Potential for dissolution breccia in Paleozoic carbonate units in the Rosemont area of the northern Santa Rita Mountains

Transcription

1 Potential for dissolution breccia in Paleozoic carbonate units in the Rosemont area of the northern Santa Rita Mountains Jon Spencer, Senior Geologist Arizona Geological Survey 416 W Congress St., #100 Tucson, AZ jon.spencer@azgs.az.gov Introduction The Santa Rita Mountains are a small, rugged mountain range in the Basin and Range tectonic and physiographic province of southwestern North America, and are located on the southeast flank of the Tucson Basin in southeastern Arizona. The geology of the range is highly complex and lithologically diverse, and includes the Rosemont porphyry copper deposit in the northern Santa Rita Mountains (Fig. 1). The proposed Rosemont mine site includes Paleozoic limestone units that are known at several locations in southeastern Arizona to contain caves that were produced by natural dissolution of limestone over geologic time. This brief article addresses the potential for cave and karst features in the area of the proposed Rosemont mine, and the potential for groundwater flow in caves and dissolution breccias in these limestone units. Caves and Karst The word karst refers to A type of topography that is formed on limestone, gypsum, and other rocks by dissolution, and that is characterized by sinkholes, caves, and underground drainage (Bates and Jackson, 1980). Rainwater interacts with carbon dioxide in the atmosphere to produce carbonic acid. When slightly acidic rain water falls on limestone or percolates through limestone, it slowly dissolves the rock, and can produce underground caves, sinkholes (collapsed underground caves), and dissolution breccia (the rubbly remains of dissolved limestone). Limestone dissolution to produce karst topography is especially effective with more pure limestone, such as the Escabrosa Limestone in southeastern Arizona, and in high rainfall climates, which are more characteristic of other areas such as the southeastern U.S. (e.g., Tihansky and Knochenmus, 2001; Brezinski and Reger, 2002; Orndorff and Harlow, 2002). Groundwater may flow readily through dissolution breccias, especially where caves are present. Some dissolution breccias are millions of years old and deep underground. Typically in this situation, dissolution is no longer occurring because the deep groundwater is not acidic, and karst topography may no longer be present (the dissolution breccia might be referred to as a paleokarst ). In general, karst topography results from active dissolution, while a paleokarst refers to buried karst rubble that formed 1

2 long ago. The Redwall Limestone in the Grand Canyon region contains paleokarst breccia that is conducive to high volume water flow. Regional Setting During the Paleozoic Era, Arizona was flat (compared to the modern landscape) and intermittently blanketed by limestone and dolomite deposited from seawater. The layered sequence of Paleozoic strata, which also includes sandstone and siltstone, was greatly modified by younger faulting and igneous activity. Remaining limestone is generally tilted and faulted, and in some cases is severely disrupted. Furthermore, some limestone has been altered by hot mineralizing fluids or metamorphosed by high temperatures and pressures. Three limestone caverns (Colossal Cave, Kartchner Caverns, and Cave of the Bells) are known within 40km of the Rosemont copper deposit (Fig. 1). All three are in areas where limestone is complexly faulted (Drewes, 1971, 1972; Jagnow, 1999; Richard et al., 2001; Johnson and Ferguson, 2007; Ferguson, 2009; Ferguson et al., 2009; Brod, 2010). Limestone does not form extensive or obvious karst topography in any of these areas, probably because the arid desert environment is not generally conducive to dissolution and karst development (Peck et al., 1988). Rosemont Several Paleozoic limestone units are present along the west side of the area of the planned open pit Rosemont mine (Fig. 2). These units dip generally eastward beneath the pit area. In the western planned pit area, the limestone is intruded by an igneous rock unit ( quartz feldspar porphyry of Johnson and Ferguson, 2007). Intrusion of this unit is suspected to be related to the widespread copper mineralization that is the target of planned mining. Indeed, such porphyry rock units are the reason copper deposits like Rosemont are known as porphyry copper deposits. Prospecting activity in southeastern Arizona was especially active in the late 1800s and early 1900s, and has continued to more recent times with greater emphasis on drilling. Prospecting activity in the Rosemont area has left a legacy of prospect pits, adits (tunnels), and mine shafts, as well as a network of dirt roads (Fig. 3). If caverns produced by limestone dissolution, with surface openings, were present in the Rosemont area, it is very likely that prospectors would have found them. Even if such a cavern did not have a surface opening, it might have been found by digging, drilling, and road building. If karst breccias are present in east dipping Paleozoic carbonate units in the Rosemont area, and these breccias capture surface and/or shallow groundwater flowing generally eastward down the upper slopes of the Santa Rita Mountains, such breccias might have been penetrated by water well drillers. If penetrated, they likely would have been productive sources of groundwater, with high well yields. Furthermore, such breccia zones, continuing down dip within the limestone units and into areas of sulfide mineralization, would allow shallow groundwater to oxidize and dissolve sulfide minerals at the Rosemont copper deposit. If this process is active, sulfate (an anion, or negatively charged ion) and perhaps cations (positively charged ions) such as iron, manganese, and copper should be carried from the Rosemont sulfide ore body by groundwater, and would be detectible in water wells. Oxidation of the 2

3 sulfide minerals and mobilization of oxidation products would gradually produce a supergene enrichment blanket including oxide copper minerals that would be a target for drilling and, perhaps, mining. This process is natural, is characteristic of porphyry copper deposits in arid regions, and not by itself indicative of cave or karst breccias and associated groundwater flow paths. Rather, the process could be accelerated by the voluminous water flow possible in subsurface karst breccias, and dissolved elements carried farther by groundwater. If karst breccias are significant in the subsurface hydrology at Rosement, a long history of mineral and water exploration and production would have had a very good chance of revealing these breccias and associated hydrology and aqueous geochemistry. Much new information about groundwater quantity and quality, and movement of ions produced by dissolution of sulfide minerals, may have been obtained by drilling during recent evaluation of the mineral potential of the Rosemont deposit. This report did not investigate or access hydrologic or mineral exploration information specifically associated with the Rosemont copper deposit. I note only that, if limestone dissolution breccias are present in the subsurface, there is a good chance that they have been identified by those engaged in mineral or water exploration or production. Detection of subsurface karst breccias and caves by geophysical techniques is possible. Primary methods are electromagnetic (such as ground penetrating radar), seismic, and potential field (subtle changes in gravity and magnetic fields associated with subsurface voids or conductive materials) (Smith, 2005). Seismic techniques are better suited to areas, unlike Rosemont, where host rocks form a layered sequence not significantly affected by faulting, tilting, and disruption by intruding igneous rocks (Doll et al., 2005). The likely effectiveness of electromagnetic and potential field surveys in detecting subsurface karst or caves is not evaluated in this report (see, for example, Smith et al., 2005). Such surveys may have been conducted during the course of mineral deposit evaluation at Rosemont, but results were not evaluated for this report. Groundwater depletion at Cave of the Bells Cave of the Bells and Onyx Cave are located approximately 10 km south of Rosemont near the crest of the Santa Rita Mountains (Fig. 1). The caves are hosted by carbonate rocks within the northwest striking Sawmill Canyon shear zone (Fig. 4; Brod, 2010). A stalagmite from Cave of the Bells yielded oxygen isotopic values that reveal changes in meteoric water composition during the past 75 thousand years (Wagner et al., 2010). These oxygen isotope analyses indicate that water entering the cave from above consists of meteoric water that retains its rainwater isotopic geochemistry. The water composition is a sensitive indicator of meteoric water composition and is not influenced by groundwater. Water table decline due to dewatering the proposed Rosemont pit should therefore have no influence on water flow through Cave of the Bells. Conclusion Caves and dissolution features are present at scattered localities in the Escabrosa Limestone and associated Paleozoic carbonate units in southeastern Arizona. Paleozoic carbonate units, including the Escabrosa Limestone, are exposed at the Rosemont copper deposit, and project in the subsurface to the 3

4 east beneath the proposed pit area (Johnson and Ferguson, 2007). Detailed geologic mapping by Johnson and Ferguson (2007) did not identify any caves, but would not necessarily have identified them if the openings were small or concealed by vegetation. Much new information obtained by drilling and geophysical investigations, done to evaluate the extent and quality of the Rosemont copper deposit, should provide a more definitive basis for identifying caves or karst, but was not examined for this report. Furthermore, if caves or karst zones extend into the top of the copper deposit, they should be conduits groundwater that would be productive for water wells and effective in disseminating sulfate and metals following interaction with sulfide minerals. Information about water well productivity and quality was not examined for this study. References Cited Bates, R.L., and Jackson, J.A., eds., 1980, Glossary of geology, 2 nd ed., Falls Church, Virginia, American Geological Institute, 751 p. Brezinski, D.K., and Reger, J.P., 2002, Stratigraphy karst rrelationships in the Frederick Valley of Maryland, in Kuniansky, E.L., ed., U.S. Geological Survey Karst Interest Group Proceedings, Shepherdstown, West Virginia, August 20 22, 2002: U.S. Geological Survey Water Resources Investigations Report , p Brod, L.G., Jr., 2010, Geology of selected areas, Sawmill Canyon fault zone, northeast Santa Cruz County, Arizona: Arizona Geological Survey Contributed Report CR 10 A, 65 p., and six plates. Doll, W.E., Carr, B.J., Sheehan, J.R., and Mandell, W.A., 2005, Overview of karst effects and karst detection in seismic data from the Oak Ridge Reservation, Tennessee, in Kuniansky, E.L., ed., U.S. Geological Survey Karst Interest Group Proceedings, Rapid City, South Dakota, September 12 15, 2005: U.S. Geological Survey Scientific Investigations Report , p Drewes, H., 1971, Geologic map of the Sahuarita Quadrangle, southeast of Tucson, Pima County, Arizona: U.S. Geological Survey Miscellaneous Geologic Investigations Map I 613, scale 1:48,000. Drewes, H., 1972, Structural geology of the Santa Rita Mountains, southeast of Tucson, Arizona: U.S. Geological Survey Professional Paper 748, 35 p., with map sheets, scales 1:100,000 (1 sheet) and 1:12,000 (3 sheets). Ferguson, C.A., 2009, Bedrock geologic map of the Empire Ranch 7 ½' Quadrangle, Pima County, Arizona: Arizona Geological Survey Digital Geologic Map DGM 78A, scale 1:24,000. Ferguson, C.A., Johnson, B.J., Pearthree, P.A., Spencer, J.E., Shipman, T.C., and Cook, J.P., 2009, Geologic map of the Helvetia 7 ½' Quadrangle, Pima County, Arizona: Arizona Geological Survey Open File Report 09 06, version 1.0, scale 1:24,000. Jagnow, D.H., 1999, Geology of Kartchner Caverns State Park, Arizona: Journal of Cave and Karst Studies, v. 61(2), p Johnson, B.J., and Ferguson, C.A., 2007, Geologic Map of the Rosemont area, northern Santa Rita Mountains, Pima County, Arizona: Arizona Geological Survey Digital Geologic Map DGM 59, version 1.1, layout scale 1:12,000, with 10 p. text. Orndorff, R., and Harlow, G.E., Jr., 2002, Hydrogeologic framework on the northern Shenandoah Valley carbonate aquifer system, in Kuniansky, E.L., ed., U.S. Geological Survey Karst Interest Group 4

5 Proceedings, Shepherdstown, West Virginia, August 20 22, 2002: U.S. Geological Survey Water Resources Investigations Report , p Peck, D.L., Troester, J.W., and Moore, J.E., 1988, Karst hydrogeology in the United States of America: U.S. Geological Survey Open File Report , 23 p. Richard, S.M., Spencer, J.E., Ferguson, C.A., Youberg, A., 2001, Geologic map of southern part of the Vail 7 ½' Quadrangle, eastern Pima County, Arizona: Arizona Geological Survey Digital Geologic Map DGM 12, layout scale 1:24,000, with 29 p. text (revised July, 2002). Shipman, T.C., and Ferguson, C.A., 2003, Geologic Map of the McGrew Spring 7½' Quadrangle, Cochise County, Arizona: Arizona Geological Survey Digital Geologic Map DGM 35, 1 sheet, scale 1:24,000. Smith, B.D., Gamey, J.T., and Hodges, G., 2005, Review of airborne electromagnetic geophysical surveys over karst terrains, in Kuniansky, E.L., ed., U.S. Geological Survey Karst Interest Group Proceedings, Rapid City, South Dakota, September 12 15, 2005: U.S. Geological Survey Scientific Investigations Report , p Smith, D.V., 2005, The state of the art of geophysics and karst: A general literature review, in Kuniansky, E.L., ed., U.S. Geological Survey Karst Interest Group Proceedings, Rapid City, South Dakota, September 12 15, 2005: U.S. Geological Survey Scientific Investigations Report , p SWCA Environmental Consultants, 2012, Biological Assessment, Rosemont Copper Company Project, Santa Rita Mountains, Nogales Ranger District: SWCA Project No , 194 p. Tihansky, A.B., and Knochenmus, L.A., 2001, Karst features and hydrogeology in west central Florida A field perspective, in Kuniansky, E.L., ed., U.S. Geological Survey Karst Interest Group Proceedings: Water Resources Investigations Report , p Wagner, J.D.M., Cole, J.E., Beck, J.W., Patchett, P.J., Henderson, G.M., and Barnett, H.R., 2010, Moisture variability in the southwestern United States linked to abrupt glacial climate change: Nature Geoscience, v. 3, p ; doi: /ngeo707 Westland Resources, 2009, Lesser Long nosed Bat Survey of the Rosemont Holdings and Vicinity. Project No A. Prepared for Rosemont Copper Company. Tucson, Arizona: WestLand Resources, Inc. March 11. 5

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7 Figure 2. Geologic map of the Rosemont mine area showing outline of proposed open pit mine and distribute of map units. Paleozoic map units are labeled. The Escabrosa, Horquilla, Colina, and Concha limestone units contain less siliceous material and are more likely to be sites of dissolution. TKp: quartzfeldspar porphyry. Tg: Gila Conglomerate. Map data from Johnson and Ferguson (2007). Scale is indicated by section lines (red) that cross at one mile intervals, and UTM lines (gray) that cross at one kilometer intervals. Contour interval is 40 feet. Topographic base map is derived from the USGS Helvetia and Empire Ranch 7 ½ topographic quadrangle maps. 7

8 Figure 3. Topographic map of the Rosemont mine area showing primitive roads, prospect pits, adits, and shafts identified on the USGS Helvetia and Empire Ranch 7 ½ topographic quadrangle maps. Also shown are Devonian to Permian map units that include limestone units potentially susceptible to dissolution. Prospecting and mining activity, and road building, likely would have identified any open caves and, potentially, underground caves without surface openings. 8

9 Figure 4. Location of Cave of the Bells and Onyx Cave within the Sawmill Canyon shear zone. Fault slivers of Devonian to Permian carbonate and related units are shown in blue. Thick lines are faults. Groundwater movement is likely directed along the shear zone and not toward the Rosemont mine site. Geologic map from Drewes (1971). 9