New publications from my research group (02/06/14)

QA_QC

It’s been a while since I’ve updated this blog. The combination of fieldwork, teaching, and just being generally busy, hasn’t provided much time for extra-curricular writing.

I’m taking a bit of a departure from previous posts, and just providing an update post of some recent papers and research coming out of my research group. When I get some more time I’ll update the blog with some more paper reviews, and I’m always interested in comments and suggestions from people.

My group has been working on a variety of projects, including massive sulphide research in Newfoundland (and elsewhere), work on portable X-ray fluorescence, and general work on quality control and quality assurance in lithogeochemical data.

My MSc student Shannon Gill has just published her first fieldwork article in Geological Survey of Canada (GSC) Current Research.  Her research is on mineralogy, metal zoning, and evolution of the Zn-Pb-Ba-Ag-Au-bearing Lemarchant volcanogenic massive sulphide deposit.  The paper breaks out the various facies and illustrates how the deposit is different than most bimodal felsic VMS deposits in  having abundant sulphosalts and a likely epithermal input to the metal budget of the deposit.  The paper can be downloaded here.

My PhD student Jean-Luc Pilote has just published his first fieldwork article of his PhD also in GSC Current Research.  His thesis is aimed at reconstructing the stratigraphy, structure, and hydrothermal alteration of the Au-bearing Ming VMS deposit.  His work is also utilizing lithogeochemistry, U-Pb geochronology, and radiogenic isotopes to understand the petrology, chemostratigraphy, alteration and timing of various events in the deposit, and the evolution of the deposit to the regional tectonic and metallogenic framework of the Baie Verte Peninsula.  The initial results of his mapping last year on the 1807 Zone of the Ming Deposit are presented here.

In addition to students, I have has two papres recently published.  The first paper is an invited contribution to Geoscience Canada and is a review article on quality assurance and quality control (QA/QC) in lithogeochemistry.  The paper is written for a non-expert and covers various topics including sampling, precision, accuracy, contamination, and general monitoring of lithogeochemical data during a QA/QC program.  For those interested in this you can get a copy of the paper here.

The second paper that has come out is on the utilization of portable X-ray fluorescence and published in Geochemistry: Exploration, Environment, Analysis.  My group has been spending a lot of time developing pXRF for utilization in lithogeochemistry and this paper reports on our approach to calibrating the instrument (single point calibration method) and provides results on international reference materials and samples that had previously been analyzed by other methods.  The results illustrate that the pXRF has considerable potential as an initial means of screening data and providing ‘fit for purpose’ information, but it is not a substitute for traditional methods.  The other major outcome of this paper is that it provides a calibration method that is relatively straightforward, and yields reasonably precise and accurate data to understand alteration, chemostratigraphy, and general metal values for samples.  The approach is now being used in conjunction with collaborators on archived assay pulps from other sites/deposits for alteration mapping and chemostratigraphy.  Those interested in a copy of the paper can find it here.  There are also numerous other great papers recently published in a thematic set on pXRF in Geochemistry: Exploration, Environment, and Analysis.  This group of papers is worth checking out as it is the current state of pXRF knowledge applied to lithogeochemistry and exploration geochemistry.

Stay tuned as well as there some more papers coming out of our group in the coming months on gold-rich massive sulphide deposits and emplacement mechanisms in VMS deposits.  I’ll also do a quick overview of these papers when they come out.

Posted in Analytical Geochemistry, Appalachians, Economic Geology, Exploration Geochemistry, Geochemistry, Geology, Gold, Gold Deposits, Lead, Lithogeochemistry, Mineral Resources, Quality Control, Seafloor Massive Sulfides, Volcanogenic Massive Sulfides, Zinc | Tagged , , , , , , , , , , , , | Leave a comment

Noble Metals, Subcontinental Lithosphere, and Ni-Cu-PGE Deposits

Merensky_Reef

Recent research by Kamenetsky et al. published in Geology provides insight into the nature of the parental magmas potentially responsible for the formation of nickel-copper-platinum group element (Ni-Cu-PGE) deposits in intra-plate environments.  Previous researchers have illustrated the importance of the mantle-derived magmas and subcontinental lithospheric mantle in the formation of Ni-Cu-PGE deposits (e.g., Arndt et al., 2005); however, finding the nature of occurrence and concentrations of chalcophile elements in primary magmas from the subcontinental lithosphere has been problematic.  New research by Kamentsky et al. provide significant insight into this problem.

The work of Kamenetsky studied unusual rocks from the Bouvet Triple Junction of the Mid-Atlantic Ridge.  These rocks are unusual and unlike normal basaltic oceanic crust, having continental (i.e., evolved) radiogenic isotope signatures suggesting derivation from the subcontinental lithospheric mantle.   Moreover, they are exceptionally well preserved, glassy, and contain sulfide globules within the glasses (Figure 1).  It is these sulfide globules that provide critical insight into the nature and composition of chalcophile elements in the subcontinental lithospheric mantle.

Kamentsky_F1.large

Figure 1.  Reflected-light photograph sulfide globule in basalt glass from the Bouvet Triple Junction  and X-ray maps of elemental distribution within the globule.  Hotter colours reflect higher concentrations. From Kamenetsky et al. (2013).

Most sulfide globules within the basalt glass are less than five microns; however, one globule was large enough to allow detailed mineralogical and geochemical study.  The large globule contains Fe-Ni sulfides interpreted to be immiscible sulfides that quenched during basalt eruption  (Figure 1).  The globules also contain Fe-oxyhydroxides, minor Cr-rich magnetite, and micro-nuggets rich in Pt, Pd, Au, and Sn, including: pure Pt; Pt-Sn alloys; Au, Pt-Au, Pt-Au-Sn alloys; and rustenburgite (Pt,Pd)3Sn (Figure 2).  In addition, laser ablation inductively coupled plasma mass spectrometric (LA-ICP-MS) analysis of the droplet has elevated chalcophile element contents (46 ppm Pt; 41 ppm Pd; 3 ppm Rh; 15 ppm Ru; 7.5 ppm Os; 14 ppm Au; and 15 ppm Ag); the samples also have chondrite-normalized noble metal signatures broadly similar to PGE-bearing layered intrusive complexes (e.g., Merensky Reef).

Kamentsky_F2.large

Figure 2.  Scanning electron images of sulfide globules wtih sulfides (light grey), Fe-oxyhydroxide (dark grey), Cr-magnetite (grey) and nuggets of noble metals (white).  From Kamenetsky et al. (2013).

While there are many features that are required to form an economic Ni-Cu-PGE-rich sulfide deposits (e.g.,  Naldrett, 1997), the results from Kamenetsky et al. illustrate that melts from the subcontinental lithospheric mantle contain Ni, Cu and PGE contents up to 2-times the values present in melts derived from the depleted mantle (e.g., N-MORB).  Hence, subcontinental lithospheric mantle melts are charged in Ni-Cu-PGE and therefore are important potential targets for exploration in the ancient geological record (see also Arndt et al., 2005 and Begg et al., 2010).

Posted in Copper, Economic Geology, Geochemistry, Geology, Layered intrusions, Magmatic Sulfides, Mineral Resources, Nickel, Platinum Group Elements, Recently Published | Tagged , , , , , , , , , , , , | Leave a comment

Classic papers in Economic Geology: Campbell and Naldrett (1979) – The Influence of Silicate-Sulfide Ratios on the Geochemistry of Magmatic Sulfides

euhedral mag on po edge, pseudo-sub-ophitic, rl, FOV = 4.8mm

Magmatic sulfide deposits are critical sources of nickel (Ni), copper (Cu), and platinum group elements (PGE) globally.  These deposits form from the segregation of a sulfide liquid from a silicate liquid, similar to oil separating from water, due to the sulfide saturation of a mantle-derived (i.e., mafic or ultramafic) silicate melt.  While igneous fractionation can cause sulfide saturation in a mafic-ultramafic melt, it often produces minor amounts of sulfide mineralization.  To form significant quantities of sulfide mineralization requires addition of external sulfur (and/or silica) to the mafic-ultramafic magma, typically via crustal contamination or crustal devolatilization.  If sulfide saturation of the mafic-ultramafic magma occurs there is a partitioning of elements between the silicate magma and the segregating sulfide liquid.  In particular, the chalcophile elements, such as Ni, Cu, and the PGE, are scavenged from the silicate liquid and partition into the sulfide liquid, thereby creating a sulfide liquid enriched in chalcophile elements and a silicate liquid depleted in chalcophile elements (e.g., Naldrett, 2010 and references therein).  The final concentration of the chalcophile metals in the sulfide liquid is dependent on a number of factors including the initial concentration of chalcophile metals in the parental silicate liquid, how readily the element partitions into the sulfide liquid (i.e., the partition coefficient), and the mass of silicate liquid to sulfide liquid (e.g., Campbell and Naldrett, 1979; Barnes et al., 1997Lesher and Burnham, 2001).

In the late 1970s, while researchers understood that Ni, Cu, and PGE had high partition coefficients and preferentially partitioned into sulfide liquids (e.g., Rajamani and Naldrett, 1978), there was still a problem yet to be solved: how could a sulfide melt become enriched to percent levels in Ni and Cu  when the parental silicate magma contained only par parts per million or parts per billion chalcophile elements? This problem was even more significant for the PGE, which were present in parts per billion levels in parental silicate magmas.  The paper by Campbell and Naldrett (1979) was a critical paper in addressing this metal enrichment problem and introduced the term the R-factor.

The R-factor represents the mass of silicate magma that a segregated sulfide liquid has equilibrated with.  In essence, if we have a litre of sulfide liquid and an R-factor of 10 it means that the sulfide liquid equilibrated with 10 litres of silicate magma.  My colleague Michael Lesher provides an outstanding analogy for the R-factor by comparing it to wearing a sweater (sulfide liquid) while running through a forest (silicate liquid).  As you run through the forest burrs from the trees (burrs = Ni, Cu, or PGE) get stuck to your sweater.  If you run through the forest once (low R-factor) only a few burrs will stick to your sweater (i.e., low grade mineralization).  In contrast, as you run through the woods more and more (high R-factor), more and more burrs will stick to the sweater (i.e., high grade mineralization).  It’s great analogy for teaching students the concept of the R-factor and how it can control metal grades in magmatic sulfides.

Campbell and Naldrett (1979) provided a mathematical formulation of the above illustrating that the grade (tenor) of a sulfide liquid (Cl) is dependent on the initial concentration of metal in the silicate magma (Co), the degree to which an element partitions into the sulfide liquid (i.e., the partition coefficient given by Di = [i]sulfide liquid/[i]silicate liquid, where i = element of interest, such as Ni, Cu, PGE), and the R-factor:

Cl = [CoDi(R+1)]/(R+Di)…….(1)

(see also Barnes et al., 1997, Lesher and Burnham, 2001, and Naldrett, 2010 for further reviews).  It is obvious from equation (1) that with an increase in Co there will be an increase in Cl, all other things being equal (Figures 1).  Therefore, the grade of Ni-Cu-PGE in a sulfide liquid (and deposit) will be proportional the amount of metal in the starting liquid. However, if we look at R=1 to 100, even with a metal-rich magma we cannot create sulfide liquids that have ore grades (Figure 2).  This is where the R-factor comes in. In order to achieve ore grade sulfide mineralization a very high R-factor is required: the sulfide liquid must equilibrate with significant quantities of metal-bearing silicate magma (Figures 1 and 2).  In Figure 2, an example of Ni, Cu, and Pt is used with rough ore grade values shown.  In the case of Ni and Cu, they are in sufficient abundance in mantle-derived magmas that they only require R-factors of 100s to 1000s to yield ore grade mineralization (Figure 2).  In contrast, low concentration elements like Pt (and other PGE) require significantly higher R-factors, ~10000 or higher, to achieve ore grade (Figure 2).

R_Factor_1

Figure 1.  Concentration of Ni in a sulfide liquid in equilibrium with a silicate magma as a function of R-factor and initial concentration (Co); DNi = 500.  Notably, at a given R-factor the initial concentration of Ni the silicate melt will control the tenor (grade) of the sulfide liquid.  It is also notable, that with increasing R-factor there is an increase in the grade of Ni in the sulfide liquid.

R_Factor_2

Figure 2.  R-factor models for various Ni, Cu, and Pt (DNi = 500 , DCu = 1500, and DPt = 10,000).  Also shown is an approximate ore grade for the various commodities.  This diagram illustrates the importance of both the starting concentration of an element (Co) and also how increased R-factor is required to generate ore grade mineralization.  Also evident is that for PGE-rich deposits the R-factor requires is an order of magnitude higher (or more) than it is for the base metals Ni and Cu, and explains why many PGE-rich deposits are associated with large igneous provinces with high volumes of magmatism.

While the R-factor explained how we could achieve grade in mineralization, it also had (and has) broader implications for the exploration for Ni-Cu-PGE sulfide deposits. While obvious, it illustrates the importance of identifying areas that contain potential Ni-Cu-PGE-rich parental magmas (e.g,. picritic to komatiitic magmatic belts; Keays, 1995).  The high R-factor also implies that  large igneous provinces (LIP), especially those in continental environments, are obvious targets for Ni-Cu-PGE mineralization (e.g., Lightfoot and Hawkesworth, 1997Lightfoot 2007, and references thereinBegg et al., 2010 and references therein).  For example, the largest Ni-Cu-PGE resources outside of Sudbury, are hosted in the Noril’sk-Talnakh area within flood basalts of the Siberian Traps, Russia (e.g., Lightfoot and Hawkesworth, 1997), and the largest PGE resources are hosted within the layered intrusive complexes of the Bushveld Igneous Complex, South Africa (e.g., Cawthorn, 1999; Maier et al., 2013).  It also led to recognition of geological settings within LIP that exhibited evidence for repeated magma flow through the Ni-Cu-PGE sulfide forming environment, such as channelized flows in komatiite fields (e.g., Kambalda – Lesher and Arndt, 1995) and magma conduits in intrusions (e.g., Voisey’s Bay; Naldrett, 2010 and references therein).

My next blog post will build this one and cover recent research on metal-rich magmas from the subcontinental lithosphere and their importance for magmatic Ni-Cu-PGE sulfide genesis.

Posted in Classic Papers, Copper, Economic Geology, Geochemistry, Geology, Layered intrusions, Magmatic Sulfides, Mineral Resources, Nickel, Platinum Group Elements | Tagged , , , , , , , , , | 2 Comments

Earthquake-Induced Geochemical Anomalies

A recent review article by Cameron illustrates the importance of earthquakes in the generation of geochemical anomalies above buried mineral deposits.  The paper builds on previous work by Cameron and colleagues (e.g., Cameron et al., 2002Cameron and Leybourne, 2005; Leybourne and Cameron, 2006; Leybourne and Cameron, 2010) and investigates how earthquakes generate faults, reactivate existing faults, change groundwater movement, and how these interrelated to form near surface geochemical anomalies above buried mineralization.

The paper reviews the hydrology and fluid movement as a function of proximity to earthquakes.  Cameron illustrates that primary earthquakes can trigger secondary earthquakes, reactivation of existing faults, and associated hydrologic changes both proximal (i.e., near field) and also at great distances (up to 1000s of km) from the earthquake foci (i.e., far field).  For example, the M9.2 Alaskan earthquake in March 1964 resulted in changes in water levels up to 12-23 feet in water wells in the Midwest United States.

The latter review sets the stage for three case studies from Chile (Spence deposit), Nevada (Mike, Gap and Pipeline deposits), and Saskatchewan (Athabasca Basin).  The Chilean example illustrates how subduction-related earthquakes resulted in the mobilization and expelling of saline groundwaters along faults above porphyry Cu mineralization. Prior to movement along faults the groundwaters interacted with porphyry mineralization and extracted metals from the deposits resulting in Cu, Mo, Re, As, and Se enrichments in groundwater (Figure 1).  The saline groundwaters travelled along faults several hundreds of meters above mineralization and deposited salts and metals (e.g., gypsum, Cu-oxides) along the fault surfaces and within the gravel near the fault surfaces (Figure 1). This was also accompanied by the discharge of metal-rich groundwater (Figure 1).

Leybourne_Cameron_2010_seismics_Cu

Figure 1.  Model for the development of anomalies above buried mineralization due to the movement of groundwater during seismic events.  Saline groundwaters interact with mineralization extracting metals from buried mineralization and are released upwards during seismic events.  The interaction of these fluids with meteoric (i.e., surface waters) results in fluids enriched in metals and the deposition of mineral salts and metals (Cu, Se, Re, Mo, As) in surficial gravels 100s of meters above mineralization.  From Cameron and Leybourne (2005) and Leybourne and Cameron (2010).

The second case study comes from the Mike and Gap-Pipeline Carlin-type deposits in Nevada. These deposits are associated with seismic activity associated with Basin and Range Province crustal extension; many faults have also been reactivated by far field effects from distal earthquakes (secondary earthquakes).  Mineralization in the Mike deposit is near faults and reactivation of these faults during seismic activity has resulted fault-proximal surficial materials being enriched in elements associated with mineralization (Cu-Au) and the supergene blankets that overlie the deposit (Zn-Cd).  Similarly, the Gap and Pipeline deposits have very strong enrichments in Zn, As, and to a lesser extent Au, in the surficial materials immediately above faults and mineralization (Figure 2). In both cases reactivation of faults due to seismic activity resulted in the upward migration of mineralization-related elements in groundwaters and subsequent deposition in the near surface environment (Figure 2).

Muntean_Taufen_2011

Figure 2.  Enrichments in Zn, As, and Au in soils immediately above the Gap deposit, Nevada.  The anomalies are spatially associated with both mineralization and faults that intersect the surface and sub-surface mineralization.  Diagram from Muntean & Taufen (2011) and Cameron (2013).

The final case study involves unconformity-type uranium mineralization in the Athabasca Basin, Saskatchewan.  Unconformity-type deposits are spatially associated with faults in basement rocks and often occur either in faults within basement rocks or at the contact of these faults with overlying Proterozoic Athabasca sandstones (Figure 3).  These faults not only controlled the formation of mineralization, but were also reactivated numerous times following ore formation, likely by ancient (to modern?) sesimic activity.  In both the sandstones and surficial materials along these faults are anomalous enrichments in 206Pb and 207Pb derived from the breakdown of 238U and 235U in the ores, respectively (Figure 3).  Similarly, there are enrichments along both the faults and surficial materials above mineralization of elements associated with the ores, including U, Ni, V, Co, and As.

Cameron_2013_AthabascaFigure 3.  Schematic diagram showing the dispersion of material, including radiogenic lead, from Athabasca uranium ores. Diagram from Cameron (2013) with geology based on Jefferson et al. (2007).

The above case studies illustrate how long-lived faults and repeated seismic activity can result in the transfer of metals in groundwater from the subsurface to the near-surface environment.  It represents a potential method for the targeting buried mineralization, a major challenge in modern mineral exploration.

Posted in Copper, Economic Geology, Exploration Geochemistry, Geochemistry, Gold, Gold Deposits, Mineral Resources, Porphyry Copper, Recently Published, Uranium | Tagged , , , , , , , , , , , , | 1 Comment

Recently published: Economic Geology Special Issue on the Pebble Porphyry Cu-Au-Mo Deposit

Econ_Geol_Pebble

The May issue of Economic Geology is a special issue on the Pebble Cu-Au-Mo deposit,  located near Iliamna, Alaska, edited by Karen Kelley, James Lang, and Robert Eppinger.  The issue is the culmination of a multi-year, multi-disciplinary collaborative research project between the USGS and Northern Dynasty Minerals Ltd.  The deposit area has been the focus of mapping and exploration since the 1950s, but the main discoveries were in the 1980 to late 1990s by Cominco (Pebble West zone), and in the mid- to late 2000s by Northern Dynasty Minerals (Pebble East zone).  At present the deposit contains National Instrument 43-101 (NI-43-101) compliant measured and indicated resources total 5,942 million metric tons (Mt) at 0.42% Cu, 0.35 g/t Au, and 250 ppm Mo with an inferred resource of 4,835 Mt at 0.24% Cu, 0.26 g/t Au, and 215 ppm Mo (Kelley et al., 2013), and is the fifth largest porphyry deposit in terms of contained Cu and contains more Au than any other porphyry deposit.

The special issue covers quite a range of topics ranging from regional tectonics (Goldfarb et al., 2013), interpretation of aeromagnetics (Anderson et al., 2013), deposit geology and mineralization (Lang et al., 2013), geometallurgy (Gregory et al., 2013), alteration and infrared spectroscopy (Harraden et al., 2013), exploration geochemistry and deposit mineralogy (Eppinger et al., 2013), and Cu, Pb, Sr, and Nd isotope geochemistry of ores and surficial materials (Mathur et al., 2013 and Ayuso et al., 2013).

This issue is a worthwhile read for anyone interested in magmatic hydrothermal mineral deposits and the geology, geophysics, geometallurgy and exploration for porphyry Cu-Au-Mo deposits.

Posted in Alaska, Copper, Cordillera, Economic Geology, Exploration Geochemistry, Geochemistry, Geochronology, Geology, Geophysics, Gold, Gold Deposits, Mineral Resources, Orogenesis, Porphyry Copper, Recently Published, Stable Isotopes, Tectonics | Tagged , , , , , , , , , | 1 Comment

Classic papers in Economic Geology: Fallick et al. (2001) – Bacteria were responsible for the magnitude of the world-class hydrothermal base metal base metal sulfide deposit at Navan, Ireland

Navan_Hypocentral.com

Irish-type zinc-lead deposits represent a distinctive sub-class of the carbonate-hosted zinc-lead deposit family, having geological features and genetic models that are hybrids between sedimentary exhalative (SEDEX; also known as clastic zinc-lead deposits) and Mississippi Valley-type (MVT) deposits.  They are important sources of global zinc and lead production, and the Navan Irish-type deposit (also known as the Tara deposit) has been an important contributor to global metal supply (e.g., ~105 Mt @8.1% Zn and 2%Pb; Ashton et al., 2010).

The paper by Fallick et al. (2001) in Economic Geology argued that the size of the Navan deposit was due to bacteria.  Their study utilized sulfur and lead isotopes to understand the sources of sulfur and lead in the deposit.  While there had been previous studies on sulfur isotopes in the Navan deposit (e.g., Anderson et al., 1998), these studies were focused on a limited number of samples that were texturally very coarse, and representative to only part of the ores being mined at Navan.  The study by Fallick et al. utilized concentrates from large, metallurgical bulk samples from the mine and argued that the concentrates were more statistically representative of the ore deposit than previous samples were.  The lead isotopic data for the concentrates was relatively straightforward and suggested the lead (and likely other metals) were derived from hydrothermal fluid leaching of metals from the basement rocks.  The sulfur isotopic story, in contrast, was much more interesting.

Previous sulfur isotopic work had illustrated that there were two main populations of sulfur: one population with δ34S that was negative and derived from bacterial sulfate reduction of seawater sulfate (BSR); and a second population with δ34S that was positive (hydrothermal) and derived from thermochemical sulfate reduction of seawater sulfate (TSR)(Anderson et al., 1998).  While the previous work illustrated there were two sources of sulfur in the deposits,  Fallick et al. quantified the proportions of bacterial versus hydrothermal sulfur in the deposit.  Their work illustrated that nearly 90% of the ores had biological sulfur signatures and that the enhanced biological activity within the Navan sedimentary basin was critical to forming the large size of the deposit.

So where do these various sulfur and lead sources fit into a model of how the deposit formed (Figure 1)?  The authors build on previous models for the generation of Irish-type (and SEDEX) deposits (e.g., Russell et al., 1981), but add very important additional constraints on deposit forming processes (Figure 1).  Hydrothermal circulation cells in an extensional environment resulted in basinal brines (bittern brines) descending and recharging through basement rocks (Figure 1A).  During this process the brines leached metals from the basement rocks (Figure 1A).  Additionally, seawater sulfate in the brine was reduced via thermochemical sulfate reduction as the brines interacted with basement rocks (Figure 1).  These processes resulted in a hydrothermal fluid that was saline, warm (90o-270oC; Wilkinson et al., 2010), metal-rich, with reduced hydrothermal sulfur (i.e., H2S; Figure 1).  Coincident with hydrothermal fluid generation was ongoing bacterial sulfate reduction at the sediment-water interface within the Navan sedimentary basin, which resulted in abundant reduced sulfur (i.e., bacterial H2S) and cool seawater (<25oC?) to be present in pore spaces in host carbonate rocks (Figure 1).  The upwelling of the saline hydrothermal fluids along synsedimentary basement faults resulted in the mixing of the warm, metal- and hydrothermal H2S-bearing hydrothermal fluids with the cooler, bacterial H2S-bearing near-surface pore fluids (Figure 1B).  The mixing of these two fluids resulted in rapid cooling and dilution of the metal-bearing brine (Figure 1B), and the complexing of metals with both hydrothermal and bacterial H2S to form the ore minerals sphalerite (ZnS) and galena (PbS):

PbCl2(aq) + H2S(aq) = PbS(s) + 2HCl(aq) (galena formation); and

ZnCl2(aq) + H2S(aq) = ZnS(s) + 2HCl(aq) (sphalerite formation).

Wilkinson_2010_1

Figure 1.  Generalized model for the generation of mineral deposits in the Irish  Midlands.  A)  Schematic cross section illustrating the circulation of hydrothermal fluids through basement rocks.  This circulation of fluids resulted in leaching of metals and the reduction of seawater sulfate the sulfide (H2S) by thermochemical sulfate redution (i.e., generation of hydrothermal sulfur).  B) Three-dimensional cartoon representation of the ore forming environment with basement faults, seafloor topography, and potential mixing processes between various fluids.  C. Representation of paleogeography in the Irish Midlands with deposits (red areas) associated with fault controlled shelves/islands where strongly evaporated bittern brines could be generated.  From Wilkinson et al. (2011).

So how did bacteria result in the Navan deposit being so large?  The answer for this comes from the nature of seafloor hydrothermal vents, which the Irish-type deposits clearly were (e.g., Boyce et al., 1983). Hydrothermal vents are extremely inefficient at forming sulfide mineralization.  In fact, the majority of metals present in hydrothermal vents go “up in smoke” and are not precipitated as sulfides within the sulfide chimneys (e.g., Converse et al., 1984).   One of the major reasons for this is that the hydrothermal vent fluids are generally deficient in the H2S required to precipitate the metals as sulfide minerals that form sulphide chimneys (e.g., sphalerite, galena, pyrite, chalcopyrite). In the case of Navan, there was abundant bacterial H2S present at the site of deposition; therefore, much of the metal that would normally go “up in smoke” complexed with the bacterial H2S and formed sulfide mineralization.  Therefore, without bacterial sulfate reduction and bacteria present in the Navan sedimentary basin it is likely the Navan deposit would have been a much smaller deposit!

The paper is also a great example of how a title can have great impact on the reader.  Furthermore, this paper has one of the best lines at the end of the abstract that explains the essence of the manuscript: “…..no bacteria, no giant ore deposit.”  A great example of impact-oriented writing.

Posted in Bacteria, Biogeochemistry, Carbonate-hosted deposits, Classic Papers, Economic Geology, Geochemistry, Geology, Geophysics, Ireland, Irish-type deposits, Lead, Mineral Resources, Seafloor, Seafloor Massive Sulfides, Sedimentary exhalative deposits, Sedimentology, Stable Isotopes, Zinc | Tagged , , , , , , , , , , , , , | Leave a comment

Sedimentary origin of gold and uranium in the Archean Witwatersrand Supergroup, South Africa

Wits_Conglom

New research by Depiné et al. published in Mineralium Deposita provides critical evidence to the origin of gold and uranium in the Archean Witwatersrand Supergroup in South Africa. The Witwaterstrand Basin in South Africa accounts for 40% of the world’s gold and is a critical source of uranium for the the planet (Frimmell, 2008).   There are two competing models for the origin of the gold and uranium in the Witwatersrand.  The first model, the paleoplacer model, suggests that the gold and uranium were derived from weathering of Archean basement rocks, and subsequently deposited as detrital grains in conglomerates (i.e., a paleoplacer model; Minter, 1999).  The second model suggests that the uranium and gold were deposited after conglomerate deposition and were introduced by hydrothermal fluids (i.e., hydrothermal model; Phillips and Powell, 2011).

Depiné et al. utilize a combination of petrography, scanning electron microscope (SEM), electron microprobe, and laser ablation inductively coupled plasma mass spectrometry of  uraninite grains that are associated with gold to provide support for a detrital origin for both the uraninite and gold in the Witwatersrand Basin.  Depiné et al.  utilize these techniques to show that the gold and uranium were deposited together, and they also obtain mineral chemical data, including rare earth element (REE) data, on uraninite grains associated with gold.  Their work builds on the work of Mercadier et al., who recently illustrated that uraninite derived from different sources have distinct rare earth element signatures.  In particular, Mercadier’s work illustrates that deposits formed from igneous sources have higher REE contents than those from hydrothermal sources.

Depiné et al.’s work illustrate that REE in uraninites from Witwatersrand have very high REE concentrations consistent with formation from high temperature, igneous sources, and have values that are much higher than hydrothermally derived uraninite (Figure 1).  Furthermore, the uraninite have enrichments in elements commonly associated with magmatic activity (e.g., Bi, W; Figure 2).  These features suggest that the uraninite are unlikely of hydrothermal origin, but were derived from igneous sources.

Wits_uraninite_REE

Figure 1.  Concentration of rare earth elements versus the light rare earth element(LREE) to heavy rare earth element (HREE) ratio.  Notably the Witwatersrand uraninites have very high REE and are indicative of a high temperature origin.  From Depiné et al. (2013).

Wits_uraninite_REE_2

Figure 2. Coloured element maps of various elements for grains of uraninite from Witwatersrand.  Note that the grains have enrichments in Bi and W, two elements commonly associated with igneous activity.  From Depiné et al. (2013).

The occurrence of uraninite in the conglomerate units with igneous sources is consistent with the uraninite, and by association gold, being derived from weathering of Archean igneous basement rocks and subsequent deposition in conglomerate beds.  Assuming that the work of Mercadier et al. holds up to further tests, it implies that the paleoplacer model for the Witwatersrand is the best model to explain gold and uranium enrichment in this important mining district.

Posted in Archean, Economic Geology, Geochemistry, Geology, Gold, Gold Deposits, Mineral Resources, Paleoplacer, Sedimentology, Uranium | Leave a comment

The origin of banding in banded iron formations (BIF)

wikipedia_bif

Banded iron formations (BIF) and iron ore are globally important resources of iron that have very distinctive textures.  Well-preserved BIF contain iron-rich bands consisting of iron oxides (e.g., hematite, magnetite), iron silicates (e.g., stilpnomelane), and iron oxyhydroxides (e.g., greenalite), that are inter-layered with siliceous chert bands.  New research by Rasmussen et al. recently published in Geology uses a simple, yet elegant approach to explain the origin of the distinctive banding found in banded iron formations.

The researchers utilized samples from the Dales Gorge Member from the Hammersley Group in Western Australia.  Samples of BIF from the Dales Gorge Member are important because they are exceptionally well-preserved and have much of their primary minerals and textures are intact, therefore allowing one to infer primary processes that lead to banding in BIF.

Utilizing petrography and scanning electron microscopy Rasmussen et al. illustrate the microgranules in the BIF are composed of the iron-silicate mineral stilpnomelane (Figure 1),  which was likely iron silicates like chamosite or nontronite at the time of original BIF formation that have subsequently been diagenetically transformed to stilpnomelane.  They argue that these granules formed via flocculation of iron-silicates in an iron-rich ocean, where the iron was supplied by hydrothermal venting (i.e., black smokers), forming a colloidal suspension of iron microgranules.  These suspensions of iron microgranules were subsequently deposited as distinct laminae via sedimentary processes (i.e., density currents), resulting in the iron-rich bands found in BIF (Figure 2). These periods of iron flocculation and deposition alternated with periods of low hydrothermal activity in which sedimentation was dominated by silica deposition and chert formation (Figure 2).  These alternating periods of hydrothermal venting and quiescence ultimately resulted in alternating layers of iron-rich and chert-rich material so diagnostic of BIF.

Rasmussen_Stilp_Microgranules

Figure 1.  Stilpnomelane (brown) surrounded by quartz and dolomite-ankerite in plane polarized and cross-polarized light.  These textures are typical microgranules from BIF. From Rasmussen et al. (2013).

They also illustrate that preservation of the well developed banding is also dependent on whether or not there has been early (i.e., diagenetic to hydrothermal) silicification of the BIF (Figure 2).  In cases where the silificiation is early the rock has strength and cannot be compacted, hence, the diagnostic banding is preserved (Figure 2B).  In cases where silicification does not take place the BIF can be compacted resulting in iron-rich bands with very high concentrations of iron, with the original microgranular textures and delicate banding all but obliterated (Figure 2C).  If there is formation of chert nodules during diagenesis this microgranular texture can be preserved in the nodules, but may be surrounded by compacted, higher iron concentration layers (Figure 2D).

Rasmussen_2

Figure 2.  Idealized diagram illustrating the development of layering in BIF.  A) Flocculated iron microgranules are deposited as layers.  B) If silicification takes place early there will be alternating layers of chert and iron-rich layers.  C) If silicification does not take place then compaction of sediment can occur and there can be larger iron-rich bands forming without alternating chert layers.  D) If chert nodules form during diagenesis there can be local preservation of BIF layering, but there will also be compaction and formation of iron-rich bands around the nodules.  From Rasmussen et al. (2013).

The paper also has obvious implications for the transformation of BIF to iron ore.  In particular, BIF that have early silicification and preservation of microgranules would be very difficult to compact, and therefore would have lower iron grades than BIF that did not endure early silicification (e.g., Figure 2C and D).

The elegance and beauty of this paper is that it relies on rather basic observations using petrography and scanning electron microscopy, coupled with a lot of intuition and reasoning to solve a fundamental problem.  A great read for those interested in BIF, iron ore, chemical sedimentation, and really good science!

See also – very interesting graphics and information on iron ore formation from Minerals Downunder.

Posted in Banded Iron Formations, Economic Geology, Geology, Iron Ore, Mineral Resources, Submarine Volcanism | Tagged , , , , , , , | 2 Comments

Recently published: Elements – One Hundred Years of Geochronology

Elements_Geochronology

This is more of a notification than a post.  The February issue of Elements magazine has a great collection of papers on One Hundred Years of Geochronology.  While geochronology is not a field of economic geology, it is a tool that economic geologists use to understand the age of ore hosting rocks, the duration of mineralizing events, and in some cases the direct age of mineralization.  The issue of Elements has the following papers of interest:

The issue provides a great overview for those interested in the current state of knowledge in geochronology.  Readers may also want to check out the Earthtime site; this issue of Elements is an outcome of the Earthtime initiative.

Posted in Economic Geology, Geochemistry, Geochronology, Geology, Geophysics, Recently Published | Tagged , , , , , , | Leave a comment

Classic Papers in Economic Geology: Hekinian et al. (1980) – the first description of seafloor massive sulfide deposits

This is my first post in the classic papers series.  I have a passion for volcanogenic massive sulfide (VMS) deposits, so I thought it would be fun to write a short blog on a classic paper that described and documented the first seafloor massive sulfides  – Hekinian et al. (1980)(Figure 1).

Science_March_28_1980_Cover

Figure 1.  Cover of the May 28, 1980 issue of Science with the venting of sulfide-rich black smoke from hydrothermal vents near 21oN on the East Pacific Rise (from Science Magazine – see associated paper by Spiess et al., 1980).

Prior to this paper many people had surmised that hydrothermal exhalation had occurred on the seafloor, including inferences from the presence of metalliferous sediments (e.g., Bostrom and Peterson, 1966) and predictions from VMS deposits on land (e.g., Solomon and Walshe, 1979). Additionally, while there had been the discovery of hydrothermal venting in the 1970s (e.g., Edmond et al., 1979 and Spiess et al., 1980), up to that point no sulfide mineralization had been found nor documented on the seafloor.

The paper documented the sulfide occurrences on the seafloor, illustrating their mode of occurrence; their sulfide and silicate mineralogy, textures, sulfide mineral chemistry and paragenesis; bulk sulfide chemistry; and sulfur isotope geochemistry.  They provided key arguments as to how the deposits formed.  They argued that the metals present in the deposits were likely leached from basaltic oceanic crust by convecting hydrothermal fluids.  They illustrated the importance of both leaching of wall rock (igneous) sulfur and thermochemical sulfate reduction of seawater sulfate to provide the reduced sulfur needed to form the sulfides.  They also surmised that magmatism may play a role in the metal budget of the seafloor massive sulfide deposits, but were uncertain as to the exact mechanism of how it contributed to the metal budgets. Ironically, we are still debating the role and mechanisms of magmatism in the metal budgets of VMS deposits (e.g., Dube et al., 2007, and references therein; Wysoczanski et al., 2012).  Probably most important with this paper, however, was that the authors made the link between active seafloor venting, seafloor massive sulfide deposits, and on-land Cyprus-type VMS deposits (e.g., Spooner and Fyfe 1972Solomon and Walshe, 1979).

It was a major breakthrough that not only led to an enhanced understanding of VMS deposits on land, but also resulted in significant research in the area of seafloor hydrothermal vents and massive sulfide deposits that has greatly enhanced our understanding of Earth geological, hydrothermal, and biological processes (see video by Dr. Susan Humphris below). The work has also set the stage for the potential marine mining of seafloor massive sulfide deposits (e.g., Nautilus Minerals).

Vents from the East Pacific Rise

Great overview of the significance of hydrothermal vents by Dr. Susan Humphris  (Woods Hole Oceanographic Institution)

 

Posted in Classic Papers, Copper, Economic Geology, Geochemistry, Geology, Lead, Mineral Resources, Seafloor, Seafloor Massive Sulfides, Submarine Volcanism, Volcanogenic Massive Sulfides, Zinc | Tagged , , | Leave a comment