Some examples of the usage of the term extend to recent times:
Geo-Alchemy: Turning Sand into Sandstone and other Microbiological and Bio-Inspired Ground Improvement Technologies.
The abstract for the article Evolution of Knowledge and Ideas in Petroleum Geochemistry - From "GeoAlchemy" to Petroleum Systems Analysis and Beyond (Creaney and Isakson, 2001) indicate that petroleum geochemistry was developed out of earlier disciplines that were geological and chemistry based.
Earlier, in a 1984 article Windows to the Earth (Comrie), has on page 35 a subheading 'Geoalchemy', in it the author focuses on the isotopic geochemistry of volcanics.
Much earlier, in the time of Isaac Newton, the article Newton's Alchemy and early Geochemistry describes how he and other naturalists of the day saw the crystalisation of saltpeter (potassium nitrate) as a model for mineral growth and ore genesis.
It seems from these examples, past and present, that the term 'geoalchemy' stems from the terms geology and alchemy and was (and in some cases, still is) used as a fancy way of saying 'geochemistry'.