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The Empire Cu-Zn Mine, Idaho: Exploration Implications of Unusual Skarn Features Related to High Fluorine Activity

机译:爱达荷州帝国铜锌矿:与高氟活度有关的异常矽卡岩特征的勘探意义

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The Empire Cu-Zn skarn deposit is unusual because of the proximal position of Zn mineralization, abundance of endoskarn, and the extremely vermicular texture of quartz phenocrysts in the related intrusive rocks. Cu-Zn skarn occurs at the contact between Upper Mississippian White Knob limestone and the granite porphyry phase of the Mackay Stock which consists, from early to late, of quartz monzodiorite, granophyre, granite porphyry, porphyritic granite, and many dikes. The late phases have high F and also extremely vermicular quartz phenocrysts. Endoskarn is more abundant than exoskarn. The earliest alteration of the intrusive rocks consists of disseminated diopsidic pyroxene (Di_(64)Hd_(36) to Di_(88)Hd_(12)), actinolite, and titanite. This assemblage was cut by early scapolite (Me_(18) to Me_(35), mostly Me_(18-26)) and/or green pyroxene (Di_(14)Hd_(80) to Di_(20)Hd_(77)) vein-lets, with or without wollastonite halos. These early veins were then cut by main-stage endoskarn veins that typically have a garnet + minor pyroxene inner zone, a wollastonite and/or pyroxene +- Ca-rich plagioclase (Anse to An_(89)) envelope, and a halo containing disseminated, fine-grained alteration minerals of the same assemblage as the envelope. Some veins contain only the envelope assemblage and are interpreted to represent the alteration front. The inner zone locally contains vesuvianite. Where many veins intersect, endoskarn is massive. Pyroxene is zoned around fluid conduits; the distal pyroxene is Fe rich (hedenbergitic) whereas the proximal pyroxene is Fe poor (diopsidic). The garnet changes in the opposite way, being Fe poor-Al rich (grossularitic) in locations distal to the fluid conduits, and Fe rich (andraditic) in proximal locations. In contrast, in the exoskarn, all pyroxene is diopsidic and garnet is andraditic. Weak, retrograde alteration composed of quartz + calcite + chlorite with minor fluorite, talc, and epidote overprinted both endoskarn and exoskarn. Magnetite precipitated after garnet-pyroxene in both endoskam and exoskarn. Sphalerite precipitated together with chalcopyrite in proximal locations and is associated with retrograde alteration. Other ore minerals include minor molybdenite, bornite, pyrite, galena, arsenopyrite, native Au, as well as supergene minerals such as chrysocolla, malachite, azurite, native Cu, and limonite. Fluid xenoliths from pyroxene in early endoskarn veinlets homogenize at >600 deg C. Massive endoskarn and exoskarn replacing limestone inclusions in granite porphyry formed at 500 deg to >700 deg C, whereas the highest temperature inclusions, >700 deg C, occur in narrow garnet + minor pyroxene veins. Fluid inclusions in exoskam replacing wall rock have homogenization temperatures of 500 deg to 650 deg C. Retrograde alteration and Cu-Zn mineralization occurred at 250 deg to 300 deg C. Fluid inclusions in prograde minerals contain daughter minerals, whereas fluid inclusions in retrograde minerals do not, indicating a decrease in salinity with time. Late-stage fluids have low eutectic temperatures, indicating the possible presence of KCl, NaCl, FeCl_2, CaCl_2, MgCl_2, K_2CO_3, and/or Na_2CO_3. Formation of the unusually abundant endoskarn, the proximal position of Zn mineralization, and the extremely vermicular texture of quartz phenocrysts are all believed to have been promoted by the high F content of the magmatic fluid. These features may serve as exploration indicators of associated high F mineralization such as buried porphyry Mo deposits.
机译:帝国铜锌矽卡岩矿床之所以罕见,是因为锌矿化的近端位置,内矽卡岩的丰富性以及相关侵入岩中石英表晶的极蠕虫质。铜锌矽卡岩出现在上密西西比州的白旋钮石灰岩与Mackay Stock的花岗岩斑岩相之间的接触,该相从早到晚由石英辉闪石,花岗岩,花岗岩斑岩,斑岩花岗岩和许多堤防组成。后期具有较高的F,并且还具有极高的蠕虫状石英隐晶。 Endoskarn比exoskarn丰富。侵入岩的最早蚀变由弥散的二足辉石(Di_(64)Hd_(36)到Di_(88)Hd_(12)),阳起石和钛铁矿组成。该组合被早期的方沸石(Me_(18)至Me_(35),主要是Me_(18-26))和/或绿色辉石(Di_(14)Hd_(80)至Di_(20)Hd_(77))切割。小静脉,有或没有硅灰石光晕。然后,这些早期静脉被主阶段的内矽卡岩脉切开,这些脉通常具有石榴石+次要辉石内部区域,硅灰石和/或辉石+-富含Ca的斜长石(Anse至An_(89))包膜,以及含弥散的晕,与外壳相同组成的细粒蚀变矿物。一些静脉仅包含包膜集合,并被解释为代表蚀变前沿。内部区域局部包含维苏石。在许多静脉相交的地方,内膜arn很庞大。辉石围绕流体导管分区;远端的辉石富含铁(勃艮第),而近端的辉石则缺乏铁(双生)。石榴石以相反的方式变化,在流体导管的远处为贫铁富铝(粗晶),在近处为富铁(放射状)。相反,在外骨骼中,所有的辉石都是双生的,石榴石是雄性的。由石英+方解石+亚氯酸盐与少量萤石,滑石和附子组成的弱逆行蚀变覆盖了内生矽卡岩和外骨骼。石榴石-rox石在磁铁矿和外骨骼中都沉淀出磁铁矿。闪锌矿与黄铜矿一起沉淀在近端,并与逆行蚀变有关。其他矿石矿物包括次要辉钼矿,褐铁矿,黄铁矿,方铅矿,毒砂,天然金,以及超基因矿物,如金水晶,孔雀石,蓝铜矿,天然铜和褐铁矿。早期内生矽卡岩细脉中来自辉石的流体异岩在> 600℃下均质化。大量内生矽卡岩和外骨骼取代了500到> 700℃形成的花岗岩斑岩中的石灰岩夹杂物,而温度最高的内含物,> 700℃,出现在狭窄的石榴石中+次要的辉石脉。外骨骼替代围岩中的流体包裹体的均质温度为500摄氏度至650摄氏度。逆向蚀变和Cu-Zn矿化发生在250摄氏度至300摄氏度。前移矿物中的流体包裹体包含子代矿物,而逆向矿物中的流体包裹体具有否,表明盐度随时间下降。后期流体的低共晶温度,表明可能存在KCl,NaCl,FeCl_2,CaCl_2,MgCl_2,K_2CO_3和/或Na_2CO_3。岩浆液中的高F含量促进了异常丰富的内矽卡岩的形成,Zn矿化的近端位置以及石英表晶的极高的蠕虫质地。这些特征可以作为与之相关的高F矿化作用的勘探指标,例如埋藏的斑岩钼矿床。

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