Transcript Carlin type Gold Deposits

Geologic Overview of the
Carlin Trend Gold Deposits
Pukyung Nat. University
Maeng-Eon Park
Purpose
• To understand sedimentary
rock–hosted deposits

• To understand Carlin trend gold
deposits

• Search whether to apply or not
Carlin type deposits in Korea
Sedimentary rockhosted deposits
Distal
disseminated
deposits
Carlin-type
Au
deposits
What is ‘Carlin-type deposit’?
• Hosted sedimentary rocks
– Carbonate-replacement gold deposits
– Micron size  ‘invisible gold’
• 70% of U.S. and 10% world (1998)
 3rd producing region in the
world
• China, Australia, Spain, Russia,…
Carlin trend
Location of Carlin type deposits
Geological map, Carlin trend, Nevada
Stratigraphy
• Robert Mountain Formation
old
• Popovich Formation
• Rodeo Creek Unit
young
Geological history
• The Antler events
– Compressional tectonism, folding,
– Imbricate thrusting
• The Permian Sonoma events
– Uplift, folding
• The Sevier orogeny
– Folding, thrusting
Characteristics
• Host rocks
– Fine-grained, laminated dolomatic
limestone which contain some silt
– Age : Cambrian~Triassic
• Structural setting
– High-angle, normal fault
increased host-rock permeability,
creating breccias
– overthrust belt
• Igneous rocks
– Alkaline igneous intrusion
– Granodiorite to granite
• Occurrence of gold
– In Pyrite, arsenian pyrite
– Au is associated with anomalous
concentrations of As, Sb, Ba,Ti, Hg
• Fluid-inclusion geochemistry
(Average)
– Depth : 500~800 bar, 2~3km
– Low Salinity : 1~7% Nacl equiv.
– Temp. : 180~245°
Alteration
• Decalcification, Decarbonization
• Argilic alteration
• Silicification
Metallization
(During hydrothermal system)
• Pre-ore stage
– Jasperoid replaced carbonate
• Gold ore stage
– Quarz veins and silicified breccias formed
• Post-ore stage
– Calcite vein formed
– Contain barite, orpiment, reagar, stibnite,
cinnabar
Schematic model for the development of a Carlin-type gold deposit
Exploration guides
• Silty to argillaceous, thinly laminated,
marine, carbonate rocks or carbonatebearing silicilclastic rocks
• An area that has undergone
extensional tectonics
• Concurrent mid-to upper-crustal
plutonism
• The occurrence of W, Mo deposits
• Jasperoid
• The characteristic trace-element suite
of Au, As, Sb, Hg, Tl
Summary
• Carlin-type deposits formed above
active, deep-penetrationg extensional
faults where plutons were emplaced
into the middle and upper crust in
regions of thick marine carbonatesiliciclastic sequences by overthrusting
• slightly to highly acidic, low-salinity,
high-CO2, high H2S fluids
• depths of up 3km by high density, CO2
bearing fluid inclusion
• Gold precipitation was related to a
mixing process, and , for the most part,
was not due to boing