(Non-quartzitic gold ore.)
In this post I'll be continuing on with my discussion of the various types of gold occurrences (minus placer occurrences due to their commonality). As always, I'll do my best to keep the scientific gobbledegook to a minimum so that that the info herein is understandable to all you "deplorables" and "irredeemables" out there.
TYPES of GOLD OCCURRENCES
2. Oxidized Ores
I've mentioned oxidation many times in Bedrock Dreams because it's one of the primary mineralization factors when it comes to gold-bearing ores and ore bodies. Oxidized ores typically contain gold in free-milling form (although some ores are refractory as well). Chemical alteration is the main driver behind the formation of gold in oxidized ores which invariably show significant evidence of the presence of sulfides like iron (Fe), iron pyrites (FeS2), copper oxide (Cu2O), and silver oxide (Ag2O) as well as other sulfides/oxides. The gold carrier or host with sulfide ores can vary widely and once again I need to stress that quartz alone is not the only sulfide host for natural gold. Sulfide ores or rocks showing the presence of oxidation are often heavily iron stained with a reddish, rusty look sometimes mixed together with other colors such as blue, green, black, yellow, and so on. If you're out in the field these are the sorts of rocks that should catch your eye due to their distinctive coloration. Remember this, though. I'm not talking about rocks that are a solid reddish hue, solid blue, or green, etc. Rocks that are heavily mineralized with sulfides will usually show an array of colors or shades of that rusty look ranging from very light to very dark in hue. Your best bet? Pick that rock up if it's coloration follows what I've described here and subject it to further, more detailed examination, especially if you've recovered it in a known gold region.
(Silver sulfide in host rock. Note the dark grey color which can sometimes also be a dark blue or bluish-green.)
3. Silver-Rich Ores
In ores that are primarily silver hosts, the gold contained within the ore or ore body is often found in the form of electrum. The term electrum dates back at least as far as the early Roman Empire and was probably used long before that. Electrum is a natural alloy where the gold content runs about 80%, silver runs close to 18, 19, or 20%, and any remaining small fractional amounts are typically copper or iron. The presence of larger amounts of silver in natural gold gives the Au a paler color than you'd normally see in oxidized ores or placer gold. The best way I can put it is that it's a bit duller looking than regular gold. Many silver-rich ores also contain native silver and kustelite which is about 60-70% silver with the remainder being gold minus a small percentage of "contaminates" like iron and copper. One thing to remember about natural silver (or even silver in general) is that it will oxidize and deteriorate over time when it's exposed to the elements, air, and saline environments. As it changes back into a sulfide (i.e., undergoes oxidation) silver will turn a dark grey or black or even a dark blue. One major U.S. strike of silver-rich ores containing electrum occurred near Tombstone, Arizona in the 1870s (by the way, the 1993 western movie "Tombstone" with Val Kilmer and Kurt Russell is one of my all-time favorites). An even greater strike of silver-rich ore took place in the late 1850s in Nevada. That was, as you correctly surmised, the famous "Comstock Lode."
(Silver-gold ore showing heavy iron sulfide presence.)
The Comstock Lode
I'll digress a bit here and tell you an interesting tale about the discovery of the Comstock Lode. This tale is entirely true and has as much to tell you about the richness of the Comstock Lode as it does about the nature of silver oxidation. During the mid-to-late 1850s many would be Argonauts were making their way across Nevada to reach the California gold fields. The area where the present day Virginia City, Nevada is located was a stopping and re-supply point for many of these would-be prospectors and miners. Some of these men (and a few of the women who accompanied them) knew a bit about gold and how to find it. A number of these individuals decided to test the slopes and drainages of nearby Mount Davidson and try their luck while they were resting up or waiting to move on to the Motherlode Region in California.
(A "Peace" silver dollar composed of 90% silver showing trace oxidation. Note the dark greys and traces of blue.)
Well lo and behold a few lucky individuals found bits of elluvial and alluvial (placer) gold showing up in their pans and rockers and something of a minor rush took place as other miners (mostly inexperienced) stepped in to get their fair share of the gold at Mount Davidson. Their initial gold fever was quickly subdued by two things. Even though gold could be found in decent quantity in and around Mount Davidson, it wasn't the sort of gold these neophytes expected to find nor was it the sort of gold the more experienced of them expected. The gold that showed up in their pans, rockers, sluice boxes, and Long Toms was very pale in color and not the sunny yellow metal glowing in the sun that they were used to seeing. Even worse, their gold pans and sluices were filling up with heavy, coarse sands that were a dark blue in color and that were much more prevalent than the usual heavy black sands found with gold. They cursed those coarse, blue sands and the chunky heavies that were part and parcel of them because they were an absolute bitch to get the gold out of. At each and every mining site along Mount Davidson these hapless Argonauts tossed those blue sands aside into ever-accumulating piles until they became so frustrated with the blue intruder they pulled up stakes and headed for California where there wasn't any blue sand to clog their works and force them to swear like sailors on drunken liberty in a foreign port.
(Mount Davidson with present-day Virginia City, Nevada at its base.)
Then one fine day two men came along who knew a little something about silver oxidation and electrum. The Grosh borthers sampled those blue sands and determined they were composed of pure oxidized silver that had eroded out of Mount Davidson's veins over the course of millenia. They squatted on what turned out to be one of the richest lode silver mines on the mountain. Mount Davidson went on to become the richest silver-gold strike in the entire United States and made multi-millionaires of the likes of George Hearst, daddy to William Randolph Hearst. The Grosh brothers? Well, one died from septicemia (blood poisoning) after injuring his foot at the mine and the other died after trying to cross California's Sierra Nevada mountains during wintry conditions to validate the brothers' claim. A friend named Henry Comstock usurped the Grosh brothers works at Mount Davidson after their deaths and claimed them as his own. Comstock was eventually bought out of his holdings for the grand sum of $16,500 which he promptly blew. He died at his own hand in Montana in 1870, broke and dispirited. But the rich strike the Grosh brothers found as well as the strikes of others on or near Mount Davidson ended up forever after being named after Henry Comstock.
I guess the message here is: 1) Know what you're looking at, and 2) Be careful what you wish for.
Till next time...
(c) Jim Rocha 2016
Questions? E-mail me at firstname.lastname@example.org