ペルー中央アンデスワイワシ山系多金属鉱床帯の生成機構 : 特にワンサラ鉱床群について

Genesis of the Polymetallic Ore Deposits in the Cordillera Huayhuash, Central Andes, Peru.

Fukahori Yasumasa

The Cordillera Huayhuash, situated in the Central part of the Cordillera Occidental of Peru, is one of the famous polyrnetal1ic provinces in Peru. The area consists mainly of the Cretaceous sediments and Tertiary volcanics and intrusives.

Major ore deposits in the Cordillera Huayhuash are : from north to south, Huanzala, Pachapaqui, Hilarion, Raura, Chanca, Uchucuchacua and Iscay Cruz.

They have many characteristics in common:
(1) being of polymetallic (Ag-Pb-Zn-Cu) vein and skarn-type;
(2) occurring in the Lower to Middle Cretaceous limestone and in Tertiary volcanies;
(3) having a genetical relation to the Late Miocene felsic rocks intruding into the sedinnentary rocks and volcanics;
(4) having similar ore and gangue mineral assemblages (galena, sphalerite, chalcopyrite, pyrite, pyrrhotite, enargite, tennantite, tetrahedrite, argentite pyrargyrite; quartz, calcite, rhodochrosite; garnet, diopside, epidote);
(5) silver and antimony being concentrated in ores hosted by the volcanics.

Of the major polyrnetallic ore deposits, the Huanzala deposits have been studiedin much more detail by the present author. Therefore, the results obtained are summarized as follows.

The Huanzala ore deposits, bedded or lenticular in form, occur mainly in the Lower Cretaceous limestone (Santa Formation). The main ore minerals are galena, sphalerite, chalcopyrite, enargite, tetrahedrite-tennantite series, argentite, pyrite, and pyrrhotite. Silverbearing minerals are characteristically associated with chalcopyrite and tetrahedrite. The gangue minerals consist mainly of quartz, calcite and sericite with garnet diopside and epidote as skarn silicates. Since sheeted quartz porphyries sometimes occur in the ore horizons, it is indicated that the polymetallic mineralization is genetically related to the Late Miocene quartz porphyry (7.7~9. 2Ma).

According to the field and microscopic observations, the mineralization is divided into three stages. The first stage is characterized by pyritization. The second stage, continued frompyritization, is characterized by zinc, lead and copper mineralization accompanied by skarnization. The last stage is characterized by copper and silver mineralization continued from Shiroji alteration.

Based on textures and mineral compositions, the pyritic ores are grouped into two kinds; one is compact and fine-grained, another is loose and coarsegrained. The zinc ores are grouped into iron-rich sphalerite (red sphalerite) and iron-poor sphalerite (black sphalerite). The iron-rich sphalerite, occurring in the pyritic and skarn ores, includes a few blebs of copper minerals whereas the iron-poor sphalerite, generally accompanied with the Shiroji ores, is characterizedby abundant inclusions of copper-bearingminerals such as chalcopyrite and bornite.

The distribution of the loose and coarse-grained pyrite generally coincides spatially with that of the iron-poor sphalerite. The compact and fine-grained pyrite was transformed into the loose and coase-grained pyriteduring the format ion of the iron-poor sphalerite. The ore solution responsible for, the coppermineralization of the last stage permeated into the grain boundary and cracks of the iron-rich sphalerite resulting in the format ion of the iron-poor sphalerite. The copper ores are composed mainly of chalcopyrite, bornite chalcocite and tennantite-tetrahedrite series.

The sequence of the mineralization and the related alteration in the Huanzala deposits is considered as follows:
(1) pyritization:
• format ion of compact and fine-grained pyrite, nearly simultaneous with quartz: porphry intrusion.
(2) skarnization and zinc mineralization
• formation of skarn minerals such as garnet, diopside and epidote,
• formation of iron-rich sphalerite (pyritic ores, skarn ores).
(3) Lead and copper mineralization:
• galena and chalcopyrite formation followed by iron-rich sphalerite.
(4) Shiroji alteration (hydrothermal alteration) and zinc mineralization:
• format ion of iron-poor sphalerite (Shiroji ores).
• format ion of the Shiroji ores (the pyritic and skarn ores changed partly into the Shiroji ores),
• format ion of loose and coarse-grained pyrite (alteration of compact and fine-grained pyrite).
(5) Late copper mineralization:
• bornite and chacocite were disseminated in the pyritic and Shiroji ores,
• veining into the compact and fine-grained pyrite.
(6) Latest copper and silver mineralization:
• characterized by the formation of tennantite and silver-bearing minerals such as canfieldite, argentite and pyrargyrite,
• veining into the pyritic, skarn, Shiroji ores, compact fine-grained, loose coarse-grained pyrite and quartz porphyry sheets.

Although the mineralization and alterat ion patterns are not simple, zonal distributions of elements and ores related to the paragenetic sequence, are recognized in the Huanzala deposits. Systematic analysis of these data leads to a useful exploration guide. On this basis, many encouraging results have been obtained.

Earth sciences. Geology [ 450 ]

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広島大学