The Brockman rare metals deposit is hosted by a fine-grained volcaniclastic unit informally termed the Niobium Tuff. This volcaniclastic unit is the lowermost unit of a sequence of lavas, subvolcanic rocks, and volcaniclastic units of the Brockman Volcanics.
There is ample outcrop within the Brockman area, and the Niobium Tuff can be traced over a strike length of 3.5km. It occurs here on the western flank and northern closure of a major south-west plunging synclinal structure. The Niobium Tuff varies in width to 35m, and has a vertical or steep easterly dip.
Only minor faulting is evident, and drilling has established continuity of the unit to a vertical depth of 250m. Weathering is limited, with oxidation observed only down to depths of 20-30m.
Two cross sections provided as Figure 3 and Figure 4 show the interpreted shape of the mineralised zone towards the southern and northern end of the drilled portion of the deposit, respectively. The major fold structure shown in Figure 3 was intersected in a number of holes providing intersections exceeding 100 metres down hole.