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The pipeline extension is now slated to be completed this quarter, the Calgary-based energy infrastructure firm said.
Archeological finds are a frequent impediment to infrastructure development in Mexico.
Construction of an assembly plant for car-maker Audi in Puebla state was recently halted by finds, as was the planned widening of the Mexico City-Tepoztlán highway.
The 235km pipeline extension will run from the terminus of an existing 130km pipeline in San Luis Potosí state, through the states of Hidalgo and Querétaro, and on to San José Iturbide, connecting to Mexico's national gas pipeline system.
Tamazunchale, which will have a capacity of 630Mf3/d (17.8Mm3/d), will supply Mexican utility CFE's 610MW El Sauz combined cycle plant.
Construction, through TransCanada's local unit Transportadora de Gas Natural de la Huasteca (TGNH), began in November 2012 and was due to wrap in the first quarter of this year.