高级矿床学8找矿案例斑岩矿床

1、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 worlds moly. Both deposit types will be discussed separately, but they share many characteristics and are formed in somewhat similar manners.

2、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U.S.S.R 、 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1.5 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

3、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

4、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0.4-1% 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 2.5:1。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

5、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

6、 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

7、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 P

8、luton Quartz Monzonite to Granodiorite (S)Qtz. Diorite to Diorite (I) AlterationPotassicPhyllicArgillicPropyliticPotassicPropyliticMineralizationQuartz in fractures CommonCommonErraticOrthoclase in fractures CommonErraticMagnetiteMinorCommonPyrite in fracturesCommonLess CommonMolybdeniteCommonRareCh

9、alcopyrite/bornite3:13:1GoldRareImportantStructureBrecciaMay OccurRareStockworkImportantImportant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 GranitesFEATURE S TYPE I TYPEGabbro:diorite: granite2:18:8015:50:35Na2O (felsi

10、c)3.2%Al2O3/Alkalis+CaO1:1.706.704-.706NormativecorundumdiopsideAssoc. metalsSn, WAuGenesisCrustal anatexis of sedimentsPartial melt of mantleS 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

11、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（0.5-2 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 ，

12、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, UtahLocationLies about 30 km southwest of Salt Lake City, Utah at an

13、altitude of 2000 meters. From a historic perspective one of the most famous mines in the United States.Regional GeologyBingham 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 cou

14、ntry 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

15、 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 Mountainsalluvial gold Ag-rich galena and sphalerite veins in limestones porphyry

16、copper mineralizationGeology of the Bingham Canyon Porphyry Copper DepositThe deposit occurs in a triangular zone of disseminated and veinlet copper sulfide mineralization 1.5 x 2.5 km in plan and at least 500 meters in thickness. The majority of the ore is in the granodiorite, but substantial reser

17、ves 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

18、 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 t

19、he 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 en

20、richment 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.GenesisNo specific genetic model has been proposed for the Bingham Canyon deposit. Rather its genesis is thought to be similar to that of al

21、l the southwestern porphyry copper deposits.Porphyry Moly and Tin DepositsIn 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

22、 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 th

23、e second type is the Climax type.Porphyry Moly Subtype1）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 (0.1-0.2% 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 su

24、bduction 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 deposit

25、s. 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. Howe

26、ver, 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

27、 the picture in the Andes.Climax Type Moly DepositsClimax 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0.35% MoS2, 某些矿床高达0.7%。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

28、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

29、。 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1.5 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 Ph

30、ase 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 earl

31、iest, 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 z

32、one 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.GenesisThought to be similar in many respects

33、 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 m

34、arked contrast to moly deposits where each intrusive has an associated ore zone.Tin DepositsPorphyry 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

35、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 0.1-0.2% SnO2. The ore occurs as an early dissemina

36、ted 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. GenesisThought

37、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

38、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 Dep

39、osits1Restricted 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 M

40、o 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

41、and cassiterite. 6Isotopic evidence suggests shallow depths of magma generation and significant groundwater interaction.Vein-type DepositsThese 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-t

42、ype 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 worlds silver and tungsten and some gold and base metals. The search for this type of deposit h

43、as 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 1930s, but with the exception of tem

44、perature data, the information has changed very little.General CharacteristicsAlthough 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 wit

45、h 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

46、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

47、of vein-type deposits have been proposed, but none have been adopted uniformly. Most geologists have switched to type groups of deposits (e.g. Carlin-type, Creede-type). This often leads to a certain degree of confusion. Hypothermal Deposits300 - 500 C (high pressure; great depth)Character of Veins

48、- Marked development of replacement textures. Gradational to pegmatitic ore deposits.Diagnostic Ore Minerals cassiteriteSnO2hematiteFe2O3graphiteCilmeniteFeTiO3magnetiteFe3O4molybdeniteMoS2marmatite(Zn,Fe)SpyrrhotiteFe1-x SrutileTiO2scheeliteCaWO4topazAl2SiO4(OH,F)woiframite(FeMn)WO4Other Common Met

49、allic MineralsarsenopyriteFeAsSbismuthBibismuthiniteBi2S3chalcopyriteCuFeS2galenaPbSgoldAupyriteFeS2Characteristic Gangue MineralsfeldsparpyroxeneamphibolegarnetmicasspineltourmalineWall 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

50、 been described, but is very rareMesothermal 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) As3smaltite(Co,Ni)As3chloanthite(Ni ,Co) As3arsenopyriteFeAsSbismuthiniteBi2S3bournonitePbAuSbS3borniteCu5FeS4chalcociteCu2Schalc

51、opyriteCuFeS2covelliteCuScobaltiteCoAsSenargiteCu3AsS4galenaPbSgoldAumolybdeniteMoS2pyriteFeS2niccoliteNiAstetrahedriteCu12Sb4S4tennantiteCu12As4S4sphaleriteZnSQuartzSiO2ankeriteCa(MgFe)(CO3)2CalciteCaCO3dolomiteCaMg(CO3)2FluoriteCaF2sideriteFeCO3Wall Rock Alteration - Dependent on the type of host

52、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 commo

53、n 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 cinnabarHgSacanthiteAg2SstibniteSb2S3argentit

54、eAg2SchalcociteCu2SproustiteAg3AsS3pyargyriteAg3SbS3arsenopyriteFeAsSborniteCu4FeS5bismuthiniteBi2S3chalcopyriteCuFeS2copperCuelectrumAuAggalenaPbSgoldAumarcasiteFeS2polybasite9Ag2S-Sb2S3orpimentAs2S3silverAgpyriteFeS2realgarAsStetrahedriteCu12Sb4S13tennantiteCu12As4S13adulariaKAlSi3O8aragoniteCaCO3

55、aluniteKAl(OH)12(S04)bariteBaSO4calciteCaCO3chalcedonySi02dolomiteCaMg (CO3)2fluoriteCaF2rhodochrositeMnCO3quartzSi02Silicification 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 l

56、imited aerial extent.Alunitization - Near surface alteration associated with descending meteoric waters. Characterized by the formation of alunite.Mesothermal Base-Metal VeinsOccur in rocks of all ages, but most important districts are of Paleozoic/Mesozoic age. Seem to be restricted to orogenic bel

57、ts, in particular the Rocky Mountains. As such, the western Cordillera is famous for the concentration of this type of deposit.FormMost 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

58、 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 。MineralogyCu, Fe, Pb, Zn 硫化

59、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

60、矿物与岩浆水有关。GenesisEarly 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 i