DESCRIPTIVE MODEL OF DETACHMENT-FAULT-RELATED POLYMETALLIC DEPOSITS
By Keith R. Long
BRIEF
DESCRIPTION
SYNONYM:
Detachment-fault-related gold, flat-fault gold.
DESCRIPTION:
Massive replacements, stockworks, and veins of iron and copper oxides
and locally sulfides along
detachment-fault
structures. These deposits sometimes contain economic concentrations of
gold and silver. Distal veins of quartz-barite-fluorite-Mn
oxides emplaced along high-angle faults in the upper plate of detachment-faulted
terranes.
GENERAL
REFERENCE: Wilkins
and others (1986).
TYPICAL
DEPOSITS: Bullard
(Roddy
and others, 1988), Copperstone
(Spencer and others, 1988),
Osborne (Allen, 1985),
Planet
(Lehman
and Spencer, 1989), Harris (Roddy
and others, 1988), Tiger Wash (Allen,
1985).
COMMODITIES:
Cu + Au + Ag + Pb + Zn.
OTHER
COMMODITIES: Fe-Ba-F-Mn-Mo-V.
ASSOCIATED
DEPOSIT TYPES (*suspected to be genetically related): *Lacustrine
Mn.
REGIONAL
GEOLOGIC ATTRIBUTES
TECTONOSTRATIGRAPHIC
SETTING: Extensional terranes characterized by regional detachment faulting.
REGIONAL
DEPOSITIONAL ENVIRONMENT: Half-graben
mountain ranges and hydrographically closed basins that formed syntectonically
with extensional deformation above detachment faults.
AGE
RANGE: Known deposits range from middle to late Tertiary in age.
LOCAL
GEOLOGIC ATTRIBUTES
HOST
ROCKS: (1) Lower-plate mylonitic rocks, chlorite breccias, and structurally
emplaced slivers of upper-plate rocks. (2) Upper-plate Paleozoic to Mesozoic
(meta)sedimentary and (meta)volcanic rocks, Mesozoic to early Tertiary
felsic intrusive rocks, middle to late Tertiary mafic to intermediate
lavas, silicic tuffs, and sedimentary rocks deposited in alluvial fan,
Alluvial, and saline lake environments.
ASSOCIATED
ROCKS: Syn-
to post tectonic alkali basalts, microdiorite dikes, and sedimentary rocks
deposited in half-graben
basins.
ORE
MINERALOGY: Specular and earthy hematite, chrysocolla, and gold or electrum.
Locally abundant chalcopyrite and other copper sulfides. Rare galena,
sphalerite, and tetrahedrite. Sulfides accompany chloritic alteration
in early-stage mineralization along and below detachment fault. Quartz-hematite-calcite-chrysocolla
mineralization follows along and above detachment fault. Late-stage quartz-barite-clorite-manganese
oxide veins with locally abundant copper oxides, cerargyrite,
argentite, gold, and hematite occur above detachment fault.
GANGUE
MINERALS: Quartz (sometimes chalcedonic or amethystine), calcite (often
ferrous and (or) manganiferous), barite, fluorite, and manganese oxides.
Locally abundant pyrite, jasperoid, gypsum, and clays.
ZONING:
Intensity of mineralization and alteration decreases away from detachment
fault. Many districts zoned from polymetallic deposits outward to Ba-F-Mn
veins. Mineralization tends to be base metal-rich and precious metal-poor
near the detachment fault but precious metal-rich/base metal-poor away
from the detachment fault.
ORE
CONTROLS: Deposits commonly located above axis or flanks of synformal
structures in underlying detachment surface. Ore bodies are localized
along high-angle fault zones below the detachment fault, along the detachment
fault, and in high-angle, sometimes listric, normal faults in the upper
plate. (]old is often associated with local silica flooding and amethystine
quartz veins in brittle, fractured upper-plate rocks. Massive specularite
replacements and chrysocolla veins occur in reactive calcareous units
in both the upper and lower plates.
ISOTOPIC
SIGNATURES: Quartz associated with oxide ore minerals has 6 to 8 per mill
[delta] 180 and that associated with sulfide ore minerals has 10 to 12
per mill [delta] 18O. Calcite associated with oxide ore minerals has 4
per mill [delta] 180 and -4 to -6 per mill [delta] 13C PDB.
K-metasomatized
rock has lower [delta] 18O than unaltered rock by 2 to 4 per mill.
FLUID
INCLUSIONS: In quartz, calcite and barite associated with sulfide ore
homogenization temperatures are higher (220 to 350 °C)
than those associated with oxide ore (150 to 350 °C).
Salinities, however, are similar at 10 to 23 equivalent weight percent
NaCl. These
fluids are thought to be saline brines derived from syntectonic, hydrographically
closed, arid basins. Quartz in distal Ba-F-Mn
veins have low-temperature (90 to 200 °C)
and saline (6 to 20 weight percent equivalent NaCl) fluid inclusions.
STRUCTURAL
SETTING: Local flexures of a regional detachment fault with strong development
of upper-plate, high-angle, listric and planar normal faults.
ORE
DEPOSIT GEOMETRY: (1) Narrow fracture and fault fillings that are 3 cm
to 12 m in
width with strike lengths of 30 to 2,000 m.
(2) Irregular, pod-shaped massive replacements of reactive lithologies
up to 900 m long, 100 m wide and 3 to 30 m thick (4)Pods end anastomosing
veins along low-angle faults. (5) Veinlets and breccia clasts in fault
breccia.
ALTERATION:
Wall rock dependent. Distinct alteration suites are observed: (1) Pre-ore
to early chloritic (chlorite-epidote-hematite)
alteration of lower-plate mylonites and fault breccias, sometimes with
associated quartz-pyrite-chalcopyrite+galena
mineralization. (2) Pre-ore to early K-metasomatism
of upper-plate volcanic rocks. Mafic rocks are converted into K-feldspar-hematite-calcite-chlorite-epidote
rocks, and silicic rocks are converted into K-feldspar-hematite-quartz
rocks. (3) Pre-ore to early massive carbonate replacement of carbonate
rocks. (4) Propylitic (chlorite-calcite-epidote-sericite-clay)
alteration envelopes around veins hosted by mafic rocks. Quartz-chrysocolla
veins often have clay selvages. (5) Weak sericite-silica-dolomite
envelopes around Ba-F-Mn
veins in calcareous rocks.
TYPICAL
ALTERATION/OTHER HALO DIMENSIONS: (1) Chloritic alteration may extend
from the top of the detachment fault down to 300 m below the detachment
fault. (2) K-metasomatism
may extend more than 2 km above the detachment fault in zones more than
10 km in extent. (3) Massive carbonate replacements range up to 900 m
in length, 100 m wide, and about 30 m thick. (4) Propylitic alteration
halos are narrow, up to a few centimeters around veins and fracture fillings.
EFFECT
OF WEATHERING: Most ore consists of primary oxides. Locally abundant sulfides
may be oxidized.
EFFECT
OF METAMORPHISM: Metamorphosed deposits are not known.
GEOCHEMICAL
SIGNATURES: Host rocks are enriched in Cu. Pb, Zn, Au, Ag, and Ba and
depleted in Mn, Sr, Ni, and Rb. As, Sb, Hg, and Tl are also very low.
GEOPHYSICAL
SIGNATURES: There may be a resistivity contrast between oxide ores along
and above the detachment fault and the mylonite zone beneath the detachment
fault. Silica flooded zones may have high resistivity. Massive hematite
ore bodies may produce a magnetic dipole anomaly. Shallow reflection seismic
might detect detachment-fault structures.
OTHER
EXPLORATION GUIDES: Conodont alteration of upper-plate Paleozoic sediments
may serve as a guide to regional paleo-heat
flow related to fluid movement along and above detachment faults.
OVERBURDEN:
Variable, owing to differing degrees of uplift along half-graben
structures and regional warps. Polymetallic deposits are thought to have
formed at a depth of 1 to 3 km (Spencer
and Welty,
1986), and Ba-F-Mn
veins at a depth of 0.5 km (Allen,
1985).