the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Apr 2023
Cobalt distribution in the Boleo stratiform sediment-hosted ore deposit: a transtension rift basin in the central Baja California peninsula, Mexico
Abstract. The Miocene Santa Rosalia Basin, located in Baja California, Mexico, is a unique basin that contains the Boleo copper deposit. This basin provides a remarkable opportunity to study the early stages of tectonic evolution, volcanic activity, ore genesis, and sedimentary deposition in the incipient Gulf of California. Previous research suggested the presence of Co in areas between Cu and Zn, but the factors responsible for cobalt distribution in the basin remained unclear. This study utilized multiple methods, including historical geologic maps, core drilling, field measurements, XRF readings, petrographic analysis, and mineral composition analysis, to comprehend the Co mineralization of the Boleo Formation. Our research found Co anomalies in five localities, with the highest values situated at the base of a slump block in manto 3 near the Neptuno mine. The presence of elevated cobalt readings in areas with uplifted and eroded gypsum suggests the possibility of uplift and dissolution of earlier gypsum deposits, and an association with brine waters. Moreover, the result of the study indicates that gypsum interfingers with both the clastic facies and ore mineralization in mantos 1, 2, and 3, signifying a north-to-south expansion of the basin. The study also identified manganese oxides and Cu-Fe sulfides, including villamaninite, along laminated gypsum beds. The findings of this study suggest that cobalt enrichment is influenced by structural factors and occurs near subsurface evaporitic deposits. This supports the notion that cobalt mineralization forms near the redox front and through sulfate reduction processes. The study's findings offer valuable insights for further exploration and understanding of cobalt mineralization in similar geological settings.
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RC1: 'Comment on egusphere-2023-607', Anonymous Referee #1, 12 May 2023
I revised the manuscript entitled: “Cobalt Distribution in the Boleo Stratiform Sediment-Hosted Ore Deposit; A Transtension Rift Basin in Central Baja California Peninsula, Mexico”.
The manuscript is an interesting contribution to the understanding of the distribution of Cobalt in the Boleo copper deposit (in Central Baja California Peninsula, Mexico).
In addition, to cobalt distribution mapping, this article extends the distribution of gypsum in the Santa Rosalia Basin to the surface and subsurface. In general, this article provides a valuable insight for further exploration and understanding of cobalt mineralization in similar geological settings.
The article is well written in English and structured, so the research work can be accepted.
Zaineb Hajjar
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RC2: 'Comment on egusphere-2023-607', Anonymous Referee #2, 07 Jun 2023
Dear authors and editor
I have now reviewed the manuscript “Cobalt distribution in the Boleo stratiform sediment-hosted ore deposit: a transtension rift basin in the central Baja California peninsula, Mexico” by Valente Octavio Salgado Munoz et al. The paper uses field observations, field XRF and SEM-EDS data, and drill core assays to investigate the distribution of Co (and Cu) grade in relation to basin rocks. It is somewhat unclear what the main conclusion of the manuscript is that is based on the authors’ own data and results, but much emphasis in the manuscript is put onto the presence of gypsum at the basin margins and interfingered with the ore-bearing “mantos”. In the discussion, the authors also propose that the precipitation of the ore mineralisation was syn-sedimentary, triggered by fluid mixing with basinal sulphate fluids, although this seems to be drawn from literature rather than own data.
Overall this paper needs a huge amount of work although there is potential. There seems to be a good amount of data here that could, if analysed and presented with a different approach, make a really interesting and valuable contribution. However, unfortunately the paper is poorly written and disorganised; many of the figures (particularly the maps and the plots) are poor and uninformative; there is plenty of material the relevance and importance of which is not clear (particularly on gypsum); whilst on the other hand most of the key background information (literature review) is not presented at all or is presented in a very cursory manner (most importantly: lacks key references and crucial information about conditions of Co transport and precipitation; very insufficient description of the structural evolution of the sub-basins and the faulting within them; and no description of the local mineralisation and what is known about the detailed mineralogy, ore paragenesis, and the presence of any zonation of the Co species within each mantos and wrt to the faults). There is also evidence of superficial understanding of Co mineralisation: the presence of linnaeite strongly implies fluid temperatures in excess of 200C, yet the authors seem to suggest a near-surface precipitation.
With respect to the approach to the data analysis, I think it is severely misdirected. The abstract suggests that the work includes a structural analysis but this is not at all presented in the rest of the manuscript. There is a cursory brush on the mineralogy, the source of the metals and the fluid properties, but the data, if it exists, are not shown. The analysis of the gypsum does not really seem to lead anywhere apart from the observation that the gypsum is interfingered with the clastic sediments, and that there are some slumps. The data, as presented, suggests to me that the occasional spatial association of gypsum and ore mineralisation is coincidental rather than an important control on the grade distribution as seems to be suggested by the authors. The Discussion on the role of basinal fluids does not change this as the importance of the gypsum for fluid properties is not demonstrated with the available data or even with the (very limited) discussion on what is known about Co precipitation conditions and the possible range of fluid chemistries.
I think this paper needs to be completely rewritten, including taking a very different approach to formulating the research question and analysing the available data. There is also a clear need for a better background literature research. For that reason I would strongly recommend rejecting this manuscript. Thinking about the role of the structures and basin evolution, and how the fluids migrated within the sub-basins would seem to me a much more fruitful route of investigation than invoking a poorly constrained model of the importance of the gypsum and poorly evidenced (by your own data) model on fluid mixing. It seems there is sufficient data from the drill core to analyse the grade distribution wrt faults and basin structure in a reasonably robust manner, but this is currently severely under-developed. The mineralogy of particularly the Co species and other Co-bearing minerals needs to be properly described, ideally with the help of good petrographic analysis with SEM-BSE/CL and paragenetic interpretation. Any re-submission also needs to include much better maps and images to show the data and analysis results properly. The manuscript also needs to be completely reorganised to give all the essential background information before describing the results, and the results need to start with field observations and build from there. I have included a pdf with detailed comments that I hope will be useful for the authors.
Best regards, Reviewer #2
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RC3: 'Comment on egusphere-2023-607', Anonymous Referee #3, 24 Aug 2023
The manuscript entitled “COBALT DISTRIBUTION IN THE BOLEO STRATIFORM SEDIMENT-HOSTED ORE DEPOSIT: A TRANSTENSION RIFT BASIN IN CENTRAL BAJA CALIFORNIA PENINSULA, MEXICO” addresses the intriguing topic of the role of cobalt (Co) in the mineralization evolution within the Boleo Basin in Baja California Sur. The manuscript contributes substantial analytical information (XRF, SEM-EDS, XRD), along with various sections and maps. One concern is the absence of analytical controls to assure precision and accuracy for the XRF data provided, which weakens the overall findings; this is concerning since most of the interpretation is based on this data set. Therefore, authors must incorporate a reference material that includes Co, Mn, Cu, Zn, to ensure reliable results and further interpretations. Furthermore, the manuscript's structure needs refinement. Some sections read more like discussions and lack proper integration within their respective segments. In the conclusions section, the authors assume that with field observations and the data provided in the manuscript (mostly XRF), it can be assumed that "Distribution of cobalt mineralization from trace mineralogy of the gypsum beds, high cobalt values from drill hole data and XRF field data suggest an interface of localized metal-rich magmatic hydrothermal fluids mixing with highly saline fluids of surficial or connate source."; authors should check on this subject or better explain the findings. Also, a Co background highlighting the significance of the genesis of ore deposits should be nice to include to broaden the discussion.
In general, the manuscript has potential, but the authors should ensure the quality of their data, show tables that include mineral species and abundance percentages determined by XRD, and also improve the DRX figures and the quality of analysis and interpretation of SEM-EDS.
Status: closed
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RC1: 'Comment on egusphere-2023-607', Anonymous Referee #1, 12 May 2023
I revised the manuscript entitled: “Cobalt Distribution in the Boleo Stratiform Sediment-Hosted Ore Deposit; A Transtension Rift Basin in Central Baja California Peninsula, Mexico”.
The manuscript is an interesting contribution to the understanding of the distribution of Cobalt in the Boleo copper deposit (in Central Baja California Peninsula, Mexico).
In addition, to cobalt distribution mapping, this article extends the distribution of gypsum in the Santa Rosalia Basin to the surface and subsurface. In general, this article provides a valuable insight for further exploration and understanding of cobalt mineralization in similar geological settings.
The article is well written in English and structured, so the research work can be accepted.
Zaineb Hajjar
-
RC2: 'Comment on egusphere-2023-607', Anonymous Referee #2, 07 Jun 2023
Dear authors and editor
I have now reviewed the manuscript “Cobalt distribution in the Boleo stratiform sediment-hosted ore deposit: a transtension rift basin in the central Baja California peninsula, Mexico” by Valente Octavio Salgado Munoz et al. The paper uses field observations, field XRF and SEM-EDS data, and drill core assays to investigate the distribution of Co (and Cu) grade in relation to basin rocks. It is somewhat unclear what the main conclusion of the manuscript is that is based on the authors’ own data and results, but much emphasis in the manuscript is put onto the presence of gypsum at the basin margins and interfingered with the ore-bearing “mantos”. In the discussion, the authors also propose that the precipitation of the ore mineralisation was syn-sedimentary, triggered by fluid mixing with basinal sulphate fluids, although this seems to be drawn from literature rather than own data.
Overall this paper needs a huge amount of work although there is potential. There seems to be a good amount of data here that could, if analysed and presented with a different approach, make a really interesting and valuable contribution. However, unfortunately the paper is poorly written and disorganised; many of the figures (particularly the maps and the plots) are poor and uninformative; there is plenty of material the relevance and importance of which is not clear (particularly on gypsum); whilst on the other hand most of the key background information (literature review) is not presented at all or is presented in a very cursory manner (most importantly: lacks key references and crucial information about conditions of Co transport and precipitation; very insufficient description of the structural evolution of the sub-basins and the faulting within them; and no description of the local mineralisation and what is known about the detailed mineralogy, ore paragenesis, and the presence of any zonation of the Co species within each mantos and wrt to the faults). There is also evidence of superficial understanding of Co mineralisation: the presence of linnaeite strongly implies fluid temperatures in excess of 200C, yet the authors seem to suggest a near-surface precipitation.
With respect to the approach to the data analysis, I think it is severely misdirected. The abstract suggests that the work includes a structural analysis but this is not at all presented in the rest of the manuscript. There is a cursory brush on the mineralogy, the source of the metals and the fluid properties, but the data, if it exists, are not shown. The analysis of the gypsum does not really seem to lead anywhere apart from the observation that the gypsum is interfingered with the clastic sediments, and that there are some slumps. The data, as presented, suggests to me that the occasional spatial association of gypsum and ore mineralisation is coincidental rather than an important control on the grade distribution as seems to be suggested by the authors. The Discussion on the role of basinal fluids does not change this as the importance of the gypsum for fluid properties is not demonstrated with the available data or even with the (very limited) discussion on what is known about Co precipitation conditions and the possible range of fluid chemistries.
I think this paper needs to be completely rewritten, including taking a very different approach to formulating the research question and analysing the available data. There is also a clear need for a better background literature research. For that reason I would strongly recommend rejecting this manuscript. Thinking about the role of the structures and basin evolution, and how the fluids migrated within the sub-basins would seem to me a much more fruitful route of investigation than invoking a poorly constrained model of the importance of the gypsum and poorly evidenced (by your own data) model on fluid mixing. It seems there is sufficient data from the drill core to analyse the grade distribution wrt faults and basin structure in a reasonably robust manner, but this is currently severely under-developed. The mineralogy of particularly the Co species and other Co-bearing minerals needs to be properly described, ideally with the help of good petrographic analysis with SEM-BSE/CL and paragenetic interpretation. Any re-submission also needs to include much better maps and images to show the data and analysis results properly. The manuscript also needs to be completely reorganised to give all the essential background information before describing the results, and the results need to start with field observations and build from there. I have included a pdf with detailed comments that I hope will be useful for the authors.
Best regards, Reviewer #2
-
RC3: 'Comment on egusphere-2023-607', Anonymous Referee #3, 24 Aug 2023
The manuscript entitled “COBALT DISTRIBUTION IN THE BOLEO STRATIFORM SEDIMENT-HOSTED ORE DEPOSIT: A TRANSTENSION RIFT BASIN IN CENTRAL BAJA CALIFORNIA PENINSULA, MEXICO” addresses the intriguing topic of the role of cobalt (Co) in the mineralization evolution within the Boleo Basin in Baja California Sur. The manuscript contributes substantial analytical information (XRF, SEM-EDS, XRD), along with various sections and maps. One concern is the absence of analytical controls to assure precision and accuracy for the XRF data provided, which weakens the overall findings; this is concerning since most of the interpretation is based on this data set. Therefore, authors must incorporate a reference material that includes Co, Mn, Cu, Zn, to ensure reliable results and further interpretations. Furthermore, the manuscript's structure needs refinement. Some sections read more like discussions and lack proper integration within their respective segments. In the conclusions section, the authors assume that with field observations and the data provided in the manuscript (mostly XRF), it can be assumed that "Distribution of cobalt mineralization from trace mineralogy of the gypsum beds, high cobalt values from drill hole data and XRF field data suggest an interface of localized metal-rich magmatic hydrothermal fluids mixing with highly saline fluids of surficial or connate source."; authors should check on this subject or better explain the findings. Also, a Co background highlighting the significance of the genesis of ore deposits should be nice to include to broaden the discussion.
In general, the manuscript has potential, but the authors should ensure the quality of their data, show tables that include mineral species and abundance percentages determined by XRD, and also improve the DRX figures and the quality of analysis and interpretation of SEM-EDS.
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