The regolith of the Portia and North Portia prospects, Benagerie Ridge Magnetic Complex, Curnamona, South Australia

Abstract

This dissertation characterizes the stratigraphic units (sediments and underlying Proterozoic bedrock and its satprolite) across the Portia and North Portia prospects, Situated on the Benagerie Ridge Magnetic Complex (BRMC), bssed on drill core and chip samples. The distribution of mineralization related elements (Cu, Au, Mo, Pb, Zn, and As) in the saprolite and the overlying sediments, their host and secondary minerals, and the factors controlling their distribution (e.g. Eh and pH of the groundwater) have been determined, The nature and origin of gold at the Portia prospect were investigated, and the threshold Au concentrations in the overlying sediments established. Finally, a landscape evolution model at the Portia and North Portia prospects is developed, including weathering and sedimentatiotion chronology, and implications for mineral exploitation are evaluated. The stratigraphic units include the Quaternary sediments, the Tertiary Namba Fornmation (NF) and Portia Unit (PU), and the saprolite of the Proterozoic bedrock. The Quaternary sediments include fluvial sediments, and soils developed in dune sand and present-day floodplain deposits. The sediments are unconsolidated, and range from 2 to 10 m thick. The top l to 2 m are light brown sandy soil with regolith carbonate and groundwater gypsum in the subsoil. Underlying is 3 to 8 m of moderately sorted, fine- to medium-grained olive brown sand and clayey sand. The Namba Formation covers the Benageric Ridge Magnetic Complex, and the thicker Nambu sequence (≈ 50 m) in the north hns been informally named as the upper-, middle-, and lower-Numban Formation (NPU, NFM und NFL respectively) and consists of various grey coloured heavy clays with red and brown mottles. The sand-rich NFU is interpreted as fluvial and flood plain deposits whereas the clay-rich NFM and NFL were deposited In low-energy shallow lucustrine environments. The NF sediments contain threshold element concentrations (up lo 14 ppb Au), except for the NFL sediments at depths greater than 5O m, which accommodate elevated concentrations of chalcophile elements derived from the mineralized saprolite through chemical and physical hydromorphic dispersions. The geochemistry delineates smectite·bearing sediments from kaolinite- and mica- bearing sediments. During diagenesis the NF sediments underwent varying degrees of compaction, pressure and tension from wetting and drying of the smectite-bearing sediments, which resulted in the absence of voids and the presence of masepic fabrics and argillans formed around quartz grains. The Portia Unit comprises light grey c1ay with abundant kaolinite and a varying proportion of mica, and very low in quartz (< 2 %), except at the base of thicker parts which are quartz·rich. Poorly stacked pseudohexagonal kaolinite plates of varying sizes are indicative of a transported origin. Nevertheless, the PU is sourced from proximal saprolite and fills a north-flowing palaeo-channel across the North Portia, Portia and Lorenzo prospects. PU is also present at the Shylock prospect, but is mostly absent or sporadically distributed across the BRMC. The fining upward sequence of the PU results in partitioning of elements into groups that show either upwardlly increasing (Sr, Tl, Te, Cu, Zn, Ni, Ce, La, Pr, Nd, P und S) or downwardly increasing (Zr, Hf, Ti, Sn, Ta, Nb K, Rb, Cs, Au, Y, Se and Si) trends. The bedrock comprises phyllite (carbonaceous) and various albitite lithologies. The major minerals present include carbonate (vein and ellipsoid calcite and dolomite), feldspars (K-feldspar and albite), biotite, chlorite, scapolite, tremolite, pyrite, Fe-oxides, rutile and quartz. XRD analysis shows that the mineral abundances vary significantly within each unit, establishing significant spatial heterogeneity of minerals present. The saprolites developed over the Proterozoic bedrock range from 5 to up to 100 m thick and are dominated by quartz and kaolinite, with varying amounts of goethite and hematite, and mica. The presence of stacks of kaolinite "books" suggests in situ weathering of the saprolite. Zircon and rutile are residually concentrated in highly weathered Al-rich suprolite. Overall, the clement distribution in the saprolite is charecterized by the factors: (As, Pb, V); (Cu, Au, Mo); (Au); (Co. Ni); and (Fe, P). The first three factors represents mineralization in the albitite and phyllite saprolites, whereas (Ni, Co) indicates the presence of pyrite, which is preserved under reducing conditions in the water saturated zone. The fifth group (Fe, Pl represents the presence of ferruginous zones a-nd REEphosphates. Native Cu, covcelite and chalcocite are present in the lower saprolite and remain stable under the weakly reducing and circum-neutral pH conditions of the current groundwater, whereas uraniunite and coffinite are likely to precipitate, as suggested by the positive saturation indices. The surfaces of Au grains with numerous eich pits, dissolution grooves and voids, indicate prolonged dissolution. The presence or galena and altaite fully enclosed within the grains, and the Ag contents similar (<10 %) to those from the Proterozoic bedrock suggest a primary origin for the Au grains. Hence, the Au grains in the saprolite are interpreted as in situ primary Au, whereas those in the PU are placer Au. In comparison, the presence of Ag-depleted rims on the Au grains and small colloidal Au particles (1 μm) are interpreted as secondary Au. The placer Au concentrated at the base of the PU is sourced from the bedrock, preserved in the saprolite, and eroded together with the saprolitic materials and deposited as sediments. Carbonaceous phyllite with abundant quartz, muscovite, and K-feldspar, hosts coarse-grained Au in veins With low sulfide and has the potential to preserve primary Au in the saprolite. Rapid erosion of the saprolite and quick burial by the PU would lower the oxidation potential and preserve the detrotal Au. The curremt reducing and neutral pH groundwater does not favour the formation of chloride, thlosulphate and monohydroxide complexing ligands, but allows the precipitation of Au to form Au colloids, and for detrital and in situ primary Au to remain stable. The BRMC cropped out during the Eocene and weathered under oxidizing conditions to produce a thick and mature saprolite. Subsequently, a period or landscape instability occurred with the incision of the paeleo-channel across the North Portia, Portia and Lorenzo prospects. Sedimentation of the shallow lacustrine Namba Formation followed under reducing conditions and the sedimentary facies changed to f1uvial and flooplain deposits. A change to arid climate in the Quaternary resulted in the formation of dunes, and playa lakes, and the precipitation of groundwter gypsum und regolith carbonate. It