Comment on “Changes in sulfides and platinum-group minerals with the degree of alteration in the Roby, Twilight, and High Grade Zones of the Lac des Iles Complex, Ontario, Canada” by MLN Djon and SJ Barnes

Abstract

I wish to raise three main issues arising out of a carefulreading of the paper by Djon and Barnes (2012):1. It employs a fundamentally flawed technique by usingthin sections/polished blocks to measure the absoluteand relative abundances of base metal sulfide (BMS)minerals and trace platinum group minerals (PGMs) insilicate whole rocks, resulting in errors in calculationsfor mass balance and PGE deportment;2. It describes poor analytical methods for electron probemicro-analysis (EPMA) of PGM and laser ablationinductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of the BMS, resulting in errors inthe reported concentrations for some elements;3. It shows poor preparation of the manuscript, omittingimportant details on which values for PGE concentra-tions were used in calculations for mass balance andPGE deportment, and why. This makes for the inabilityof the results to be checked and evaluated critically byreaders.It is also unfortunate that the authors do not acknowledgeany referees for their efforts, which make it impossible forthe reader to judge whether (a) the referees were knowl-edgeable in applied/process mineralogy and EPMA/LA-ICP-MS, and (b) the authors implemented the obvious rec-ommendations that would have been made by such experts.Because this journal is mainly devoted to the study ofmineral deposits, some background is relevant to put theabove criticisms in context. Studies of PGE mass balancesand their deportment in rocks is a well-developed techniquein the discipline of applied/process mineralogy. Similarly,well-established procedures have been developed in theanalytical geochemistry community for in situ analyses ofPGEs in sulfide minerals by LA-ICP-MS. Nonetheless, bothapplied/process mineralogy and LA-ICP-MS analyses ofminerals are relatively new areas of scientific endeavor formany mineral deposit geologists and academics, perhapsexplaining some of the mistakes made.Applied/process mineralogy only evolved internationallywith the first International Council for Applied Mineralogy(ICAM) conference in 1981, having been developed in someof the major mining companies and some governmentresearch laboratories [e.g., Canmet (Canada), CSIRO (Aus-tralia), and Mintek (South Africa)]. Unfortunately, there areveryfew universitieswherestudentscanbecomeproficientinappliedandprocessmineralogy;mostexpertshavinglearntonthe job (Schouwstra and Smit 2011). There are also very fewbooks devoted to the subject, a rare one being by Petruk(2000); most case studies being found in conference proceed-ings [e.g., Canadian Mineral Processors, ICAM, and TheMinerals, Metals, and Material Society (TMS-AIME)], ormore recently in the Conference of Metallurgists (e.g., Fordet al. 2009, 2011) and Minerals Engineering (e.g., Lotter2011). Thus, proper procedures for PGE mass balances anddeportment are not well known outside the applied/processmineralogy community. Secondly, LA-ICP-MS of mineralswasdevelopedonly20yearsago,fromthepioneeringworkofJackson et al. (1992), and proper procedures for analyses ofBMS has been only widely possible more recently.