Prof Colin Godwin, of the Department of Geological Sciences, University of British Columbia, has developed a new approach to applying galena lead isotope models. Interpretation of lead isotopes from even minute galena samples often can clearly distinguish different ore-forming episodes that produce similar looking prospects.
The same conclusions may be reached conventionally with assays, mineralogy and so on. Unfortunately, a few fist-sized, sulphide-rich grab samples seldom are reliable indicators of the overall gold and silver potential of an entire zone, and they rarely allow the origin and type of deposit to be uniquely identified. On the other hand, lead isotope ratios from two or three pinhead-sized samples of galena can often definitively classify a showing.
A lead isotope analysis from galena in a small exposure or in a weakly mineralized vein in the Alice Arm/ Stewart/Iskut area will indicate whether the mineral occurrence was related to an earlier gold-silver-base- metal event or a later lead-zinc-silver (-molybdenum) mineralizing episode. Because the earlier gold-silver mineralization episode formed such major economic deposits as the Dolly Varden mine, the Premier mine and the Johnny Mountain deposit, this distinction between sources of mineralization can be used to govern the sense of urgency, intensity or even necessity with which further exploration work is carried out.
Early work on the analyses of galena lead isotopes was limited because of the high cost, difficulty and inaccuracy of analyses. With the advent of modern, automated mass spectrometers, analysing galena lead is now accurate, rapid and relatively cheap. Early interpretations were limited, not only by analyses, but by models that were not designed to discriminate among different types of deposits.
The “lead/lead” or “common lead” method of radiometric dating is based on the accurate measurement of lead isotope abundances and on the changing ratios of radiogenic lead isotopes over periods of geologic time. Galena is used because, once it has crystallized, its lead isotopic composition remains constant because of the absence of any radioactive elements in its crystal lattice. The reference isotope lead 204 is not produced by radioactive decay and so the contained amount of this isotope has always been constant. Isotope lead 206 and lead 207 are formed by the radioactive decay of uranium 238 and uranium 235 respectively. Isotope uranium 235 has a much shorter half-life than uranium 238; therefore, most of the uranium 235 that was originally present has decayed. Consequently, in the Paleozoic, the ratio of the radiogenic lead 207 to lead 204 changes progressively less with younger time, compared with the ratio of radiogenic lead 206 and lead 204.
One of the new galena lead isotope models on which Prof Godwin relies in the Alice Arm/Stewart/Iskut area involves simple comparison of lead isotope ratios of galena from a showing of unknown age and origin, to ratios for galena from known ore deposits. This “fingerprinting” technique relies on the recognition of isotopic differences in galena from different mineralizing episodes. A detailed study of the lead characteristics of the major deposits and showings in the Alice Arm/Stewart/Iskut area shows that deposits can be grouped into two statistically distinct clusters. A sampling of the data is presented in Table 1 and the fingerprinted difference — into Groups 1 and 2 — is plotted in Figure 1. Group 1 data are from deposits that are of the same age as their host Hazelton Group volcanic rocks. Group 2 data are from vein deposits that are later and related to post-accretionary Tertiary granitic intrusions of the Coast Crystalline Complex that were generated from subduction under the Intermontane Belt. Deposits with isotope ratios in Group 1 include the Dolly Varden mine in the Alice Arm area, the Premier mine in the Stewart area and the Johnny Mountain deposit in the Iskut River area. Group 2 deposits include the British Columbia Moly deposit at Alice Arm and numerous veins. Historically, deposits related to Group 1 have been more productive and currently are more attractive to explorationists.
Given that many related deposits in the Alice Arm/Stewart/Iskut area are overburden-covered and in mountainous terrain, it is extremely difficult to determine by either normal geological observations or other isotopic techniques whether mineral showings are related genetically to the Hazelton Group. Analyses of galena lead isotopes, however, allow deposits to be classified unambiguously into either Group 1 or Group 2.
The lead isotope model in the Alice Arm/Stewart/Iskut area is an effective way of evaluating the commodity potential of a small mineral showing or of setting exploration priorities on properties that host several varied mineral occurrences.
Prof Godwin’s research, supported mainly by the British Columbia Science Council, has defined a number of models that are applicable to the different terranes of the Canadian Cordillera. These models offer advantages, similar to those described above, to explorationists evaluating properties. REFERENCES Alldrick, D. J., Gabites, J. E. and Godwin, C. I. (1987): “Lead Isotope Data from the Stewart Mining Camp.” British Columbia Ministry of Energy, Mines and Petroleum Resources, Geological Fieldwork, 1986, Paper 1987-1. Godwin, C. I., Gabites, J. E. and Andrew, A. (1988): “LEADTABLE: A Galena Lead Isotope Data Base for the Canadian Cordillera.” British Columbia Ministry of Energy, Mines and Petroleum Resources, Paper 1988-4. Joyce Musial is a Toronto-based geologist and freelance writer.
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