System type: Sediment-hosted Carlin-type gold along Carlin Trend
Target: Replacement-style disseminated gold
The Richmond Summit Project is within the central Carlin Trend 4 miles northwest of the Mike copper-gold deposit within the Gold Quarry (Maggie Creek) district and 5 miles south of the Carlin gold mine, both areas operated by Newmont Mining Corporation. Gold production and reserves/resources in the Maggie Creek district total at least 29 million ounces, with at least 10 million ounces in the Carlin mine area.
The property lies at the southern tip of the Lynn window, an area exposing carbonate (calcareous) rocks in the lower plate of the regional Roberts Mountains thrust fault. Lower-plate carbonates host most of the gold mineralization along the Carlin trend, although mineralization locally extends into the overlying upper-plate rocks. Although rocks exposed on the project are dominantly upper-plate siliciclastics, thick sections of carbonates believed to part of the lower plate have been intersected in drilling and are locally exposed as thrust slices within the upper-plate rocks. In addition to this favorable stratigraphic setting, the project lies on the southern extension of the Post fault, which crosses the northeastern portion of the project. The northerly-trending Post fault system is a major structural control to world-class gold deposits in the northern Carlin trend north of the project such as Post-Betze (Goldstrike) and Meikle. The project contains numerous north trending, Late Eocene rhyolite and andesite dikes which follow the Post fault trend. These dikes can be traced north along the Post fault system from the project through the Genesis, Deep Post, Goldstrike and Meikle gold deposits. Carlin-trend gold mineralization occurred during emplacement of these dikes, which are common within the gold deposits. At Richmond Summit, known gold mineralization occurs along the margins of these dikes and within adjacent wallrock.
Carlin-type gold mineralization on the project occurs in several widely-spaced areas within upper-plate siltstones and greenstone along the margins of dikes. The strongest gold mineralization identified to date occurs in the Main Zone, where select surface samples reach 7.75 ppm Au. The mineralization is poorly exposed, but historic sampling in trenches returned 3.048 ppm gold over 20 feet, 4.960 ppm over 10 feet and 3.59 ppm over ten feet. Limited historic drilling (prior to 1991) intersected shallow gold mineralization within 200 feet of surface, including 2.540 ppm over 10 feet. Mineralization is hosted in greenstone and shale adjacent to and locally within a north-trending andesite dike and is accompanied by strongly-elevated arsenic (to 2.7%), antimony (to 450 ppm) and mercury (to 3.3 ppm), a signature consistent with Carlin-type gold mineralization. Other mineralized areas include the Ridge Zone, 1,200 feet west of the Main Zone, which contains anomalous gold at surface to 1.97 ppm in siliciclastic rocks and greenstone adjacent to a rhyolite dike; a float sample from Ridge returned 3.7 ppm.
Two deep core holes have been completed in the Main Zone and one at the Ridge Zone. One of the Main-Zone holes intersected anomalous gold to 400 ppb with strongly-anomalous As to 2,600 ppm along the sulfidized and argillized to silicified margins of a dike and within adjacent shales. The dike occupies a newly-recognized north-trending fault, which is apparently a significant control to the Main Zone mineralization. A second, deeper follow-up hole was unmineralized but successfully intersected lower-plate carbonates (based on conodont ages). Unfortunately, the hole largely followed the dike to depth without testing enough of the surrounding country rock. One deep core hole at the Ridge Zone also intersected lower-plate carbonates. Although unmineralized, the hole is believed to have failed to test the structure controlling the gold at surface. Therefore, drilling to date has not successfully intersected the roots of the two anomalies within the lower plate.
The target at Richmond Summit is structurally-focused gold mineralization with surrounding replacement-style mineralization similar to that in other gold deposits along the Carlin Trend. The exploration history at the Ren deposit near the Meikle mine on the northern Carlin Trend is a good example of the importance of persistence is locating lower-plate gold mineralization beneath mineralization in overlying upper-plate rocks. Ren, with a high-grade resource of approximately 2 Moz gold, was discovered on the 24th drill hole.