Mineralogic alteration patterns in volcanic rocks of the La Libertad gold mining district and its surroundings, Nicaragua

Abstract

Several alteration zones characterized by secondary mineral assemblages of successively higher formation temperatures can be recognized in the Tertiary volcanic country rocks surrounding a swarm of auriferous quartz veins at La Libertad, Nicaragua. The outermost zone (I) is affected by regional burial diagenesis at low zeolite facies (mordenite subfacies) and a thermal gradient < 50 degrees C/km. The secondary phases are smectite and cristobalite-tridymite (+ or - mordenite and heulandite). Passing to zone II cristobalite-tridymite are replaced by quartz, and heulandite becomes more abundant (heulandite subfacies). Next zone (III; laumontite subfacies) is marked by the appearance of laumontite instead of heulandite and mordenite, swelling chlorite instead of smectite (in drill core samples), and calcite. Chlorite then replaces swelling chlorite, and wairakite and epidote become local constituents at depth (zone IV; transition to wairakite subfacies).A steep thermal gradient at La Libertad, exceeding 150 degrees C/km, reveals that the mining district is a fossil geothermal field. The influence of the geothermal system (zones II-IV) can be noticed more than 5 km from the district. Strongly altered rocks forming aureoles around quartz veins define a local alteration pattern characterized by quartz, chlorite, adularia, illite, and pyrite, suggesting that the deposit is of adularia-sericite type. An illite-kaolinite-bearing supergene cap (zone V) covers most of the mining district. The geologic history can be inferred from the study of the regional to local alteration patterns.