MVT Deposit Genesis at the Start of the 21st Century
“I didn’t know until recently that stratigraphy is dead. Many schools don’t teach it any more. To me that’s writing the story without knowing the alphabet." - John McPhee, 1986: Rising from the Plains
Theorising about the origin of Mississippi Valley Type Lead Zinc Deposits came to an abrupt climax in the middle 1980s and interest declined soon after. Quantitatively, there were fewer students and supervisors, and less stratigraphy. Many, if not all of us, switched to gold exploration or retired from the field.
I have had nearly 40 years to think, but not much research was available for input. The field appears to be fallow from disuse. Considering the states of art in 1980 and 2017; there may be a place for cryogenics after all. Last year the price of zinc and lead, principal metals of MVTs rose to a dollar a pound and more, riding the particularly strong US dollar. As many large zinc producers closed, zinc and lead greenfield deposits have had their Easter and are rising from the dead.
MVTs are a subset of base metal deposits competing with volcanogenic (VMS) and SEDEX deposits, but are lower in iron content. All smelters will find MVT concentrates desirable.
The typical Frankenstein genetic model (constructed from many parts) has structural control, reefs or not, facies fronts, basinal brines or not, fluid mixing or not, complex breccia, disconformities and karst, dolostone or limestone; remind yourself of the fable of the blind men examining an elephant. In Jainism, it is explained that truth can be stated in seven different ways.
There is something old (Sloss sequence stratigraphy) and something new (cathode-luminescence) that narrows my personal 2017 model from the 1980 genetic model. Sloss sequence stratigraphy coincides with continent-wide erosion surfaces (disconformities) which have controlled regolith porosity and permeability and provides a locus not only for MVT deposits but probably for sediment hosted gold deposits.
Cathodoluminescence in carbonate rocks is activated by manganese and inhibited by the presence of iron. In many MVT cases C-L distinguishes mineralisation paragenesis because the precipitation of iron sulphide as pyrite allows visualisation of carbonate gangue history. At Robb Lake in the Muskwa range of the Canadian Rockies C-L shows that the brecciation of epigenetic dolostone collapse features was dynamic and is penecontemporaneous with mineralisation and the replacement of evaporite minerals; selenite gypsum for certain and very possible wholesale replacement/removal of a thicker nodular anhydrite unit.
There is a lot of catching up to do.
Dr. Francis Tucker Manns is a geologist/stratigrapher and the sole practitioner of Artesian Geological Research in Toronto, Canada. He has had a long year career in geology of base metals and gold in Canada, US and Borneo. Fran has worked as a consultant at Derry, Michener and Booth in Toronto, as an exploration geologist at whatever he was told to look for, and as a mining analyst. He recommended Eldorado Gold Corp. when it had one mine in Mexico, and recommended Virginia Gold because of the people, but thought Bre-X was too lean and too deep. He became a geologist at the Toronto Stock Exchange during the launch of NI 43-101 and retired to MVT land after 14 years in 2014.
PhD - University of Toronto Dissertation (1982) on the stratigraphic aspects of the Robb Lake lead/zinc deposit, NE British Columbia. In 1972, he was part of a team exploring for lead/zinc on Cornwallis Island near Polaris. He contracted each summer at Robb Lake and throughout the main range of the Rockies from the Peace River north with Texasgulf, Brinex, and later into Alaska for Northgate Exploration Ltd. While in grad school he managed drill programs in the Rockies and Newfoundland.
He participated in a large working group specialised in Mississippi Valley type lead and zinc deposits at UofT. He visited and sampled operating mines in Missouri, Tennessee, New York, Newfoundland, Pennsylvania, Nova Scotia and Ireland.
BA, MA – Temple University, Philadelphia, Pennsylvania Specialised in stratigraphy and paleoenvironments of limestone and dolostone of the central Appalachians and western New York.
On resource estimation and quantification of the nugget effect.