There are four major ore bodies within the CSA mine, the Western System, Eastern System, QTS North System and the QTS South System (Fig. 2.5 and 2.8). All ore systems within the CSA deposit are hosted in the CSA siltstone within quartz veins and have been subjected to chloritization and silicification.
2.3.3.1 Western System
The Western System was the first ore body at CSA to be mined and was discovered by a gossan above Pb-Zn ore (Stegman and Stegman, 1996; Shi and Reed, 1998). The ore body has multiple lenses averaging 45 m in length and 7 m in width with a down plunge extent roughly 200 m in length (Shi and Reed, 1998; Stockton et al., 2006). The Western System contains high grade lead-
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Figure 2.8 Plan view showing location of major ore systems within the CSA deposit.
zinc ores as well as copper ores. Lead-zinc ore lenses occur from present day surface to a depth of 850 m and consist of mainly MSGS with Po-rich type ores. The MSGS type mineralization is often banded (Shi and Reed, 1998). The Po-rich ore type occurs between the MSGS rich lenses and the copper rich lenses below. Copper-rich lenses occur on the eastern side of the Western System and have also been encountered in discrete patches below the lead-zinc rich ore lenses The copper-rich ore consists of sulfide rich-veins. Major sulfide minerals present within the Western System include sphalerite, galena, pyrite, pyrrhotite, and chalcopyrite along with trace amounts of cubanite. Major gangue minerals include quartz, chlorite, plagioclase and muscovite.
2.3.3.2 Eastern System
Mineralization in the Eastern System begins 250 m below surface. The ore system contains multiple lenses with a strike length between 50 to 80 m, a width averaging 10 m and a plunge in excess of 400 m (Stockton et al., 2006). It is located 100 m east of the Western System and 100 m west of the QTS North System. The Eastern System contains high grade copper ore with very little lead-zinc ore. The copper-rich ore lenses occur between 500 to 1200 m below present day surface. Mineralization within this system is primarily composed of chalcopyrite, pyrrhotite, quartz and pyrite
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veins hosted in strongly cleaved chlorite altered siltstone (Stockton et al., 2006). While the Eastern system contains all three copper-rich ore types (MS, SMSQ, and SMSC; Fig. 2.7), the most common are MS in the cores of the lenses and SMSQ on the fringes of the lenses. However SMSQ also can be the only constituent of a given lens..
2.3.3.3 QTS North System
The QTS North System is the largest, most northerly and deepest of the four ore bodies at the CSA deposit and is currently the main source of ore at the mine. This ore body occurs roughly 100 m east of the Eastern System and 500 m north of the QTS South System. The QTS North System consists of 13 lenses of massive chalcopyrite roughly 15 to 130 m in length and 5 to 16 m wide (Stockton et al., 2006). Mineralization begins roughly 600 m below surface and extends down-plunge from 400 m to more than a kilometre (Stockton et al., 2006). Mineralization in the QTS North System is copper-rich only. Mineralization is composed primarily of massive (MS, Fig. 2.7) to semi-massive (SMSQ and SMSC, Fig. 2.7) chalcopyrite and pyrrhotite with no lead-zinc mineralization (Shi and Reed, 1998). Similar to the Eastern System, the MS ore within the QTS North System generally occurs in the core of the lenses while SMSQ and SMSC occur on the periphery of the lenses. Many lenses within the QTS North System lack the MS cores and are composed solely of either SMSQ, SMSC or a transition from one to the other. Unlike the Eastern System, however, the QTS North System contains much more SMSC than SMSQ and, in general, is associated with more intense alteration than any other system.
2.3.3.4 QTS South System
The QTS South System is a north-south trending, steeply sub-vertical series of lenses with strike lengths of 200 m and average widths of 10 m (Shi and Reed, 1998). This ore body occurs roughly 600 m below surface and 500 m south of the other three ore bodies. QTS South resides 100 m to the east and 500 m to the south of QTS North (Stockton et al., 2006). The QTS South System is composed of massive (MS, Fig. 2.7) and disseminated sulfides (Robertson, 1974; Shi and Reed, 1998). Again, both SMSQ and SMSC are present within the QTS South system, but in much lower quantities than the other systems. QTS South contains the most massive sufide rich lenses even though it is a smaller ore system by volume. The major sulfide minerals present within this ore body are chalcopyrite and pyrrhotite with isolated pods of galena and sphalerite at its extremities (Shi and Reed, 1998).
2.3.4 ALTERATION
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of alteration and silicification. The chloritization and silicification occur as alteration halos around mineralization (Fig. 2.9). Talc forms locally along some faults.
2.3.4.1 Chloritization
Two forms of chlorite exist within the CSA mine. The Fe-rich chlorite (largely pyrochlore) gives the rock a bright green appearance against the grey of the unaltered sedimentary host rocks (McDermott et al., 1996). The Fe-chlorite is generally aligned in the direction of the regional S2 cleavage. Around mineralization the Fe-rich chlorite alteration generally obscures bedding and
Figure 2.9 Alteration present at CSA. [A] Sample 1000208 shows typical chloritization and is characterized by both a Fe-rich and a Fe-Mg-rich end member. [B] Sample 913210 shows typical silicification, the second most intense alteration within the deposit, which occurs much more selectively than chloritization and mainly as veins and cementation of breccia (See section 3.4.5 for details on breccias). Some pervasive silicification occurs near to major mineralization but disappears very quickly away from ore zones. [C] Sample 909701 shows talc appearing in discrete shear zones generally found in the western portion of the deposit between the Western System and the Eastern System. This alteration is the least intense variety at the CSA deposit.
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intensifies the regional S2 cleavage (Kirk, 1983). In areas of less strain, the Fe-chlorite anastomoses around other grains. A more intermediate chlorite (Mg/Mg*Fe = <0.55) is largely found in late shear zones which cut across the deposit. This intermediate chlorite is recognized by a dark green, almost black, colour that contrasts with the grey of the unaltered sedimentary rocks (McDermott et al., 1996). Much work has been done by workers such as Robertson and Taylor (1987), Brill, (1988), Jeffery, (1994), McDermott et al. (1996) and Stegman (2000) to understand the different types of chloritic alteration within the Cobar-style Deposit as potential targeting tools for more mineralization both within the different mine sites as well as the region as a whole. A detailed investigation and discussion surrounding the chlorite alteration will be presented in Chapter 4 section 4.6.1.
2.3.4.2 Silicification
According to Brill (1988) the silicification of the host rock at CSA replaces feldspar clasts. Intense silicification is referred to as the SAS or “silica-altered sediment” by mine geologists (McDermott et al., 1996). These SAS and other intense silicification zones are generally bounded by intense chlorite alteration, both Fe- and Mg-rich. A detailed investigation and discussion surrounding silicification will be presented in Chapter 4 section 4.6.2.
2.3.4.3 Talc
Talc alteration, while occurring within the deposit, is poorly understood. At CSA, the talc alteration is widely accepted to occur in a large shear zone ranging from meters to 10’s of meters in thickness and running the vertical extent of the deposit (Derek Webb, Pers. Comm). The Talc shear zone is thought to occur between the Western and Eastern Systems and be a major controlling structure on the placement of the ore zones within the deposit (Derek Webb, Pers. Commu, 2009). While talc has been shown to occur in the mine, the extent to which it has altered country rock or has affected mineralization within the deposit is not well understood.