By Dan Oancea - Twitter
“We built a small, rude cabin in the side of the crevice and roofed it with canvas, leaving a corner open to serve as a chimney, through which the cattle used to tumble occasionally, at night, and mash our furniture and interrupt our sleep. It was very cold weather and fuel was scarce. Indians brought brush and bushes several miles on their backs; and when we could catch a laden Indian it was well--and when we could not (which was the rule, not the exception), we shivered and bore it.
I confess, without shame, that I expected to find masses of silver lying all about the ground. I expected to see it glittering in the sun on the mountain summits. I said nothing about this, for some instinct told me that I might possibly have an exaggerated idea about it, and so if I betrayed my thought I might bring derision upon myself. Yet I was as perfectly satisfied in my own mind as I could be of anything, that I was going to gather up, in a day or two, or at furthest a week or two, silver enough to make me satisfactorily wealthy--and so my fancy was already busy with plans for spending this money.
... Then I began my search with a feverish excitement that was brimful of expectation--almost of certainty. I crawled about the ground, seizing and examining bits of stone, blowing the dust from them or rubbing them on my clothes, and then peering at them with anxious hope. Presently I found a bright fragment and my heart bounded! I hid behind a boulder and polished it and scrutinized it with a nervous eagerness and a delight that was more pronounced than absolute certainty itself could have afforded. The more I examined the fragment the more I was convinced that I had found the door to fortune. I marked the spot and carried away my specimen.”
Roughing it by Mark Twain
In 1861, Unionville, Humboldt County, Nevada comprised “eleven cabins and a liberty pole”; at least this is how Mark Twain described it. A few years later the city reached a whooping 3,000 people, a church, a schoolhouse, stores, hotels and many more saloons.
Nowadays, the city boasts a population of about 20 permanent residents, relics from the pioneer epoch, old silver mines and a hospitable B&B: The Old Pioneer Garden.
The Exploring Forgotten Nevada site (them or us?) has a few words of advice for any unsuspected visitor:
“…private property abounds, so you have to respect NO TRESSPASSING signs unless you want to end up as a rug in someone's living room.”
One of their pictures displays another interesting caption:
“Snarling Dobermans, machine gun nests, and barbed wire fences protect the old Unionville School. “
Despite all those Indians and claim jumpers, it definitely looked more peacefully back in 1861 than it looks today. But let’s get back to business.
The town’s development has been fueled by the Arizona silver mine’s development. A USGS paper states that:
“… Some are relatively large vein and replacement zones in limestone, as the original Cortez silver mine or the Arizona mine near Unionville.”
Mark Twain’s mountain actually hosted a vein and CRD silver deposit type, i.e. the same type of deposit which constituted the focus of Dr Peter Megaw’s dissertation at the Vancouver Silver Conference.
A summary of his presentation follows.
CRD deposits are Phanerozoic, high-temperature (>250 C) deposits, consisting of stratigraphically discordant Pb-Zn-Ag-Cu skarn and massive sulfide bodies associated with intrusive stocks and hosted by carbonate rocks.
- In average contain about 10-15 M tones of ore, but could go up to 50 Mt;
- High-grade polymetallic ores: 60-600 g/t Ag, 2-12% Pb, 2-18% Zn, trace to 2% Cu and trace to 6 g/t Au;
- Types: Pb-Zn-Ag (+/- Mn); Cu-Zn (+/- Ag, Au, W); Cu-Au (+/- Ag);
- The carbonate rock contact with sulfide or skarn is razor sharp;
- Gangue is different for: skarn (garnets, pyroxenes, amphiboles, wollastonite, rhodonite, magnetite, serpentine); and non-skarn (calcite, fluorite, quartz, dolomite, baritone, gypsum, rhodocrosite, ankerite) deposits;
- Are amenable to low-cost mining methods and have a minimum environmental footprint (limestone is an ARD inhibitor).
- Metals were sourced deep in the crust/mantle; as the fluids moved away from the heat source, a contamination process (exotic elements, sulfur and isotopes) became more important resulting in masking the original character of the mineralization;
- They are intrusion related deposits located in close contact or proximity with intrusive stocks;
- The dissolution of carbonate (host rock) and deposition of sulfides was a nearly simultaneous process;
- The replacement caused a reduction in volume, collapse, voids apparition; subsequently, lower temperature sulfide depleted fluids that altered the wallrock, and deposited residual metals (Mn, Au, etc), silica, and at the very end the carbonate (dissolved and transported away by mineralized fluids - telescoping);
- Having been located on major structural conduits, which supplied a strong and constant load of metals, the ore bodies grew rapidly; there is evidence to support the theory that fluids migrated through previously-formed ore to sites of ore growth;
- Additional pulses of mineralization (linked to additional magmatic events) overprinted the previously formed mineralization;
- Deposition of skarn versus sulfides was controlled by sulfur and CO2 content, and P/T characteristics of the plumbing system, and not by the silica content;
CRDs are similar, sometimes overlap and are associated with:
- Clastic-hosted Pb-Zn-Ag deposits;
- Epithermal veins;
- Porphyry deposits (sometimes mineralized porphyry absent).
CRDs are not:
- Mississippi Valley Type deposits – MVT (Pb-Zn richer);
- Irish Type Deposits (more silver and not intrusion related);
- VMS type deposits.
- They are hosted by deformed miogeoclinal platform or shallow-shelf carbonate rocks (limestone, dolomite, dolomitized limestone as major hosts) overlying ancient continental crust (remember that metals were sourced deep in the crust); limestone was deposited along platform margins or basement highs (reefs);
- Within a specific district CRDs are usually associated with limestone or dolomite, not with both of them;
- Generally located near the bottom of the thick carbonate sequence (relative to the local intrusion);
- Many CRDs are covered (masked) by volcanic or non-reactive rocks (shales); when absent, the volcanic cover might have been removed by erosion;
- Igneous associations: intrusions (stocks, sills, dykes, breccia pipes) and volcanic (very common);
- The central intrusion could be a barren stock, a porphyry copper or molybdenum deposit;
- Intrusion might be in direct contact or in proximity to ore; it might also be lacking (not known);
- Alteration of the carbonate rock: dolomitization, marbelization, secondary calcite introduction, AMOM, iron/oxide with minor sulfides, silicification/jasperoid, fluorite and sanding, skarn and skarnoids;
- Alteration of capping rocks: argilization, bleaching; fracture filling and coatings;
- Ore and alteration types could be superimposed and overlapping;
- CRDs are well-zoned but telescoping could obscure the pattern; zoning could be seen at different scales: from hand sample to district; even though a certain type might be predominant in an area, we should not exclude the presence of transitional types (gradation);
- Geometry of the mineralization: lenses, chimneys, mantos; influenced by ore controls and changes with distance from the source;
- CRD-Skarn spectrum model: a gradation/spectrum ranging from stock contact skarns, dike and sill contact skarn, dike and sill massive sulfide, chimneys, mantos to AMOM deposits;
- Ore controls: structural, porosity and permeability (primary and secondary);
- Geographic types: the U.S.A. type is characterized by dolomitization, limited skarn (magnetite-serpentine) and the presence of jasperoids; the Mexican type almost lacks the jasperoids and the secondary dolomitization, but boasts a major garnet-pyroxene skarn;
- Tectonic setting: continental margins and/or fold-thrusts, orogenic belts (e.g.. Cordillera);
What else to look for (at different scales):
- Regional: major regional structures and their parallel replicas (grain);
- District/deposit: analyze all of the district characteristics in order to realize where are you in the system; evaluate your target based on the proximal skarn to distant manto spectrum; identify the degree of telescoping (e.g. overlapping of skarn with manto zones); study the stratigraphic control on alteration and mineralization (e.g. is there room for more mineralization in the host section); intrusive-ore relation: proximity, geometry, symmetry, continuity;
- Geology: lithology, structure, alteration;
- Geochemistry: crack sampling;
- Geophysics: used only after detailed geological and geochemical study; NSAMT recommended.