12/29/20 | 500 Foot Level
500 Level Samples
In early December, after dewatering and preliminary exploration for safety were completed, rehabilitation efforts commenced on the 500 level of the Bates Hunter. Multiple grab samples were collected from a stope in between the Bates Hunter and Becker-Bates shafts. These samples displayed visible “peacocking,” a rainbow-like sheen that miners in the area historically associated with high-grade ore. After collection, the samples were sliced into halves for analysis of the vein structure and mineralogy. Pyrite, chalcopyrite, galena, and “grey copper” sulfosalt minerals were visually observed (Figures 1 & 2). The sulfosalt minerals are of particular interest due to their theorized association with gold in recently published research into the mineralogy of the Central City Mining District from the Colorado School of Mines (Alford, L. 2018, 2020).
Figure 1: Ore Sample, 500 Level Stope
Figure 2: Cross-section of the high-grade vein with massive chalcopyrite making up a bulk of the center with “grey copper” sulfosalts lining the edges. Pyrite surrounds the copper rich mineralization.
Two samples from the Bates vein exposure in a stope on the 500 level, east of the Bates-Hunter shaft, were sent to Hazen Research, Inc. in Golden, Colorado, where fire assays were performed with a gravimetric finish. Results for the samples appear in Table 1, from Analytical Report 20M03300. Values reported are in ounces per short ton (opt).
Table 1: 500 Level Sample Assay Results
An extensive channel sampling program is planned for early 2021 to further quantify the grade of ore on the 500, 550 and 640 levels and to determine future areas of production.
Flotation Testing Samples
Initial process testing of materials from the 240 level stope fill began in early 2020, with crushing, wet-grinding and gravity recovery of gold from jig and table concentrates. Material was ground to p80-100 mesh (80% passing a 100-mesh screen) in closed circuit (ball mill and screw classifier). A Denver 8” x 12” mineral jig produces a gravity concentrate from the closed circuit which is then tabled on a Gemeni GT60 shaking table. Overflow (sized material) leaves the grinding circuit and is decanted and stored in bulk sacks. From the sacks, samples are taken for flotation testing in a Denver D12 laboratory flotation machine. Beginning in November, 2020, the company began testing various reagent suites using legacy chemicals at the Golden Gilpin mill and samples of modern Clariant flotation reagents. Assay results from the concentrate and tailings for the initial series of tests appear in Table 1, from Analytical Report 20M03300. Sample ID numbers ending in “-FC” are flotation concentrates with corresponding tailings sample number ending in “-FT”.
Figure 3: Weighing Flotation Concentrate
Figure 4: Flotation Testing at the Golden Gilpin Mill
From the assay values and test weights, gold recovery and concentration ratios were calculated, as well as flotation head grades. Gold recovery in initial flotation ranged from 73.22 to 96.63% with an average of 88.73%. Concentration ratios ranged from 4:1 to 67:1 with an average of over 19:1. Flotation head grade averaged 0.06 ounces Au per ton after the gravity recoverable portion was removed. At this stage, the company is not yet able to calculate the percentage of gravity recoverable gold and actual average head grade of the stope fill material removed from the 240 level. Future testing of bulk samples of fresh material is planned to determine gravity recovery efficiency, final flotation concentrate grade and final tailings characteristics.
Table 2: Flotation Testing Sample Assay Results
Jared Tadla, Geologist
Tom Pietrykowski, Environmental Chemist
Matt Collins, General Manager