Biogeochemical prospecting for gold-bearing quartz veins of the Piedmont, Great Falls, Maryland

Abstract

A plant-soil chemistry exploration survey of gold-quartz mineralization in the Wissahickon Formation of the Appalachian Piedmont was done in the vicinity of the Ford Mine, near Great Falls, Maryland, USA. Leaf samples were collected from nine plant species available at 73 sites: Acer rubrum (red maple), Asimina triloba (pawpaw), Carpinus caroliniana (ironwood), Cornus florida (flowering dogwood), Fagus grandifolia (American beech), Kalmia latifolia (mountain laurel), Nyssa sylvatica (black gum), Polystichum acrostichoides (Christmas fern), and Viburnum acerifolium (maple-leaf viburnum). Soil samples from the B horizon (<0.15 mm size fraction) were collected at all locations. Seven samples of F. grandifolia and seven corresponding soil samples along one traverse across the mineralization, and two to three samples of other plant species, plus six soil samples along a second traverse (38 samples in all) were analysed for Au, As, Sb, Br, Na and K by the neutron activation method. All plant samples (150) and soils were analysed for Zn and Cu by atomic absorption spectrometry. In soils, high concentrations of Au (33 ppb), Sb (1.8 ppm), Zn (646 ppm), and a high K 2O/Na 2O value (9.6) were directly related spatially to a gold-quartz vein along one traverse. F. grandifolia had strong concentrations of Au (to 17 ppb) and Zn (to 104 ppm) and a high K 2O/Na 2O (to 136) value displaced downslope up to 140 m from the vein along this same traverse. Soil would be the best prospecting sample in these situations. Of the nine plants species analysed ( n = 25), one specimen of N. sylvatica of three analysed had the highest Au content (66 ppb vs. <0.1 ppb in the corresponding soil). Other high values observed were for V. acerifolium in both samples analysed (19 and 16 ppb), and for F. grandifolia in two of seven samples analysed (17 and 12 ppb). These species may be accumulators of Au. In the study zone, only F. grandifolia has a dense enough areal distribution so as to make it useful in biogeochemical exploration for Au.