Hurgledurgles as a guide to ore at Mt Morgan and the Dee Range, central Queensland

Spotted rocks associated with the Mt Morgan orebody are shown by petrology to be profoundly retrogressed cordierites. Similar but less retrogressed spotted rocks occur associated with VHMS mineralization at the same stratigraphic level over a strike length of 25 km along the Dee Range SE of Mt Morgan. The Mt Morgan Au-Cu orebody (50mt @5 gpt Au and 0.7 % Cu) occurs in a roof pendant (S.L.) in the Late Devonian Mount Morgan Trondhjemite. The ore body consists of massive pyrite with stringers of quartz, chalcopyrite, and gold. The "Slag Heap" and "Car Park" mineralization discovered by Perilya Limited in 1995 is centred about 600m south of the mine, and consists of banded and massive VHMS style sulphides with minor underlying stringer mineralization. Spotted rocks form an envelope around the pipe-like Mt Morgan orebody and the adjacent mineralization. The spots have been so affected by retrogression that their origin was never satisfactorily explained, being variously labelled as spherulites, concretions, or the local non-genetic term of "hurgledurgles". The texture is developed in volcanolithic siltstones, fine-grained intrusive porphyry, coarse mass-flow or debris-flow deposits, and in minor strongly chloritic zones. Almost all the spots are thoroughly retrogressed to assemblages of very fine grained quartz, albite, chlorite, clay-mica, and pyrite. Petrographic investigations show that the spots are due to profound retrogression of a cordierite-amphibole zone which is a metamorphosed chloritic alteration zone related to the mineralization. The pervasive retrogression may be due to continued percolation of sea water after the emplacement of the Mt Morgan Trondhjemite. Conodont data from the "Banded Mine Sequence, the exhalite sequence associated with the Mt Morgan orebody, show the sequence to be late Eifelian (early Middle Devonian), the same age as the similar "Banded Mineralized Sequence" of the Mt Warner Volcanics of the Dee Range, 14 km SE of the mine (UNMC area). Stratabound mineralization similar to the Slag Heap mineralization (local high grade zinc rich zones within a lower grade alteration zone) occurs over a strike length of 2.5 km in the UNMC area, where most of the conodont data were obtained. Similar mineralization associated with cordierite occurs at the same stratigraphic level at the Ajax mine and the Lux, Omo, and Surf prospects further southeast. The cordierite in these areas is mostly less retrogressed than that at the Mt Morgan mine. The alteration and mineralization in the Dee Range is of VHMS style and supports the interpretation that the Mt Morgan mine is a VHMS. Exploration in the area is ongoing. Acknowledgement: Perilya Limited for permission to publish.1 page(s