Geochemical stream and soil sampling in AP 2372 and AP 2392, Huckitta, NT

Abstract

The Arinthrunga Formation forms the bulk of outcrop in AP 2372. Its thickness is given variously as 1000m (Smith, 1963) near the Marshall River in the south of the Huckitta sheet area, 90m in the Elkedra sheet area (Smith and Milligan, 1966), and 450-900m in the Turkey Creek area and surroundings (Sturmfels, 1960). In AP 2372 the thickness of the formation probably does not exceed 400m. The Eurowie Sandstone Member forms discontinuous but prominent lenses of up to 30m thick within the main dolomite near the top of the formation. It contains halite casts, ripple marks and cross bedding. To the south of the investigated area, in the vicinity of the Arthur Creek, the formation rests conformably on Middle Cambrian carbonate rocks, the Arthur Creek Beds, but in the northeast of AP 2372 it is in faulted contact with the younger Tomahawk Beds. Maximum observed thickness of these is about lOOm. The Arinthrunga Formation generally forms gently undulating country, while prominent peaks and mesas are formed by the Tomahawk Beds. Dips are shallow, except in the vicinity of faults where drag folding has in some instances produced pronounced dips. The Tomahawk Beds, apart from Tertiary lateritization products and Recent surface deposits, constitute the youngest rocks in the present area, but elsewhere they are overlain by the Ordovician Nora Formation (calcarenites and argillites) and the Devonian Dulcie Sandstone (arenites and rudites). Of some interest is the occurrence, in the north of A. to P. 2392 and in A. to P. 2381 of numerous patches of loose surface limonite rubble. These patches measure from 5O - 1000m in diameter and apparently represent residual lateritic material. Some occurrences of lateritized outcrop are described in the following section, and although these tend to act as base metal concentrators, no obvious relationship was observed between metal values in surface soil samples and occurrence of limonite rubble. However, it may be worthy of note that all anomalous lead values were obtained from these northern parts. Although comprehensive reports on the geology and mineralisation of the Turkey Creek Prospect are on file (Phillips, 1960; Sturmfels, 1960), a very brief recapitulation may not be out of place. Attention must be drawn to the fact that what has been mapped as Tomahawk Beds by the B.M.R. is shown on Sturmfels (1960) map as a Sandstone member in Arinthrunga Formation which is now known as Eurowie Beds. Mineralisation occurs in silicified dolomite interspersed with chert and algal limestone along both flanks of a gently plunging, asymmetrical and faulted syncline within the Arinthrunga Formation. Mineralisation is intermittent and extends horizontally over a distance of over 8km. Vertical thickness of mineralised outcrop varies between 30 and 60m. Strikingly rich surface occurrences of galena contrast with much poorer ore obtained from drill core where the mineral assemblage is galena, sphalerite, cerussite and barite in varying proportions. There are definite indications of stratigraphic and structural control (Sturmfels, 1960; Patterson, 1960), but the origin of mineralisation is obscure; both hydrothermal replacement and syngenetic origins may be proposed, but neither theory completely fits the observed facts. The writer tends to favour the view held by Phillips (1960), namely that environmental control played a major part and that the sulphides were deposited by bacterial action or biochemical agencies in the sediments in which they are now found, despite the fact that certain factors remain unexplained, e.g. the contrasting character and quality of the ore at surface and at depth. Tectonic and/or hydrothermal events may, however, have played a part in secondary concentration. Before embarking on the main survey, a number of soil and drainage samples were collected from the Turkey Creek Prospect and its surroundings by way of an orientation survey. Results of this survey are shown on Plan No?s NT 669 and NT 670. These results are fairly self evident, and while it is recognised that the prospect is a low grade one of substantial size, the following comments are offered; 1. Background values for Pb and Zn (based also on results from AP 2372 and AP 2392) are 5-10 ppm, and 10-20 ppm, respectively. 2. Although galena is the dominant mineral both at surface and at depth, Zn values generally reach higher levels than Pb values, even in residual soil samples. 3. Pb tends to fall off more rapidly than Zn away from the mineralised zone, reflecting the former?s relative low mobility.