Geochemistry and geology of spatially and temporally associated calc-alkaline (I-type) and K-rich (A-type) magmatism in a Carboniferous continental arc setting, Pataz gold-mining district, northern Peru

Abstract

The Pataz–Parcoy gold-mining area, in the Eastern Andean Cordillera of northern Peru, is located on the western margin of the Amazonia craton, in a transtensional jog on the Cordillera Blanca Fault. Episodic subduction, accretion and rifting have taken place in the Eastern Andean Cordillera since the Mesoproterozoic. In the Pataz district, the Cambrian–Ordovician Vijus and Atahualpa formations comprise dacitic and rhyodacitic volcaniclastic rocks that formed by fractionation and/or crustal assimilation of a regionally more abundant andesitic parent melt. Mississippian magmatism began with enriched tholeiitic magmas that lack a Ti anomaly and formed by melting undepleted metasomatised asthenosphere. These magmas assimilated variable amounts of continental crust and were emplaced as Vista Florida Group volcaniclastic rocks, and dioritic plutons of the Pataz batholith. Ponding of these mantle-derived magmas in andesitic lower crust caused partial melting, which generated the volumetrically dominant, granodioritic component of the calc-alkaline (I-type) Pataz batholith. Within a few million years but following rifting, and uplift of the Pataz batholith, relatively K-rich (A-type) magmas formed by melting of a mid-crustal tonalitic source, and were emplaced in the upper crust as the latitic to rhyolitic Lavasen Volcanics and Esperanza subvolcanic complex. Anatexis in the mid crust was promoted by a second batch of magma derived from the undepleted metasomatised asthenosphere, evidence of which is preserved as mafic components of the Esperanza subvolcanic complex and post-Esperanza dolerite dykes. A K-rich magma chamber, the source of the Lavasen Volcanics and the Esperanza subvolcanic complex, is proposed as a possible source of the ore fluid, which deposited gold in veins hosted by the Pataz batholith.

秘鲁北部的东安第斯山脉Pataz - Parcoy黄金矿区位于亚马逊克拉通西缘，Blanca山脉断层上的一个扭张割阶之内。自中元古代起，阶段性俯冲、增生和张裂发生于东安第斯山脉。在Pataz区，寒武 - 奥陶系Vijus组和Atahualpa组的组成是：由于区域上更丰富的安山质原岩熔化的分级和/或地壳同化所形成的流纹英安火山碎屑岩。密西西比岩浆活动以富含拉斑玄武岩的岩浆开始，这种岩浆缺乏钛异常，通过熔化耗竭交代软流圈而形成。这些岩浆同化不等量的大陆地壳，以Vista Florida群火山碎屑岩及Pataz岩体的闪长岩的形式存在。这些幔源岩浆淤积在安山质下地壳中，导致部分熔融，产生钙碱性（ I型） Pataz岩基中的主要组分花岗闪长岩。在几百万年之内，但在Pataz岩基张裂和隆起之后，由中地壳英云源的熔融而形成相对富钾（ A型）岩浆，并以粗安岩至流纹岩质Lavasen火山岩和Esperanza次火山岩复合体的形式存在于上地壳中。由于从非耗竭交代软流圈派生的第二次岩浆导致了中层地壳中的深熔作用作为Esperanza次火山岩复杂和后Esperanza辉绿岩岩墙的铁镁成分而保留下来。一个富钾岩浆室是Lavasen火山岩和Esperanza次火山岩复合体的来源，也认为是矿石流体的可能来源，这种矿石流体在Pataz基岩岩脉中将金沉积下来。

KEY WORDS:

• geochemistry
• geology
• calc-alkaline
• I-type
• K-rich
• A-type
• magmatism
• Carboniferous
• Pataz
• gold
• Eastern Andean Cordillera Peru

ACKNOWLEDGEMENTS

The content of this paper benefited from conversations and e-mail exchange between the authors and Diego Sologuren, Alex Miskovic, Thomas Angerer and Tony Kemp. Constructive advice and reviews of the manuscript prior to submission were provided by Kevin Cassidy, Roger Bateman, Paul Duuring and Tony Kemp. Col Steele assisted with drafting of some of the figures. The paper is published with permission of Compania Minera Poderosa S.A., who funded the study reported here and provided logistic support in the field. The authors also acknowledge constructive reviews by David Champion and an anonymous journal reviewer.