Institute of Applied Geoscience (IAG)

Mineralized veins: how did they get there? A combined physiochemical approach to vein formation.

PhD project (Mr Georgian Manuc)

Supervisors: Dr Taija Torvela and Dr Rob Chapman

Fluids play the central role in formation of ore deposits. Most ores form essentially because of fluid flow either through crystalline rocks (e.g. orogenic gold, porphyry copper) or through sedimentary layers (e.g. uranium ores, sedimentary lead-zinc deposits). “Orogenic” gold is almost invariably hosted by quartz or quartz-carbonate veins, precipitated from crustal fluids. These veins can be several meters thick and tens of meters long. The formation mechanism of these veins and, therefore, the formation of the entire deposit type is still obscure. Components of ore fluids may be derived from a variety of sources (i.e. from the deep crust or even the mantle, from shallow crust, seeping down from the surface, or a combination of these; e.g. Goldfarb & Groves, 2015). Previous research has also shown that orogenic gold is often associated with large fault zones, and the so-called fault-valve model where earthquakes essentially pump fluids through the crust, has been invoked as the main mechanism for crustal-scale fluid transport (Sibson et al., 1988). However, in other cases mineralization is remote from the main fault zones, and is located within secondary structures such as so-called fault splays, fault jogs, and other secondary faults or folds (e.g. Cox, 2007; Micklethwaite et al., 2010). It remains partly unclear how and why the fluids get directed into these secondary structures and in many cases why these structures are ideal as mineralization traps.

In addition to the structural questions, the importance of multiple pulses of fluid in the formation of economically important ores is unresolved, with recent work suggesting that only specific episodes are responsible for the majority of the metal values. The reasons for the differences in mineralizing regime between episodes in the same system remains unclear, but have implications for our fundamental understanding of the generic mineralization process.

This project conducts a detailed investigation into mineralized vein formation, using a combination of state-of-the-art geochemical and structural techniques. The project will characterise the paragenesis and geochemistry of mineralized veins and their associated alteration which may then be mapped into a structural evolution model. This combined technique allows addressing two of the key questions: why do the veins form where they do, and is the vein growth episodic or continuous?

A range of both structural and geochemical techniques have been brought to bear in the study of mineralized vein systems in attempts to illuminate specific aspects of the ‘source- transport-trap’ mechanism. Structural analysis, as applied to mineralisation, has been successfully used to show how deformation either enhances or decreases the permeability in rocks, both spatially and over time. The original fault-valve model by Sibson et al. (1988) has been utilized and refined by many authors. It is now recognized that the detailed interactions of e.g. fluid pressure gradients, buoyancy effects, and permeability distribution govern the fluid pathways between fluid sources, metal sources, and sites of mineral precipitation (e.g. Cox et al., 2001). In crystalline rocks, permeability is chiefly controlled by fractures, the geometry and distribution of which is determined by stress and fluid pressure states in the rock, but also by pre-existing fabric anisotropies (e.g. lithological contacts or pre-existing structures; e.g. Cox et al., 2001).

Generic processes by which ore minerals precipitate have been widely studied and reported and geochemical characteristics have been proposed to infer the sources of the fluids (e.g. Groves and Goldfarb 2015 for a summary). For example, the origins of metals and their associated anions within individual mineral within ore mineral assemblages have been investigated by sulphur and/or carbon isotope studies. Fluid inclusion studies of mineralized veins reveal broad groupings according to salinity and CO2 content according to mineralization type, but very often such studies are inconclusive on a deposit scale. Unfortunately, uncertainties in using isotopes studies tend to be high (e.g. Goldfarb & Groves, 2015). Alternative geochemical approaches are, therefore, needed.

The proposed study comprises a novel holistic approach through the use of both geochemistry and structural techniques to study the genesis of hydrothermal, mineralized veins in gold deposits in Ireland and Scotland, in a wider geological context.

A combined geochemical-structural approach as a solution

Micklethwaite et al. (2010) outlined key knowledge gaps regarding hydrothermal mineralizations in their study. Among these gaps were the "need to improve constraints on the scaling characteristics of faults, shear zones and veins specifically related to mineralisation", and "the integration of stress change and damage concepts with 3D lithological observations and reactive transport modelling". Critically, bigger is not necessarily better, i.e. bigger fractures may not form the best traps as fluid may flow through them too quickly to allow precipitation. This project proposes to address these two key knowledge gaps.

1)         Scaling: what is the relative importance of fractures of various dimensions and connectivity as traps for mineralizing fluids? And:

2)         Integration: what are the feedback relationships between structural features (stress and damage) and lithological/chemical changes associated with fluid transport?

These main scientific questions be addressed by a detailed study and modelling of the evolution of the stress field around structural disturbances, such as fault splays, which are typically also host structures for orogenic gold; combining these models with detailed vein growth models, achieved through microstructural and geochemical studies:

Main objectives:

1)        Construct 3D structural models for vein distributions

2)        Perform stress field modelling and fracture network modelling

3)        Conduct microstructural investigations into vein structure

4)        Use geochemical analyses to study vein paragenesis and fluid flow patterns

The structural studies are enhanced by detailed investigations using the state-of-the-art Scanning Electron Microscope (SEM) facility at Leeds. The deformation associated with the vein formation, alongside with any wall-rock alteration patterns, can be studied using SEM techniques. Micro-scale structures are a central part of the scaling problem: at which scale do the fractures cease to be significant for fluid transport and wall-rock alteration? Detailed crystal lattice deformation maps for the vein walls will also aid in estimating any possible response of the wall rock to vein formation, and as such support the geochemical studies addressing the temperature and pressure conditions of the vein formation.

The structural approaches are combined by geochemical investigations, in order to inform flow and fracture propagation models and generate paragenetic sequences. We envisage that cathodoluminesence (CL) will be key in this study. The technique permits identification of different episodes of quartz deposition not apparent from classical petrological investigations of thin sections. At Curraghinalt this approach constrained the timing of economic mineralization to specific generations of quartz, (Wilkinson et al. 1999) and the approach has been successfully applied to illuminate paragenesis of gold-bearing veins in the Klondike District, Yukon (ongoing studies in Leeds).

Identification of individual mineralizing phases within a vein permits study of specific sub-systems both in terms of fluid chemistry and associated mineralogy. Existing expertise at the School of Earth and Environment underpins fluid inclusion studies and trace element analysis by LA-ICP-MS of vein components identified by CL. In addition, the in-house world-leading expertise in gold characterization will be applied to well constrained episodes of mineralization within vein systems to gain a better understanding of the pressure-temperature-chemical controls on gold composition.

CL imaging also provides textural evidence which may be linked to wider influences on fluid flow regimes. For example brecciation of pre-existing vein quart is indicative of a highly pressurised system, whereas the presence of large euhedral vein quartz candidate is indicative of protracted growth under relatively steady state conditions. Textural and mineralogical and chemical information (from fluid inclusion studies and isotope studies) information can be synthesised to aid interpretation of both fluid origins and depositional conditions.

References:

Chapman, R.J., Mortensen, J.K., Crawford, E. and LeBarge, W., 2010, Microchemical studies of placer and lode gold in the Klondike District, Yukon, Canada: 1. Evidence for a small, gold-rich, orogenic hydrothermal system in the Bonanza and Eldorado Creek area: Economic Geology, v. 105, p. 1369–1392

Chapman, R.J., and Mortensen, J.K. 2016:   Characterization of gold mineralization in the northern Cariboo Gold District, British Columbia through integration of compositional studies of lode and detrital gold with historical placer production: a template for evaluation of orogenic gold districts. Economic Geology, 111, p. 1321-1345

Chapman, R.J, Mileham, T.J, Allan, M.A and Mortensen, J.K., 2017a.  A distinctive Pd-Hg signature in detrital gold derived from alkalic Cu-Au porphyry systems. Ore Geology Reviews, 83, 84-102.

Chapman, R.J., Banks, D.A. and Spence-Jones, C. 2017b: Detrital gold as a deposit-specific indicator mineral, British Columbia: analysis by laser-ablation inductively coupled plasma–mass spectrometry; in Geoscience BC Summary of Activities 2016, Geoscience BC, Report 2017-1, p. 201–212.

Cox SF, 2007. Structural and isotopic constraints on fluid flow regimes and fluid pathways during upper crustal deformation: an example from the Taemas area of the Lachlan Orogen, SE Australia. Journal of Geophysical Research 112, doi:10.1029/2006JB004734.

Cox SF, Knackstedt MA, and Braun J, 2001. Principles of structural control on permeability and fluid flow in hydrothermal systems. In: Richards JP and Tosdal RM, Structural Controls on Ore Genesis. Reviews in Economic Geology 14, 1-24.

Goldfarb RJ and Groves DI, 2015. Orogenic gold: common or evolving fluid and metal sources through time. Lithos 233, 2-26.

Micklethwaite S, Sheldon HA, and Baker T, 2010. Active fault and shear processes and their implications for mineral deposit formation and discovery. Journal of Structural Geology 32, 151-165.

Sibson RH, Robert F, and Poulsen KH, 1988. High-angle reverse faults, fluid pressure cycling, and mesothermal gold-quartz deposits. Geology 13, 551-555.

Wilkinson, J.J., Boyce, A.J., Earls, G. and Fallick, A.E., 1999. Gold remobilization by low-temperature brines; evidence from the Curraghinalt gold deposit, Northern Ireland. Economic Geology, 94(2), pp.289-296.

 

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