I don't know how the high grade shoots were formed however the...

I don't know how the high grade shoots were formed however the assumption that they are the babies of a mother lode of similar grade is not necessarily valid. Hydrothermal fluids aren't high grade and don't come from high grade igneous rocks. Under pressure they are forced laterally and upwards from the igneous source through weaknesses in the surrounding rocks and accumulate in zones where porosity is high and/or where the geochemistry and temperature-pressure regime allow the metals to precipitate. This accumulation is slow...very low grade fluids being stripped of their metals over millions of years. A mother lode, if it exists, is more probably a big baby, i.e. a larger zone where the conditions were right for the hydrothermal fluids to drop their metals.