Potential for dissolution breccia in Paleozoic carbonate ... · no longer be present (the...
Transcript of Potential for dissolution breccia in Paleozoic carbonate ... · no longer be present (the...
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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 85701
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
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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
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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
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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, 2nd 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 02‐4174, p. 59‐65.
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.
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detection in seismic data from the Oak Ridge Reservation, Tennessee, in Kuniansky, E.L., ed., U.S.
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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: quartz‐
feldspar 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.
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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.
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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).