Base metal exploration of the Mount Read Volcanics, western Tasmania; Pt. II, Lead isotope signatures and genetic implications

Abstract

Lead isotope measurements of 13 deposits and prospects from western Tasmania have been undertaken to establish a framework for the use of the lead isotope technique in exploration and to gain an understanding of the processes of ore formation. Lead isotope signatures for the Cambrian volcanogenic deposits (Mount Lyell, Rosebery, Hercules, Que River, Hellyer) differ by more than 1 percent from those characterizing the post-Cambrian mineralization, thought to be related to Devonian-Carboniferous granitoids and/or Tabberabberan metamorphism (e.g., Renison Bell, Queen Hill, Spray mine, Farrell lodes). The differences in lead isotope signatures allow discrimination of the two contrasting styles of mineralization in both surface material and drill core; the isotopic signatures have been utilized in an exploration program at Elliott Bay which is described in companion papers.The following important genetic conclusions can be drawn from the results of this exploration-oriented investigation: (1) A whole-rock Pb-Pb age of 580 + or - 170 m.y. (2 Sigma ) was measured for polymictic volcaniclastic rocks from the Que River area, which, although not tightly constrained because of the large error limits, is consistent with paleontological evidence. (2) The intersection of this whole-rock line with the average Que River ore lead isotope value is consistent with a common source for the lead. (3) The lead isotope data are consistent with an origin of the post-Cambrian mineralization by remobilization from the host volcanic pile in response to Devonian-Carboniferous metamorphism and/or plutonism. (4) All isotopic data for the volcanic rocks, massive sulfides, and post-Cambrian mineralization lie on linear arrays, and on the uranogenic lead plot ( 207 Pb/ 204 Pb vs 206 Pb/ 204 Pb) they intersect the lead evolution (growth) curve for other massive sulfide deposits at approximately 1,000 m.y. This is interpreted as evidence of a Precambrian source for the metals and also of the cogenesis of the volcanics, massive sulfide, and later mineralization. (5) The Proterozoic source age preserved in the lead isotope data for the volcanics (and mineralization) is consistent with their formation in a rifting environment or a continental margin. (6) The 206 Pb/ 204 Pb values of 18.27 to 18.35 are unusually high for Cambrian massive sulfides and the calculated model ages of approximately 270 m.y. are about 50 percent too young and possibly result from an increase in the U/Pb ratio of the source in the last billion years. A calculated 238 U/ 204 Pb (mu ) value for the stage from 1,000 (or 1,500) to 540 m.y. ago is high at approximately 13, similar to that measured in the volcanic rocks today, and may account for the unusual 206 Pb/ 204 Pb ratios. (7) The Th/U value of 4 for volcanics and mineralization is normal for continental crustal rocks but is markedly different from the low Th/U of 1 to 2 in Devonian granitoids. (8) The unusual lead isotope signatures for the deposits cast doubt on the proposed conjunction of Tasmania with the Australian mainland in Cambrian times; the relationships with Antarctica await future investigations.