By Dan Oancea - Twitter
“On one occasion, being in Tlaltenango with a group of Spaniards, he was shown a piece of ore of some promise, and asked the local Indians where it came from. On being told, he led out an expedition consisting of Indian slaves, and Spaniards of his own force and that of one Miguel de Ibarra, on a search which resulted in the Zacatecas strike.”
This piece of action took place in 1547 at a time when the Spaniards were searching for the fabled Cibola Golden Cities. The newly discovered silver veins were part of the famed Veta Grande Vein, located in the Zacatecas mining district of Mexico. For centuries to come the white metal has been pulled out of this vein system, which proved to be one of the world’s richest primary silver vein deposits.
Concerned about the lack of commerce between Indians and Spaniards, as well as the fact that the locals intended to pay tribute in corn and blankets and not in silver, gold or any other kind of convertible currency, the King of Spain decided to:
"…give orders for money to be coined there, though only of silver and vellon for the present. Thus in accordance with the order and instructions that will be given you by My Council of the Indies, I order you immediately to have the said money coined."
Native silver vein deposits naturally occurring on top of the mountains, a need for a precious metal to be coined as a valid local currency (which translates in means to collect taxes), and a readily available work force (i.e. enslaved natives), not to mention the conquistadores greed, were the main ingredients responsible for triggering the great silver rush, a mining rush that engulfed the American Cordillera, a mighty geological terrain which nowadays still provides us some of the world’s finest hunting grounds for primary silver deposits.
Epithermal silver veins once represented 75% of the world’s silver deposits; nowadays, some 52% of the world’s silver comes from the Cordillera. If you need more reasons to start looking for them, let’s just say that they’re extremely rich (>500g/t Ag) and display a simple geometry and metallurgy, not to mention that many of the old silver mining districts are extremely under explored, at least by modern techniques. Historically speaking, most of the discoveries have been outcrop driven or at least unintentionally made (the cross-cuts simply happened to intercept non-outcropping silver veins). The concept of exploration for blind epithermal veins is a recent one.
Dr. Peter Megaw championed the geology and silver exploration topic. His extensive experience in Mexico made this country and several of its silver deposit types the focus of his presentation (LSEVD and CRD type deposits).
Silver Low Sulphidation Epithermal Vein Deposits (LSEVD) are:
- Silver veins characterized by a 350-500 m zoned mineralized interval (base metals grade increase with depth), developed as mineralized structures up to 10 km long and generally less than 10 m wide. At depth the veins pinch down in the batholith;
- Were formed near the surface in low temperature (< 350 C) and pressure conditions (i.e. the mineralized fluids rose on ingress structural channels and ‘dumped’ their mineral load whenever pressure dropped suddenly as a result of faulting or intersection of any other type of a larger physical conduit; this represents the so called ‘boiling’ process). Metals are dropped first followed by gangue minerals. Sulphides and sulfosalts represent <5-25% of vein volume;
- Distinctive minerals include pyrite, sphalerite, galena, arsenopyrite and sulfosalts. Gangue is made of quartz, adularia and calcite with some illite, barite, fluorite also being present.
- Low sulphidation mineralized fluids do not strongly react with wallrock, but a widespread alteration pattern could be observed for some distance in the surrounding rocks (i.e. silicification, argilization and propyllic alteration). Alteration is zoned in respect to boiling levels and paleosurface (the alteration tends to spread outwards and upwards);
- Display a banded and brecciated texture resulting from periodic and successive episodes of fracture filling, followed by internal pressure build-up and another vigorous episode of ripping of vein fillings and subsequent mineral filling deposition. Long-lived epithermal vein systems display larger metal budgets than short-lived system and also various boiling levels;
- Structures are the most fundamental control on location of veins. The location of major silver vein deposits indicates that they were generated by hydrothermal fluids circulating through a series of opened/reopened parallel and/or intersecting structures most of the time related with the regional stress field. Different centers of mineralization existed at roughly the same time along hundreds of kilometers within these regional structures;
- Weathering of outcropping silver veins resulted in the destruction of the sulphides and a migration of the silver metal downward. The result is a misleading barren vein outcrop which overlies an area of supergene enrichments (bonanza grades);
- The ones located in the Cordillera of Western Americas are hosted by highly evolved and slightly eroded Tertiary volcanic-intrusive belts (Peru, Bolivia, Mexico, USA and Canada); they are located in close proximity to magma, volcanic centers within major structural zones.
An analysis of the order of favorability:
- How favorable is the system’s location within the district/region?
- How much of the vertical mineralized interval has been eroded away?
- Is it a strong, long lived system?
- Could we identify the centers of mineralization based on alteration patterns?
- How competent is the wallrock and what is it made of?
- Is it possible to have multiple, stacked, composite veins?
- Is it possible that younger cover masks other logically ‘missing’ parts of the system?
Methods that should be employed:
- Biogeochemistry (San Carlos vein discovery) and MMI;
- Alteration map analysis (satellite images – ASTER). Blind veins discoveries have been made based on characteristic alteration feature recognition (e.g. MAG-Penoles JV at Fresnillo, Mexico)
- An integration of local with regional and district geological data; a structural integration analysis (faults, intersections, stockworks, breccias, district grain);
- Drill for structure, drift for grade.
A couple of things worthy to be highlighted:
- There are no high-sulphidation silver deposits in Mexico;
- And, listen to Peter when he says that while in Mexico, you have to ‘Stay away from the poppy fields at harvest time’.
CRD silver deposits will be showcased in a further article.