Three-Dimensional, Km-Scale Hyperspectral Data of Well-Exposed Zn–Pb Mineralization at Black Angel Mountain, Greenland

Round 1

Reviewer 1 Report

Summary:

The manuscript describes a workflow to acquire, correct, and process oblique imaging spectroscopic data from far ranges and ways to decipher the alteration assemblages associated with a Pb-Zn mineralization in Greenland.

General comments:

The manuscript is poorly organized and the methodology is not well-described. The abstract is not well written and does convey the message of the work properly. The way and the aim of obtaining the spectral mineral products are not clearly stated and the results are not interpreted in the context of geology and Pb-Zn mineralization. For example, the relationships between carbonate mineralogy and ore emplacement as well as the role of metamorphism on spectral products are not clearly delineated. The processing methodology to obtain mineral maps is vaguely explained and some of the results do not match the described methodology and vice versa.

While the minimum wavelength mapping results appear to be continuous at first glance, each wavelength range could be easily assigned to a species of carbonate mineralogy (Mg, Fe, and Ca). So I recommend assigning each color pallet to a specific mineral supported by spectral plots from the imagery. Moreover, no ground-truth data is provided to support the finding of the imaging spectroscopic data. Finally, for me, it is not clear which part of the workflow was developed before and which part is the contribution of this article.

Specific comments:

- In the title, “Zn-Pb mineral exploration target” could be changed to “Zn-Pb mineralization” or “Zn-Pb mineralization target”.

- Line 34: you can correct to read: oblique (non-nadir) hyperspectral imaging has… (delete “Previously a niche application” from the beginning of the sentence).

- Line 36: provide a citation for this statement.

- Line 36-37: correct to read “Yet despite recent advances in imaging technology, the processing of the data continues to be a complex task”.

- Line 39: correct to read: “the underlying spectroscopic and geologic processes. The best practices for…”

- Line 57: correct to read “mineral abundances”.

- Lines 96-97: correct to read: “mineral compositional variations.”

- Lines 116-117: correct to read: “visualization and processing of imaging spectroscopic data”.

- Line 148: correct to read: “reflectance spectra of the reference targets used during data acquisition”.

- Section 2-5: What has been cited (reference 11) has no relevance to the methodology used in the manuscript. Please consider citing more relevant articles here.

- Section 3-2: given the pixel size of 2-5 m for the imagery, could you please clarify the size of the reference panel that was used. Was it big enough to be seen at such a low resolution?

- Lines 254-256: tremolite has a double absorption feature between 2300 to 2400 nm. One is centered at 2318 nm and the other at around 2386. A shift towards wavelengths shorter than 2320 nm can be due to tremolite occurrences only when it is associated with an additional feature around 2386 nm, otherwise, it might be due to Mg carbonate (magnesite) which has its diagnostic absorption feature centered around 2302 nm. Please consider revising the mineralogical interpretations.

- Section 3-4: the ferrous iron feature, which here is most likely arising from Fe-carbonates, appears beyond 1000 nm. So I doubt the ratio used here (730 nm / 880 nm) to be rational to map ferrous (Fe2+) minerals. It might be better to reprocess the data using spectral plots extracted from the imagery. Moreover, by using the 2280 nm / 2245 nm ratio, it is not clear to me which mineral you expect to highlight. Fe-OH minerals like jarosite and nontronite are unlikely to appear in this environment. If it is meant to map chlorite, then you should clearly state that. Moreover, while Al-OH minerals are shown to be present in the area, there is no mention of how they were mapped.

- Lines 274-284: I cannot agree with the authors that the target area has complex mineralogy so needs a more sophisticated methodology for spectral processing. The obtained results are rational and they need to be carefully interpreted in the context of the geology of the area. There is an untold but interesting story behind these colorful maps.

- For me, it is not clear if this is an abundant mining site or if there is potential for exploration and exploitation in the future. Please clarify.

- It is not easy for me to understand the ternary plots shown in Fig. 6.

- Fig 7d: consider redoing the spectral plots for the reader to easily see the absorption features. Consider having a discrete legend bar for the reader to easily assign each color to a wavelength value and a potential mineral in the scene. The color bars could be stretched for the reader to easily see the trends.

In Fig. 1a, what do you mean by RGB photogrammetry?

In Fig. 1c, consider adding a bar to describe the colors.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Providing hyperspectral and point cloud data extracted from photogrammetry techniques to the community is worthwhile. Hyperspectral imaging and the application of drones for mineral mapping are on the rise. Several new satellite-based hyperspectral sensors are launched or being launched in recent years. Sample datasets for Lead-zinc in carbonate rocks could be helpful to the scientific community. There is high-resolution, high-quality AVIRIS-NG for many locations, including this part of Greenland. Does AVIRIS-NG cover this location? If that’s the case, that could be valuable. Perhaps in figure 2, the footprint of AVIRIS-NG, PRISMA, and other hyperspectral sensors could be shown.

But in any case, this data could be handy in steep terrains with oblique views. Moreover, several researchers will benefit from this data to calibrate and integrate different datasets.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors have done a great job of revising the manuscript. I still have some notes that needs to be addressed before the paper being accepted for publication.
-  I still think the abstract is not well written and does convey the message of the work properly.

- I doubt the ratio used here (730 nm / 880 nm) to be rational to map ferrous (Fe2+) minerals. Please consider revising and using another band ratio.
-  the absorption features assigned to tremolite (Fig. 7D), lack the additional feature around 2386 nm. I would like to see the VNIR spectral plot to be convinced that the mineral is truly identified.
- Fig. 7D, please add the minimum absorption features to the plots to facilitate the interpretation. also consider showing the geo-location of each spectral plot on Fig. 7C using a tag.

Author Response

Please see attachment.

Author Response File: Author Response.pdf