Summary:
The Jacinth-Ambrosia (J-A) Project area occurs in Tertiary age sediments of the Eucla Basin. North of the J-A Project area, the Eucla Basin is underlain by the older Palaeozoic Officer Basin, which subcrops and outcrops north of the railway. South east of the J-A Project area, Eucla Basin sediments overlie the Precambrian Gawler Craton, which subcrop and outcrop to the east (Iluka 2007b).
The Jacinth deposit is a north-south oriented palaeo-sedimentary sand deposit approximately 5 km long by 900 m wide and up to 42 m deep. The average thickness of overburden is approximately 8 m. Ore thickness ranges from approximately 20 - 45 m.
The Ambrosia deposit consists of a larger central zone up to 700 m wide, approximately 2.2 km long and up to 30 m deep as well as three smaller satellite pits to the east and north. The average ore thickness is 12 m; overburden thickness varies, averaging 8 m.
Soils
The soil distribution across the J-A Project area reflects the geological history of the area. At least five marine transgression and regression events have occurred in the Eucla Basin (depositing 40–50 m of sediments), the most recent event forming the Nullarbor Limestone found in the borefield and pipeline areas. The sedimentary sequence overlies partially weathered granitic and gneissic rocks of the Gawler Craton. The characteristics of the sediments from the various marine regression and transgression events vary sufficiently to form distinct stratigraphic units.
The thickness of these stratigraphic units varies across the J-A Project area and individual units may be absent at some locations. West of the airstrip and village, the stratigraphic units are not present and the soils are dominated by the Nullarbor Limestone, with occasional Aeolian dunal sand ridges.
The regolith in the J-A Project area and adjacent areas is highly heterogeneous, with thickness and physio-chemical characteristics varying significantly spatially, with depth, and between soils. The soil surface is fragile, with the high percentage of fine sand particles in the surface (and some regolith) samples being particularly susceptible to wind erosion. Soil strength measurements indicate that weak soil crusts develop within the topsoil material which offers some protection from wind erosion.
The soil profile above the orebody and barren Ooldea Sands can be broadly subdivided into soil materials that have been termed Brown Loam and Red Loam, occurring beneath topsoil and, in places, dunal sand, termed Yellow Sand (Soil Water Consultants 2008). A conceptual model of the soil profile, which re-interprets the geological stratigraphy through additional survey and data from a soil science perspective, is shown in Figure 9. The physio-chemical characteristics of the soil materials can vary significantly (Table 9), especially Red Loam which represents an amalgam of clayey- and sandy-members of the Quaternary Sand Unit. Red Loam generally has higher clay content than Brown Loam and can be dispersive as a result of a higher exchangeable sodium percentage (ESP). Areas of higher pH are generally associated with the presence of calcium carbonate (CaCO3) that can manifest as calcrete in the profile, although this is not a continuous layer. Beneath the topsoil, which is non- to slightly saline, the soils are classed as slightly to extremely saline. Plant available water capacity (PAWC) is low in topsoil and Yellow Sand, increasing in the Brown Loam and Red Loam layers due to their higher clay content, but PAWC is moderated by these materials’ higher salinity.