Fluid-Inclusion Characteristics of Oligocene to Miocene Iron-Oxide Cu-Au (IOCG) Deposits in Central Western Arizona

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Fluid-inclusion characteristics of Oligocene to Miocene iron-oxide Cu-Au (IOCG) deposits in central western Arizona

Jon E. Spencer1 and John T. Duncan2 1Arizona Geological Survey | 2Geologist

Clara Peak Klippe (Photo by J. Spencer)

OPEN-FILE REPORT OFR-15-05

August 2015

Arizona Geological Survey

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M. Lee Allison, State Geologist and Director

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Recommended Citation: Spencer, J.E. and Duncan, J.T., 2015, Fluid-inclusion characteristics of Oligocene to Miocene iron-oxide Cu-Au (IOCG) deposits in central western Arizona. Arizona Geological Survey Open File Report, OFR-15-05, 16 p, appendices A, B, and C. Fluid-inclusion characteristics of Oligocene to Miocene iron-oxide Cu-Au (IOCG) deposits in central western Arizona Jon E. Spencer, Arizona Geological Survey, #100, 416 W. Congress St., Tucson, AZ 85701 John T. Duncan, 3579 N 2500 W, Cedar City, UT 84721

Abstract Extreme Late Oligocene to middle Miocene tectonic extension in central western Arizona was associated with exhumation of deep crustal rocks to form metamorphic core complexes, felsic to mafic magmatic activity, and formation and subsidence of sedimentary basins. Tectonic extension was accompanied by genesis of iron-oxide copper gold (IOCG) deposits along and near extensional detachment faults. Fluid-inclusion analyses from these deposits indicate derivation from saline fluids at moderate temperature, as identified in previous studies. Many other Miocene epithermal mineral deposits in the region were derived from high-salinity fluids, indicating that brines were characteristic of mineralizing fluids regionally and were not limited to areas affected by detachment faulting. Some outlying deposits have a range of fluid-inclusion salinities within a single sample, suggesting changing ratios of contrasting-salinity fluids that mixed during deposit genesis. Manganese mineralization and K-metasomatism were widespread over the same area and occurred at about the same time, but it has not been possible to demonstrate a relationship between these disparate geochemical processes even though such a relationship seems likely. Introduction The Colorado River Extensional Corridor is one of the most severely extended regions in western North America (Fig. 1; Howard and John, 1986; Spencer and Reynolds, 1991). Felsic to mafic igneous activity occurred intermittently during the 15-20 million year period of extension (Shafiqullah et al., 1980; Spencer et al., 1989b, 1995). Widespread iron-oxide and copper mineralization during extension produced deposits that have yielded significant copper and gold and minor silver, lead, and zinc. These deposits are a type of iron-oxide copper gold (IOCG) deposit (e.g., Barton, 2014) with the distinction that they are associated with extensional detachment faults. Numerous bedded and fault-zone-related manganese-oxide deposits of the same age form a separate group of deposits in the same general area (Fig. 2). Sedimentary basins that formed during tectonic extension accumulated sedimentary and volcanic rocks that, in many areas, were moderately to severely affected by potassium metasomatism, with K/Na ratios of 10:1 to even 100:1 (e.g., Chapin and Lindley, 1986; Brooks, 1986). It is suspected that all three alteration and mineralization processes are related (Fig. 3), but it has not been possible to demonstrate genetic relationships with existing data. A PowerPoint file with pictures and graphics of the geology and mineral deposits of central western Arizona is included with this report as Appendix A, and two Excel files of fluid inclusion data and associated graphs are included as Appendices B and C.

Mineral deposits related to detachment faults The association of mineral deposits with extensional detachment faults in the lower Colorado River extensional corridor has been known since the 1980s (e.g., Wilkins and Heidrick, 1982; Ridenour et al., 1982; Wilkins et al., 1986; Spencer and Welty, 1986, 1989; Spencer et al., 1988; Duncan, 1990; Salem, 1993; Evenson et al., 2014). These deposits generally consist of massive specular hematite with associated quartz, calcite, barite, and rare fluorite. Pyrite and chalcopyrite are present locally and formed early during mineralization, whereas common fracture-filling chrysocolla and malachite formed late. Of the ~53 million pounds of copper produced historically from the deposits, most came from chalcopyrite at the Swansea, Planet, and Mineral Hill Mines in the Buckskin Mountains (Spencer and Welty, 1989). Production at Swansea came largely from replacement deposits in metamorphosed Paleozoic carbonates directly above the detachment fault, but the structural and lithologic associations of the deposits are diverse (Fig. 4). Gold mineralization was by far most significant at Copperstone mine, which yielded ~516 thousand ounces of gold (mostly during 1988-1993) primarily from amethystine quartz veins within specular hematite. Gold production in other deposits is minor and mostly came from four mines, at least two of which contain more quartz than typical (Spencer and Welty, 1989). Heating and freezing experiments with fluid inclusions from quartz and fluorite in mineral deposits associated with detachment faults indicate a distinctive high salinity and moderate temperature mineralizing environment (Wilkins et al., 1986; Roddy et al., 1988; Duncan, 1990; Salem, 1993; Appendix B). Mineralizing fluids circulated along detachment faults, and were probably heated by footwall rocks which were uplifted from middle-crustal depths during tectonic exhumation and isostatic uplift. Tectonic ascent of mylonitic footwall rocks now exposed in metamorphic core complexes was analogous to ascent of large, moderate- to low- temperature intrusions (Fig. 5). Faulting repeatedly crushed rocks in fault zones and by this process probably maintained open spaces for fluid circulation. The origin of the consistently saline fluids is less well understood. In the mid to late l980s one of us (Spencer) collected a large number of mineral specimens that were analyzed (by Duncan) for fluid-inclusion salinity and temperature. Sampling was done over a broad area that included both detachment-related deposits and other epithermal deposits of similar age (Fig. 1). Part of the rationale for the sampling plan was to determine the extent of saline groundwaters that had possibly accumulated in numerous extensional basins and been mobilized in various mineralizing hydrothermal systems. Saline fluids had been attributed to geologic environments associated with detachment-faulting, but possibly were much more widespread. Methods. Procedures for fluid-inclusion analysis were as follows (Duncan, 1990). Samples of gangue material collected from outcrops and mine dumps were prepared as doubly polished thick (100-200 µm) sections. Fluid-inclusion analysis was performed in the fluid-inclusion laboratory of The University of Arizona. The system used was manufactured by S.G.E., Inc., of Tucson, Arizona, using a modified U.S. Geological Survey design. The system was calibrated using

2 synthetic fluid inclusions obtained from Synflinc, Inc., at Penn State University. Heating (homogenization) measurements reported here are considered to be accurate to within ±1.0°C, and cooling (melting) temperatures are accurate to ±0.3°C. The temperature at which the fluid-inclusion contents homogenize to one phase upon heating (Th) and the temperature at which the inclusion contents melt upon warming after freezing (Tm) were recorded from both primary and secondary fluid inclusions. Eutectic first-melting temperatures were difficult to ascertain because of both the limited optical capabilities of the system and the subtle nature of the early stages of the phase change itself. Where the appearance of liquid was observed during melting, the temperature measurement is probably a maximum estimate of the eutectic temperature. The eutectic for a water-NaCl solid-ice solution falls at - 21.8°C. Any simple NaCl-water solution in which the final melting temperature for ice is below - 21.8°C should also contain crystalline NaCl. No NaCl solid phase was observed in any fluid inclusion in this study, but only a few inclusions had melting temperatures slightly below - 21.8°C and these are within analytical uncertainty. A +3.67% pressure correction was applied to homogenization temperatures based on the assumption that the deposits formed at depths of approximately 1km and under hydrostatic pressure. The corrected values are plotted (Figs. 6, 7, 8), but the raw data are given for most samples (Appendices B and C). Results. As expected, fluid-inclusion salinities represented as NaCl equivalent for deposits related to detachment faults are generally 10-22%, with a few samples slightly higher or lower (Fig. 6A; Appendix B). Other deposits in surrounding areas include many inclusions with similar characteristics and many others with distinctly lower salinities (Fig. 6B; Appendix C). Homogenization temperatures are generally 175-300°C for all deposits. Examination of results for individual fluid inclusions for detachment-related deposits show almost no evidence of mixing between low and high salinity fluids or between warm and hot fluids (Fig. 7). One sample from the Cleopatra district (Fig. 7D) has a linear trend with salinities ranging from ~9 to ~15% NaCl equivalent, but is the only such trend apparent from the 40 samples analyzed. Note also that deposits from the Harcuvar and Cunningham Pass districts are in the footwall of the Bullard detachment fault, but the fault is nowhere exposed in the western Harcuvar Mountains. Designation of the districts as detachment-fault related was speculative, especially considering that at least some mineralization appeared to be related to mafic dikes (Reynolds and Spencer, 1993). Fluid-inclusion characteristics, however, are firmly within the range for deposits related to detachment faults. Similar-age deposits that are far from exposed detachment faults or core complexes show a range of salinities. Two of five samples from the U.S. Mine in the Big Horn Mountains (Fig. 8A; Allen, 1985) and two of three from the Heiroglyphic Mountains (Fig. 8D) show well developed mixing trends over a large range of salinities but with no significant difference in temperatures associated with the end members. It thus appears that mineralizing fluids had a great range of salinities, but mixing of hydrothermal fluids with different salinities is not a general

3 characteristic of these outlying deposits. This is unlike the detachment-related deposits which are more consistently characterized by high-salinity fluids. Manganese mineralization Bedded and shear-zone-hosted manganese-oxide deposits are widespread in central western Arizona and adjacent southeastern California, largely in the same area as detachment-fault- related deposits (Fig. 2). These deposits, present in about 24 mining districts, have yielded over 200 million pounds of Mn, mostly during a US government buying program in 1953-1955 (Spencer, 1991). Genesis of manganese-oxide deposits is generally thought to begin with iron and manganese mobilization by warm to hot, mildly acidic solutions, followed by selective precipitation of iron and retention of manganese until manganese-rich solutions deposit manganese at locations removed from the iron deposits (e.g., Krauskopf, 1957; Hewett, 1966). Analysis of fluid inclusions in calcite, barite, and quartz from four samples associated with manganese deposits in the Artillery Mountains indicated fluid salinities of <3% NaCl equivalent (Spencer et al., 1989a). This means that the saline fluids that yielded iron oxide in detachment- fault-related deposits were not the same fluids that yielded manganese oxides in near-surface or surficial environments, at least for the two manganese deposits studied. In addition, manganese- oxide deposits in the Metate district in Arizona (east of Blythe) are hosted by Pliocene strata (AZGS DGM map of the Dome Rock Mountains SW 7 ½ʹ Quadrangle, in preparation). These observations indicate that manganese mineralization is not associated with saline, detachment- related mineralizing fluids in a simple manner, and in at least one district is not related at all. Potassium metasomatism K-metasomatism has drastically altered the chemistry of volcanic and sedimentary rocks in Miocene basins and volcanic fields in western Arizona, including in areas of detachment-fault mineralization and manganese mineralization (Roddy et al., 1988; Brooks, 1988; Spencer et al., 1989b; Hollocher et al., 1994; Caudill, 2015). Drastic chemical modification of large volumes of volcanic and sedimentary rocks occurred at about the same time as detachment and manganese mineralization, leading to the inference that K-metasomatism mobilized metals that were later precipitated in detachment deposits and manganese deposits (Spencer and Welty, 1989; Hollocher et al., 1994). It has not been possible to establish geochemical links between these different processes of mineralization and alteration (Caudill, 2015), however, although more studies could conceivably establish such links. Conclusion Detachment-fault-related mineralization, manganese mineralization, and potassium metasomatism affected Miocene rocks over a wide area in central western Arizona. Although it has been proposed that the three phenomena are related, it has not been possible to conclusively demonstrate this with available data. New fluid-inclusion data presented here indicate that saline fluids were widespread, but mixing trends between different salinity or different temperature fluids are not apparent from data derived from deposits associated with detachment faults. Such

4 mixing trends are more common, but still rare, in other epithermal deposits of similar age in western Arizona. High salinities in some of these other deposits indicate that basin brines were widespread and present in basins not obviously related to detachment faults. References Cited

Allen, G.B., 1985, Economic geology of the Bighorn Mountains of west-central Arizona: Arizona Geological Survey Open-File Report 85-17, 140 p. Barton, M.D., 2014, Iron oxide(-Cu-Au-REE-P-Ag-U-Co) systems: Elsevier, Treatise on Geochemistry, 2nd edition, v. 11, chap. 23, p. 515-541. Brooks, W.E., 1986, Distribution of anomalously high K2O volcanic rocks in Arizona: Metasomatism at the Picacho Peak detachment fault: Geology, v. 14, n. 4, p. 339-342. Brooks, W.E., 1988, Recognition and geologic implications of potassium metasomatism in upper-plate volcanic rocks at the detachment fault at the Harcuvar Mountains, Yavapai County, Arizona: U.S. Geological Survey Open-File Report 88-17, 9 p. Caudill, C.M., 2015, K-metasomatism as related to manganese and copper deposits in upper- plate Miocene sedimentary units in central-western Arizona: Tucson, University of Arizona M.S. thesis, 183 p. Chapin, C.E., and Lindley, J.I., 1986, Potassium metasomatism of igneous and sedimentary rocks in detachment terranes and other sedimentary basins: economic implications, in Beatty, Barbara, and Wilkinson, P.A.K., eds., Frontiers in geology and ore deposits of Arizona and the Southwest: Arizona Geological Society Digest 16, p. 118-126. Duncan, J.T., 1990, The geology and mineral deposits of the Northern Plomosa mineral district, La Paz County, Arizona: Arizona Geological Survey Open-File Report 90-10, 55 p., 1 sheet, scale 1:9,318. Evenson, N.S., Reiners, P.W., Spencer, J.E., and Shuster, D.L., 2014, Hematite and Mn oxide (U-Th)/He dates from the Buckskin-Rawhide detachment system, western Arizona: Gaining insights into hematite (U-Th)/He systematics: American Journal of Science, v. 314, p. 1373- 1435; DOI 10.2475/10.2014.01. Hewett, D.F., 1966, Stratified deposits of the oxides and carbonates of manganese: Economic Geology, v. 61, n. 3, p. 431-461. Hollocher, K., Spencer, J.E., and Ruiz, J., 1994, Composition changes in an ash-flow cooling unit during K-metasomatism, west-central Arizona: Economic Geology, v. 89, p. 877-888. Howard, K.A., and John, B.E., 1987, Crustal extension along a rooted system of imbricate low- angle faults: Colorado River extensional corridor, California and Arizona, in Coward, M.P., Dewey, J.F., and Hancock, P.L., eds., Continental Extensional Tectonics: Geological Society Special Publication No. 28, p. 299-311. Krauskopf, K.B., 1957, Separation of manganese from iron in sedimentary processes: Geochimica et Cosmochimica Acta, v. 12, p. 61-84.

Reynolds, S.J., and Spencer, J.E., 1993, Geologic map of the western Harcuvar Mountains, La Paz County, west-central Arizona: Arizona Geological Survey Open-File Report 93-8, 9 p., scale 1:24,000. Ridenour, J., Moyle, P.R., and Willett, S.L., 1982, Mineral occurrences in the Whipple Mountains Wilderness Study Area, San Bernardino County, California, in Frost, E.G., and Martin, D.L., eds., Mesozoic-Cenozoic tectonic evolution of the Colorado River region, California, Arizona, and Nevada: San Diego, Cordilleran Publishers, p. 69-75. Roddy, M.S., Reynolds, S.J., Smith, B.M., and Ruiz, J., 1988, K-metasomatism and detachment- related mineralization, Harcuvar Mountains, Arizona: Geological Society of America Bulletin, v. 100, p. 1627-1639. Salem, H.M., 1993, Geochemistry, mineralogy, and genesis of the Copperstone gold deposit, La Paz County, Arizona: Tucson, Arizona, University of Arizona Ph.D. dissertation, 206 p. Shafiqullah, M., Damon, P.E., Lynch, D.J., Reynolds, S.J., Rehrig, W.A., and Raymond, R.H., 1980, K-Ar geochronology and geologic history of southwestern Arizona and adjacent areas, in Jenny, J.P., and Stone, C., eds., Studies in Western Arizona: Arizona Geological Society Digest, v. 12, p. 201-260. Spencer, J.E., 1991, The Artillery manganese district in west-central Arizona, in Arizona Geology: Arizona Geological Survey, v. 21, no. 3, p. 9-12. Spencer, J.E., and Reynolds, S.J., 1991, Tectonics of mid-Tertiary extension along a transect through west-central Arizona: Tectonics, v. 10, p. 1204-1221. Spencer, J.E., Duncan, J.T., and Burton, W.D., 1988, The Copperstone Mine: Arizona's new gold producer, in Arizona Geology: Arizona Geological Survey, v. 18, no. 2, p. 1-3. Spencer, J.E., and Welty, J.W., 1986, Possible controls of base- and precious-metal mineralization associated with Tertiary detachment faults in the lower Colorado River trough, Arizona and California: Geology, v. 14, p. 195-198. Spencer, J.E., and Welty, J.W., 1989, Geology of mineral deposits in the Buckskin and Rawhide Mountains, in Spencer, J.E., and Reynolds, S.J., Geology and mineral resources of the Buckskin and Rawhide Mountains, west-central Arizona: Arizona Geological Survey Bulletin 198, p. 223-254. Spencer, J.E., Grubensky, M.J., Duncan, J.T., Shenk, J.D., Yarnold, J.C., and Lombard, J.P., 1989a, Geology and mineral deposits of the Central Artillery Mountains, in Spencer, J.E., and Reynolds, S.J., Geology and mineral resources of the Buckskin and Rawhide Mountains, west-central Arizona: Arizona Geological Survey Bulletin 198, p. 168-183. Spencer, J.E., Richard, S.M., Reynolds, S.J., Miller, R.J., Shafiqullah, M., Grubensky, M.J., and Gilbert, W.G., 1995, Spatial and temporal relationships between mid-Tertiary magmatism and extension in southwestern Arizona: Journal of Geophysical Research, v. 100, p. 10,321- 10,351.

Spencer, J.E., Shafiqullah, M., Miller, R.J., and Pickthorn, L.G., 1989b, K-Ar geochronology of Miocene extensional tectonism, volcanism, and potassium metasomatism in the Buckskin and Rawhide Mountains, in Spencer, J.E., and Reynolds, S.J., eds., Geology and mineral resources of the Buckskin and Rawhide Mountains, west-central Arizona: Arizona Geological Survey Bulletin 198, p. 184-189. Wilkins, J., Jr., and Heidrick, T.L., 1982, Base and precious metal mineralization related to low-angle tectonic features in the Whipple Mountains, California and Buckskin Mountains, Arizona, in Frost, E.G., and Martin, D.L., eds., Mesozoic-Cenozoic tectonic evolution of the Colorado River region, California, Arizona, and Nevada: San Diego, Cordilleran Publish- ers, p. 182-203. Wilkins, J., Jr., Beane, R.E., and Heidrick, T.L., 1986, Mineralization related to detachment faults: A model, in Beatty, Barbara, and Wilkinson, P.A.K., eds., Frontiers in geology and ore deposits of Arizona and the Southwest: Arizona Geological Society Digest, v. 16, p. 108-117.

Figure 1. Simplified geologic map of central western Arizona and adjacent southeastern California, showing locations of fluid-inclusion samples (Appendices B and C).

Figure 2. Simplified map of central western Arizona and adjacent southeastern California showing locations of manganese mineral districts.

Figure 3. Schematic cross section through an area of extensional detachment faults, potassium metasomatism, alteration and mineralization along and near the detachment fault, and bedded manganese deposits. It is possible that potassium metasomatism liberated iron, manganese, copper, and gold that were deposited near detachment faults or in overlying manganese deposits (modified from Spencer and Welty, 1989).

Figure 4. Sites of mineralization along and near detachment faults in western Arizona (modified form Spencer and Welty, 1989): (A) replacement specular hematite within tectonic slivers of carbonate rocks within quartzo-feldspathic footwall gneiss, (B) specular hematite along high- angle faults within lower-plate rocks, (C, D, E) replacement specular-hematite deposits along the detachment fault in (C) hydrothermal carbonate, (D) Paleozoic carbonate tectonites, (E) Triassic to Jurassic fine-grained calcareous clastic rocks, (F) fracture-filling specular hematite deposits along and near detachment faults, (G) replacement specular hematite in Paleozoic carbonates adjacent to high-angle normal faults, (H) fracture-filling deposits along high-angle normal faults and within adjacent Mesozoic clastic metasedimentary rocks, (I) bedded manganese, (J) specular hematite vein deposits along faults and fractures within Miocene strata.

Figure 5. Schematic cross-section evolution of a metamorphic core complex showing exhumation and isostatic uplift of gneissic and mylonitic rocks originally at middle crustal depth.

Figure 6. Fluid-inclusion homogenization temperature upon heating (Th) versus salinity represented as equivalent weight-percent NaCl (derived from melting temperature upon heating after freezing). Each point is an average for a rock sample.

Figure 7. Fluid-inclusion homogenization temperature upon heating (Th) versus salinity represented as equivalent weight-percent NaCl. Data are from deposits related to detachment faults. Each point represents a fluid inclusion.

Figure 7 continued.

Figure 8. Fluid-inclusion homogenization temperature upon heating (Th) versus salinity represented as equivalent weight-percent NaCl. Data are from deposits distant from detachment faults and not obviously related to them. Each point represents a fluid inclusion.