Carbonate-hosted lead–zinc occurrences in northeastern British Columbia with emphasis on the Robb Lake deposit: Discussion

Abstract

Paleotemperature Three methods were employed by the authors in an attempt to evaluate the geothermal history of the Robb Lake area. (a) Bitumen data: An analysis of four bitumen samples indicated a maximum of 200-250°C, described as rough estimates at best (p. 1755) by Powell, the person who made the (subjective?) rough estimates. ( 6 ) Illite crystallinity of three samples gave temperature ranges of 175185, 195-205 and 2 15-230C, described as approximations (p. 1755). Inasmuch as none of these temperature ranges overlap and, therefore, the verall range of 175-230°C is entirely dependent on he number of samples studied, one is left to wonk er what the range would have been if four. six, or ten samples had been used. r (c) FTuid inclusions in on1 y two quartz crystals pave a filling range of 210-260°C (p. 1756). Their Fig. 4 (p. 1742) indicates considerable overlap of huartz, galena, sphalerite, and white dolomite deposition, thereby implying that quartz and sulphides were formed at much the same temperature (i.e., 210-260~). The authors later, however, assume the sulphides formed at 100150°C. the common temperature range of Mississippi Valley-type carbonate-hosted lead-zinc deposits (p. 1759). These two assertions are obviously at variance with one another. On the basis of the determined by these three techniques, the authors conclude that temperatures in the approximate range of are paleotemperatures (p. 1756) for that part of the stratigraphy under discussion in northeastern British Columbia, a conclusion drawn after analysis of a total of only nine samples. As shown above, because results by two methods carried very substantial qualifications as to their validity and, in addition, no error limits were given, the ultimate temperature range possible by bitumen and illite analysis is left to the reader to decide. A fluid inclusion filling temperature range is provided, but without backup data as to the size, nature or kind of inclusions, or the possible error limits of the determination. Thus, in spite of the authors' claim to the contrary, three weak lines of evidence do not combine to produce a reasonable estimate of in the Robb Lake area. Having arrived at estimated maximum of 200-230°C and an estimated burial depth of 5 km, Macqueen and Thompson proceed to construct a range of geothermal gradients (presented in their Fig. 15) stating that paleotemperature data, because they are derived from several indicators, are considered more reliable than the burial history data, which are fraught with uncertainties . (p. 1758). Nevertheless, the authors then combine the suspect data, the assumed 100-150°C sulphide deposition temperature, and the uncertain burial-thermal history of the host rocks to draw conclusions concerning time and depth of mineralization. Such conclusions can only be as valid as the assumptions upon which they are based.