Summary:
GEOLOGICAL SETTING
The CSA deposit is one of several base metal- and Au-bearing deposits of the Cobar Mineral Field. It occurs on the E margin of the early Devonian Cobar Basin (Central Lachlan Orogen) and is hosted by the CSA Siltstone (turbiditic siltstones and sandstones). The CSA Siltstone (Glen, 1991) is part of the Cobar Supergroup, consisting of. lower syn-rift sediments (Nurri Group) and upper post-rift sag phase sediments (Amphitheatre Group). These rocks have undergone lowergreenschist metamorphism.
The CSA deposit lies near the stratigraphic top of the CSA Siltstone (Glen, 1994). It is bound to the W by the Footwall Fault (Kappelle, 1970) and located immediately N of the WNW-trending Plug Tank Fault and W of a NNE-trending right lateral fault (Glen, 1988). Between these structures, ore zones occur as a series of N-trending, steeply E-dipping lenses within the W- dipping host sequence. The lenses are parallel to the regional, steeply E-dipping cleavage. Rocks enclosing the mineralization show extensive alteration. Green Fe-rich chlorite and silica are prominent alterations.
Minor gossans occur close to the CSA mineralization. Weathering commonly reaches 20-100 m. Strong oxidation reaches 60 m and moderate to weak oxidation penetrates to 100 m. Secondary hematite and goethite mottling extends from surface to 75 m. Deeper oxidation commonly extends to >100 m along faults and shears and to >160 m over the CSA deposit itself.
MINERALIZATION
Mineralization is as structurally controlled, epigenetic vein-complexes containing semi- massive to massive sulphide bodies. There are four main ore systems striking approximately N and dipping steeply E, subparallel to the cleavage. Each system contains many sub-parallel ore lenses that extend along strike for up to 120 m and range to 20 m in thickness. They plunge steeply N for several hundreds of m. In addition to the four main ore zones, a large number of smaller zones of variable size and similar composition occur (Kappelle, 1970). The ore lenses are predominantly chalcopyrite, cubanite (CuFe2 S3 ), pyrrhotite, pyrite and sphalerite with minor galena. Minor and trace ore minerals include arsenopyrite, bisthmuthinite (Bi2 S3 ), bornite, galenobismutite (PbBi2 S4 ), guanajuatite (Bi2 Se3 ), native Bi, native Ag, stannite (Cu2 FeSnS4 ) and tetrahedrite ([Cu,Fe]12Sb4 S13) (McDermott et al., 1996). The ore zones also contain a large number of quartz-sulphide veins subparallel to the crenulation foliation. The sulphides and quartz are hosted in intensely cleaved chloritic siltstones. There are also numerous flat, W-dipping, strike slip faults. Fault zones are generally unmineralized.
Alteration associated with the mineralization includes a broad halo of green, Fe-rich chlorite that surrounds the ore zones and extends outwards for up to 50 m (Scott and Phillips, 1990). Pervasive silicification occurrs as a broad zone surrounding the entire mineralized area. Extensive but cryptic Li-depletion haloes (up to 500 m across strike) and less extensive depletions of Na, K, Sr and Ba (up to 100 m) have been documented around the CSA ore zones (Robertson and Taylor, 1987). These are coincident with reduced feldspar and sericite in the rocks. Small areas of intensely siliceous rock known as ‘elvan’ (Scott and Phillips, 1990) were encountered around the Western System. Black chlorite is developed along late-stage shear zones. Large bodies of talc, commonly containing euhedral pyrite, are associated with most black chloritic shear zones (Scott and Phillips, 1990).
REGOLITH EXPRESION
Only the Western System Cu ore body and the adjacent pyrite-Pb-Zn lodes are exposed at surface (Kappelle, 1970). Here, surface gossans are surrounded by elongate haloes of silicification. The upper parts of the gossans are strongly leached of base metals with only traces of secondary Cu and Pb carbonates (Andrews, 1913). Malachite and azurite veining, native Cu (psudomorphing pyrite and infilling cavities), minor cerussite (PbCO3 ) and chlorargyrite (AgCl) occur from 85-115 m. In the original CSA workings, a large mass of supergene cerussite (up to 10 m thick) occurs at the base of oxidation, near the water table (Andrews, 1913). A supergene chalcocite zone also occurred in the northern shoots of the Western System between 140-146 m (Brooke, 1975). Some sulphides, particularly pyrite, occur in the oxidized zone, where they are protected by silica alteration. Iron and Mn oxide staining extends across the surface projection of the blind Eastern System. The other CSA ore bodies are well below the weathering zone and have no weathering expression, although a zone of hematite and goethite above the QTS North lenses occurs at a depth of 1.5 m and probably reflects the near-surface periphery of this system.
Saprolite
Ferruginous veins and mottles in the saprolite and saprock are an important sampling medium for CSA style mineralization with contents of Cu >140 ppm, Pb >450 ppm and Zn >600 ppm in near surface saprolite, up to 50 m from mineralization. Around the CSA deposit, mottled saprolite contains numerous well-defined multi-element geochemical anomalies. Copper and Cd anomalies clearly define the main mineralized zones with abundances of up to 265 ppm and 8 ppm respectively. Zinc has been more widely dispersed. These anomalies are short in strike and narrow in width, and are consistent with the N-trending ore structures. Arsenic has a limited distribution over the main mineralized zone, but it is particularly enriched around the Western System. Barium has a patchy distribution over the main ore systems. Lead is particularly enriched (up to 3000 ppm) in the Western System, with a limited distribution over the Eastern System. Bismuth, Sb, Hg and Zn generally show an even distribution over the QTS North mineralization and the surrounding area, with respective maximum concentrations of 1.9, 15, 0.29 and 336 ppm.
Soil
The regional, red-brown silty loam contains a significant transported component, including wind-blown dust. Important pathfinder elements for soil exploration here include Ag, Ba, Cd, Cu, Pb and Zn, with threshold values of 0.6 ppm, 255 ppm, 0.7 ppm, 75 ppm, 435 ppm and 360 ppm respectively.
Lag
Lag around the CSA deposit is a mixture of coarse (>2 mm), locally derived bedrock fragments, quartz clasts, gossan fragments and variably transported, highly ferruginous pisoliths. The lithic fragments show variable ferruginization and the ferruginous pisoliths commonly consist of cemented sand and silt, typically with a homogeneous internal structure. The pisoliths commonly have a smooth, varnished surface. A proportion of this ferruginous lag is magnetic (maghemite-bearing).
The lag from around the CSA deposit generally has elevated Pb, As, Sb and, in some, Ag in the more ferruginous fractions and Cu and Zn typically in the lithic materials relative to regional background. Contents of Pb, As, Sb and Ag are up to 3096 ppm, 509 ppm, 227 ppm and 5 ppm respectively and of Cu and Zn up to 690 ppm and 537 ppm respectively. There is a well- defined positive correlation between the abundance of Fe and hematite with the contents of Pb, As and Sb. This is related to a previously recognized weathering-controlled fractionation of these elements during surface maturation of the lag (McQueen and Munro, 2003). Copper and Zn show a strong negative correlation with Fe and hematite abundance, except in lag derived from gossanous outcrops. A lag sample collected near an outcropping Pb-Zn gossan associated with the Western System has distinctly anomalous concentrations of Pb (330-3100 ppm), As (61-510 ppm), Bi (7-77 ppm), Cu (100-690 ppm), Zn (150-540 ppm) and Ag (0.3-3.1 ppm) in the different lag fractions (lithic and ferruginous).