KINEMATIC ANALYSIS OF MINERALIZED VEINS IN THE IDAHO SPRINGS MINING DISTRICT, CENTRAL COLORADO
Steven T. Beach (M.S. 2000)
ABSTRACT
The Idaho Springs mining district forms the central portion
of a structurally controlled hydrothermal base and precious metal
mineralization system in the Front Range mineral belt in central
Colorado. The extent of mineralization in the hydrothermal system
is controlled by Precambrian structures, namely the Central City
anticline, which accommodates the chalcopyrite-pyrite core of
the mineralizing system, the northwest trending Blackhawk fault
which bounds the system to the north, and the Idaho Springs-Ralston
shear zone which controls the extent of mineralization to the
southeast and controls the orientation of mineralized veins in
the Idaho Springs mining district. Lateral movement of ore forming
fluids to the west was accommodated by two northwest trending
Precambrian faults, the Idaho Springs fault and the Emerson-Gem
fault. Two younger major faults are present in the area, trending
north-northeast. There is evidence for a Cretaceous age for these
faults. One of these faults, the Dory Hill fault, is spatially
related to the core of early mineralogical zoning.
Mineralized veins in the Idaho Springs mining district formed
as conjugate faults in response to a compressive stress oriented
approximately 076° + 12°. In the Idaho Springs district,
the strong northeast striking, northwest dipping structural fabric
of rocks in the two mile wide Idaho Springs Ralston shear
zone causes right lateral veins to dominate. These veins are the
result of laterally extensive anastomosing fissures, which form
open space when their strike cuts across the host rock foliation
and approaches the maximum compressive stress orientation.
The southeastern extent of mineralization is controlled by the
Idaho Springs Ralston shear zone. The regional orientation
of folds and foliation changes sharply at the southeastern margin
of the Idaho Springs Ralston shear zone. Northwest of this
margin, regional folding trends northeastward and local foliation
strikes northwestward. The orientation of this rock fabric accommodates
shearing and the formation of northeast trending fissures in response
to an east-northeastward compressive stress. Southeast of the
Idaho Springs Ralston shear zone, regional folds trend northwest
and local foliation strikes to the northwest. This orientation
of regional fabric is more resistant to the formation of faults
and fissures trending northeast, thus economic veins are not present
southeast of the Idaho Springs Ralston shear zone.
(Co-advisors: Eric Nelson & M. Hitzman)
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ALTERATION AND MINERALIZATION AT THE CENTENARIO MANTO-TYPE COPPER DEPOSIT, NORTHERN CHILE
Alexis Tawn Cupo (M.S. 2000)
ABSTRACT
The Centenario deposit is a manto-type copper deposit, located
just east of the Coastal Range in northern Chile. The deposit
has estimated reserves of 33.8 million tons at 0.7% Cu in a 30
to 60 meter thick blanket of supergene copper oxides. The primary
copper sulfides at Centenario are hosted in a sequence of sub-aerially
erupted andesite to basaltic-andesite flows and minor fluvial
to lacustrine sedimentary rocks of the Cretaceous Cerros Florida
Formation.
Mineralization occurs in amygdules, veins and as disseminations
in the groundmass. Fluids were focused vertically through the
deposit along fractures to stratigraphically favorable horizons,
that are more vesicular or coarse grained, within the volcanic
sequence, creating a series of peneconformable, flat-lying "manto"
shaped ore bodies. Three stages of alteration have been recognized
at the Centenario deposit. Stage I alteration corresponds to the
regional burial metamorphic event that produced a sub-greenschist
facies assemblage containing chlorite-quartz-epidote-prehnite-calcite
(+ albite). Hypogene precipitation of bornite also occurred towards
the end of this stage of alteration. Stage II alteration is characterized
by alteration of the groundmass and plagioclase phenocrysts to
"dirty" calcite, "sericite", and corrensite.
The end of the Stage II event is characterized by a fine-grained
quartz-chalcocite-specularite veining event, which was the primary
stage of copper sulfide mineralization at the deposit. The oxygen
and hydrogen isotopic composition of the fluid changed between
Stage I and end Stage II. This change may be related to the circulation
of a fluid more "meteoric" in origin.
Supergene enrichment of the deposit yielded a blanket of copper
oxides, containing atacamite, chrysocolla, and minor malachite.
The supergene minerals are concentrated less than 60 meters below
the present ground surface, but extend to greater than 100 meters
along faults and fractures. Earthy hematite and goethite also
flooded and replaced the groundmass of the andesites during this
stage. The genetic model for mineralization and alteration at
the Centenario deposit involve an initial hydrothermal event associated
with the circulation of fluids that were released from an underlying
sedimentary sequence during burial metamorphism. The circulation
of cooler "meteoric" fluids, which introduced chalcocite
into favorable, more permeable horizons, followed this event.
The fluids leached the copper from the surrounding country rocks
and deposited it upon reaction with earlier formed calcite.
(Advisor: M. Hitzman)
CARBONATE-HOSTED BRECCIAS IN THE MEIKLE MINE, NEVADA AND THEIR RELATIONSHIP WITH GOLD MINERALIZATION
Dane Evans (M.S. 2000)
ABSTRACT
Carbonate-hosted breccia bodies contain most of the gold mineralization
at the Meikle deposit. Geological mapping and laboratory studies
have delineated the physical character, mineralogy, morphology,
distribution and relationship with gold mineralization of five
distinct breccia types. Breccias formed in a range of environments
from karsting by low temperature meteoric fluids to hydrothermal
dissolution, collapse, and replacement.
The first breccia event, which formed Breccia Type 1 (Bx-1), occurred
within the massive limestone facies of the Roberts Mountains Formation.
Prior to this event, the massive limestone facies had undergone
micritization, marine cementation, and authigenic mineral growth.
Breccia Type 1 is monolithic and ranges from matrix- to clast-supported
with a light to dark grey carbonate matrix. The breccia bodies
usually occur as less than 2 meter-wide irregular, vein-like bodies.
Breccia formation is attributed to karsting by meteoric water
during exposure of the shelf margin during relative sea level
drop.
A base metal hydrothermal event occurred within the massive limestone
facies sometime during the Paleozoic (?). A hydrothermal dolomitization
stage, associated with base metal mineralization, altered a large
portion of the massive limestone to ferroan dolomite. Reactive
Fe-rich dolomite within the dolomite-altered limestone was of
paramount importance for later high-grade gold mineralization.
The iron within ferroan dolomite was important for later sulfidation
which precipitated auriferous pyrite during Breccia Type 2 formation.
Intrusions of monzonite and lamprophyre were emplaced during Jurassic
time along high-angle structures that largely bound the dolomite-altered
body. Bracketed between the base metal and intrusive events, bitumen+/-
quartz or calcite veins and abundant stylolites formed within
the massive limestone and dolomitized limestone.
The second breccia event, which is associated with most of gold
mineralization in the system, was developed mostly within the
dolomitized limestone body during the Tertiary. Breccia Type 2
(Bx-2) is commonly matrix-supported, varies from monolithic to
heterolithic with dolomite, intrusive, and/or mudstone clasts,
and often contains a matrix of residual dolomite, quartz, and
sulfides along with very fine-grained auriferous pyrite and/or
hydrothermal quartz. The main body of Bx-2 is largely vein-like
and steeply dipping, exhibiting a thicker, central portion. Breccia
Type 2 resulted from hydrothermal dissolution, collapse, and intense
replacement of the permeable ferroan dolomite body along structures.
The third breccia event occurred along the edges of Bx-2 within
mostly dolomitized limestone after ore deposition. Breccia Type
3 (Bx-3) is predominantly matrix-supported and varies from monolithic
to heterolithic. It typically contains Bx-2, intrusive, dolomite,
and/or mudstone clasts in a quartz- to dolomite-rich matrix with
highly luminescent hydrothermal quartz and locally auriferous
pyrite; sedimentary features are common. Breccia type 3 was formed
by hydrothermal dissolution and collapse with partial post-breccia
replacement by quartz.. Formation of Bx-3 was apparently associated
with the introduction of meteoric fluids into the hydrothermal
system after the ore event.
The fourth breccia event represents large-scale carbonate dissolution
with cavernous void development and subsequent collapse and cavity
sedimentation below the water table. Breccia Type 4 (Bx-4) ranges
from matrix- to clast supported, is heterolithic with numerous
clast types, has a calcite-rich to argillaceous matrix, and often
contains sedimentary features. The breccia bodies, which cut previous
breccia types and unaltered limestone, are commonly bulbous to
tabular in shape with variable thickness.
Breccia Type 5 (Bx-5), which is gradational with Bx-4, is characterized
by abundant calcite precipitation in open space within highly
fractured and collapsed rock. The breccia type is usually clast-supported
with a coarsely crystalline, white calcite matrix. Voids were
lined with calcite and less commonly honey-colored barite. Precipitation
of these minerals is still l occurring today.
(Advisor: M. Hitzman)
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Fe-OXIDE (Cu-U-Au-REE) MINERALIZATION AND ALTERATION AT THE PRODUCTURA PROSPECT, CHILE
Kari Ann Fox (M.S. 2000)
ABSTRACT
The Productura Cu-Au Prospect lies within the Chilean Iron
Belt, 625 km north of Santiago and 15 km south-southwest of Vallenar.
The hydrothermal mineralization and alteration at Productura is
similar to that associated with the giant Candelaria deposit (>1
billion tons, 0.7% Cu), 120 km to the north.
Lithologies in the Productura area are predominantly volcanic
flows and ash-flow tuffs of Jurassic-Lower Cretaceous Bandurrias
Formation. The volcanic rocks are intruded by the dioritic Cachiyuyito
intrusion (130 Ma), and the granodioritic Ruta 5 composite batholith
(border phase, 96 Ma).
Three hydrothermal systems are present in the Productura area:
(1) Cachiyuyito system, (2) Productura systems, and (3) Ruta 5
system. The Cachiyuyito system produced massive magnetite and
apatite mineralization similar to that of other iron deposits
of the Chilean Cretaceous iron belt. The magnetite and apatite
mineralization occurred within andestitic tuff adjacent to the
Cachiyuyito intrusion. The Productura system produced silicification
around fault zones followed by iron oxide (Cu-U-Au-REE) mineralization
with significant chlorite, potassium feldspar (90 Ma), tourmaline,
and epidote alteration. Iron oxides include magnetite and specularite.
Hypogene copper sulfides consist solely of chalcopyrite. This
type of mineralization is similar in style to that of the Candelaria
deposit and is the primary exploration target. The Ruta 5 system
produced propylitic alteration of late basalt-andesite blows and
late quartz-hematite veins.
Supergene alteration consists mainly of alteration of magnetite
to hematite, quartz, calcite, alunite, and dumortierite veins,
and goethite alteration of hematite, magnetite and sulfides. Minor
supergene copper mineralization consists of copper oxides, carbonates,
and silicates and digenite, covellite, and chalcocite.
(Advisor: M. Hitzman)
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GEOLOGY AND STRUCTURE OF THE LOWER SOUTHWEST OREBODY, GREENS CREEK MINE, ALASKA
Katja Freitag (Ph.D. 2000)
ABSTRACT
The Greens Creek Zn-Pb-Ag-(Cu-Au) volcanogenic massive sulfide
deposit, in the Alexander terrane in southeastern Alaska, formed
by structurally-controlled hydrothermal activity in a back-arc
basin environment. Numerous sulfide orebodies are located discontinuously
along the contact between highly altered mafic volcanic/volcaniclastic
stratigraphic footwall rocks and stratigraphically overlying graphitic
metasiltstones and metashales. Subsequent to formation, the orebodies
were multiply deformed during accretionary and later wrench tectonic
activity. The complex structure of the deposit includes open to
isoclinal folds that are refolded by open to closed folds, low-angle
faults, and high-angle strike-slip faults.
Detailed rib maps and constructed plan maps, cross sections and
long sections were used to define the structural geometry and
style, and enable a nonrigorous palinspastic reconstruction of
the Lower Southwest orebody. Petrographic, electron backscatter
diffraction, and stable isotope studies were used to characterize
sulfide and carbonate assemblages.
The earliest deformation features, inferred from ore thicknesses
and high Au and Cu grades, are late Triassic, syndepositional
normal (?) faults (Dmineral deformation). During diagenesis (D0
deformation), early "ladder" veins formed perpendicular
to metasiltstone beds and, during partial subduction of the Alexander
terrane in the Jurassic, these beds were offset along small thrust
faults (D1 deformation). Late Jurassic to early Tertiary accretion
of the Alexander terrane resulted in south-southwest- to southeast-plunging,
open to isoclinal folds (D2 deformation) and southeast plunging,
open folds (D3 deformation), with orebody thickness and footwall
and ore lithologies controlling fold geometry. The D4 deformation
event signified a change from ductile to brittle deformation,
and formed thrust faults and gentle folds. Starting in the Eocene,
wrench tectonics resulted in high-angle faults (D5), which locally
reactivated the thrust faults.
Despite intense folding, most pyrite, galena, and sphalerite textures
are predominantly syndepositional, with local remobilization of
galena, sulfosalts, chalcopyrite, and gold into fragmented pyrite,
and only subtle deformation textures observed along open to isoclinal
fold limbs. A comparison to sulfide textures of undeformed and
deformed volcanogenic massive sulfide deposits indicate that sulfide
textures at Greens Creek are similar to deformed massive sulfides
that have been metamorphosed to lower greenschist facies conditions.
Palinspastic reconstruction of the Lower Southwest orebody reveals
spatially distinct, coeval units of massive sulfide and hydrothermal,
mineralized dolomite. Although stockwork zones are not found beneath
the orebody, locally developed, thickened sections of the sulfide
and dolomite units, which coincide with highest grade:thickness
ratios and highest Cu and Au grades, are interpreted to indicate
proximity to hydrothermal fluid vent sites.
(Co-advisors: M. Hitzman & Eric Nelson)
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SEQUENCE STRATIGRAPHIC FRAMEWORK FOR THE SILURIAN-DEVONIAN BOOTSTRAP LIMESTONE, ROBERTS MOUNTAINS, AND DEVONIAN POPOVICH FORMATIONS, NORTHERN CARLIN TREND, ELKO AND EUREKA COUNTIES, NEVADA
Roger A. Furley (M. S. 2001)
ABSTRACT
Sediment-hosted ("Carlin-type") gold deposits are
present in Siluro-Devonian rocks of the Carlin trend. Although
the major stratigraphic relations have been previously investigated
and are well known, the objective of this study is to define a
finer-scaled stratal and sequence stratigraphic framework, and
reconstruct the paleogeography of the study area. The objectives
of this study were met through detailed facies analysis of recently
acquired diamond drill-hole cores consisting of non- to weakly
hydrothermally altered rocks.
The study are is located 27 miles northwest of Carlin, NV, in
the northern portion of the Carlin trend, from Barrick Goldstrike
north past the Dee-Rossi property. The local stratigraphy consists
of Ordovician Hanson Creek, Silurian-Devonian Roberts Mountains,
Silurian-Devonian Bootstrap Limestone, Devonian Popovich, Devonian
Rodeo Creek, Ordovician Vinini, and Tertiary Carlin Formation.
This study described the Roberts Mountains Laminated Micritic
Limestone and Apron (from this study); Bootstrap Limestone reef
and shoal (from this study); and Popovich Wispy, Planar, Soft-sediment
Deformation, and Micritic facies in detail and modeled them using
a systems tracts analysis.
The sequence framework constructed from this study shows that
during a highstand systems tract (HST), a massive Bootstrap Limestone
platform facies was deposited adjacent to the Roberts Mountains
Laminated Micritic Limestone member, representing slope and basinal
facies. A topographic high developed, resulting in deposition
of an apron facies at the base-of-slope. The subsequent fall in
relative sea level resulted in a sequence boundary and deposition
of the overlying lowstand systems tract (LST) Popovich Wispy basinal
facies. The overlying Popovich Planar facies signifies another
change in relative sea level and the beginning of the transgressive
systems tract (TST). Rapid rise in relative sea level during the
TST led to submergence of the platform and starvation into the
basin. A Monograptus sp. and dendroidal variety graptolite
zone found in the upper portions of the Planar facies represents
a condensed section of the maximum flooding surface. During the
subsequent HST, reactivation of the carbonate platform factory
resulted in deposition of additional Bootstrap Limestone platform
facies. The Popovich Soft-Sediment Deformation slope facies represents
the instability of carbonate muds rapidly deposited onto the slope
during the early HST. Sea level continued to rise, eventually
drowning the system, resulting in retrogradation of the shoreline
and deposition of the overlying Popovich Micritic basinal facies
member. The contact between the Micritic member and the overlying
Rodeo Creek unit represents another sequence boundary and a change
from carbonate to more of a clastic influence.
The chronostratigraphic framework constructed from this study
shows that initial deposition of the Bootstrap Limestone, Roberts
Mountains LL, and Roberts Mountains Apron facies began in late
Llandoverian to early Wenlockian time and abruptly ended in the
early-middle Lochkovian. The overlying Popovich WS facies was
deposited during middle Lochkovian time. During the middle Lochkovian,
Bootstrap Limestone facies was rejuvenated and was emplaced laterally
adjacent to the Popovich PL facies, with continued deposition
through to early-middle Emsian time. Deposition of the overlying
Popovich SSD followed and continued through the Pragian. During
the Emsian, deposition of the Popovich UM facies began and continued
through to early Frasnian time. A 6-7 m.y. hiatus spanned from
early Frasnian through late-middle Famennian time, with deposition
of the overlying Rodeo Creek not occurring until Frasnian to late
Famennian time.
Past depositional models for the Bootstrap, Roberts Mountains
and Popovich facies only provided static representations of the
carbonate system for the northern Carlin trend based on an instant
in geologic time. This study successfully used systems tracts
to overcome the static problem by integrating time and relative
sea-level changes to track migration of facies.
(Co-advisors: John Humphrey & M. Hitzman)
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WALL ROCK ALTERATION OF THE METALINE FORMATION AROUND THE PEND OREILLE Zn-Pb DEPOSIT, METALINE DISTRICT, NE WASHINGTON
Craig R. McClung (M.S. 2001)
ABSTRACT
The carbonate-hosted Zn-Pb Pend Oreille deposit is located
in northeastern Washington. The deposit occurs near the top of
the Ordovician Metaline Formation along the contact between peritidal
dolostones to the east and deeper water limestone to the west.
The Metaline Formation is overlain by deep water Ledbetter Slate
which appears to have formed a barrier to vertical fluid flow
in the area.
The host carbonate rocks at Pend Oreille display a complex sequence
of diagenesis and alteration. Dolomitization, silicification,
and sulfide mineralization formed roughly linear, NNE-trending
zones. The linearity of these zones and their persistence through
time suggest a fundamental structural control to fluid flow.
Both limestone and early diagenetic, peritidal dolomite (dolomite-I)
in the mine area are locally replaced by a medium- to course-grained,
variably-textured dolomite (dolomite-II). Carbon and oxygen isotopic
values for dolomite-II indicate that it formed in a rock-dominated,
burial environment system. Dolomite-II displays textural evidence
of repeated dissolution events during its formation including
breccia zones which contain dolomite II clasts in a dolomite II
matrix. The lack of evidence for pre-existing meteoric karst
or tectonic brecciation in the Pend Oreille mine area suggests
that the repeated precipitation and dissolution of dolomite-II
resulted from the introduction of either H2S-rich or acidic CO2-rich
fluids.
Within the Josephine Breccia, and adjacent to the overlying Ledbetter
Slate, dolomite-II is locally replaced by a later generation of
ferroan dolomite (dolomite-III) which displays a distinctly heavier
carbon signature, and elevated Fe2+ and Mn2+ values, relative
to earlier carbonates. Dolomite-III probably formed by the interaction
of a reduced, organic carbon-poor, connate water from the Ledbetter
Slate with pre-existing carbonate rocks.
Base metal mineralization of the Pend Oreille deposit occurred
in three stages. Stage-1 mineralization, which post-dates the
development of dolomite-II breccia zones, produced the Yellowhead-type
(massive, pyritic) sulfides bodies along a series of north-northeast-trending
zones, as well as early disseminated Zn-Fe-Ba mineralization in
the Josephine Breccia. Stage-2 mineralization deposited a quartz-sphalerite-galena
+/- copper sulfide assemblage, primarily in the Josephine Breccia.
Stage-3 mineralization produced the volumetrically minor pyritic
sulfide bodies of the Josephine Breccia and Ledhead Horizon.
GEOLOGY OF THE GOLD-BEARING L1 AND L2 LIESE QUARTZ ZONES, POGO DEPOSIT, EAST CENTRAL ALASKA
Keri H. Moore (M.S. 2000)
ABSTRACT
Gold mineralization at Pogo is hosted within the subhorizontal,
subparallel L1 and L2 Liese quartz zones, which crosscut Proterozoic
to Paleozoic para- and orthogneisses of the Yukon-Tanana terrane.
Intrusion of granite and diorite brackets quartz vein emplacement
and mineralization between 107 and 94.5 Ma. Detailed core logging
and petrography have identified mappable quartz vein types within
the Liese zones. These types have been related to alteration and
mineralization through definition of a paragenetic sequence for
formation and mineralization of the Liese zones.
Formation of the Liese zones consisted of precipitation of granular quartz (Type 1) and then massive white quartz (Type 2A). As the granular quartz is similar in mineralogy to pegmatite bodies and locally displays igneous textures, it is probably related to granitic intrusions. The massive white quartz records a major episode of fracturing and quartz precipitation. Both granular and massive white quartz are accompanied by weak potassic alteration of surrounding wallrocks and were probably initially nonmineralized.
The "Liese event" followed emplacement of the granular
and massive white quartz veins and includes the following stages:
1) precipitation of iron-arsenic sulfides (arsenopyrite, pyrite,
and pyrrhotite) along fractures, 2) formation of ankerite veins
with haloes of white mica ("sericite") and precipitation
of chalcopyrite, gold, and bismuth, 3) deformation, and 4) formation
of minor base metal veins (sphalerite, galena). Sulfides, ankerite,
gold, and bismuth occur in fractures in both granular and massive
quartz. Deformation following stage 1 and 2 resulted in both massive
gray, strained quartz (Type 2B) and dark gray to black, recrystallized
quartz (Type 3). Type 2B quartz is gradational with massive white
quartz (Type 2A). Type 3 quartz forms thin zones that typically
follow the lower contacts of the Liese zones.
Limited fluid inclusion studies and the presence of abundant carbonate
indicate that the fluids responsible for mineralization were probably
CO2-rich. These fluids appear to have been related to magmatic
processes, as evidence by mineralogical and morphological similarities
between the quartz veins and granite and pegmatite bodies, the
timing of Liese zone emplacement (postdates peak metamorphism),
sulfur isotopic values (around zero permil) and the strong correlation
between gold and bismuth.
GEOLOGY AND GEOCHEMISTRY OF THE JERONIMO GOLD DEPOSIT, POTRERILLOS DISTRICT, III REGION, CHILE
Maximino E. Simian (M.S. 2000)
ABSTRACT
Jeronimo is a newly discovered (1994) sedimentary rock-hosted, disseminated gold deposit (combined oxide and refractory sulfide delineated reserves: 16.29 Mt averaging 5.5 g/t). It is located in the Potrerillos porphyry copper district of northern Chile. The deposit consists of disseminated auriferous pyrite with submicron size gold in a zone of decalcified and locally silicified rock. Stratigraphy and structure are the major controls on the location of altered and mineralized zones. The deposit occurs within marine carbonate rocks of the mid-Jurassic Asientos Formation. The Jeronimo deposit is restricted to a particular 8 to 12 meter thick, richly fossiliferous bed. Unlike other beds within the local stratigraphic sequence, the mineralized bed is characterized by abundant ferroan calcite cement. This suggests that sulfidation reactions involving replacement of diagenetic iron-rich calcite by iron sulfide may have played a critical role in gold deposition. Underground mapping indicates that late, high-angle faults and fractures served as ore fluid conduits and that mineralizing solutions did not travel far from these conduits into the host rocks. The mineralized zones are dominated by pyrite but also contain minor orpiment, realgar, marcasite, arsenopyrite, pyrrhotite, sphalerite, bournonite, and pyrargyrite. The alteration and mineralization paragenetic sequence was decalcification, silicification, sulfide and gold precipitation, argillization, and localized supergene weathering. Supergene weathering has resulted in the breakdown of sulfides to oxides and the liberation of gold. The Jeronimo orebody is similar in many respects to the "Carlin-type" deposits of western North America. However, the close spatial relationship of Jeronimo with mineralized porphyry copper centers suggests a direct relationship to magmatism.
(Advisor: M. Hitzman)
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MINERALIZATION AND ALTERATION OF THE KANSANSHI COPPER DEPOSIT, ZAMBIA
Heidie I. Torrealday (M.S. 2000)
ABSTRACT
The Kansanshi copper deposit, located in the Pan-African Damaran-Lufilian
fold belt of northern Zambia, consists of high-angle, sheeted
veins and spatially and genetically associated disseminated sulfides
which cut and replace metamorphosed Katangan sediments of Late
Proterozoic age. Crosscutting relationships and geochronology
have been used to delineate two stages of subparallel veins. The
first vein set is chalcopyrite-rich with relatively abundant molybdenite
while the second vein set contains minor chalcopyrite and bornite
and relatively abundant molybdenite-monazite-brannerite.
Sulfide-bearing veins crosscut an interlayered sequence of phyllites,
biotite garnet schists, and marbles that have reached the biotite
isograd of upper greenschist/lower amphibolite facies metamorphism.
The 350 meter thick mineralized section at Kansanshi has been
informally divided into the Upper Mixed Clastics, the Upper Marble,
the Middle Mixed Clastics, the Lower Calcareous Sequence, the
Lower Marble, and the Pebble Schist. The majority of delineated
sulfides occur within the Upper Mixed Clastics and the Middle
Mixed Clastics. Mineralization at Kansanshi occurs within a NE-trending
antiform, which folds an earlier NW-trending antiform. At least
two of these broad domal features are observed within the Kansanshi
area and appear to step to the NW across the concession area.
Alteration at Kansanshi consists of moderate to intense albitization
and weak to intense dolomitization that has been introduced during
episodes of vein-hosted mineralization. Albitization is the dominate
alteration style in phyllites and schists while dolomitization
is the dominant alteration style in the Upper Marble and calcareous
units.
40Ar/39Ar dating of metamorphic biotite indicate that metamorphic
biotite in the Kansanshi area cooled below its blocking temperature
between 490 to 500 Ma. Re-Os dating of molybdenite, U-Pb analysis
of monazite, and previous U-Pb dating of brannerite, indicate
that mineralization occurred between 512 and 503 Ma. 40Ar/39Ar
dating of muscovite intergrown with albite and ferroan dolomite
in altered zones adjacent to veins yield an age of 473 Ma. These
results indicate that mineralization was synchronous with metamorphism.
Limited fluid inclusion analysis indicates that the mineralizing
fluids were highly saline (<30 wt. % NaCl equivalent), and
CO2-rich. One homogenization temperature,
derived from studies with a hydrothermal diamond-anvil cell, was
400°C and is compatible with temperature estimates for peak
metamorphism. Forty sulfide separates were analyzed for sulfur
isotopes. The sulfides all display relatively heavy values (mean
+4 permil). The results suggest that the sulfur may have been
derived from mixing of isotopically heavy sulfur from dissolution
of Neoproterozoic evaporites (+11 to +23 permil) with sulfur from
diagenetic sulfides within the local Kansanshi sedimentary sequence.
The available evidence suggests that the Kansanshi deposit formed
from highly saline (sodium- and chlorine-rich) brines, capable
of transporting significant amounts of metals. The absence of
significant igneous bodies in the Kansanshi area, combined with
the geochronological evidence that mineralization and metamorphism
were synchronous, strongly suggests that the mineralizing brines
were metamorphic fluids.
(Advisor: M. Hitzman)
(top)
ALTERATION AND MINERALIZATION OF THE SAN PEDRO PORPHYRY, CERRO SAN PEDRO, SAN LUIS POTOSÍ, MEXICO
David Winterbourne (M.S. 2000)
ABSTRACT
Cerro San Pedro, located near the capital city of San Luis
Potosí in the state of San Luis Potosí, Mexico,
is a historic mining district that has undergone continuous mining
since 1575. Recent efforts have focused on delineating a bulk
mineable resource of porphyry-hosted gold and silver mineralization.
The result of this exploration activity is a global resource containing
an estimated 251 million tonnes (Mmt) averaging o.49 g/t Au and
15.3 g/t Ag for a total of approximately 123 tonnes of gold and
3.825 tonnes of silver. Based on a gold price of US$350 per ounce
and silver at US$6.00 per ounce, the current reserve is estimated
to contain a total of 56 Mmt grading 0.57 g/t Au and 21.8 g/t
Ag, representing 32 tonnes of gold and 1,220 tonnes of silver.
Minera San Xavier, S.A. de C.V., currently operates the San Pedro
project, though development efforts have been halted due to unfavorable
market conditions.
The Cerro San Pedro district is located on the western margin
of the Sierra Madre Oriental. District stratigraphy consists of
a transgressive sequence of Cretaceous limestone units belonging
to the Aptian age La Pena and Albian age Cuesta del Cura formations.
The limestone units have undergone extensive folding and faulting
in the Cerro San Pedro region through a system of conjugate NE-
and NE-striking wrench faults and subordinate east-striking fault
splays. These faults are superimposed on a system of NNW-striking,
east-verging, overturned folds and associated thrusts and reverse
faults. The San Pedro porphyry was emplaced into a zone of complex
wrench faulting during a period when the style of tectonism shifted
from predominantly compressional to extensional. The timing of
the emplacement of the porphyry is approximately 64-64 Ma based
on various radiometric dates. Regional extensional deformation
in the district continued after emplacement, resulting in extensive
brittle deformation of both the porphyry and the surrounding limestone
units. Nearly all of the district faults and fractures are mineralized,
indicating the importance of structural preparation both prior
to, and during, primary mineralization. The lack of displacement
along mineralized structures indicates that mineralization post-dates
principal deformation events.
Alteration and mineralization in the Cerro San Pedro district
appear to have occurred during two distinct periods. The first
episode of mineralization closely followed the intrusion of the
San Pedro porphyry. This early event is classified as a low-sulfidation
event, and is characterized by extensive propylitic alteration
that grades into intense sericitic alteration towards the center
of hydrothermal activity. Base-metal mineralization (Pb-Zn-Au-Ag+Cu)
associated with low-sulfidation alteration is present at deeper
of the system, and is interpreted to have been present prior to
supergene oxidation in the shallow levels of the system based
on the presence of secondary oxidation minerals and physical characteristics
of the oxidized zones. The shallow levels of the system are now
dominated by gold and silver with minor base metals. The distribution
of mineralization is distinct on whether the host material is
porphyry or limestone, and is a direct results of contrasting
rheology between the two materials. Faults and fractures in the
limestone units occur as narrow zones of deformation. Pipe-like
or cylindrical deformation zones and breccia bodies develop where
two or more structures intersect. The same structure broadens
and becomes less distinct in the porphyry, resulting in wide zones
of deformation and random orientation of fractures within the
deformation zones. In addition, the San Pedro porphyry has a well-developed
system of stockwork fractures related to emplacement, cooling,
and continued stress from below the porphyry following emplacement.
Precious-metal mineralization within the San Pedro porphyry is
broadly distributed at a lower grade than in the limestone units
as a result of the deformation style. Mineralization in the porphyry
is largely controlled by stockwork fractures with increasing gold
and silver concentrations along the contact with the overlying
limestone units and along the trace of larger faults and fractures.
Mineralization in the limestone units occurs a stratiform carbonate
+ jasperoid replacement bodies, open-space filling along zones
of structural dilatancy, stockwork veining and recrystallization
along vein selvages, and carbonaceous+sulfide replacement within
carbonaceous sections of the La Pena Formation; it is typically
higher grade than in the porphyry.
The second alteration event occurred nearly 40 million years after
the first, at approximately 24-25 Ma. This second event is characterized
by widespread and deep oxidation of primary mineralization in
both the limestone units and the San Pedro porphyry. The second
alteration event is interpreted to be the result of supergene
weathering. In the limestone units, massive sulfide bodies were
nearly entirely oxidized, leaving secondary oxides, hydroxides,
sulfates, and carbonates, including abundant iron and manganese
oxides (hematite, goethite, pyrolusite), jarosite, gypsum, anglesite,
smithsonite, and cerrusite, along with remobilization of silver
as cerargyrite. Gold, which occurred primarily in its native state
during the first alteration event, remained as native gold.
The San Pedro porphyry was also extensively oxidized. Oxidation
reaches depths of up to several hundred meters below the current
topography along the trace of major district faults and along
the contact between the porphyry and limestone units. Acid sulfate
alteration developed in the center of the district, and is characterized
by veins and disseminations of alunite+ kaolinite, widespread
disseminated jarosite, and intense acid leaching and silica flooding
at shallow levels. Acid sulfate alteration occurs to depths of
approximately 300 meters. Sulfur stable isotopic analyses of alunite
and spatially associate pyrite are nearly identical, and range
from approximately 0 permil to 6 permil. Alunite is extremely
fine-grained to cryptocrystalline and occurs as porcelaneous to
chalky veins and dull, earthy masses, is typically intergrown
with kaolinite and is chemically homogeneous.
(Advisor: M. Hitzman)
(top)
STRUCTURAL CONTROLS AND GEOCHEMISTRY OF CARLIN-TYPE GOLD MINERALIZATION IN THE GOLD BAR DISTRICT, EUREKA COUNTY, NEVADA
Ozcan Yigit (Ph.D. 2001)
ABSTRACT
The Gold Bar district with over 1.5 million ounces of gold
resource contains five deposits and four resources that are sedimentary
rock-hosted, disseminated, replacement gold deposits with alteration
and mineral assemblages typical of Carlin-type systems. Tertiary
gold mineralization is controlled by high-angle faults and lithology,
specifically the Devonian thinly bedded lime mudstone of Unit
2 of the Upper Member of the Denay Formation and Bartine Member
of the McColley Canyon Formation within the lower plate of the
Roberts Mountains thrust. Decalcification and silicification with
clay recrystallization are the most common hypogene alteration
types. Supergene alteration is characterized by oxidation of sulfide
minerals, mostly pyrite. This study synthesizes results of detailed
geological and structural mapping. It also includes lithogeochemistry,
stable isotope geochemistry and structural analyses to examine
spatial and temporal relations between structures and the distribution
of gold mineralization in the district. The nature of the structures
associated with gold mineralization and their possible origin
are discussed from deposit to district scale.
In the Gold Bar district, the importance of inherited pre-Tertiary
structures is displayed within the Gold Canyon deposit and at
a regional scale within the west-northwest trend of satellite
orebodies. Paleozoic and/pr Mesozoic contractional structures
were reactivated as northwest- and possibly northeast-trending
high-angle faults that focused hydrothermal fluid flow during
Tertiary mineralization. Intersection of these fault trends formed
loci for Tertiary gold mineralization within favorable lithologies.
Gold mineralization in the largest deposit (Gold Bar) occurs preferentially
northeast of a major northwest-trending high-angle fault that
intersects a series of east and northeast trending faults within
argillaceous lime mudstone. The model that best explains the present
geometry of alteration and mineralization suggests that beds were
horizontal during the gold deposition in the hanging wall of a
65-degree dipping normal fault. Post-mineral tilting rotated the
major northwest-trending fault to near vertical and beds to 25
degrees. Later, the almost horizontal erosional surface of the
orebody was covered by Tertiary volcanic rocks and thin alluvium.
The Gold Bar district contains a variety of jasperoid bodies.
The largest, the Wall fault zone, consists of four types of mineralized
jasperoids, J0, J1, J2, and J3 distinguished on the basis of their
texture, petrography, lithogeochemistry, stable isotope geochemistry,
and geologic and structural setting. Field relations, geochemistry,
and d18O values
of quart from these different jasperoid types suggest jasperoid
formed during three distinct periods. Early J0 jasperoids in the
Webb Formation, with relatively heavy d18O values, may be related to an early (Antler
orogenic age) hydrothermal event. Late J1, J2, and J3 jasperoids
from the Wall fault zone have relatively low d18O values. J3 jasperoids are the distal product
of fluid flow along high-angle feeder faults and may indicate
the outflow zones of the hydrothermal system. Jasperoids related
to the main gold event have d18O values between 9.6 and 21.3 permil. A strong
positive correlation between sizes of gold deposits and d18O composition
of jasperoids related to the main gold event in the district suggests
that d18O enriched fluids produced
relatively larger deposits, and late stage relatively depleted
fluid produced relatively smaller deposits.
Stable isotope studies suggest that sulfur in barite samples associated
with gold mineralization is derived from Devonian to Mississippian
seawater sulfate and oxygen values suggest possible mixing with
middle Tertiary meteoric water. High d34S values in late ore stage realgar and orpiment
exclude a magmatic origin for sulfur. The d13C values in calcite and dolomite associated
with jasperoid are similar to d13C values in ore and late-stage calcites from
other Carlin-type deposits in Nevada. However, the d18O values in the calcite and dolomite have
a relatively broader range than other ore carbonates. The calcite
and dolomite samples with the lowest d18O values correspond to the most altered samples.
Lithogeochemical studies of the lower plate carbonate rocks from
the Gold Bar district indicate that Unit 2 of the Upper Member
of the Denay Formation, one of the most favorable host rock for
mineralization, has high Al2O3,
Fe2O3, and SiO2 contents and a low CaO content. A seven-factor
R-mode factor model suggests that the main episode of gold mineralization
was characterized by increase in Au, As, Sb, Tl and decrease in
CaO and volatiles. The decreased CaO and LOI reflect decalcification,
whereas increase in SiO2 reflects silicification associate with
the main stage gold deposition event. Mass transfer studies using
isocon diagrams suggest that decalcification caused mass losses
of 44 to 51 percent in Unit 2, accompanied by enrichment in Au,
Hg, Zn, Sb, As, and organic C, and depletion in CaO, CO2, Mn,
P, and volatiles. The alteration diagrams indicate that decalcification
was accompanied by jasperoid formation, and that illite-group
clays are the principal phyllosilicates related to alteration
and mineralization in the Gold Bar deposit.
(Co advisors: Eric Nelson, Murray Hitzman)
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