Quartz from Allchar as monitor for cosmogenic 26 Al: Geochemical and petrogenetic constraints

Results of a multidisciplinary study on quartz concentrates (mineralogically separated) and etched concentrates (stoichiometric quartz) from three locations at Allchar (Macedonia) are presented. The investigation of quality and composition of these quartz samples is of great interest because the same material has been previously used as monitor for 26Al Acceleration Mass-Spectrometry (AMS) erosion rate estimates. Two genetically different types of quartz are distinguished in the studied samples which petrologically can be described as hydrothermally altered dacites or quartz latites; i.e. volcanic (QV) and hydrothermal (QH) quartz with relative proportions of QH:QV around 3:2. QH is genetically related to the Allchar Sb–As–Tl–S mineralization having very high Sb (85–785 ppm), As (7.6–78 ppm) and (Tl 3.3–4.0 ppm) contents. This type of quartz is also characterized by very high Li (129–138 ppm), Al (2424–2520 ppm) and Ti (153–219 ppm) concentrations. QV appears to be much less enriched in trace elements having Al and K contents ranging from 0 to 280 ppm and from 50 to 85 ppm, respectively. 26Al AMS measurements were done on the samples containing two genetically different types of quartz but this had no effects on the interpretation and erosion rate determinations. However, the extremely high Al concentrations in the analyzed quartz have generally negative effects, mainly by decreasing 26Al/27Al ratios and thus causing an increase of the detection limit. The disagreement between the results of 26Al AMS analyses and quantitative geomorphologic data for one location is probably caused by different geographical position with respect to the direction of cosmic rays