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

Abstract

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.

秘鲁北部Pataz区所产黄金，来自Pataz岩基中的温热矿脉、基底中的表热矿脉和碳酸盐碱金属矿脉。在区域规模上，对先进星载热发射和反射辐射资料的处理可用来确定地区规模泥化蚀变状态。这样一个区域在Maranon河谷的Vijus向北北西方向延伸几十公里。在这个区域的南端，基底断层中的石英 - 碳酸盐 -硫化物矿脉保证手工金矿开采业务。扫描电镜分析表明，围绕这些断层的蚀变包层以伊利石粘土为主。这些个体金矿巷道显示北部小区规模泥化蚀变带的很大未开发部分所具有的经济潜力。在地区范围，根据Pataz区新的1:25万地质图，我们运用古应力模型对石炭纪ENE -WSW缩短的低最小应力区制图。由此产生的低最小应力分布被用来预测因高流体压力而产生的岩石断裂的部位，和随之而形成的矿脉。这些较低的最低应力区域占据了模型面积的11％，但包含50％的Pataz区已知矿脉。低最小应力的部分地区不包含已知矿脉，这些地区探索不佳或暴露不佳，被提出作为黄金勘探的潜在目标。与扫描电镜微观分析相结合，钻井岩芯样品的ASD高光谱反射率分析显示，围绕岩体中含金矿脉的可见近端绢云母蚀变主要是多硅白云母伊利石。利用光谱助器进行的ASD反射光谱自动化软件的解释显示远离含金矿脉的蚀变中的绢云母，从多硅白云母伊利石带附近的伊利岩为主到更远的白云母，变化不一。在新生代Andean造山运动过程中断层再活动和矿化脉接触导致绿泥石蚀变，局部地叠覆近端多硅白云母伊利石蚀变。ASD光谱识别绿泥石化蚀变带中一些残留多硅白云母伊利石，从而标示矿床规模矿脉的存在。在基石内矿脉周围的近端多硅白云母伊利石蚀变带内导向元素富积在可见蚀变圈之外延伸不超过若干米。在一个考察孔中，碱变指数[(Rb ϸ Cs)/Th] N升高到背景水平之上，达可见绢云母蚀变带之外15米。在另一个孔中，碱变指数[(Rb ϸ Cs)/Th] N和3K/Al都可以作为可见绢云母蚀变宽交叉点内含金脉的脉矿级指标。

Key words:

• exploration targeting
• gold
• Peru
• ASTER
• spectral geology
• stress modelling
• geochemistry

ACKNOWLEDGEMENTS

The authors acknowledge the support and encouragement of Compania Minera Poderosa S.A. (CMPSA) in the funding the research presented here and providing excellent assistance in the field. CMPSA also provided processed ASTER imagery and funded geochemical analyses of drill core samples. ASD hyperspectral analysis of drill-core samples was provided by Scott Halley of Mineral Mapping Limited. Julia Kornikova is thanked for her comments on an earlier version of this paper. The paper also benefited from the thoughtful and constructive comments of two anonymous journal reviewers.

Supplementary Papers

Table 1 Results of XRD analysis of samples from Vijus-Santa Filomena area.

Table 2 SEM analyses of white mica and clay minerals from mineralised quartz–carbonate–sulfide veins, Viju-Santa Filomena area.

Table 3 Results of stress modelling, Pataz district.

Table 4 SEM analyses of white mica and clay, batholith-hosted proximal alteration.

Table 5 Average and range of selected elements from 8 whole-rock geochemical samples of granodiorite from the Pataz district.