An Exploration Application for Lead Isotope Ratios, Stewart Mining Camp, Northwestern British Columbia

Exploration & Mining Geology, Vol. 2, No. 2, 1993

DANI J. ALLDRICK, British Columbia Geological Survey Branch, Ministry of Energy, Mines and Petroleum Resources, Victoria, British Columbia, Canada, COLIN I. GODWIN and ALASTAIR J. SINCLAIR Department of Geological Sciences, The University of British Columbia, Vancouver, British Columbia, Canada

The Stewart mining camp has a long history of precious and base metal mining. The camp is abundantly mineralized, with more than 200 widespread, varied, vein prospects, deposits and orebodies. These show wide variations in the mineralogy of ore, gangue and alteration assemblages, and wide variations in textures and structural settings. This report establishes the timing of the different metallogenic epochs and suggests a specific exploration application for systematic lead isotope analyses. Galena lead isotope ratios from a range of mineral occurrences indicate that all these varied deposits formed during two main mineralizing events. Lead isotope data from the Stewart mining camp do not provide absolute ages for the formation of mineral deposits, but the relative distributions of data are consistent with dates determined in other studies. One ore-forming episode occurred in the Early Jurassic and the other in the Middle Eocene. Both metallogenic epochs were brief, regional-scale phenomena. Some deposits from the younger mineralizing episode were em-placed adjacent to or superimposed on the older deposits. Field and laboratory studies show that the two metallogenic epochs are characterized by different base and precious metal suites: Early Jurassic gold-silver-zinc-lead-copper; and Middle Eocene silver-lead-zinc. Therefore, showings of different ages have different base and precious metal potentials and deserve different exploration emphasis. Lead isotope studies provide a reliable, cost-effective method for evaluating the age and commodity potential of small or recently discovered mineral showings and for setting exploration priorities on properties with several varied mineral occurrences. The determination of the age and metal potential of a prospect at an early stage of exploration may . govern the urgency, intensity or even the necessity for further exploration work.