Geology and Mineralization of the McLean Lode, Monte Cristo Property, Esmeralda County, Nevada

摘要

The Monte Cristo property is located approximately 40 km (24 mi.) west-northwest of Tonopah, Nevada in the Walker Lane structural belt and is host to epithermal gold and silver mineralization in at least three, sub-parallel, northeast-trending fault zones. These fault zones are spatially associated with a similarly striking,> 11.3 km (7 mi.) district-scale fault placing slightly older, Tertiary, rhyolitic rocks against younger Tertiary, andesitic rocks. Historic production was from both underground workings, in the two easternmost mineralized fault zones, and a small open-pit operated by KEMCO in the late 1980s. The McLean Lode is a low-sulfidation epithermal system hosted in the westernmost of these three fault zones. The near surface portion of this mineralized fault was open-pit mined by KEMCO. This northeast-trending, northwest-dipping structure hosts high-grade gold and silver within a much larger zone of anomalous gold and silver values. The fault zone, with left lateral and dip-slip motion, shows evidence of episodic alteration, mineralization and movement, including post-mineralization movement. Alteration within the McLean Lode is dominated by moderate to intense silicification with potassic alteration occurring both as potassium feldspar flooding and quartz-adularia veins/veinlets. Argillic alteration occurs sporadically within the lode zone but is common in both the hanging- and footwalls. Mineralization is hosted in the following domains; vein breccias, fluidized breccias, granulated-shattered quartz zones, bull quartz veins and cross-cutting quartz (±adularia) veins and veinlets. The vein breccias host the majority of the precious metals and show episodic brecciation, silicification, replacement of bladed calcite by quartz, recrystallization of chalcedonic quartz, and emplacement of cross-cutting quartz and quartz-adularia veins. The granulated domain, a feature that occurs in the Comstock Lode, is composed of intensely shattered quartz veins and silicified rock displaying a texture similar to coarse-grained salt or sugar. This domain is the second most common host to gold and silver in the McLean Lode with grades up to 1.1 oz/ton Au (34 g/t) and 1.65 oz/ton Ag (51.7 g/t). The fluidized breccia hosts a smaller portion of the gold and silver, with locally very high gold grades (up to 8.1 oz/ton Au/253.6 g/t Au) and moderate silver grades (up to 3.7 oz/ton Ag/116.2 g/t Ag). Within the fluidized breccia visible gold is present in cross-cutting, grayish, banded quartz and quartz-adularia veins/veinlets. Bull quartz veins host only a minor amount of banding and bladed calcite replaced by quartz, with trace amounts of amethyst and sphalerite.