The Laramide Caborca Orogenic Gold Belt of Northwestern Sonora, Mexico: White Mica 40Ar/ 39Ar Geochronology from Gold-Rich Quartz Veins

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Open-File Report 2016–1008

U.S. Department of the Interior U.S. Geological Survey

The Laramide Caborca Orogenic Gold Belt of Northwestern Sonora, Mexico: White Mica 40Ar/ 39Ar Geochronology from Gold- Rich Quartz Veins

By Aldo Izaguirre, Michael J. Kunk, Alexander Iriondo, Ryan McAleer, Juan Antonio Caballero-Martínez, and Enrique Espinosa-Arámburu

Open-File Report 2016–1008

U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior SALLY JEWELL, Secretary U.S. Geological Survey Suzette M. Kimball, Director

U.S. Geological Survey, Reston, Virginia: 2016

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Suggested citation: Izaguirre, Aldo, Kunk, M.J., Iriondo, Alexander, McAleer, Ryan, Caballero-Martínez, J.A, and Espinosa Arámburu, Enrique, 2016, The Laramide Caborca orogenic gold belt of northwestern Sonora, Mexico; white mica 40Ar/39Ar geochronology from gold-rich quartz veins: U.S. Geological Survey Open-File Report 2016–1008, 30 p., http://dx.doi.org/10.3133/ofr20161008.

ISSN 2331-1258 (online) iii

Contents

1. Introduction...... 1 2. Methods...... 2 2.1. Sample Preparation...... 2 2.2. Packing and Irradiation Method...... 2 2.3. Sample Analysis...... 2 2.4. Isotopic Data Reduction...... 3 3. Results of 40Ar/39Ar Data...... 3 3.1. Group 1 Plateau Ages...... 3 3.2. Group 2 Isochron Ages...... 3 3.3. Group 3 Average or Single-Step Ages...... 3 3.4. Histogram and Cumulative Probability Diagram of Ages...... 4 4. Conclusions...... 4 5. Acknowledgments...... 4 6. References Cited...... 5

Figures

1. Sample locations for dated white micas of gold-rich quartz veins...... 7 2. Histogram and cumulative probability distribution diagram of 40Ar/39Ar ages determined from hydrothermal white mica samples of gold-rich quartz veins from the Caborca orogenic gold belt (COGB), northwestern Sonora, Mexico...... 8 3. Age spectra and inverse-isotope correlation diagrams from group 1 of white mica samples of gold-rich quartz veins, with plateau ages as best age, from the Caborca orogenic gold belt (COGB), northwestern Sonora, Mexico...... 9 4. Age spectra and inverse-isotope correlation diagrams from group 2 of white mica samples of gold-rich quartz veins, with isochron ages as best age, from the Caborca orogenic gold belt (COGB), northwestern Sonora, Mexico...... 16 5. Age spectra and inverse-isotope correlation diagrams from group 3 of white mica samples of gold-rich quartz veins, with average and (or) single-step ages as best age, from the Caborca orogenic gold belt (COGB), northwestern Sonora, Mexico...... 22 iv

Tables

[Available as separate files for download in Excel format from http://dx.doi.org/10.3133/ ofr20161008.]

1. Summary of 63 40Ar/39Ar ages determined from white micas of gold-rich quartz veins of the Caborca orogenic gold belt (COGB), northwestern Sonora, Mexico. 2. 40Ar/39Ar step-heating data and plateau ages determined from white micas of gold-rich quartz veins of the Caborca orogenic gold belt (COGB), northwestern Sonora, Mexico. 3. 40Ar/39Ar step-heating data and isochron ages determined from white micas of gold- rich quartz veins of the Caborca orogenic gold belt (COGB), northwestern Sonora, Mexico. 4. 40Ar/39Ar step-heating data and average or single step ages determined from white micas of gold-rich quartz veins of the Caborca orogenic gold belt (COGB), northwestern Sonora, Mexico.

Conversion Factors

Multiply By To obtain Length kilometer (km) 0.6214 mile (mi)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows: °F=(1.8×°C)+32 The Laramide Caborca Orogenic Gold Belt of Northwestern Sonora, Mexico: White Mica 40Ar/ 39Ar Geochronology from Gold-Rich Quartz Veins

By Aldo Izaguirre,1,2 Michael J. Kunk,3 Alexander Iriondo,2,4 Ryan McAleer,3 Juan Antonio Caballero-Martínez,5 and Enrique Espinosa-Arámburu5

1. Introduction

This report contains reduced 40Ar/39Ar geochronological data from hydrothermal white mica (63 samples) separated from orogenic quartz gold-rich veins in the Laramide Caborca orogenic gold belt (COGB) of northwestern Sonora, Mexico (fig. 1). The main objective of this report is to present the sample locations, 40Ar/39Ar experimental methodology, and 40Ar/39Ar isotopic data (tables 1–4). We also include age spectra and inverse-isotope correlation diagrams for all white mica samples (figs. 3–5). The age spectra are separated into three groups based on the type of age used for geologic interpretation, including (1) plateau ages, (2) isochron ages, and (3) average or single-step heating ages. These age spectra are interpreted to represent the time of mineralization for each sample locality, and together, all the ages help to establish the age of mineralization for the entire COGB (fig. 2). Another objective of this Open-File Report (OFR) is to organize the data in a systemic way so that they can be integrated into future scientific publications. The COGB is approximately 600 kilometers (km) long and 60 to 80 km wide, trends northwest, and extends from west-central Sonora to southern Arizona and California (fig. 1). The COGB contains mineralized gold-rich quartz veins that contain free gold associated with white mica “sericite” and carbonate minerals (calcite and ankerite) plus sulfides such as pyrite and galena (Izaguirre and others, 2012). Limited geochronologic studies (K-Ar, Ar-Ar (white mica), and Re-Os (magnetite)) exist for parts of the COGB, and previous work was concentrated in mining districts (Pérez-Segura, 1993; Pérez-Segura and others, 1996; Araux-Sánchez, 2000; Iriondo and Atkinson, 2000; Iriondo, 2001; Poulsen and others, 2008; Quintanar-Ruíz, 2008). These scattered studies recorded mineralization ages of approximately 70 to 40 Ma (mega-annum, or million years ago). Therefore, some workers proposed that the orogenic gold mineralization in the region occurred during a single pulse that was associated with the Laramide Orogeny that took place during Cretaceous to early Eocene in the western margin of North America (Damon and others, 1964; Coney, 1976; Dickinson and others, 1988). However, the geochronologic dataset was quite limited, making any regional interpretations tenuous. Accordingly, one of the objectives of this geochronology study was to get a better representative sampling of the COGB in order to obtain a more complete record of the mineralization history. The 63 samples presented in this work are broadly distributed throughout the area of the COGB (fig. 1) and allow us to better test the hypothesis that mineralization occurred in a single pulse.

1U.S. Geological Survey, Mail Stop 973, Denver Federal Center, Denver, CO 80225 2Centro de Geociencias, Universidad Nacional Autónoma de Mexico, Campus Juriquilla, Boulevard Juriquilla 3001, Juriquilla, 76230, Querétaro, Mexico 3U.S. Geological Survey, Mail Stop 926A, National Center, Reston, VA 20192 4Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, TX 78712 5Servicio Geológico Mexicano, Boulevard Felipe Ángeles, km 93.50-4, Colonia Venta Prieta, 42080, Hidalgo, Mexico 2 The Caborca Orogenic Gold Belt of Sonora, Mexico: Mica 40Ar/39Ar Geochronology from Gold-Rich Veins 2. Methods

2.1. Sample Preparation

All of the quartz vein samples were crushed, ground, and sized using 250-, 180-, and 150-micrometer (μm) sieves. White mica separations were performed using a magnetic separation instrument (Frantz™), paper shaking, and hand picking with a goal of getting greater than 99 percent purity on the white mica. The resulting separates were washed three times in acetone, alcohol, and deionized water in an ultrasonic cleaner (Branson™) to remove dust and other impurities, and then re-sieved using 250- and 180-μm sieve sizes.

2.2. Packing and Irradiation Method

The samples were irradiated in five separate irradiation packages (DD60, KD12, KD52, KD53, and KD55).Two copper packets were loaded for each white mica sample, with weights of approximately 1 milligram (mg) and 20 mg, respectively. The packets were sealed under vacuum in fused silica tubes and irradiated in the central thimble facility at the Training Reactor Iso- topes General Atomics (TRIGA) reactor at the U.S. Geological Survey in Denver, Colorado. The monitor mineral used in all the irradiation packages was hornblende (sample MMhb-1) with an age of 519.4±2.5 Ma (Alexander and others, 1978; Dalrymple and others, 1981). The type of container, and the geometry of samples and standards are similar to those described by Snee and others (1988).

2.3. Sample Analysis

White mica samples were analyzed in the U.S. Geological Survey Argon Thermochronology Laboratory in Reston, Vir- ginia. The large white mica samples (approximately 20 mg) were analyzed in a VG Isotopes, Ltd. model 1200 mass spectrometer fitted with an electron multiplier using the40 Ar/39Ar step-heating method. All samples were heated for 10 minutes per step (11‒20 steps) in a small-volume, molybdenum-lined, low-blank tantalum resistance furnace similar to that described by Stau- dacher and others (1978). The temperature was monitored by a W5Re-W26Re thermocouple and controlled by a proportional, programmable controller. The furnace and the rear manifold were pumped between steps with a turbo molecular pump. Two isolated ion pumps evacuated the front manifold and the mass spectrometer flight-tube between each incremental step. Prior to analysis in the mass spectrometer, the gas was purified in the rear manifold by a SAES ST707 Zr-V-Fe (zirconium-vanadium- iron) getter operated at room temperature and a hot Re (rhenium) filament. Gas was equilibrated with the front manifold, then isolated and cleaned in the front manifold with a SAES ST101 Al-Zr (aluminum-zirconium) getter operated at 400°C and a Ti (titanium) getter operated at 350°C. An activated charcoal finger submerged in a thermally equilibrated mixture of dry ice and acetone in the front manifold was used to remove gasses with a molecular weight greater than 60 or 80 (primarily other noble gasses) prior to the expansion of the Ar (argon) dominated gas into the mass spectrometer. The gas was further purified in the mass spectrometer by a second SAES

ST101 getter operated at room temperature. Ar isotopes with atomic weights of 40 through 36 and CO2 (atomic weight of 44) were analyzed as a function of time in five or six analytical cycles.40 Ar, 39Ar, 38Ar, and 37Ar peaks and their baselines were measured for five, 1.28-second integrations in each of the five cycles.36 Ar and its baselines were measured for twenty, 1.28-second integrations in each cycle. After the analysis, the mass spectrometer was evacuated with an isolated ion pump. All phases of the sample heating, cleanup, equilibration, and analysis were performed under computer automation. Smaller white mica samples (weights of approximately 1 mg) were analyzed using a MAP-216 mass spectrometer fitted with an electron multiplier using the 40Ar/39Ar step-heating dating method. Heating for 10 minutes per step followed a schedule of 11 to 20 steps. The heating schedules were designed such that the percentage of 39Ar released per step was limited to less than 20 percent of the total released for most samples. The samples were heated in the same manner and in the same type of furnace as was described earlier. The furnace and the rear and front manifolds were pumped between steps with a turbo molecular pump. An isolated ion pump was used to pump the mass spectrometer. Prior to the mass spectrometer analysis, the gas was purified in the manifold by two SAES ST101 Al-Zr getters, one operated at room temperature and a second one using a hot Re filament at 400°C. The Ar-rich gas was further purified by a third SAES ST101 getter operated at room temperature in the flight tube of the mass spectrometer. Ar isotopes with atomic weights of 40 through 36 were analyzed as a function of time in six analytical cycles. Baselines were measured for 39Ar and 36Ar. The 36Ar baselines were subtracted from the 40Ar, 38Ar, 37Ar, and 36Ar peaks. The 39Ar baseline was subtracted from the 39Ar peak to reduce the influence of the tail of the40 Ar peak. 3. Results of 40Ar/39Ar Data 3 2.4. Isotopic Data Reduction

All the isotopic data produced on the VG-1200 spectrometer were reduced using an updated version of the computer program ArAr* (Haugerud and Kunk, 1988). Isotopic data from the MAP-216 spectrometer was reduced using the computer program Mass Spec (Deino, 2001). We used the decay constants recommended by Steiger and Jäger (1977). The isotopic measure- ments made in the five- or six-cycle analysis were regressed to time zero (2/3 of the inlet time) using standard linear regression techniques. These regressed peak values, and associated analytical uncertainties, were used in data reduction. For the VG-1200 and the MAP-216, full system blanks were measured prior to the suite of analyses made on each sample and then subtracted from the analytical results. Error estimates of the blanks were quadratically combined with the analytical errors. Corrections for interfering reactor-produced Ar isotopes from Ca (calcium), K (potassium), and Cl (chlorine) in the sample were made using the production ratios given in Dalrymple and others (1981) and Roddick (1983). Errors in calculated ages or ratios include the measurement uncertainty in the analysis, decay factor uncertainties, uncertainties in measured atmospheric 40Ar/36Ar ratios, the irradiation parameter J, the production ratios of the various reactor induced Ar producing reactions, the initial 38Ar/36Ar ratio, and the age of the monitor mineral (Haugerud and Kunk, 1988). The data in tables 2, 3, and 4 include individual step ages and total gas ages. Total gas ages represent the age calculated from the addition of all the measured Ar peaks for all steps in a single sample. The total gas ages are roughly equivalent to con- ventional K/Ar ages; however, no analytical precision is calculated for these total gas ages.

3. Results of 40Ar/39Ar Data

White mica 40Ar/39Ar geochronological results of 63 samples presented in this report (tables 1–4 and figs. 3–5) are sub- divided into the following three age groups: plateau ages (group 1), isochron ages (group 2), and average or single-step ages (group 3). The reported ages are plotted in figure 1, and improve our knowledge of the timing of hydrothermal gold-rich quartz vein formation in the COGB.

3.1. Group 1 Plateau Ages

Group 1 is made up of 21 samples (tables 1 and 2; fig. 3) of coarse flakes of white mica separated to an optical purity of greater than 99 percent. The data from step-heating experiments on these samples meet the criteria for a plateau age defined by Fleck and others (1977) and modified by Haugerud and Kunk (1988).All of the white mica samples in this group released argon 39 ArK in three steps or more (accounting for a minimum of 50 percent of the total released gas) at furnace temperatures between approximately 800 to 1250°C (sample Quitovac-5; fig. 3U). In addition, isochron ages calculated for these samples, shown in inverse isotope diagrams of 36Ar/40Ar vs. 39Ar/40Ar (sample Quitovac-5; fig. 3V), support within analytical error, the more precise plateau ages (summarized in table 1). Therefore, these white mica plateau ages that range from approximately 66 to 48 Ma are interpreted to be the best approximation for the age of hydrothermal activity that resulted in this group of gold-rich quartz veins.

3.2. Group 2 Isochron Ages

Group 2 is made up of 16 samples (tables 1 and 3; fig. 4). White mica from these samples includes both coarse flakes and fine-grained aggregates, separated to an optical purity of greater than 99 percent.The flat age spectra from step-heating experiments on these samples did not meet the statistical definition of a plateau (Fleck and others; 1977). For these slightly irregular age spectra (sample Trin-10; fig. 4AC), inverse isotope correlation diagrams of36 Ar/40Ar vs. 39Ar/40Ar were generated and isochron ages calculated. A regression (and the resulting age) is considered meaningful if it includes more than three contiguous 39 heating steps that accounts for at least 50 percent of the total ArK released and has a MSWD (mean square weighted deviation) of ≤ 2.5.The age range determined for this group of white micas is approximately 62 to 36 Ma, and interpreted as the age inter- val for hydrothermal mineralization along the COGB.

3.3. Group 3 Average or Single-Step Ages

Group 3 is made up of 26 samples of white mica (tables 1 and 4; fig. 5) that consist of mostly fine-grained aggregates of white mica. These samples were prepared to a variety of purity levels (in a few instances less than 99 percent purity). It is possible that many of the impurities in the white mica separates included quartz, plagioclase, K-feldspar, or even igneous or meta- morphic white micas from the quartz vein host rocks. These impurities made the 40Ar/39Ar data results more difficult to interpret. 4 The Caborca Orogenic Gold Belt of Sonora, Mexico: Mica 40Ar/39Ar Geochronology from Gold-Rich Veins

The data for group 3 white mica samples did not meet the statistical criteria for a plateau or isochron age. However, for a group of 15 of these samples we calculated average ages for a set of contiguous steps (selected steps provided in figure 5).The selected steps define a flat portion of each age (spectrum similar to a plateau), but failed to meet one or more of the requirements established by Fleck and others (1977). This behavior is well represented in white mica sample Pit-4 (fig. 5AI) where 6 steps, 39 representing 87 percent of the total ArK released, were used to calculate the weighted-average age. The resulting weighted- average ages are very similar to the calculated total gas ages for the same samples. This group of 15 white micas yielded average ages in a range between approximately 65 to 39 Ma, consistent with the ages determined from groups 1 and 2. For the remaining 11 samples from group 3 the age of a single-step was used as the preferred age (ages summarized in table 1). The resulting age spectra from these 11 samples are difficult to interpret meaningfully from the step heating results, but it is important to illustrate that not all the 40Ar/39Ar data for these gold-rich quartz veins is considered “well behaved” or “ideal,” which would generally yield a flat spectrum. A good example of a “non-ideal” staircase age spectrum is shown in figure 5S (sample El Chanate) in which step C is selected as the assigned age of the sample. The single-step ages of the 11 samples are chosen by selecting the first heating step of the staircase age spectrum in which we observe a tendency of the spectrum to flatten out. The selected single-step age is interpreted as representing the maximum possible age for the formation of the white mica. Group 3 average and single-step ages range from approximately 69 to 39 Ma. The resulting age spectrum data from group 3 are more complex than groups 1 and 2, however, the determined ages are consistent with the more reliable ages of groups 1 and 2.

3.4. Histogram and Cumulative Probability Diagram of Ages

In order to evaluate and interpret the resulting 40Ar/39Ar data of the 63 white mica samples, we generated a histogram and a cumulative probability diagram (fig. 2) of all the determined ages.The diagram shows the age range as well as apparent ages of more abundant gold mineralization along the COGB. The Late Cretaceous to late Eocene age range (69 to 36 Ma) for the quartz veins represents a mineralization event that lasted for more than 30 million years. However, in the histogram a probability peak at approximately 61 Ma is interpreted as the climax of mineralization. The climax of mineralization at 61 Ma occurred within the main pulse of mineralization between approximately 63 to 56 Ma (age range of the majority of analyzed white mica samples; 35 white micas and approximately 56 percent of the total samples). After approximately 48 Ma, only 5 scattered ages of mineralization are identified in figure 2. The youngest age is 36 Ma and represents the youngest mineralization age determined for the gold veins along the COGB.

4. Conclusions

Organization of this large 40Ar/39Ar dataset into three age groups has facilitated the interpretation of the timing of mineralization of gold-rich quartz veins in the COGB (table 1). The data presented in this report will serve as a data repository to facilitate the integration of the age data in future manuscripts for publication in scientific journals, where space restrictions are common. We conclude that the ages between approximately 69 to 36 Ma (figs. 1 and 2), determined from the 63 white micas, represent the timing of orogenic gold mineralization in northwestern Sonora, Mexico, supporting previous interpretations that these gold-rich quartz veins are restricted to the Late Cretaceous to Eocene time. The vast majority of the sample ages (>56 percent) were determined to be between approximately 63 to 56 Ma, which we interpret as the climax of gold mineralization along the COGB. Moreover, the frequency of mineralization gradually decreased from approximately 56 to 48 Ma, followed by a final stage of vein formation that occurred between approximately 45 to 36 Ma. The end of mineralization most likely relates to the contemporaneous cessation of the Laramide orogeny along this portion of the North American Cordillera.

5. Acknowledgments

We acknowledge Consejo Nacional de Ciencia y Tecnología (CONACyT) for providing a doctoral scholarship to Aldo Izaguirre. Also we are grateful to CONACyT for supporting scientific projects CB-82518 and CB-129370 and for financing part of this work. Furthermore, we would like to thank Universidad Nacional Autónoma de Mexico grant PAPIIT IN-116709. We are thankful to the Servicio Geológico Mexicano (SGM) Gerencia Hermosillo, Sonora, for help in the logistics to undertake fieldwork, and in particular, we would like to thank Javier Hernández-Rojas (former SGM geologist) for his helpful companion- ship in the field while Aldo Izaguirre collected the samples of gold-rich quartz veins. Thanks to Bethany Stackhouse and Wright Horton Jr., for reviewing and improving this manuscript. 6. References Cited 5 6. References Cited

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114° 113° 112° 111°

Phoenix Tempe Sample number and location North C o l o r a d o 1. Alamo-733°Mesa 27. Lluvia de Oro 3b 51.Globe Sargenta R i v e r 2. American Boy 28. Noche Buena Casa 52. SE Sonoyta California 3. Ana Celia 29. Noche Buena 53. Sonoyta-1 4. Betty Lee Mine Workings 54. SVE-1 5. Cantera 30. Pinor-1 55. Taj-3 33° 6. Carretera-1 31. Pinta-1a 56. Tajitos 2 33° 7. Carretera-2 32. Pinta-2 57. Tres Mojones 2 Gila Bend Florence 8. Celaya-1 33. Pit-3 58. Trin-10 51 Casa9. Centauro-3Grande 34. Pit-4 59. Trin-11 10. Cerro Colorado 35. Prieta-1 60. Vidrios-1 11. Chip Mine 36. Prieta-4 61. Vidrios-2 12. Costa-1 37. Prieta-5 62. Yaqui-3 Yuma 68 13. Costa-3 38. Quitovac-2 63. 10 de Mayo ? 14. Costa-4 39. Quitovac-3 ? ? 65 66 48 15. Cubabi 40. Quitovac-4 44 Age queried (?) for samples 4 25 16. Dolores 41. Quitovac-5 in process to determine San Luis Ajo 17. El Capitán Mine 42. Quitovac-6 40 39 Río Colorado Ar/ Ar ages 57 18. El Chanate 43. SanFran 4d 11 19. El Tiro 44. Sanfran-1 64. Bell Mine 17 20. Estación-1 45. Sanfran-2 65. Copper Mine 67 51 64 Tucson 32° ? 21. Jerónimo 46. Sanfran-3 66. Fortuna Mine 32° 56 60 ? 22. La Colorada 47. Sanfran-4 67. Rasmussen Mine 58 23. La Esperanza 48. Sanfran-6 68. Welton Mine 61 53 24.Sells La Herradura 49. Sanfran-7 23 Lukeville 25. La Pinta USA Mine 50. Sanluisito-2 Sonoyta 57 57 52 26. LH08-1 51 59 53 21 59 44 3 50 8 60 62 49 15 48 61 57 47 42 Arizona 30 46 60 62 50 41 69 50 54 32 60 62 65 62 57 44 40 z e t r Co O f a e S 5 64 57 Sonora 53 45 59 38 Nogales 16 58 31 35 60 39 63 50 Puerto Peñasco 59 61 54 Nogales (Rocky Point) 36 60 62 48 37 61 62 26 31° San Felipe 62 9 56 55 31° 24 61 53 53 1 61 29 56 Cananea 59 28 36 18 13 56 48 52 27 Baja 12 Caborca Imuris California 14 61 Altar Magdalena Explanation Santa Ana Area of the Caborca orogenic 19 55 43 39 gold belt (COGB) Sample location number 56 58 within circle. Age of 10 20 60 58 42 7 sample in millions 34 Pre-COGB bedrock of years (Ma) in bold 59 57 Post-COGB bedrock 65 Benjamin Hill 66 33 30° 66 48 Confirmed locations of orogenic 52 6 30° quartz-carbonate veins Puerto Libertad C o l o r a d o ? ? R i v e r 100 km

USA Phoenix México Ures ? Mexicali Canada COGB z e t Cr o f o a e S USA

Pa México Isla del Tiburón United States n a Oe c c c i f i 29° Hermosillo ? 29° Map Hermosillo México 29° Area 22 ? 50 km North America Bahía Kino 65 114° 113° 112° 111° World Geodetic System (WGS) 1984 Datum

Figure 1. Sample locations for dated white micas of gold-rich quartz veins. Numbers in bold represent 40Ar/39Ar ages determined in this open-file report from the Caborca orogenic gold belt (COGB), northwestern Sonora, Mexico. Shades of gray represent pre-COGB mineralization (dark-gray) and post-COGB mineralization (light-gray) bedrock outcrops after Iriondo (2001). 8 The Caborca Orogenic Gold Belt of Sonora, Mexico: Mica 40Ar/39Ar Geochronology from Gold-Rich Veins

8 40Ar/39Ar geochronology of hydrothermal n = 63 white mica samples representing the mineralization of the COGB

Age probability curve 4

Number of Samples Number 2

0 35 40 45 50 55 60 65 70 Mineralization Age (Ma)

Figure 2. Histogram and cumulative probability distribution diagram (blue line) of 40Ar/39Ar ages determined from hydrothermal white mica samples of gold-rich quartz veins from the Caborca orogenic gold belt (COGB), northwestern Sonora, Mexico. Abbreviations: Ma, mega-annum; n, number of samples. 9

80 A) * B) 0.003 B 70 Betty Lee Mine Betty Lee Mine white mica flakes white mica flakes from quartz vein O from quartz vein 60 Plateau 0.002

47.9 ± 0.2 Ma Ar C 50 N 40 L H I J K M-O Ar/ M D 40 B-G 36 Plateau Age = 0.001 Apparent Age (Ma) Isochron Age = 47.8 ± 0.3 Ma 40 36 E 47.9 ± 0.2 Ma [ Ar/ Ar]i = 307 ± 18 F 30 39 69 % of ArK from steps H to K MSWD = 1.4 G 39 69 % of ArK from steps H to K H I-J 20 0 L 0 20 40 60 80 100 0 0.05 0.10 0.15 0.20 0.25 39 39 40 Cumulative % ArK Released Ar/ Ar 80 C) * D) K-O Cantera 0.003 A 70 Plateau B-D white mica flakes 64.3 ± 0.4 Ma from quartz vein E F G H I J 60 Cantera 0.002

50 white mica flakes Ar B from quartz vein 40 Ar/

40 36 Plateau Age = 0.001 Apparent Age (Ma) Isochron Age = 63.6 ± 1.1 Ma 40 36 64.3 ± 0.4 Ma [ Ar/ Ar]i = 422 ± 76 D C 39 E 30 88 % of ArK from steps F to J MSWD = 0.3 39 M-O F-L 92 % of ArK from steps F to O 20 0 0 20 40 60 80 100 0 0.05 0.10 0.15 0.20 0.25 39 39 40 Cumulative % ArK Released Ar/ Ar 120 E) G F) Celaya-1 Celaya-1 white mica flakes 0.002 white mica flakes from quartz vein from quartz vein 100 H

Plateau Age = F G 80 60.5 ± 0.2 Ma Ar 61 % of 39Ar from steps J to L Q 40 K 0.001

P Ar/ Q

Plateau 36 O H 60.5 ± 0.2 Ma

Apparent Age (Ma) I 60 I M J K L N Isochron Age = 60.3 ± 0.2 Ma [40Ar/36Ar]i = 329 ± 2 MSWD = 2.7 N 39 P O 67 % of ArK from steps H to L JK M 40 0 L 0 20 40 60 80 100 0 0.015 0.030 0.045 0.060 0.075 39 39 40 Cumulative % ArK Released Ar/ Ar

Figure 3. Age spectra (A, C, E) and inverse-isotope correlation diagrams (B, D, F) from group 1 of white mica samples of gold-rich quartz veins, with plateau ages as best age, from the Caborca orogenic gold belt (COGB), northwestern Sonora, Mexico. * is the assumed limit of the atmospheric argon ratio. Abbreviations: Ma, mega-annum; MSWD, mean square weighted deviation. 10 The Caborca Orogenic Gold Belt of Sonora, Mexico: Mica 40Ar/39Ar Geochronology from Gold-Rich Veins

80 G) * H) B Chip Mine Chip Mine 0.003 70 white mica flakes white mica flakes from quartz vein N from quartz vein

60 Plateau 51.2 ± 0.3 Ma 0.002 M 50 G Ar H I J K 40 C C-F L-N Ar/ 40 Plateau Age = 36 L 0.001 D Apparent Age (Ma) Isochron Age = 50.6 ± 0.3 Ma E B 51.2 ± 0.3 Ma [40Ar/36Ar] = 391 ± 10 39 i F 30 55 % of ArK from steps H to J MSWD = 1.4 39 G 95 % of ArK from steps F to K H-K 20 0 0 20 40 60 80 100 0 0.05 0.10 0.15 0.20 0.25 39 39 40 Cumulative % ArK Released Ar/ Ar 80 Dolores I) *C Dolores J) white mica aggregates 0.003 white mica aggregates from quartz vein D from quartz vein 60 Plateau 52.8 ± 0.3 Ma

N O L M E K 0.002

40 Ar F 40

C-I Plateau Age = Ar/

I 36 52.8 ± 0.3 Ma 0.001 H G Apparent Age (Ma) Age Apparent 39 Isochron Age = 53.8 ± 0.3 Ma 20 58 % of ArK from steps N to O [40Ar/36Ar]i = 159 ± 10 MSWD = 3.4 K 39 98 % of ArK from steps H to O L J M-O 0 0 0 20 40 60 80 100 0 0.015 0.030 0.045 0.060 0.075 39 39 40 Cumulative % ArK Released Ar/ Ar 80 K) * L) El Capitán Mine 0.003 El Capitán Mine 70 white mica flakes M-N white mica flakes from quartz vein B from quartz vein 60 N G H I J K L 0.002 50 Plateau Ar

B-F 40 56.9 ± 0.3 Ma Ar/ C 40 36 Plateau Age = 0.001 M Apparent Age (Ma)

Isochron Age = 56.8 ± 0.3 Ma D 56.9 ± 0.3 Ma 40 36 30 79 % of 39Ar from steps H to N [ Ar/ Ar]i = 319 ± 10 F K MSWD = 0.5 E 39 100 % of ArK from steps B to N G H-L 20 0 0 20 40 60 80 100 0 0.05 0.10 0.15 0.20 0.25 39 39 40 Cumulative % ArK Released Ar/ Ar

Figure 3—Continued. Age spectra (G, I, K) and inverse-isotope correlation diagrams (H, J, L) from group 1 of white mica samples of gold-rich quartz veins, with plateau ages as best age, from the Caborca orogenic gold belt (COGB), northwestern Sonora, Mexico. * is the assumed limit of the atmospheric argon ratio. 11

80 M) * A N) Pinor-1 0.003 70 K-O white mica flakes Plateau from quartz vein 61.6 ± 0.3 Ma E 60 F G H I J

B-D Pinor-1 0.002 B 50 Ar

white mica flakes 40 from quartz vein Ar/

40 36 0.001 Apparent Age (Ma) Plateau Age = Isochron Age = 61.4 ± 0.8 Ma C-O 61.6 ± 0.3 Ma [40Ar/36Ar]i = 308 ± 15 30 39 MSWD = 0.1 88 % of Ar from steps F to J 39 K 99 % of ArK from steps C to O

20 0 0 20 40 60 80 100 0 0.05 0.10 0.15 0.20 0.25 39 39 40 Cumulative % ArK Released Ar/ Ar 80 O) * P) A Pit-3 0.003 white mica flakes 70 Plateau from quartz vein 65.9 ± 0.3 Ma N

H I J K L G F 60 Pit-3 M 0.002

white mica flakes Ar from quartz vein 40 B N

50 Ar/ 36 M Plateau Age = 0.001 Apparent Age (Ma) A-E Isochron Age = 66.0 ± 0.7 Ma [40Ar/36Ar]i = 260 ± 30 C 40 65.9 ± 0.3 Ma MSWD = 2.4 F 39 39 E D 57 % of ArK from steps I to L 91 % of ArK from steps G to M G L H-K 30 0 0 20 40 60 80 100 0 0.02 0.04 0.06 0.08 0.10 39 39 40 Cumulative % ArK Released Ar/ Ar 80 Q) * R) Prieta-1 A Prieta-1 white mica flakes/aggregates 0.003 white mica flakes/aggregates 70 from quartz vein N from quartz vein A-D Plateau 59.0 ± 0.3 Ma M 60 K L G H I J E F B 0.002

50 Ar 40

Plateau Age = Ar/

40 36 0.001

Apparent Age (Ma) 59.0 ± 0.3 Ma 39 56 % of ArK from steps E to I Isochron Age = 59.4 ± 0.9 Ma [40Ar/36Ar] = 300 ± 40 30 i C MSWD = 19, all steps M L D-K 20 0 0 20 40 60 80 100 0 0.03 0.06 0.09 0.12 39 39 40 Cumulative % ArK Released Ar/ Ar

Figure 3—Continued. Age spectra (M, O, Q) and inverse-isotope correlation diagrams (N, P, R) from group 1 of white mica samples of gold-rich quartz veins, with plateau ages as best age, from the Caborca orogenic gold belt (COGB), northwestern Sonora, Mexico. * is the assumed limit of the atmospheric argon ratio. 12 The Caborca Orogenic Gold Belt of Sonora, Mexico: Mica 40Ar/39Ar Geochronology from Gold-Rich Veins

80 A S) * T) Prieta-5 N 0.003 70 white mica flakes from quartz vein Plateau 59.7 ± 0.3 Ma B 60 F E G H I J L D K M Prieta-5 0.002 N 50 C white mica flakes Ar from quartz vein 40 Ar/

40 36 0.001 Apparent Age (Ma) Plateau Age = Isochron Age = 59.6 ± 0.8 Ma 40 36 [ Ar/ Ar]i = 290 ± 50 C 30 59.7 ± 0.3 Ma MSWD = 2.4 B 39 39 M D 73 % of ArK from steps E to J 94 % of ArK from steps D to L E F-L 20 0 0 20 40 60 80 100 0 0.02 0.04 0.06 0.08 0.10 39 39 40 Cumulative % ArK Released Ar/ Ar 80 U) * A V) Quitovac-5 Quitovac-5 0.003 70 white mica flakes/aggregates white mica flakes/aggregates from quartz vein from quartz vein

B 60 N Isochron Age = 49.7 ± 0.5 Ma A-E Plateau [40Ar/36Ar]i = 293 ± 18 49.6 ± 0.3 Ma 0.002 MSWD = 0.9 39 50 Ar 92 % of ArK from steps F to L F G H I J K M 40 L

Ar/ C

40 36 M Plateau Age = 0.001 F Apparent Age (Ma) E N D 49.6 ± 0.3 Ma L 30 39 93 % of ArK from steps F to M G J IH 20 0 K 0 20 40 60 80 100 0 0.03 0.06 0.09 0.12 0.15 39 Cumulative % Ar Released 39 40 K Ar/ Ar 80 * W) A Quitovac-6 X) Quitovac-6 0.003 white mica flakes 70 white mica flakes from quartz vein from quartz vein A-E N B 60 Plateau 49.9 ± 0.3 Ma 0.002

50 Ar

F K 40 G H I J L M Ar/

40 36 Plateau Age = 0.001 D Apparent Age (Ma) E F N Isochron Age = 49.9 ± 0.5 Ma 49.9 ± 0.3 Ma [40Ar/36Ar]i = 307 ± 14 C 30 39 G 70 % of ArK from steps E to J MSWD = 1.0 L 39 M 78 % of ArK from steps E to K H-K 20 0 0 20 40 60 80 100 0 0.03 0.06 0.09 0.12 39 39 40 Cumulative % ArK Released Ar/ Ar

Figure 3—Continued. Age spectra (S, U, W) and inverse-isotope correlation diagrams (T, V, X) from group 1 of white mica samples of gold-rich quartz veins, with plateau ages as best age, from the Caborca orogenic gold belt (COGB), northwestern Sonora, Mexico. * is the assumed limit of the atmospheric argon ratio. 13

80 Y) * Z) Sanfran-2 A Sanfran-2 white mica flakes 0.003 70 from quartz vein white mica flakes from quartz vein A-E Plateau 60 57.2 ± 0.3 Ma F G K L H I J 0.002 N B Ar 50 M 40 M-N

Plateau Age = Ar/ 40 36 E 0.001 F Apparent Age (Ma) 57.2 ± 0.3 Ma C 39 70 % of Ar from steps H to L Isochron Age = 57.1 ± 0.6 Ma D K 40 36 30 [ Ar/ Ar]i = 309 ± 15 MSWD = 1.8 G 39 99 % of ArK from steps C to M J LK I 20 0 H 0 20 40 60 80 100 0 0.03 0.06 0.09 0.12 39 39 40 Cumulative % ArK Released Ar/ Ar 80 * AA) A AB) 70 M-N 0.003 Sanfran-3 Plateau B white mica flakes 59.5 ± 0.3 Ma from quartz vein 60 H I K L F G J N Sanfran-3 0.002 50 white mica flakes Ar A-E from quartz vein 40 C Ar/ M 40 36 0.001 F Apparent Age (Ma) Plateau Age = E 30 59.5 ± 0.3 Ma Isochron Age = 59.6 ± 0.7 Ma D 39 [40Ar/36Ar] = 291 ± 14 62 % of ArK from steps D to I i G MSWD = 4.3, all steps J H L I K 20 0 0 20 40 60 80 100 0 0.02 0.04 0.06 0.08 0.10 39 39 40 Cumulative % ArK Released Ar/ Ar 80 AC) 0.004 AD) N 70 Sanfran-4 A-E Plateau M L * white mica flakes 61.6 ± 0.3 Ma 0.003 from quartz vein 60 F G H I J K Sanfran-4 50 white mica flakes Ar B 40 0.002 from quartz vein L Ar/ 40 Plateau Age = 36 Apparent Age (Ma) 0.001 Isochron Age = 61.5 ± 0.6 Ma C 61.6 ± 0.3 Ma [40Ar/36Ar]i = 330 ± 30 30 39 MSWD = 1.2 D 79 % of ArK from steps G to J 99 % of 39Ar from steps C to K E F K G-K 20 0 0 20 40 60 80 100 0 0.02 0.04 0.06 0.08 0.10 39 39 40 Cumulative % ArK Released Ar/ Ar

Figure 3—Continued. Age spectra (Y, AA, AC) and inverse-isotope correlation diagrams (Z, AB, AD) from group 1 of white mica samples of gold-rich quartz veins, with plateau ages as best age, from the Caborca orogenic gold belt (COGB), northwestern Sonora, Mexico. * is the assumed limit of the atmospheric argon ratio. 14 The Caborca Orogenic Gold Belt of Sonora, Mexico: Mica 40Ar/39Ar Geochronology from Gold-Rich Veins

80 L-N AE) * A AF) Sanfran-6 70 0.003 A-E Plateau white mica flakes 60.4 ± 0.3 Ma from quartz vein L N 60 F G H I J K Sanfran-6 0.002

50 white mica flakes Ar B from quartz vein 40 M Ar/

40 36 Plateau Age = 0.001 Apparent Age (Ma) Isochron Age = 60.6 ± 0.7 Ma C 60.4 ± 0.3 Ma 40 36 30 39 [ Ar/ Ar]i = 295 ± 20 F 80 % of ArK from steps F to J MSWD = 5.4, all steps G D L E H-K 20 0 0 20 40 60 80 100 0 0.02 0.04 0.06 0.08 0.10 39 39 40 Cumulative % ArK Released Ar/ Ar 80 * AG) A AH) N Sanluisito-2 70 Sanluisito-2 0.003 white mica flakes white mica flakes from quartz vein from quartz vein B 60 A-E Plateau Isochron Age = 48.0 ± 0.5 Ma 48.1 ± 0.2 Ma M 0.002 [40Ar/36Ar]i = 310 ± 30

Ar MSWD = 1.7 50 39 G K L 40 94 % of ArK from steps E to L F H I J Ar/

40 36 Plateau Age = 0.001 N Apparent Age (Ma) C D E 48.1 ± 0.2 Ma F 30 39 L 75 % of ArK from steps C to J M

G-K 20 0 0 20 40 60 80 100 0 0.03 0.06 0.09 0.12 0.15 39 39 40 Cumulative % ArK Released Ar/ Ar 80 AI) * A AJ) Sonoyta-1 Sonoyta-1 70 white mica flakes 0.003 white mica flakes from quartz vein from quartz vein A-E Plateau M-N 57.3 ± 0.3 Ma B 60 H F G I J K L 0.002

50 Ar 40 N Ar/

40 36 C Plateau Age = 0.001 M Apparent Age (Ma) D Isochron Age = 57.5 ± 0.6 Ma E 57.3 ± 0.3 Ma 40 36 F 30 39 [ Ar/ Ar]i = 290 ± 6 L 54 % of ArK from steps I to N MSWD = 1.8, all steps GJ IH 20 0 K 0 20 40 60 80 100 0 0.02 0.04 0.06 0.08 0.10 39 39 40 Cumulative % ArK Released Ar/ Ar

Figure 3—Continued. Age spectrum (AE, AG, AI) and inverse-isotope correlation diagrams (AF, AH, AJ) from group 1 of white mica samples of gold-rich quartz veins, with plateau age as best age, from the Caborca orogenic gold belt (COGB), northwestern Sonora, Mexico. * is the assumed limit of the atmospheric argon ratio. 15

80 Plateau AK) * AL) 52.6 ± 0.8 Ma I-L 0.003 70 Taj-3 Taj-3 A-B white mica flakes white mica flakes from quartz vein 60 from quartz vein 0.002

F G Ar 50 C D E H 40 Ar/ B 40 Plateau Age = 36 0.001 Apparent Age (Ma) 52.6 ± 0.8 Ma C-L 39 Isochron Age = 51.8 ± 2.8 Ma 30 100 % of ArK, all steps [40Ar/36Ar]i = 318 ± 111 MSWD = 0.5, all steps A 20 0 0 20 40 60 80 100 0 0.05 0.10 0.15 0.20 0.25 39 39 40 Cumulative % ArK Released Ar/ Ar 80 Trin-11 AM) * A AN) 70 white mica flakes 0.003 Trin-11 from quartz vein A-E white mica flakes Plateau from quartz vein 60 56.9 ± 0.3 Ma B G H I L M F J K N 0.002

50 Ar 40

D Ar/ Plateau Age = C 40 36 E 0.001 N Apparent Age (Ma) 56.9 ± 0.3 Ma 39 54 % of ArK from steps H to K Isochron Age = 57.3 ± 0.7 Ma F 30 [40Ar/36Ar]i = 288 ± 18 M MSWD = 4.1, all steps G-L

20 0 0 20 40 60 80 100 0 0.03 0.06 0.09 0.12 39 39 40 Cumulative % ArK Released Ar/ Ar 90 AO) * AP) A Vidrios-2 Vidrios-2 0.003 white mica flakes 80 white mica flakes from quartz vein A-E from quartz vein N Isochron Age = 57.7 ± 0.6 Ma [40Ar/36Ar]i = 308 ± 19 70 MSWD = 0.9 0.002 39 N 95 % of ArK from steps F to L M Ar

Plateau 40 57.7 ± 0.3 Ma B

60 Ar/

F G H I J K L 36 0.001 M D Apparent Age (Ma) Plateau Age = C F 50 E 57.7 ± 0.3 Ma L 39 G 95 % of ArK from steps F to L J H I 40 0 K 0 20 40 60 80 100 0 0.02 0.04 0.06 0.08 0.10 39 39 40 Cumulative % ArK Released Ar/ Ar

Figure 3—Continued. Age spectrum (AK, AM, AO) and inverse-isotope correlation diagrams (AL, AN, AP) from group 1 of white mica samples of gold-rich quartz veins, with plateau age as best age, from the Caborca orogenic gold belt (COGB), northwestern Sonora, Mexico. * is the assumed limit of the atmospheric argon ratio. 16 The Caborca Orogenic Gold Belt of Sonora, Mexico: Mica 40Ar/39Ar Geochronology from Gold-Rich Veins

60 A) * A B) Alamo-7 0.003 Alamo-7 50 white mica flakes/aggregates N white mica flakes/aggregates from quartz vein N from quartz vein 40

H I J K 0.002 B G M E L F Ar 30 D L 40 Ar/

20 36 M A-C 0.001 C Apparent Age (Ma) Isochron Age = G F Isochron Age = 36.1 ± 1.2 Ma H-K D 36.1 ± 1.2 Ma [40Ar/36Ar]i = 300 ± 30 E 10 MSWD = 2.4 39 60 % of ArK from steps H to N 0 0 0 20 40 60 80 100 0 0.03 0.06 0.09 0.12 0.15 39 39 40 Cumulative % ArK Released Ar/ Ar 80 C) * D) A 70 0.003 Carretera-1 B Carretera-1 white mica flakes white mica flakes 60 from quartz vein N from quartz vein

A-E M-N 0.002 50 Ar 40 G H I J K L

F Ar/ E C

40 36 D 0.001 Apparent Age (Ma) F Isochron Age = M Isochron Age = 47.8 ± 0.6 Ma 30 47.8 ± 0.6 Ma [40Ar/36Ar]i = 290 ± 30 MSWD = 1.3 G 39 L 68 % of ArK from steps G to J H-J K 20 0 0 20 40 60 80 100 0 0.03 0.06 0.09 0.12 0.15 39 39 40 Cumulative % ArK Released Ar/ Ar 80 E) *A F) N 70 0.003 Estación-1 B Estación-1 white mica flakes white mica flakes 60 from quartz vein M from quartz vein 0.002

50 M-N Ar C 40 A-F Ar/

K 36 40 G H I J L E 0.001 D Apparent Age (Ma) Isochron Age = 42.2 ± 0.5 Ma F Isochron Age = 40 36 G [ Ar/ Ar]i = 299 ± 19 30 MSWD = 2.1 L 42.2 ± 0.5 Ma 39 87 % of ArK from steps E to K H-K 20 0 0 20 40 60 80 100 0 0.03 0.06 0.09 0.12 0.15 39 39 40 Cumulative % ArK Released Ar/ Ar

Figure 4. Age spectra (A, C, E) and inverse-isotope correlation diagrams (B, D, F) from group 2 of white mica samples of gold-rich quartz veins, with isochron ages as best age, from the Caborca orogenic gold belt (COGB), northwestern Sonora, Mexico. * is the assumed limit of the atmospheric argon ratio. Abbreviations: Ma, mega-annum; MSWD, mean square weighted deviation. 17

80 G) * A H) Jerónimo Jerónimo 0.003 white mica flakes 70 white mica flakes M-N B B-E from quartz vein from quartz vein

60 F L G H I J K Isochron Age = 58.9 ± 0.7 Ma 0.002 [40Ar/36Ar]i = 330 ± 40 MSWD = 2.3 Ar 50 39

40 87 % of ArK from steps D to K

Ar/ N Isochron Age = C 40 36 0.001 Apparent Age (Ma) 58.9 ± 0.7 Ma D E M 30 F G L H-K 20 0 0 20 40 60 80 100 0 0.03 0.06 0.09 0.12 39 39 40 Cumulative % ArK Released Ar/ Ar 80 I) * B J) La Pinta USA Mine 0.003 La Pinta USA Mine 70 white mica flakes white mica flakes P from quartz vein O from quartz vein 60

M-P 0.002 C Ar 50 N 40

L Ar/ G H I J K 40 36 B-F 0.001 D

Apparent Age (Ma) M Isochron Age = Isochron Age = 43.9 ± 0.2 Ma E 40 36 30 [ Ar/ Ar]i = 290 ± 6 F 43.9 ± 0.2 Ma MSWD = 2 39 74 % of ArK from steps B to J G L H-J 20 0 K 0 20 40 60 80 100 0 0.06 0.12 0.18 0.24 0.30 39 39 40 Cumulative % ArK Released Ar/ Ar 80 K) * L) LH08-1 A 0.003 70 white mica flakes LH08-1 A-E from quartz vein white mica flakes K from quartz vein 60 F J B G H I L-O 0.002 50 Ar 40 Ar/

40 36 0.001 Apparent Age (Ma) Isochron Age = D Isochron Age = 54.1 ± 1.7 Ma E C 40 36 30 54.1 ± 1.7 Ma [ Ar/ Ar]i = 522 ± 172 O MSWD = 2.1 N F-J 39 87 % of ArK from steps F to J K M L 20 0 0 20 40 60 80 100 0 0.06 0.12 0.18 0.24 0.30 39 39 40 Cumulative % ArK Released Ar/ Ar

Figure 4—Continued. Age spectra (G, I, K) and inverse-isotope correlation diagrams (H, J, L) from group 2 of white mica samples of gold-rich quartz veins, with isochron ages as best age, from the Caborca orogenic gold belt (COGB), northwestern Sonora, Mexico. * is the assumed limit of the atmospheric argon ratio. 18 The Caborca Orogenic Gold Belt of Sonora, Mexico: Mica 40Ar/39Ar Geochronology from Gold-Rich Veins

80 M) * A N) Pinta-1a 0.003 Pinta-1a 70 white mica flakes white mica flakes A-E from quartz vein N from quartz vein 60 B Isochron Age = 58.3 ± 0.6 Ma F G H I J K L M 0.002 [40Ar/36Ar]i = 300 ± 20 MSWD = 1.8 50 Ar 39

40 98 % of ArK from steps C to M Ar/

40 36 N C 0.001 D-F Apparent Age (Ma) Isochron Age =

30 M 58.3 ± 0.6 Ma G H-L 20 0 0 20 40 60 80 100 0 0.03 0.06 0.09 0.12 39 39 40 Cumulative % ArK Released Ar/ Ar 80 O) * A P) Pinta-2 Pinta-2 N 0.003 70 white mica flakes white mica flakes A-E from quartz vein from quartz vein

60 G J K F H I L M 0.002 50 Ar 40 B N Ar/

40 36 0.001 Apparent Age (Ma) Isochron Age = C-F Isochron Age = 56.9 ± 0.8 Ma 30 56.9 ± 0.8 Ma [40Ar/36Ar]i = 390 ± 100 M MSWD = 1.8 G 39 L 64 % of ArK from steps G to J H J 20 0 K I 0 20 40 60 80 100 0 0.03 0.06 0.09 0.12 39 39 40 Cumulative % ArK Released Ar/ Ar 80 Q) * A R) Prieta-4 Prieta-4 70 white mica flakes 0.003 white mica flakes from quartz vein B from quartz vein A-F

M 60 G H I J K L N 0.002

50 Ar 40

Ar/ C 40 Isochron Age = 36 N 0.001 Apparent Age (Ma) 60.6 ± 0.7 Ma Isochron Age = 60.6 ± 0.7 Ma D [40Ar/36Ar] = 280 ± 30 E 30 i F MSWD = 2.1 39 G 68 % of ArK from steps C to K M H-K 20 0 L 0 20 40 60 80 100 0 0.03 0.06 0.09 0.12 39 39 40 Cumulative % ArK Released Ar/ Ar

Figure 4—Continued. Age spectra (M, O, Q) and inverse-isotope correlation diagrams (N, P, R) from group 2 of white mica samples of gold-rich quartz veins, with isochron ages as best age, from the Caborca orogenic gold belt (COGB), northwestern Sonora, Mexico. * is the assumed limit of the atmospheric argon ratio. 19

80 S) * A T) Quitovac-2 Quitovac-2 0.003 70 white mica flakes N white mica flakes A-E from quartz vein L-N from quartz vein B 60 J F G H I K 0.002

50 Ar 40

M Ar/ C 36 40 D 0.001 Apparent Age (Ma) Isochron Age = E Isochron Age = 59.4 ± 0.7 Ma L 40 36 30 59.4 ± 0.7 Ma [ Ar/ Ar]i = 298 ± 14 F MSWD = 0.9 39 G 62 % of ArK from steps C to I J H 20 0 K I 0 20 40 60 80 100 0 0.03 0.06 0.09 0.12 39 39 40 Cumulative % ArK Released Ar/ Ar 80 M-N U) * V) Sanfran-7 A Sanfran-7 0.003 70 white mica flakes white mica flakes A-E from quartz vein from quartz vein

60 B L G H I J K 0.002

50 F Ar 40 C F Ar/ E D 40 36 N 0.001 Apparent Age (Ma) Isochron Age = Isochron Age = 56.5 ± 0.6 Ma M 30 56.5 ± 0.6 Ma [40Ar/36Ar]i = 300 ± 13 MSWD = 2.5 L G 39 72 % of ArK from steps A to J H-J 20 0 K 0 20 40 60 80 100 0 0.03 0.06 0.09 0.12 39 39 40 Cumulative % ArK Released Ar/ Ar 80 W) *A X) Sargenta Sargenta N 0.003 70 white mica flakes white mica flakes from quartz vein B from quartz vein A-F

60 M G H I J K L 0.002

50 Ar 40 Ar/

40 Isochron Age = 36 0.001 C Apparent Age (Ma) Isochron Age = 59.8 ± 0.4 Ma 59.8 ± 0.4 Ma N [40Ar/36Ar]i = 294 ± 6 D-G 30 MSWD = 2.2 39 79 % of ArK from steps A to K L H-K M 20 0 0 20 40 60 80 100 0 0.03 0.06 0.09 0.12 0.15 39 39 40 Cumulative % ArK Released Ar/ Ar

Figure 4—Continued. Age spectra (S, U, W) and inverse-isotope correlation diagrams (T, V, X) from group 2 of white mica samples of gold-rich quartz veins, with isochron ages as best age, from the Caborca orogenic gold belt (COGB), northwestern Sonora, Mexico. * is the assumed limit of the atmospheric argon ratio. 20 The Caborca Orogenic Gold Belt of Sonora, Mexico: Mica 40Ar/39Ar Geochronology from Gold-Rich Veins

80 Y) * A Z) SE Sonoyta SE Sonoyta 0.003 70 white mica flakes white mica flakes A-E from quartz vein N from quartz vein

60 M-N B 0.002 F G H K 50 I J L Ar 40 Ar/

40 36 0.001 C M

Apparent Age (Ma) Isochron Age = Isochron Age = 52.5 ± 0.4 Ma D [40Ar/36Ar]i = 340 ± 20 30 52.5 ± 0.4 Ma G MSWD = 0.3 E F J-K 39 57 % of ArK from steps F to I H I 20 0 0 20 40 60 80 100 0 0.03 0.06 0.09 0.12 0.15 39 39 40 Cumulative % ArK Released Ar/ Ar 80 AA) * A AB) Tres Mojones 70 N 0.003 white mica flakes from quartz vein B G M 60 H I J K L A-F Tres Mojones 0.002 Isochron Age = 62.2 ± 0.4 Ma [40Ar/36Ar] = 318 ± 17 50 white mica flakes Ar i

40 MSWD = 1.2 from quartz vein 39 55 % of ArK from steps E to J Ar/

40 36 0.001 C Apparent Age (Ma) F Isochron Age = E 30 D G 62.2 ± 0.4 Ma N H I K-M J 20 0 0 20 40 60 80 100 0 0.03 0.06 0.09 0.12 0.15 39 39 40 Cumulative % ArK Released Ar/ Ar 80 AC) * AD) Trin-10 N A Trin-10 70 white mica flakes 0.003 white mica flakes A-E from quartz vein from quartz vein

B 60 G K L M H I J F 0.002

50 Ar 40 N Ar/

40 36 0.001 C Apparent Age (Ma) Isochron Age = Isochron Age = 58.3 ± 1.0 Ma 40 36 30 58.3 ± 1.0 Ma [ Ar/ Ar]i = 350 ± 70 M MSWD = 1.6 E-L 39 D 95 % of ArK from steps E to M 20 0 0 20 40 60 80 100 0 0.03 0.06 0.09 0.12 39 39 40 Cumulative % ArK Released Ar/ Ar

Figure 4—Continued. Age spectra (Y, AA, AC) and inverse-isotope correlation diagrams (Z, AB, AD) from group 2 of white mica samples of gold-rich quartz veins, with isochron ages as best age, from the Caborca orogenic gold belt (COGB), northwestern Sonora, Mexico. * is the assumed limit of the atmospheric argon ratio. 21

80 AE) * A AF) Vidrios-1 0.003 Vidrios-1 70 white mica flakes L-N white mica flakes from quartz vein B from quartz vein

60 Isochron Age = 56.3 ± 0.6 Ma F [40Ar/36Ar] = 304 ± 14 G H I J K 0.002 i N MSWD = 2.0 Ar 39 50 98 % of ArK from steps D to L 40 F A-E D Ar/ C 40 36 0.001 M Apparent Age (Ma) Isochron Age = E L 30 56.3 ± 0.6 Ma G H-K 20 0 0 20 40 60 80 100 0 0.03 0.06 0.09 0.12 39 39 40 Cumulative % ArK Released Ar/ Ar

Figure 4—Continued. Age spectra (AE) and inverse-isotope correlation diagrams (AF) from group 2 of white mica samples of gold-rich quartz veins, with isochron ages as best age, from the Caborca orogenic gold belt (COGB), northwestern Sonora, Mexico. * is the assumed limit of the atmospheric argon ratio. 22 The Caborca Orogenic Gold Belt of Sonora, Mexico: Mica 40Ar/39Ar Geochronology from Gold-Rich Veins

80 A) A B) American Boy American Boy white mica flakes 0.003 white mica flakes from quartz vein from quartz vein 60

Average N A 50.6 ± 0.4 Ma 0.002 Ar 40 K I J L 40 E F G H D M B

C Ar/

36 L B Average Age =

Apparent Age (Ma) 0.001 30 50.6 ± 0.4 Ma 39 83 % of ArK from steps F to M Isochron Age = 50.4 ± 0.1 Ma [40Ar/36Ar]i = 293 ± 1 C MSWD = 170, all steps N D M E-K 20 0 0 20 40 60 80 100 0 0.015 0.030 0.045 0.060 0.075 39 39 40 Cumulative % ArK Released Ar/ Ar 80 C) 0.004 D) Ana Celia Ana Celia 70 white mica flakes white mica flakes A-C from quartz vein * A from quartz vein L-N 0.003 60 Isochron Age = 43.2 ± 0.8 Ma [40Ar/36Ar]i = 290 ± 110 Average B MSWD = 4.5, all steps 50 Ar

43.6 ± 1.1 Ma 40 0.002

H Ar/ F G I J 40 K 36 M N D E Apparent Age (Ma) Average Age = 0.001 C 30 43.6 ± 1.1 Ma 39 D-K 80 % of ArK from steps F to K L

20 0 0 20 40 60 80 100 0 0.04 0.08 0.12 0.16 0.20 39 39 40 Cumulative % ArK Released Ar/ Ar 80 E) * A F) 0.003 Carretera-2 70 white mica flakes Average B 59.7 ± 1.3 Ma from quartz vein M 60 G H L I K J N F 0.002

50 Carretera-2 Ar white mica flakes 40 C

from quartz vein Ar/ A-E 40 36 N D 0.001 E Apparent Age (Ma) Average Age = F Isochron Age = 59.6 ± 1.2 Ma 30 [40Ar/36Ar] = 290 ± 20 59.7 ± 1.3 Ma i M G 39 MSWD = 76, all steps 57 % of ArK from steps G to J H-L 20 0 0 20 40 60 80 100 0 0.03 0.06 0.09 0.12 39 39 40 Cumulative % ArK Released Ar/ Ar

Figure 5. Age spectra (A, C, E) and inverse-isotope correlation diagrams (B, D, F) from group 3 of white mica samples of gold-rich quartz veins, with average ages as best age, from the Caborca orogenic gold belt (COGB), northwestern Sonora, Mexico. * is the assumed limit of the atmospheric argon ratio. Abbreviations: Ma, mega-annum; MSWD, mean square weighted deviation. 23

140 M G) * H) Centauro-3 L A Centauro-3 white mica aggregates 0.003 white mica aggregates 120 from quartz vein from quartz vein

K 100

0.002 B

80 J Ar 40 Isochron Age = 50 ± 40 Ma 40 36 I Ar/ [ Ar/ Ar]i = 7,000 ± 18,000

G H 36 MSWD = 17,000, all steps 60 F Single Step Age = 0.001 Apparent Age (Ma) C A-E ~62 Ma 40 Step F D M L K J E-I 20 0 0 20 40 60 80 100 0 0.015 0.030 0.045 0.060 39 39 40 Cumulative % ArK Released Ar/ Ar 80 Cerro Colorado I) * Cerro Colorado J) white mica aggregates Q 0.003 B white mica aggregates from quartz vein from quartz vein 60 K L G H I J F M-P D E C Q B 0.002 40 Ar Single Step Age = 40 O C Ar/

~56 Ma 36 D Step H 0.001 Apparent Age (Ma) 20 P E Isochron Age = 57.8 ± 0.2 Ma N [40Ar/36Ar]i = 280 ± 1 F MSWD = 150, all steps M 0 0 G-L 0 20 40 60 80 100 0 0.015 0.030 0.045 0.060 0.075 39 39 40 Cumulative % ArK Released Ar/ Ar 80 K) * A L) Costa-1 0.003 70 B white mica flakes A-F Average 61.0 ± 1.0 Ma from quartz vein N 60 H K L M I J G Costa-1 0.002 50 white mica flakes Ar from quartz vein 40 C Ar/

40 36 N Average Age = 0.001 Apparent Age (Ma) D-F Isochron Age = 61.1 ± 0.9 Ma 61.0 ± 1.0 Ma [40Ar/36Ar] = 290 ± 20 30 39 i M 81 % of ArK from steps H to L MSWD = 30, all steps G

H-L 20 0 0 20 40 60 80 100 0 0.03 0.06 0.09 0.12 39 39 40 Cumulative % ArK Released Ar/ Ar

Figure 5—Continued. Age spectra (G, I, K) and inverse-isotope correlation diagrams (H, J, L) from group 3 of white mica samples of gold-rich quartz veins, with average and single-step ages as best age, from the Caborca orogenic gold belt (COGB), northwestern Sonora, Mexico. * is the assumed limit of the atmospheric argon ratio. 24 The Caborca Orogenic Gold Belt of Sonora, Mexico: Mica 40Ar/39Ar Geochronology from Gold-Rich Veins

80 M) * N) A Costa-3 0.003 70 M-N white mica aggregates from quartz vein Average B 58.9 ± 0.7 Ma 60 G H K N I J L 0.002

50 Costa-3 Ar 40 A-F white mica aggregates

from quartz vein Ar/ M 40 36 0.001 C Apparent Age (Ma) Average Age = Isochron Age = 57.5 ± 0.7 Ma 30 58.9 ± 0.7 Ma [40Ar/36Ar] = 304 ± 17 D-L 39 i 85 % of ArK from steps H to L MSWD = 10, all steps

20 0 0 20 40 60 80 100 0 0.03 0.06 0.09 0.12 39 39 40 Cumulative % ArK Released Ar/ Ar 80 O) * A P) Costa-4 Costa-4 70 white mica flakes 0.003 white mica flakes A-E from quartz vein from quartz vein B 60 0.002

L N Ar 50 K M H 40 I J G Ar/ F 40 36 C Single Step Age = 0.001 Apparent Age (Ma) N ~48 Ma E 30 Isochron Age = 48.7 ± 1.4 Ma F D Step I 40 36 G [ Ar/ Ar]i = 320 ± 70 M MSWD = 61, all steps L H-K 20 0 0 20 40 60 80 100 0 0.03 0.06 0.09 0.12 0.15 39 39 40 Cumulative % ArK Released Ar/ Ar 80 Q) * A R) B Cubabi Cubabi 0.003 70 white mica flakes white mica flakes from quartz vein from quartz vein 60 Average M-N 50.0 ± 0.5 Ma 0.002 N 50 L Ar G H I J K 40 A-F C

Ar/ D 40 Average Age = 36 0.001 E Apparent Age (Ma)

50.0 ± 0.5 Ma M F 39 Isochron Age = 50.2 ± 0.7 Ma 30 81 % of ArK from steps G to K [40Ar/36Ar] = 290 ± 30 i G-L MSWD = 12, all steps 20 0 0 20 40 60 80 100 0 0.03 0.06 0.09 0.12 0.15 39 39 40 Cumulative % ArK Released Ar/ Ar

Figure 5—Continued. Age spectra (M, O, Q) and inverse-isotope correlation diagrams (N, P, R) from group 3 of white mica samples of gold-rich quartz veins, with average and single-step ages as best age, from the Caborca orogenic gold belt (COGB), northwestern Sonora, Mexico. * is the assumed limit of the atmospheric argon ratio. 25

80 * I-L S) T) El Chanate 0.003 El Chanate 70 white mica bad aggregates white mica bad aggregates from quartz vein from quartz vein H A 60 G E F 0.002 D 50 C Ar 40 B Ar/

Single Step Age = 36 J K 40 M 0.001 B Apparent Age (Ma) ~52 Ma A Step C Isochron Age = 55.9 ± 0.3 Ma I 30 40 36 L [ Ar/ Ar]i = 246 ± 1 C-G H MSWD = 240, all steps G 20 0 0 20 40 60 80 100 0 0.06 0.12 0.18 0.24 0.30 39 39 40 Cumulative % ArK Released Ar/ Ar 120 U) *A V) Q El Tiro N-O El Tiro white mica aggregates 0.003 white mica aggregates 100 from quartz vein B from quartz vein P Isochron Age = 59.0 ± 0.2 Ma M [40Ar/36Ar] = 625 ± 2 80 Single Step Age = i 0.002 MSWD = 5,000; all steps

~55 Ma Ar Step G 40 L C

60 Ar/ J K F G H I 36 0.001 Apparent Age (Ma) A-E D 40 Q N E F

O P M 20 0 G-L 0 20 40 60 80 100 0 0.015 0.030 0.045 0.060 0.075 39 39 40 Cumulative % ArK Released Ar/ Ar 100 white mica aggregates W) * C X) La Colorada D La Colorada from quartz vein 0.003 E white mica aggregates R F 80 Average G from quartz vein 64.7 ± 0.7 Ma

60 K L M N O P J 0.002 Q H I Ar 40

40 H Average Age = Ar/ 36 0.001 Apparent Age (Ma) 64.7 ± 0.7 Ma R D-G 83 % of 39Ar from steps K to P Q 20 K I Isochron Age = 64.0 ± 0.2 Ma 40 36 [ Ar/ Ar]i = 270 ± 60 J-P MSWD = 2,400; all steps 0 0 0 20 40 60 80 100 0 0.015 0.030 0.045 0.060 39 39 40 Cumulative % ArK Released Ar/ Ar

Figure 5—Continued. Age spectra (S, U, W) and inverse-isotope correlation diagrams (T, V, X) from group 3 of white mica samples of gold-rich quartz veins, with average and single-step ages as best age, from the Caborca orogenic gold belt (COGB), northwestern Sonora, Mexico. * is the assumed limit of the atmospheric argon ratio. 26 The Caborca Orogenic Gold Belt of Sonora, Mexico: Mica 40Ar/39Ar Geochronology from Gold-Rich Veins

80 Y) * Z) N A La Esperanza 70 0.003 white mica aggregates Average L from quartz vein 58.7 ± 0.8 Ma 60 K N I J F H M E G 0.002 A-D

La Esperanza Ar B 50 M white mica aggregates 40 from quartz vein Ar/

40 36 0.001 Apparent Age (Ma) Average Age = Isochron Age = 59.1 ± 1.6 Ma 30 58.7 ± 0.8 Ma 40 36 39 [ Ar/ Ar]i = 340 ± 70 C 78 % of ArK from steps D to J MSWD = 74, all steps L D E-K 20 0 0 20 40 60 80 100 0 0.03 0.06 0.09 0.12 39 39 40 Cumulative % ArK Released Ar/ Ar 120 AA) * A AB) La Herradura La Herradura 100 0.003 white mica bad aggregates white mica bad aggregates M from quartz vein from quartz vein L J K 80 N Isochron Age = 65.0 ± 12 Ma I 40 36 [ Ar/ Ar]i = 1,400 ± 1,700 0.002 MSWD = 540, all steps H

G Ar 60 F

E 40 N D B C Ar/

40 Single Step Age = 36 0.001 Apparent Age (Ma) ~61 Ma A-B L 20 Step F M

I-K C-H 0 0 0 20 40 60 80 100 0 0.03 0.06 0.09 0.12 39 39 40 Cumulative % ArK Released Ar/ Ar 120 S AC) * Lluvia de Oro 3b AD) Lluvia de Oro 3b 0.003 white mica aggregates 100 white mica aggregates D from quartz vein from quartz vein Isochron Age = 48.0 ± 13 Ma E [40Ar/36Ar]i = 1,500 ± 1,400 MSWD = 8,100; all steps 80 0.002 G R F Ar 40 Q O P 60 L N Ar/

36 H K 0.001 Apparent Age (Ma) Single Step Age = I 40 J J K ~61 Ma S I Step L R N L M-O D-H Q 20 0 P 0 20 40 60 80 100 0 0.02 0.04 0.06 0.08 0.10 39 39 40 Cumulative % ArK Released Ar/ Ar

Figure 5—Continued. Age spectra (Y, AA, AC) and inverse-isotope correlation diagrams (Z, AB, AD) from group 3 of white mica samples of gold-rich quartz veins, with average and single-step ages as best age, from the Caborca orogenic gold belt (COGB), northwestern Sonora, Mexico. * is the assumed limit of the atmospheric argon ratio. 27

80 D E AE) * F AF) Noche Buena Casa G Noche Buena Casa 70 white mica aggregates 0.003 white mica aggregates from quartz vein from quartz vein

Average H 60 R 52.9 ± 0.8 Ma 0.002 O Q 50 L M N P Ar I

K 40 J Ar/ I 40 Average Age = 36 E-H 0.001 Apparent Age (Ma) J 52.9 ± 0.8 Ma Isochron Age = 53.0 ± 2.0 Ma 39 40 36 30 72 % of ArK from steps L to Q [ Ar/ Ar]i = 290 ± 30 R Q K-P MSWD = 2,700; all steps

20 0 0 20 40 60 80 100 0 0.015 0.030 0.045 0.060 0.075 39 39 40 Cumulative % ArK Released Ar/ Ar 80 AG) * AH) Noche Buena Workings D Noche Buena Workings white mica aggregates 0.003 E white mica aggregates 70 from quartz vein from quartz vein

Average F 60 55.7 ± 1.4 Ma R

P 0.002 G L M N O Q 50 K Ar J 40 I Ar/ H 40 H Average Age = 36 E-G 0.001 Apparent Age (Ma) I 55.7 ± 1.4 Ma Isochron Age = 56.1 ± 1.6 Ma 39 40 36 30 90 % of ArK from steps J to Q [ Ar/ Ar]i = 280 ± 70 MSWD = 2,100; all steps O J R K-P 20 0 0 20 40 60 80 100 0 0.015 0.030 0.045 0.060 0.075 39 39 40 Cumulative % ArK Released Ar/ Ar 80 N AI) 0.004 AJ) Average Pit-4 70 65.0 ± 0.6 Ma A white mica flakes/aggregates * from quartz vein J M H I K G L 0.003 60

Pit-4 N C-F white mica flakes/aggregates B 50 Ar from quartz vein 40 0.002 Ar/ 40 Average Age = 36 C Apparent Age (Ma) 0.001 Isochron Age = 65.1 ± 1.2 Ma 65.0 ± 0.6 Ma [40Ar/36Ar] = 270 ± 30 30 39 i D 87 % of ArK from steps G to L MSWD = 27, all steps F E M L G A-B H-K 20 0 0 20 40 60 80 100 0 0.02 0.04 0.06 0.08 0.10 39 39 40 Cumulative % ArK Released Ar/ Ar

Figure 5—Continued. Age spectra (AE, AG, AI) and inverse-isotope correlation diagrams (AF, AH, AJ) from group 3 of white mica samples of gold-rich quartz veins, with average ages as best age, from the Caborca orogenic gold belt (COGB), northwestern Sonora, Mexico. * is the assumed limit of the atmospheric argon ratio. 28 The Caborca Orogenic Gold Belt of Sonora, Mexico: Mica 40Ar/39Ar Geochronology from Gold-Rich Veins

80 N AK) 0.004 AL) A-F Quitovac-3 70 M white mica flakes Average * A from quartz vein 60.2 ± 0.4 Ma 0.003 60 J L G H I K Quitovac-3 50 Ar

white mica flakes 40 0.002 from quartz vein B Ar/

40 36 N

Apparent Age (Ma) Average Age = 0.001 D F E 30 60.2 ± 0.4 Ma Isochron Age = 60.1 ± 0.8 Ma C 39 M 92 % of ArK from steps G to L [40Ar/36Ar] = 300 ± 20 i G-L MSWD = 12, all steps 20 0 0 20 40 60 80 100 0 0.02 0.04 0.06 0.08 0.10 39 39 40 Cumulative % ArK Released Ar/ Ar 80 AM) * AN) 0.003 A Quitovac-4 70 white mica flakes N from quartz vein J K L I 60 H F G D E M A-C Quitovac-4 0.002

50 white mica flakes Ar from quartz vein 40 Ar/

40 36 B 0.001 N Apparent Age (Ma) Single Step Age =

30 ~62 Ma Isochron Age = 62.0 ± 2.0 Ma 40 36 Step H [ Ar/ Ar]i = 330 ± 120 L C MSWD = 88, all steps M K D-J 20 0 0 20 40 60 80 100 0 0.03 0.06 0.09 0.12 39 39 40 Cumulative % ArK Released Ar/ Ar 80 M AO) 0.004 AP) Sanfran-1 70 H I L A white mica flakes G J K N * F from quartz vein 0.003 60 B A-E Sanfran-1 white mica flakes 50 Ar

from quartz vein 40 0.002 N Ar/ 40 Single Step Age = 36 Apparent Age (Ma) ~69 Ma 0.001 30 Step K Isochron Age = 69.4 ± 1.0 Ma C [40Ar/36Ar]i = 280 ± 20 MSWD = 11, all steps M D-L 20 0 0 20 40 60 80 100 0 0.02 0.04 0.06 0.08 0.10 39 39 40 Cumulative % ArK Released Ar/ Ar

Figure 5—Continued. Age spectra (AK, AM, AO) and inverse-isotope correlation diagrams (AL, AN, AP) from group 3 of white mica samples of gold-rich quartz veins, with average and single-step ages as best age, from the Caborca orogenic gold belt (COGB), northwestern Sonora, Mexico. * is the assumed limit of the atmospheric argon ratio. 29

80 AQ) * AR) SanFran 4d SanFran 4d 0.003 70 white mica aggregates white mica aggregates from quartz vein from quartz vein

60 Average Age = 38.9 ± 0.7 Ma 0.002

39 Ar 50 97 % of ArK from steps L to Q J 40 Average Ar/ J K K 38.9 ± 0.7 Ma 36 40 P L M N Q 0.001 L

Apparent Age (Ma) O

Isochron Age = 38.5 ± 0.8 Ma Q 30 40 36 [ Ar/ Ar]i = 330 ± 50 M MSWD = 93, all steps P O N 20 0 0 20 40 60 80 100 0 0.02 0.04 0.06 0.08 0.10 39 39 40 Cumulative % ArK Released Ar/ Ar 100 * AT) SVE-1 AS) SVE-1 white mica aggregates 0.003 white mica aggregates from quartz vein from quartz vein 80 C-G Average 64.9 ± 0.5 Ma

0.002 C H I J K L M 60 N-Q Ar 40 Q P

Average Age = Ar/ 36 D E 0.001 Apparent Age (Ma) 64.9 ± 0.5 Ma 40 39 F 95 % of ArK from steps G to N O Isochron Age = 64.7 ± 0.2 Ma N [40Ar/36Ar]i = 336 ± 3 G MSWD = 18, all steps H I-M 20 0 0 20 40 60 80 100 0 0.015 0.030 0.045 0.060 39 39 40 Cumulative % ArK Released Ar/ Ar 100 AU) *C AV) Tajitos-2 Tajitos-2 white mica aggregates 0.003 white mica aggregates Q from quartz vein D from quartz vein 80 Q

C-G Average 0.002 60 55.6 ± 1.1 Ma Ar

40 E N O P H M

I J K L Ar/ F 36 0.001 Apparent Age (Ma) Average Age = 40 Isochron Age = 55.6 ± 0.1 Ma P 55.6 ± 1.1 Ma G 88 % of 39Ar from steps I to O [40Ar/36Ar]i = 309 ± 1 K MSWD = 160, all steps H I-O 20 0 0 20 40 60 80 100 0 0.015 0.030 0.045 0.060 0.075 39 39 40 Cumulative % ArK Released Ar/ Ar

Figure 5—Continued. Age spectra (AQ, AS, AU) and inverse-isotope correlation diagrams (AR, AT, AV) from group 3 of white mica samples of gold-rich quartz veins, with average ages as best age, from the Caborca orogenic gold belt (COGB), northwestern Sonora, Mexico. * is the assumed limit of the atmospheric argon ratio. 30 The Caborca Orogenic Gold Belt of Sonora, Mexico: Mica 40Ar/39Ar Geochronology from Gold-Rich Veins

120 AW) * AX) Yaqui-3 A Yaqui-3 white mica aggregates 0.003 white mica aggregates from quartz vein from quartz vein 100 Q B Isochron Age = 76.0 ± 8.0 Ma 40 36 N [ Ar/ Ar]i = 700 ± 500 MSWD = 21,000; all steps M 0.002 L P 80 Ar

K O 40 Q C

J Ar/ 36 Single Step Age = 0.001 P Apparent Age (Ma) 60 I F G H ~61 Ma D Step H E B-E O F-I K-N J 40 0 0 20 40 60 80 100 0 0.015 0.030 0.045 0.060 39 39 40 Cumulative % ArK Released Ar/ Ar 120 AY) * AZ) 10 de Mayo A 10 de Mayo white mica aggregates Q 0.003 white mica aggregates 100 from quartz vein B from quartz vein O

N M 80 P L 0.002

K Ar Q C 40 J F G H I P 60 Ar/

Single Step Age = 36 0.001 Apparent Age (Ma) A-E D 40 ~62 Ma Step F Isochron Age = 76.0 ± 5.0 Ma E [40Ar/36Ar]i = 430 ± 160 O F MSWD = 4,800; all steps G-N 20 0 0 20 40 60 80 100 0 0.015 0.030 0.045 0.060 39 39 40 Cumulative % ArK Released Ar/ Ar

Figure 5—Continued. Age spectrum (AW, AY) and inverse-isotope correlation diagram (AX, AZ) from group 3 of white mica samples of gold-rich quartz veins, with single-step age as best age, from the Caborca orogenic gold belt (COGB), northwestern Sonora, Mexico. * is the assumed limit of the atmospheric argon ratio.

Izaguirre and others—The Caborca Orogenic Gold Belt of Sonora, Mexico: Mica 40Ar/39Ar Geochronology from Gold-Rich Veins—Open-File Report 2016–1008 ISSN 2331-1258 (online) http://dx.doi.org/10.3133/ofr20161008