Can magnetite be formed in a deposit form? I found a very interesting phenomenon in Wuding County, Chuxiong Prefecture, Yunnan Province. The lenticular hematite in the overlying sedimentary quartz sandstone in direct contact with the diabase body had a strong magnetic anomaly. For a long time, we all think that the quartz sandstone and the fai green rock mass is the sedimentary contact relations , but when this magnetic anomaly was discovered, it changed our understanding. We propose that the age of the diabase should be later than the overlying set of quartz sandstone and the magnetic anomaly may be caused by thermal contact and metamorphism of the quartz sandstone containing lenticular hematite near the contact zone due to the intrusion of the diabase mass. . The redox reaction of the hematite partially changes into magnetite.
Hi,
I suppose that what you call lenticular hematite are the rounded pebbles or nodules in your sandstone. A hypothesis is that those nodules have been a previous ferricrete (paleosol), the latter has been eroded and reworked in a fluvial channel. Pieces of this paleosol have been resedimented in your sandstone. You should have a look to the upper lanscape or in geological maps if the in situ ferricrete is not still present, potentially capping the upperhills.
Dear Jérôme Juilleret ,
Thank you for sharing your opinion. A cherry red scratch can be clearly seen on the streak. We made a light sheet to clearly see the metal minerals. I think these are stable metal minerals.
Best wishes,
Liu Chenming
Hi Liu Chenming:
The photos indicate hematite/magnetite are possibly detrital minerals, derivded from a local provenance deposited in a restricted basin (e.g., river valley). Contact metamorphism due to intrusion of the diabase is likely to elevate magnetic features manifested by mag high zone along the contact between the hematite-magnetite-bearing sandstone and diabase.
Regards,
Xue-Ming Yang
Dear Xue-Ming Yang ,
Thank you. I think your explanation is the same as my opinion. But I still have a question: If the magnetic mineral itself is present in the diabase, will the magnetic component be retained after the diabase weathering, and it will appear in the form of sedimentary quartz sandstone? We have not yet performed high-precision mineralogy analysis.
Best wishes,
Liu Chenming
Dear Chenming:
It is likely that magnetite in the diabase may survive weathering of the host given it is not too old - the possible consequence of magnetite exposed on the surface for long time is that it is transformed to hydrous-iron oxides and lost magmatism. Therefore, it is possible that magnatite as detritus and deposited in sandstone.
Best regards,
Xue-Ming Yang
Dear Xueming ,
So, I give two possibilities for explaining the magnetic anomaly in this area. One is that the diabase body invaded after this set of quartz sandstones, and the thermal contact metamorphism of magmatic rocks resulted in magnetic anomalies; the second is that the magnetic minerals in the diabase were preserved after the parent rock weathered, and Quartz sandstone deposits together. But I support the first explanation.
Best wishes,
Chenming
Dear Chenming:
Agree with your point. I think that the field relationship between the diabase and sandstone is the key, i.e. intrusive contact (the diabase is younger), or disconformity (the diabase is older than the overlying sandstone).
Best regards,
Xue-Ming
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