Walter Witt

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

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.

Multiple methods for regional- to mine-scale targeting, Pataz gold field, northern Peru

Gold production in the Pataz district, northern Peru, is derived from mesothermal veins hosted by the Pataz batholith and basement-hosted epithermal and carbonate–base metal veins. At the regional scale, processing of Advanced Spaceborne Thermal Emission and Reflection Radiometer data can be used to delineate district-scale argillic alteration. One such area extends for tens of kilometres NNW of Vijus in the Maranon Valley. At the southern end of this area, basement-hosted quartz–carbonate–sulfide veins in faults support artisanal gold-mining operations. SEM analyses show that the alteration envelopes around these faults are dominated by illitic clays. These artisanal gold workings highlight the economic potential of the largely unexplored parts of the district-scale argillic alteration zone, further north. At the district scale, paleostress modelling maps areas of low minimum stress during Carboniferous ENE–WSW shortening, based on a new 1:25 000 geological map of the Pataz district. The resulting distribution of low minimum stress is used to predict sites of rock fracture under high fluid pressure, and consequent vein formation. These areas of low minimum stress occupy 11% of the modelled area but contain 50% of the known veins in the Pataz district. Some areas of low minimum stress contain no known veins, and where these are poorly explored or poorly exposed, they are proposed as potential targets for gold exploration. In combination with SEM microanalysis, ASD hyperspectral reflectance analysis of drill core samples shows that visible proximal sericitic alteration around batholith-hosted auriferous veins is predominantly phengitic illite. Automated software interpretation of ASD reflectance spectra using The Spectral Assistant shows that sericite in cryptic alteration distal from auriferous veins varies from mainly illite adjacent to the phengitic illite zone, to more distal muscovite. Reactivation of faults and mineralised vein contacts during the largely Cenozoic Andean orogeny produced chlorite alteration that locally overprints proximal phengitic illite alteration. ASD spectrometry identifies relict phengitic illite in some chloritic alteration zones and thus indicates proximity to mineralised veins at the deposit scale. Elevated pathfinder element concentrations within proximal phengitic illite alteration zones around batholith-hosted veins do not extend more than a few metres beyond the visible alteration envelope. The alkali alteration index [(Rb + Cs)/Th]N is elevated above background levels for up to 15 m beyond the visible sericite alteration zone in one of two holes investigated. In the other hole, both [(Rb + Cs)/Th]N and 3K/Al can be used as a lode-scale vector to gold-bearing veins within broad intersections of visible sericite alteration.

Introduction to using research to benefit mineral exploration

This thematic volume on ‘Using Research to Benefit Mineral Exploration’ provides a series of papers that summarise the results of applied geological research conducted on various commodities, at different scales and using a combination of established and modern analytical techniques. In all papers, an attempt is made to link geological data to exploration targeting. The Centre for Exploration Targeting (CET) was founded in 2005 with the specific mission to build a link between fundamental and applied research that would deliver new exploration targeting methods and models for the mining and exploration industry. CET consists of tenured staff members, postdoctoral fellows, PhD, MSc and honours students, as well as adjunct fellows. The papers in this volume were written by this team and represent a sampling of research conducted at CET over the past five years. This thematic issue contains nine papers, and these are organised around commodities of gold copper, nickel, zinc and iron. The majority of papers deal with gold, and this topic is covered at different scales, from tectonic scale metallogenesis to country-scale evaluation of gold systems, to camp and deposit scale analyses of gold-bearing structures and alteration zones. The first paper by Loucks (2014) investigates magma fertility and the generation of porphyryand highsulfidation copper ore deposits in his paper on ‘Distinctive composition of copper-ore-forming arc magmas.’ The paper provides petrochemical evidence for geodynamically imposed compressive stress that fosters entrapment of mantle-derived basaltic magmas in chambers near the Moho. The paper also offers two principal criteria for exploration geologists to select geologically favourable target regions: (1) chemical screening of whole-rock analyses to identify arc segments, and individual igneous complexes within them, and stages in the evolution of long-lived magmatic centres that match the chemical features of copper-ore forming arc magmas; and (2) targeting arc intervals that have experienced recent compressive orogenic deformation in the lead-up to the magmatic activity being assessed. A paper by Witt et al. (2014a) describes and discusses the ‘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.’ The Eastern Andean Cordillera in Peru has been the site of almost continuous subduction, from the latest Neoproterozoic to the Triassic, and yet significant gold mineralisation is restricted to the Mississippian (lower Carboniferous). The K-rich magma is proposed as a possible source of ore fluids for the Pataz gold. ‘Multiple methods for regionalto mine-scale targeting, Pataz gold field, northern Peru’, by Witt et al. (2014b), describes a range of methods for targeting gold at regionalto mine scales in the Pataz district. ASTER spectral imagery applied at the regional scale highlights widespread hydrothermal (argillic) alteration and gold mineralisation in basement rocks of the Maranon Complex, to the north and west of the Pataz batholith, which hosts the main producing gold deposits. Paleostress modelling, based on a new 1:25 000 geological map of the Pataz district, identify areas of low minimum stress that occupy 11% of the modelled area but contain 50% of the known veins in the Pataz district. ASD hyperspectral reflectance analysis of drill core samples, in combination with SEMmicroanalysis, identifies zoning of phyllosilicate compositions from proximal phengitic white mica through illite to distal, cryptic muscovite. Finally, at the lode scale, it is shown that the alkali alteration index [(Rb þ Cs)/Th]N provides a potential vector to ore within and beyond the visible alteration halo. The paper on ‘Gold systems in Uganda’ by Nyakecho & Hagemann (2014) represents the first systematic presentation of gold systems in Uganda. Detailed analyses of historic and recent information on active and abandoned gold mines and alluvial workings, in combination with new regional geochronology, lithostratigraphic,