Great Deposits – The Comstock Lode

Mining & Minerals - Evaluations / Silver Exploration & Mining

Andrew Watson reports that flying over Nevada on a clear day is one of the great joys of being an economic geologist. The Basin and Range country has abundant structural features, intrusions, deformed bedding and pits and plenty to keep one occupied. Every so often the occasional grid drilling pattern, hinting of what will or what just didn’t quite comes into view. The driver is of course gold, with Nevada dominating US gold production claiming 78% of the total. The perceptive might wonder why then is it called the “Silver State” – the answer is the Comstock Lode.

Like all great deposits, the Comstock Lode did many things; it broke Nevada away from Utah, it pushed mining technology ahead by leaps and bounds, it was critical in kick-starting the study of hydrothermal alteration, it brought the first elevator to Nevada, and it made a few people very, very rich.  Not the original discoverers, they died of gangrene and starvation1, not the second discoverers, they sold out for fractions of the true value. As the archetype for all that came after it, (the term “Bonanza” entered the English language as a result of the Comstock Lode), the people who made the money were the promoters, whiskey sellers, lawyers, and assorted hangers-on that colonize every new discovery. Shares in what we would consider to be “moose pasture” were selling for $40 a share in 1860 – about $1100 each in today’s dollars. The first whiskey salesman on scene, did a cool $200 ($5,600)2 his first night. The lawyers did even better, with some 265 lawsuits in the first 10 years of operation and over $10,000,000 ($281,000,000) spent on legal dueling.

Behind all this was the biggest silver discovery since the Spanish knocked off the Inca and claimed Potosi. Assays of 3,000 oz Silver per tonne where not uncommon. When the Ophir Company shipped its first ore to San Francisco, it made $112,000 ($3,150,000) gross on 38 tons for an average of $78,000 (2017 dollars) per ton. Later in 1873, mining really got going when The Consolidated Virginia and California mines hit “The Big Bonanza” 1000 feet below the surface. It produced $116,500,000 ($3.3 Billion) of gold and silver in 7 years, and was responsible for a quarter of the Lode’s total gold production and a third of the silver.

What is now known as a low-sulphidation epithermal deposit. It has the typical quartz-adularia+/- pyrite alteration with elements of high sulphidation alteration as shown by the presence of dickite, kaolinite, pyrophyllite, and diaspore. In 2003 Hudson3 proposed 4 broad zones grading from distal illitic-anhydrite to quartz-adularia with stockwork (vein systems) and massive quartz, to massive and stock work quartz with a sericitic phase coming in from the north. Pre-faulting reconstruction puts the ore bodies on roughly the same horizon, and with bases of massive calcite. This makes intuitive sense as, ore deposition was accomplished by “boiling” off of hydrothermal fluidsand deposition of metals. Hudson suggested that unusual superposition of high sulphidation and low sulphidation was due to two distinct episodes occurring in close proximity in time and space. Interestingly the high sulphidation system was essentially barren and deposited only pyrite.