LARAMIDIC CU-AU PORPHYRY-STYLE MINERALIZATION IN MOCHITLAN, GUERRERO, MEXICO

Calcic to calc-alkaline, metaluminous, volcanic-arc porphyritic granodioritic sills and stocks are present at Mochitlán and El Molino creek, Guerrero. A swarm of sills with Cu-Au mineralization crops out north of Mochitlán town. They intrude dolomites of the Cretaceous Morelos Formation and anhydrite interbedded with slates of the Huitzuco Anhydrite. Quartz veinlets in both the granodiorite and the dolomite are widely distributed around and above a potassic alteration zone of biotite and orthoclase veinlets. Sericitization and propylitization (epidote, chlorite, and actinolite) are restricted to the igneous rocks. Replacement bodies are located at the contact between the sills and the dolomite. Jasperoid hydrothermal breccias contain the higher Au values. The main minerals identified in the mineralized zone are: Pyrite, arsenopyrite, bornite, covellite, enargite, molibdenite, tetrahedrite, pyrargirite, chalcopyrite, fluorite, digenite, chalcocite, and malachite. Eleven drill-holes sunk in the Santa Rosa claim intercepted 52 m @ 0.7% Cu, 0.4g/t Au, 0.015% Mo, and 11.3g/t Ag or 202 m @ 0.3% Cu and 0.3g/t Au in drill-hole 01. Drill-hole 05 intercepted 56 m @ 0.4% Cu and 1.0g/t Au including 30 m @ 0.62% Cu and 1.5g/t Au. New K-Ar dating of magmatic biotite (drill-hole-SR00-03-182-184) is 65.8+/-1.7 Ma.

A porphyritic granodiorite stock crops out at the Molino creek, approximately 5 km to the south of Santa Rosa. There, a well developed stockwork with quartz and orthoclase and rare biotite veinlets contains abundant pyrite, molibdenite and rare chalcopyrite. New K-Ar dating in hydrothermal orthoclase is 62.1+/-1.6 Ma; and 64.9+/-1.6 Ma in hydrothermal biotite.

Similar Laramidic igneous activity and hydrothermal systems have been recognized in Guerrero State. Gold-Cu skarn and stockworks with porphyry-copper deposits characteristics at El Bermejal and Nukay, Mexcala, Guerrero, are 65 to 63Ma and Las Salinas, Guerrero, porphyry copper deposit is 63 Ma.