Mapping bedrock lithologies through in situ regolith using retained element ratios: a case study from the Agnew-Lawlers area, Western Australia

Abstract

Large, high-quality multi-element geochemical datasets are becoming widely available in the exploration industry, and afford excellent opportunities to investigate geochemical processes. A dataset of over 2500 analyses of unweathered and variably weathered mafic and ultramafic rocks for over 50 elements has been collected by Gold Fields Ltd. in the auriferous Agnew-Lawlers area of the eastern Yilgarn Craton of Western Australia. This dataset is used to investigate changes in element abundances and inter-element ratios through varying degrees and styles of weathering in an area of thick regolith characterised by deep in situ weathering. Systematic interrogation of the data, using lithostratigraphic controls derived from regional mapping and geophysics, reveals that a suite of elements, including Ti, Al, Zr, Th, La, Sc and Nb, and to a lesser extent Cr and Ni, behave as essentially immobile components during saprolite formation. In some cases diagnostic element ratios persist into siliceous duricrust. Ratios of these elements are used as reliable discriminants of bedrock type, and delineate features such as cryptic layering within fractionated sills and subtle geochemical variants in a sequence of tholeiitic and komatiitic basalts. Mapping on the basis of discriminant element ratios greatly extends previous trace-element ratio-based schemes for rock type discrimination. The potential to determine several of these elements with adequate precision and accuracy using portable XRF technology opens a potentially useful technique for rapid geochemical bedrock mapping in residual terrains.

大且高质的多元素地球化学资料在勘探业正变得普遍，给调查地球化学过程提供了极好的机会。这些资料包括超过2500个对未风化和风化度不一的镁铁质和超镁铁质岩石进行的50多个元素的分析，由西澳Yilgarn克拉通东部的含金Agnew-Lawlers地区的金田有限公司收集。该套资料用来研究由于原地高度风化的厚风化层的风化程度和方式而导致的元素丰度变化和元素间比例。通过使用从区域测绘和地球物理学得出的岩石地层控制，对资料的系统性检查，揭示了有一套元素，包括钛、铝、锆、钍、镧、钪和铌，以及较小程度上铬和镍，在腐岩形成的过程中基本上原地不动。在有些情况下鉴定元素比值持续到硅质硬壳。这些元素的比值用作基岩类型的可靠判别标准和区别划定特征，如粒级岩床内的成层拉斑玄武岩和科马提玄武岩地层中的潜在的地球化学变数。在判别元素比的基础上进行的制图将以前的微量元素比例为基础的岩石类型判别极大地扩展了。用便携式XRF技术来准确确定若干这样的元素的潜力在为残留地体中快速进行地球化学基岩制图提供了一个潜在的有用的技术。

KEYWORDS:

• weathering
• regolith
• greenstone
• saprolite
• komatiite
• basalt
• greenstone

ACKNOWLEDGMENTS

We are very grateful to Gold Fields Ltd for allowing access to their multi-element database, and for financial support for CSIRO research at Agnew from 2009 to 2012. Ian Robertson, Melvyn Lintern and Ian Pegg provided very helpful reviews of an early draft, and we are grateful to Steve Beresford and Nick Arndt for reviews of the submitted manuscript. This is a contribution from the Minerals Down Under National Research Flagship.