高级矿床学-斑岩型矿床
斑岩铜矿
斑岩铜矿(Porphyry copper deposits)为世界提供了 50% 以上的
铜金属(over 100 producing mines)。 Their close relatives the
porphyry moly deposits produce 70% of the world's moly. Both
deposit types will be discussed separately, but they share many
characteristics and are formed in somewhat similar manners.
全球斑岩铜矿分布图
绝大多数斑岩铜矿形成于中新生代，与离散板块边缘上及附近的火
山链有关（环太平洋、古地中海和古亚洲洋）。极少数为古生代，
分布在古生代板块边缘上，如 、 United States。
全球已知最大的88个斑岩铜矿产于美国西南部，成矿时
代为58-72 Ma。
岩体形态
典型斑岩铜矿床呈圆筒状，产于岩株状岩体中，出露面
积为 x 2 km （椭圆状），核部为斑岩质，向外到边
部为中—粗粒等粒的成分相似的岩石。
主岩岩石学
一般地，容矿主岩为长英质侵入体，成分为石英正长岩、
石英二长岩、花岗闪长岩系列；闪长岩—正长岩系列
热液蚀变
Lowell and Guilbert 总结的斑岩铜矿蚀变模式，围绕斑岩株依次出现4个蚀
变晕
•钾化带- 都存在。特征：次生的钾长石、黑云母和/或绿泥石交代原生的
钾长石、斜长石和镁铁质，及微量的绢云母。
绢英岩化带 – 可存在。特征：脉石英、绢云母、黄铁矿和微量绿泥石、
伊利石和金红石，交代钾长石和黑云母。
泥化带（Argillic ） - 可存在。特征：高岭石、蒙脱石等粘土矿物及微量
浸染黄铁矿。斜长石强烈蚀变、钾长石不受影响，黑云母绿泥石化。
青盘岩化带（Propylitic ） - 都存在。特征：绿泥石、方解石、微量绿帘
石。镁铁质矿物强烈蚀变，斜长石消失。
在深部，上述各带融为一体，构成石英-钾长石-绢云母-绿泥石组合。
Hypogene Mineralization
矿体产出环境:
1）整个岩株内；2）部分岩株部分围岩；3）仅在围岩内
矿体通常为陡壁圆筒状，也有板状、扁圆锥状（tabular to flat conical） 矿
化为浸染状或细网脉状，铜品位-1% Cu ，含微量Mo 和 gold。
矿化呈带状：
Inner Zone – 与钾化带吻合，一般直径数百米，相对地硫低，但钼最高。
黄铁矿 2-5% ， py/cp比约 3:1。矿化为浸染状而非细网脉状。
Ore Zone – 大致分布在potassic-phyllic 带边界上，黄铁矿 5-10% ， py/cp
比约 :1。主要矿物为黄铜矿，呈细网脉状。其他矿物有斑铜矿、硫砷
铜矿和辉铜矿。
Pyrite Zone – 包括phyllic and argillic (若存在) 的大部。黄铁矿相当高，可
达 10-15， py/cp 比约 15:1。矿化即有浸染状又有脉状。许多附加的外来
硫化物相开始出现在上部。
Outer Zone – 与 propylitic zone吻合。黄铁矿微量，铜矿物稀有。闪锌矿和
方铅矿常见，但常为边界品位级。 矿化为脉状。
Breccia Zones - Often major ore carriers in the porphyry
system. Have very high grades (2-5% Cu) and can occur both
in the porphyry or the country rock. May be formed by
hydrothermal activity, gravitational collapse or later explosive
volcanism.
在成矿区带上，斑
岩铜矿、钼矿和锡
矿明显呈带状分布，
与距板块俯冲带的
距离有关，铜矿离
海沟最近，而钼锡
依次分布在海沟内
侧。
Vertical Extent of
Porphyry Bodies
斑岩铜矿与小高
位小岩株和陆相
钙碱性火山作用
有关。因而侵入
体为层火山所覆
盖，青盘岩化延
展到火山岩中，
其它蚀变于其界
面处。一般地，
此模式说明斑岩
铜矿是更大的成
矿体系的一部分，
包括高位浅成低
温贵金属矿床。
闪长岩模式
斑岩矿床有两个不同的体系：
其一为“Lowell and Guilbert type”， 以美国西南部以石英二长
岩-花岗闪长岩为特征。
其二为Andes and Pacific Islands. 主岩为闪长岩，偶为正长岩。
两者的特征对比见表。
闪长岩体系的特征为：低硫逸度，存在磁铁矿，蚀变仅有钾化
带和青盘岩化带，金为重要组分，而钼稀有或缺失。
Comparison of the Lowell-Guilbert and Diorite Types of Porphyry Copper Deposits
FEATURE LOWELL-GUILBERT DIORITE
Host Pluton Quartz Monzonite to Granodiorite (S) Qtz. Diorite to Diorite (I)
Alteration Potassic
Phyllic
Argillic
Propylitic
Potassic
Propylitic
Mineralization
Quartz in fractures Common Common Erratic
Orthoclase in fractures Common Erratic
Magnetite Minor Common
Pyrite in fractures Common Less Common
Molybdenite Common Rare
Chalcopyrite/bornite >3:1 <3:1
Gold Rare Important
Structure
Breccia May Occur Rare
Stockwork Important Important
这些差异可以理解为：日本岛弧侵入体没有斑岩铜矿，在岩相学上他
们属于两类花岗岩（“S” and “I” type ）。前者为陆壳深熔作用形成的，
后者为板块俯冲期间部分熔融的最后阶段的分异产物。
Characteristics of S & I Type Granites
FEATURE S TYPE I TYPE
Gabbro:diorite: granite 2:18:80 15:50:35
Na2O (felsic) <% >%
Al2O3/Alkalis+CaO >1:1 <1:1
Iron oxide ilmenite magnetite
87Sr/86Sr >.706 .
Normative corundum diopside
Assoc. metals Sn, W Au
Genesis Crustal anatexis of sediments Partial melt of mantle
S and I type granites 的特征对比，与Lowell and Guilbert and Diorite 斑
岩矿床所揭示的特征相似。 闪长岩模式的矿床形成于大洋岩石圈俯
冲所致的部分熔融作用，而 Lowell and Guilbert 模式矿床代表远离板
块边界陆壳的熔融作用。
成因
典型斑岩体系的最明显特征是其大小。热液不仅渗透到母侵入
体中，而且渗透至围岩中。认为容矿岩体侵入至浅部（-2
km深)。侵入体结晶开始时，岩浆房的蒸汽压随不相容元素进
入蒸汽相而增大， 当蒸汽压超过上覆岩石的围压时发生退化沸
腾。快速沸腾的液体最终克服岩石的拉伸强度从硅酸盐熔体中
分离出来，导致强烈的网脉状角砾化 (如: water at 2 Kb 压力和
5000C的水由于沸腾体积增加至少 10%)。此外，沸腾是吸热反
应 ，正在逸出的蒸汽膨胀时需要岩浆中的热，因而，快速降低
了岩浆房中的温度, ，形成侵入体中心的斑状结构。
流体包裹体证据
氧同位素研究表明
钾化带黑云母的值与
岩浆水相当，而绢英
岩化带的绢云母 亏
损18O ，表明为天水。
泥化带也是如此。
1）侵入体上升至地壳开始结晶；
2）岩浆热液对流在侵入体内和附近形成钾化带；
3）围岩中的天水对流形成青盘岩化，对流由侵入体热所驱动；
4）随着侵入体的冷却，天水体系叠加至岩浆体系之上，形成 phyllic-
argillic alteration zones。
问题：1）为什么闪长岩模式矿床仅有potassic-
propylitic alteration?
2）为什么闪长岩模式岩浆水-天水系统从不互相
侵入？
Bingham Canyon, Utah
Location
Lies about 30 km southwest of Salt Lake City, Utah at an
altitude of 2000 meters. From a historic perspective one of
the most famous mines in the United States.
Regional Geology
Bingham is situated in the Basin and Range tectonic province.
The Oquirrah Mountains form a horst block of folded Paleozoic
rocks bounded by north trending faults. The principal country
rocks are Pennsylvanian quartzites and limestones folded and
intruded during the Mesozoic. The ore body itself occurs in one
of a series of smaller horsts bounded by the northwest trending
Bear and Occidental Faults which uplift the northeast trending
Copperton Anticline.
Mineralization is in or adjacent to the Bingham stock, a
porphyritic granodiorite. The slightly younger (?) Last Chance
stock to the south is barren. Both have been age dated at
Eocene.
There are three types of mineralization in the Oquirrah
Mountains
alluvial gold
Ag-rich galena and sphalerite veins in limestones
porphyry copper mineralization
Geology of the Bingham Canyon Porphyry Copper Deposit
The deposit occurs in a triangular zone of disseminated and veinlet copper
sulfide mineralization x km in plan and at least 500 meters in thickness.
The majority of the ore is in the granodiorite, but substantial reserves are
present in the metamorphosed Paleozoics.
The original host intrusive was a granodiorite, but subsequent hydrothermal
alteration has resulted in a rock more closely approximating a granite. The
stock has a well defined potassic alteration zone characterized by secondary
biotite, poorly defined phyllic alteration and a spotty and irregular propylitic
alteration zone. Argillic alteration is absent. Extensive brecciation of both the
intrusive and country rock is common.
The primary mineralization averages 1-4% of the rock. The core of the
intrusive is moly rich with pyrite abundant only in the phyllic zone.
Chalcopyrite and minor bornite are the only primary copper sulfides recovered.
Peripheral skarn deposits consisting of enargite, galena, sphalerite and
tetrahedrite are important in the surrounding metasediments.
Typical of most American deposits, early mining was from a supergene
enrichment zone of chalcocite, malachite and native copper lying beneath a
barren limonite cap 10-100 meters thick. Much of this supergene ore has been
mined out.
Genesis
No specific genetic model has been proposed for the Bingham
Canyon deposit. Rather it's genesis is thought to be similar to that
of all the southwestern porphyry copper deposits.
Porphyry Moly and Tin Deposits
In addition to copper there are other porphyly related types of
mineralization, most notably molybdenum and tin. While each of
the three types of porphyry deposits contain appreciable quantities
of the other elements, they are nonetheless geologically distinct
enough to constitute end members of the porphyry spectrum.
Originally it was thought that all moly deposits belong to a single
group. It is now accepted that there are two subtypes. The
nomenclature is confusing since one type is termed simply
porphyry moly while the second type is the Climax type.
Porphyry Moly Subtype
1）与花岗闪长岩—石英二长岩类I型花岗岩有关。与I型斑岩铜矿（闪
长质）相比，略显酸性。矿物只有辉钼矿，脉状和浸染状，品位低
(% MoS2)。
2）蚀变不易识别，但钾蚀变普遍。由于岩石富钾，难以识别钾化。有
些硅化仅出现在钼矿脉周围。Ore bodies are sheet-like to tabular.
Deposits range in age from 30-80 MY and occur in association with
Mesozoic to recent subduction zones, particularly those with steep angles.
No simple genetic model seems to fit all of the deposits in this subgroup.
Clearly they are related to subduction and evidence from fluid inclusions
and isotopes favors a genesis similar if not identical to the diorite type
porphyry copper deposits. An unanswered question is the relationship of the
moly deposits to the copper deposits. Quartz Hill in Alaska lies less than 50
km from an active trench leading some geologists to suggest that porphyry
moly deposits form at shallower depths nearer the trench than do porphyry
copper deposits. However, in the Andes where both moly and copper
deposits have been extensively mapped the moly deposits lie in a belt to the
east of the copper deposits and farther from the trench. Unfortunately, age
differences between the copper and moly belts as well as changes in the
angle of subduction complicate the picture in the Andes.
Climax Type Moly Deposits
Climax type 钼矿与A型(S型亚类)花岗岩有关。其与S型不同点是富K2O 和
F。 矿体均在岩体中，辉钼矿为主矿物，但可回收锡钨。品位高于斑岩
钼矿，平均约% MoS2, 某些矿床高达%。
蚀变为弥散型，强烈的硅化及钾化、青盘岩化、磁铁矿-黄玉化，偶见弱
的绢英岩化/泥化。矿体为伞形。成矿年龄约 30 Ma。已知的 Climax type
moly deposits 位于西 Cordillera, 大致从中 Montana 至北 New Mexico。若
与中生代俯冲带有关（尚有争议），矿床则分布于板缘 1000 km内。
A-type 或非造山花岗岩认为是大陆裂谷的产物（尚有争议）。 另一可能
性是与俯冲有关, 矿床形成在俯冲浅角度的地区。锶同位素比大于0 .706
，常超过 0 .740 。高氟说明陆壳混染。俯冲带深部的热使加厚的陆壳基
底发生熔融。这些岩浆慢慢混染陆壳物质而富集K和Mo。
Climax, Colorado
位置
位于Denver西部约100 km ，海拔 4000 m的陆地分界线。
概述
位于Colorado Front Range，由元古代片岩和片麻岩 (Idaho Springs Group)
所组成，被元古代Sliver Plume Granite所侵入。 Front Range构造复杂，
但褶皱却以北东向为主 。古生代沉积岩不整合于元古代基底之上。
Mosquito Fault切割Climax岩株，为区内主要断裂 ，它从古生代活动至今。
早第三世浅侵入体和流纹质火山作用是本区最后的地质事件。
Geology of the Climax Deposit
主要矿化带直径约 km ，成为了侵入于Idaho Springs Group中的岩株
和岩脉的顶帽。Mosquito Fault 切过矿带的西部。4个岩体控制了矿化，
时代为Oligocene 世。岩株西南部最老，次为中央岩株 ，共同构成了
Climax 花岗闪长岩体。最后它们被 Aplite Phase and the Porphyritic
Granite所侵入。
Alteration consists of early potassic alteration, weak sericitic alteration
and late, intense silicification.
Four phases of mineralization are recognized, each correlated with an
intrusion. The first three form the thick umbrella-like ore bodies (Figure).
The earliest, and uppermost, Ceresco ore body has been almost
completely removed by erosion. Beneath the Ceresco lie the Upper and
Lower (younger) ore bodies, correlating with the second and third
intrusions. The fourth intrusion produced silicification, but no
mineralization. Mineralization consists of a zone of irregular veinlets
containing quartz, K-spar and fine-grained molybdenite. Some ore
occurs in larger veinlets (pseudo-pegmatites) and along joint surfaces.
Fluorite and topaz are common accessory minerals, as are wolframite,
pyrite and cassiterite.
Genesis
Thought to be similar in many respects to porphyry copper deposits,
however the rocks are more silicic suggesting either significant crustal
contamination or an entirely crustal origin for the intrusives. While some
copper deposits occur in settings suggestive of multiple intrusive phases
there appears to be only a single ore body, in marked contrast to moly
deposits where each intrusive has an associated ore zone.
Tin Deposits
Porphyry tin deposits are restricted to a linear belt in the
Andes Mountains east of the copper-moly belts. Host
intrusives are generally latite porphyry stocks to true
granites. There is a close association with stratovolcanoes
indicating very shallow levels of emplacement. Rather
than the typical flaring stock, intrusions associated with
tin deposits appear to be narrow finger-like projections.
The main ore mineral is cassiterite, ore grades averaging
% SnO2. The ore occurs as an early disseminated
phase that is sub-ore grade followed later veins.
Alteration consists of silicification, sericitization,
propylitization and deep level quartz-tourmaline. There is
an absence of potassic alteration. In some instances, there
are associated peripheral base metal vein-type deposits.
Genesis
Thought to be a two stage model (Figure).
Emplacement of shallow plug-like stock in the vent of a felsic
stratovolcano. Magmatic-meteoric water system develops as
igneous activity wanes causing pervasive alteration and the
disseminated tin mineralization.
As temperature declines cooling causes fracturing of the rocks.
These fractures widen and deepen eventually tapping the deeper
magma chamber feeding the volcano. This allows volatiles to
escape the chamber and stream upward. Cooling causes
deposition on fracture walls generating the vein-type tin
mineralization.
Characteristics of Porphyry Cu-Mo-Sn Deposits
1Restricted to the Phanerozoic (mostly Mesozoic-Tertiary).
Closely associated with felsic intrusives of qtz diorite to qtz
monzonite composition.
2Accompanied by characteristic alteration envelope. potassic-->
phyllic--> argillic--> propylitic.
3Strong zoning in relationship to plate boundaries. Cu > Mo >
Sn moving inland from the plate margin.
4Closely associated with island arc development and subduction
zones at convergent ocean-continent plate boundaries.
5Ore occurs in a large, low-grade shell adjacent to the potassic
zone. Chalcopyrite and bornite most common minerals with
lesser molybdenite and cassiterite.
6Isotopic evidence suggests shallow depths of magma
generation and significant groundwater interaction.
Vein-type Deposits
These were once one of the most sought after and heavily mined
types of ore deposits. The famous mining districts of the West are
mainly vein-type deposits.
Vein-type deposits represent a progression into "true" hydrothermal
deposits that lie at some distance from the "igneous" source. Today
this group has declined markedly in importance, but still produces
much of the world’s silver and tungsten and some gold and base
metals. The search for this type of deposit has been abandoned
largely because they tend to be small and difficult to find.
Lindgren Classification - Still much in vogue for the description of
vein-type deposits (See Handouts for Hypothermal, Mesothermal
and Epithermal deposits). These handouts date from the 1930’s, but
with the exception of temperature data, the information has changed
very little.
General Characteristics
Although as a group the deposits vary considerably in size, mineralogy and
geologic environments, all have certain unifying features:
association in space and time with calc-alkaline igneous activity;
clearly epigenetic with ores having vein-like form and close spatial relationship
to faulting;
ore minerals are deposited as open space filling along dilatant zones or as
replacements of carbonate host rocks;
zonation is characteristic and always present (Figure);
deposition is from hydrothermal fluids at depths of less than 3 km;
sulfur is usually of magmatic origin (34S = 0);
districts are structurally complex;
some form of alteration is always present, often sericitic at a minimum;
often see a progression from magmatic water to meteoric water later in the
paragenetic sequence.
Various more recent
classifications of vein-type
deposits have been
proposed, but none have
been adopted uniformly.
Most geologists have
switched to "type" groups
of deposits (. Carlin-
type, Creede-type). This
often leads to a certain
degree of confusion.
Hypothermal Deposits
300 - 500 °C (high pressure; great depth)
Character of Veins - Marked development of replacement textures. Gradational to pegmatitic ore deposits.
Diagnostic Ore Minerals
cassiterite SnO2 hematite Fe2O3
graphite C ilmenite FeTiO3
magnetite Fe3O4 molybdenite MoS2
marmatite (Zn,Fe)S pyrrhotite Fe1-x S
rutile TiO2 scheelite CaWO4
topaz Al2SiO4(OH,F) woiframite (FeMn)WO4
Other Common Metallic Minerals
arsenopyrite FeAsS bismuth Bi
bismuthinite Bi2S3 chalcopyrite CuFeS2
galena PbS gold Au
pyrite FeS2
Characteristic Gangue Minerals
feldspar pyroxene amphibole garnet
micas spinel tourmaline
Wall Rock Alteration
Iron-magnesium Metasomatism –以富铁黑云母、电气石、钙铁榴石（石榴石）、角闪石和辉石为特征。
Sericitic – 存在但不广泛。
Propylitization - Has been described, but is very rare
Mesothermal Deposits 200 - 300 °C (moderate pressure; moderate depth)
脉体特征 – 均匀板状脉，脉壁较平直。 交代常见，以碳酸盐为盛。少见张性充填，分带良好， 脉系长而大。
skutterrudite (Co,Ni) As3 smaltite (Co,Ni)As3
chloanthite (Ni ,Co) As3
arsenopyrite FeAsS bismuthinite Bi2S3
bournonite PbAuSbS3 bornite Cu5FeS4
chalcocite Cu2S chalcopyrite CuFeS2
covellite CuS cobaltite CoAsS
enargite Cu3AsS4 galena PbS
gold Au molybdenite MoS2
pyrite FeS2 niccolite NiAs
tetrahedrite Cu12Sb4S4 tennantite Cu12As4S4
sphalerite ZnS
Quartz SiO2 ankerite Ca(MgFe)(CO3)2
Calcite CaCO3 dolomite CaMg(CO3)2
Fluorite CaF2 siderite FeCO3
Wall Rock Alteration - Dependent on the type of host rock
Sericitic - Develops in feldspathic rocks Feldspars alter to sericite and mafic minerals to pyrite.
Dolomitization - Develops in carbonate rocks. Characterized by ankerite and secondary calcite and dolomite.
Silicification - Also most commonly developed in carbonates, but none-the-less also common in feldspathic rocks.
Jasperization - Characterized by the appearance of jasper, an iron-rich variety of amorphous silica.
Epithermal Deposits 50 - 200 °C ( low pressure; shallow depth)
Character of Veins – 受主岩构造控制张性充填，脉常呈皮壳状、条带状、梳状 。
Diagnostic Ore Minerals
cinnabar HgS acanthite Ag2S
stibnite Sb2S3 argentite Ag2S
chalcocite Cu2S proustite Ag3AsS3
pyargyrite Ag3SbS3
arsenopyrite FeAsS bornite Cu4FeS5
bismuthinite Bi2S3 chalcopyrite CuFeS2
copper Cu electrum AuAg
galena PbS gold Au
marcasite FeS2 polybasite 9Ag2S-Sb2S3
orpiment As2S3 silver Ag
pyrite FeS2 realgar AsS
tetrahedrite Cu12Sb4S13 tennantite Cu12As4S13
adularia KAlSi3O8 aragonite CaCO3
alunite KAl(OH)12(S04) barite BaSO4
calcite CaCO3 chalcedony Si02
dolomite CaMg (CO3)2 fluorite CaF2
rhodochrosite MnCO3 quartz Si02
Silicification – 不一定直接与成矿流体有关，早阶段蚀可紧随矿化之后。
Propylitization – 发育绿泥石、绿帘石、方解石，似与矿化有关，岩石呈特征的绿色。
Sericitic - Formation of the assemblage sericite plus pyrite. Not always present and usually of limited aerial extent.
Alunitization - Near surface alteration associated with descending meteoric waters. Characterized by the formation of alunite.
Mesothermal Base-Metal Veins
Occur in rocks of all ages, but most important districts are of
Paleozoic/Mesozoic age. Seem to be restricted to orogenic belts,
in particular the Rocky Mountains. As such, the western
Cordillera is famous for the concentration of this type of deposit.
Form
Most of the large deposits are replacement-type associated with
major fault systems. Overall the shape of most districts is linear
reflecting the strong control faulting has on ore deposition.
Setting
矿床与钙碱性火山作用及其侵入体密切相关，尽管大部分地区地质情况
复杂，但成矿时代却集中在侵入事件的峰期后不久。典型岩株为浅成相
石英闪长岩至花岗岩。断裂作用是区内主要特征，可填出至少一条区域
断裂。一般地矿体与主断裂无关，但与其平行的次级断层或年轻的交切
断层有关。
Alteration
变化大，决定于主岩性质。砂岩中蚀变限于矿脉两侧数厘米，而碳酸盐内
可达数千米。 碎屑岩内为简单的硅化，而碳酸盐和变质岩内有绢英岩化+
泥化+青盘岩化 。蚀变通常是复杂的，难以区分，特别是脉体之间的蚀变
相互叠加 。
Mineralogy
Cu, Fe, Pb, Zn 硫化物为主，次为硫盐矿物。常见矿物为黄铁矿、黄铜矿、
斑铜矿、方铅矿、闪锌矿、砷黝铜矿tennantite 、黝铜矿tetrahedrite、硫
砷铜矿enargite、辉铜矿 chalcocite 和蓝辉铜矿digenite。磁黄铁矿缺失，由
此限定了其上限。 所列矿物大多是银的重要载体，为世界银资源的主体。
共生序列极其复杂，显示其形成历史长 。在中温区沿走向矿物分带，从
而呈现出亚带来。
Gangue
脉石矿物种类少，主要为方解石、石英、菱铁矿、白云石、铁白云石
Geochemistry
流体包裹体表明成矿温度为250-4000C ，盐度较低 (1-4 wt%)。稳定同位
素数据支持岩浆硫的观点 ，说明至少早期矿物与岩浆水有关。
Genesis
Early genetic models suggested that the major faults within the
districts served as conduits for magmatic waters which flowed
along the faults in search of easily replaceable rocks (carbonates)
where deposition occurred. This model is not without its
problems. First stable isotopes indicate that in many districts
meteoric water has played a substantial role. Second there is the
problem of carrying sulfur and ore minerals in the same fluid.
Despite these problems the model has been little changed or
modified. The lack of emphasis on these deposits in recent years
is probably a major reason for the rather incomplete model. For
instance, compare them to epithermal deposits that will be
discussed in a separate lecture. Models for epithermal deposits
are complex and leave few unanswered questions.
Coeur d‘Alene District, Idaho
The Coeur d‘Alene district is one of the most famous in the
United States. It has produced almost continuously since it was
discovered in 1857. The district produces mainly silver with
byproduct lead, zinc and copper.
Location
The Coeur d‘Alene district extends east-west about 50 km and
north-south about 25 km along the west flank of the northern
Rocky Mountains.
Regional Geology
The basal rocks in the region are a part of the
weakly metamorphosed sandstones and shales
of the Proterozoic Belt Supergroup. These are
unconformably overlain by Cambrian rocks.
All rocks are highly folded and faulted. The
folds trend generally north-south, although
immediately south of the Osborne Fault the
folds strike more nearly east-west. The largest
fault in the region is the east-west trending
Osborne Fault (Figure) which has been traced
for 800 km along strike and is known to have
a displacement of 15,000 meters. Many other
smaller subparallel faults have been mapped
many of which seem to control the major vein
systems.
Intrusives are not conspicuous in the Coeur d‘Alene district,
but several small Cretaceous age stocks of granodiorite and
moazonite are present to the north of the Osborne Fault. These
are thought to be offshoots of the much larger Idaho batholith
to the south.
Ore Deposits
Hundreds of vein and lode deposits occur within the district, most along
dilatant fault zones and as replacements along bedding planes of the host
rocks. Veins trend subparallel to the Osborne Fault. Quartzites tend to be
better ore hosts than argillites since the former has a greater density of brittle
fractures. These fractures are thought to have formed as a consequence of
oblique-slip along the Osborne Fault. Some deposits appear to be clearly
associated with the Cretaceous age intrusives, but for others that association
is more tenuous.
Silver is the most significant mineralization, but there is also recoverable
lead, zinc, copper and gold. Associated gangue minerals are pyrite,
arsenopyrite, quartz, siderite, and calcite. There is a strong mineral zoning
within the districts:
silver-zinc with pyrite near the stocks
silver-copper with siderite along the south and southeast edge of the district
silver-lead east and west sides of the district
Alteration is largely restricted to sericitization. Other alteration includes local
silicification and weak bleaching.
Genesis
The Coeur d’Alene veins were long thought to be related to
intrusion of the Mesozoic Idaho batholith, however U/Pb isotopes
indicated ages that were Proterozoic. This presented a major
problem. Discovery of undeformed stratiform ores in the Belt
Supergroup to the east in Montana that are clearly unrelated to
igneous activity have caused some geologists to propose a
modified genetic model. They believe that the Coeur d扐lene ores
were originally deposited as stratiform sediment-hosted sulfides
during the Precambrian and remobiized to their current locations
during the Mesozoic by heat from the intrusives.
