Pre-Columbian Gold and Silver
Pre-Columbian Gold and Silver: Lecture Notes
Introduction to Pre-Columbian Civilizations and Mysteries
There is significant unknown information regarding Latin American civilizations prior to the Spanish conquest in the 1500s.
The Nazca Lines in southern Peru exemplify these unknowns.
Location and Creation: Large symbols made in the soil of the Nazca Desert.
Time Period: Created between 500 BCE and 500 CE.
Method: Shallow incisions or depressions were made in the desert floor by removing pebbles, exposing differently colored dirt.
Visibility: Best viewed from the air (around 500 m [1,600 ft] altitude), but also visible from surrounding foothills.
Designs: Most lines are straight; some are figurative designs of animals and plants. Individual figurative geoglyphs measure between 400 and 1,100 metres (440-1,200 yd) across.
Characteristics: Shapes are usually made from one continuous line; the largest are about 370 m (400 yd) long.
Examples include a hummingbird, a spider, and enigmatic figures (possibly with 4 fingers and 5 fingers).
Significance of Metals in Pre-Columbian Latin America
Metals were highly valued by pre-Columbian cultures.
Gold and silver were considered to be of divine origin.
Primary Reasons for Preference: Color, shine, and sound were crucial factors for their use in ornaments and sacred artifacts.
Later Uses: Copper and bronze were later employed for tools and weapons, but they never fully replaced the existing use of stone and obsidian.
Stages in the Evolution of Pre-Iron Metallurgy (Worldwide)
Four distinct stages of metallurgical evolution are recognized (Patterson, 1971):
Stage I: Native Metals
Characterized by hammering and annealing of naturally occurring metals.
Gold, copper, and silver were the first metals used due to their availability in native (pure) forms.
Stage II: Reduction, Smelting, Melting, and Casting
Achieved through the processing of oxide ores.
Stage III: Use of Alloys
Examples include bronze, which is a mixture of metals.
Stage IV: Smelting of Sulfide Ores
Limitation in Pre-Columbian Latin America: Iron and steel production was never developed in pre-Columbian Latin America.
Times of Metallurgical Development in the World
Euro-Asiatic: 7800 B.C. to 1200 B.C. (Achieved Stages I, II, III, and IV).
Great Lakes Region: 4500 B.C. to 1000 A.D. (Achieved Stage I).
Latin America: 600 B.C. to 1500 A.D. (Achieved Stages I, II, and III, with possible limited Stage IV development).
Geology and the Use of Ores
Stage I: Native Metals
The use of metals is contingent on both availability and technological capacity.
Native metals were the initial metals exploited.
These metals were concentrated at the Earth's surface through groundwater leaching.
Placer Deposits: Nuggets are transported and concentrated by rivers through erosion and transport processes.
Native metals are pure enough for direct use but are not highly abundant.
Stage II: Oxides and Carbonates
Concentrated in the Oxidation Zone, often mined with relative ease.
Smelting oxides involves a simple reduction process (removing oxygen).
Example Reaction (from transcript): Cu2O ext{ (Cuprite)} + CO ext{ (Carbon monoxide)} = Cu ext{ (Copper)} + CO2
Stage III: Use of Alloys (Bronze)
Marks the beginning of material science.
Alloy Definition: A substance formed by mixing two or more metals, or a metal with a nonmetallic material.
Bronze Composition: An alloy primarily of copper, with additions of tin, arsenic, phosphorus, and small amounts of other elements.
Properties: Bronzes are harder than brasses (alloys of copper and zinc).
Production: Most bronzes are created by melting copper and then adding the desired quantities of other elements like arsenic, tin, and phosphorus.
Pre-Columbian Context: Most pre-Columbian cultures were in the Bronze Age when the Spanish conquest occurred. However, the use of metals was not universal, and stone artifacts and weapons remained extensively utilized.
Stage IV: Sulfides
Formula: MetalX SX
Location: The sulfide zone is found below the water table.
Challenges: Sulfide deposits contain high metal content but are more difficult to mine due to their deeper location and presence below the water table. This requires pumping or draining water and ensuring proper ventilation.
Smelting Process (Two Stages):
Roasting: Eliminating sulfur through oxidation.
Reaction: 2CuS + 2O2 = 2SO2 + 2CuO
Reduction by Carbon Monoxide:
Reaction: CuO + CO = Cu + CO_2
Ore Deposit Formation and Plate Tectonics
Plate tectonics provides a valuable model for understanding the formation and location of ore deposits.
Magmas associated with subduction zones are responsible for creating mineralization.
Ore deposits in the Americas are predominantly concentrated in the western regions (e.g., California Gold Rush).
Further discussion on plate tectonics and Latin American ore deposits will occur in subsequent lectures.
Properties of Gold (Au)
Chemical Element: Metallic, transition element, chemical symbol Au.
Purity: Expressed in karats (K), where 24-karat is pure gold, and 12-karat is 50\%
gold.Melting Point (24 K): 1945^ullet F or 1063^ullet C
Specific Gravity: 19.32 (a measure of density relative to water).
Distribution: Widely distributed in all igneous rocks at low concentrations.
Historical Significance: Recovery from ores and deposits has been a primary concern since ancient times.
Major Supply Leaps: The world's gold supply has seen three significant increases:
Spanish conquest of the Americas in the 1500s.
Discoveries in California and Australia (1850-1875).
Discoveries in Alaska, Yukon, and South Africa (1890-1915).
Workability: Pure gold is too soft for prolonged handling and is typically used in alloys with silver, copper, and other metals.
Key Properties:
Resistance to Corrosion: Gold is the most non-reactive metal, benign in all natural and industrial environments. It does not react with oxygen, meaning it will not rust or tarnish.
Ductility and Malleability: Gold is the most ductile of all metals, allowing it to be drawn into fine wires or threads without breaking. One ounce of gold can be hammered into a 100-square-foot sheet or drawn into a wire five miles long.
Properties of Platinum (Pt)
Chemical Element: Metallic, chemical symbol Pt, atomic number 78.
Physical Characteristics: Very heavy (specific gravity 21.4), silvery-white, precious metal. It is soft and ductile.
Melting Point: High melting point (3,216^ullet F or 1,769^ullet C).
Resistance: Good resistance to corrosion and chemical attack.
Occurrence: Usually found as alloys of 80-90\%
purity in placer deposits, or more rarely combined with arsenic or sulfur.Current Price: Approximately \$1000/oz
.
Placer Deposits
Definition: River deposits or sedimentary concentrations.
Formation: Sedimentary processes involving transport and deposition, such as sand and gravel from old river channels, glacial deposits, and deltas.
Concentration Mechanism: Placer deposits concentrate minerals that are resistant to weathering and are heavy, including gold, silver, diamonds, and garnets.
Most placer gold occurs as