The geology and mineralogical zoning of the H.Y.C. Ag-Pb-Zn deposit, McArthur River, Northern Territory, Australia

Abstract

The H.Y.C. Pyritic Shale Member is composed of interbedded potassic bituminous pyritic shale, sedimentary breccia and tuff. The shales contain dolomite nodules, dolomite concretions and black chert globules. The H. Y. C. Pb-Zn deposit, a sedimentary stratiform sulfide deposit, occurs at the base of the member in the H. Y. C. sub-basin. It is postulated that the member was deposited in a saline lacustrine environment, which was subject to oscillations in water level. During periods of low water level the area was emergent. The Cooley Dolomite Member, adjacent to the H.Y.C. sub-basin, is a fault bounded block (the Western Fault Block) of Mara Dolomite. Between the Western Fault Block and the H.Y.C. sub-basin it is suggested that a talus breccia was deposited, composed of dolomite clasts derived from the Western Fault Block. The contact between the Western Fault Block and the talus breccia is delineated by a major syndepositonal fault, the Western Fault. The tu££ beds and shale in the H.Y.C. sub-basin are composed of quartz, ferroan dolomite, K-feldspar, albite, calcite, illite, kaolinite, chlorite and sulfides. The non-sulfide minerals outline a mineral halo above the tt. Y. C. deposit. The halo is recognized by changes with increasing distance from the H.Y.C. deposit and Cooley Dolomite, by decreases in the: (a) K-feldspar to albite ratio (b) dolomite to calcite ratio (c) crystallinity of illite (d) percentage alteration of feldspar: to clay The mineral halo is consistent with solutions which travelled from east to west across the sub-basin, and with falling temperatures away from the H.Y.C. deposit. The chalcophile elements in the H. Y. C. deposit and in the shales above the deposit are zoned with respect to the ratios Pb/Zn, Cq/(Pb+Zn), . Cu/Zn, Ag/Pb, Fe/(Pb+Zn), As/Fe, Co/Spy (where Spy is the amount of sulfur in pyrite), Cd/Zn and Tl/Spy. The zonal distribution of the base metal sulfides in the lower orebodies is consistent with mineralization having formed from solutions which entered the H.Y.C. sub-basin at its northeastern corner and travelled in a southwestern direction. The zonal distribution in the upper orebodies and the shales above the deposit, is consistent with mineralization having formed from solutions which entered the sub-basin along its eastern margin and travelled from east to west across the sub-basin. The zonal distribution and mineral textures of the sulfide and non-sulfide minerals in the H. Y. C. sub-basin indicate that they formed during an extended time period and/or from several pulses of solution with varying chemical compositions. The sulfide minerals are postulated to have formed during diagenesis below, but within about 100 meters of the sediment-water interface, and the non-sulfide minerals during diagenesis and also after deposition of the entire H.Y.C. Pyritic Shale Member. The chemical composition of the solutions and the minerals in equilibrium with the solutions, both in time and space, changed as the solutions travelled across the sub-basin away from the Cooley Dolomite. The major chemical changes in the solution(s) as they moved across the sub-basin were: (a) a decrease in the Cu/(Pb+Zn) ratio (b) a decrease in the Pb/Zn ratio (c) a decrease in the K/Na ratio (d) a decrease in the Mg/Ca ratio (e) an increase in the K⁺/H⁺ ratio. The formation of the sulfide and non-sulfide minerals in the H.Y.C. sub-basin were integral parts of a multistage hydrothermal history, in which solutions discharged intermittently from the Emu Fault Zone and/or Cooley Dolomite over the time period encompassing the deposition of almost all of the ll.Y.C. Pyritic Sha],e Hember exposed in the !LY.C. sub-basin