Doğu Kazakistan'da "green field" bir projede damar tipi Pb-Zn oluşumlarının haritalanmasına saha çalışmalarından örnek
Green field mapping of vein type Pb-Zn ore occurences. Field case sharing. Eastern Kazakhstan.
DISCLAIMER: This LinkedIn article is not a scientific publication and doesn't reflect broad research. It's a field sharing from my recent regional reconnaissance campaign. I'm using an article format for its convenience in writing long text and inserting images, which is better than a normal LinkedIn post.
Intro.
Vein or massive Pb-Zn deposits are polymetallic targets that are structurally controlled. This article shares my experience with vein-style mineralization in Eastern Kazakhstan. Its important to note that this is not a porphyry-related system but rather a case of mineralization occurring within tectonic fault settings.The Pb-Zn mineralization forms along ore-hosting channels created by tectonic faults. Hydrothermal fluids are transported upward along open and accessible pathways produced by tectonic activity. Subsequent crystallization and deposition of ore minerals in these open channels—such as cracks, cavities, and faults—determine the complex shape of the ore body, which can be represented by vertically dipping bodies or stockworks. (Fig "AAA").
The diagram above illustrates how ore mineralization occurs across accessible open channels, forming the final ore bodies. For any Pb-Zn mineralization to form, a favorable lithological environment must also be present.
The key factors are:
1) Tectonic Settings: Simply put, we cant have ore bodies without tectonics, as faults are essential ore-hosting channels. Ascending hydrothermal fluids move upward along open and accessible cracks and cavities produced by tectonic pressure within the host rocks. Therefore, mapping and studying the tectonics around an exploration area is a crucial part of targeting Pb-Zn. If we are talking about a large mineral deposit, we are looking for 'big guy' faults that are several to ten kilometers long, while lower-order faults are less likely to host large deposits due to their smaller size. However, these smaller faults can act as additional ore channels. We must also consider post-ore tectonics, which can break and complicate the shape of the ore body.
2) Lithology: Not all rock units develop cracks and fractures during tectonic stress. Some rocks, such as sedimentary units, are highly plastic and tend to bend under tectonic stress instead of fracturing. For cracks and fractures to form, rocks must have low shear limits, allowing for brittle rather than plastic deformation. Such rocks include intrusive, volcanic, and subvolcanic units, making them a favorable environment for vein or stockwork mineralization. In folded systems like geosynclines or geoanticlines, ore formation is localized in the hinges of folds, where the greatest bending and subsequent brittle fracturing occur.
Let's get some bullet points of above:
- Tectonic Control on Mineralization: Tectonic structures, particularly faults and fractures, serve as potential channels for hosting ore bodies. The more open and accessible these channels are, the better the conditions for ore deposition, as they allow hydrothermal fluids to ascend and deposit minerals.
- Favorable Host Rocks: Intrusive and subvolcanic lithologies are considered favorable environments for ore deposition. This is because these rocks are brittle, which allows for the formation of numerous cracks and fractures under tectonic stress, creating the necessary pathways and spaces for mineralizing fluids.
Supergene mapping
In real field work, we deal with eroded and weathered rocks and minerals. The crucial part is to reconstruct the primary picture and determine whether it could be a potential ore target.
As you can see from the image above, vein-style mineralization occurs within a volcanic unit with complete quartz-sericite alteration. Its difficult to reconstruct the primary sulfide sequence, but in the hypogene zone, it would be pyrite-chalcopyrite-galena-sphalerite. In the supergene zone, however, it transforms into hematite, goethite, limonite, and oxidation products of copper, zinc, and lead. Iron oxidation, or red ores, is an indicator that the original fluids were enriched with iron (pyrite).
Let's talk a bit more about alteration. Ascending hydrothermal fluids rich in sulfides move up along open channels and alter the host rocks. Subsequently, ore sulfides are deposited within these same open channels.
In Fig 2, we can see a volcanic unit that is completely altered with sulfide veining. The unit is thoroughly altered to quartz-sericite and shows silicification, which is typical for lead-zinc (Pb-Zn) targets. When we discuss Pb-Zn style alteration, quartz-sericite is the key indicator.
Hydrothermal alteration and its intensity tend to be strongest close to faults and weaken with distance from them. This suggests that the tectonic fault was likely the ore-hosting channel. In the field, its crucial to find this alteration near tectonic features. This is why geologists often cant see the primary textures of the original host rocks near faults but can find them further away where the intensity of alteration decreases.
Supergene minerals
In the field, we mostly deal with hypogene rather than primary sulfidation. Our challenge is to identify the primary mineral assemblage and reconstruct the mineral paragenesis. Fig 3 shows an oxidized polymetallic stockwork exposure of pyrite, goethite, hemimorphite, hematite, and limonite, hosted by a completely quartz-sericite and clay-altered volcanic unit I mapped recently.
A high goethite-to-jarosite (Gt/Ja) ratio indicates initial sulfide mineralization enriched in pyrite. This is what we actually deal with in the field, as opposed to the ideal photos from textbooks and papers. Mapping such iron oxidation within altered rock units is a good indicator that we have found a potential polymetallic ore occurrence.
In the case of lead-zinc (Pb-Zn) targets and supergene enrichment, it's not necessary that we would see primary galena or sphalerite. On the surface, it is more probable to find their oxidation products, which must be distinguished in situ so as not to miss a potential ore occurrence.
Fig 4 shows what an oxidized mineralized unit looks like in the field. Primary galena oxidizes into hemimorphite, which is a zinc silicate. Supergene mineralization can appear as "red ore" due to the enrichment of iron oxides and hydroxides from pyrite oxidation. It's worth noting that one can also encounter "white ore," which shows no significant iron oxidation, indicating the absence of pyrite. These "white ore" oxidation products mostly represent oxidized galena and sphalerite. However, when iron is present, it is critical to distinguish the lead-zinc (Pb-Zn) oxidation products from all the other supergene minerals.
Time to get bullet points:
- Polymetallic veining: Lead-zinc (Pb-Zn) mineralization is typically polymetallic and occurs within accessible open channels in quartz-sericite and silica-altered units.
- Alteration patterns: Alteration intensity tends to be highest close to tectonic faults and decreases with distance from them.
- Supergene ore occurrences: Supergene Pb-Zn ore is represented by oxidation products rather than primary sulfides. Galena (PbS) and sphalerite (ZnS) oxidize to form their respective silicate or carbonate products. For example, galena can form cerussite or anglesite, while sphalerite can form hemimorphite.
- "Red Ore" Indicator: The "red color" or "red ore" appearance is due to the enrichment of iron oxides and hydroxides from the oxidation of pyrite. A high goethite-to-jarosite (Gt/Ja) ratio is a key indicator of this process.
Quartz veins
While porphyry geologists often use quartz veining as an indicator of alteration and a guide to the mineralization history, the case is different for vein-style lead-zinc (Pb-Zn) targets. In this context, extensive quartz veining is often not a positive indicator. As mentioned, ascending hydrothermal fluids are typically rich in sulfides, which fill open spaces to form veins or massive ore. However, when we observe significant quartz veining within a Pb-Zn target, we can hardly expect rich ore mineralization.
There are several reasons for this assumption. Quartz mineralization may represent an earlier stage that occurred prior to the sulfide mineralization. The quartz fills and occupies the open spaces, which then prevents the subsequent sulfide-rich fluids from gaining full access to all channels. This results in poor ore mineralization. Although sulfide mineralization often overprints the earlier quartz, creating quartz-sulfide veins, it's important not to be confused, as this overprinting simply indicates that the sulfides were deposited after the quartz.
In Fig 5 one can observe a quartz (qtz) veining pattern within a quartz-sericite (qtz-ser) altered volcanic host rock. On the left, a pure quartz vein is poorly filled with sulfides. Early quartz mineralization occupied cracks and channels, preventing later sulfide mineralization from producing rich ore due to a lack of access to the open channels.The middle image shows a more unusual case where sulfide mineralization is present at the top and quartz at the bottom of the same vein.The right side shows a mixed quartz-sulfide mineralization, which is predominantly quartz-dominant.All three cases are clear examples of poor mineralization.
Thus quartz veinin of Pb-Zn target is the good question for discussion and evaluating of its role in ore mineralization.
Conclusions
Key Factors for Vein-Type Pb-Zn Deposits
Host Rocks: Intrusive and volcanic rocks are favorable hosts for vein-type Pb-Zn deposits because their brittle nature allows for the development of cracks and fissures under tectonic stress.
Structural Control: Tectonics is a crucial factor. Large, regional-scale faults (several to tens of kilometers long) act as primary channels for ascending hydrothermal fluids. Smaller, lower-order faults are less likely to host large deposits due to their limited size.Localization of Mineralization: In geological structures like geoanticlines and geosynclines, mineralization often localizes in areas of maximum bending. These zones undergo brittle deformation, creating the necessary open spaces for ore formation.
Supergene Environment: The supergene environment on the surface is significantly different from what is shown in textbooks. It is characterized by the alteration of primary sulfides into their oxidation products (e.g., galena and sphalerite converting to carbonates or silicates).
Role of Quartz Veining: The role of quartz veining is a key point of discussion. Early-stage quartz mineralization can fill open channels and fractures, making it difficult for later-stage sulfide-rich fluids to access and deposit rich ore. This can result in limited or poor sulfide mineralization.
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