PARAGENESIS, GEOCHEMISTRY, AND TEMPERATURES OF FORMATION OF ALTERATION ASSEMBLAGES AT THE SIERRITA DEPOSIT, PIMA COUNTY, ARIZONA

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1 PARAGENESIS, GEOCHEMISTRY, AND TEMPERATURES OF FORMATION OF ALTERATION ASSEMBLAGES AT THE SIERRITA DEPOSIT, PIMA COUNTY, ARIZONA by Richard Kellar Preece, III 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

2 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. Requests 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 scholarship. In all other instances, however, permission must be obtained from the author. SIGNED: X:.c.er APPROVAL BY THESIS DIRECTOR This thesis has been approved on the date shown below: Rich d E. Beane Associate Professor of Geosciences 7/245/7 9 Date

3 ACKNOWLEDGMENTS I wish to thank Dr. R. E. Beane for selecting and guiding the research phase of this project, for the many discussions with him on the implications of the fluid inclusion data, and for his critical editing of this thesis. I would also like to thank Drs. D. L. Norton and S. R. Titley for their suggestions that improved this manuscript. Assistance with fluid inclusion and electron microprobe research was given by Robert Bodnar and Valerie Walker, respectively, whom I gratefully acknowledge. I am grateful for the financial aid received during this project which came from The University of Arizona in the form of a research assistantship, and from the Duval corporation in the form of a scholarship. Research was also partially funded by National Science Foundation grant EAR I am especially indebted to my wife, Eustolia, for her incredible patience and support during what must have seemed an interminable progression of days and nights that I spent in the laboratory. iii

4 TABLE OF CONTENTS Page LIST OF ILLUSTRATIONS LIST OF TABLES ABSTRACT vi viii ix 1. INTRODUCTION 1 2. ALTERATION MINERALOGY AND PARAGENESIS 7 Alteration Paragenisis 8 Harris Ranch Quartz Monzonite 11 Biotite Quartz Diorite Microprobe Data 18 Feldspars 19 Muscovite 21 Biotite 21 Chlorite 24 Epidote 24 Summary FLUID INCLUSION STUDIES 30 Temporal Classification of Fluid Inclusions 30 Compositional Classification of Fluid Inclusions 32 Type I: Moderate Salinity, Liquid -rich. 32 Type II: Vapor -rich 32 Type III: Halite -bearing 33 Fluid Inclusion Homogenization Data 35 Sample HR Sample HR Sample BQD Sample BQD Secondary Inclusions 44 Salinities 44 Composition of Halite- bearing Inclusions 50 Na /K Mole Ratio of Moderate -Salinity Fluids 55 Pressure Corrections 57 Discussion and Results 60 iv

5 V TABLE OF CONTENTS -- Continued Page 4. SUMMARY AND CONCLUSIONS 66 APPENDIX A: THIN AND POLISHED SECTION DESCRIPTIONS 68 HR HR HR HR HR HR HR BQD BQD BQD-01-02A 75 BQD BQD BQD BQD APPENDIX B: APPENDIX C: ELECTRON MICROPROBE PROCEDURES AND ANALYSES 81 FLUID INCLUSION HOMOGENIZATION AND FREEZING EQUIPMENT AND PROCEDURES. 99 Optics 101 Sample Preparation 102 LIST OF REFERENCES 103

6 LIST OF ILLUSTRATIONS Figure Page 1. Location Map of the Sierrita Porphyry Copper Deposit 3 2. Generalized Geological Map of the Sierrita Open Pit Showing Sample Location 4 3. Paragenesis and Relative Abundances of Minerals in Sample HR Schematic Diagram of Sample HR -02 Showing Geometric Vein Relations and Mineralogy Paragenesis and Relative Abundances of Minerals in Sample BQD Paragenesis and Relative Abundances of Minerals in Sample BQD Compositions of Alkali and Plagioclase Feldspars Portion of Compositional Triangle of Biotite Octahedral Site Occupancy Portion of Compositional Triangle of Chlorite Octahedral Site Occupancy Portion of Compositional Triangle of Octahedral Site Occupancy of Coexisting Biotites and Chlorites from Sample HR Histogram of Fe3+ /Fe3 + +A1) Mole Ratios in Epidotes Histograms of Primary Fluid Inclusion Homogenization Temperatures (Th) from Sample HR Distribution of Secondary Fluid Inclusion Homogenization Temperatures in Quartz Grain Cut by Vein in Sample HR vi

7 vii LIST OF ILLUSTRATIONS -- Continued Figure Page 14. Histogram of Primary Fluid Inclusion Homogenization Temperatures (Th) from Samples HR -01 and BQD Histograms of Primary Fluid Inclusion Homogenization Temperatures (Th) from Sample BQD Comparison among Secondary Fluid Inclusion Homogenization Temperatures (Th) from Each Sample Studied Relationship between Salinity and Homogenization Temperature of Fluid Inclusions at Sierrita Histogram of Measured Fluid Inclusion Salinities at Sierrita Temperature of Vapor Disappearance (Tv) vs. Temperature of Halite Dissolution (TNaC1) for Halite -bearing Inclusions from Sierrita Distribution of Daughter Products with Respect to Salinity in Halite -bearing Inclusions Comparison between Homogenization Temperatures and Pressure- corrected Trapping Temperatures of Moderate -salinity Liquid -rich Fluid Inclusions 22. Hypothetical Histograms of Fluid Inclusion Homogenization Temperatures from Three Crosscutting Veins Temporal Evolution of Temperature, Salinity, and Vein Formation at Sierrita 64

8 LIST OF TABLES Table 1. Modal Mineralogy of the Harris Ranch and the Biotite Quartz Diorite Page 9 2. Daughter Products Observed in Fluid Inclusions at Sierrita, with Optical and Physical Properties 34 B.1. Feldspar Microprobe Analyses 83 B.2. Muscovite Microprobe Analyses 88 B.3. Biotite Microprobe Analyses 89 B.4. Chlorite Microprobe Analyses 93 B.5. Epidote Microprobe Analyses 95 viii

9 ABSTRACT The Sierrita porphyry copper orebody is thought to be contemporaneous with the emplacement of a Laramide -age quartz monzonite porphyry into two older intrusives of distinctly different compositions: a quartz monzonite and biotite quartz diorite. Wallrock chemistry appears to have influenced the mineralogy of alteration and vein assemblages, in that minerals typical of potassic and propylitic assemblages attend main -stage sulfide deposition in the quartz monzonite and biotite quartz diorite, respectively. In addition, late quartz + muscovite + sulfide veining was observed to cut potassic veins only in the quartz monzonite. Although the presence and relative abundances of hydrothermal minerals are governed by host rock lithology, electron microprobe analyses of vein and alteration minerals indicate that compositional variations are independent of wallrock composition. Fluid inclusion studies revealed that early veining in both wallrocks occurred at temperatures of C, from two chemically distinct fluids. The earliest veining observed was associated with hypersaline brines ( - 12 molal NaCl equivalent), which was followed by an initially hotter, locally boiling 2 molal NaCl equivalent solution. The bulk of mineralization in both rocks was ix

10 X associated with the low salinity fluid at temperatures of C, although sulfide deposition occurred at temperatures as low as 190 C in the center of late phyllic veins.

11 CHAPTER 1 INTRODUCTION Field and laboratory studies of porphyry copper - type alteration and mineralization systematics have suggested that significant variations in temperature, pressure, and hydrothermal fluid composition had occurred both temporally and spacially during ore - forming processes (i.e., Norton and Knight 1977, Gustafson and Hunt 1975, Bodnar 1978). It has also been recognized that the nature of alteration assemblages may be greatly influenced by the chemical and mineralogical makeup of the host rock (Guilbert and Lowell 1974, Beane 1979). This study was designed to monitor the temporal evolution of the thermal and chemical environment of hydrothermal mineral deposition, and the effects of host rock composition on alteration assemblages at the Sierrita porphyry copper deposit. Relative temporal relationships among alteration and vein assemblages were established by detailed examination of crosscutting veins. The scale of observation ranged from hand lens inspection of rock slabs to detailed thin and polished section studies utilizing the petrographic microscope. Standard fluid inclusion heating and freezing techniques 1

12 were used to determine the temperature and gross salinity of 2 hydrothermal fluids associated with the alteration assemblages. The compositions of alteration and vein minerals were established by electron microprobe analyses. The Duval -Sierrita porphyry copper -molybdenum deposit, located 40 km south southwest of Tucson, Arizona (Fig. 1), was selected for study because supergene effects are generally lacking, and because fracture -controlled hypogene alteration and mineralization occurs in two chemically distinct wallrocks. The deposit which is part of the Sierrita- Esperanza porphyry copper system lies on the southeastern flank of the Sierrita Mountain range. The mining history and geology of the complex has been previously described by Lynch (1967), Cooper (1973), Smith (1975), and Aiken and West (1978). As such, only the general geology exposed in the Sierrita pit will be discussed here. The Sierrita deposit is localized in three intrusive bodies: Jurassic -Triassic Harris Ranch quartz monzonice,an early Paleocene biotite quartz diorite, and the Laramide Ruby Star quartz monzonite porphyry (Fig. 2). The Harris Ranch quartz monzonite is medium grained equigranular to slightly porphyritic and outcrops in the southwestern part of the pit. This rock unit has been dated at about 200 million years, making it the oldest intrusive in the pit (Cooper 1973). Laramide intrusive activity commenced with

13 3 ARIZONA -*PHOENIX TUCSON X'SIERRITA km Figure 1. Copper Deposit. Location Map of the Sierrita Porphyry

14 4 E Ó rn W -J 4 c.

15 the emplacement of the biotite quartz diorite, dated at 67 million years ago (Cooper 1973). The diorite, located in the northwestern part of Sierrita, appears to be in intrusive contact with the Harris Ranch quartz monzonite. It is a fine to medium grained porphyritic rock, although it may have a highly variable texture (Smith 1975). The eastern half of the Sierrita pit consists almost entirely of Laramide -aged Ruby Star quartz'monzonite porphyry, which is thought to be temporally related to mineralization (Smith 1975, Aiken and West 1978). This quartz monzonite porphyry, dated at around 57 million years ago, is a late -stage differentiate of the 69 million year old Ruby Star granodiorite batholith which extends north of the Esperanza- Sierrita complex, occupying most of the Sierrita Mountain range (Cooper 1973). The quartz monzonite porphyry enhibits several textural variations which grade into one another, suggesting either a complex cooling history or multiple intrusive events (Smith 1975). An intrusive breccia (apparently related to the emplacement of the quartz monzonite porphyry), Triassic volcanics, and post- mineralization quartz latite dikes also outcrop in the pit (Fig. 2). Samples for petrographic and fluid inclusion studies were collected along the contact between the Harris Ranch quartz monzonite and the biotite quartz diorite, about 250 m from the contact of the two with Ruby Star quartz monzonite

16 6 porphyry (Fig. 2). Single and crosscutting veins were studied from four samples collected within 50 m of each other from the quartz monzonite and the biotite quartz diorite. Because the samples were collected from a relatively restricted area, the results should not be considered to characterize the entire deposit. However, the results of this study are compatible with other studies at Sierrita (Denis 1974, Fellows 1976, Aiken and West 1978).

17 CHAPTER 2 ALTERATION MINERALOGY AND PARAGENESIS Previous studies of hydrothermal alteration distribution and paragenesis at Sierrita -Esperanza have been carried out by Smith (1975), Denis (1974), Fellows (1976), and Aiken and West (1978). These studies have shown that while the spatial distribution of hydrothermal alteration assemblages at Sierrita is broadly comparable to other porphyry copper deposits (Lowell and Guilbert 1970, Rose 1970), the distribution and mineralogy of alteration assemblages are strongly influenced by the composition of the host rock. The most notable deviations from the generally accepted alteration model of Lowell and Guilbert (1970) are the absence of a pervasive quartz + sericite + pyrite assemblage, and the abundance of epidote and chlorite associated with mineralization in the center of the deposit. The general sequence of vein formation at Sierrita is (from early to late): biotite --quartz + K- feldspar quartz + sericite gypsum + zeolites. Each of these stages of vein formation consists of several generations of similar mineralogy and relative abundances of phases, each of which may or may not contain sulfides (Aiken and West 1978). 7

18 Slabbed surfaces of four rock samples of quartz monzonite (2) and biotite quartz diorite (2) were examined by hand lens and stereomicroscope in order to determine the 8 relative ages of crosscutting veins. Thin sections of selected veins and intersections of veins were made for petrographic examination in order to establish the paragenesis and mineralogy of vein and alteration assemblages. Selected thin sections were stained with sodium cobaltinitrite to facilitate the identification of potassium feldspars. When necessary, polished sections were made so as to identify sulfide and oxide minerals, utilizing vertically- incident reflected microscopy. Electron microprobe analyses were used to obtain the compositions of biotite, feldspars, chlorite, muscovite, and epidote. Alteration Paragenesis Mineral abundances were visually estimated from thin section examination, and modified from average estimates by Denis (1974) and Smith (1975). The approximate volume abundance of the original rock -forming minerals of the quartz monzonite and the biotite quartz diorite are presented in Table 1, illustrating the difference between the two rock types. Tourmaline was not seen in this study, consistent with the observation of Aiken and West (1978) that the tourmaline content of the Harris Ranch quartz monzonite decreases as the contact with the Ruby Star quartz monzonite

19 Table 1. Modal Mineralogy of the Harris Ranch and the Biotite Quartz Diorite The estimated volume percentages are from this study, as modified from Denis (1974) and Smith (1975). Harris Ranch Quartz Monzonite Biotite Quartz Diorite 25 % Quartz 45 % Andesine 37 % Orthoclase 25 % Hornblende 29 % Oligioclase 20 % Biotite 4 % Biotite 5 % Quartz 4 % Tourmaline 5 % Orthoclase 1 % Amphibole (Tremolite?) Trace Magnetite Trace Sphene

20 10 porphyry is approached. The total groundmass biotite content observed in altered quartz monzonite is close to the combined igneous biotite, amphibole, and tourmaline content reported by Denis (1974). Although no compelling evidence was seen, this is permissive evidence that the missing tourmaline, as well as amphibole, may have been replaced by hydrothermal biotite.

21 11 The following summary of vein and alteration assemblages in each of the four rock samples is derived from thin and polished section petrographic descriptions in Appendix A, as well as observations of textural relations made during the fluid inclusion study. Harris Ranch Quartz Monzonite The two samples from the quartz monzonite are HR -01, consisting of a single vein, and HR -02 which contains several crosscutting veins, five of which were suitable for fluid inclusion studies. The paragentic sequences interpreted from the petrographic studies are shown in Figures 3 and 4. The paragenesis and relative abundances of hydrothermal phases of Sample HR -01, a single mineralized vein, are shown in Figure 3. Early microcline and biotite alteration of the wallrock was contemporaneous with cloudy quartz and microcline deposition in the vein. Pyrite was then deposited on the surfaces of the cloudy quartz, slightly overlapping deposition of the quartz. Chalcopyrite and lesser amounts of clear quartz, epidote, chlorite, K- feldspar, and anhydrite fill open spaces, vein, and partially replace the earlier assemblages. Figure 4 is a schematic diagram of geometric relationships in Sample HR -02. The earliest vein assemblage

22 Kspar : Çç,, '3,;;'.;:.:J:L.:F:<v. :;.tt;>;::iv::. vw..::'tr.. biotite hematite quartz `.1:!C;..:.; o`c. i: :.: ^'..ivti::;.rrti:'èn`:s:;:á':w'<+:':c j. ;'i` :.. "; :'c`':;';'ÿ,: ;};.. ; i3:"c`%:"i,?6't`ì -:+r`..^ BARs jyi`..mm.. M4 pyrite chalcopyrite. >p.....t.. anhydrite epidote chlorite ATAM.da: >.tst.i,,- `.`á,.`i>uí' '." sericite EARLY > LATE Figure 3. Paragenesis and Relative Abundances of Minerals in Sample HR -0l.?3....x

23 13 VEIN A: qtz + Kspar + blot VEIN D ( VEIN G: qtz+ Kspar + blot VEIN E: qtz + musc+ Kspar+ py + cp + bn Figure 4. Schematic Diagram of Sample HR -02 Showing Geometric Vein Relations and Mineralogy.

24 14 documented in this sample is composed of quartz + micro - cline + biotite (Vein A, Fig. 4). This was followed by at least two more generations of quartz + microcline + biotite veining (Veins G and C, Fig. 4). Vein filling in the potassic veins was apparently uninterrupted except for Vein C, where evidence of reopening and deposition of quartz + K- feldspar is present. Flakes of hematite, tens of microns in diameter, in quartz and K- feldspar are present in all potassic veins. Quartz + muscovite veining (Veins D and E, Fig. 4) cuts all potassic veins in Sample HR -02. Filling of the two veins initiated with an early assemblages of quartz + K- feldspar + chlorite with minor muscovite, chalcopyrite, and bornite. Continued vein filling was dominated by muscovite with minor amounts of quartz, chlorite, pyrite, chalcopyrite, and bornite. Late stage deposition is primarily pyrite and chalcopyrite, with minor bornite and quartz. As no reopening or brecciation textures were observed, filling of Veins D and E is interpreted as having evolved continuously. Alteration of Sample HR -02 may have commenced with biotitization of amphibole and tourmaline (?) and the development of thin discontinuous stringers of biotite. This is taken from the observations of Aiken and West (1978), but petrographic evidence on the scale of a single sample is inconclusive.

25 15 Biotite Quartz Diorite The paragenesis of Sample BQD -03, a single vein, is very similar to that of HR -01. The paragenesis of BQD -03 is shown in Figure 5, with the width of the bars proportional to the relative abundances of the respective mineral. Early alteration and vein filling consisted of biotite, K- feldspar, sodic plagioclase, and quartz. Continued deposition of quartz was accompanied by anhydrite, and overlapped with the development of bladed specular hematite. Pyrite was deposited on exposed surfaces of the previously formed minerals, probably contemporaneously with the later stages of hematite deposition and the subsequent replacement of hematite by magnetite. Chalcopyrite, epidote, and the other associated phases (Fig. 5) filled open spaces, and veined and replaced previous assemblages. As biotite is confined to a narrow selvage and to microfractures in the wallrock, the time frame of biotitization relative to vein deposition is incompletely known. Sample BQD -01 contains five veins which crosscut and offset each other. As seen in Figure 6, each vein exhibits much the same mineralogy and paragenesis as each other and as BQD -03 (Fig. 5). The major difference between the five veins in BQD -01 and that of BQD -03 is the alteration of wallrock plagioclase to sericite in Sample BQD -01. Sericite is interpretated to be an intregal part of vein

26 p-qzjg ajdtusg uz stu.zau-ry4 Jo saouepunqy an-rt.-eau pue. sisauabp.zed c, a.znbta AlL7V3 alltaubow elluepqmow.ir"v'..._% kw:.{.... 0:. >>::t;.}::::.:,,: VT WA. Aro { j.f{i ; >:@ fu= ä.,`í",çy,. ;<,; r::f, ì.' :3,ii '.:. epixdooloyo altad ewow etoptde zl.ionb emwey , MaginMir~aWA eiiapayuo b ; :#,}3:r`>:.::;;>;:<'<v»'s:{{ii>_' w.:;,xh.\'::^.sg::i:yjli':;r`+s{: 6 anion aodsn bold-dn

27 LATE No -plag VEIN A mom VEIN C VEIN D K spar biotite hematite anhydrite quartz epidote INV -em NMI a chlorite pyrite chalcopyrite molybdenite magnetite sericite Figure 6. Paragenesis and Relative Abundances of Minerals in Sample BQD -01.

28 18 formation, rather than as a later, separate event, for three reasons. 1. The abundance of sericitization decreases dramatically away from the veins. 2. No veinlets or stringers of sericite alone were seen. 3. Only igneous plagioclase was replaced by sericite; secondary feldspars were unaltered. The timing of sericitization of walirock plagioclase with respect to the evolution of vein mineralogy is unknown. The paragenesis and relative abundance of vein and alteration minerals for each vein in Sample BQD -01 are presented in Figure 6. In each case, vein filling initiated with an assemblage of quartz + biotite + K- feldspar, that may or may not include sodic plagioclase and anhydrite. This was followed by pyrite which commonly overlaps chalcopyrite + epidote + chlorite + quartz deposition. Magnetite, molybdenite, and anhydrite are usually associated with the later assemblage. Microprobe Data The compositions of alteration and vein minerals capable of solid solution were determined by electron microprobe analyses. Of particular interest in this study are the variations of mineral compositions through time in a

29 19 particular sample, and correlations between walirock chemistry and alteration mineral composition. Microprobe procedures and tabulated analyses are given in Appendix B, and the resulting mineral compositions are summarized below. Feldspars As noted in the thin section studies, two solid solution series of feldspars are present: potassium -rich alkali feldspar, and sodium -rich plagioclase. Microprobe analyses were made of 22 secondary K- feldspars and 7 plagioclases occurring in veins and in vein selvages. In addition, two igneous feldspars in the diorite were analyzed. The feldspar analyses are plotted in terms of end -member component mole fractions on the orthoclase -albite- anorthite compositional diagram in Figure 7A. Plagioclase compositions generally fall near the albite- anorthite tieline, but also include three theoretically impossible K- feldspar concentrations for low - temperature feldspars (Deer, Howie, and Zussman 1966, p. 291). These "impossible" compositions correspond to bulk compositions of perthitic feldspars which are the result of exsolution (Deer et al. 1966). Plagioclase analyses from Samples BQD -01 and BQD -03 fall into three groups distinguishable by both composition and mode of occurrence. Plagioclase feldspars deposited in

30 Figure 7. Feldspars. Compositions of Alkali and Plagioclase The symbols I, S, and V designate igneous, selvage, and vein feldspars, respectively. A. Compositional Triangle of Alkali and Plagioclase Feldspars. B. Orthoclase Content of Alkali Feldspars.

31 20 A. + 8QD-03 BQD-01 X HR-02 O HR-01 Ab FREQ B. 8QD-03 BQD-0I HR-02 HR xory Figure 7. Compositions of Alkali and Plagioclase Feldspars.

32 21 veins average Ab72An26Or2, the average of two igneous plagioclase feldspars in the diorite is Ab58An41Or1. The two alteration feldspars located on the vein- wallrock contact average Ab67An31Or2, plotting between igneous and vein feldspar compositions. K- feldspar compositions plot along the orthoclase - albite tieline, ranging from Or99Ab1 to Or83Ab17 (Fig. 7A). These compositions are plotted against frequency in Figure 7B. The majority of microcline compositions fall between Or100 and Or92' with four K- feldspars containing less than 90 mole percent orthoclase component. The range of vein and selvage K- feldspar compositions in both rock types are similar, with no trend observed with respect to paragenetic position. The average composition is Or96Ab4. Muscovite Five muscovites from the quartz + muscovite + sulfide veins in HR -01 (Veins D and E) were analyzed. The compositions are very similar to each other, with an average of: (K1.84'Na0. 16) 2.00 (A13.57'Ti0.03'Fe3+0.16'Mg0.21) 3.97 (Al1. 76' Si6.24) (OH) 4 Biotite The results of microprobe analyses of 21 biotites from the quartz monzonite and biotite quartz diorite are

33 portrayed in Figure 8. The biotites may be broken into 22 three groups on the basis of octahedral site distribution and mode of occurrence. Biotites associated with early potassic veins in HR -02 contain a significantly higher Mg/ (Mg + Fe) mole ratio than the rest of the analyzed biotites, with an average mole ratio of Shreddy biotites and recrystallized igneous biotites from Sample HR -02 contain only slightly more Mg than Fe, with an average Mg /(Mg + Fe) mole ratio of The compositional shift between these and the more phlogopitic selvage biotites is shown in Figure 8. All biotites in the biotite quartz diorite have very similar compositions to each other. In addition, the Mg/ (Mg + Fe) mole ratios are essentially identical to alteration biotites in the quartz monzonite, with an average mole ratio of One analyzed biotite in Sample HR -02 contains 4.50 wt. % TiO2, similar to the TiO 2 content of igneous biotites at Bingham, Utah (Moore and Czmanske 1973). As this biotite was located on a polished section, the petrographic difference between it and biotites identified as recrystallized igneous biotites was not established. The composition is listed in Appendix B (Biotite HR- 02-4I, Table B.3).

34 50,is:,It 40 DioriM ` xq HR - 02 (vein) X X HR - 02 (alteration) x\ HR-02 (igneous) :.i 20 N 4% 10 N Mg \ Mg/(Mg+Fe) Figure 8s Portion of Compositional Triangle of Biotite Octahedral Site Occupancy. --Arrow shows compositional shift between selvage and alteration biotites in Sample HR -02.

35 24 Chlorite Analyzed chlorites include both vein chlorites and alteration chlorites after biotite. As seen in Figure 9, the chlorites have a relatively constant octahedral (Al + Ti), mole proportion, with variable Mg /(Mg + Fe) mole ratios. Chlorites from the quartz + muscovite + sulfides veins have an average Mg /(Mg + Fe) mole ratio of 0.46, and fall in the ripodolite compositional field according to the classification of Hey (1954). The remainder, occurring as both vein and alteration chlorites, are classified as pycnochlores (Hey 1954), with an average Mg /(Mg + Fe) mole ratio of The compositions of alteration chlorites and parent biotites from Sample HR -01 are shown in Figure 10. The limited data presented here suggest that the chlorite composition is relatively independent of parent biotite composition. Epidote Twenty -two epidote analyses include both vein and selvage epidotes from Samples HR -01, BQD -01, and BQD -03, and are plotted in Figure 11. The average Fe3 + /(Fe3+ + Al) is 0.30, which compares favorably with the average value of 0.29 from 1382 analyses done by Fellows (1976).

36 BQD-03 X HR-01 HR-02 (vein) HR-02 (alteration) Mg Mg/(Mg+Fe) Figure 9. Portion of Compositional Triangle of Chlorite Octahedral Site Occupancy.

37 50 (AI+Ti) X Biotite Chlorite 10 Mg! / / 1 I 1 1 \ \ Mg /(Mg +Fe) Figure 10. Portion of Compositional Triangle of Octahedral Site Occupancy of Coexisting Biotites and Chlorites from Sample HR Coexisting phases are connected by tielines.

38 27 HARRIS RANCH DIORITE FREQ I XFe 0.40 Figure 11. in Epidotes. Histogram of Fea+/ Fe3 + +A11 Mole Ratios

39 28 Summary Petrographic studies have shown that the presence and relative abundances of alteration and vein minerals at Sierrita bear a strong resemblance to the host rock mineralogy. In the quartz monzonite, alteration and vein assemblages are dominated by microcline and muscovite. Alteration of the biotite quartz diorite, on the other hand, is dominated by calcium -, magnesium-, and iron -rich minerals: biotite, chlorite, and epidote. The paragenetic sequences that were developed from this study demonstrate that vein filling typically involved an evolution of hydrothermal mineral assemblages. In the mineralized veins, sulfide deposition appeared late during the development of vein assemblages, in most cases associated with an assemblage that altered earlier vein minerals. The general sequence of vein and alteration mineralogy in the quartz monzonite is an early quartz + K- feldspar assemblage which may or may not be associated with mineralization followed by the formation of quartz + muscovite veining. The biotite quartz diorite as seen in this study is cut by a series of mineralized veins, all of which exhibit an evolution from an early assemblage dominated by quartz and biotite to a later epidote + chlorite + sulfide assemblage.

40 Electron microprobe analyses indicate that the compositions of vein and alteration minerals were not affected 29 by distinctly different wallrock chemistries. Compositional variations of plagioclase, biotite, and chlorite were seen to be more a function of the mode of occurrence within a particular rock type rather than the rock in which the minerals appeared. The compositions of K- feldspar and epidote showed no systematic variation within a single sample or between samples. This would seem to indicate that the overriding factors dictating the most stable mineral composition are the prevailing temperature, pressure, and fluid chemistry, rather than wallrock composition.

41 CHAPTER 3 FLUID INCLUSION STUDIES Fluid inclusions from each vein discussed in the previous chapter were examined utilizing standard fluid inclusion techniques. Details of sample preparation and heating and freezing methods used are in Appendix C. The purpose of this fluid inclusion study is to determine the temperature and salinity of hydrothermal fluids associated with particular mineral assemblages. By correlating fluid characteristics with the paragenetic positions of the respective alteration assemblages, the evolution of temperature and salinity of hydrothermal fluids through time can be established for each sample. In addition, as the samples were selécted in order to minimize any difference in the sourceregions of the hydrothermal fluids (Norton 1978), and the thermal gradient between samples, it may be expected that time equivalency can be established between alteration assemblages of different samples on the basis of similar temperatures and salinities of fluids. Temporal Classification of Fluid Inclusions The relative time frame of a particular fluid inclusion depends on the position of the host crystal in a 30

42 31 paragenetic sequence, and on the relationship of the inclusion to the host. A three -fold system, based on the timing of the formation of the inclusion relative to the host, is commonly used to classify fluid inclusions. Primary fluid inclusions were trapped at the same time as the enclosing minerals by irregularities in crystal growth or fluid hetereogenity (Roedder 1967). Pseudo - secondary inclusions were formed during crystal growth by the development of a fracture, which subsequently filled with fluid and rehealed. Further crystal growth resulted in a plane of inclusions which abruptly ends within the host crystal. Primary and pseudosecondary inclusions represent conditions during crystal growth. Secondary inclusions were formed during a fracturing event at some time after the growth of the host crystal. Subsequent rehealing of the fluid - filled fracture resulted in a train of fluid inclusions that records the temperature of the fracturing event (Roedder 1967). Fluid inclusions of all three temporal types were selected for heating and freezing studies, in order to determine the character of hydrothermal fluids, both during and after vein formation. Determination of the temporal nature of fluid inclusions was based on the criteria given by Roedder (1976).

43 32 Compositional Classification of Fluid Inclusions The classification used here was taken from Nash (1976), and is based on phase relationships observable at room temperature. Of the four general types listed by Nash (19 76) as commonly found associated with porphyry copper - type mineralization, three were observed at Sierrita. Type I; Moderate- Salinity, Liquid -rich The most numerous type of inclusion observed in this study consists of two phases: a liquid and a small vapor bubble (10 to 40 vol %). Freezing tests indicate that the liquid is a low to moderately saline NaC1 solution. Rarely, small daughter products of hematite or unknown opaques be present. This type of inclusion homogenizes to the may liquid by contraction and disappearance of the vapor bubble with increasing temperature. Type II: Vapor -rich This type of inclusion, only present in one vein of this study, contains a large vapor bubble (over 55 vol %) and is a liquid phase. Vapor -rich inclusions are commonly believed to form by entrapment of low- salinity steam, or by necking down of Type I inclusions (Roedder 1967). Although attempts at freezing the liquid in such inclusions were unsuccessful, the mode of occurrence, as discussed later, is

44 consistent with the former interpretation, rather than the 33 latter. With increasing temperature, the vapor phase expands to completely fill the inclusion at the homogenization point. Type III: Halite -bearing These locally abundant inclusions contain a cube of halite in addition to a salt- saturated solution and a small vapor bubble. This type of inclusion is the result of trapping a fluid of a higher salinity than 6.1 molal NaC1, which is halite saturation at room temperature. At least one other daughter product is regularly present, but as many as three have been observed. Those daughter products observed in inclusions in this study are listed in Table 2.. Halite- bearing inclusions homogenized by contraction of the vapor bubble and dissolution of the daughter minerals are noted in Table 2. The homogenization point is marked by the disappearance of either halite or the vapor bubble, whichever temperature is higher, but other daughter minerals may remain.

45 34 Table 2. Daughter Products Observed in Fluid Inclusions at Sierrita, with Optical and Physical Properties Mineral Properties Halite (NaC1) Sylvite (KC1) Hematite (Fe2O3) Anhydrite (CaSO4) Unknown opaque(s) Unknown A Colorless; high relief; isotropic; cubic. Colorless; moderate relief; isotropic; cubic, with rounded corners; seen in one inclusion only. Red to orange; high relief; hexagonal to irregular; does not dissolve upon heating. Colorless; high relief; highly birefringent; rectanl gular; occasionally corners become rounded when heated. Opaque; doesn't respond to magnet; too small to establish morphology; does not dissolve upon heating. Colorless; moderate relief; low or no birefringence; no distinctive morphology; seen in one inclusion, only; dissolved at 309 C.

46 Type IV inclusions of Nash (1976) contain a third 35 phase of liquid CO2. No inclusions of this type were observed during this study at Sierrita. Fluid Inclusion Homogenization Data Histograms summarizing primary fluid inclusion homogenization temperatures of each vein studied are shown in Figures 12, 14 (p. 40), 15 (p. 43). Inclusions are shown in three categories according to the phase relationships as discussed above. Type I inclusion homogenization temper - atures are signified by the dotted pattern, vapor -rich inclusions by the V- pattern, and halite -bearing inclusion homogenization temperatures are indicated by the line pattern. In addition, a shaded pattern is used to distinguish inclusions of different mineral assemblages, where evolution of alteration is observed in a single vein. Sample HR -02 Histograms of primary fluid inclusion homogenization temperatures for quartz monzonite sample HR -02 are shown in Figure 12, arranged in accordance with the crosscutting relations (earliest vein at top). Both halite - bearing and moderate- salinity, liquid -rich primary inclusions were observed, indicating that both hypersaline and moderately saline fluids were present during vein filling.

47 Figure 12. Histograms of Primary Fluid Inclusion Homogenization Temperatures (Th) from Sample HR -02. The dotted pattern represents liquid -rich inclusions; the lined pattern represents halite- bearing inclusions (total homogenization); and the shaded pattern in the bottom histogram represents inclusions from selvage quartz (see text).

48 36 EARLY 5- FREQ. VEIN A 13 5 VEIN G 9 5 VEIN C 31 0 T V 5 LATE - Q wi aaaa VEINS D & E 18 Th ( C)- I I I Figure 12. Histograms of Primary Fluid Inclusion Homogenization Temperatures (Th) from Sample HR -02.

49 37 Halite- bearing inclusions occur in all three potassic veins, and to a minor extent, in selvage quartz associated with the quartz + muscovite veining. The homogenization temperatures of fluid inclusions from the potassic veins exhibit considerable overlap, indicating that early potassium feldspar- stable veining occurred at relatively constant temperature. Moderate -salinity, liquid -rich primary inclusions are present in Vein C and in both selvage and vein quartz in the late phyllic veins. In Vein C, two groups of homogenization temperatures of the Type I inclusions are present, one from 290 C to 330 C, and the other from 350 to 380 C. The bottom histogram in Figure 12 consists of primary fluid inclusion filling temperatures from Veins D and E. An important observation is that the range of primary inclusion homogenization temperatures from 190 to 280 C is unique to the quartz + muscovite veining. Primary inclusions were observed in quartz grains in the selvage adjacent to the veins, and in quartz within the vein intergrown with muscovite. Inclusions in the selvage quartz (shaded pattern, Fig. 12) homogenized between 240 and 280 C, while those in the veins homogenized between 190 and 240 C. An apparently pre -vein quartz crystal is cut by healed and unhealed fractures paralleling Vein D (Fig. 13A). The intense fracturing and the sulfide grains in the middle

50 Figure 13. Distribution of Secondary Fluid Inclusion Homogenization Temperatures in Quartz Grain Cut by Vein D in Sample HR-02. A. Camera lucida of fractured quartz grain and sulfide grains (shaded pattern) showing homogenization temperature ranges of inclusions found in the center and near the edges of the quartz grain. B. Histogram of secondary inclusion homogenization temperatures (Th) from quartz grain.--the dotted pattern represents liquid-rich inclusions; and the lined pattern represents halite-bearing inclusions.

51 38 quartz grain A. Center of Vein D Center of Vein D 1mm B. 29 inclusions Th (*C)- Figure 13. Distribution of Secondary Fluid Inclusion Homogenization Temperatures in Quartz Grain Cut by Vein D in Sample HR -02.

52 39 of the quartz grain are coincident with the center of the vein. A fine- grained selvage of quartz + muscovite + pyrite surrounds and partially embays the quartz grain. Figure 13B presents a histogram of homogenization temperatures of secondary inclusions found in the quartz crystal. The higher temperature group from 200 to 260 C is from inclusions located near the edge of the phenocryst, while the inclusions homogenizing around 140 to 150 C are in the highly fractured areas in the center of the quartz grain (Fig. 13A). The distribution of secondary inclusions in the quartz phenocryst is compatible with the interpretation that those homogenizing between 140 and 150 C represent the last fluids flowing through Vein D, presumably responsible for deposition of the sulfides observed filling the center of the vein. Sample HR -0l As noted in the previous chapter, Vein HR -01 consists of two vein assemblages (Fig. 3). Primary inclusions from cloudy quartz from the early assemblage and from later clear quartz intergrown with sulfides were studied; the results of homogenization tests are summarized in Figure 14A. All inclusions are moderate salinity, liquid -rich, and those associated with the earlier assemblage are shaded. Early vein filling occurred between 330 and 410 C. Later sulfide

53 40 A. HR inclusions FREQ B. BQD inclusions 0 V. :: :: :,....;..: r:13.` V V" Th ( C) Figure 14. Histogram of Primary Fluid Inclusion Homogenization Temperatures (Th) from Samples HR -01 and BQD The dotted pattern represents liquid -rich inclusions; the V- pattern represents vapor -rich inclusions; and the shaded pattern represents inclusions found in the earlier assemblages (see text).

54 41 deposition persisted down to 290 C, with a slight overlap in temperatures between the early and late alteration assemblages. Sample BQD -03 In a similar fashion to the paragenesis of Sample HR -01, two mineral assemblages were observed in the petrographic study of the single vein. Primary fluid inclusions from the early cloudy quartz and from clear quartz and epidote associated with later sulfide deposition are plotted in Figure 14B. Primary inclusions associated with the early assemblage (shaded pattern, Fig. 14B) include both Type I (dotted pattern, Fig. 14B) and vapor -rich (V- pattern, Fig. 14B) inclusions. Vapor -rich inclusions were uncommon in Vein BQD -03, appearing in groups of two or three randomly distributed inclusions, or in planes of secondary or pseudosecondary inclusions. Although, only four were suitable for homogenization, they appeared to be characteristic of those observed. In one case, two vapor -rich and an adjacent liquid -rich inclusion homogenized in the temperature range of 418 to 427 C. These indicate that the vapor -rich inclusions are a manifestation of boiling of moderately saline fluids during early quartz deposition. Deposition of the early assemblage continued with cooling to 350 C, as

55 42 evidenced by four inclusions that homogenized at temperatures from 350 C to 410 C (Fig. 14B). Homogenization temperatures of primary inclusions in later clear quartz and epidote vary between 310 and 350 C. Only moderate -salinity, liquid -rich inclusions are associated with the later mineralizing event. Sample BQD -01 The rather complex paragenesis of this sample (Fig. 6) may be generalized into an early and a late assemblage for each of the five veins. Figure 15 portrays histograms of homogenization temperatures of primary fluid inclusions from each of the veins, with the earliest vein at the top. The inclusions associated with each of the early vein assemblages is in the shaded pattern. With the exception of halite -bearing inclusions present in the early assemblage of the earliest vein (Vein E), fluid inclusions are moderate -salinity, liquid -rich. Homogenization temperatures generally decrease with time both within a particular vein, and from older to younger veins. Exceptions to this cooling trend are apparent in the earliest vein (Vein E), and in the transition from the late assemblage of a particular vein to the early assemblage of the subsequent vein.

56 43 FREQ. 5 o VEIN E 7 EARLY 5- VEIN A tj ::..T. e t i Si VEIN C 17 VEIN D o Th( C)- LATE Figure 15. Histograms of Primary Fluid Inclusion Homogenization Temperatures (Tb) from Sample BQD The dotted pattern represents liquid -rich inclusion; the lined pattern represents halite- bearing inclusion; and the shaded pattern represents inclusions found in the early assemblage of each vein (see text).

57 44 Secondary Inclusions Homogenization temperatures of secondary fluid inclusions from veins in the four samples are shown in Figure 16. Each data point in the histograms consists of a representative temperature of a plane of inclusions, determined from the most common homogenization temperature of each plane. Locally, individual planes of secondary inclusions could not be distinguished due to large amounts of inclusions. In those cases, the reliability of the temperatures obtained is not as good as those obtained from a recognizable plane, due to common -place necking -down of secondary inclusions (Roedder 1967). Three features are shown in Figure 16. First, the range of homogenization temperatures is similar in each sample, from ti 130 C to ti 370 C. Also three peaks consistently appear in each histogram: at C, at C, and at C. Finally, only Sample HR -02 contains secondary halite- bearing inclusions. Salinities Observations of fluid inclusions at room temperature attest to the presence of at least two compositionally distinct fluids during vein formation at Sierrita. Halite - bearing inclusions are present only during early potassic veining. Lower salinity fluids, evidenced by Type I

58 FREQ. HR BQD BQD Th ec} Figure 16. Comparison among Secondary Fluid Inclusion Homogenization Temperatures (Th) from Each Sample Studied. - -The dotted pattern represents liquid -rich inclusions; and the lined pattern represents halite- bearing inclusions.

59 46 inclusions, were also present, although at a later time. Figure 17 presents measured salinities of 234 primary and secondary inclusions plotted against homogenization temperature. Salinity is given in moles NaC1 equivalent per 1000 grams H2O (molality). As sylvite was seen as a daughter product in only one inclusion, the total salinity of the remainder of the halite -bearing inclusions may only be approximated from halite solubility, due to an unknown KC1 content. The salinities of these inclusions are more closely approximated by expressing the salinity in terms of molality rather than weight percent salt. In addition, the freezing point depression of NaC1 + KCI aqueous solutions are nearly identical to those of NaC1 solutions of equiva -. lent total molality. The procedure used to determine the salinity of an inclusion depends on the phases present. Salinities of halite- bearing inclusions were determined by heating the inclusion and noting the temperature at which the salt dissolved. The salinity was approximated by comparing this temperature with the NaCl solubility data compiled by Potter, Babcock, and Brown (1977). Freezing point depression determinations were carried out on selected Type I inclusions, employing the procedure outlined in Appendix C. The salinities were calculated from equations regressed on experimentally

60 47 I I 1 X X X XX X X X X. I O 00 o 00 O o N O O o 0 v 8 0 d" o 0 0 $ o Ir) o Ó N do O ß0 á % ocaso pp,, o o Z o G O 0 o o Sao DI C" 4 o 0 o0 o o I- 4 o 00 o o 00 Z.3 o v Z I II. I I N O lfl co tt (ninbe DDNw) JIlINI1dS N o O

61 48 derived freezing point depression curves for aqueous NaC1 solutions (Potter, Clynne, and Brown 1978). The linear distribution of most halite- bearing inclusions in Figure 17 is the result of the salt dissolving at a higher temperature than the vapor bubble. As a result of defining the homogenization temperature as the temperature of the disappearance of the higher- temperature phase, and of defining the salinity by the dissolution temperature of halite, the inclusions must plot along the NaCl -H2O saturation curve. As experimental solubility data in the NaC1- H2O system is limited to vapor- saturated determinations (Potter and Brown 1975), the salinities defined in this manner are approximate values only. Unknown pressure dependency, if any, on NaC1 solubility cannot be taken into account. Liquid -rich inclusions undersaturated with respect to NaC1 at room temperature vary considerably in salinity, from 0.25 to 4.7 molal NaC1 equivalent. The great majority of inclusions, however, cluster between 1 to 3 molal NaC1 equivalent, irregardless of the homogenization temperature. As seen in Figure 18, two basis populations of fluid salinities are obvious. The pronounced bimodal nature of the salinity distribution is best explained by a discrete change, rather than a continuous evolution, of fluid salinity.

62 FREQ inclusions IO SALINITY (m NacI equiv) Figure 18. Histogram of Measured Fluid Inclusion Salinities at Sierrita. -- The dark pattern represents primary inclusion; and the lined pattern represents secondary inclusions.

63 50 Composition of Halite- bearing Inclusions As indicated above, halite (with rare exceptions) dissolved at a higher temperature than the disappearance of the vapor bubble. The persistence of daughter products at temperatures higher than the liquid -vapor homogenization point has been ascribed to four mechanisms by Roedder (1972). 1. Equilibrium was not attained during the heating run. 2. The mineral was a solid inclusion, trapped accidently by the inclusion. 3. The inclusion has necked down, trapping the daughter product in a smaller inclusion, thus increasing the salt - liquid ratio. 4. The dissolution temperature is the actual temperature of halite saturation. The first three explanations can be dismissed for the inclusions included as data points. In the first case, halite equilibrates with the solution rapidly (Roedder 1972), and several inclusions were heated several times at different rates with no change in the salt -dissolution temperature. In the case of the second and third explanations, trapped solid inclusions and necked -down inclusions would exhibit diverse phase ratios and resultant homogenization temperatures. The consistency of both temperatures of vapor

64 51 disappearance and halite dissolution, as seen in Figure 19, argue against these explanations. Conversely, hematite probably does occur as an accidently trapped mineral in a number of the halite- bearing inclusions. A diversity in the volume proportion of hematite compared to the vapor bubble and salt crystal was observed. In a few cases, the hematite was seen to apparently cross the inclusion walls. Hematite flakes appear as precipitated phases within quartz, and some may have trapped fluid inclusions as the host quartz grew around the hematite (Roedder 1967; 1972). Not all hematite was trapped, however, as trains of secondary inclusions in quartz phenocrysts were seen to contain hematite flakes in consistent phase proportions. This is diagnostic of a true daughter mineral (Roedder 1972). Although the gross salinity of halite -bearing fluid inclusions exhibit a relatively continous range from 8 to 13 molal NaCl equivalent, the distribution of additional daughter products (Table 2) varies with respect both salinity and time. Figure 20 presents the occurrence of daughter products as a function of salinity. Inclusions with halite only, halite + hematite, and halite + opaque(s) in the salinity range of 10.5 to 13 molal NaCl equivalent are found as primary inclusions in Veins A, G, and C in Sample HR -02, and in Vein E, Sample BQD -01. Those in the range of