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24-MMP-A4 Mine Valuation and Mineral Resource Estimation · December 2018

Question 9 of 29: VMS and SEDEX Deposit Models — Geologic Setting

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

EGBC National Exam — Mining and Mineral Processing Engineering, 09-MMP-A4 Mine Valuation and Mineral Resource Estimation, 2018-Dec. 3 hours duration; one handwritten 8.5×11 in reference sheet permitted (not an open-book exam); only approved Sharp or Casio calculators allowed. Question 1 is compulsory (40 marks, parts 1.1–1.8); candidates then select THREE of the five optional Questions 2–6 (20 marks each) to complete the paper.

Reference texts: Isaaks & Srivastava, An Introduction to Applied Geostatistics (variogram modelling, kriging, anisotropy); Hustrulid, Kuchta & Martin, Open Pit Mine Planning and Design (mine valuation, NPV/IRR and cut-off grade methodology); Gentry & O'Neil, Mine Investment Analysis (smelter/refining contract terms, net smelter return, taxation and risk); Guilbert & Park, The Geology of Ore Deposits, and Evans, Ore Geology and Industrial Minerals (VMS/SEDEX and porphyry deposit models); SME Mining Engineering Handbook, 3rd ed. (mineral exploration/evaluation stages, ore reserve classification); CIM Best Practice Guidelines and NI 43-101 (Canadian Securities Administrators).

Question 2.1: VMS and SEDEX Deposit Models — Geologic Setting (12 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

VMS (e.g. Bathurst, NB)SEDEX (e.g. Sullivan, BC)massive sulphide lensstringer/feederfelsic volcanic pileseawater convectionlaminated Zn-Pb-Ag +/- bariteclastic sedimentary basin fillbasinal brine venting
Schematic cross-sections: VMS mound with underlying stringer/feeder alteration pipe in a felsic volcanic pile (left) vs. SEDEX laminated sulphide layer conformable within a clastic sedimentary basin fill (right).

Both deposit types form by metalliferous hydrothermal fluid discharging onto or near the seafloor and precipitating sulphides on contact with cold seawater/basin water, but their HOST and DRIVING environment differ fundamentally. VMS deposits form at or near submarine volcanic centres (mid-ocean ridges, back-arc or arc-related felsic-dominated volcanic piles — the Bathurst Camp, NB is a classic Ordovician bimodal felsic-mafic arc setting): seawater convects deep into the hot volcanic pile, leaches metals, and vents at the seafloor through a focused feeder/stringer zone, precipitating a stratiform massive sulphide MOUND directly on the volcanic-sedimentary contact. SEDEX deposits instead form within actively subsiding, extensional (rifted) sedimentary basins (the Sullivan deposit, BC, in the Purcell/Belt Basin): metal-bearing BASINAL BRINES, concentrated by evaporation/diagenetic processes in restricted sub-basins and driven by geothermal/rift heat rather than direct volcanic heat, migrate along growth faults and vent into anoxic bottom waters, producing thinly LAMINATED sulphide beds interbedded WITHIN the normal clastic sedimentary sequence rather than atop a volcanic edifice. The "ocean/brine" distinction is therefore central: VMS is a seawater-convection-through-volcanic-rock system, while SEDEX is a basinal-brine-expulsion-through-sedimentary-rock system, and this is why VMS deposits are always found within (or immediately overlying) a volcanic pile while SEDEX deposits are hosted entirely within fine clastic sedimentary rocks (shale, siltstone) with at most minor associated volcanism.

Constituent economic minerals and products: VMS ore is dominated by pyrite, pyrrhotite, chalcopyrite and sphalerite (± galena, minor Au/Ag), typically split by differential flotation into separate copper and zinc concentrates for shipment (with Au/Ag credits in one or both). SEDEX ore is dominated by sphalerite and galena with abundant pyrite and often barite, split into zinc and lead concentrates with significant silver credit in the lead concentrate. Alteration: VMS deposits carry a well-developed, structurally focused chlorite–sericite–silica ALTERATION PIPE directly beneath the massive sulphide lens (the fossil feeder conduit, itself sometimes ore-grade as a stringer/stockwork zone), grading outward to a broader propylitic halo in the volcanic pile. SEDEX alteration is comparatively SUBTLE — diagenetic silicification, carbonatization and sometimes a footwall feeder-vein stockwork, but without the intense, laterally-restricted alteration pipe typical of a VMS feeder zone, because heat and fluid focusing are weaker in a sediment-hosted brine system than in a volcanic-heat-driven convective one.