In Ian's Keeble Trinity blog he writes:
Gold is not about giving the icon value, in the worldly sense. The value of the icon is its subject, in this case the Most Holy Trinity. Rather, Gold has a very particular set of qualities, which speak most eloquently of the nature and presence of God, which makes its use in iconography eminently suitable. According to Christian mystical theology, especially in the teachings of the early fathers and the Hesychasts of the eastern Church, these are fundamental ways in which we can describe God: as Light that can be perceived and reveals Itself, but yet as utterly Unknowable, without limits or definition. "True Light itself, Eternal Day, you are far brighter than the sun Illuminating with your grace, The deep recesses of each heart" (VIth century hymn attri. to St Hilary of Poitiers). Gold speaks of this divine light: it is a 'bright darkness', capable of capturing and intensifying the power of light to the point that it is blinding, while at the same time so dark it seems to have no end.
I really like the quote attributed to St. Hilary: God as light that illuminates the deep recesses of our hearts. When we reflect on the gold in the icon it should draw our mind - and heart - towards contemplating the divine light of God's love as expressed in the Heart of Jesus.