06
Dec
13

The Book

LodeVeinCover
LAST UPDATE: 4 APRIL 2014

How do gold deposits form and how do you find more ore?

THE METALLOGENY OF LODE GOLD DEPOSITS: A SYNGENETIC PERSPECTIVE

This book is a treatise on the formation of vein or lode gold deposits, which historically have accounted for about 75% of worldwide gold production. Each of the 13 chapters also has a brief summary for brokers, investors, promoters and geology students. It describes all aspects of the geology of gold vein formation. Existing formation theories are reviewed but the main thrust of the book centers on discovery of a paradigm shifting geological process. This which was unearthed over a period of 30 years by following subtle clues that the mainstream geological fraternity has interpreted differently. Very powerful new exploration tools are the result. While they cannot be patented, they have proven useful in a worldwide search for gold deposits and to find extensions of existing mines and mining districts. One potential mine has already been found. The book is over 80% complete and all key chapters are finished. The book expands on a paper published by the author in the conference proceedings for WorldGold2011 held in Montreal under the auspices of The Canadian Institute of Mining and Metallurgy.(Downloads) There are numerous diagrams and photos to illustrate the theme. Anyone interested in how gold deposits form and how to find more ore will welcome this book. It is essential reading for gold mining exploration companies, brokerage firms specializing in gold, entities such as the World Gold Council or Sprott Equities. In particular it will be of interest to out-of-the-box thinking exploration geologists, entrepreneurs and financiers who appreciate more than most, the advantages to early adoption of new ideas. The book provides a practical guide to ore!

Continue reading ‘The Book’

24
Mar
14

Lode Gold Vein Deposits: Preface and Summary

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Preface and Summary 14 April 2014

Preface and Summary

This book records a journey of trying to understand the metallogeny (ie genesis in time and space) of lode gold deposits. For me this began in 1982 when I saw excellent sea-shore exposures in SE Nova Scotia, where ubiquitous quartz outcrops that is characteristic of Meguma turbidite-hosted gold deposits. I had read much of the literature on their origin, including the comments of T. Sterry Hunt in 1868 who said that: “ I think that to describe them otherwise than as interstratified beds would be to give a false notion of their geognostic relations. The laminated structure of many of the lodes, and the intercalation between their layers of thin continuous films or layers of argillite, can hardly be explained in any other way than by supposing these lodes to have been formed by successive deposition of what was, at the time, the surface of the earth.”

I found no convincing contrary evidence during a summer of field work nor during extensive discussions with the late structural geologist Jack Henderson nor the late Simon Haynes both of whom worked in the same area at the same time. Published research from authorities who hold other, mainly epigenetic views on their genesis was also not compelling. A report to the Geological Survey of Canada summarized my findings. Based on detailed trench and outcrop mapping and vein textures, I proposed that sea-floor hydrothermal hot springs seemed to provide a simple and compelling origin for the majority of Meguma quartz and lode gold “ veins”. There are also clearly later cross-cutting quartz veins, some of which are probable feeders. Quartz is widespread in Meguma but gold districts are near euxinic pyrite-rich basins. The report did not find favour and was not published at the time. It is reproduced in its entirety as Appendix A of this book, because some of the ideas are relevant today, and presage the current excellent comprehensive work of Kontak and co-workers. Together with Appendix B, descriptions of veins constitute essential background to understanding the syngenetic ideas which underpin this current book. Interestingly, in the intervening time, my findings have not been duplicated by other researchers, although much excellent analytical and dating work has been done on the Meguma. I think the observations and conclusions of T. Sterry Hunt are as valid today as in 1868, because a fundamental process is addressed. The book you are reading today is based on accepting his basic hypothesis and examining its ramifications in the light of subsequent findings.

Over a period of several years, I worked intermittently in the Quebec portion of the Abitibi belt, Chester Twp (Côté Gold deposit) and the Beardmore gold districts in Ontario as well as the Maskwa batholith in Manitoba. During detailed mapping and core logging, it became clear these 3.2 to. 2.7 Ga Archean rocks were also showing evidence for synvolcanic sea-floor hot spring formation. The gold-sulfide-quartz veins formed at the same time as the enclosing host rocks and some form of syngenesis not only underlies Meguma turbidite-hosted deposits, but Archean volcanic-hosted lode vein deposits and indeed many lode deposits of all ages on a world-wide basis. This book records the steps that constitute proof of this claim. I present my evidence, with some trepidation, while trying to navigate objectively what I consider a hinterland of preconceptions in much lode gold literature. The task is made more difficult because deformation can result in saddle-reef, en-echelon and crack-seal veins. Also both outcrops and much geochemical data permit alternate and opposite interpretation. Sorting out the critical variables which relate to gold deposit formation is the key to understanding their genesis. Indeed some fundamental crustal processes are revealed and I hope the reader will not be put off by my fearless interpretations.

My career in geology started as a geological assistant on regional Geological Survey of Canada Arctic mapping parties with Ken Eade and Al Fraser (and Paul Hoffmann) at a time when regional scale mapping project were common and student assistants were in demand. I also collected minerals and in trying to understand how they formed, developed a research interest in oxide and sulfide phase chemistry, especially of iron-titanium oxides (M. Sc. Kretschmar & McNutt, 1971) and arsenopyrite (Ph.D, Kretschmar & Scott, 1976). This led to a lifelong quest to try to understand the physical and chemical conditions of ore deposition and how to apply this knowledge in a practical way during a career in exploration geology.

The vast expansion of research and ready access to almost all technical papers through the Internet is unparalleled in the history of science and should in theory by now, have resulted in a complete understanding of the lode vein formation process. While many would not agree, I think that gold deposit research is model-rich and data-poor, and models are galloping off in all directions, in an apt analogy by Canadian humorist Steven Leacock.

The fragmented nature of gold formation models is in part due to the declining importance of field work in the geosciences. There is a loss of data due to mining, as lower gold grades become economic and evidence is destroyed in open pits on the surface or in large underground drifts. As the gold price fluctuates, the fortunes of junior mining companies whose websites and reports often contain excellent descriptive data also fluctuate or disappear. Availability of research funding mirrors gold price fluctuations. On the positive side, the development of instrumental and analytical techniques e.g. XANES has permitted independent insights into early Archean sea-floor hydrothermal P-T conditions. The quest to understand primordial carbon in 3.2 Ga Strelly Pool cherts by NASA scientists, led to the realization that graphite and stylolite-like textures, both common in lode veins, have an abiotic origin. The carbon originates under P-T conditions that are similar to the Fischer-Tropsch industrial process. Ethane in fluid inclusions has been used as another tracer to show fluids and gold were sourced from deeply buried carbon-rich, and pyrite-gold-bearing sedimentary rock. The common mineral pyrite frequently shows zoning which may be due to two periods of gold mineralization. In another direction, theoretical heat-flow calculations and the thermodynamic properties of sea-water lead to sea-floor spacing of hydrothermal cells, that corresponds quite well with spacing and geometry of lodes observed from grade-thickness contours in gold mining camps.

Which elements of current lode gold formation models are compatible with syngenesis? How does one differentiate a brilliant academic construct from a model driving process? Correlation is not necessarily causation. I think it is clear that interpretations and models are unlikely to be valid if they are not solidly grounded in careful field work.

What is this book? This book is intended as a synthesis of lode gold vein forming processes and to point out the commonality in similar deposits worldwide. I construct an empirical model which incorporates widely known and accepted principles of ore deposition and extends them into the felsic “intrusion” environment. A large part of the book is based on detailed outcrop maps and photos of field occurrences and textures. All my interpretations flow directly from field work, the querying of underlying physical processes and interpretations in the literature. It should be kept in mind that geological descriptions represent a slice in time. Often key outcrops in mining camps are only exposed for a limited time and much detailed and potentially valuable geological data may be lost. Stripped outcrops are covered again because of environmental regulations, abandonment of the project or they may be mined out in increasing large and lower grade open pits.

In recent history, there have been many developments that have aided ore deposit research. These include: the development of microprobe and sophisticated analytical instruments which produce increasingly lower detection limits and more accurate and precise analyses, the large field of stable isotopes, age dating, detailed understanding of continental drift, discovery of sea-floor hydrothermal systems, and the development of T-fS2 space and Paul Barton’s concept of ore forming environment. Much excellent recent work has illuminated geological and chemical conditions accompanying vein gold formation. e.g. recent Witwatersrand mineral mapping and the mineral zoning studies on Carlin deposits showing two separate mineralizing events. In parallel, there is in evidence a disturbing disconnect between exploration geologists and academe. In the 1970s and 1980s field-oriented explorationists tended to more readily accept syngenetic concepts for lode gold deposits. Today, such ideas are heretical and have been virtually expunged from the literature. There is increased specialization resulting often in substantially different interpretation of a given outcrop or map area.

While I try to be objective, the book is balanced in favour of syngenesis and a focus on the origin of the early quartz, the existence of which is acknowledged in current multi-stage formation models of Dick Hutchison for volcanic-hosted gold, Ross Large for Carlin-type and Dan Kontak for Meguma turbidite-hosted veins. I think the majority of lode veins -a definition of which is important – formed at the same time as the enclosing rocks. Fluid remobilized during lithification, deformation and metamorphism has been suggested to add to gold endowment in some deposits and may upgrade a sub-economic resource to mineable status. Excellent evidence for the magnitude and importance of this latter process is now emerging. I hope it will also become apparent that subsequent events such as deformation, “shearing” and metamorphism neither invalidate a syngenetic origin nor are required for the formation of a gold deposit. In a hydrothermal seafloor environment with elevated geothermal gradients, considerable complexity may be expected.

A highly disputed feature of lode vein ores has been the time and mode of emplacement in relation to the surrounding country rocks I,. e., are they
epigenetic or syngenetic? To some extent, the distinction between syngenetic and epigenetic becomes meaningless in the light of the apparent complexity of the origin of these deposits. As Ross Large has pointed out for VMS deposits, the important factor is that the mineralization is coeval with volcanism (or sedimentation in the case of turbidite-hosted veins such as Meguma) whether it be epigenetic or syngenetic.

The book has thirteen chapter and four appendixes.

In Chapter 1, review of numerous models in the literature reveals consensus that there is an elusive process, which has prevented complete understanding of lode gold formation process. It has even been recently stated by Franco Pirajno that lode gold deposits are amagmatic or unknown. This is unlike for instance in komatiitic nickel formation, for which in an equivalent time period, say from 1968 when komatiites were first identified, to the present, an excellent understanding has emerged as well as a general agreement on a formation model. Reviewing the historical aspects of syngenetic ideas for gold deposit formation it becomes apparent that they are almost completely absent from current main stream thought. There is a strong polarization of ideas towards a metamorphogenetic origin of fluids, deep seated faults to focus the fluids, and structural traps where the lode veins are sited. Persistent and key questions remain: what is the timing of vein formation and the origin of the hydrothermal fluid?

In Chapter 2, I explain the basis of the synformational perspective on textures that I bring to this book. A main contention is that unless there is evidence to the contrary, what we see in outcrops is primary. Imagine the proverbial man from Mars looking at an A-E turbidite with a quartz “vein” on a sea-shore in the Meguma of Nova Scotia. In the absence of preconceptions (also known as previous work with a contrary view), he could considered the white rock as either primary or secondary. From a purely academic perspective the idea that the quartz formed at the same time as the enclosing rocks is a valid direction of inquiry and this is my second and most important prejudice: most of the quartz was deposited at the same time as the enclosing rocks. Detailed outcrop maps and numerous photos of textures from the Meguma clearly show that silica was deposited often as a gel, and always constitutes an integral element of turbidite sedimentation. A rapid deposition rate may result in massive structureless quartz (bull quartz). Fine-scale monomineralic layering in quartz veins (crack-seal texture) reflects slow deposition from dense ponded cooling brines and evolution of solutions over time. Primary vein and sulfide textures are essentially preserved during deformation and metamorphism and what we see in outcrop provides crystal clear constraints on hypotheses for their origin. Cross-cutting veins and textures do not invalidate a syngenetic origin for most of the quartz and understanding the P-T conditions for quartz formation is of prime importance (Chapter 7).

In Chapter 3, my observation that a majority of lode veins occur at the top
of graded bedding cycles is expanded to the concept of gold cycles (GC), loosely based on turbidite formation processes and terminology. Most quartz occurs with within or on top of E-division pelagic-pelitic (or tuffaceous) units in Bouma-like A-E turbidites. In gold camps, I give “E” division units the name Gold Cycle “E” or GCE which in another large leap, I suggest that these interflow sediments or “auriferous exhalites” are often mistaken as “shear zones”, “lamprophyre” or “mafic dikes”. In volcanic terrain “E” units have a basaltic composition. GC “A” division lithologies include gabbro, diorite, granite, syenite, TTG suite and epiclastics. The GC concept systematizes description of bedding- parallel quartz units (“veins”) which permit them to be widely recognized in the literature.

Chapter 4 provides further background field observations for the syngenetic concepts that form the basis of this book. First, I describe and show by photos of quartz in the “E” division of Bouma turbidites in outcrops how Meguma and by analogy, Ballarat and other bedding-parallel turbidite-hosted gold veins, can be interpreted as forming at the same time as the enclosing rocks. This concept was developed in a previously unpublished manuscript (reproduced in entirety as Appendix A) and is illustrated by photos (Appendix B). In the second part of Chapter 4 , I show that in drill core and in outcrop, GCE also occur in the much older 2,640 Ma Chester tonalite Complex, which hosts the multimillion ounce Côté Gold deposit currently being developed by Iamgold. The presence of GC lithologies and graded bedding clearly demonstrates that the veins and gold were formed under water. Gold-bearing strata can be traced for hundreds of metres through a thick pile of felsic flows. By using the asymmetry of GCs, faults and structure can be mapped and stratigraphy can be traced. This is followed by descriptions and a brief discussion of GCs and lode veins from the spectacularly high grade Archean Sage Gold-Prodigy (Kodiak) “Hercules” vein systems and extensions in the Elmhirst “intrusion” in the Beardmore-Geraldton greenstone belt of Ontario. From very close (50 m) drill spacing and logging the core on sometimes millimetre scale, it is possible to unravel structure, stratigraphy and sea-floor vent spacing parameters. High grade veins in the Little Bear Lake area in the centre of the Maskwa tonalite batholith in Manitoba are synvolcanic, their asymmetry can be used to unravel regional structure. They are stated to be hosted by rocks of the trondhjemite-tonalite-granodiorite (TTG) suite.

Highly relevant for lode vein formation models is an outcrop in the Chester Complex in which a lamprophyre (L) cross-cuts a Gold Cycle “E” (GCE) unit. I mapped this in detail and difference between them is explored extensively in Chapter 5, in which I examine the purported relationship between lode gold deposits and lamprophyres (L) using REE, trace element and whole rock analyses to clarify the role of Ls. L and GCE units are subtly but clearly different. They share broadly similar calc-alkaline affinity and a high degree of fractionation with elevated LREE and incompatible trace elements. Primitive Mantle and NMORB normalized plots also show significant depletion of Nb, Ta and Ti. However, basaltic (GCE) samples are more primitive, with higher concentrations of Mg, Cr, Ni & Ca and lower concentrations of Si, Al, Na and P. Lamprophyres also have higher concentrations of LREE and considerably lower concentrations of Au. I concur with previous workers that there is no genetic connection between lode gold deposits and lamprophyres, but if a true lamprophyre intrudes a gold-bearing sequence, assimilation may lead to above background gold concentrations. A well-documented example occurs in Red Lake. Confusing lamprophyre and GCE is not possible in a good outcrop exposure, but how do you know what is the origin of that fragmented green xenolith in a deformed felsic host? Many so-called lamprophyres in Abitibi-belt, Yilgarn block and other gold camps appear to be CGE units. This is the most important conclusion based on some 20 trace element ratios and petrographic plots. Unsurprisingly, chemistry alone cannot unequivocally distinguish between CGE and lamprophyre.

My claim that GCEs are time-stratigraphic volcano-sedimentary markers that subdivide flows and flow sequences, naturally leads to question the nature of their felsic host rocks. The presence of GCs in felsic-hosted gold deposits is further clear evidence of felsic submarine extrusive volcanism. This is explored in Chapter 6 in context of an area within the Chester Complex that is a stratigraphic extension of the Côté Gold deposit. Major lithologies in the Chester “trondhjemite-diorite laccolith”, which hosts the Côté Gold deposit of IMG, are bi-modal volcanics with SiO2 contents ranging from 51-77%. A database of >200,000 assays, trace elements and high resolution photos of >4,000 m of core from drilling on Clam Lake (Appendix C) was used to correlate stratigraphy and mineralization over hundreds of metres between drill holes. Scandium is a useful proxy for SiO2 and its concentration is inversely related to SiO2. Gold is hosted mainly by GC interflow sediments which may contain quartz. The Côté Gold deposit is not an Archean copper-gold porphyry, as proposed in current publications, but it occurs within one fold limb of a northwest trending steeply south-dipping overturned bi-modal volcanic sequence.

In Chapter 7, I selectively examine the vast literature of chemical, isotopic and age dating data that underlie gold deposit formation models. Some lode vein textures and especially quartz in Meguma turbidite-hosted deposits (Appendix B), suggest sea-floor deposition from silica gel. Experimental and theoretical calculations from the seawater-quartz system shows that gel precipitation is possible in depressions where hot dense silica and salt-saturated fluids (hopefully with some gold) are ponded. Cooling of these by diffusion favours gel formation and significantly, if the same fluid is merely diluted by mixing with seawater, silica may not precipitate. A P-T-x window for lode vein formation is therefore apparent. Formation conditions can be further narrowed down by considering Ag/Au ratios in electrum, Fe content of sphalerite and As/S ratios in arsenopyrite. What causes gold to come out of solution? fO2 and fS2 diagrams show a solubility cliff for gold and sulfide phase chemistry shows the general position of lode gold forming conditions within Barton’s ore forming environment. Zoning in arsenopyrite in Meguma veins and sediments is considered the result of non-equilibrium growth, differences in S and As availability and diffusion kinetics. I do not think temperatures obtained from the arsenopyrite geothermometer are meaningful where arsenopyrite grows at low temperature in a sedimentary environment. XANES analyses of stromatolite-like structures in 3.45 Ga Strelly Pool cherts in the Pilbara craton show that carbonaceous material and graphite – common in gold-bearing lode veins – was abiotically generated at 2-300o C and 550 bars pressure. This provides independent evidence for a link to sea-floor vents because such carbon is common in lode gold veins. Age dates for bedding parallel lode veins and host rocks coincide in several well-studied examples. In other instances where they do not, what was dated and the techniques should be re-examined before discrepancies can be considered as evidence against syngenesis. Fluid P-T-X parameters for lode veins are well constrained. Both metamorphic and magmatic hydrothermal fluids are compatible with syngenesis and the vast field of stable isotope evidence from fluid inclusion analyses in quartz is briefly addressed.

My empirical model for syngenetic formation of lode veins is presented in Chapter 8. It is characterized by simplicity. Sea-floor hydrothermal vents and seeps are buried by periodic influx of detrital, pyroclastic material or flows. This “new” empirical syngenetic model for LV formation focuses on the role of silica, stratigraphy, sedimentology, fertile “recycled” crust as a gold source, bimodal felsic-mafic volcanism and combines new insights from sea-floor hydrothermal systems and crustal formation processes. All you need is a source bed and heat from an elevated geothermal gradient (mantle plume?) or crustal subsidence or subduction. Key variables are gold, base metal, SiO2 and CO2 content of the original fluid. Economic gold deposits result from a favourable interplay of gold-rich hydrothermal solution, vent geometry parameters, boiling, depressurization and dilution at the site, sedimentation and burial by volcanics or clastic sediments. Keep the hydrothermal cell operating long enough (say 40 my) and you may get a large or giant deposit (e.g. Homestake). Even 10 million years is long enough as is shown by recent dating of Blake River volcanics which host major Abitibi belt Au-rich VMS deposits.

In Chapter 9, I summarize literature on some gold deposits mainly from Archean shield areas that clearly show syngenetic aspects. Drill cross-sections and mine grade-thickness plots, allow detailed delineation of vent geometry, bottom topography and whether hydrothermal solutions emanate from a point source, multiple diffuse seeps or parallel fractures. Lode veins generally range from 250 to 600 m and up to 1,800 m in length. Larger lode veins may be approximately 1-3 km long, 1-3 m thick and can be disc-shaped, lobate or porous tuffaceous substrate may be silica saturated, which reflects diffuse venting. A brief summary of some world lode gold deposits, mainly from Archean shields, focuses on aspects of a syngenetic origin, such as their stratabound nature, the presence of Gold Cycles, asymmetric alteration and the role of “lamprophyres” or GCE units. Mafic hosted deposits include Akasaba, Clavos, Eau Claire, Eagle River, Omai and Lamaque. Felsic hosted deposits include Goldlund, Bourlamaque, Côté Gold, Magino, Hammond Reef, Goldex, Maskwa, and Kodiak Golden Mile.

Chapter 10 examines and presents exploration applications and examples how Gold Cycles, asymmetry and the geopetal nature of veins can be used in the field to determine regional stratigraphy, structure and aid with interpretation of geophysical data. Exploration applications of a syngenetic formation mode for lode vein deposits are numerous and to delineate them is one of the main objectives of this book. Recognition based on decades of dating and mapping by John Ayer, Phil Thurston, the OGS, GSC and Quebec collaborators that the Abitibi belt developed autochtonously, has resulted in a correlative unconformity-bounded stratigraphic model which provides regional and deposit-scale stratigraphic intervals to explore for syngenetic mineralization. TTG rocks are targets and GCE and asymmetry can help unravel stratigraphy and structure. Theoretical and observed sea-floor fracture spacing (commonly 200-450 m) can reduce the cost of exploration drilling. Basin analysis and pyroclastic fragment mapping can be used for exploration. District-wide alteration halos may be detectable from remote sensing, rock geochemistry and oxygen isotopes. Vent tracing techniques can be developed by using interflow (GCE) sediment composition, Fe/Mg ratio in carbonate, copper content in pyrite, iron content of sphalerite, proportion of quartz in GCE and vertical and horizontal mineral changes in staked veins.

Chapter 11 briefly deals with my findings that the chemistry of mafic dikes, or enclaves in TTG suite “intrusions”, such as the Maskwa batholith, the Bourlamaque batholith and the Chester Complex is equivalent to those that in the TTG literature are stated to be mafic components of andesite-dacite-rhyolite (ADR) suites. TTG rocks are widely considered to have a magmatic origin, with magmas formed under conditions of low pressure, dry melting and fluxing by hydrous fluids derived from subducted lithosphere. How is this possible when we find ubiquitous GCE pelagic-pelitic strata in these “intrusions” ? TTG suite rocks often are older than surrounding volcanics and I think one piece of the puzzle might consist of early felsic volcanism, which now forms the basement to Archean greenstone belts and this basement is exposed by folding and doming. An excellent example is provided by the Maskwa batholith in Manitoba and the Webb Lake stock which hosts the Magino deposit. Consequences for TTG formation models are significant.

In Chapter 12 I address bi-modal volcanic and turbidite-hosted sedimentary environments of lode vein formation. For felsic-hosted deposits such as Côté Gold, fine grained pelagic and hemi-pelagic sediments (GCE) in a volcanic setting indicate an intra-oceanic setting away from landmasses. Massive felsic and mafic lavas indicate high discharge rates. Absence of pyroclastics or explosive fragmentation and peperite indicate deep water. Fischer-Tropsch P-T parameters and the P-T-X “window” from quartz indicate water depths of 500- 2000 m and open space fillings with euhedral quartz in veins are indicative of sea-floor or sub-seafloor boiling. A short section is devoted to comparing the chemistry of GCE to Archean interflow sediments and GLOSS, the average global subducting sediment in order to understand palaeogeological environments of gold deposition. Studies of the Grenvillian orogenic cycle yield important clues about the formation of lode vein deposits. Large tectonic plates previously formed during dispersal of Nuna ca 1.8 Ga were made significantly larger by addition of juvenile crust over wide areas from 1.7 to 1.3 Ga, within marginal plate setting of arcs and back arcs. Large areas of the continental crust were also thicker. The earth’s mantle was 55o C hotter than today and the rate of continental drift immediately prior to Grenvillian orogenesis was nearly an order of magnitude larger than at any subsequent time in Earth history. Combined, rapid drift rate, enhanced plate size and strength, and elevated temperature characterize high orogenic intensity. How is this reflected in the size, frequency of occurrence and location of lode gold deposits? Archean sagduction, isoclinal folding and deformation accompanying granitic intrusion accounts for the steep dip of most lode veins. Deformation exposes veins but has no genetic importance.

Conclusions and suggestions for further work form the subject of Chapter 13.

Much confusion in the gold deposit literature and generations of scientific data is resolved if the origin of lode veins origin is considered from a syngenetic perspective. Vent tracing techniques can be developed or refined. Gold Cycles provide stepping stones to an enhanced understanding of stratigraphy, structure, and deformation, basin analysis, age dating, volcanic architecture and the lode vein forming environment. Inevitably, many questions arise from the proposed formation model. These can now be addressed within the revitalizing context of syngenesis.

Dedication

This book is dedicated to those who have told me that until they heard my ideas, they did not understand the origin of gold deposits. It is dedicated to the students with million dollar instruments at their disposal but who have never walked through the bush while trying to understand quartz vein formation. It is dedicated to the student with an advance degree from a prestigious university who when asked to write a field description of a gold showing, returned to me with a one page list of the deformation alphabet, but failed to mention what was the host rock type.

My advice above all, is that you must go into the field and let what you see underfoot lead you to your theories. You will not be able to distinguish felsic flows from intrusive granite with a similar chemistry, unless you get out in the field. You will not be able to distinguish intrusive lamprophyres from pelagic-pelitic interflow sediments unless you see these rocks the field.

Finally, I dedicate this book to those geologist everywhere who are interested in understanding and finding lode gold deposits. I cannot claim originality, since every single observation has been previously made, but the significance has not necessarily been realized. My quest started out by simply stating that quartz veins in the Meguma formed at the same time as the enclosing rocks. This is neither a popular nor a balanced view, but I think the rocks themselves have provided eloquent evidence in favour of syngenesis.

Acknowledgements

This book has had a long gestation period. My 1982 report which contained what I considered abundant evidence for a syngenetic origin of Meguma gold deposits was not published by the Geological Survey of Canada. Since then encouragement from gold experts has failed to materialize but exploration geologists have been more receptive. A genuine ray of light was provided by George Langdon and Gerard Edwards of Shoal Point Energy and Bill Love of Sage Gold, who generously funded some of my iconoclastic ideas and gave permission to publish them. Sanatana Resources Inc. paid for chemical analyses and imposed minimal reporting requirements during an extended consultancy. Frank Racicot helped significantly with mapping and sampling. Steve Scott provided invaluable access to University of Toronto library facilities and made helpful comments on aspects of an earlier draft. Greg Anderson provided insight into behaviour of quartz in the silica-seawater system. A review by Stephanie Brueckner of the World Gold 2011 paper helped me to improve the manuscript. John Ayer helped with petrographic plots and along with Hadyn Butler, Walter Pohl, Peter Laznicka and Andreas Mueller provided insightful comments on parts of the manuscript. Dick Hutchinson urged me to never give up. All errors are entirely my own.

06
Dec
13

Meguma

14-HarriganCove

This section is taken from:
Kretschmar, U.H. (2011) Syngenetic Gold: Lode Vein Geology and Exploration Implications. (in) World Gold 2011. Proceedings of the 50th Annual Conference of Metallurgists of CIM. Montreal, Canada. Edited by G. Deschênes. R. Dimitrakopoulos, J. Bouchard. pp 849-863.

 

A summary of the excellent work on Meguma veins extending over many years  by Dan Kontak and co-workers  is :  Kontak D.J.& Horne, R.J. (2011) Chapter 4. A Multi-Stage Origin for the Meguma Lode Gold Deposits, Nova Scotia, Canada: A Possible Global Model for Slate Belt-Hosted Gold Mineralization. p. 58-82. In Gold Metallogeny: India and Beyond. Ed. Mihir Deb and Richard J. Goldfarb. Alpha Science International Ltd. ISBN 978-1-84265-646-4.

EXAMPLES OF “GOLD CYCLES”

Meguma Turbidite-Hosted Gold Veins and The Sedimentary Environment

Cambro-Ordovician Meguma gold deposits have been the subject of numerous studies. For widely accepted views of their genesis, see Kontak, Horne, and Smith [23], and Sangster and Smith [24]. They are considered part of the “greenstone clan” of gold veins, formed at depth from solutions, remobilized during deformation. The host Meguma Supergroup, discussed by Waldron et al. [25] is described as a thick (>10 km thick Cambrian to Early Ordovician turbiditic clastic succession, with a lower, coarser grained Goldenville Formation containing numerous quartz and lode gold veins and an upper, dominantly fine-grained Halifax Formation.

During a 1982 Canada-Nova Scotia Mineral Agreement project in NTS 11D16 and 11E1, detailed mapping was carried out by the author in gold districts and on seashores with excellent exposures, within the area shown in Fig. 2.

Fig. 2

Figure 2 – Location of seashore exposures and Meguma study areas, South Coast, Nova Scotia.

This unpublished work (Kretschmar [26, 27]) is referenced by Crocket et al. [28] and incorporated in part by Haynes [29]. The main claims are that bedding-parallel veins, which host 90% or more gold deposits are silica crusts deposited mainly in the form of a gel from hydrothermal solutions on the Cambro-Ordovician sea floor. The author suggests that deposition of quartz is terminated by episodic influx of detrital material from turbidity currents. This empirical model was based on the subjective interpretation of textures: bedding parallel veins formed at the same time as the enclosing rocks. For the Meguma, Mawer [30] and virtually all other researchers express diametrically opposed views, with the notable exception of T. Sterry Hunt [op.cit.]. In Nova Scotia, low metamorphic grade, excellent seashore exposures and abundant drill cores yield much information. Bedding parallel veins are found almost invariably within the upper E division of Bouma turbidites, e.g. Fig. 3.

fig-3a

Figure 3A – Markie Point. Typical shoreline exposure of Meguma turbidites with synsedimentary quartz veins (tops are to the left of the photo).

 

Markie Point, Nova Scotia. Synsedimentary quartz veins at top of Bouma A-E turbidites. Quartz was deposited probably as gel during ongoing pelagic sedimentation.  Precipitation from silica-saturated hydrothermal solution was terminated by next turbidite. Hydrothermal cell re-established itself and cycle was repeated, resulting in stacked quartz crusts or "veins".


Markie Point, Nova Scotia. Synsedimentary quartz veins at top of Bouma A-E turbidites. Quartz was deposited probably as gel during ongoing pelagic sedimentation. Precipitation from silica-saturated hydrothermal solution was terminated by next turbidite. Hydrothermal cell re-established itself and cycle was repeated, resulting in stacked quartz crusts or “veins”.

Figure 3B.– Markie Point.  Upper Ec division of Bouma turbidite (GC for gold-bearing veins) showing quartz vein with complex internal structure, overlain by 30 cm thick turbidite with graded bedding. Massive base of next overlying turbidite (T3A) is at top of photo. Individual units are numbered from base upwards adapting turbidite terminology. Outcrop location: NAD 83. UTM 20T, 0543478/496511 and Figure 2.

13-HarriganCove

Figure 4 – Harrigan Cove Gold District, trench map of turbidite-hosted gold-bearing quartz veins. Turbidites and A, E, and Q subdivisions are numbered individually from base upwards. (Mapping by U. Kretschmar, 1982)

Detailed mapping in Harrigan Cove (Fig. 4) provides significant insights into veins, quartz, and gold and turbidite sedimentation. Location, history, geology, Au and As distribution in Harrigan Cove turbidites are described by Crocket et al. [op. cit.]. Fig. 4 illustrates twelve turbidites in a Harrigan Cove trench. Quartz occurs within the E division of turbidites or at the contact with the overlying A. Arsenopyrite occurs throughout in A division greywacke as a diagenetic mineral. Crosscutting quartz consists of thin “angulars” that transgress stratigraphy for 3 m. or less. Bedding-parallel quartz shows complex internal structure, bulbous bases that appear to contour onto the pelagic-pelitic substrate and chlorite, sericite, and graphite on internal partings (described in detail by Mawer [op.cit.] and Kontak et al [op.cit]).

BeaverDamNSSyngeneticQuartzVein

Fig. 5 is a polished slab of vein stratigraphy from Beaver Dam (location on Fig. 2) with four or more domains – representing different episodes of deposition. The bottom 7 cm. bed is massive white Q with a few apparent shale wisps.

Figure 5 – Polished slab of quartz vein, Beaver Dam Gold Deposit. The vein is a crust on pelitic substrate (upper E division of a turbidite). Monomineralic sulfide layers and probable abiotic carbon require multiple episodes of deposition (see Strelly Pool chert discussion)

Overlying are thin shale partings deposited on an indented surface – hollows are filled with 3 or 4 mm thick accumulations of black shale, chlorite or pelitic material. Q2 is 1-2 cm of darker re-
crystallized quartz . They are separated by a thin, poorly-developed bedding surface with small amounts of clayey material. Separating the lower three quartz “domains” from the overlying fourth is a 3 mm thick pyrite-galena cap. The upper quartz “domain” consists of 7.5 cm of coarsely crystalline quartz with irregularly shaped conical inclusions, generally elongated perpendicular to bedding. The fragments are delicately laminated and individual laminae consist of graphite, sericite, or chlorite. The quartz filling interstices between these structures is whiter and less crystalline. Our empirical field- based observations on quartz veins in the Meguma support the 1868 conclusion of T. Sterry Hunt [op. cit.] who wondered how anyone could doubt that they formed at the same time as the enclosing rocks. Current research (e.g. Sangster and Smith [op.cit.]) will yield significant new insights into Meguma gold deposits if they are re-interpreted on that basis. Further support is provided by Archean lode veins from Beardmore.

06
Dec
13

Downloads

AbitibiRegionalGeology LODE GOLD VEIN BOOK

2014 May. GAC-MAC Meeting, Fredericton: Abstract

2014MayGAC-MAC Abstract

2012 November. University of Western Ontario, Colloquium Series: Abstract

2011: World Gold 2011, Montreal

1985: GSA Meeting. Providence, R.I. Abstract. 

1984: Meguma TGDG: Abstract

28
Nov
13

Exploration Applications

bourlamaque-5

This is one example of how facing direction indicators provided by the asymmetry of Q-E-A Gold cycles can be used to unravel regional structure, narrow down the  search area  and point to potential targets.

The example is taken from the publication:  Kretschmar, U.H. (2011) Syngenetic Gold: Lode Vein Geology and Exploration Implications. (in) World Gold 2011. Proceedings of the 50th Annual Conference of Metallurgists of CIM. Montreal, Canada. Edited by G. Deschênes. R. Dimitrakopoulos, J. Bouchard. pp 849-863.

Eight mines in the Bourlamaque batholith are located on one or more  folded stratigraphic horizons. Structural correlation is based on Gold Cycle top determinations.  
The Bourlamaque Batholith and TTG Lithologies. The 10 x 25 km Bourlamaque batholith contains 8 mines. Campiglio and Darling [60] studied its chemistry and characterized it as a synvolcanic quartz tonalite. Its age is 2700 Ma (Wong et al. [61]). In a gravity study, Jebrak et al. [62], point out that it resembles a dyke rather than a sill – and confirm a distinct density difference between the northern and southern parts. Belkabir et al. [63] discuss the relation between “dikes” and “auriferous shear zones.” They indicate that complex shear zone and vein patterns within the pluton reflects the influence of diorite dikes. These acted as weak layers that were activated during subsequent deformation and illustrate the importance of layer anisotropy in auriferous shear zone development. The Beaufort mine was studied by Tremblay [64], the Ferderber mine by Vu et al. [65] and the gold potential of the Bourlamaque batholith by Taner and Trudel [66]. Current investigations on dating of veins are being carried out by Lemarchand et al. [67] showing strike, dip and spatial relationship of gold-bearing quartz veins to dikes. Daigneault and Gaboury [68] present a compilation of these studies.

Continue reading ‘Exploration Applications’

01
Oct
13

Formation Models

fig-15

This syngenetic model is based on grade-thickness contours of ore lenses, which shows how different ore-shoot geometry provides information about the sea-floor on which silica crusts are deposited. Hot dense saline brines sit in depressions or hollows and silica (hopefully with gold) precipitates as gel during conductive cooling. Only heat is needed as quartz and gold are leached from various rock types. Burial of the quartz crust may involve pyroclastics, volcanic flows or turbidity currents.

Traditionally Gold Formation Models fall into three main categories:

  1. those based on fluid composition and source of fluid.
  2. those based on structure and deposition site and mechanics of deposition e.g brittle-ductile transition or dilational jogs and
  3. those based on grade of metamorphism eg. greenschist-amphibolite transition.

Difficulties in the systematic classification of ore deposits stem from the large number of variables (lithological, structural, chemical and tectonic), which interact to make the observed data difficult to interpret. According to Frimmel (2007), a given deposit may be classified according to host rock (e.g. sediment-hosted), or according to a preferred genetic model (e.g. orogenic), the classification may emphasize a specific metal association (e.g. iron oxide copper gold), or it may be based on a comparison with a large prototype (e.g. Carlin type)’’. According to Bierlein (), who places importance on metamorphism, states there is still no consensus on a genetic model for gold deposits in metamorphic belts.  One point that can be stressed with some degree of certainty is that surface waters are unlikely to be important in the formation of ores. However, components of metamorphic, magmatic, and/or mantle processes may all play some role in ore genesis.

One purpose of this website is to provide a source of information on gold deposit models and in particular to update syngenetic concepts, which clearly have not been updated recently.




Welcome to Syngenetic Gold

This website is dedicated to making better known the syngenetic aspects of lode gold vein genesis and to provide a source of information on theories of gold deposit formation.

In the 1970s and 1980s when models for VMS deposits invoked continental drift and sea-floor hydrothermal vents, models for gold deposit formation followed suit. Current formation models favour epigenetic origins. Over many years of research, I have accumulated much evidence that most lode gold veins represent silica and gold crusts that were deposited from sea-floor hydrothermal vents. These were buried by periodic influx of turbidites, volcanic flows, or epiclastic equivalents. The strata were later folded and the original flat-lying beds are now steeply dipping "veins".

This simple model is at variance with current thinking but the implications for exploration are profound and the entire field of gold deposit genesis will benefit from applying syngenetic concepts to current models.

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