Geochemical analyses of possible marketplace activities on an ancient plaza floor in Coba, Mexico were conducted by chelate extraction coupled with ICP (Inductively coupled plasma) determination, and by pXRF (portable x-ray fluorescence). The DTPA chelate procedure provided a measure of extractable trace elements on the surfaces of soil particles. The pXRF analysis of air-dried, crushed and sieved samples provided total element analysis. The DTPA extraction method has proven to be a reliable soil analysis procedure for anthropogenic resides of ancient activities but the requirement for ICP analysis precludes this procedure from field laboratory work.  The pXRF instrument can be transported to a field laboratory where only drying, grinding and sieving are required for total element analysis.  Correction coefficients for analysis of samples contained in plastic bags are determined.  Extractable and total element analyses helped to determine differences in plaza floor materials and to locate areas of ancient feasting and marketplace activities.