
The existence of this early event
is suggested by the spatial distribution of altered fractures, fluid
inclusion temperatures, styles of fracturing, and the absence of this
assemblage in the Ruby Star Granodiorite.
35
36
In conclusion, it is clear that to fully understand an active
or fossil geothermal system, a detailed study of the altered fractures
in that system is necessary.
In addition to characterizing the nature
and extent of such a hydrothermal event, it is possible to distinguish
that event from other hydrothermal events which might have been hosted
by the same rock body.
This type of study has obvious applications in
the exploration and development of porphyry copper deposits and active
geothermal reservoirs.
This study also provides further insight into
the general nature and origins of geothermal systems and the factors
which influence their development.
REFERENCES
Aiken, M. A., and West, R. J., 1978, Some geologic aspects of the
Sierrita -Esperanza copper - molybdenum deposit, Pima County,
Arizona: Ariz. Geol. Soc. Dig., 11:117 -128.
Anderson, C. A., 1948, Structural control of copper mineralization,
Bagdad, Arizona: Trans. Am. Inst. Min. Met. Eng., 178:170 -180.
Batzle, M. L., and Simmons, G., 1976, Microfractures in rocks from two
geothermal areas: Earth Planet Sci. Let., 30:71 -93.
Batzle, M. L., and Simmons, G., 1977, Geothermal systems: Rocks, fluids,
fractures, in The Earths Crust, J. G. Heacock, ed., Am. Geophys.
Union Mon. 20, pp. 233 -242.
Cooper, J. R., 1960, Some geologic features of the Pima Mining District,
Pima County, Arizona: U. S. Geol. Surv. Bull. 112 -C.
Cooper, J. R., 1971, Mesozoic stratigraphy of the Sierrita Mountains,
Pima County, Arizona: U. S. Geol. Surv. Prof. Pap. 658 -D,
pp. D1 -D -40.
Cooper, J. R., 1973, Geological map of the Twin Buttes quadrangle,
southwest of Tucson, Pima County, Arizona: U. S. Geol. Surv.
Misc. Geol. Inv. Na p I -745.
Damon, P. E., and Mauger, R. L., 1966, Epeirogeny- orogeny viewed from
the Basin and Range Province: Soc. Min. Eng. Trans., 235:99 -112.
Haynes, F. M., and Titley, S. R., 1980, The evolution of fracture related permeability within the Ruby Star granodiorite, Sierrita
porphyry copper deposit, Pima County, Arizona: Econ. Geol.,
in press.
Knapp, R. B., and J. E. Knight, 1977, Differential thermal expansion of
pore fluids; fracture propagation and microearthquake production
in hot pluton environments: Jour. Geophys. Res., 82:2515 -2522.
Koide, H., and Bhattacharji, S., 1975, Formation of fractures around
magmatic intrusions and their role in ore localization: Econ.
Geol., 70:781 -799.
Lacy, W. C., 1959, Structure and ore deposits of the east side of the
Sierrita Mountains: Ariz. Geol. Soc. Dig., vol. 2, p. 185.
37
38
Lootens, D. J., 1966, Geology and structural evolution of the Sierrita
Mountains, Pima County, Arizona: Ariz. Geol. Soc. Dig.,
8:33 -56.
Lynch, D. W., 1966, The economic geology of the Esperanza Mine and
vicinity, in Geology of the Porphyry Copper Deposits--Southwestern United States, S. R. Titley and C. L. Hicks, eds.,
Tucson: University of Arizona Press, pp. 267 -279.
Lynch, D. W., 1967, The geology of the Esperanza mine and vicinity,
Pima County, Arizona: unpublished thesis, The University of
Arizona, Tucson, Arizona.
Manske, S. L., 1980, Fracturing events in the Ruby Star Granodiorite
adjacent to the Esperanza porphyry copper deposit, Pima County,
Arizona: unpublished thesis, The University of Arizona, Tucson,
Arizona.
Norton, D., and Knapp, R., 1977, Transport phenomena in hydrothermal
systems: The nature of porosity: Am. Jour. Sci., 277:913 -936.
Norton, D., and Knight, J., 1977, Transport phenomena in hydrothermal
systems: Cooling plutons: Am. Jour. Sci., 277:937 -981.
Preece, R. K., and Beane, R. E., in prep., Contrasting evolutions of
hydrothermal alteration in quartz monzonite and quartz diorite
at the Sierrita porphyry copper deposit,
Rehrig, W. A., and Heidrick, T. L., 1972, Regional fracturing in Laramide stocks of Arizona and its relationship to porphyry copper
mineralization: Econ. Geol., 67:198 -213.
Smith, V. L., 1975, Hypogene alteration at the Esperanza mine, Pima
County, Arizona: unpublished thesis, The University of Arizona,
Tucson, Arizona.
Snow, D. T., 1970, The frequency and apertures of fractures in rock:
Intnl. Jour. Rock Mech. Min. Sci., 7:23 -40.
Titley, S. R., 1978, Geologic history, hypogene features, and processes
of secondary sulfide enrichment at the Plesyumi copper prospect,
New Britain, Papua New Guinea: Econ. Geol., 73:768 -784.
Titley, S. R., Fleming, A. W., and Neale, T. I., 1978, Tectonic evolution of the porphyry copper system at Yandera, Papua New Guinea:
Econ. Geol., 73:810 -828.
Werre, R. W., Jr., Bodner, R. J., Bethke, P. M., and Barton, P. B., Jr.,
1979, A novel gas -flow fluid inclusion heating /freezing stage:
GSA Abstr., vol. 11, no. 7, p. 539.
Figure 3
O F-202
O F-201
e71
a / \ 1,
i
OF-19905.8 KM
OF-198
LOCATION MAP OF FRACTURE DATA STATIONS
1i
WITH GEOLOGY MODIFIED FROM COOPER (1973), EASTERN
SIERRITA MOUNTAINS, PIMA COUNTY, ARIZONA
OF-I9
OF-4>
OF-55
OF-49
John L. White
Jhr
0,98
O F-50
Geosciences
1980
OF1e3
O F-54
OF-51
._
OF_I92
0 F-l54
O F-le
OF_18 1
'
Scale 1:10,000
OF-155
OF-5
O F-14
.J
OF-IS
N
It
of-156 of-150
Tql....
0E-I49
O F-92
One
OF-128
OF.Í29
pF-I37
OF-12r
0,1513
OF-1I4
0E-119
OF -196
/^)
Elevation Contour Interval 200 Feet
OF -99
OF-61
OF 62
OF-Be
OF-36
3
11
119
Kilome ler
O F-70
O F-80
OF-85
O
Explanation
OF_63
OF-57
0E-94
OF-69/
OF-91
O F-64
OF-rà
O
O F-191
pF-188
OF-102
0,103
Quaternary
OF-76
Alluvium and fill
OF-7
1
OF-101 OF100 -.
OF-99
7r
OF-104
%
O F-21
0 F20
OF-182 i
oFel
O F a OF
o
O F-94
t511
Paleocene
Ruby Star Granódiorite
O
o
Tertiary
Quartz /alite
OF-105
OF-111
OF-4
O F-36
--
_
0,107
O F-37
o
_-,.
_
4
XOe!
-:OF-1s7
.
.
0,17
OF-le
0,168
5,0E-0
Formation of Tinaja Peak
Rhyolitic to andesitic floes, tuff, and
related sedimente
-
°F-19
o F-/50
Oligocene /Miocene -
rt
0E-176
O F-I>:9
t,.f!.t'::.
o F-z9
OF-78
O F-14
OF-90
OF-26
O -137
Biotite qua
OF-166
oF-35
4
OF-25
OF27-.
OF-2
-I
3,770
rC
Jhr
6
diorite
dF-135
OF-Y9
D
F-450
OF-3
o
//
OF-224
OF-23
OF-22
h;
//
, -
/
-148
Cretaceous
OF-39
OF-40
Jhr
of-225 \<++
F228
volcanic rocks
Andesitla and dacitic brecc las, rhyolite
tuff, and related sediments
/O9,1.1 I OF-41
OF-151
O
Cretaceous
Sierrita Granite
OF-33
Jurassic
Jhr
i
]h
4t^' r
O F-2e3
0,159
A^ r.> , ^
OF-534
OF-227
°>
OF22e
o
O F-240
0,165
//
4/09
6.5:7,f15°7
433
O F-23P
O F-234
J
yf
<^r >
'
O F-287
O F-235
,
> eo5.,
l
\c
c>nc
,`f>
O F-260
>v7 n
v/
w
<
!
<
a
4
<
nLC7<
°
,
f`
-
,
J
cr
SG.
'
5
^
r
vnf
<I
°"^
°^i
>a<
//.
Ìe
F-262 /
v
.
1<
>
Áe
Cele teve We ada,g 9250nt
O F-E.
--
Q 3.5 KM
o_
Fluid inclusion sample station
Contact
Dashed where approximately
O F-207
O F-221
o
located
Fault, showing dip
Dashed where opproximately located
Arrows, D, and U indicate relative
OF-249
F-zoe
eisploeemen1
O F223
O F-z97
°F-2I7
13'IS
O F-222
O F-209
Fault zone, showing dip
F-267
/
/
/
OF216
-215
O
O F-266
OF-2I2 O
Breccia pipe
O F-277
OF-299
Of-278
r
06-210
OF-213
OF-214
OF-299
O F-275
J
0 F-279
OF-263
<,
Outline of open pit mine
O
O F-265
O F-2e2
//11111111\O\
o F-279
,y]
OF-264
Outline of mine dumps
O F-lel
John L, White
nr
s+
Road
°o°
19 B 0
.I
c
c
FVn
M.S. Thesis
Geosciences
i>]
-',, °
IIè
,c
n
F-206
OF-z19
OF-273l
G.
c
O
O F-295
^> <hv
<
{+
O F-205
O F-220
OF-300.
O-F272I
n
Jhr
'
F-247 O
5,
O F-z58
>
o Fee
0,164
Fracture dota station
,
F.2509.,
and quartzite
O F_2I2
]
-
o
30
OF-271
A., A
i
oFaol
.
O F-269
OF-87
DEPARTMENT
OF GEOSCIENCES
UNIVERSITY OF ARIZONA
-
0,281
n
07246
O F-246
/
OF-294
OF-292
OF-270
.
O,60
._
o F 28
0,293
c
Rhyolitic flows and tuff, andesl te,
OF-288
OF-291
rys°>
r
OF-29
- Triassic
Ox Frame Formation
4.
? e<
OF-244
O F-290
;^
k,-'/
^<,
O F-252
Jhr
-
-6772;7
ePo
sa,7
O F-243
I
J
4
q
e
44.4
OF-256
OF132
Harris Ranch Quartz Monzonite
OF-2eF.
OF-13(
0,133
/ 0E-242
7
-
0,230
OF-23I
\
Or-24r
OF-239
5KM
F-629
OF-285
OF Iè9
r
o
o
O F-z64
OF-160
o
>
nn
yc<y
.l
6]V
tel
o
°
°
Figure 10
MAP OF CONTOURED, AVERAGED FRACTURE DENSITIES
OF THE SECOND FRACTURE - CONTROLLED ALTERATION
ASSEMBLAGE
IN THE HARRIS RANCH QUARTZ MOA'ZONITE
PIMA COUNTY, ARIZONA
John
L.
White
1980
Scale 1:10,000
o
One Kilometer
j
Contour Interval 0.01 cm-1
39m
John L White
M.S. Thesis
Geoscien ces
19 8 0
7
is
opo
14
13
13
24
0.00