The iron deposits at Lammin, Piddig, Ilocos Norte, are replacement bodies in metamorphosed volcanic and sedimentary rocks. Calcareous horizons were preferentially replaced. Metamorphism of host rocks and eventual mineralization were caused by late ore solutions genetically related to a diorite intrusion.

Quartz, calcite, epidote, pyroxene, magnetite, and hematite are the chief minerals. Primary magnetite replaced altered rocks along grain boundaries and cleavage planes. Hematite generally occurs along fractures and as intergrowths with magnetite. The mineralized belt, 1 2/4 kilometers long, contains 13 groups of magnetite outcrops which average about 3 meters in width. Based upon surface indications, the estimated iron reserve is about 850,000 metric tons. Analyses indicate from 50 to 71% Fe and 0.085% S in outcrops.