INITIAL INVESTIGATION OF SAMPLES OF THE VIRTUE MINE, OREGON: A STUDY INTO THE TEXTURAL, MINERALOGICAL, AND GEOCHEMICAL PROPERTIES OF GOLD BEARING QUARTZ VEIN DEPOSITS

Gold is increasingly important in medical, technological, and environmental applications globally. Efficient exploration for new gold deposits and reevaluation of previously mined deposits is warranted. One type of deposit that should be reevaluated is gold bearing quartz veins (GBQV). This project focuses on a GBQV deposit located in the Virtue mining district which gains its name from its most productive mine, the Virtue mine, 12 miles east of Baker city, Oregon. This district is defined by 8 ore bearing quartz veins containing predominantly quartz with minor calcite, scheelite, and sericite as well as trace amounts of pyrite, chalcopyrite, and free gold favoring brecciated areas of the vein. The mine was operated on and off under varying management from 1862-1907 with the total production from the mine, accounting for inflation, being ~$2.2 million.

Three thin sections from the mine were investigated using Polarized Light Microscopy (PLM) and Scanning Electron Microscopy (SEM) paired with electron dispersive spectroscopy (EDS). PLM indicated one thin section dominated by approximately 90% quartz and 10% calcite, another by 20% quartz and 80% calcite, and a third by 65% quartz and 35% calcite. Most quartz crystals were sub-anhedral at 3-5mm in diameter and were intruded by calcite veins throughout, most veins at a width of 0.5-1mm.The intruded areas were brecciated and altered forming sulfides and oxides around the affected zones. Sulfides and oxides in each thin section were 90% sub-anhedral with 10% being euhedral and all were 0.1-1mm in width. Detailed SEM analysis highlighted sulfides and oxides near the brecciated areas as well as carbonate intrusions. Most sulfides and oxides showed zonation. Acicular oxides radially surrounded the edges of these crystals. SEM identified a beam sensitive Hg-S phase within brecciated areas. Au was not detected in any SEM analyses, but this could be due to the detection limit of 0.1 wt% for EDS. The current working paragenesis for the set of samples is the initial formation of quartz veins, a later stage hydrothermal alteration mineralizing carbonates, sulfides, and then oxides. Results provide new constraints on the textural, mineralogical, and geochemical properties of the Virtue Mine and will aid in the development of potential proxies when evaluating similar deposits.