Rubicon Minerals Provides an Updated Preliminary Interpretation of the Structural Geology of the F2 Gold Deposit

by ahnationtalk on March 12, 2018105 Views

TORONTO, March 12, 2018  – Rubicon Minerals Corporation (TSX: RMX | OTCQX: RBYCF) (“Rubicon” or the “Company”) provides an updated preliminary interpretation of the structural geology of the F2 Gold Deposit (the “Preliminary Structural Interpretation”) at the Phoenix Gold Project (the “Project”).

Key Takeaways from the Preliminary Structural Interpretation:

  • The primary structural controls on the gold mineralization at the F2 Gold Deposit are the well-established dextral Riedel vein system of quartz-actinolite veins (the “Riedel vein system”) that occur within the HiTi Basalt Units (the main host rock) and the Felsic Intrusive Units (to a lesser extent).
  • Elevated gold grades appear to be spatially associated with the R’ veins within the Riedel vein system and the multiple Quartz Breccia Zones, which share the same geometry as the R’ veins.
  • The Preliminary Structural Interpretation appears to show more continuity of gold mineralization within the HiTi Basalt Units compared to the 2016 structural interpretation, with the main difference being the identification of the Riedel vein system.
  • In the 2016 structural interpretation, the discrete D2 east-west shear zones (previously termed as the “D2 structures”) were interpreted as the main controls of high-grade gold mineralization at the F2 Gold Deposit. However, in the Preliminary Structural Interpretation, these discrete D2 east-west share zones appear to be localized, discontinuous, and not seen as having significant controls on the gold mineralization at the deposit.
  • The Preliminary Structural Interpretation has simplified and improved future modelling and allows for the evaluation of bulk mining methods.

CEO’s Comments

Rubicon President and Chief Executive Officer George Ogilvie, P.Eng., stated, “the Preliminary Structural Interpretation provides an encouraging step towards advancing our understanding of the F2 Gold Deposit at the Phoenix Gold Project. The Preliminary Structural Interpretation benefits from the data and observations we collected in 2017 from the detailed mapping, oriented drilling, and core re-logging, which, collectively, provide critical information that was not available when the previous 2016 structural interpretation and geological model were developed. The Preliminary Structural Interpretation simplifies our understanding of the F2 Gold Deposit, bringing to light the gold-bearing Riedel vein system, which appears to create more continuity of gold mineralization within the HiTi Basalt Units when compared to the 2016 geological model.”

“We will continue to refine and support the Preliminary Structural Interpretation from the data and observations we plan to collect from our 2018 Exploration Program. Underground development at site is progressing well. We are continuing to stockpile low-grade material on surface, which will be used for bedding for our mill before we process higher-grade mineralized material from the test stopes.”

“We are on track to commence the processing of mineralized material from the test stopes in mid-2018. We are currently blasting the #977 test stope, production drilling the #015 test stope, and developing the #161 test stope. We remain on schedule to provide an updated geological model and Mineral Resource Estimate in the second half of 2018.”

Preliminary Structural Interpretation Details

Rubicon and its consultants, Golder Associates Ltd., Terrane Geoscience Inc. and T. Maunula and Associates (together, “Consultants”), have developed the Preliminary Structural Interpretation. The Preliminary Structural Interpretation has not been finalized; it will continue to be refined, enhanced, and supported with data and observations collected from the 2018 Exploration Program, prior to potentially serving as the basis of an updated geological model and Mineral Resource Estimate. The Preliminary Structural Interpretation utilizes historical data and benefits from the additional data and observations collected from the Company’s 2017 Exploration program, including more than 10,000 metres (“m”) of historical core re-logging for structural geology, more than 3,500 m of targeted oriented structural drilling, over 20,000 m of infill and step-out drilling (which included an additional 6,500 m of oriented drilling), and detailed underground mapping on the 183-, 244-, and 305-metre levels. The 2017 Exploration Program included oriented drilling in both the north and east directions (all orientations referenced are relative to mine grid; mine grid north is oriented 45 degrees east of true north) to allow for evaluation of both north-south (“N-S”) and east-west (“E-W”) oriented structures and associated trends in gold mineralization.

Host Rock

The local geology in the F2 Gold Deposit area appears to comprise of a series of N-S trending, steeply dipping to sub-vertical alternating volcanic units of talc-altered komatiitic ultramafic flows (“Ultramafic Flow Units”) and biotite and silica altered basaltic mafic volcanic flows (“HiTi Basalt Units”). There are three predominant HiTi Basalt lenses that comprise the F2 Gold Deposit (from west to east, mine-grid): The Hanging Wall Basalt, the West Limb Basalt, and the F2 Basalt. These volcanic units were later intruded by a series of quartz-feldspar porphyry felsic dykes and sills (“Felsic Intrusive Units”) as well as less abundant intermediate and mafic dykes and sills.

The geometry and distribution of the HiTi Basalt lenses are a result of regional scale deformation events, resulting in the boudinage (the stretching and brittle-ductile deformation of more competent units relative to ductile deformation of surrounding less competent units) of the HiTi Basalt lenses in the N-S direction.

Please see Diagrams 1 and 2 for the conceptual images of the local geology of the F2 Gold Deposit, plan and section views.

Deformation History

Similar to the 2016 structural geological interpretation, the updated Preliminary Structural Interpretation recognizes three primary deformation events associated with the F2 Gold Deposit, which include:

  • D1: Regional shortening related to thrusting and folding.
  • D2: Dextral transpression (right lateral oblique sense of shear due to a component of shortening perpendicular to the shear plane) along the regional scale N-S oriented East Bay Deformation Zone. This is the main deformation event associated with gold mineralization.
  • D3: Regional folding related to a later broad open fold with a N-S trending, sub-horizontal fold axis.

 

Please see Diagram 3 for a conceptual summary of the deformation events and related structural elements that comprise the Preliminary Structural Interpretation. While the D1 and D3 deformation events are important for understanding the initial structural modification of the deposit stratigraphy and the present-day geometry of the deposit, respectively, the D2 deformation event represents the main deformation event associated with gold mineralization at the F2 Gold Deposit.

Rubicon and its Consultants believe that the strain partitioning (heterogeneous distribution of strain type and intensity due to the variability in composition, mechanical properties, pre-existing deformation features and geometry of geological units or domains) during the D1 and D2 deformation resulted in the ductile deformation of the talc-rich Ultramafic Flow Units and the brittle-ductile deformation of the more resistant HiTi Basalt and Felsic Intrusive Units.  The ductile behaviour of the Ultramafic Flow Units resulted in the generation of the pervasive N-S oriented, steeply dipping to sub-vertical S1 penetrative foliation during the D1 deformation event. The brittle-ductile behaviour of the HiTi Basalt Units resulted in the boudinage of these units, with the primary stretching direction paralleling the N-S orientation, with a lesser vertical component of stretching such that the boudin necks that bound the HiTi Basalt lenses are arranged in both N-S, shallowly dipping and sub-vertical orientations.

Structural Controls on Gold Mineralization

The 2016 structural interpretation and the Preliminary Structural Interpretation are similar in that the high-grade gold mineralization is associated with structural features during the D2 deformation event. However, there appears to be significant differences in terms of the key controlling features and resultant geometries between the two interpretations.

The 2016 structural interpretation identified the D2 E-W oriented shear zones (previously termed as the “D2 structures”) cutting across the deposit as the primary structural feature controlling the gold mineralization at the F2 Gold Deposit. Please see Diagram 4 for a conceptual summary of the 2016 structural interpretation. Modelled gold mineralization trends were generally oriented N-S, in part due to the predominant E-W orientation of most drill holes drilled prior to the 2017 Exploration Program.

Data and observations collected from the 2017 detailed underground mapping and oriented core drilling programs indicate that, while D2 E-W oriented shear zones and brittle faults do exist, they are generally more localized and discontinuous in both their lateral and vertical extents than previously interpreted. They do not appear to represent deposit-scale features. Additionally, the D2 E-W oriented shear zones are not necessary to explain the geometry and continuity of the HiTi Basalt Unit panels and the Felsic Intrusive Units, or the gold mineralization hosted within these units.

The updated Preliminary Structural Interpretation identifies the Riedel vein system as the primary structural control on gold mineralization at the F2 Gold Deposit. The Riedel vein system is interpreted to have occurred during D2 regional dextral transpression. The gold mineralization occurs in association with disseminated sulphide replacement and vein mineralization, both of which developed in the more competent HiTi Basalt Units, and, to a lesser degree, in the Felsic Intrusive Units. The 2017 drilling program and subsequent new interpretation suggest that the continuity of gold mineralization within the HiTi Basalt Units is better than previously modelled. Please see Diagram 3 for a conceptual image of the Riedel vein system within the HiTi Basalt Units.

The Riedel vein system, comprised of the R’, R, and P shear veins (each representing a different orientation; see bottom left quadrant of Diagram 3 for further details), and the Quartz-Breccia Zones, all host gold mineralization. The highest gold grades generally occur within the R’ veins and the Quartz-Breccia Zones (both share the same E-W striking, sub-vertical dipping geometry), located in the thickest portions of the HiTi Basalt Units. The Quartz-Breccia Zones are interpreted to have developed as multiple opening and sealing events of the E-W striking sub-vertical R’ veins. A possible explanation for the development of the R’ veins related to the Quartz-Breccia Zones is that their sinistral sense of shear is opposed to the dextral bulk sense of shear, resulting in limited displacement but allowing for the development of zones of intense deformation, where the repeated fracturing and comminution of the vein and wall rock material resulted in the creation of high-porosity and permeability zones for mineralizing fluids. The thickest portions of the HiTi Basalt Unit presented both favourable structural traps for the R’ veins and Quartz-Breccia Zones and chemical traps where disseminated sulphides and associated gold mineralization are developed. These areas are the focus of the Company’s future exploration efforts. Table 1 below summarizes the key differences between the 2016 structural interpretation and the Preliminary Structural Interpretation.

Table 1: Apparent Key Differences between the 2016 Structural Interpretation and the 2018 Preliminary Structural Interpretation

2016 Structural Interpretation

2018 Preliminary Structural Interpretation

Host Rock

HiTi Basalt Units (main);

Felsic Intrusive Units (lesser extent);

Used multiple descriptors for lithological units.

HiTi Basalt Units (main);

Felsic Intrusive Units (lesser extent);
Simplified library of lithological units, removing some apparent geological complexity.

Continuity
and geometry
of HiTi Basalt
Units

HiTi Basalt Units modelled as relatively continuous and constant thickness panels of basalt offset by D2 E-W shear zones and late-brittle faulting.

HiTi Basalt Units modelled as discontinuous lenses of basalt formed as a result of strain partitioning during D2 dextral transpression, resulting in the boudinage of the HiTi Basalt lenses and ductile deformation of Ultramafic Flow Units.

Structural
controls on
gold mineralization

Discrete D2 E-W shear zones;

Quartz-Breccia Zones (1 zone).

D2 dextral transpression related Riedel vein system (R’, R, P shear veins) developed throughout the HiTi Basalt Units and, to a lesser degree, in the Felsic Intrusive Units;

Higher-grade gold mineralization hosted in R’ veins and Quartz-Breccia Zones (multiple zones) within the HiTi Basalt Units;

D2 E-W shear zones identified as being localized, discontinuous and not seen as significant control on gold mineralization.

Continuity of
gold
mineralization

Discontinuous within the HiTi Basalt Units, associated with vein and sulphide replacement mineralization in proximity to discrete D2 E-W shear zones

More continuous within the HiTi Basalt Units when compared to 2016 due to the development of the Riedel vein system throughout the HiTi Basalt Units

 

Potential Impact of Preliminary Structural Interpretation

The Preliminary Structural Interpretation has a few potential implications with regard to an updated geological model and how the Company views the F2 Gold Deposit:

  • Potential simplification of the geological model: As alluded to in prior news releases, the 2016 geological model had multiple descriptors for lithological units, which didn’t necessarily match with what was observed from the data collected in 2017. The Preliminary Structural Interpretation has grouped and simplified the lithological descriptors into three main units consisting of the main HiTi Basalt Units, Felsic Intrusive Units and Ultramafic Flow Units.
  • The appropriate modelling of the Riedel vein system and its focus for future exploration: The Preliminary Structural Interpretation, based on data and observations from the detailed underground mapping and oriented core drilling in 2017, recognizes that the Riedel vein system, which include the higher-grade R’ veins and Quartz-Breccia Zones, were either non-existent or not adequately captured in the 2016 structural interpretation. These areas are the focus of future exploration efforts. The Preliminary Structural Interpretation shows a more continuous mineralized envelope compared to the 2016 geological model.
  • Evaluating the amenability of bulk mining: Upon review of the Preliminary Structural Interpretation, along with the current understanding of the distribution of the gold mineralization within the deposit, the Company and its Consultants are evaluating whether bulk mining methods are more amenable to the F2 Gold Deposit compared to narrow-vein, selective methods. The planned test mining in 2018 will provide more direction on the potential predominant mining methods.

As mentioned, the Preliminary Structural Interpretation has not been finalized and will continue to be refined and enhanced with data and observations collected from the 2018 Exploration Program.

Qualified Persons and Quality Assurance and Quality Control (QA/QC)

The content of this news release has been read and approved by George Ogilvie, P.Eng., President and CEO for Rubicon, and Jerry DeWolfe, P.Geo., M.Sc. and Brian Thomas, P.Geo. for Golder Associates Ltd. All three are Qualified Persons as defined by NI 43-101.

Underground drilling was conducted by Boart Longyear Drilling of Haileybury, Ontario and was supervised by the Rubicon exploration team. Oriented core drilling was performed using the Boart Longyear TruCore™ orientation system.  All assays reported are uncut unless otherwise stated. All samples reported herein were performed by SGS Mineral Services of Red Lake, Ontario.  All NQ core assays reported were obtained by fire assay with AA-finish or using gravimetric finish for values over 10.0 g/t Au.

Intercepts cited do not necessarily represent true widths, unless otherwise noted, however drilling is generally intersecting interpreted mineralized zones at a high angle. True width determinations are estimated at 65-80% of the core length intervals for the 305-metre level drilling, and estimated at 75-95% of the core length for the 610- and 685-metre level drilling.  Rubicon’s quality control checks include insertion of blanks, certified reference standards and blind duplicates to ensure laboratory accuracy and precision.

About Rubicon Minerals Corporation
Rubicon Minerals Corporation is an advanced gold exploration company that owns the Phoenix Gold Project, located in the prolific Red Lake gold district in northwestern Ontario, Canada. Additionally, Rubicon controls over 280 square kilometres of prime exploration ground in Red Lake and more than 900 square kilometres of mineral property interests in the emerging Long Canyon gold district that straddles the Nevada-Utah border in the United States. Rubicon’s shares are listed on the Toronto Stock Exchange (RMX) and the OTCQX markets (RBYCF). For more information, please visit our website at www.rubiconminerals.com.

RUBICON MINERALS CORPORATION
George Ogilvie, P.Eng.
President, CEO, and Director

Allan Candelario, CFA, Director, Investor Relations and Corporate Development, Phone: 1 (416) 766-2804 | E-mail: ir@rubiconminerals.com | www.rubiconminerals.com, Rubicon Minerals Corporation | Suite 830-121 King St. W. | Toronto ON, CANADA M5H 3T9

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