Annual report on exploration, EL 4774 Umbrewarra Tin Field.

Abstract

The current exploration programme was instigated to determine the presence of cassiterite and xenotime in economic quantities within the Umbrewarra region. Main targets were Umbrewarra Creek, and major creeks to the south such as Wesleyan, Harts, No Name and Copperfield Creek. Alluvials in floodplains adjacent to the active channels were reasoned to be most prospective. Also, tributaries and catchment areas were sampled to find the extent of alluvial and eluvial cover. Exploration activities were conducted from 18th July to 3rd August, 1986. The chief tool employed in the programme was a backhoe/ front-end loader. It was utilized to excavate series of pits in areas of potential cassiterite-and xenotime-bearing gravels. Areas of potential were defined by the occurrence of old workings and the presence of cassiterite in panned concentrates from active creek sediments. The number of pits and spacing at each site depended on the surface expression of alluvials, the depth of coarse gravels, occurrence of old workings, and the potential for eluvials. Along the main target, Umbrewarra Creek sites were spaced approximately 250m apart. Pits were excavated to bedrock and were typically 3m x im wide. Granite commonly constituted bedrock. Alluvials were classified into gravels, sands, and silts 20-30 kg channel samples were taken over various intervals from the walls of the pits, subdivided at the geologist's discretion. The depth of individual units and their boundaries determined the sample interval with particular attention being paid to the coarser alluvals. However, intervals for channel samples did not exceed 1.5m, nor were they less than 0.5m. Smaller, approximately 10 kg, silt overburden samples were taken. A total of 96 samples from 34 sites were despatched to Normet, Darwin for gravity concentration of the heavy mineral suite. Full details of site spacing, pit spacing, and sample intervals are included accompanying plans and graphic logs.