PRELIMINARY LAVA FLOW MORPHOLOGY STUDIES AT THE FIVE MILE LAKE VMS PROSPECT, ARCHEAN VERMILION DISTRICT, NE MINNESOTA: IMPLICATIONS FOR VOLCANIC PROCESSES, VOLCANIC PALEOENVIRONMENTS, AND VMS EXPLORATION

We have undertaken a field-based, detailed investigation of Archean (~2.7 billion year old) basalt pillow lava facies at the Five Mile Lake volcanic-associated massive sulfide (VMS) prospect located 15 miles southwest of Ely, Minnesota. Detailed morphological (facies) mapping provides a means to identify the locations of volcanic eruptive centers in both modern and ancient volcanic rocks. Studies have also shown that VMS-mineralization is most often associated with vent-proximal volcanic facies. Our detailed facies mapping included measuring various features of pillow lavas, including pillow shape, pillow horizontal and vertical dimensions, pillow vesicularity, and the thickness of formerly glassy pillow margins. Several studies have found that the glassy margins on vent proximal sheet flows tend to be thicker than the glassy margins on more distal, pillow lavas formed from the same eruption. This occurs because a) vent proximal lavas are hotter, and therefore a thicker quenched rind is produced; and b) more distal pillow lavas generally contain more crystals of crystal nuclei that inhibit the formation of their glassy outer margins. Therefore, the thickness of the glassy margins on these lava flows is also a general indicator of proximity to the volcanic vent. We have been able to identify several vent-proximal locations where mafic dikes can be seen undergoing vertical, then lateral transformations into pillow lavas. We have found that the formerly glassy pillow margins are clearly thicker at these vent-proximal sites than in pillows that are distal from the vents. This is the same relationship observed in modern subaqueous pillow lavas. This has significant implications for mineral exploration, as measurements of formerly glassy pillow margins can be easily and quickly completed during field mapping, and may provide an effective method to identify regions within monotonous pillow sequences that are more likely to contain VMS mineralization.