Neutron activation analysis of gold in Archean igneous and metamorphic rocks of the Rice lake-Beresford lake area, southeastern Manitoba

Abstract

Gold abundances determined by the method of neutron activation are reported for 26 replicate analyses of a variety of igneous and metamorphic rocks. Samples analyzed come from scattered locations in and adjacent to an Archean greenstone belt containing numerous low-grade gold-quartz veins. Most of the richer veins are enclosed by mafic intrusions. The average gold content of each of the rock types is as follows:Rock Type No. of analyses Au (ppb)Northern Granites; quartz diorite: 4 1.9 + or - 1.2Manigotagan Gneissic Belt; paragneiss: 2 0.6 + or - 0.4associated quartz monzonite: 2 0.3 + or - 0.1Rice Lake Group; metavolcanics: 3 0.3 + or - 0.2Mafic intrusives; gabbro, diabase: 2 18.2 + or - 1.5Central plutons; quartz diorite: 5 1.1 + or - 0.6related felsic porphyry dykes: 2 6.1, 1.1Altered diabase wall-rock and vein zone: 6 0.5-325Gold abundances for the quartz diorite, quartz monzonite and metavolcanic groups are fairly typical of younger rocks of similar composition. Explanations are put forward to account for the unexpectedly low gold content of the paragneiss samples and relative gold enrichment in one felsic dyke sample.There is evidence to indicate that the mafic intrusions may be the source rocks of the gold. Thus, in addition to the spatial relationship of the richer veins to the mafic intrusions the latter also contain an order of magnitude more background gold than other rocks in the area. Furthermore, gold is apparently depleted in altered diabase wall-rock adjacent to a vein in the San Antonio gold mine.Non-repeatable replicate analyses observed in some mafic samples may be due to inhomogeneous distribution of gold between powder aliquots resulting from an erratic distribution of gold-rich host minerals.