Paper No. 36-3
Presentation Time: 9:00 AM-3:30 PM
CHARACTERIZING THE PARAGENETIC SEQUENCE OF ALTERATION AT THE MOONLIGHT IOCG DEPOSIT, PLUMAS COUNTY, CA
The Moonlight deposit is a hematite-rich Iron Oxide Copper Gold (IOCG) deposit which is located in Indian Valley, California. It is associated with the Lights Creek Stock, along with two other deposits that were historically mined, Engels and Superior. The Moonlight deposit and the regional geology are not well characterized and have not been studied in detail. The objective of this project is to characterize the alteration assemblages and determine the order of alteration by studying the cross-cutting vein relationships. Drill core was analyzed in hand sample to describe the cross-cutting vein relationships on a macro scale. Core samples were then selected to be made into thin sections based on the different stages of alteration visible. The thin sections were analyzed to produce petrographic descriptions and observe the cross-cutting vein relationships in microscopic view. Cross-cutting veins with different mineral assemblages indicate that the deposit has undergone multiple phases of alteration. The host rock is quartz monzonite as it is made up of mostly plagioclase and orthoclase with some quartz. The host rock also contains large tourmaline crystals that are most likely from an alteration phase. The black veins are of similar mineralogy to the host rock however quartz is more widespread, making up approximately 70% of the vein, and the vein has a finer texture with quartz crystals that are approximately 90% smaller than the host rock. The first phase(s) of alteration created the black veins which cross-cut each other and the final phase(s) created a light brown vein that cross-cut everything else. Hand sample view suggests that there are two phases of alteration, but closer examination in thin section suggests that there were at least four stages of alteration as there are additional veins and inclusions that can only be seen under a higher magnification.