Mineralization related to the biotite syenite at the Money Knob intrusion-related gold deposits, Livengood, Alaska

Abstract

The recently discovered Money Knob Au deposit represents a bulk tonnage deposit with a measured, indicated, and inferred resource of 20.6 million ounces of gold at an average grade of 0.52 g/t and a cut-off grade of 0.22 g/t. The deposit is located in the central Alaskan portion of the Tintina Gold Province. The low-grade, high tonnage resource of the Money Knob deposit is hosted in a tectonically complex package of Devonian sedimentary and volcanic rocks that have been overthrust by Cambrian ophiolitic rocks. Gold mineralization formed in close association with a mid-Cretaceous felsic dike complex that intruded into this package of sedimentary and volcanic rocks and is thought to be part of the Tombstone Plutonic Suite. A major phase of the felsic dike complex is a biotite syenite, which occurs late in the deposit development and has a U/Pb zircon age of 92.04±0.14 Ma. The dikes account for about less than 10 percent of the total rock volume in the deposit, with individual dikes ranging from several centimeters to over 10 meters in width. The biotite syenite crosscuts preexisting ore zones and associated altered rocks, but has clearly also been affected by strong hydrothermal alteration and can be mineralized (0.25 to 1.0 g/t Au). The observed field relationships indicate that deposit formation at Money Knob occurred in a tectonically complex environment characterized by tectonic relaxation along reactivated thrust faults. Utilizing cathodoluminescence microscopy coupled with fluid inclusion petrography and microthermometry, four different quartz types were recognized in the quartz veins that formed at progressively decreasing temperatures. Homogenization temperatures gathered from secondary fluid inclusion assemblages suggest that early quartz formed at temperatures of at least 305-310 degrees C from hydrothermal fluids that contained a high amount of CO2 while the next quartz type formed at temperatures below 200 degrees C at the same pressure conditions. This low-temperature quartz and subsequent quartz types formed at even lower temperatures also contain significant amounts of CO2. It is estimated that the vein quartz formed at a minimum depth of 3 kilometers. Lithostatic pressure conditions were maintained throughout the formation of the different quartz types. The gold in the quartz veinlets occurs as fracture-controlled free gold within the earliest type of quartz. Gold formation clearly post-dated the formation of the high-temperature quartz. Outside the veins, much of the gold occurs as fine-grained native gold disseminations within the intensely altered host rock. Optical microscopy, cathodoluminescence microscopy, and scanning electron microscopy were used to identify the different alteration mineral associations related to quartz veining. Hydrothermal alteration occurred in a generally retrograde environment. The earliest alteration product observed is hydrothermal biotite. Formation of biotite alteration was followed by sericite alteration and then albite alteration. Ankerite and lesser dolomite overprint biotite and sericite alteration and can overprint or accompany albite alteration. At lower temperatures, these alteration mineral association were overprinted by clay alteration, involving the formation of smectite and kaolinite as well as ankerite and lesser dolomite. Geochemical data gathered from the different alteration mineral associations suggest that biotite and sericite alteration were accompanied by slight enrichment in potassium while albite alteration was typified in a pronounced enrichment of sodium and silica. Albite alteration appears to have coincided with gold enrichment while igneous rocks affected by the low-temperature clay alteration have the lowest gold grades. Although the biotite syenite at Money Knob does not represent the direct source of the mineralizing hydrothermal fluids, the close spatial and temporal relationship between igneous activity resulting in the formation of the dike complex and mineralization suggests that Money Knob can be classified as an intrusion-related gold deposit.