Regional settings of structurally hosted gold mineralization in the Mudgee-Gulgong District, N.S.W.

Abstract

The Mudgee-Gulgong district is located within the exposed northeastern margin of the Lachlan Fold Belt in New South Wales. The district was an important gold mining centre in the 1800's and produced up to 1 million ounces of gold, mostly from deep leads. Re-mapping of the area has resulted in a major revision to the stratigraphy and structural knowledge of the area. Significant changes include the recognition of the formerly known Early Devonian Burranah Formation as a Late Ordovician volcanic unit with significant exploration potential for Au-Cu mineralization. Also recognised is a Late Silurian shelf sequence with potential for Au-Cu and base metals overlying the Burranah Formation. The Burranah Formation is a dominantly submarine, volcano-sedimentary succession with a complex internal stratigraphy. Two main lithofacies associations can be recognised on the magnetic images. A lower package, composed mostly of primary volcanic rocks and minor volcaniclastics is overlain by a package composed dominantly of volcaniclastics and sediments. Small elongate intrusive bodies occur throughout the sequence. Structural interpretation of the area reveals one dominant D2 deformation (Early Carboniferous) that produced meridional to northwest-trending folds, cleavage, thrust faults and oblique-slip faults. A zone of higher strain is developed within the Burranah Formation and is characterised by overturned, tight, F2 folds and considerable shortening. Mapping and interpretation of the area has been greatly assisted by the availability of high resolution gravity, magnetic and radiometric data. The interpretation of magnetic data has considerably enhanced the structural interpretation. Volcanic and intrusive rocks of the Burranah Formation comprise a coherent calcalkaline suite with a dominantly shoshonitic character. In MORB-normalized plots, they display patterns typical of many modern subduction-related volcanics with a marked depletion of Ta and Nb and similar or lower abundances of the heavy REE and Ti. Positive εNd values indicate a mantle source for the shoshonites with little or no crustal contamination. Primary gold mineralization in the district occurs in veins and as disseminations in the structurally more competent rocks adjacent to faults and shear zones in the higher strain zone. Host rocks are generally intrusive monzodiorites, diorites or coherent volcanics and range in age from Late Ordovician to Early Devonian. Sulphur and lead isotope data support a syndeformational model for mineralization with fluids and gold derived from the host rock sequence.