Geophysical responses for copper mineralisation associated with iron deposits at Nowa Nowa

Abstract

Significant iron deposits in Victoria are concentrated in the Buchan Rift in eastern Victoria. The deposits occur as replacement deposits in Silurian sediments and basal Snowy River Volcanics, with minor iron occurrences associated with oxidized stratabound and syngenetic pyrite lenses. The ironstone deposits were initially investigated for iron ore and later for copper and gold minerahsation. The highest-grade magnetite-haematite deposits are located north of Nowa Nowa.

Several intense magnetic responses identified from 1950s airborne magnetic surveys indicated possible iron deposits. Subsequent ground surveys separated these responses into three prospects of which the Five Mile deposit is the largest. A 33-hole diamond-drilling program between 1955 and 1959 led to resource estimates of about 6 Mt of magnetite-haematite minerahsation with 52% iron beneath an overburden of brecciated ignimbrite of the Snowy River Volcanics. The best copper intersection was 21.5 m @ 0.59% copper.

In 1983, the subsurface Three Mile prospect to the southeast was identified from its magnetic response in an airborne survey. Tertiary sediments up to 70 m thick overlie intermediate volcanics and minor rhyolite of the Snowy River Volcanics. Minerahsation consists of magnetite bands (~10 m thick) with pyrite. The best copper intersections were 5.2 m @ 4.8% copper as chalcopyrite from 204.4 m; including 1.0 m @ 14.6% copper. The chalcopyrite appears to form a copper rich zone concentrated at the outer contact of the magnetite-chlorite rocks with the talc-magnetite rocks.

At both prospects, induced polarization defined broadly mineralised areas usually coincident with the magnetic responses. The drillholes targeted areas of high magnetic response and coincident induced polarization responses. The magnetic response is clearly from the high magnetite content in the minerahsed zone. The high induced polarization responses are caused by pyrite which is typically 3 to 5% but up to 15%.

The shapes and mineralogy of these deposits present difficult geophysical targets. While drilling and magnetic surveys show ironstones have small plan areas, the bodies are generally open at depth. The subtle induced polarization responses expected from chalcopyrite could easily be missed in the high responses from the pyrite, and recent advances in induced polarization interpretation procedures may help to define drilling targets.

Although the deposits have been previously described as skarns they are unrelated to any known intrusion and limestone only occurs in a single drillhole. The quartz diorites in the area are typically magnetic but interpretation of the recent magnetic data gives no indication of a nearby intrusion. Moore et al. (1998) suggested that they may be sulphur-poor massive sulphide deposits, similar to those at Tennant Creek. This model, together with existing untested geophysical anomalies, provides opportunities for further investigation of these prospects.

• Lachlan Fold Belt
• Buchan Rift
• Snowy River Volcanics
• base metals
• ironstone
• iron
• copper
• chalcopyrite
• Tennant Creek style
• geophysics
• geophysical interpretation
• electrical surveys
• induced polarization
• ground electromagnetic surveys
• Turam
• SIROTEM
• petrophysics
• Three Mile prospect
• Five Mile deposit
• Nowa Nowa
• Bairnsdale 1:250 000
• Victoria