Dispersion into the Tertiary Southern Cross Formation sedimentsfrom the Scott and Cindy Lodes, Pajingo, NE Queensland,Australia

Abstract

Auriferous epithermal quartz veins occur at Pajingo, in NEQueensland, Australia, within a basement of relatively flat-lying Devonian– Carboniferous andesitic lava, tuff and volcaniclastic sediments. The host rocks are relatively fresh on the Mt. Janet Range but are weathered and mottled on the pediments, where they are partly covered by the immature, coarse-grained Tertiary Southern Cross Formation sediments and by more recent colluvia and alluvia. The Tertiary sediments were derived from saprock, saprolite and pisolitic detritus from the volcanic basement and were then mottled by later weathering. Dispersion of Au from the basement into the thick (20–80 m) Southern Cross Formation sediments was investigated at the Scott and Cindy Lode deposits. Within the Southern Cross Formation, regional background Au concentrations are at 5–10 ppb but, at just over 1 km from known Au mineralization, they are elevated to >30 ppb. The sediments proximal to the Scott Lode, exposed by mining, are Au-rich (100 ppb locally reaching 700 ppb) as they were largely derived from the Scott Lode and its environs. Here, local Au anomalies (500–700 ppb) occur both near the base and well above the base of the sedimentary column. In contrast, sediments proximal to the Cindy Lode mineralization contain significantly less Au, as their detritus was derived from up-slope. A very localized Au anomaly (150–500 ppb) occurs near the base of a palaeochannel that drained the eastern side of the Cindy Lode. All the evidence from the Scott and Cindy lodes suggests mechanical Au dispersion. However, partial extractions of sediments over the Scott Lode indicate that about 18% of the Au is soluble in 0.1 M KI (a weak Au extractant) so there may have been minor relocation of Au by weathering. Compared with total Au analysis (instrumental neutron activation analysis), only about half the Au in the Southern Cross Formation at Scott Lode can be dissolved by KCN. Analysis of exploration data confirmed that Au is dispersed into the Southern Cross Formation around Scott Lode at several levels. Dispersions of 100–300 m in these sediments occur not only at Scott Lode but also near zones of numerous sub-economic auriferous quartz veins. Implications for exploration are as follows. (1) Dispersion from buried mineralization into the Southern Cross Formation was initially mechanical but did not necessarily occur at the base of the sediments. There was minor hydromorphic relocation of Au during subsequent weathering. (2) Dispersion may be traceable for 100–300 m down-slope from mineralization, or from areas of numerous sub-economic auriferous quartz veins. (3) Large dispersion patterns are most likely where mineralization was exposed for a long period during sedimentation (Scott Lode) and less likely where mineralization was covered by early sediments or quickly smothered by material from up-slope (Cindy Lode) so an understanding of palaeotopography is essential. (4) Elevated Au at 25–35 ppb may indicate distal mineralization, whereas anomalies of 35–70 ppb and, more specifically, >70 ppb Au may indicate proximal mineralization. (5) Total Au analysis of the Southern Cross Formation is important as about half of its Au is occluded from cyanide attack.